U.S. patent application number 14/355943 was filed with the patent office on 2014-10-02 for method for producing metal by molten salt electrolysis and apparatus used for the production method.
The applicant listed for this patent is SUMITOMO ELECTRIC INDUSTRIES, LTD.. Invention is credited to Tomoyuki Awazu, Masatoshi Majima.
Application Number | 20140291161 14/355943 |
Document ID | / |
Family ID | 48191866 |
Filed Date | 2014-10-02 |
United States Patent
Application |
20140291161 |
Kind Code |
A1 |
Awazu; Tomoyuki ; et
al. |
October 2, 2014 |
METHOD FOR PRODUCING METAL BY MOLTEN SALT ELECTROLYSIS AND
APPARATUS USED FOR THE PRODUCTION METHOD
Abstract
Provided is a method for obtaining a particular metal at high
purity, with safety, and at low cost, from a treatment object
containing two or more metal elements. The present invention
provides a method for producing a metal by molten salt
electrolysis, the method including a step of dissolving, in a
molten salt, a metal element contained in a treatment object
containing two or more metal elements; and a step of depositing or
alloying a particular metal present in the molten salt, on one of a
pair of electrode members disposed in the molten salt containing
the dissolved metal element, by controlling a potential of the
electrode members to a predetermined value.
Inventors: |
Awazu; Tomoyuki; (Itami-shi,
JP) ; Majima; Masatoshi; (Itami-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUMITOMO ELECTRIC INDUSTRIES, LTD. |
Osaka-shi, Osaka |
|
JP |
|
|
Family ID: |
48191866 |
Appl. No.: |
14/355943 |
Filed: |
October 22, 2012 |
PCT Filed: |
October 22, 2012 |
PCT NO: |
PCT/JP2012/077223 |
371 Date: |
May 2, 2014 |
Current U.S.
Class: |
205/348 ;
204/230.2; 205/363; 205/367; 205/368; 205/369; 205/371; 205/397;
205/399; 205/402; 205/407 |
Current CPC
Class: |
C25C 3/02 20130101; C25C
3/28 20130101; C25C 3/26 20130101; C25C 3/34 20130101; C25C 3/36
20130101; C25C 3/00 20130101; C22C 21/02 20130101; C25C 7/06
20130101; C22C 1/02 20130101 |
Class at
Publication: |
205/348 ;
205/367; 205/363; 205/371; 205/407; 205/368; 205/399; 205/397;
205/402; 205/369; 204/230.2 |
International
Class: |
C25C 3/00 20060101
C25C003/00; C25C 7/06 20060101 C25C007/06; C25C 3/28 20060101
C25C003/28; C25C 3/26 20060101 C25C003/26; C25C 3/34 20060101
C25C003/34; C25C 3/02 20060101 C25C003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 4, 2011 |
JP |
2011-242381 |
Nov 4, 2011 |
JP |
2011-242457 |
Nov 4, 2011 |
JP |
2011-242473 |
Dec 22, 2011 |
JP |
2011-281477 |
Claims
1. A method for producing a metal by molten salt electrolysis, the
method comprising: a step of dissolving, in a molten salt, a metal
element contained in a treatment object containing two or more
metal elements; and a step of depositing or alloying a particular
metal present in the molten salt, on one of a pair of electrode
members disposed in the molten salt containing the dissolved metal
element, by controlling a potential of the electrode members to a
predetermined value.
2. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the treatment object is an ore or a
crude metal ingot obtained from the ore.
3. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the method is a method for producing
tungsten, a metal element contained in the treatment object is
tungsten, in the step of dissolving, in a molten salt, a metal
element from a treatment object, tungsten is dissolved from the
treatment object, and in the step of depositing or alloying a
particular metal, tungsten present in the molten salt is deposited
on one of a pair of electrode members disposed in the molten salt
containing dissolved tungsten, by controlling a potential of the
electrode members to a predetermined value.
4. The method for producing a metal by molten salt electrolysis
according to claim 3, wherein the treatment object is a metal
material containing the tungsten.
5. The method for producing tungsten a metal by molten salt
electrolysis according to claim 3, wherein the treatment object is
a metal material containing tungsten and a transition metal.
6. The method for producing a metal by molten salt electrolysis
according to claim 3, wherein the treatment object is a cemented
carbide product.
7. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the method is a method for producing
lithium, a metal element contained in the treatment object is
lithium, in the step of dissolving, in a molten salt, a metal
element from a treatment object, lithium is dissolved from the
treatment object, and in the step of depositing or alloying a
particular metal, lithium present in the molten salt is deposited
on one of a pair of electrode members disposed in the molten salt
containing dissolved lithium, by controlling a potential of the
electrode members to a predetermined value.
8. The method for producing a metal by molten salt electrolysis
according to claim 7, wherein the treatment object is a material
containing lithium and a transition metal.
9. The method for producing a metal by molten salt electrolysis
according to claim 7, wherein the treatment object is a battery
electrode material containing lithium.
10. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the treatment object contains a
transition metal or a rare earth metal.
11. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the treatment object contains one or
more metals selected from the group consisting of V, Nb, Mo, Ti,
Ta, Zr, and Hf.
12. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the treatment object contains Sr
and/or Ba.
13. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the treatment object contains one or
more metals selected from the group consisting of Zn, Cd, Ga, In,
Ge, Sn, Pb, Sb, and Bi.
14. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the molten salt is selected such
that, in the step of depositing or alloying a particular metal, a
difference between a standard electrode potential of a simple
substance or alloy of the particular metal and a standard electrode
potential of a simple substance or alloy of another metal in the
molten salt is 0.05 V or more.
15. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein, in the step of depositing or
alloying a particular metal, the potential of the electrode members
is controlled to the predetermined value so that the particular
metal in the molten salt is selectively deposited or alloyed.
16. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein, in the step of dissolving, in a
molten salt, a metal element contained in a treatment object, the
metal element is dissolved in the molten salt by a chemical
procedure.
17. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein, in the step of dissolving, in a
molten salt, a metal element contained in a treatment object, a
cathode and an anode that is formed of an anode material containing
the treatment object are disposed in the molten salt, and a
potential at the anode is controlled to a predetermined value so
that a metal element corresponding to the controlled potential is
dissolved in the molten salt from the treatment object.
18. The method for producing a metal by molten salt electrolysis
according to claim 17, wherein the molten salt is selected such
that, in the step of dissolving, in a molten salt, a metal element
contained in a treatment object, a difference between a standard
electrode potential of a simple substance or alloy of the
particular metal and a standard electrode potential of a simple
substance or alloy of another metal in the molten salt is 0.05 V or
more.
19. The method for producing a metal by molten salt electrolysis
according to claim 17, wherein, in the step of dissolving, in a
molten salt, a metal element contained in a treatment object, the
potential at the anode is controlled to a predetermined value so
that the particular metal element is selectively dissolved in the
molten salt.
20. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein, in the step of dissolving, in a
molten salt, a metal element contained in a treatment object, one
or more metals each serving as the particular metal are dissolved
in the molten salt.
21. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the particular metal deposited or
alloyed is a transition metal.
22. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the particular metal deposited or
alloyed is a rare earth metal.
23. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the particular metal deposited or
alloyed is V, Nb, Mo, Ti, Ta, Zr, or Hf.
24. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the particular metal deposited or
alloyed is Sr or Ba.
25. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the particular metal deposited or
alloyed is Zn, Cd, Ga, In, Ge, Sn, Pb, Sb, or Bi.
26. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the molten salt is a chloride or
fluoride molten salt.
27. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the molten salt is a molten salt
mixture containing a chloride molten salt and a fluoride molten
salt.
28. The method for producing a metal by molten salt electrolysis
according to claim 1, wherein the treatment object has a form of
particles or powder.
29. The method for producing a metal by molten salt electrolysis
according to claim 28, wherein the treatment object having the form
of particles or powder is compacted to form the anode.
30. A method for producing a metal by molten salt electrolysis, the
method being a method for producing a particular metal by molten
salt electrolysis from a treatment object containing two or more
metal elements, wherein a cathode and an anode that is formed of an
anode material containing the treatment object are disposed in a
molten salt, and a potential at the anode is controlled to a
predetermined value so that a particular metal is left in the anode
by performing a step of dissolving a metal element corresponding to
the controlled potential in the molten salt from the treatment
object.
31. The method for producing a metal by molten salt electrolysis
according to claim 30, wherein the treatment object is an ore or a
crude metal ingot obtained from the ore.
32. The method for producing a metal by molten salt electrolysis
according to claim 30, wherein the method is a method for producing
tungsten by molten salt electrolysis from a treatment object
containing tungsten, a cathode and an anode that is formed of an
anode material containing the treatment object are disposed in a
molten salt, and a potential at the anode is controlled to a
predetermined value so that a metal element corresponding to the
controlled potential is dissolved in the molten salt from the
treatment object and tungsten is left in the anode.
33. The method for producing a metal by molten salt electrolysis
according to claim 30, wherein the molten salt is selected such
that, in the step of dissolving a metal element in the molten salt
from the treatment object, a difference between a standard
electrode potential of a simple substance or alloy of the
particular metal and a standard electrode potential of a simple
substance or alloy of another metal in the molten salt is 0.05 V or
more.
34. An apparatus used for a method for producing a metal by molten
salt electrolysis, the apparatus comprising: a container containing
a molten salt; a cathode immersed in the molten salt contained
within the container; and an anode that is immersed in the molten
salt contained within the container and that contains a treatment
object containing two or more metal elements, wherein the molten
salt is movable into and out of the anode, the apparatus further
comprises a control unit configured to control a potential of the
cathode and the anode to a predetermined value, and a value of the
potential is changeable in the control unit.
35. An apparatus used for a method for producing a metal by molten
salt electrolysis, the apparatus comprising: a container containing
a molten salt containing two or more dissolved metal elements; a
cathode and an anode that are immersed in the molten salt contained
within the container; and a control unit configured to control a
potential of the cathode and the anode to a predetermined value,
wherein a value of the potential is changeable in the control
unit.
36. The apparatus according to claim 34, wherein the two or more
metal elements include at least one of tungsten and lithium.
37. The apparatus according to claim 35, wherein the two or more
metal elements include at least one of tungsten and lithium.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
metal by molten salt electrolysis; and an apparatus used for the
production method.
BACKGROUND ART
[0002] Known methods of smelting ores to provide particular metals
are pyrometallurgy and hydrometallurgy.
[0003] The pyrometallurgical smelting is a method of melting an ore
in a high temperature furnace to separate a target metal. For
example, a concentrate, a roasted ore, or a sintered ore is melted
in a high temperature furnace, concentrated into a crude metal
ingot while gangue, impurities, and the like are separated as slag
(Non Patent Literature (NPL) 1, p. 46).
[0004] In smelting, since the specific gravity difference between
molten metals is utilized to separate the metals from an ore, the
specific gravity difference between the metals to be separated
needs to be large. In addition, separation targets need to have low
solubility in each other. Since elements that satisfy such
conditions between metal materials are limited, target elements
separated by pyrometallurgy are limited, which is problematic.
[0005] The hydrometallurgy is a method of dissolving an ore in, for
example, an alkaline or acidic solution and separating and
extracting a target metal from the solution. A method for
separating and extracting the target metal from this aqueous
solution is, for example, a method using ion exchange, a method
using solvent extraction, or a method using aqueous solution
electrolysis.
[0006] In the method using ion exchange, a solid substance that
partially has a group of ions allowing ion exchange and is referred
to as an ion exchanger is used to perform reversible ion exchange
(NPL 1, p. 194).
[0007] Ion exchange, which uses the adsorption capability and
exchange capability of an ion-exchange resin, is an excellent
treatment. However, since this treatment is performed by repeating
of adsorption and dissociation of ions, ion exchange is not
suitable for economically and efficiently treating a large amount
of substance, which is problematic.
[0008] The method using solvent extraction is a separation method
using the difference in the distributions of different solutes in
two solvents that are immiscible with each other (NPL 1, p.
199).
[0009] In this solvent extraction, for example, an acid treatment
is performed to achieve ionization; and, in separation, a large
number of treatment processes need to be performed. In these
processes, large amounts of acid and alkali are required and a
large amount of wastewater is generated, which is problematic.
[0010] In the method of electrolysis smelting using aqueous
solution electrolysis, the presence of a difference between
elements in tendency for anode dissolution or cathode deposition is
used and a pure metal is produced. Simultaneously, in the
electrolytic solution used, reactions of generating slightly
soluble salts from impurity ions are also used (NPL 1, p. 219).
[0011] However, metal elements that can be separated and deposited
by purification using aqueous solution electrolysis are limited.
For example, deposition of rare earth materials cannot be
theoretically achieved, which is problematic.
[0012] Regarding Al, electrolytic smelting utilizing molten salt
electrolysis is also known. In this method, three layers of Al
(purification target material) alloyed so as to have a decreased
melting point, a molten salt, and the recovered metal, are formed
and the specific gravity difference is utilized to perform
smelting. Since the specific gravity difference is thus utilized,
smelting needs to be performed while all the three layers are
melted (NPL 1, p. 254).
[0013] The target metal of this method is Al. In addition, when the
potential of an impurity present with the purification target metal
is close to the potential of the purification target metal, entry
of the impurity into the deposited target metal occurs, which is
problematic.
[0014] On the other hand, a method for recovering tungsten is
described in, for example, NPL 2 as follows.
[0015] Hard scrap or soft scrap of cemented carbide tools is made
to react with sodium nitrate molten salt and then dissolved in
water to produce an aqueous solution of sodium tungstate. The
aqueous solution of sodium tungstate is treated by an ion-exchange
method using an ion-exchange resin to produce an aqueous solution
of ammonium tungstate. From the aqueous solution of ammonium
tungstate, ammonium paratungstate (APT) is crystallized. After
that, the thus-crystallized ammonium paratungstate is calcined,
reduced, and carbonized to provide tungsten carbide.
[0016] The hard scrap collectively denotes pieces of scrap still
having the shapes of products. The soft scrap denotes powder-form
scrap such as powder dust and cutting dust generated during
processing for producing cemented carbide tools.
[0017] Patent Literature (PTL) 1 proposes, in the production of
sodium tungstate by oxidizing hard alloy scrap and/or heavy metal
scrap in a molten salt bath, use of a molten salt containing 60 to
90 wt % of sodium hydroxide and 10 to 40 wt % of sodium sulfate.
PTL 1 also proposes that the reaction between such scrap and the
molten salt is performed in a rotary kiln that is operated as batch
processes and can be directly heated.
[0018] However, in the above-described method described in NPL 2,
the reaction between hard scrap or soft scrap of cemented carbide
tools and sodium nitrate molten salt occurs very vigorously.
Accordingly, the reaction is difficult to control and the operation
has safety problems. In addition, when hard scrap or soft scrap of
cemented carbide tools is made to react with sodium nitrate molten
salt, metals contained in the hard scrap or soft scrap of cemented
carbide tools, such as vanadium and chromium, take the form of
water-soluble metal oxide ions and enter the aqueous solution of
sodium tungstate. As a result, because of the presence of such
metals as impurities, high purity is difficult to achieve, which is
problematic.
[0019] In the above-described method described in PTL 1, sodium
sulfate molten salt serving as an oxidizing agent has a high
melting point of 884.degree. C. Accordingly, the temperature during
reaction needs to be set to a high temperature of 884.degree. C. or
more. As a result, metal materials are corroded, which is
problematic. In addition, the reaction proceeds slowly and hence
the reaction is time-consuming and involves a large energy loss,
which is problematic.
[0020] On the other hand, lithium is mainly extracted from
lithium-containing ores (such as spodumene, amblygonite, petalite,
and lepidolite), and salt lakes and underground brine that have
high lithium concentrations. However, Japan does not have
lithium-containing ores or salt lakes. Accordingly, almost the
whole amount of lithium is actually supplied by imports.
[0021] Thus, recently, studies were started regarding separation
and recovery of lithium from, for example, lithium-containing waste
generated in the production steps of lithium-containing products
such as lithium batteries or waste of used lithium-containing
products.
[0022] The following method for recovering lithium has been
proposed: lithium cobalt oxide serving as the positive electrode
material of lithium secondary batteries, together with metallic
lithium, is subjected to a reduction reaction in lithium chloride
molten salt, so that lithium oxide is generated and cobalt or
cobalt oxide is separated by precipitation; after that, lithium
oxide is electrolyzed in the lithium chloride molten salt, so that
metallic lithium is deposited on the cathode and recovered (PTL 2:
Japanese Unexamined Patent Application Publication No.
2005-011698).
[0023] However, in this method, in order to separate cobalt
contained in the treatment object by reduction, addition of
metallic lithium is required. The method employs the step of adding
metallic lithium in order to recover metallic lithium and hence is
inefficient, which is problematic.
[0024] A method for recovering lithium has been proposed in which a
mixture of carbon and lithium manganese oxide serving as the
positive electrode material of lithium secondary batteries is
roasted in any one of the air atmosphere, an oxidizing atmosphere,
an inert atmosphere, and a reducing atmosphere, to turn the lithium
into lithium oxide; and this roasted substance is immersed in water
so that lithium is leached in the form of lithium hydroxide and
lithium carbonate (PTL 3).
[0025] However, in this method, since lithium hydroxide and lithium
carbonate do not have high solubility, the recovery efficiency is
low. In addition, a large amount of water is required to leach
lithium hydroxide and lithium carbonate into water and, as a result
of the treatment, a large amount of wastewater is generated, which
is problematic.
[0026] Furthermore, tantalum (Ta) is mainly used in tantalum
capacitors and can be recovered from tantalum capacitor scrap.
Specifically, tantalum is recovered by processes of oxidation
treatment, magnetic separation, screening, separation with running
water, grinding, screening, leaching, oxidation treatment,
reduction treatment, and leaching (refer to NPL 3, pages 319 to
326).
[0027] Vanadium (V) is used as an additive to steel or a
desulfurization catalyst in oil refining. Vanadium used as an
additive to steel is collected in the form of steel scrap and
recycled as steel. Spent catalysts can be sequentially subjected to
steps of classification, roasting, grinding, leaching, filtration,
leaching solution, dehydration, thermal decomposition, and melting,
so that vanadium pentoxide can be obtained (NPL 3, pages 391 to
396).
[0028] Molybdenum (Mo) is also used as an additive to steel, alloy,
or a desulfurization catalyst in oil refining. Molybdenum used as
an additive to steel or an alloy element is collected in the form
of steel or alloy and used, without being extracted, in the form of
steel or alloy. Spent catalysts can be sequentially subjected to
steps of roasting, removal of oil, water, and sulfur, leaching in
basic condition, and recovery, so that Mo can be obtained (NPL 3,
pages 301 to 303).
