U.S. patent application number 14/770060 was filed with the patent office on 2016-04-14 for device for reducing a metal ion from a salt melt.
The applicant listed for this patent is SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Bernd FRIEDRICH, Marc HANEBUTH, Alexander TREMEL, Hanno VOGEL.
Application Number | 20160102411 14/770060 |
Document ID | / |
Family ID | 50976614 |
Filed Date | 2016-04-14 |
United States Patent
Application |
20160102411 |
Kind Code |
A1 |
FRIEDRICH; Bernd ; et
al. |
April 14, 2016 |
DEVICE FOR REDUCING A METAL ION FROM A SALT MELT
Abstract
Between an anode and a cathode, a salt melt containing a metal
ion is separated from the anode by a gap across which an electric
arc can be formed. The metal ion is deposited on the anode and
subsequently removed.
Inventors: |
FRIEDRICH; Bernd; (Aachen,
DE) ; HANEBUTH; Marc; (Nuremburg, DE) ;
TREMEL; Alexander; (Erlangen, DE) ; VOGEL; Hanno;
(Monheim, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIEMENS AKTIENGESELLSCHAFT |
Munchen |
|
DE |
|
|
Family ID: |
50976614 |
Appl. No.: |
14/770060 |
Filed: |
June 12, 2014 |
PCT Filed: |
June 12, 2014 |
PCT NO: |
PCT/EP2014/062216 |
371 Date: |
August 24, 2015 |
Current U.S.
Class: |
204/246 ;
204/243.1 |
Current CPC
Class: |
C25C 3/34 20130101; C25C
7/025 20130101; C25C 7/06 20130101; C25C 7/005 20130101 |
International
Class: |
C25C 7/02 20060101
C25C007/02; C25C 3/34 20060101 C25C003/34; C25C 7/06 20060101
C25C007/06; C25C 7/00 20060101 C25C007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 24, 2013 |
DE |
102013211922.4 |
Claims
1-10. (canceled)
11. An apparatus for reducing a metal ion in a salt melt,
comprising: a cathode; and an anode disposed above the salt melt
with a gap for formation of an electric arc therebetween.
12. The apparatus as claimed in claim 11, wherein the salt melt
comprises oxygen ions.
13. The apparatus as claimed in claim 11, wherein the salt melt
comprises a rare earth metal ion.
14. The apparatus as claimed in claim 11, wherein the anode is
inert toward materials in the salt melt.
15. The apparatus as claimed in claim 11, further comprising an
electrolysis vessel accommodating the salt melt; and wherein the
cathode is electrically connected to a wall of the electrolysis
vessel.
16. The apparatus as claimed in claim 15, wherein the cathode is
arranged at a bottom of the electrolysis vessel.
17. The apparatus as claimed in claim 11, wherein a plasma prevails
above the salt melt.
18. The apparatus as claimed in claim 17, wherein an inert gas
which forms the plasma is present above the salt melt.
19. The apparatus as claimed in claim 11, further comprising an
inert gas feed line and an offgas outlet, and wherein the salt melt
has a surface separated from air surrounding the apparatus.
20. The apparatus as claimed in claim 11, wherein the salt melt
comprises an oxide of the metal ion to be reduced and additional
oxides.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. national stage of International
Application No. PCT/EP2014/062216, filed Jun. 12, 2014 and claims
the benefit thereof. The International Application claims the
benefit of German Application No. 10 2013 211 922.4 filed Jun. 24,
2013, both applications are incorporated by reference herein in
their entirety.
BACKGROUND
[0002] Described below is an apparatus for reducing a metal ion
from a salt melt.
[0003] Rare earth elements, which are also referred to as
lanthanides in chemistry, are required in many electronic
components and in the production of magnets. For example, the rare
earth element neodymium is an important constituent of permanent
magnets which are used in wind generators. The work-up and
separation of rare earth elements is in principle chemically
complicated since the rare earth elements occur in nature in very
finely distributed and associated (especially with one another)
form and in low concentrations. The rare earth elements are
frequently present in phosphate compounds, in particular in the
crystal structure of monazite or xenotime or as separate
constituents in apatite, which are again finely distributed in
deposits, which can also contain iron. A substep of this
complicated process for obtaining rare earth elements in pure form
is an electrolysis process in which chlorides or fluorides of the
rare earth element in molten form may be used as electrolyte.
