U.S. patent application number 11/792954 was filed with the patent office on 2008-05-15 for method for producing metal.
Invention is credited to Masahiko Hori, Tadashi Ogasawara, Toru Uenishi, Makoto Yamaguchi.
Application Number | 20080110765 11/792954 |
Document ID | / |
Family ID | 36587786 |
Filed Date | 2008-05-15 |
United States Patent
Application |
20080110765 |
Kind Code |
A1 |
Ogasawara; Tadashi ; et
al. |
May 15, 2008 |
Method For Producing Metal
Abstract
A method for producing a metal by an electrolytic process using
an yttria-containing porous ceramic body as a diaphragm is
provided; the calcium formed by electrolysis cannot pass through
the diaphragm, hence the back reaction can be effectively
inhibited. Preferably, to be used is a diaphragm comprising a
porous ceramic body having a purity of yttrium of 90 mass % or more
(more preferably, 99% or more), a porosity of 1% or more and a pore
diameter of 20 .mu.m or less, and having a thickness of 0.05-50 mm
and a metal halide is used as the electrolytic bath. The method can
be utilized for producing metals such as calcium or rare earth
elements, in particular. For example, when the method is applied to
the production of calcium, metallic calcium can be produced with
ease and at low cost without the need for enormous heat energy.
Inventors: |
Ogasawara; Tadashi; (Hyogo,
JP) ; Yamaguchi; Makoto; (Hyogo, JP) ; Hori;
Masahiko; (Hyogo, JP) ; Uenishi; Toru; (Hyogo,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW, SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
36587786 |
Appl. No.: |
11/792954 |
Filed: |
December 9, 2005 |
PCT Filed: |
December 9, 2005 |
PCT NO: |
PCT/JP05/22651 |
371 Date: |
June 13, 2007 |
Current U.S.
Class: |
205/402 ;
205/560 |
Current CPC
Class: |
C25B 1/26 20130101; C25C
3/02 20130101; C25C 5/04 20130101; C25C 7/04 20130101; C25C 3/34
20130101 |
Class at
Publication: |
205/402 ;
205/560 |
International
Class: |
C25C 3/02 20060101
C25C003/02; C25C 1/22 20060101 C25C001/22 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 15, 2004 |
JP |
2004-363708 |
Claims
1. A method for producing a metal by an electrolytic process,
comprising using an yttria-containing porous ceramic body as a
diaphragm.
2. The method for producing a metal as claimed in claim 1, wherein
said porous ceramic body is made of a material which is mainly
constituted by yttria.
3. The method for producing a metal as claimed in claim 1, wherein
a metal halide is used as an electrolytic bath.
4. The method for producing a metal as claimed in claim 1, wherein
said porous ceramic body has a porosity of not less than 1%.
5. The method for producing a metal as claimed in claim 1, wherein
said diaphragm has a thickness of 0.05-50 mm.
6. The method for producing a metal as claimed in claim 1, wherein
said porous ceramic body has a pore diameter of not more than 20
.mu.m.
7. The method for producing a metal as claimed in claim 1, wherein
said porous ceramic body has an yttria purity of not less than 90
mass %.
8. The method for producing a metal as claimed in claim 1, wherein
a current density in electrolysis is 0.1-100 A/cm.sup.2.
9. A method for producing a metal, comprising using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 1 to obtain metallic calcium or a molten salt
containing metallic calcium.
10. The method for producing a metal as claimed in claim 9, wherein
said metallic calcium is obtained in the form of a solid.
11. The method for producing a metal as claimed in claim 9, wherein
said metallic calcium is obtained in the form of a molten
substance.
12. A method for producing a metal, comprising using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 2 to obtain metallic calcium or a molten salt
containing metallic calcium.
13. A method for producing a metal, comprising using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 3 to obtain metallic calcium or a molten salt
containing metallic calcium.
14. A method for producing a metal, comprising using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 4 to obtain metallic calcium or a molten salt
containing metallic calcium.
15. A method for producing a metal, comprising using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 5 to obtain metallic calcium or a molten salt
containing metallic calcium.
16. A method for producing a metal, comprising using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 6 to obtain metallic calcium or a molten salt
containing metallic calcium.
17. A method for producing a metal, comprises using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 7 to obtain metallic calcium or a molten salt
containing metallic calcium.
18. A method for producing a metal, comprising using a calcium salt
or a mixed salt containing calcium salt in a molten state as an
electrolytic bath and applying the method for producing a metal as
defined in claim 8 to obtain metallic calcium or a molten salt
containing metallic calcium.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
metal by an electrolytic process, in particular to a method of
producing a metal which can be applied to the production of such a
metal as calcium or a rare earth element.
BACKGROUND ART
[0002] Metallic calcium is strong in reducing power and is a
valuable metal usable as a reducing agent in producing other
metals. Rare earth metals are used in various ways in a wide range
of fields of industry, such as glasses (coloring agents), ceramics
and, further, magnetic materials, nuclear materials, metallurgical
additives, catalysts, etc.