[0029] Niobium (Nb) is mainly used as an additive to steel. Niobium
used as an additive to steel is collected in the form of steel
scrap. However, the niobium content of high-tensile steel,
stainless steel, and the like is very low and niobium itself is not
recycled (NPL 3, page 339).
[0030] Manganese (Mn) is mostly used for steel and aluminum alloy
and collected in the form of steel scrap and aluminum alloy scrap,
respectively. In the case of recycling of steel, a high proportion
of manganese is left in various slags and such manganese forming
slags is not suitable for recycling. Manganese in slag is partially
used in, for example, a manganese-calcium silicate fertilizer.
[0031] Aluminum cans containing such aluminum alloy are collected
and then recycled (NPL 3, pages 343 to 344).
[0032] Chromium (Cr) used for steel (stainless steel) and
superalloy is collected in the form of steel scrap and superalloy
scrap, respectively, and then recycled; and extraction and recovery
of elemental chromium is not performed (NPL 3, pages 219 to
221).
[0033] In the above-described recycling techniques, recovery
involves a large number of processes such as roasting (heating),
grinding, leaching, and reduction. And the processes are
complicated and hence the treatment is time-consuming and costly,
which is problematic.
[0034] In addition, the treatment requires roasting and, in the
treatment, substances that are not the extraction target are also
treated, resulting in unnecessary energy consumption. Furthermore,
by subjecting substances that are not the treatment target to the
roasting treatment, unnecessary oxides are generated, resulting in
a large amount of waste. In addition, since acid treatment or base
treatment is performed, the treatment produces acid or base
wastewater, which exerts a load on the environment.
[0035] In summary, existing metal recycling techniques have
problems as follows: for example, the treatment is costly, energy
loss is large, the amount of waste is large, and the environmental
load is heavy. In addition, because of problems in terms of cost or
technique, some metals are not recycled as simple substances.
CITATION LIST
Non Patent Literature
[0036] PTL 1: Japanese Unexamined Patent Application Publication
(Translation of PCT Application) No. 11-505801 [0037] PTL 2:
Japanese Unexamined Patent Application Publication No. 2005-011698
[0038] PTL 3: Japanese Unexamined Patent Application Publication
No. 2011-094227 [0039] NPL 1: Courses of Contemporary Metallurgy,
Smelting Part, Vol. 2, Nonferrous Metal Smelting, edited by The
Japan Institute of Metals and Materials (1980), pages 46, 194, 199,
219, and 254 [0040] NPL 2: Rare-Metal
High-Efficiency-Recovery-System Development Project "Recovery of
tungsten etc. from waste cemented carbide tools", Metal Resource
Report, Vol. 38, No. 4, November 2008, pp. 407-413 [0041] NPL 3:
Compilation of Noble Metal and Rare Metal Recycling Techniques,
published by NTS Inc., planned and edited by Bookers Ltd., the
first impression of the first edition, Oct. 19, 2007
SUMMARY OF INVENTION
Technical Problem
[0042] In view of the above-described problems, an object of the
present invention is to provide a method for producing a metal, the
method being applicable to any ore and providing high purity metal
at low cost; and an apparatus used for the production method. An
object of the present invention is to provide a method for
producing a metal, the method providing a particular metal at high
purity, with safety, and at low cost, from a treatment object
containing two or more metal elements; and an apparatus used for
the production method.
Solution to Problem
[0043] An embodiment of the present invention is a method for
producing a metal by molten salt electrolysis, the method including
a step of dissolving, in a molten salt, a metal element contained
in a treatment object containing two or more metal elements; and a
step of depositing or alloying a particular metal present in the
molten salt, on one of a pair of electrode members disposed in the
molten salt containing the dissolved metal element, by controlling
a potential of the electrode members to a predetermined value.
[0044] In another embodiment of the present invention, the
treatment object is an ore or a crude metal ingot obtained from the
ore.
[0045] Another embodiment of the present invention is a method for
producing tungsten, wherein a metal element contained in the
treatment object is tungsten, in the step of dissolving, in a
molten salt, a metal element from a treatment object, tungsten is
dissolved from the treatment object, and in the step of depositing
or alloying a particular metal, tungsten present in the molten salt
is deposited on one of a pair of electrode members disposed in the
molten salt containing dissolved tungsten, by controlling a
potential of the electrode members to a predetermined value.
[0046] In another embodiment of the present invention, the
treatment object is a metal material containing the tungsten.
[0047] In another embodiment of the present invention, the
treatment object is a metal material containing tungsten and a
transition metal.
[0048] In another embodiment of the present invention, the
treatment object is a cemented carbide product.
[0049] Another embodiment of the present invention is a method for
producing lithium, wherein a metal element contained in the
treatment object is lithium, in the step of dissolving, in a molten
salt, a metal element from a treatment object, lithium is dissolved
from the treatment object, and in the step of depositing or
alloying a particular metal, lithium present in the molten salt is
deposited on one of a pair of electrode members disposed in the
molten salt containing dissolved lithium, by controlling a
potential of the electrode members to a predetermined value.
[0050] In another embodiment of the present invention, the
treatment object is a material containing lithium and a transition
metal.
[0051] In another embodiment of the present invention, the
treatment object is a battery electrode material containing
lithium.
[0052] In another embodiment of the present invention, the
treatment object contains a transition metal or a rare earth
metal.
[0053] In another embodiment of the present invention, the
treatment object contains one or more metals selected from the
group consisting of V, Nb, Mo, Ti, Ta, Zr, and Hf.
[0054] In another embodiment of the present invention, the
treatment object contains Sr and/or Ba.
[0055] In another embodiment of the present invention, the
treatment object contains one or more metals selected from the
group consisting of Zn, Cd, Ga, In, Ge, Sn, Pb, Sb, and Bi.
[0056] In another embodiment of the present invention, the molten
salt is selected such that, in the step of depositing or alloying a
particular metal, a difference between a standard electrode
potential of a simple substance or alloy of the particular metal
and a standard electrode potential of a simple substance or alloy
of another metal in the molten salt is 0.05 V or more.
[0057] In another embodiment of the present invention, in the step
of depositing or alloying a particular metal, the potential of the
electrode members is controlled to the predetermined value so that
the particular metal element in the molten salt is selectively
deposited or alloyed.
[0058] In another embodiment of the present invention, in the step
of dissolving, in a molten salt, a metal element contained in a
treatment object, the metal is dissolved in the molten salt by a
chemical procedure.
[0059] In another embodiment of the present invention, in the step
of dissolving, in a molten salt, a metal element contained in a
treatment object, a cathode and an anode that is formed of an anode
material containing the treatment object are disposed in the molten
salt, and a potential at the anode is controlled to a predetermined
value so that a metal element corresponding to the controlled
potential is dissolved in the molten salt from the treatment
object.
[0060] In another embodiment of the present invention, the molten
salt is selected such that, in the step of dissolving, in a molten
salt, a metal element contained in a treatment object, a difference
between a standard electrode potential of a simple substance or
alloy of the particular metal and a standard electrode potential of
a simple substance or alloy of another metal in the molten salt is
0.05 V or more.
[0061] In another embodiment of the present invention, in the step
of dissolving, in a molten salt, a metal element contained in a
treatment object, the potential at the anode is controlled to a
predetermined value so that the particular metal element is
selectively dissolved in the molten salt.
[0062] In another embodiment of the present invention, in the step
of dissolving, in a molten salt, a metal element contained in a
treatment object, one or more metals each serving as the particular
metal are dissolved in the molten salt.
[0063] In another embodiment of the present invention, the
particular metal deposited or alloyed is a transition metal.
[0064] In another embodiment of the present invention, the
particular metal deposited or alloyed is a rare earth metal.
[0065] In another embodiment of the present invention, the
particular metal deposited or alloyed is V, Nb, Mo, Ti, Ta, Zr, or
Hf.
[0066] In another embodiment of the present invention, the
particular metal deposited or alloyed is Sr or Ba.
[0067] In another embodiment of the present invention, the
particular metal deposited or alloyed is Zn, Cd, Ga, In, Ge, Sn,
Pb, Sb, or Bi.
[0068] In another embodiment of the present invention, the molten
salt is a chloride or fluoride molten salt.
[0069] In another embodiment of the present invention, the molten
salt is a molten salt mixture containing a chloride molten salt and
a fluoride molten salt.
[0070] In another embodiment of the present invention, the
treatment object has a form of particles or powder.
[0071] In another embodiment of the present invention, the
treatment object having the form of particles or powder is
compacted to form the anode.
[0072] Another embodiment of the present invention is a method for
producing a metal by molten salt electrolysis, the method being a
method for producing a particular metal by molten salt electrolysis
from a treatment object containing two or more metal elements,
wherein a cathode and an anode that is formed of an anode material
containing the treatment object are disposed in a molten salt, and
a potential at the anode is controlled to a predetermined value so
that a metal element corresponding to the controlled potential is
dissolved in the molten salt from the treatment object and a
particular metal is left in the anode.
[0073] In another embodiment of the present invention, the
treatment object is an ore or a crude metal ingot obtained from the
ore.
[0074] Another embodiment of the present invention is a method for
producing tungsten by molten salt electrolysis from a treatment
object containing tungsten, wherein a cathode and an anode that is
formed of an anode material containing the treatment object are
disposed in a molten salt, and a potential at the anode is
controlled to a predetermined value so that a metal element
corresponding to the controlled potential is dissolved in the
molten salt from the treatment object and tungsten is left in the
anode.
[0075] In another embodiment of the present invention, the molten
salt is selected such that, in the step of dissolving a metal
element in the molten salt from the treatment object, a difference
between a standard electrode potential of a simple substance or
alloy of the particular metal and a standard electrode potential of
a simple substance or alloy of another metal in the molten salt is
0.05 V or more.
[0076] Another embodiment of the present invention is an apparatus
used for a method for producing a metal by molten salt
electrolysis, the apparatus including a container containing a
molten salt; a cathode immersed in the molten salt contained within
the container; and an anode that is immersed in the molten salt
contained within the container and that contains a treatment object
containing two or more metal elements, wherein the molten salt is
movable into and out of the anode, the apparatus further includes a
control unit configured to control a potential of the cathode and
the anode to a predetermined value, and a value of the potential is
changeable in the control unit.
[0077] Another embodiment of the present invention is an apparatus
used for a method for producing a metal by molten salt
electrolysis, the apparatus including a container containing a
molten salt containing two or more dissolved metal elements; a
cathode and an anode that are immersed in the molten salt contained
within the container; and a control unit configured to control a
potential of the cathode and the anode to a predetermined value,
wherein a value of the potential is changeable in the control
unit.
[0078] In another embodiment of the present invention, the two or
more metal elements include at least one of tungsten and
lithium.
Advantageous Effects of Invention
[0079] A method for producing a metal and an apparatus used for the
production method according to the present invention are applicable
to any ore. Use of a production method or an apparatus used for the
production method according to the present invention can provide a
particular metal at high purity, with safety, and at low cost, from
a treatment object containing two or more metal elements.
BRIEF DESCRIPTION OF DRAWINGS
[0080] FIG. 1 is a flow chart for explaining an embodiment of the
present invention.
[0081] FIG. 2 is a schematic view describing examples of deposition
potentials of rare earth metals in a molten salt.
[0082] FIG. 3 is a graph illustrating examples of a relationship
between treatment time and concentration of ions of a rare earth
metal in a molten salt in an embodiment of the present
invention.
[0083] FIG. 4 is a schematic sectional view for explaining the
configuration of an apparatus according to an embodiment of the
present invention.
[0084] FIG. 5 is a schematic sectional view for explaining the
configuration of an apparatus according to an embodiment of the
present invention.
[0085] FIG. 6 is a flow chart for explaining another embodiment of
the present invention.
[0086] FIG. 7 is a schematic sectional view for explaining another
embodiment of the present invention.
[0087] FIG. 8 is a schematic sectional view for explaining another
embodiment of the present invention.
[0088] FIG. 9 is a schematic sectional view for explaining another
embodiment of the present invention.
[0089] FIG. 10 is a schematic sectional view for explaining another
embodiment of the present invention.
[0090] FIG. 11 is a schematic sectional view for explaining a
modification of another embodiment of the present invention.
[0091] FIG. 12 is a schematic sectional view for explaining a
modification of another embodiment of the present invention.
[0092] FIG. 13 is a schematic sectional view for explaining a
modification of another embodiment of the present invention.
[0093] FIG. 14 is a photograph for explaining an anode electrode
used in examples according to the present invention.
[0094] FIG. 15 is a graph illustrating the relationship between the
value of anode current and time in an example according to the
present invention.
[0095] FIG. 16 is a scanning electron micrograph of a surface
portion of a cathode electrode used in an electrolysis step in an
example according to the present invention. The scale in the lower
right of the micrograph indicates a length of 8 .mu.m.
[0096] FIG. 17 is a scanning electron micrograph illustrating Dy
distribution status in the regions of the micrograph illustrated in
FIG. 16.
[0097] FIG. 18 is a schematic sectional view for explaining an
example of the configuration of an apparatus according to an
embodiment of the present invention.
[0098] FIG. 19 is a schematic sectional view for explaining an
example of the configuration of an apparatus according to an
embodiment of the present invention.
DESCRIPTION OF EMBODIMENTS
[0099] An embodiment of the present invention is a method for
producing a metal by molten salt electrolysis, the method including
a step of dissolving, in a molten salt, a metal element contained
in a treatment object containing two or more metal elements; and a
step of depositing or alloying a particular metal present in the
molten salt, on one of a pair of electrode members disposed in the
molten salt containing the dissolved metal element, by controlling
a potential of the electrode members to a predetermined value.
First Embodiment
[0100] In the first embodiment, the treatment object is an ore
containing two or more metal elements or a crude metal ingot
obtained from the ore (hereafter sometimes simply referred to as a
crude metal ingot).
[0101] That is, roughly speaking, this embodiment includes a
process of dissolving, in a molten salt, a metal contained in an
object (the ore or crude metal ingot), and a process of depositing
a metal or an alloy of a separation-extraction target element on
one of electrodes (cathode) from a molten salt containing the
dissolved metal by molten salt electrolysis. A feature of this
embodiment is that, by controlling the potential of the electrodes,
a particular target element is selectively dissolved or deposited
to achieve separation and smelting.
[0102] The process of dissolving, in a molten salt, a metal element
contained in an object will be first described.
[0103] A procedure for dissolving, in a molten salt, a metal
element contained in an ore or a crude metal ingot is, for example,
a chemical procedure for dissolution. Specifically, an ore or a
crude metal ingot is ground into particles or powder, mixed with a
salt, and heated. As a result, two or more metal elements contained
in the ore or the crude metal ingot can be dissolved in the molten
salt. Alternatively, the treatment object may be placed in a molten
salt and dissolved.
[0104] Another procedure is an electrochemical procedure.
Specifically, an object is disposed as an anode in a molten salt
and the value of the potential at the object is controlled to
selectively dissolve an element contained in the object: molten
salt electrolysis is characterized in that different elements are
dissolved at different potentials; and such characteristics are
utilized to selectively separate metals corresponding to
potentials. In this way, by using an object as an anode and
controlling the potential during dissolution, a metal element that
is a smelting target can be selectively dissolved in a molten
salt.
[0105] In the process of dissolving, in a molten salt, a metal
element contained in an object, the potential is preferably
controlled such that impurities contained in the object remain
undissolved. As a result, entry of impurities in the subsequent
deposition process can be reduced.
[0106] For this purpose, the molten salt is preferably selected
such that, in the step of dissolving, in the molten salt, a metal
element contained in an ore or a crude metal ingot, the difference
between the standard potential of a simple substance or alloy of a
particular metal (metal element to be dissolved) and the standard
electrode potential of a simple substance or alloy of another metal
in the molten salt is 0.05 V or more. As a result, the metal
element that is dissolved in the molten salt can be sufficiently
separated from the metal element that is left in the anode. The
difference between the standard electrode potentials is more
preferably 0.1 V or more, still more preferably 0.25 V or more.
[0107] The value of the potential controlled at the anode can be
calculated by Nernst equation described below.
[0108] In the case where a plurality of target particular metals
are contained in an ore or a crude metal ingot used, the potential
may be controlled such that respective metals are dissolved in a
molten salt. Alternatively, after one of the particular metals is
dissolved, the ore or crude metal ingot (anode) containing the
remainder of the metals may be moved to another molten salt and the
potential may be similarly controlled to a predetermined value so
that the remainder of the particular metals is dissolved in the
molten salt.
[0109] Some metals are more easily separated by deposition
described below. In such cases, the entire treatment object may be
dissolved or only a particular metal and some other metals may be
dissolved.
[0110] From the standpoint of reduction of entry of impurities, in
the step of dissolving, in a molten salt, a metal element contained
in an ore or a crude metal ingot, the potential at the anode is
preferably controlled to a predetermined value so that the
particular metal element is selectively dissolved in the molten
salt.
[0111] The molten salt can be selected from chlorides and
fluorides. Examples of chloride molten salts include KCl, NaCl,
CaCl.sub.2, LiCl, RbCl, CsCl, SrCl.sub.2, BaCl.sub.2, and
MgCl.sub.2. Examples of fluoride molten salts include LiF, NaF, KF,
RbF, CsF, MgF.sub.2, CaF.sub.2, SrF.sub.2, and BaF.sub.2. In the
cases where rare earth elements are subjected to molten salt
electrolysis, chloride molten salts are preferably used in view of
efficiency; in particular, KCl, NaCl, and CaCl.sub.2 are preferably
used because they are inexpensive and easily available.
[0112] Among such molten salts, a plurality of molten salts can be
combined and used as a molten salt having a desirable composition.
For example, a molten salt having a composition such as
KCl--CaCl.sub.2, LiCl--KCl, or NaCl--KCl may be used.
[0113] The cathode is formed of carbon or a material that tends to
form an alloy with an alkali metal such as Li or Na constituting
cations in the molten salt. For example, aluminum (Al), zinc (Zn),
gallium (Ga), cadmium (Cd), indium (In), tin (Sn), antimony (Sb),
lead (Pb), or bismuth (Bi) may be used.