Application of a voltage between immersed graphite anode and inert
tungsten cathode results in the rare earth oxides dissolved in the
electrolyte being converted into metal and CO/CO.sub.2. However,
perfluorocarbons such as CF.sub.4 or C.sub.2F.sub.6, which
frequently have the greenhouse potential of CO.sub.2, are also
formed at the carbon anode. Furthermore, highly toxic hydrofluoric
acid can be formed in the presence of water. All these undesirable
products which are formed in the electrolysis have to be got rid of
again by complicated purification and neutralization processes,
which considerably increases the total process costs. Similar
problems occur in principle in the electrolysis of salt melts using
graphite electrodes, for which reason application to the
preparation of rare earth elements can be considered to be
illustrative.
SUMMARY
[0004] Described below is an apparatus which provides for the
reduction of metal ions from metal-containing melts, in which there
is a lower emission of damaging greenhouse gases compared to the
prior art.
[0005] The apparatus for reducing a metal ion in a salt melt has an
anode and a cathode. The apparatus is wherein a gap for formation
of an electric arc is present between the anode and the salt melt.
The metal ion may be a rare earth metal ion which is frequently
prepared by electrolysis of salt melts. However, the apparatus is
not restricted to the use of rare earth metal ions. Furthermore,
the salt melt also contains oxygen ions which is due to the rare
earth metal ion originally being present in solid form in the form
of an oxide. An oxide is for the present purposes also subsumed
under the term salt.
[0006] Compared to a known electric arc melting pot, the apparatus
described has the difference that the electric arc is present
across a gap between the anode and the surface of the salt melt.
This in turn means, in contrast to the prior art in which graphite
electrodes for the reduction of rare earth ions are dipped into the
melt, that no carbon compounds which would form compounds with the
anions, i.e., halide ions or oxygen ions, are formed. Thus, no
carbon halides which are damaging particularly in terms of the
greenhouse effect are formed. Furthermore, no hydrogen fluoride,
i.e., no hydrofluoric acid, which is likewise highly toxic is
formed in the case of this apparatus.
[0007] The term rare earth elements refers, in particular, to the
lanthanides, including, inter alia, lanthanum, cerium,
praseodymium, neodymium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, ytterbium and lutetium, but yttrium
and scandium are also counted as rare earth elements in this case
because of their chemical similarities. Rare earths are in turn
compounds of rare earth elements, in particular the oxides thereof,
but no rare earth phosphates are included here.
[0008] It has been found to be advantageous for the anode to be
formed of a chemically inert material having good conductivity, for
example copper, which if necessary is cooled from the inside. This
avoids any compound between anions which are oxidized to the
corresponding elements in the region of the electric arc and the
material of the anode. It has been found to be particularly
advantageous for the salt melt to contain oxygen ions, particularly
instead of halide ions. The oxidation of the oxygen ions forms pure
oxygen which is discharged as O.sub.2 via the offgas.
[0009] In an advantageous embodiment, an electrolysis vessel which
serves to accommodate the salt melt is provided. This electrolysis
vessel or the vessel wall thereof is in direct electrical contact
with the cathode. In principle, electrically conductive
constituents of the electrolysis vessel can likewise serve as
cathode. This means that in an electrolysis operation, the
positively charged cations, i.e., the metal ions, in particular
rare earth metal ions, are deposited on the vessel wall and as a
result of their high specific gravity settle at the bottom of the
electrolysis vessel. This in turn leads to the elemental rare earth
metal constituents, whether in solid or liquid form, being in
electrical contact with the vessel wall and thus with the cathode
and in turn acting as cathode. At the phase interface between the
particles already precipitated as elemental metal and the salt
melt, ever more metal atoms are deposited, so that a phase of pure
metal is present in the lower region of the electrolysis vessel and
can be separated off after the electrolysis process.
[0010] A plasma may be present above the salt melt, i.e., in the
region of a hollow space above the salt melt, in which the anode is
also arranged. For the present purposes, a plasma is an ionized
gas, for example an ionized noble gas. As plasma gas, a mixture of
argon and nitrogen may be used. This gas is also referred to as
inert gas since it undergoes a chemical reaction neither with the
salt melt nor with the material of the anode. In a further
advantageous embodiment, the salt melt includes not only the oxide
of the metal to be reduced, i.e., generally the rare earth metal,
but also further oxides. These are oxides of metals which are more
stable in respect of the electrolysis than the rare earth metal
oxide and at the same time reduce the melting point of the salt
melt. In principle, other salts can also be employed for reducing
the melting point as long as these are sufficiently stable, in
particular in respect of their anions, for no damaging halides to
be formed at the anode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] These and other aspects and advantages will become more
apparent and more readily appreciated from the following
description of the exemplary embodiments, taken in conjunction with
the accompanying drawings of which:
[0012] FIG. 1 is a sequence chart using schematic drawings of a
process for extraction of rare earth metals from an ore; and
[0013] FIG. 2 is a schematic block diagram of the electrolysis of a
salt melt.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0014] Reference will now be made in detail to the preferred
embodiments, examples of which are illustrated in the accompanying
drawings, wherein like reference numerals refer to like elements
throughout.