[0114] When the ore or crude metal ingot is used as an anode, for
example, the ore or crude metal ingot contained in a conductive
basket formed of a metal or the like may be disposed in the molten
salt. An opening may be formed in an upper portion of the basket so
that the ore or crude metal ingot serving as the treatment object
can be inserted through the opening into the basket; and a large
number of holes may be formed in the side and bottom walls of the
basket so that the molten salt can flow into the basket. The basket
may be constituted by a desired material such as a mesh member
knitted from metal wires or a sheet member that is a sheet-shaped
metal plate having a large number of holes. In particular, it is
effective that the material is formed of C, Pt, Mo, or the
like.
[0115] In the cases where the object is an ore or the like and has
a high electric resistance, the contact area between the object and
the conductive material is preferably increased. The object is
effectively used as an electrode by, for example, wrapping the
object with a metal mesh member or filling the object into spaces
within a metal porous member.
[0116] When the cathode and the basket containing an ore or crude
metal ingot are disposed in the molten salt and the potential at
the anode (basket) is controlled from the outside as described
above, a target metal can be dissolved in the molten salt from the
ore or crude metal ingot.
[0117] In the subsequent deposition process, molten salt
electrolysis is performed with a pair of electrode members disposed
in the molten salt so that a metal element dissolved in the molten
salt is deposited on one of the electrode members (cathode). In
this case, by controlling the potential value in the molten salt
electrolysis, a particular metal element can be selectively
deposited as metal or alloy on the cathode.
[0118] As in the dissolution process, in this deposition process,
molten salt electrolysis is characterized in that different
elements are deposited at different potentials as metal or alloy on
the cathode; and such characteristics are utilized to separate the
metals. Thus, even when a plurality of target particular metals are
contained in the molten salt, by controlling the potential, the
metals can be individually deposited on cathodes one by one.
[0119] The electrode members may be formed of, for example, nickel
(Ni), molybdenum (Mo), or glassy carbon (C).
[0120] In this embodiment, the above-described two processes are
used to separate and extract from an object a particular metal
element that is a smelting target. In this embodiment, since a
molten salt is used, the system needs to be heated such that the
temperature of the system in the processes is equal to or more than
the melting point of the molten salt.
[0121] A feature of the two processes is use of a molten salt.
Thus, the fact that different molten salts have different
dissolution-deposition potentials for elements is utilized and the
processes can be designed by selecting a molten salt such that the
dissolution-deposition potentials of a particular metal element
that is a target element and the other impurity metal elements are
values that allow easy performance of the processes. Specifically,
the molten salt is preferably selected such that, in the step of
depositing or alloying the particular metal, the difference between
the standard electrode potential of a simple substance or alloy of
the particular metal and the standard electrode potential of a
simple substance or alloy of another metal in the molten salt is
0.05 V or more. In the molten salt, the difference between the
standard electrode potential of a simple substance or alloy of the
particular metal and the standard electrode potential of a simple
substance or alloy of another metal is more preferably 0.1 V or
more, still more preferably 0.25 V or more.
[0122] In this way, in the step of depositing or alloying the
particular metal, the potential of the electrode members is
preferably controlled to a predetermined value so that the
particular metal element in the molten salt is selectively
deposited or alloyed.
[0123] The deposition potential of a simple substance or alloy of a
metal to be deposited on the cathode can be determined by
electrochemical calculation. Specifically, the calculation is
performed with Nernst equation.
[0124] For example, the potential at which a simple substance of
praseodymium (Pr) is deposited from trivalent Pr ions (hereafter
represented by Pr(III)) can be determined with the following
equation.
E.sub.Pr=E.sup.0.sub.Pr+RT/3Fln(a.sub.Pr(III)/a.sub.Pr(0)) Equation
(1)
[0125] In Eq. (1), E.sup.0.sub.Pr represents the standard
potential, R represents the gas constant, T represents absolute
temperature, F represents the Faraday constant, a.sub.Pr(III)
represents the activity of Pr(III) ions, and a.sub.Pr(0) represents
the activity of Pr simple substance.
[0126] When Eq. (1) is rewritten in view of activity coefficient
.gamma..sub.Pr(III), since a.sub.Pr(0)=1, the following equations
are provided.
E Pr = E Pr 0 + RT / 3 F ln a Pr ( III ) = E Pr 0 + RT / 3 F ln (
.gamma. Pr ( III ) C Pr ( III ) ) Equation ( 2 ) E Pr = E Pr 0 ' +
RT / 3 F ln C Pr ( III ) Equation ( 3 ) ##EQU00001##
[0127] In Eq. (3), C.sub.Pr(III) represents the concentration of
trivalent Pr ions, and E.sup.0'.sub.Pr represents formal electrode
potential (here, equal to E.sup.0.sub.Pr+RT/3Fln
.gamma..sub.Pr(III)).
[0128] Similarly, the potential at which PrNi alloy is deposited on
the electrode surface (deposition potential: E.sub.Pr.Ni) can be
determined with the following equation.
E.sub.Pr.Ni=E.sup.0'.sub.Pr.NiRT/3Fln C.sub.Pr(III) Equation
(4)
[0129] In Eq. (4), E.sup.0'.sub.Pr.Ni represents formal electrode
potential (here, equal to E.sup.0.sub.Pr.Ni+RT/3Fln
.gamma..sub.Pr(III)).
[0130] Similarly, by using the above-described equations,
deposition potentials of all deposits corresponding to different
molten salts can be determined. In the process of depositing or
alloying a particular metal on the cathode, in view of the
deposition potential value of this particular metal or an alloy
thereof, a deposit that provides a potential difference with
respect to another metal or an alloy thereof is selected or the
order of depositions is determined.
[0131] Voltage and current during operation vary depending on the
size or positional relationship of electrodes. Accordingly,
reference values of voltage and current are determined on the basis
of conditions and subsequently voltage and current are determined
in each step on the basis of the potential value and order
determined by the above-described method.
[0132] As described above, in a method for producing a metal by
molten salt electrolysis according to this embodiment, the
potential value is controlled to thereby electrochemically dissolve
and deposit a target metal. Accordingly, the steps can be
simplified, compared with, for example, the existing wet treatment
involving repeating of processes of dissolution and extraction
using acid or the like; and a particular element can be selectively
separated and recovered. In addition, adjustment of the specific
gravity of the molten salt is not necessary; and, by selecting a
low-temperature molten salt in which an object can be treated in
the solid state, a simple apparatus configuration can be employed.
Moreover, the operation pattern can also be simplified. As a
result, the steps can be performed efficiently at low cost.
[0133] Alternatively, a particular metal can be smelted on the
basis of an idea that is totally contrary to the above-described
idea of depositing or alloying a particular metal on the
cathode.
[0134] That is, a method for producing a metal according to this
embodiment is a method for producing a particular metal by molten
salt electrolysis from an ore containing two or more metal elements
or a crude metal ingot obtained from the ore, wherein a cathode and
an anode that is formed of an anode material containing the ore or
crude metal ingot are disposed in a molten salt, and the potential
at the anode is controlled to a predetermined value so that a metal
element corresponding to the potential is dissolved in the molten
salt from the ore or crude metal ingot and a particular metal is
left in the anode.
[0135] In this method, the object (the ore or crude metal ingot) is
used as the anode and metal elements other than a particular metal
element, that is, only metal elements serving as impurities are
dissolved in the molten salt, so that the particular metal is left
in the anode. In this case, by also controlling the potential at
the anode, such a phenomenon is caused in which the metal element
that is the smelting target is left in the anode and impurity
elements are dissolved in the molten salt. As a result, a smelted
metal material is obtained in the anode.
[0136] In this method, the molten salt is also preferably selected
such that, in the step of dissolving, in the molten salt, a metal
element from the ore or crude metal ingot, the difference between
the standard electrode potential of a simple substance or alloy of
the particular metal and the standard electrode potential of a
simple substance or alloy of another metal in the molten salt is
0.05 V or more. As a result, the particular metal can be
sufficiently separated from the other metal and the particular
metal alone can be left in the anode. The difference between the
standard electrode potentials is more preferably 0.1 V or more,
still more preferably 0.25 V or more.
[0137] The value of the potential controlled at the anode can be
calculated by Nernst equation as described above.
[0138] Ores usable in the method for producing a metal by molten
salt electrolysis according to this embodiment are ores containing
target particular metals. Examples of the ores include gold ore,
silver ore, copper ore, iron ore, aluminum ore, lead ore, zinc ore,
tin ore, mercury ore, sulfur ore, phosphorus ore, nickel ore,
cobalt ore, manganese ore, chromium ore, molybdenum ore, tungsten
ore, antimony ore, arsenic ore, bismuth ore, strontium ore,
beryllium ore, magnesium ore, barium ore, and calcium ore. For
example, rare earth metals can be obtained from bastnaesite,
monazite, loparite, apatite, xenotime, fergusonite, and
eudialyte.
[0139] The crude metal ingot obtained from the ore denotes a metal
containing a target particular metal at a low purity, such as a
metal obtained by smelting the ore.
[0140] The method for producing a metal by molten salt electrolysis
according to this embodiment is suitably applied to an ore or crude
metal ingot obtained from the ore that is used as the anode and
contains a transition metal or a rare earth metal.
[0141] The transition metal is not particularly limited and may be
any element among from group 3 (group IIIA) to group 11 (group IB)
of the periodic table. The rare earth metal is also not
particularly limited and may be any element among scandium (Sc),
yttrium (Y), and 15 lanthanoid elements in group 3 (group IIIA) of
the periodic table.
[0142] The method for producing a metal by molten salt electrolysis
according to this embodiment is also suitably applicable to the
cases where particular metals deposited or alloyed on cathodes are
rare earth metals. In this embodiment, appropriate selection of the
composition of the molten salt allows even deposition of rare earth
metals that cannot be deposited by aqueous solution electrolysis.
Thus, rare earth metals that are difficult to mine as resources can
be easily obtained.
[0143] In this embodiment, the ore or crude metal ingot obtained
from the ore preferably has the form of particles or powder. When
the ore or crude metal ingot to be treated is prepared so as to
have the form of particles or powder, the surface area is increased
and the treatment efficiency can be increased. From this viewpoint,
the maximum particle size of the ore or crude metal ingot is
preferably 0.01 mm to 2 mm, more preferably 0.01 mm to 1 mm, still
more preferably 0.01 mm to 0.2 mm.
[0144] In addition, the ore or crude metal ingot in the form of
particles or powder is preferably compacted to form the anode. The
ore or crude metal ingot in the form of powder can be compacted
and, as a result, can be used as the anode. In this case, between
the particles, there are desirably spaces that the molten salt can
easily enter.
[0145] Hereinafter, this embodiment will be described with
reference to drawings. In the drawings below, the same or
corresponding parts are designated by the same reference signs and
repetitive descriptions thereof are omitted.
First Embodiment-1
[0146] An example of this embodiment will be described that is a
method for obtaining neodymium (Nd), dysprosium (Dy), and
praseodymium (Pr) by molten salt electrolysis from an ore
containing Nd, Dy, and Pr. Examples of the ore include monazite,
apatite, xenotime, fergusonite, and eudialyte.
[0147] As illustrated in FIG. 1, a preparation step (S10) is first
performed.
[0148] In this step, for example, an ore that is a treatment
object, a molten salt to be used, and an apparatus including, for
example, electrodes and a container for containing the molten salt
are prepared. Optionally, in order to promote dissolution of the
treatment object in the molten salt, the treatment object may be
finely ground for the purpose of increasing the contact area
between the treatment object and the molten salt.
[0149] The ore containing Nd, Dy, and Pr may be, for example,
xenotime ore. For example, a xenotime ore has a composition of 3.0%
Nd, 7.9% Dy, and 0.5% Pr.
[0150] Subsequently, a dissolution step in the molten salt (S20) is
performed.
[0151] In this step (S20), the ore and (another) electrode member
are immersed in the prepared molten salt; the ore and the electrode
member are connected via a power supply so that the potential of
the ore and the electrode member is controlled. By controlling the
potential at the ore, rare earth elements (Nd, Dy, and Pr) in the
ore are selectively dissolved in the molten salt. The molten salt
used may be a molten salt having a desired composition.
[0152] For example, the molten salt may be LiF--NaF--KF; the other
electrode member may be an electrode formed of glassy carbon; and
the above-described ore may be used as the treatment object.
[0153] In this case, for example, while the molten salt is heated
at 700.degree. C., Nd, Dy, and Pr can be selectively dissolved in
the molten salt from the ore. The potential is controlled to a
value at which elements other than Nd, Dy, and Pr are scarcely
dissolved in the molten salt but Nd, Dy, and Pr are dissolved in
the molten salt.
[0154] Subsequently, as illustrated in FIG. 1, a separation
extraction step (S30) is performed.
[0155] Specifically, in the molten salt in which Nd, Dy, and Pr are
dissolved as described above, a pair of electrodes are inserted and
the potential of the electrode members is controlled to a
predetermined value. For example, in the case of using LiCl--KCl
molten salt, as illustrated in FIG. 2, the potential value is
controlled to potentials corresponding to deposition potentials
determined for respective rare earth metals. As a result, by
controlling the potential, the rare earth metal deposited on the
electrode can be selected. Thus, the rare earth metals can be
selectively recovered element by element.
[0156] For example, as illustrated in FIG. 2, rare earth elements
such as Nd, Dy, and Pr have different deposition potential values
for respective elements. Specifically, as illustrated in FIG. 2,
the deposition potential of Nd is about 0.40 V (vs. Li.sup.+/Li);
the deposition potentials of Pr and Dy are about 0.47 V (vs.
Li.sup.+/Li); and the deposition potential of DyNi.sub.2, which is
a Dy compound, is about 0.77 V (vs. Li.sup.+/Li).
[0157] The deposition potentials in FIG. 2 are described with
reference to Li. In FIG. 2, the ordinate axis indicates deposition
potential (unit: V). Such deposition potentials are values in the
case where the molten salt is LiCl--KCl and the temperature of the
molten salt is set at 450.degree. C.
[0158] As described above, elements and compounds have different
deposition potentials. Accordingly, by immersing a pair of
electrodes in a molten salt in which particular metals are
dissolved and by controlling the potential at the cathode so as to
correspond to the above-described deposition potentials, particular
rare earth elements can be selectively deposited on the cathode. By
changing the potential value at the cathode (for example,
sequential potential changes), particular metals to be deposited
can be selected.
[0159] For example, as illustrated in FIG. 3, different voltages
are sequentially applied across a pair of electrodes immersed in
the molten salt in which Nd, Dy, and Pr are dissolved. The
concentrations (ion concentrations) of Nd, Dy, and Pr in the molten
salt are each 0.5 mol %.
[0160] When data described in FIG. 2 are used as deposition
potential values, for example, LiCl--KCl is used as the molten salt
and the temperature of the molten salt is set at 450.degree. C. In
FIG. 3, the abscissa axis indicates treatment time and the ordinate
axis indicates the ion concentrations of rare earth elements in the
molten salt. The unit in the ordinate axis is mol %.
[0161] In STEP 1, when Ni is first used as a cathode material and
the potential at the cathode is set to a value that is less noble
than 0.77 V (vs. Li+/Li) and slightly more noble than 0.63 V (vs.
Li.sup.+/Li) (for example, the potential difference is set to 0.631
V (vs. Li.sup.+/Li)), Dy ions are alloyed with the cathode material
Ni so that DyNi.sub.2 is deposited on the cathode surface. As a
result, as illustrated in FIG. 3, the Dy ion concentration in the
molten salt is sharply decreased. Dy can be recovered until the Dy
ion concentration in the molten salt becomes about
3.6.times.10.sup.-4 mol %.
[0162] Subsequently, in STEP 2, when another electrode (for
example, a Mo electrode) is used as a cathode and the potential at
the cathode is set to a value that is slightly more noble than 0.40
V (vs. Li.sup.+/Li) (for example, the potential difference is set
to 0.401 V (vs. Li.sup.+/Li)), Pr is deposited on one of the
electrodes (cathode). As a result, as illustrated in FIG. 3, the Pr
ion concentration in the molten salt is sharply decreased. Pr can
be recovered until the Pr ion concentration in the molten salt
becomes about 0.017 mol %.
[0163] The electrode used in STEP 2 is different from the electrode
on which DyNi.sub.2 has been deposited in STEP 1. For example, the
electrode on which DyNi.sub.2 has been deposited in STEP 1 may be
removed from the molten salt before STEP 2 is started, and another
electrode may be immersed in the molten salt; alternatively, the
electrode on which DyNi.sub.2 has been deposited may be left
unremoved and, in STEP 2, the potential at another electrode may be
controlled.
[0164] Subsequently, in STEP 3, when the potential at still another
electrode (for example, a Mo electrode) is set to 0.10 V (vs.
Li.sup.+/Li), Nd is deposited on this electrode (cathode). As a
result, as illustrated in FIG. 3, the Nd ion concentration in the
molten salt is sharply decreased. Nd can be recovered until the Nd
ion concentration in the molten salt becomes, for example, about
2.7.times.10.sup.-7 mol %.
[0165] The electrode on which Pr has been deposited in STEP 2 may
be removed from the molten salt before STEP 3 is started, and
another electrode may be immersed in the molten salt;
alternatively, the electrode on which Pr has been deposited in STEP
2 may be left immersed in the molten salt and, in STEP 3, another
electrode may be used.
[0166] DyNi.sub.2 recovered in STEP 1 is treated in STEP 4: the
electrode on the surface of which DyNi.sub.2 has been deposited and
another electrode (for example, a Mo electrode) are immersed in a
molten salt; and the potential at the DyNi.sub.2 electrode is set
to be in a potential range in which Dy is dissolved but Ni is not
dissolved (0.77 or more and 2.6 or less V (vs. Li.sup.+/Li)), so
that Dy can be dissolved in the molten salt and Dy alone can be
deposited on the surface of the other electrode.
[0167] As has been described above, the target particular metals
can be individually recovered from the molten salt.
Apparatus Used for Method of this Embodiment
[0168] Hereinafter, an apparatus used for the method of this
embodiment in FIG. 1 will be described with reference to FIGS. 4
and 5. A recovery apparatus illustrated in FIG. 4 includes a
container 1 containing a molten salt, a molten salt 2 contained
within the container 1, a basket 4 containing a treatment object
(the ore or crude metal ingot) 3, electrodes 6 to 8, a heater 10
for heating the molten salt 2, and a control unit 9 electrically
connected to the basket 4 and the electrodes 6 to 8 via conductive
wires 5. The control unit 9 is configured to control the potential
(change the potential) of one electrode that is the basket 4 and
the other electrode that is one of the electrodes 6 to 8. In the
control unit 9, the value to which the potential is controlled is
changeable. The heater 10 is disposed so as to circularly surround
the container 1. The electrodes 6 to 8 may be formed of desired
materials. For example, the electrode 6 may be formed of nickel
(Ni). For example, the electrodes 7 and 8 may be formed of carbon
(C). The container 1 may have a bottom surface that has a circular
shape or a polygonal shape. The basket 4 may be the above-described
basket.
[0169] The basket 4 and the electrodes 6 to 8 are controlled by the
control unit 9 to predetermined potential values. By controlling
the electrodes 6 to 8 to different potentials, different particular
metals corresponding to the controlled potential values are
deposited on the surfaces of the electrodes 6 to 8 as described
below. For example, as described below, the potential value set for
the electrode 6 can be adjusted so that a DyNi.sub.2 film 11 is
deposited on the surface of the electrode 6. By adjusting the
potential set for the electrode 7, a Pr film 12 can be deposited on
the surface of the electrode 7. By adjusting the potential set for
the electrode 8, a Nd film 13 can be deposited on the surface of
the electrode 8.
[0170] The electrode 6 on which the DyNi.sub.2 film 11 is deposited
is then placed in a container 1 containing a molten salt 2 as
illustrated in FIG. 5. Furthermore, another electrode is placed in
the molten salt 2 so as to face the electrode 6 on the surface of
which the DyNi.sub.2 film 11 is deposited. The electrodes 6 and 15
are connected to a control unit 9 via conductive wires 5. While the
molten salt 2 is heated with a heater 10 disposed so as to surround
the container 1, the control unit 9 is used to control the
potential of the electrodes 6 and 15 to a predetermined value. At
this time, the potential is controlled such that the potential at
the cathode (electrode 15) is the deposition potential of Dy.
[0171] As a result, from the DyNi.sub.2 film 11 deposited on the
surface of the electrode 6, Dy is dissolved in the molten salt 2
and a Dy film 16 is deposited on the surface of the electrode 15.
The heating temperature for the molten salt 2 with the heater 10
may be, for example, 800.degree. C. in both of the treatments using
the apparatuses illustrated in FIGS. 4 and 5. In this way,
particular metals can be deposited as simple substances on the
surfaces of the electrodes 7, 8, and 15.
[0172] In the case where the method of this embodiment is performed
with the apparatuses illustrated in FIGS. 4 and 5, for example, the
method may be performed in the following manner.
[0173] The ore (9 kg) is first prepared as the treatment object 3
and LiF--NaF--KF is prepared as the molten salt 2. For example, the
ore may contain 3.0 wt % of Nd, 0.5 wt % of Pr, and 7.9 wt % of Dy.
The ore is ground and placed within the basket 4. From the
viewpoint of enhancement of the treatment efficiency, the size of
the ore serving as the treatment object 3 is preferably minimized
by grinding. For example, the ore is ground into particles having a
maximum particle size of 2 mm or less, preferably 1 mm or less,
more preferably 0.2 mm or less. The amount of the molten salt 2 is
about 16 liters (mass: 25 kg).
[0174] The treatment object 3 contained in the basket 4 and one of
the electrodes 6 to 8 are used as a pair of electrodes and STEP 1
to STEP 3 of the method of this embodiment described with reference
to FIGS. 2 and 3 are performed. Specifically, in the
above-described STEP 1, the treatment object 3 contained in the
basket 4 and the electrode 6 are used as a pair of electrodes and
the potential of the electrodes is controlled to a predetermined
value. As a result, DyNi.sub.2 is deposited on the surface of the
electrode 6. In the above-described STEP 2, the treatment object 3
contained in the basket 4 and the electrode 7 are used as a pair of
electrodes and the potential of the electrodes is controlled to a
predetermined value. As a result, Pr is deposited on the surface of
the electrode 7. The mass of a Pr film deposited on the surface of
the electrode 7 in FIG. 4 is, for example, about 30 g to about 50
g.
[0175] In the above-described STEP 3, the treatment object 3
contained in the basket 4 and the electrode 8 are used as a pair of
electrodes and the potential of the electrodes is controlled to a
predetermined value. As a result, Nd is deposited on the surface of
the electrode 8. The mass of a Nd film deposited on the surface of
the electrode 8 is, for example, about 200 g to about 300 g.
[0176] In the above-described STEP 4, the electrode 6 and the
electrode 15 are placed in the apparatus illustrated in FIG. 5 and
the potential of the electrodes in the molten salt is controlled to
a predetermined value. As a result, Dy is deposited on the surface
of the electrode 15. The mass of a Dy film 16 deposited on the
surface of the electrode 15 is, for example, 600 g to 800 g.
[0177] As described with reference to FIG. 4, the step of
dissolving target metals in the molten salt 2 and the step of
depositing particular metals as simple substances on the surfaces
of the electrodes 7, 8, and the like can be performed within the
same apparatus (with the same molten salt 2). On the other hand,
the step of separating and extracting Dy from DyNi.sub.2 described
above in STEP 4 is preferably performed in an apparatus (apparatus
illustrated in FIG. 5) other than the apparatus (apparatus
illustrated in FIG. 4) used for the step of dissolving metals in
the molten salt 2 described with reference to FIG. 4.
[0178] As has been described above, particular metals (for example,
Dy, Pr, and Nd can be recovered from an ore or crude metal ingot
serving as the treatment object 3.
First Embodiment-2
[0179] An example of this embodiment will be described that is a
method for obtaining neodymium (Nd), dysprosium (Dy), and
praseodymium (Pr) by molten salt electrolysis from a crude metal
ingot obtained by smelting an ore containing Nd, Dy, and Pr.
[0180] The crude metal ingot containing Nd, Dy, and Pr may be, for
example, mixed rare earth metal (didymium). A smelting method for
obtaining the mixed rare earth metal is not particularly limited
and may be selected from publicly known methods.
[0181] As illustrated in FIG. 6, a step (S11) of preparing a crude
metal ingot serving as a treatment object is first performed.
Specifically, as illustrated in FIG. 7, a crude metal ingot serving
as a treatment object 3 is immersed in a molten salt 2 contained
within a container 1; and a conductive wire 5 is connected to the
treatment object 3, the conductive wire 5 being used for connection
to a power supply in a control unit 9. The salt used was
LiCl--KCl.
[0182] In the molten salt 2, an electrode material 25 contained
within a basket 24 and serving as the other electrode is immersed
together with the basket 24. The electrode material 25 is a
material that tends to form an alloy with an alkali metal such as
Li and Na constituting cations in the molten salt. Examples of the
electrode material 25 include aluminum (Al), zinc (Zn), gallium
(Ga), cadmium (Cd), indium (In), tin (Sn), antimony (Sb), lead
(Pb), and bismuth (Bi).
[0183] Subsequently, as illustrated in FIG. 6, a step (S21) of
dissolving Nd, Dy, and Pr in a molten salt is performed.
[0184] Specifically, as illustrated in FIG. 7, the potential of the
treatment object 3 and the electrode material 25 contained within
the basket 24 is controlled with the control unit 9, so that the
potential at the treatment object 3 is adjusted to a predetermined
value. As a result, rare earth elements such as Nd, Dy, and Pr are
dissolved in the molten salt 2 from the crude metal ingot serving
as the treatment object 3.
[0185] Subsequently, as illustrated in FIG. 6, a step (S31) of
depositing DyNi.sub.2 by electrolysis is performed. Specifically,
instead of the electrode material 25 contained in the basket 24 in
FIG. 7, as illustrated in FIG. 8, an electrode 6 formed of nickel
is immersed in the molten salt 2. This electrode 6 is connected to
the control unit 9 via a conductive wire 5. In this state, the
control unit 9 is used to control the potential of the treatment
object 3 serving as one electrode and the electrode 6 serving as
the other electrode, to a predetermined value.
[0186] As a result, rare earth elements such as Dy are dissolved in
the molten salt 2 from the treatment object 3 and DyNi.sub.2 is
deposited on the surface of the electrode 6 from the molten salt
2.
[0187] Subsequently, as illustrated in FIG. 6, a step (S32) of
recovering Pr by electrolysis is performed. Specifically, as
illustrated in FIG. 9, instead of the treatment object 3, an
electrode 27 formed of carbon is immersed as one electrode in the
molten salt 2. In addition, instead of the electrode 6 in FIG. 8,
an electrode 7 formed of carbon is placed at a position so as to
face the electrode 27 and be immersed in the molten salt 2. The
electrode 27 and the electrode 7 are electrically connected to the
control unit 9 via conductive wires 5. In this state, the potential
of one electrode 27 and the other electrode 7 is controlled to a
predetermined value.
[0188] As a result, Pr dissolved in the molten salt 2 is deposited
on the surface of the electrode 7. When a chloride is used as the
molten salt 2, chlorine gas (Cl.sub.2) is released from the region
around the electrode 27.
[0189] Subsequently, as illustrated in FIG. 6, a step (S33) of
recovering Nd by electrolysis is performed. Specifically, instead
of the electrode 7, as illustrated in FIG. 10, an electrode 8
formed of carbon is placed so as to face the electrode 27 and be
immersed in the molten salt 2. This electrode 8 is electrically
connected to the control unit 9 via a conductive wire 5. The
control unit 9 is used to control the potential of the electrode 8
and the electrode 27 to a predetermined value. As a result, Nd is
deposited on the surface of the electrode 8. At this time, chlorine
gas is released from the region around the electrode 27.
[0190] Subsequently, a step (S34) of recovering Dy by electrolysis
from DyNi.sub.2 recovered in the step (S31) is performed.
Specifically, as illustrated in FIG. 5, the electrode 6 on the
surface of which DyNi.sub.2 is deposited (refer to FIG. 8) is
immersed in the molten salt 2; the other electrode 15 is disposed
so as to be immersed in the molten salt 2; and the control unit 9
is used to control the potential of the electrodes 6 and 15 to a
predetermined value. As a result, Dy is temporarily dissolved in
the molten salt 2 from DyNi.sub.2 deposited on the surface of the
electrode 6 and then a Dy film 16 is deposited on the surface of
the electrode 15. Thus, Nd, Dy, and Pr, which are rare earth
metals, can be individually recovered.
[0191] The above-described steps (S21 to S32) may be performed with
the following apparatus configurations. For example, the
above-described step (S31) may be performed with an apparatus
configuration illustrated in FIG. 11.
[0192] Specifically, instead of the treatment object 3 in the
apparatus configuration in FIG. 8, the basket 24 containing a
material 26 alloyed by the step illustrated in FIG. 7 is immersed
in the molten salt 2. As illustrated in FIG. 11, this basket 24 is
electrically connected to the control unit 9 via a conductive wire
5. The potential of the electrode 6 and the material 26 contained
within the basket 24 and alloyed by the step illustrated in FIG. 7
is controlled to a predetermined value. As a result, Dy dissolved
in the molten salt 2 is deposited as DyNi.sub.2 on the surface of
the electrode 6. Dy can be recovered as a simple substance from
DyNi.sub.2 deposited on the surface of the electrode 6, by the same
step as the step (S34) in FIG. 6.
[0193] Subsequently, the above-described step (S32) may be
performed by a treatment with an apparatus configuration
illustrated in FIG. 12. Specifically, instead of the electrode 6 in
FIG. 11, as illustrated in FIG. 12, an electrode 7 formed of carbon
is placed at a position so as to face the basket 24 and be immersed
in the molten salt 2. This electrode 7 is electrically connected to
the control unit 9 via a conductive wire 5. The control unit is
used to control the potential of the electrode 7 and the alloy 26
contained within the basket 24, to a predetermined value. As a
result, Pr dissolved in the molten salt 2 is deposited on the
surface of the electrode 7.
[0194] Subsequently, the above-described step (S33) may be
performed by a treatment with an apparatus configuration
illustrated in FIG. 13. Specifically, as illustrated in FIG. 13,
instead of the electrode 7 in FIG. 12, an electrode 8 formed of
carbon is placed at a position so as to face the basket 24 and be
immersed in the molten salt 2. The electrode 8 is electrically
connected to the control unit 9 via a conductive wire 5. The
control unit 9 is used to control the potential of the electrode 8
and the alloy 26 disposed within the basket 24, to a predetermined
value. As a result, Nd is deposited on the surface of the electrode
8.
[0195] By using the method having been described, particular metals
contained in a crude metal ingot can be sequentially individually
recovered. According to the method of this embodiment, the
apparatus configuration can be simplified and the treatment time
can also be decreased, compared with the existing wet separation
method and the like. Thus, the cost incurred for obtaining elements
such as rare earth elements can be reduced. In addition, by
appropriately setting a potential at an electrode, a particular
metal can be deposited as a simple substance on the electrode
surface and hence high purity metal can be obtained. The potentials
for depositing individual metals and alloys can be determined by
the above-described calculation.
Second Embodiment
[0196] A method for producing tungsten by molten salt electrolysis
according to this embodiment is a method for producing tungsten by
molten salt electrolysis from a treatment object containing
tungsten, the method including a step of dissolving, in a molten
salt, tungsten from the treatment object, and a step of depositing
tungsten present in the molten salt, on one of a pair of electrode
members disposed in the molten salt containing dissolved tungsten,
by controlling a potential of the electrode members to a
predetermined value.
[0197] That is, roughly speaking, this embodiment includes a
process of dissolving, in a molten salt, tungsten contained in the
treatment object, and a process of depositing tungsten on one of
electrodes (cathode) from the molten salt containing dissolved
tungsten by molten salt electrolysis. A feature of this embodiment
is that, by controlling the potential of the electrodes, tungsten
is selectively deposited from a treatment object to provide high
purity tungsten.
[0198] The process of dissolving, in a molten salt, tungsten
contained in a treatment object will be first described.
[0199] A procedure for dissolving, in a molten salt, tungsten
contained in a treatment object is, for example, a chemical
procedure for dissolution. Specifically, a treatment object is
ground into particles or powder, mixed with a salt, and heated. As
a result, tungsten contained in the treatment object can be
dissolved in the molten salt. Alternatively, a treatment object may
be placed in a molten salt and dissolved.
[0200] Another procedure is an electrochemical procedure.
Specifically, an anode formed of an anode material containing a
treatment object is placed in a molten salt and the value of the
potential at the treatment object placed as the anode is controlled
to selectively dissolve tungsten contained in the treatment object.
Molten salt electrolysis is characterized in that different
elements are dissolved at different potentials. Such
characteristics can be utilized to separate tungsten form other
metals. In this way, by using a treatment object as an anode and
controlling the potential during dissolution, tungsten can be
selectively dissolved in a molten salt.
[0201] In this step, the entire treatment object may be dissolved,
or a tungsten-containing portion of the treatment object or
tungsten alone may be dissolved. Conditions under which
non-tungsten metals contained in the treatment object are dissolved
may be employed; however, if possible, the potential is preferably
controlled so that tungsten alone is dissolved. That is, in the
step of dissolving tungsten in a molten salt, the potential of the
anode and the cathode is preferably controlled to a predetermined
value so that tungsten is selectively dissolved in the molten salt.
As a result, entry of impurities in the subsequent deposition step
can be reduced.
[0202] For this purpose, the molten salt is preferably selected
such that, in the step of dissolving, in the molten salt, tungsten
from the treatment object, the difference between the standard
electrode potential of a simple substance or alloy of tungsten and
the standard electrode potential of a simple substance or alloy of
another metal in the molten salt is 0.05 V or more. As a result,
tungsten that is dissolved in the molten salt can be sufficiently
separated from the metal element that is left in the anode. The
difference between the standard electrode potentials is more
preferably 0.1 V or more, still more preferably 0.25 V or more.
[0203] The value of the potential controlled at the anode can be
calculated by Nernst equation described below.
[0204] The cathode used in the dissolution step is formed of carbon
or a material that tends to form an alloy with an alkali metal such
as Li or Na constituting cations in the molten salt. For example,
aluminum (Al), zinc (Zn), gallium (Ga), cadmium (Cd), indium (In),
tin (Sn), antimony (Sb), lead (Pb), or bismuth (Bi) may be
used.
[0205] When the treatment object containing tungsten is used as an
anode, for example, the treatment object contained within a
conductive basket (anode material) formed of metal or the like may
be disposed in the molten salt. An opening may be formed in an
upper portion of the basket so that the treatment object can be
inserted through the opening into the basket; and a large number of
holes may be formed in the side and bottom walls of the basket so
that the molten salt can flow into the basket. The basket may be
constituted by a desired material such as a mesh member knitted
from metal wires or a sheet member that is a sheet-shaped metal
plate having a large number of holes. In particular, it is
effective that the material is formed of C, Pt, Mo, or the
like.
[0206] In the cases where the object is an oxide or the like and
has a high electric resistance, the contact area between the object
and the conductive material is preferably increased. The object is
effectively used as an electrode by, for example, wrapping the
object with a metal mesh member or filling the object into spaces
within a metal porous member.
[0207] The cathode and an anode formed of an anode material
containing the treatment object (for example, a metal basket
containing the treatment object) are disposed in the molten salt; a
control unit configured to control the potential of the electrodes
from the outside is connected; and the potential is controlled as
described above. As a result, tungsten can be dissolved in the
molten salt from the treatment object.
[0208] In the subsequent deposition process, molten salt
electrolysis is performed with a pair of electrode members disposed
in the molten salt containing dissolved tungsten so that tungsten
is deposited on one of the electrode members (cathode). In this
case, by controlling the potential value in the molten salt
electrolysis, tungsten can be selectively deposited as metal or
alloy on the cathode.
[0209] As in the dissolution process, in this deposition process,
tungsten is separated from other metals by utilizing the following
characteristics: in molten salt electrolysis, different elements
are deposited at different potentials as metal or alloy on the
cathode. Thus, even when metals other than tungsten are contained
in the molten salt, by controlling the potential, tungsten alone
can be deposited on the cathode. As a result, high purity tungsten
can be obtained.
[0210] In deposition of tungsten, when the difference between the
dissolution-deposition potential of tungsten and the
dissolution-deposition potential of another metal contained in the
molten salt is so small that tungsten is difficult to separate from
the metal, the cathode material may be selected and the potential
may be controlled such that an alloy of the cathode material and
tungsten is deposited. As a result, tungsten in the molten salt can
be separated as a tungsten alloy from the other impurity metal;
and, after that, for example, a dissolution step and a deposition
step in another molten salt can be performed with the cathode
material alloyed with tungsten to thereby provide high purity
tungsten.
[0211] The electrode members used in the deposition step may be
formed of, for example, nickel (Ni), molybdenum (Mo), or glassy
carbon (C).