[0015] Firstly, the process for extraction of rare earth metals, as
is, for example, customary for the mineral monazite, is shown
schematically in FIG. 1, without making any claims as to
completeness. The mineral monazite is a phosphate in which the
metal ions frequently occur in the form of rare earth metals, in
particular cerium, neodymium, lanthanum or praseodymium. Here,
there is not a homogeneous composition in respect of rare earth
metals within a particle, but instead the lattice sites of the
cations in the crystal structure are occupied by various rare earth
metals in different concentrations.
[0016] The starting raw materials containing the monazite mineral
are firstly milled very finely and treated in a flotation plant 2
in such a way that the monazite is separated as well as possible
from the other mineral constituents. The monazite is dried and,
according to the related art, admixed with sulfuric acid and then
treated in a furnace, for example a rotary tube furnace 4. Here,
the phosphates are converted into sulfates. This process in the
rotary tube furnace takes place at temperatures up to 650.degree.
C. The conversion of phosphate into sulfate is advantageous since
the rare earth sulfates are significantly more readily soluble in
water than the phosphates of the rare earth metals.
[0017] The sulfuric acid-containing solution of rare earth sulfates
is, after treatment in the rotary tube furnace 4 and a subsequent
leaching, neutralized in a neutralization apparatus 6, i.e., the pH
is increased by addition of a basic substance, resulting in
undesirable substances being precipitated and separated off so that
an aqueous rare earth sulfate solution is present in the remaining
liquid.
[0018] This resulting solution of a rare earth compound (sulfate,
nitrate, chloride or the like) is usually subjected to a
liquid/liquid extraction, i.e., a separation, in mixer-settler
apparatuses 8. Here, the solution is treated by mixing with an
extractant dissolved in organic solvents such as kerosene,
including possible further additives, in such a way that the rare
earth cations which in the case of the same charge have slightly
different ion diameters accumulate at different concentrations
either in the aqueous part of the solution or in the organic part
of the solution. The organic phase and the aqueous phase of the
mixture are here alternately mixed and separated again in a
multistage separation process, so that particular rare earth ions
become, depending on the extractant in the organic phase, ever more
concentrated until these ions are present in sufficient purity in
one phase. Up to 200 separation operations per element can be
necessary here.
[0019] The rare earth metals which have been separated in this way
are subsequently precipitated by addition of a carbonate or oxalate
in a process in a precipitation apparatus 10, so that the
respective rare earth carbonate or oxalate accumulates at the
bottom of the precipitation apparatus 10. This is in turn calcined
in a calcination apparatus, for example in a tunnel kiln 12,
through which a stream of hot air is passed. After this process, a
discrete rare earth oxide is thus present.
[0020] This discrete rare earth oxide is continuously added to a
molten electrolyte in the electrolysis plant 16. The electrolyte is
mainly formed of the corresponding rare earth fluoride. The oxide
compound dissociates into rare earth cations and oxygen anions in
this electrolyte. The rare earth cations are reduced to elemental
metal at the cathode and are collected in a collection vessel
underneath the cathode. The oxygen ions react with the carbon of
the anode to form CO/CO2, but fluorine ions also form compounds
with the carbon of the anode and leave the electrolysis bath
together in gaseous form.
[0021] The rare earth oxide can optionally be converted into a
lower-melting salt, e.g. an iodide, a chloride or fluoride, before
introduction into the electrolysis process and then be introduced
in molten form into an electrolysis process, with elemental rare
earth metal depositing at a cathode of the electrolysis
apparatus.
[0022] The metal 20 obtained in liquid form is pumped out from the
collection vessel underneath the cathode and cast to produce
ingots.
[0023] FIG. 2 illustrates an advantageous embodiment of an
electrolysis apparatus. This is a schematic depiction of an
electrolysis apparatus. The apparatus has an anode 26 and a cathode
28. A salt melt 24 is accommodated in an electrolysis vessel 34.
This salt melt 24 can be heated either by a resistance heating
element (not shown here) or by an electric arc 32 which generates a
plasma 33. A combination of a plurality of heating methods is also
possible. A gap 30 is provided between the anode 26 and a surface
42 of the salt melt 24 and an electric arc 32 is present in this
gap when a voltage is applied. This electric arc 32 leads to inert
gas, in particular a mixture of argon and nitrogen, which is
introduced via an inert gas feed line 36 being ionized and being
present in the form of a plasma 33 above the surface 42. In a
plasma space 44, in which the plasma 43 is present and which is
largely sealed off from an atmosphere, a positive charge prevails.