[0212] In this embodiment, the above-described two processes are
used to separate and extract tungsten from a treatment object. In
this embodiment, since a molten salt is used, the system needs to
be heated such that the temperature of the system in the processes
is equal to or more than the melting point of the molten salt.
[0213] Alternatively, smelting in the processes can be performed on
the basis of a totally contrary idea. That is, a treatment object
is used as the anode and only metal elements serving as impurities
are dissolved in a molten salt. In this case, by also controlling
the potential at the anode to a predetermined value, such a
phenomenon is caused in which tungsten is left in the anode and
impurity elements are dissolved. As a result, tungsten is provided
in the anode.
[0214] A feature of the two processes is use of a molten salt.
Thus, the characteristics of molten salt electrolysis in which
different molten salts have different dissolution-deposition
potentials for elements are utilized; and the processes can be
designed by selecting a molten salt such that the
dissolution-deposition potential of tungsten and the
dissolution-deposition potential of a non-tungsten impurity metal
are sufficiently different values that allow easy performance of
the processes.
[0215] Specifically, the molten salt is preferably selected such
that, in the step of depositing or alloying tungsten, the
difference between the standard electrode potential of a simple
substance or alloy of tungsten and the standard electrode potential
of a simple substance or alloy of another impurity metal in the
molten salt is 0.05 V or more. The difference between the standard
electrode potential of a simple substance or alloy of tungsten and
the standard electrode potential of a simple substance or alloy of
another metal in the molten salt is more preferably 0.1 V or more,
still more preferably 0.25 V or more.
[0216] In this way, in the step of depositing or alloying tungsten,
the potential of the electrode members is preferably controlled to
a predetermined value so that the tungsten in the molten salt is
selectively deposited or alloyed.
[0217] The deposition potential of tungsten to be deposited on the
cathode can be determined by electrochemical calculation.
Specifically, the calculation is performed with Nernst
equation.
[0218] For example, the potential at which a simple substance of
tungsten (W) is deposited from divalent W ions (hereafter
represented by W(II)) can be determined with the following
equation.
E.sub.W=E.sup.0.sub.W+RT/3Fln(a.sub.W(II)/a.sub.W(0)) Equation
(1)
[0219] In Eq. (1), E.sup.0.sub.W represents the standard potential,
R represents the gas constant, T represents absolute temperature, F
represents the Faraday constant, a.sub.W(II) represents the
activity of W(II) ions, and a.sub.W(0) represents the activity of W
simple substance.
[0220] When Eq. (1) is rewritten in view of activity coefficient
.gamma..sub.W(II), since a.sub.W(0)=1, the following equations are
provided.
E Wr = E W 0 + RT / 3 F ln a W ( II ) = E W 0 + RT / 3 F ln (
.gamma. W ( II ) C W ( II ) ) Equation ( 2 ) E W = E W 0 ' + RT / 3
F ln C W ( II ) Equation ( 3 ) ##EQU00002##
[0221] In Eq. (3), C.sub.W(II) represents the concentration of
divalent W ions, and E.sup.0'.sub.W represents formal electrode
potential (here, equal to E.sup.0.sub.W+RT/3Fln
.gamma..sub.W(II)).
[0222] Similarly, by using the above-described equations,
deposition potentials of all deposits corresponding to different
molten salts can be determined. Similar calculations can also be
performed in the case of depositing tungsten as an alloy. In the
process of depositing or alloying tungsten on the cathode, in view
of the deposition potential values of tungsten simple substance and
tungsten alloy, the molten salt and the cathode material are
selected such that a sufficiently high potential difference is
achieved with respect to the deposition potential of a simple
substance or alloy of another metal, and whether tungsten is
deposited or a tungsten alloy is deposited is decided.
[0223] Voltage and current during operation vary depending on the
size or positional relationship of electrodes. Accordingly,
reference values of voltage and current are determined on the basis
of conditions and subsequently voltage and current are determined
in each step on the basis of the potential value and order
determined by the above-described method.
[0224] As described above, in a method for producing tungsten by
molten salt electrolysis according to this embodiment, the
potential value is controlled to thereby electrochemically dissolve
and deposit tungsten. Accordingly, the steps can be simplified,
compared with, for example, the existing wet treatment involving
repeating of processes of dissolution and extraction using acid or
the like; and the particular element can be selectively separated
and recovered. In addition, adjustment of the specific gravity of
molten salt is not necessary; and, by selecting a low-temperature
molten salt in which tungsten can be treated in the solid state, a
simple apparatus configuration can be employed. Moreover, the
operation pattern can also be simplified. As a result, the steps
can be performed efficiently at low cost.
[0225] Alternatively, as described above, tungsten can be smelted
on the basis of an idea that is totally contrary to the idea of
depositing or alloying tungsten on the cathode.
[0226] That is, a method for producing a metal according to this
embodiment is a method for producing tungsten by molten salt
electrolysis from a treatment object containing tungsten, wherein a
cathode and an anode that is formed of an anode material containing
the treatment object are disposed in a molten salt, and the
potential at the anode is controlled so that a metal element
corresponding to the potential value is dissolved in the molten
salt from the treatment object and tungsten is left in the
anode.
[0227] In this method, the anode material containing the treatment
object is used as the anode and metal elements other than tungsten,
that is, only metal elements serving as impurities are dissolved in
the molten salt, so that tungsten is left in the anode. In this
case, by also controlling the potential at the anode, such a
phenomenon can be caused in which tungsten as the smelting target
is left in the anode and impurity elements are dissolved in the
molten salt. As a result, smelted tungsten is provided in the
anode.
[0228] In this method, the molten salt is also preferably selected
such that, in the step of dissolving, in the molten salt, a metal
element from the treatment object, the difference between the
standard electrode potential of a simple substance or alloy of
tungsten and the standard electrode potential of a simple substance
or alloy of another metal in the molten salt is 0.05 V or more. As
a result, tungsten can be sufficiently separated from the other
metal and tungsten alone can be left in the anode. The difference
between the standard electrode potentials is more preferably 0.1 V
or more, still more preferably 0.25 V or more.
[0229] The value of the potential controlled at the anode can be
calculated by Nernst equation as described above.
[0230] In a method for producing tungsten by molten salt
electrolysis according to this embodiment, the treatment object
containing tungsten is preferably, for example, a metal material
containing tungsten. Examples of the metal material containing
tungsten include tungsten heaters.
[0231] This embodiment is also suitably applicable to cases where
the treatment object is a metal material containing tungsten and a
transition metal. Such a transition metal is not particularly
limited and may be any element among from group 3 (group IIIA) to
group 11 (group IB) of the periodic table. Examples of the metal
material containing tungsten and a transition metal include
cemented carbide.
[0232] The treatment object may be, for example, cemented carbide
products. Herein, cemented carbide products collectively denote
products including cemented carbide materials, such as cutting
tools, jigs, dies, and molds including cemented carbide
materials.
[0233] The molten salt can be selected from chloride molten salts
and fluoride molten salts. A molten salt mixture containing a
chloride molten salt and a fluoride molten salt may be used.
[0234] Examples of chloride molten salts include KCl, NaCl,
CaCl.sub.2, LiCl, RbCl, CsCl, SrCl.sub.2, BaCl.sub.2, and
MgCl.sub.2. Examples of fluoride molten salts include LiF, NaF, KF,
RbF, CsF, MgF.sub.2, CaF.sub.2, SrF.sub.2, and BaF.sub.2. Chloride
molten salts are preferably used in view of efficiency; in
particular, KCl, NaCl, and CaCl.sub.2 are preferably used because
they are inexpensive and easily available.
[0235] Among such molten salts, a plurality of molten salts can be
combined and used as a molten salt having a desirable composition.
For example, a molten salt having a composition such as
KCl--CaCl.sub.2, LiCl--KCl, or NaCl--KCl may be used.
[0236] In a method for producing tungsten by molten salt
electrolysis according to this embodiment, the following
apparatuses can be preferably used. That is, an apparatus used for
a method for producing tungsten by molten salt electrolysis
according to this embodiment includes a container containing a
molten salt; a cathode immersed in the molten salt contained within
the container; and an anode that is immersed in the molten salt
contained within the container and that contains a conductive
treatment object containing tungsten, wherein the molten salt is
movable into and out of the anode, the apparatus further includes a
control unit configured to control the potential of the cathode and
the anode to a predetermined value, and the value of the potential
is changeable in the control unit. An apparatus used for a method
for producing tungsten by molten salt electrolysis according to
this embodiment includes a container containing a molten salt
containing dissolved tungsten; and a cathode and an anode that are
immersed in the molten salt contained within the container, wherein
the apparatus includes a control unit configured to control the
potential of the cathode and the anode to a predetermined value,
and the value of the potential is changeable in the control
unit.
[0237] The apparatuses for this embodiment will be described with
reference to FIGS. 18 and 19. An apparatus illustrated in FIG. 18
includes a container 1 containing a molten salt, a molten salt 2
contained within the container 1, a basket 4 containing a treatment
object 3 containing tungsten, an electrode 6, a heater 10 for
heating the molten salt 2, and a control unit 9 electrically
connected to the basket 4 and the electrode 6 via conductive wires
5.
[0238] The control unit 9 is configured to control the potential of
one electrode (anode) that is the basket 4 and the other electrode
(cathode) that is the electrode 6, to a predetermined value. In the
control unit 9, the value to which the potential is controlled is
changeable. The heater 10 is disposed so as to circularly surround
the container 1. The electrode 6 may be formed of a desired
material, for example, carbon. The container 1 may have a bottom
surface that has a circular shape or a polygonal shape. The basket
4 may be the above-described basket.
[0239] The potential of the basket 4 and the electrode 6 is
controlled by the control unit 9 to a predetermined potential
value. As a result, tungsten is dissolved in the molten salt 2 from
the treatment object 3.
[0240] After tungsten is sufficiently dissolved from the treatment
object 3, the basket 4 and the electrode 6 are removed and another
electrode 7 (cathode) and another electrode 8 (anode) are placed in
the molten salt 2. These electrodes 7 and 8 are connected to the
control unit 9 via conductive wires 5. The control unit 9 is used
to control the potential of the electrodes 7 and 8 to a
predetermined value. At this time, the potential is controlled such
that the potential at the electrode 7 is the deposition potential
of tungsten. As a result, tungsten dissolved in the molten salt 2
is deposited on the surface of the electrode 7 (cathode). The
electrodes 7 and 8 may be formed of a material such as glassy
carbon (C).
[0241] The heating temperature for the molten salt 2 with the
heater 10 may be, for example, 800.degree. C. in both of the
treatments using the apparatuses illustrated in FIGS. 18 and 19. In
this way, tungsten can be deposited as a simple substance on the
surface of the electrode 7.
[0242] The potential of the electrodes 7 and 8 may be controlled
such that an alloy of tungsten and the cathode material is
deposited on the surface of the electrode 7 (cathode). In this
case, the above-described dissolution step and deposition step may
be performed with the alloyed electrode 7. That is, the apparatus
illustrated in FIG. 18 is newly prepared and the electrode 7
alloyed with tungsten is used instead of the above-described
treatment object 3.
[0243] In the case where the method for producing tungsten of this
embodiment is performed with the apparatuses illustrated in FIGS.
18 and 19, for example, the method may be performed in the
following manner.
[0244] Cemented carbide cutting tools (9 kg) are first prepared as
the treatment object 3 and KCl--NaCl is prepared as the molten salt
2. For example, the cemented carbide cutting tools may contain 90
wt % of tungsten carbide (WC) and 10 wt % of cobalt (Co). The
cemented carbide cutting tools are ground and placed within the
basket 4. From the viewpoint of enhancement of the treatment
efficiency, the size of the cemented carbide cutting tools serving
as the treatment object 3 is preferably minimized by grinding. For
example, the cemented carbide cutting tools are ground into
particles having a maximum particle size of 5 mm or less,
preferably 3 mm or less, more preferably 1 mm or less. The amount
of the molten salt 2 is about 16 liters (mass: 25 kg).
[0245] The above-described dissolution step may be performed with a
carbon electrode serving as the electrode 6. Subsequently, the
deposition step may be performed with electrodes formed of glassy
carbon and serving as the electrodes 7 and 8.
[0246] As has been described, tungsten can be recovered from
cemented carbide cutting tools serving as the treatment object 3.
According to the method for producing tungsten by molten salt
electrolysis of this embodiment, the apparatus configuration can be
simplified and the treatment time can also be decreased, compared
with the existing wet separation method and the like. Thus, the
cost incurred can be reduced. In addition, by appropriately setting
a potential at an electrode, tungsten can be deposited as a simple
substance on the electrode surface and hence high purity tungsten
can be obtained. The potentials for depositing tungsten and a
tungsten alloy can be determined by the above-described
calculation.
Third Embodiment
[0247] A method for producing lithium by molten salt electrolysis
according to this embodiment is a method for producing lithium by
molten salt electrolysis from a treatment object containing
lithium, the method including a step of dissolving, in a molten
salt, lithium from the treatment object, and a step of depositing
lithium present in the molten salt, on one of a pair of electrode
members disposed in the molten salt containing dissolved lithium,
by controlling a potential of the electrode members to a
predetermined value.
[0248] That is, the lithium production method of this embodiment
includes a process of dissolving, in a molten salt, lithium
contained in the treatment object, and a step of depositing lithium
on one of electrodes (cathode) from the molten salt containing
dissolved lithium by molten salt electrolysis. A feature of this
embodiment is that, by controlling the potential of the electrodes
in the step of dissolving lithium, lithium is selectively dissolved
from the treatment object; and, by controlling the potential of the
electrodes to a predetermined value in the step of depositing
lithium, lithium is selectively deposited on the cathode from the
molten salt to thereby provide high purity lithium.
[0249] The step of dissolving, in a molten salt, lithium contained
in a treatment object will be first described.
[0250] A procedure for dissolving, in a molten salt, lithium
contained in a treatment object is, for example, a chemical
procedure for dissolution. Specifically, a treatment object is
ground into particles or powder, mixed with a salt, and heated. As
a result, lithium contained in the treatment object can be
dissolved in the molten salt. Alternatively, a treatment object may
be placed in a molten salt and dissolved.
[0251] Another procedure is an electrochemical procedure.
Specifically, an anode formed of an anode material containing a
treatment object is placed in a molten salt and the value of the
potential at the treatment object placed as the anode is controlled
to selectively dissolve lithium contained in the treatment object.
Molten salt electrolysis is characterized in that different
elements are dissolved at different potentials. Accordingly, in
this way, by using a treatment object as an anode and controlling
the potential during dissolution, lithium can be selectively
dissolved in a molten salt to separate lithium from the other
metals.
[0252] In this step, the entire treatment object may be dissolved,
or a lithium-containing portion of the treatment object or lithium
alone may be dissolved. Non-lithium metals contained in the
treatment object may also be dissolved; however, if possible, the
potential value is preferably controlled so that lithium alone is
dissolved. That is, in the step of dissolving lithium in a molten
salt, the potential of the anode and the cathode is preferably
controlled to a predetermined value so that lithium is selectively
dissolved in the molten salt. As a result, entry of impurities in
the subsequent deposition step can be reduced.
[0253] For this purpose, the molten salt is preferably selected
such that, in the step of dissolving, in the molten salt, lithium
from the treatment object, the difference between the standard
electrode potential of a simple substance or alloy of lithium and
the standard electrode potential of a simple substance or alloy of
another metal in the molten salt is 0.05 V or more. As a result,
lithium that is dissolved in the molten salt can be sufficiently
separated from the metal element that is left in the anode. The
difference between the standard electrode potentials is more
preferably 0.1 V or more, still more preferably 0.25 V or more.
[0254] The value of the potential controlled at the anode can be
calculated by Nernst equation described below.
[0255] The cathode used in the dissolution step is formed of carbon
or a material that tends to form an alloy with an alkali metal such
as Li or Na constituting cations in the molten salt. For example,
aluminum (Al), zinc (Zn), gallium (Ga), cadmium (Cd), indium (In),
tin (Sn), antimony (Sb), lead (Pb), or bismuth (Bi) may be
used.
[0256] When the treatment object containing lithium is used as an
anode, for example, the treatment object contained within a
conductive basket (anode material) formed of metal or the like may
be disposed in the molten salt. An opening may be formed in an
upper portion of the basket so that the treatment object can be
inserted through the opening into the basket; and a large number of
holes may be formed in the side and bottom walls of the basket so
that the molten salt can flow into the basket. The basket may be
constituted by a desired material such as a mesh member knitted
from metal wires or a sheet member that is a sheet-shaped metal
plate having a large number of holes. In particular, it is
effective that the material is formed of C, Pt, Mo, or the
like.
[0257] In the cases where the object is an oxide or the like and
has a high electric resistance, the contact area between the object
and the conductive material is preferably increased. The object is
effectively used as an electrode by, for example, wrapping the
object with a metal mesh member or filling the object into spaces
within a metal porous member.
[0258] The cathode and an anode formed of an anode material
containing the treatment object (for example, a metal basket
containing the treatment object) are disposed in the molten salt; a
control unit configured to control the potential of the electrodes
from the outside to a predetermined value is connected; and the
potential is controlled as described above. As a result, lithium
can be dissolved in the molten salt from the treatment object.
[0259] In the subsequent deposition step, molten salt electrolysis
is performed with a pair of electrode members disposed in the
molten salt containing dissolved lithium so that lithium is
deposited on one of the electrode members (cathode). In this case,
by controlling the potential value in the molten salt electrolysis,
lithium can be selectively deposited as metal or alloy on the
cathode.
[0260] As in the dissolution step, in this deposition step, lithium
is separated from other metals by utilizing the following
characteristics: in molten salt electrolysis, different elements
are deposited at different potentials as metal or alloy on the
cathode. Thus, even when metals other than lithium are contained in
the molten salt, by controlling the potential, lithium alone can be
deposited on the cathode. As a result, high purity lithium can be
obtained.
[0261] In deposition of lithium, when the difference between the
dissolution-deposition potential of lithium and the
dissolution-deposition potential of another metal contained in the
molten salt is so small that lithium is difficult to separate from
the metal, the cathode material may be selected and the potential
may be controlled such that an alloy of the cathode material and
lithium is deposited. As a result, lithium in the molten salt can
be separated as a lithium alloy from the other impurity metal; and,
after that, a dissolution step and a deposition step in another
molten salt are performed with the cathode material alloyed with
lithium to thereby provide high purity lithium.
[0262] The electrode members used in the deposition step may be
formed of, for example, nickel (Ni), molybdenum (Mo), or glassy
carbon (C).
[0263] In this embodiment, the above-described two steps are used
to separate and recover lithium from a treatment object.
[0264] In this embodiment, since a molten salt is used, the system
needs to be heated such that the temperature of the system in the
steps is equal to or more than the melting point of the molten
salt.
[0265] A feature of the two steps is use of a molten salt as the
electrolytic solution. Thus, the characteristics of molten salt
electrolysis in which different molten salts have different
dissolution-deposition potentials for elements are utilized; and
the steps can be designed by selecting a molten salt such that the
dissolution-deposition potential of lithium and the
dissolution-deposition potential of a non-lithium impurity metal
are sufficiently different values that allow easy performance of
the steps.
[0266] Specifically, the molten salt is preferably selected such
that, in the step of depositing or alloying lithium, the difference
between the standard electrode potential of a simple substance or
alloy of lithium and the standard electrode potential of a simple
substance or alloy of another impurity metal in the molten salt is
0.05 V or more. The difference between the standard electrode
potential of a simple substance or alloy of lithium and the
standard electrode potential of a simple substance or alloy of
another metal in the molten salt is more preferably 0.1 V or more,
still more preferably 0.25 V or more.
[0267] In this way, in the step of depositing or alloying lithium,
the potential of the electrode members is preferably controlled to
a predetermined value so that the lithium in the molten salt is
selectively deposited or alloyed.
[0268] The deposition potential of lithium to be deposited on the
cathode can be determined by electrochemical calculation.
Specifically, the calculation is performed with Nernst
equation.
[0269] For example, the potential at which a simple substance of Li
is deposited from lithium ions (Li.sup.+) can be determined with
the following equation.
E.sub.Li=E.sup.0.sub.Li+RT/3Fln(a.sub.Li(I)/a.sub.Li(0)) Equation
(1)
[0270] In Eq. (1), E.sup.0.sub.Li represents the standard
potential, R represents the gas constant, T represents absolute
temperature, F represents the Faraday constant, a.sub.Li(I)
represents the activity of Li ions, and a.sub.Li(0) represents the
activity of Li simple substance.
[0271] When Eq. (1) is rewritten in view of activity coefficient
.gamma..sub.Li(I), since a.sub.Li(0)=1, the following equations are
provided.
E Li = E Li 0 + RT / 3 F ln a Li ( I ) = E Li 0 + RT / 3 F ln (
.gamma. Li ( I ) C Li ( I ) ) Equation ( 2 ) E Li = E Li 0 ' + RT /
3 F ln C Li ( I ) Equation ( 3 ) ##EQU00003##
[0272] In Eq. (3), C.sub.Li(I) represents the concentration of Li
ions, and E.sup.0'.sub.Li represents formal electrode potential
(here, equal to E.sup.0.sub.Li+RT/3Fln .gamma..sub.Li(I)).
[0273] Similarly, in the case where LiM alloy (M represents an
alloyed metal) is deposited on the electrode surface, the potential
(deposition potential: E.sub.LiM) can be determined with the
following equation.
E.sub.Li.M=E.sup.0'.sub.Li.M+RT/3Fln C.sub.Li(I) Equation (4)
[0274] In Eq. (4), E.sup.0'.sub.Li.M represents formal electrode
potential (here, equal to E.sup.0.sub.Li.M+RT/3Fln
.gamma..sub.Li(I)).
[0275] Similarly, by using the above-described equations,
deposition potentials of all deposits corresponding to different
molten salts can be determined. In the step of depositing or
alloying lithium on the cathode, in view of the deposition
potential values of lithium simple substance and lithium alloy, the
molten salt and the cathode material are selected such that a
sufficiently high potential difference is achieved with respect to
the deposition potential of a simple substance or alloy of another
metal, and whether lithium is deposited or a lithium alloy is
deposited is decided.
[0276] Voltage and current during operation vary depending on the
size or positional relationship of electrodes. Accordingly,
reference values of voltage and current are determined on the basis
of conditions and subsequently voltage and current are determined
in each step on the basis of the potential value and order
determined by the above-described method.
[0277] As described above, in a method for producing lithium by
molten salt electrolysis according to this embodiment, the
potential value is controlled to thereby electrochemically dissolve
and deposit lithium. Accordingly, the steps can be simplified,
compared with, for example, the existing wet treatment involving
repeating of steps of dissolution and extraction using acid or the
like; and the particular element can be selectively separated and
recovered. In addition, adjustment of the specific gravity of
molten salt is not necessary; and, by selecting a low-temperature
molten salt in which lithium can be treated in the solid state, a
simple apparatus configuration can be employed. Moreover, the
operation pattern can also be simplified. As a result, the steps
can be performed efficiently at low cost.
[0278] In a method for producing lithium by molten salt
electrolysis according to this embodiment, the treatment object is
not limited as long as it is a material containing lithium.
Preferred examples of the treatment object include negative
electrode materials of lithium primary batteries and positive
electrode materials of lithium-ion secondary batteries.
[0279] Examples of positive electrode active materials of positive
electrode materials of thium-ion secondary batteries include
lithium cobalt oxide (LiCoO.sub.2), lithium nickel oxide
(LiNiO.sub.2), lithium nickel cobalt oxide
(LiCo.sub.0.3Ni.sub.0.7O.sub.2), lithium manganese oxide
(LiMn.sub.2O.sub.4), lithium titanium oxide
(Li.sub.4Ti.sub.5O.sub.12), lithium manganese oxide compounds
(LiM.sub.yMn.sub.2-yO.sub.4); M=Cr, Co, Ni), and lithium acid.
[0280] The molten salt can be selected from chloride molten salts
and fluoride molten salts. A molten salt mixture containing a
chloride molten salt and a fluoride molten salt may be used.
[0281] Examples of chloride molten salts include KCl, NaCl,
CaCl.sub.2, LiCl, RbCl, CsCl, SrCl.sub.2, BaCl.sub.2, and
MgCl.sub.2. Examples of fluoride molten salts include LiF, NaF, KF,
RbF, CsF, MgF.sub.2, CaF.sub.2, SrF.sub.2, and BaF.sub.2. Chloride
molten salts are preferably used in view of efficiency; in
particular, KCl, NaCl, and CaCl.sub.2 are preferably used because
they are inexpensive and easily available.
[0282] Among such molten salts, a plurality of molten salts can be
combined and used as a molten salt having a desirable composition.
For example, a molten salt having a composition such as
KCl--CaCl.sub.2, LiCl--KCl, or NaCl--KCl may be used.
[0283] In a method for producing lithium by molten salt
electrolysis according to this embodiment, the following
apparatuses can be preferably used. That is, an apparatus used for
a method for producing lithium by molten salt electrolysis
according to this embodiment includes a container containing a
molten salt; a cathode immersed in the molten salt contained within
the container; and an anode that is immersed in the molten salt
contained within the container and that contains a conductive
treatment object containing lithium, wherein the molten salt is
movable into and out of the anode, the apparatus further includes a
control unit configured to control the potential of the cathode and
the anode to a predetermined value, and the value of the potential
is changeable in the control unit.
[0284] An apparatus used for a method for producing lithium by
molten salt electrolysis according to this embodiment includes a
container containing a molten salt containing dissolved lithium;
and a cathode and an anode that are immersed in the molten salt
contained within the container, wherein the apparatus includes a
control unit configured to control the potential of the cathode and
the anode to a predetermined value, and the value of the potential
is changeable in the control unit.
[0285] The apparatuses for this embodiment will be described with
reference to FIGS. 18 and 19. An apparatus illustrated in FIG. 18
includes a container 1 containing a molten salt, a molten salt 2
contained within the container 1, a basket 4 containing a treatment
object 3 containing lithium, an electrode 6, a heater 10 for
heating the molten salt 2, and a control unit 9 electrically
connected to the basket 4 and the electrode 6 via conductive wires
5.
[0286] The control unit 9 is configured to control the potential of
one electrode (anode) that is the basket 4 and the other electrode
(cathode) that is the electrode 6, to a predetermined value. In the
control unit 9, the value to which the potential is controlled is
changeable. The heater 10 is disposed so as to circularly surround
the container 1. The electrode 6 may be formed of a desired
material, for example, aluminum. The container 1 may have a bottom
surface that has a circular shape or a polygonal shape. The basket
4 may be the above-described basket.
[0287] The potential of the basket 4 and the electrode 6 is
controlled by the control unit 9 to a predetermined potential
value. As a result, lithium is dissolved in the molten salt 2 from
the treatment object 3.
[0288] After lithium is sufficiently dissolved from the treatment
object 3, the basket 4 and the electrode 6 are removed and, as
illustrated in FIG. 19, another electrode 7 (cathode) and another
electrode 8 (anode) are placed in the molten salt 2. These
electrodes 7 and 8 are connected to the control unit 9 via
conductive wires 5. The control unit 9 is used to control the
potential of the electrodes 7 and 8 to a predetermined value. At
this time, the potential is controlled such that the potential at
the electrode 7 is the deposition potential of lithium. As a
result, lithium dissolved in the molten salt 2 is deposited on the
surface of the electrode 7 (cathode). The electrodes 7 and 8 may be
formed of a material such as glassy carbon (C).
[0289] The heating temperature for the molten salt 2 with the
heater 10 may be, for example, 800.degree. C. in both of the
treatments using the apparatuses illustrated in FIGS. 18 and 19. In
this way, lithium can be deposited as a simple substance on the
surface of the electrode 7.
[0290] The potential of the electrodes 7 and 8 may be controlled to
a value such that an alloy of lithium and the cathode material is
deposited on the surface of the electrode 7 (cathode). In this
case, the above-described dissolution step and deposition step may
be performed with the alloyed electrode 7. That is, the apparatus
illustrated in FIG. 18 is newly prepared and the electrode 7
alloyed with lithium is used instead of the above-described
treatment object 3.
[0291] In the case where the method for producing lithium of this
embodiment is performed with the apparatuses illustrated in FIGS.
18 and 19, for example, the method may be performed in the
following manner.
[0292] A lithium-containing positive electrode material of
lithium-ion batteries is first prepared as the treatment object 3
and KCl--NaCl is prepared as the molten salt 2. For example, the
positive electrode material is a powder containing lithium cobalt
oxide (LiCoO.sub.2) or lithium manganese oxide. The positive
electrode material is ground and placed within the basket 4. From
the viewpoint of enhancement of the treatment efficiency, the size
of the positive electrode material serving as the treatment object
3 is preferably minimized by grinding. For example, the positive
electrode material is ground into particles having a maximum
particle size of 5 mm or less, preferably 3 mm or less, more
preferably 1 mm or less. The above-described dissolution step may
be performed with a carbon electrode serving as the electrode 6.
Subsequently, the deposition step may be performed with electrodes
formed of glassy carbon and serving as the electrodes 7 and 8.
[0293] As has been described, lithium can be recovered from the
positive electrode material serving as the treatment object 3.
[0294] According to the method for producing lithium by molten salt
electrolysis of this embodiment, the apparatus configuration can be
simplified and the treatment time can also be decreased, compared
with the existing wet separation method and the like. Thus, the
cost incurred can be reduced. In addition, by appropriately setting
a potential value at an electrode, lithium can be deposited as a
simple substance on the electrode surface and hence high purity
lithium can be obtained.
Fourth Embodiment
[0295] This embodiment is a method for producing a metal by molten
salt electrolysis, the method including a step of dissolving, in a
molten salt, a metal element contained in a treatment object
containing two or more metal elements; and a step of depositing or
alloying a particular metal present in the molten salt, on one of a
pair of electrode members disposed in the molten salt containing
the dissolved metal element, by controlling a potential of the
electrode members to a predetermined value.
[0296] Roughly speaking, this embodiment includes a process of
dissolving, in a molten salt, a particular metal contained in the
treatment object, and a process of depositing the particular metal
on one of electrodes (cathode) from the molten salt containing the
dissolved particular metal by molten salt electrolysis. A feature
of this embodiment is that, by controlling the potential of the
electrodes to a predetermined value, the particular metal is
selectively deposited from the treatment object to provide the
particular metal at high purity.
[0297] The process of dissolving, in a molten salt, a particular
metal contained in a treatment object will be first described.
[0298] A procedure for dissolving, in a molten salt, a particular
metal contained in a treatment object is, for example, a chemical
procedure for dissolution. Specifically, a treatment object is
ground into particles or powder, mixed with a salt, and heated. As
a result, the particular metal contained in the treatment object
can be dissolved in the molten salt. Alternatively, the treatment
object may be placed in a molten salt and dissolved.
[0299] Another procedure is an electrochemical procedure.
Specifically, a cathode and an anode that is formed of an anode
material containing the treatment object are disposed in the molten
salt; and the potential at the anode is controlled to a
predetermined value so that a metal element corresponding to the
controlled potential value is dissolved in the molten salt from the
treatment object. Molten salt electrolysis is characterized in that
different elements are dissolved at different potentials; and such
characteristics are utilized to thereby separate a particular metal
from other metals. In this way, by using a treatment object as an
anode and controlling the potential during dissolution, a
particular metal can be selectively dissolved in a molten salt.
[0300] In this step, all the metals contained in the treatment
object may be dissolved. Alternatively, a particular metal and
another metal contained in the treatment object may be dissolved.
Preferably, only a particular metal contained in the treatment
object is dissolved. Conditions under which a particular metal and
another metal contained in the treatment object are dissolved may
be employed; however, if possible, the potential is preferably
controlled so that the particular metal alone is dissolved. That
is, in the step of dissolving a particular metal in a molten salt,
the potential at the anode is preferably controlled to a
predetermined value so that the particular metal element is
selectively dissolved in the molten salt. As a result, entry of
impurities in the subsequent deposition step can be reduced.
[0301] For this purpose, the molten salt is preferably selected
such that, in the step of dissolving, in the molten salt, a
particular metal from the treatment object, the difference between
the standard electrode potential of a simple substance or alloy of
the particular metal and the standard electrode potential of a
simple substance or alloy of another metal in the molten salt is
0.05 V or more. As a result, the particular metal that is dissolved
in the molten salt can be sufficiently separated from the other
metal element that is left in the anode. The difference between the
standard electrode potentials is more preferably 0.1 V or more,
still more preferably 0.25 V or more.
[0302] The value of the potential controlled at the anode can be
calculated by Nernst equation described below.
[0303] When one or more target particular metals are contained in
the treatment object, in the dissolution step, one or more
particular metals are dissolved in the molten salt.
[0304] When the treatment object contains only one particular
metal, as described above, this particular metal is dissolved and
then the deposition step is performed to provide the target metal.
When the treatment object contains two or more target particular
metals, only one of the metals may be dissolved in a molten salt; a
deposition step may be subsequently performed; and, after that,
another dissolution step may be performed so that the remainder of
the particular metals is dissolved in the molten salt. In this
case, the treatment object having been used in the initial
dissolution step may be moved from the molten salt used in this
dissolution step to another molten salt and subjected to a
dissolution step to thereby dissolve the remainder of the
particular metals.
[0305] When two or more particular metals contained in the
treatment object are dissolved in a molten salt, the subsequent
deposition step may be performed such that the particular metals
present in the molten salt are deposited or alloyed one by one on
electrode materials, so that desired particular metals can be
produced. In this case, after one particular metal is deposited or
alloyed on an electrode material, this electrode material may be
replaced by another electrode material and another particular metal
dissolved in the molten salt may be deposited or alloyed on this
electrode material.
[0306] The cathode used in the dissolution step is formed of carbon
or a material that tends to form an alloy with an alkali metal such
as Li or Na constituting cations in the molten salt. For example,
aluminum (Al), zinc (Zn), gallium (Ga), cadmium (Cd), indium (In),
tin (Sn), antimony (Sb), lead (Pb), or bismuth (Bi) may be
used.
[0307] When the treatment object containing a particular metal is
used as an anode, for example, the treatment object contained
within a conductive basket (anode material) formed of metal or the
like may be disposed in the molten salt. An opening may be formed
in an upper portion of the basket so that the treatment object can
be inserted through the opening into the basket; and a large number
of holes may be formed in the side and bottom walls of the basket
so that the molten salt can flow into the basket. The basket may be
constituted by a desired material such as a mesh member knitted
from metal wires or a sheet member that is a sheet-shaped metal
plate having a large number of holes. In particular, it is
effective that the material is formed of C, Pt, Mo, or the
like.
[0308] In the cases where the object is an oxide or the like and
has a high electric resistance, the contact area between the object
and the conductive material is preferably increased. The object is
effectively used as an electrode by, for example, wrapping the
object with a metal mesh member or filling the object into spaces
within a metal porous member.
[0309] The cathode and an anode formed of an anode material
containing the treatment object (for example, a metal basket
containing the treatment object) are disposed in the molten salt;
and the potential at the anode is controlled to a predetermined
value. As a result, a particular metal can be dissolved in the
molten salt from the treatment object.
[0310] In the subsequent deposition process, molten salt
electrolysis is performed with a pair of electrode members disposed
in the molten salt containing the dissolved particular metal so
that the particular metal is deposited on one of the electrode
members (cathode). In this case, by controlling the potential value
in the molten salt electrolysis, the particular metal can be
selectively deposited as metal or alloy on the cathode.
[0311] As in the dissolution process, in this deposition process,
the particular metal is separated from other metals by utilizing
the following characteristics: in molten salt electrolysis,
different elements are deposited at different potentials as metal
or alloy on the cathode. Thus, even when metals other than the
particular metal are contained in the molten salt, by controlling
the potential of the electrode members to a predetermined value,
the particular metal element can be selectively deposited or
alloyed on the cathode. That is, the particular metal at high
purity can be obtained.
[0312] In deposition of a particular metal, when the difference
between the dissolution-deposition potential of the particular
metal and the dissolution-deposition potential of another metal
contained in the molten salt is so small that the particular metal
is difficult to separate from the other metal, the cathode material
may be selected and the potential may be controlled such that an
alloy of the cathode material and the particular metal is
deposited. As a result, the particular metal in the molten salt can
be deposited as an alloy and separated from the other impurity
metal; and, after that, for example, a dissolution step and a
deposition step in another molten salt can be performed with the
cathode material alloyed with the particular metal to thereby
provide the particular metal at high purity.
[0313] The electrode members used in the deposition step may be
formed of, for example, nickel (Ni), molybdenum (Mo), or glassy
carbon (C).
[0314] In this embodiment, the above-described two processes are
used to separate and extract a particular metal from a treatment
object. In this embodiment, since a molten salt is used, the system
needs to be heated such that the temperature of the system in the
processes is equal to or more than the melting point of the molten
salt.