The negative charges of the salt melt 24, in particular oxygen
ions, migrate to the surface 42 of the salt melt, also referred to
as electrolyte, and are oxidized there to atomic oxygen at the
boundary between the salt melt, i.e., the electrolyte, and the
plasma. This means that the electrolyte should be conductive for
rare earth ions, oxygen ions and also electrons. The atomic oxygen
forms O2 molecules outside the plasma space 44 and leaves the
plasma space through the offgas outlet 38.
[0024] The anode is a material which is self evidently firstly
electrically conductive but on the other hand is inert to all
reactants in the electrolysis system. For this purpose, the anode
has to have internal water cooling so that it does not melt at the
high plasma temperatures. It is possible to use, for example,
copper as material here. However, the anode does not consist of
carbon since carbon together with the oxidized elements, in
particular with the oxygen but also with certain halides if they
are present in the salt melt, tends to form gases which cause great
damage to the atmosphere, in particular are strong greenhouse
gases.
[0025] In contrast to the anode arranged above the salt melt, the
cathode is electrically conductively connected to a vessel wall 40
of the electrolysis vessel underneath the salt melt. In principle,
the vessel wall 40 can also be formed of an electrically conductive
material and thus directly form the cathode 28. In this case, it
would be advantageous for upper regions of the vessel wall or of
the electrolysis vessel 34 to be electrically insulated from lower
regions. As an alternative, it is also possible to make the
electrolysis vessel of a refractory material which in its lower
region has a cutout into which a metallic or other conductive
cathode 28 is inserted. On application of an appropriate voltage,
elemental metal which has formerly been present in the form of
metal ions in the salt melt 24 is deposited at the electrically
conductive cathode 28. The surface of the cathode 28 is thus
covered very promptly by elemental metal, but this is likewise
electrically conductive and thus builds up a fresh electrically
conductive surface at which further ions can again be reduced. The
electrolysis is stopped when there is no longer any voltage or when
the salt melt 24 is present in chemical equilibrium and no further
electrolysis takes place. Depending on the temperature in the
electrolysis vessel, i.e., depending on the melting point of the
electrolyte 24 or salt melt 24 used, and depending on the melting
point of the metal being deposited, the latter can be present
either in solid form or in liquid form at the cathode 28 in the
lower region of the electrolysis vessel 34. Accordingly, the
deposited metal, i.e., the rare earth metal 20, can be drained off
when it is present in liquid form or can be taken out in pure,
solid form after solidification of the salt melt 24.
[0026] A substantial advantage of the apparatus is firstly that
there is a spacing between the anode 26 and the electrolyte 24 or
the salt melt 24, i.e., the materials of the electrode do not come
into direct contact with the salt melt 24 but are instead connected
to one another in energy terms only indirectly via the electric are
32. A further important point is that, compared to known electric
arc processes, the polarity is reversed so that the anode is
positioned above the salt melt and the electric arc 32 prevails
between the anode and the salt melt. This in turn leads to the now
elemental, oxidized anions, which are generally present in gaseous
form, rising upward and being able to escape from the apparatus via
the plasma space 44 and the offgas outlet 38. Furthermore, it is
possible as a result of this arrangement for the elemental metal to
be isolated as material value to settle on the bottom of the
apparatus at the cathode 28. Thus, a high measure of purity of the
deposited metal 20 can also be achieved here.
[0027] A further advantage is to select the material of the salt
melt 24 in such a way that very few halides and a large amount of
oxygen ions are present, so that no damaging halogen compounds or
elemental halogens occur in the oxidation of the anions. However,
since the halogen compounds are not compounds with carbon, salts
can also be present in the form of halides in the salt melt 24 when
this serves to lower the melting point of the salt melt 24.
Overall, production of CO2 is prevented and any after-treatment of
the offgas becomes significantly simpler and less costly. This
serves to make the ecologically problematical process for
extraction of rare earth metals or other metals cheaper and more
ecologically friendly.
[0028] A description has been provided with particular reference to
preferred embodiments thereof and examples, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the claims which may include the phrase "at
least one of A, B and C" as an alternative expression that means
one or more of A, B and C may be used, contrary to the holding in
Superguide v. DIRECTV, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir.
2004).
* * * * *