[0315] Alternatively, as described below, smelting can be performed
on the basis of an idea that is totally contrary to that of the
processes. That is, a treatment object is used as the anode and
only metal elements serving as impurities are dissolved in a molten
salt. In this case, by also controlling the potential at the anode,
such a phenomenon is caused in which a particular metal is left in
the anode and impurity elements are dissolved. As a result, the
particular metal is provided in the anode.
[0316] A feature of the two processes is use of a molten salt.
Thus, the characteristics of molten salt electrolysis in which
different molten salts have different dissolution-deposition
potentials for elements are utilized; and the processes can be
designed by selecting a molten salt such that the
dissolution-deposition potential of a particular metal and the
dissolution-deposition potential of an impurity metal other than
the particular metal are sufficiently different values that allow
easy performance of the processes.
[0317] Specifically, the molten salt is preferably selected such
that, in the step of depositing or alloying a particular metal, the
difference between the standard electrode potential of a simple
substance or alloy of the particular metal and the standard
electrode potential of a simple substance or alloy of another metal
in the molten salt is 0.05 V or more.
[0318] The difference between the standard electrode potential of a
simple substance or alloy of the particular metal and the standard
electrode potential of a simple substance or alloy of another metal
in the molten salt is more preferably 0.1 V or more, still more
preferably 0.25 V or more.
[0319] In this way, in the step of depositing or alloying a
particular metal, the potential of the electrode members is
preferably controlled to a predetermined value so that the
particular metal element in the molten salt is selectively
deposited or alloyed.
[0320] The deposition potential of a particular metal to be
deposited on the cathode can be determined by electrochemical
calculation. Specifically, the calculation is performed with Nernst
equation.
[0321] For example, the potential at which a simple substance of
molybdenum (Mo) serving as the particular metal is deposited from a
molten salt in which molybdenum is dissolved into tetravalent Mo
ions (hereafter represented by Mo(IV)) can be determined with the
following equation.
E.sub.Mo=E.sup.0.sub.Mo+RT/3Fln(a.sub.Mo(IV)/a.sub.Mo(0)) Equation
(1)
[0322] In Eq. (1), E.sup.0.sub.Mo represents the standard
potential, R represents the gas constant, T represents absolute
temperature, F represents the Faraday constant, a.sub.Mo(IV)
represents the activity of Mo(IV) ions, and a.sub.Mo(0) represents
the activity of Mo simple substance.
[0323] When Eq. (1) is rewritten in view of activity coefficient
.gamma..sub.Mo(Iv), since a.sub.Mo(0)=1, the following equations
are provided.
E Mo = E Mo 0 + RT / 3 F ln a Mo ( IV ) = E Mo 0 + RT / 3 F ln (
.gamma. Mo ( IV ) C Mo ( IV ) ) Equation ( 2 ) E Mo = E Mo 0 ' + RT
/ 3 F ln C Mo ( IV ) Equation ( 3 ) ##EQU00004##
[0324] In Eq. (3), C.sub.Mo(IV) represents the concentration of
tetravalent Mo ions, and E.sup.0'.sub.Mo represents formal
electrode potential (here, equal to E.sup.0.sub.Mo+RT/3Fln
.gamma..sub.Mo(IV)).
[0325] Similarly, by using the above-described equations,
deposition potentials of all deposits corresponding to different
molten salts can be determined.
[0326] Similar calculations can also be performed in the case of
depositing molybdenum as an alloy.
[0327] In the process of depositing or alloying molybdenum on the
cathode, in view of the deposition potential values of molybdenum
simple substance and molybdenum alloy, the molten salt and the
cathode material are selected such that a sufficiently high
potential difference is achieved with respect to the deposition
potential of a simple substance or alloy of another metal, and
whether molybdenum simple substance is deposited or a molybdenum
alloy is deposited is decided.
[0328] Voltage and current during operation vary depending on the
size or positional relationship of electrodes. Accordingly,
reference values of voltage and current are determined on the basis
of conditions and subsequently voltage and current are determined
in each step on the basis of the potential value and order
determined by the above-described method.
[0329] As described above, in a method for producing a particular
metal by molten salt electrolysis according to this embodiment, the
potential value is controlled to thereby electrochemically dissolve
and deposit the particular metal. Accordingly, the steps can be
simplified, compared with, for example, the existing wet treatment
involving repeating of processes of dissolution and extraction
using acid or the like; and a particular metal can be selectively
separated and recovered. In addition, adjustment of the specific
gravity of molten salt is not necessary; and, by selecting a
low-temperature molten salt in which the particular metal can be
treated in the solid state, a simple apparatus configuration can be
employed. Moreover, the operation pattern can also be simplified.
As a result, the steps can be performed efficiently at low
cost.
[0330] Alternatively, as described above, a particular metal can be
smelted on the basis of an idea that is totally contrary to the
idea of depositing or alloying a particular metal on the
cathode.
[0331] That is, a method for producing a metal by molten salt
electrolysis according to this embodiment is a method for producing
a particular metal by molten salt electrolysis from a treatment
object containing two or more metal elements, wherein a cathode and
an anode that is formed of an anode material containing the
treatment object are disposed in a molten salt, and the potential
at the anode is controlled to a predetermined value so that a metal
element corresponding to the potential is dissolved in the molten
salt from the treatment object and the particular metal is left in
the anode.
[0332] In this production method, the anode material containing the
treatment object is used as the anode and metal elements other than
the particular metal, that is, only metal elements serving as
impurities are dissolved in the molten salt, so that the particular
metal is left in the anode. In this case, by also controlling the
potential at the anode, such a phenomenon can be caused in which
the particular metal as the smelting target is left in the anode
and impurity elements are dissolved in the molten salt. As a
result, the smelted particular metal is provided in the anode.
[0333] In this method, the molten salt is also preferably selected
such that, in the step of dissolving, in the molten salt, a metal
element from the treatment object, the difference between the
standard electrode potential of a simple substance or alloy of the
particular metal and the standard electrode potential of a simple
substance or alloy of another metal in the molten salt is 0.05 V or
more. As a result, the particular metal can be sufficiently
separated from the other metal and the particular metal alone can
be left in the anode. The difference between the standard electrode
potentials is more preferably 0.1 V or more, still more preferably
0.25 V or more.
[0334] The value of the potential controlled at the anode can be
calculated by Nernst equation as described above.
[0335] In a method for producing a metal by molten salt
electrolysis according to this embodiment, the treatment object
containing two or more metal elements is not limited at all as long
as it is a metal material containing a target particular metal. For
example, Mn, Co, Sb, and the like can be obtained from collected
battery materials; Nb and the like can be obtained from metal
superconducting materials; Bi, Sr, and the like can be obtained
from oxide superconducting materials; V can be obtained from
ferrovanadium; Mo and the like can be obtained from Mo--Cu heat
spreaders; and Ge and the like can be obtained from optical fiber
materials.
[0336] This embodiment is also suitably applicable to cases where
the treatment object is a metal material containing a transition
metal or a rare earth metal. Such a transition metal is not
particularly limited and may be any element among from group 3
(group IIIA) to group 11 (group IB) of the periodic table. This
embodiment is also suitably applicable to cases where the treatment
object contains, as a transition metal, one or more metals selected
from the group consisting of V, Nb, Mo, Ti, Ta, Zr, and Hf.
[0337] In addition, this embodiment is also suitably applicable to
cases where the treatment object contains a metal that is one or
both of Sr and Ba. Furthermore, this embodiment is also suitably
applicable to cases where the treatment object contains one or more
metals selected from the group consisting of Zn, Cd, Ga, In, Ge,
Sn, Pb, Sb, and Bi.
[0338] In a method for producing a metal by molten salt
electrolysis of this embodiment, by selecting a transition metal or
a rare earth metal as the particular metal to be deposited or
alloyed, the transition metal or the rare earth metal can be
obtained. Such a transition metal is not particularly limited and
may be any element among from group 3 (group IIIA) to group 11
(group IB) of the periodic table.
[0339] Similarly, by selecting the particular metal to be deposited
or alloyed from V, Nb, Mo, Ti, Ta, Zr and Hf, or Sr and Ba, or Zn,
Cd, Ga, In, Ge, Sn, Pb, Sb, and Bi, such metals can be
obtained.
[0340] As described above, in the dissolution step, one or more of
these metals contained in the treatment object can be dissolved in
a molten salt and particular metals can be sequentially deposited
or alloyed on electrode members from the molten salt.
[0341] The treatment object preferably has the form of particles or
powder. When the treatment object is prepared so as to have the
form of particles or powder, the surface area is increased and the
treatment efficiency can be increased.
[0342] In addition, the treatment object prepared in the form of
particles or powder can be compacted and used as the anode. In this
case, between the particles, there are desirably spaces that the
molten salt can easily enter.
[0343] The molten salt can be selected from chloride molten salts
and fluoride molten salts. A molten salt mixture containing a
chloride molten salt and a fluoride molten salt may be used.
[0344] Examples of chloride molten salts include KCl, NaCl,
CaCl.sub.2, LiCl, RbCl, CsCl, SrCl.sub.2, BaCl.sub.2, and
MgCl.sub.2. Examples of fluoride molten salts include LiF, NaF, KF,
RbF, CsF, MgF.sub.2, CaF.sub.2, SrF.sub.2, and BaF.sub.2. Chloride
molten salts are preferably used in view of efficiency; in
particular, KCl, NaCl, and CaCl.sub.2 are preferably used because
they are inexpensive and easily available.
[0345] Among such molten salts, a plurality of molten salts can be
combined and used as a molten salt having a desirable composition.
For example, a molten salt having a composition such as
KCl--CaCl.sub.2, LiCl--KCl, or NaCl--KCl may be used.
[0346] In a method for producing a metal by molten salt
electrolysis according to this embodiment, the following
apparatuses can be preferably used. That is, preferably, the
apparatus includes a container containing a molten salt; a cathode
immersed in the molten salt contained within the container; and an
anode that is immersed in the molten salt contained within the
container and that contains a conductive treatment object
containing two or more metal elements, wherein the molten salt is
movable into and out of the anode, the apparatus further includes a
control unit configured to control the potential of the cathode and
the anode to a predetermined value, and the value of the potential
is changeable in the control unit. An apparatus used for a method
for producing a metal by molten salt electrolysis according to this
embodiment is preferably an apparatus that includes a container
containing a molten salt containing a dissolved particular metal;
and a cathode and an anode that are immersed in the molten salt
contained within the container, wherein the apparatus includes a
control unit configured to control the potential of the cathode and
the anode to a predetermined value, and the value of the potential
is changeable in the control unit.
[0347] The apparatuses will be described with reference to FIGS. 18
and 19. An apparatus illustrated in FIG. 18 includes a container 1
containing a molten salt, a molten salt 2 contained within the
container 1, a basket 4 containing a treatment object 3 containing
two or more metal elements, an electrode 6, a heater 10 for heating
the molten salt 2, and a control unit 9 electrically connected to
the basket 4 and the electrode 6 via conductive wires 5.
[0348] The control unit 9 is configured to control the potential of
one electrode (anode) that is the basket 4 and the other electrode
(cathode) that is the electrode 6, to a predetermined value. In the
control unit 9, the value to which the potential is controlled is
changeable. The heater 10 is disposed so as to circularly surround
the container 1. The electrode 6 may be formed of a desired
material, for example, carbon. The container 1 may have a bottom
surface that has a circular shape or a polygonal shape. The basket
4 may be the above-described basket.
[0349] The potential of the basket 4 and the electrode 6 is
controlled by the control unit 9 to a predetermined potential
value. As a result, a particular metal is dissolved in the molten
salt 2 from the treatment object 3.
[0350] After the particular metal is sufficiently dissolved from
the treatment object 3, the basket 4 and the electrode 6 are
removed and another electrode 7 (cathode) and another electrode 8
(anode) are placed in the molten salt 2. These electrodes 7 and 8
are connected to the control unit 9 via conductive wires 5. The
control unit 9 is used to control the potential of the electrodes 7
and 8 to a predetermined value. At this time, the potential is
controlled such that the potential at the electrode 7 is the
deposition potential of the particular metal. As a result, the
particular metal dissolved in the molten salt 2 is deposited on the
surface of the electrode 7 (cathode). The electrodes 7 and 8 may be
formed of a material such as glassy carbon (C).
[0351] The heating temperature for the molten salt 2 with the
heater 10 may be, for example, 800.degree. C. in both of the
treatments using the apparatuses illustrated in FIGS. 18 and 19. In
this way, the particular metal can be deposited as a simple
substance on the surface of the electrode 7.
[0352] The potential of the electrodes 7 and 8 may be controlled
such that an alloy of the particular metal and the cathode material
is deposited on the surface of the electrode 7 (cathode). In this
case, the above-described dissolution step and deposition step may
be performed with the alloyed electrode 7. That is, the apparatus
illustrated in FIG. 18 is newly prepared and the electrode 7
alloyed with the particular metal is used instead of the
above-described treatment object 3.
[0353] In the cases where the method for producing a metal of this
embodiment is performed with the apparatuses illustrated in FIGS.
18 and 19, for example, the method may be performed in the
following manner. Hereinafter, examples relating to vanadium,
molybdenum, strontium, and germanium will be described.
(Vanadium)
[0354] For example, the method for producing a metal of this
embodiment is used to obtain vanadium. Ferrovanadium (1 kg) is
first prepared as the treatment object 3 and NaCl--KCl is prepared
as the molten salt 2. For example, the ferrovanadium contains 75 wt
% of vanadium (V) and 25 wt % of iron (Fe). The ferrovanadium is
ground and placed within the basket 4. The amount of the molten
salt 2 is about 15 liters.
[0355] The above-described dissolution step may be performed with a
carbon electrode serving as the electrode 6. Subsequently, the
deposition step may be performed with electrodes formed of glassy
carbon and serving as the electrodes 7 and 8.
(Molybdenum)
[0356] The method for producing a metal of this embodiment is used
to obtain molybdenum. Mo--Cu heat spreaders (1 kg) are first
prepared as the treatment object 3 and LiCl--KCl is prepared as the
molten salt 2. For example, the Mo--Cu heat spreaders contain 50 wt
% of molybdenum (Mo) and 50 wt % of copper (Cu). The heat spreaders
are ground and placed within the basket 4. The amount of the molten
salt 2 is about 5 liters.
[0357] The above-described dissolution step may be performed with a
carbon electrode serving as the electrode 6. Subsequently, the
deposition step may be performed with electrodes formed of glassy
carbon and serving as the electrodes 7 and 8.
(Strontium)
[0358] The method for producing a metal of this embodiment is used
to obtain molybdenum. An oxide superconducting material (1 kg) is
first prepared as the treatment object 3 and LiF--CaF.sub.2 is
prepared as the molten salt 2. For example, the oxide
superconducting material contains 17 wt % of strontium (Sr) and 8
wt % of calcium (Ca). The oxide superconducting material is ground
and placed within the basket 4. The amount of the molten salt 2 is
about 4 liters.
[0359] The above-described dissolution step may be performed with a
carbon electrode serving as the electrode 6. Subsequently, the
deposition step may be performed with electrodes formed of glassy
carbon and serving as the electrodes 7 and 8.
(Germanium)
[0360] The method for producing a metal of this embodiment is used
to obtain germanium. An optical fiber material (1 kg) is first
prepared as the treatment object 3 and LiF--CaF.sub.2 is prepared
as the molten salt 2. For example, the optical fiber material
contains 3 wt % of germanium (Ge). The optical fiber material is
ground and placed within the basket 4. The amount of the molten
salt 2 is about 4 liters.
[0361] The above-described dissolution step may be performed with a
carbon electrode serving as the electrode 6. Subsequently, the
deposition step may be performed with electrodes formed of glassy
carbon and serving as the electrodes 7 and 8.
[0362] As has been described, by using ferrovanadium, Mo--Cu heat
spreaders, oxide superconducting material, and optical fiber
material as the treatment object 3, vanadium, molybdenum,
strontium, and germanium can be obtained, respectively. From the
viewpoint of enhancement of the treatment efficiency, the size of
ferrovanadium, Mo--Cu heat spreaders, oxide superconducting
material, and optical fiber material serving as the treatment
object 3 is preferably minimized by grinding: for example, the
treatment object 3 is preferably ground into particles having a
maximum particle size of 5 mm or less, more preferably 3 mm or
less, still more preferably 1 mm or less.
[0363] According to the method for producing a metal by molten salt
electrolysis of this embodiment, the apparatus configuration can be
simplified and the treatment time can also be decreased, compared
with the existing recovery methods and the like. Thus, the cost
incurred can be reduced. In addition, by appropriately setting a
potential at an electrode, a particular metal can be deposited as a
simple substance on the electrode surface and hence high purity
metal can be obtained.
[0364] The potentials for depositing vanadium, a vanadium alloy,
molybdenum, a molybdenum alloy, strontium, a strontium alloy,
germanium, and a germanium alloy can be determined by the
above-described calculation.
[0365] First to Fourth embodiments have been individually described
so far. However, for example, in order to obtain tungsten, lithium,
transition metals, and rare earth metals in Second to Fourth
embodiments, methods in other embodiments may be entirely or
partially employed.
EXAMPLES
First Embodiment
Example
[0366] Nd, Dy, and Pr were produced by molten salt electrolysis
from an ore containing rare earth metals.
(Sample)
[0367] The ore serving as a treatment object was xenotime ore. The
xenotime ore was ground with a crusher or a ball mill so as to have
a particle size of about 2 mm. The ground sample (xenotime ore) was
wrapped with a molybdenum (Mo) mesh (50 mesh).
[0368] As illustrated in FIG. 14, the sample powder contained
within the mesh was used as an anode (anode electrode).
(Details of Experiment)
[0369] A molten LiF--NaF--KF eutectic salt was employed as the
molten salt. This salt was completely melted by heating at
700.degree. C. In this molten salt, the above-described anode
electrode and a cathode electrode were wired and immersed. The
cathode electrode was formed of glassy carbon.
Dissolution Step:
[0370] While the anode electrode and the cathode electrode were
thus immersed in the molten salt, the anode electrode was
maintained at a predetermined potential. After about 4 hours
lapsed, a sample was taken from the molten salt and the sample was
subjected to composition analysis by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES).
Electrolysis Step:
[0371] After the dissolution step, a cathode electrode formed of Ni
and an anode electrode formed of glassy carbon were immersed in the
molten salt. The potential at the cathode electrode was maintained
at a predetermined potential. Specifically, the potential was
maintained such that Dy--Ni alloy was formed in the LiF--NaF--KF
molten salt. After a predetermined time lapsed, the surface status
of the cathode electrode was observed.
(Result)
Regarding Dissolution Step:
[0372] The anode current observed in the dissolution step varied
with time as illustrated in FIG. 15.
[0373] In FIG. 15, the abscissa axis indicates time (unit: min),
and the ordinate axis indicates the value of anode current (unit:
mA). As illustrated in FIG. 15, the current value decreased with
time. The change rate of current value with respect to time had the
following tendency: the change rate was the highest at the
beginning of the measurement (at the beginning of application of
current) and, after that, the change rate gradually decreased.
[0374] The sample taken from the molten salt was subjected to
composition analysis by ICP-AES. As a result, dissolution of Nd and
Dy in the molten salt was confirmed.
Regarding Electrolysis Step:
[0375] FIGS. 16 and 17 illustrate results of observation of a
section of the surface layer of the cathode electrode with a
scanning electron microscope (SEM). As illustrated in FIGS. 16 and
17, Dy--Ni alloy 32 was deposited on the surface of an electrode
body part 31 constituting the cathode electrode and formed of Ni.
This Dy--Ni alloy 32 was probably formed by the reaction between Dy
present in the molten salt and Ni constituting the cathode
electrode, and deposited on the surface of the cathode electrode.
In this way, Dy contained in the xenotime ore can be separated and
extracted as Dy--Ni alloy from the ore.
[0376] FIG. 16 illustrates a back-scattered electron image observed
with the SEM. FIG. 17 illustrates the distribution of Dy atoms in
the regions illustrated in FIG. 16 and subjected to X-ray analysis.
As illustrated in FIG. 17, Dy was scarcely detected in a region 33
corresponding to the electrode body part 31; however, Dy was
detected in a region 34 corresponding to the Dy--Ni alloy 32.
Second Embodiment
Example
[0377] Cemented carbide tools were used as the metal material
containing tungsten and tungsten was produced by molten salt
electrolysis.
(Sample)
[0378] The cemented carbide tools serving as a treatment object
were cutting tools containing 90 wt % of tungsten carbide and 10 wt
% of cobalt serving as a binder. The cutting tools were ground with
a bead mill or an attritor so as to have a particle size of about 2
mm. The ground sample (cutting tools) was wrapped with a molybdenum
(Mo) mesh (50 mesh). As illustrated in FIG. 14, the sample powder
(treatment object) contained within the Mo mesh was used as an
anode (anode electrode).
(Details of Experiment)
[0379] A molten NaCl--KCl eutectic salt was employed as the molten
salt. This salt was completely melted by heating at 700.degree. C.
In this molten salt, the above-described anode electrode and a
cathode electrode were wired and immersed. The cathode electrode
was formed of glassy carbon.
Dissolution Step:
[0380] While the anode electrode and the cathode electrode were
thus immersed in the molten salt, the anode electrode was
maintained at a predetermined potential. After a predetermined time
lapsed, a sample was taken from the molten salt and the sample was
subjected to composition analysis by ICP-AES.
Electrolysis Step:
[0381] After the dissolution step, a cathode electrode formed of
glassy carbon and an anode electrode formed of glassy carbon were
immersed in the molten salt. The potential at the cathode electrode
was maintained at a predetermined potential. Specifically, the
potential was maintained such that tungsten was deposited in the
NaCl--KCl molten salt. After a predetermined time lapsed, the
surface status of the cathode electrode was observed.
(Result)
Regarding Dissolution Step:
[0382] The anode current observed in the dissolution step varied
with time as in First embodiment (example) (FIG. 15). In FIG. 15,
the abscissa axis indicates time (unit: min), and the ordinate axis
indicates the value of anode current (unit: mA). As illustrated in
FIG. 15, the current value decreased with time. The change rate of
current value with respect to time had the following tendency: the
change rate was the highest at the beginning of the measurement (at
the beginning of application of current) and, after that, the
change rate gradually decreased.
[0383] The sample taken from the molten salt was subjected to
composition analysis by ICP-AES. As a result, dissolution of
tungsten in the molten salt was confirmed.
Regarding Electrolysis (Deposition) Step:
[0384] Observation of a section of the surface layer of the cathode
electrode with a scanning electron microscope (SEM) revealed
deposition of tungsten on the surface of an electrode body part
constituting the cathode electrode and formed of glassy carbon.
[0385] In this way, high purity tungsten was obtained from the
cemented carbide cutting tools containing tungsten.
Third Embodiment
Example
[0386] Commercially available lithium-ion secondary batteries were
used as the treatment object containing lithium and lithium was
produced by molten salt electrolysis.
(Sample)
[0387] Commercially available lithium-ion secondary batteries (the
positive electrode was formed of lithium cobalt oxide and the
negative electrode was formed of graphite, lithium cobalt oxide
content: mass %)
(Separation of Lithium Battery Positive Electrode Material)
[0388] The lithium-ion secondary batteries were immersed in an
electrolytic solution (5% NaCl) and discharged until the voltage
became 0.1 mV. After that, the positive electrode material was
taken out by manual disassembly, and ground with a cutter mill to
provide a positive electrode material powder having an average
particle size of 0.1 mm. The composition of the powder is described
in Table I. As a result of analysis, it was confirmed that the
powder obtained by the separation was lithium cobalt oxide.
TABLE-US-00001 TABLE I Composition (mass %) Li 7 Co 60
[0389] The powder was wrapped with a molybdenum (Mo) mesh (200
mesh). As illustrated in FIG. 14, the sample powder contained
within the Mo mesh was used as an anode (anode electrode).
(Preparation of Electrolysis Apparatus)
[0390] A molten NaCl--KCl eutectic salt was employed as the molten
salt. This salt was completely melted by heating at 700.degree. C.
In this molten salt, the above-described anode electrode and a
cathode electrode were wired and immersed. The cathode (cathode
electrode) was formed of carbon.
(Electrolysis Dissolution Step)
[0391] While the anode electrode and the cathode electrode were
thus immersed in the molten salt, the anode electrode was
maintained at a predetermined potential. After a predetermined time
lapsed, a sample was taken from the molten salt and the sample was
subjected to composition analysis by ICP-AES.
[0392] The anode current observed in the dissolution step varied
with time as in First embodiment (example) (FIG. 15). In FIG. 15,
the abscissa axis indicates time (unit: min), and the ordinate axis
indicates the value of anode current (unit: mA). As illustrated in
FIG. 15, the current value decreased with time. The change rate of
current value with respect to time had the following tendency: the
change rate was the highest at the beginning of the measurement (at
the beginning of application of current) and, after that, the
change rate gradually decreased.
[0393] The sample taken from the molten salt was subjected to
composition analysis by ICP-AES. As a result, dissolution of
lithium in the molten salt was confirmed.
(Electrolysis Deposition Step)
[0394] After the dissolution step, a cathode electrode formed of
glassy carbon and an anode electrode formed of glassy carbon were
immersed in the molten salt. The potential at the cathode electrode
was maintained at a predetermined potential. Specifically, the
potential was maintained such that lithium was deposited in the
NaCl--KCl molten salt. After a predetermined time lapsed, a section
of the surface layer of the cathode electrode was observed with a
scanning electron microscope (SEM).
[0395] The observation revealed deposition of lithium on the
surface of an electrode body part constituting the cathode
electrode and formed of glassy carbon.
[0396] In this way, lithium was recovered from the positive
electrode material containing lithium.
Fourth Embodiment (Example)-1
[0397] Ferrovanadium was used as the metal material containing
vanadium and vanadium was produced by molten salt electrolysis.
(Sample)
[0398] The ferrovanadium serving as a treatment object contained 75
wt % of vanadium and 25 wt % of iron. The ferrovanadium was ground
with a bead mill or an attritor so as to have a particle size of
about 2 mm. The ground sample (ferrovanadium) was wrapped with a
molybdenum (Mo) mesh (50 mesh). As illustrated in FIG. 14, the
sample powder (treatment object) contained within the Mo mesh was
used as an anode (anode electrode).
(Details of Experiment)
[0399] A molten NaCl--KCl eutectic salt was employed as the molten
salt. This salt was completely melted by heating at 700.degree. C.
In this molten salt, the above-described anode electrode and a
cathode electrode were wired and immersed. The cathode electrode
was formed of glassy carbon.
Dissolution Step:
[0400] While the anode electrode and the cathode electrode were
thus immersed in the molten salt, the anode electrode was
maintained at a predetermined potential. At this time, the
potential was set such that iron was not dissolved but vanadium
alone was selectively dissolved. After a predetermined time lapsed,
a sample was taken from the molten salt and the sample was
subjected to composition analysis by ICP-AES.
Electrolysis Step:
[0401] After the dissolution step, a cathode electrode formed of
glassy carbon and an anode electrode formed of glassy carbon were
immersed in the molten salt. The potential at the cathode electrode
was maintained at a predetermined potential. Specifically, the
potential was maintained such that vanadium was deposited in the
NaCl--KCl molten salt. After a predetermined time lapsed, the
surface status of the cathode electrode was observed.
(Result)
Regarding Dissolution Step:
[0402] The anode current observed in the dissolution step varied
with time as in First embodiment (example) (FIG. 15). In FIG. 15,
the abscissa axis indicates time (unit: min), and the ordinate axis
indicates the value of anode current. As illustrated in FIG. 15,
the current value decreased with time. The change rate of current
value with respect to time had the following tendency: the change
rate was the highest at the beginning of the measurement (at the
beginning of application of current) and, after that, the change
rate gradually decreased.
[0403] The sample taken from the molten salt was subjected to
composition analysis by ICP-AES. As a result, dissolution of
vanadium in the molten salt was confirmed.
Regarding Electrolysis (Deposition) Step:
[0404] Observation of a section of the surface layer of the cathode
electrode with a scanning electron microscope (SEM) revealed
deposition of vanadium on the surface of an electrode body part
constituting the cathode electrode and formed of glassy carbon.
[0405] In this way, high purity vanadium was obtained from the
ferrovanadium containing vanadium.
Fourth Embodiment (Example)-2
[0406] Mo--Cu heat spreaders were used as the metal material
containing molybdenum and molybdenum was produced by molten salt
electrolysis.
(Sample)
[0407] The Mo--Cu heat spreaders serving as a treatment object
contained 50 wt % of molybdenum and 50 wt % of copper. The heat
spreaders were ground with a bead mill or an attritor so as to have
a particle size of about 2 mm. The ground sample (heat spreaders)
was wrapped with a platinum (Pt) mesh (50 mesh). The sample powder
(treatment object) contained within the Pt mesh was used as an
anode (anode electrode).
(Details of Experiment)
[0408] A molten LiCl--KCl eutectic salt was employed as the molten
salt. This salt was completely melted by heating at 450.degree. C.
In this molten salt, the above-described anode electrode and a
cathode electrode were wired and immersed. The cathode electrode
was formed of glassy carbon.
Dissolution Step:
[0409] While the anode electrode and the cathode electrode were
thus immersed in the molten salt, the anode electrode was
maintained at a predetermined potential. At this time, the
potential was set such that copper was not dissolved but molybdenum
alone was selectively dissolved. After a predetermined time lapsed,
a sample was taken from the molten salt and the sample was
subjected to composition analysis by ICP-AES.
Electrolysis Step:
[0410] After the dissolution step, a cathode electrode formed of
glassy carbon and an anode electrode formed of glassy carbon were
immersed in the molten salt. The potential at the cathode electrode
was maintained at a predetermined potential. Specifically, the
potential was maintained such that molybdenum was deposited in the
LiCl--KCl molten salt. After a predetermined time lapsed, the
surface status of the cathode electrode was observed.
(Result)
Regarding Dissolution Step:
[0411] The value of anode current observed in the dissolution step
decreased with time as in the above-described case relating to
vanadium. The change rate of current value with respect to time had
the following tendency: the change rate was the highest at the
beginning of the measurement (at the beginning of application of
current) and, after that, the change rate gradually decreased.
[0412] The sample taken from the molten salt was subjected to
composition analysis by ICP-AES. As a result, dissolution of
molybdenum in the molten salt was confirmed.
Regarding Electrolysis (Deposition) Step:
[0413] Observation of a section of the surface layer of the cathode
electrode with a scanning electron microscope (SEM) revealed
deposition of molybdenum on the surface of an electrode body part
constituting the cathode electrode and formed of glassy carbon.
[0414] In this way, high purity molybdenum was obtained from the
heat spreaders containing molybdenum.
Fourth Embodiment (Example)-3
[0415] An oxide superconducting material was used as the metal
material containing strontium and strontium was produced by molten
salt electrolysis.
(Sample)
[0416] The oxide superconducting material serving as a treatment
object contained 17 wt % of strontium and 8 wt % of calcium. The
oxide superconducting material was ground with a bead mill or an
attritor so as to have a particle size of about 2 mm. The ground
sample (oxide superconducting material) was wrapped with a platinum
(Pt) mesh (50 mesh). The sample powder (treatment object) contained
within the Pt mesh was used as an anode (anode electrode).
(Details of Experiment)
[0417] A molten LiF--CaF.sub.2 eutectic salt was employed as the
molten salt. This salt was completely melted by heating at
850.degree. C. In this molten salt, the above-described anode
electrode and a cathode electrode were wired and immersed. The
cathode electrode was formed of glassy carbon.
Dissolution Step:
[0418] While the anode electrode and the cathode electrode were
thus immersed in the molten salt, the anode electrode was
maintained at a predetermined potential. At this time, the
potential was set such that strontium and calcium alone were
selectively dissolved and the other elements contained were not
dissolved. After a predetermined time lapsed, a sample was taken
from the molten salt and the sample was subjected to composition
analysis by ICP-AES.
Electrolysis Step:
[0419] After the dissolution step, a cathode electrode formed of
glassy carbon and an anode electrode formed of glassy carbon were
immersed in the molten salt. The potential at the cathode electrode
was maintained at a predetermined potential. Specifically, the
potential was maintained such that strontium was deposited in the
LiF--CaF.sub.2 molten salt. After a predetermined time lapsed, the
surface status of the cathode electrode was observed.
(Result)
Regarding Dissolution Step:
[0420] The value of anode current observed in the dissolution step
decreased with time as in the above-described case relating to
vanadium. The change rate of current value with respect to time had
the following tendency: the change rate was the highest at the
beginning of the measurement (at the beginning of application of
current) and, after that, the change rate gradually decreased.
[0421] The sample taken from the molten salt was subjected to
composition analysis by ICP-AES. As a result, dissolution of
strontium in the molten salt was confirmed.
Regarding Electrolysis (Deposition) Step:
[0422] Observation of a section of the surface layer of the cathode
electrode with a scanning electron microscope (SEM) revealed
adhesion of strontium to the surface of an electrode body part
constituting the cathode electrode and formed of glassy carbon.
Since strontium has a melting point of 768.degree. C., strontium
was in the liquid state. When the amount of strontium adhering to
the electrode body becomes large, the strontium rises to the
surface due to the specific gravity difference relative to the
molten salt. Accordingly, a jig for collecting strontium rising to
the surface was disposed on the upper side of the electrode.
[0423] In this way, high purity strontium was obtained from the
oxide superconducting material containing strontium.
Fourth Embodiment (Example)-4
[0424] An optical fiber material was used as the metal material
containing germanium and germanium was produced by molten salt
electrolysis.
(Sample)
[0425] The optical fiber material serving as a treatment object
contained 3 wt % of germanium. The optical fiber material was
ground with a bead mill or an attritor so as to have a particle
size of about 2 mm. The ground sample (optical fiber material) was
wrapped with a platinum (Pt) mesh (50 mesh). The sample powder
(treatment object) contained within the Pt mesh was used as an
anode (anode electrode).
(Details of Experiment)
[0426] A molten LiF--CaF.sub.2 eutectic salt was employed as the
molten salt. This salt was completely melted by heating at
850.degree. C. In this molten salt, the above-described anode
electrode and a cathode electrode were wired and immersed. The
cathode electrode was formed of glassy carbon.
Dissolution Step:
[0427] While the anode electrode and the cathode electrode were
thus immersed in the molten salt, the anode electrode was
maintained at a predetermined potential. At this time, the
potential was set such that germanium alone was selectively
dissolved and the other elements contained were not dissolved.
After a predetermined time lapsed, a sample was taken from the
molten salt and the sample was subjected to composition analysis by
ICP-AES.
Electrolysis Step:
[0428] After the dissolution step, a cathode electrode formed of
glassy carbon and an anode electrode formed of glassy carbon were
immersed in the molten salt. The potential at the cathode electrode
was maintained at a predetermined potential. Specifically, the
potential was maintained such that germanium was deposited in the
LiF--CaF.sub.2 molten salt. After a predetermined time lapsed, the
surface status of the cathode electrode was observed.
(Result)
Regarding Dissolution Step:
[0429] The anode current observed in the dissolution step decreased
with time as in the above-described case relating to vanadium. The
change rate of current value with respect to time had the following
tendency: the change rate was the highest at the beginning of the
measurement (at the beginning of application of current) and, after
that, the change rate gradually decreased.
[0430] The sample taken from the molten salt was subjected to
composition analysis by ICP-AES. As a result, dissolution of
germanium in the molten salt was confirmed.
Regarding Electrolysis (Deposition) Step:
[0431] Observation of a section of the surface layer of the cathode
electrode with a scanning electron microscope (SEM) revealed
deposition of germanium on the surface of an electrode body part
constituting the cathode electrode and formed of glassy carbon.
[0432] In this way, high purity germanium was obtained from the
optical fiber material containing germanium.
[0433] The embodiments and examples disclosed above are given by
way of illustration in all respects and should be considered as
non-limitative. The scope of the present invention is indicated not
by the above descriptions but by Claims and is intended to embrace
all the modifications within the meaning and range of equivalency
of the Claims.
INDUSTRIAL APPLICABILITY
[0434] The present invention is suitably applicable to a method for
obtaining a particular metal at high purity from a treatment object
containing two or more metal elements. The present invention is
also suitably applicable to a method for obtaining a desirable
metal from an ore or a crude metal ingot. The present invention is
also suitably applicable to a method for obtaining tungsten at high
purity from a treatment object containing at least one of tungsten
and lithium.
REFERENCE SIGNS LIST
[0435] 1 container; 2 molten salt; 3 treatment object; 4, 24
basket; 5 conductive wire; 6 to 8, 15, 27 electrode; 9 control
unit; 10 heater; 11 DyNi.sub.2 film; 12 Pr film; 13 Nd film; 16 Dy
film; 25 electrode material; 26 alloy; 31 electrode body part; 32
Dy--Ni alloy; 33, 34 region
* * * * *