[0003] Among those metals, rare earth metals are currently produced
by the steps of: extracting specific rare earth metals (to be
produced) from a refined ore containing them by a solvent
extracting process; converting them to oxides by an alkali
precipitation process, for instance; and then reducing the oxides
by a fused or molten salt electrolysis or metallothermic reduction
process, for instance.
[0004] On the other hand, calcium is currently produced mainly by
the aluminum reduction process which comprises heating refined
calcium carbonate with metallic aluminum and condensing the calcium
vapor thus formed to yield metallic calcium. However, for
increasing its purity, vacuum refining must be done and, for this
and other reasons, the production cost becomes very high.
Therefore, in spite of its being suitable as a reducing agent,
calcium cannot be easily applied.
[0005] If metallic calcium can be produced by electrolysis of such
a molten salt as molten calcium chloride, such a process will be
very useful as a relatively inexpensive production process without
requiring such a large quantity of thermal energy as required in
the aluminum reduction process.
[0006] Such a molten salt electrolysis process can be applied to
the production of rare earth metals as well. However, because of
the excessively strong reducing power of calcium, the so-called
back reaction readily occurs and the calcium formed on the cathode
side immediately reacts with the chlorine formed on the anode
(graphite) side to form calcium chloride again.
[0007] For preventing such a phenomenon, the application of a
diaphragm process according to which a diaphragm is provided
between the cathode and anode has been proposed. However, alkaline
earth metals such as calcium and alkali metals are strong in
reducing power and readily react with and reduce the ceramic
material used as the diaphragm material. Therefore, such a
diaphragm process has not yet been put to practical use.
[0008] In a related technical document (Waste Management, Vol. 17,
No. 7, pp. 451-461, 1997. P. D. Ferro et al.: Application of
Ceramic Membrane in Molten Salt Electrolysis of CaO--CaCl.sub.2),
it is described that magnesia (MgO) is best suited as a sheath
material surrounding the anode for inhibiting the back reaction
attributable to Ca at the time of electrolytic reduction in a
CaO--CaCl.sub.2 molten salt system.
[0009] However, when the present inventors happened to use magnesia
as a material for the diaphragm to be disposed between the cathode
and anode in electric reduction of a CaCl.sub.2-based molten salt,
the reduction attributable to Ca occurred. Further, it was also
confirmed that alumina (Al.sub.2O.sub.3), silicon nitride
(Si.sub.3N.sub.4) and zirconia (ZrO.sub.2) are also reduced by
reaction with Ca.
DISCLOSURE OF INVENTION
[0010] It is an object of the present invention to provide a method
for producing a metal which can be applied to the production of
such a metal as calcium and a rare earth element which is produced
or producible by an electrolytic process.
[0011] To accomplish the above object, the present inventors made
investigations on the application of the molten salt electrolysis
method to be carried out with a diaphragm disposed between the
anode and cathode (diaphragm method), in particular on the
diaphragm material in terms of the resistance to the reducing
action of calcium (calcium reduction resistance), the mechanical
strength and the like, among others. As the results of these, they
found that a porous ceramic body comprising yttria (Y.sub.2O.sub.3)
as prepared by firing or sintering shows such a selective
permeability that it allows the passage of calcium and chloride
ions but does not allow the passage of metallic calcium.
[0012] Although yttria has a property such that it will not be
reduced by even a metal having a strong reducing powder such as
calcium, yttria has so far been regarded as being difficult to
apply as a diaphragm from the viewpoint of strength and the like.
However, as the results of investigations made by the present
inventors, this strength problem also has been successfully
overcome. Namely, it was revealed that an yttria-based porous
ceramic body can be used on the occasion of molten salt
electrolysis as a diaphragm which is highly resistant to reduction
by calcium and has the above-mentioned selective permeability.
[0013] The gist of the present invention, which has been completed
based on the above-mentioned findings and others, consists in an
electrolytic method for producing metals wherein a porous ceramic
article or body containing yttria is used as a diaphragm.
[0014] The "diaphragm" so referred to herein has function of
allowing the passage of calcium and chloride ions through itself
but not allowing the passage of metallic calcium.
[0015] The method for producing metals according to the invention
is characterized by the following first to tenth aspects or modes
of embodiment.
[0016] In a first embodiment of the invention, the above-mentioned
porous ceramic body is made of a material which is mainly
constituted by yttria and can be highly and stably resistant to
reduction by calcium. Further, in a second embodiment of the
invention, a metal halide is used as an electrolytic bath.
[0017] In a third embodiment of the invention, the porous ceramic
body has a porosity of 1% or more so that it may function as a
diaphragm while securing the conductivity of the bath. In a fourth
embodiment of the invention, the diaphragm desirably has a
thickness of 0.05-50 mm. Further, in a fifth embodiment of the
invention, the porous ceramic body has a pore diameter of 20 .mu.m
or less so that the passage of metallic calcium through it may
effectively be inhibited.
[0018] In a sixth embodiment of the invention, the porous ceramic
body has an yttria purity of 90 mass % or more so that a further
improvement in resistance to calcium reduction may be produced.
Further, in a seventh embodiment of the invention, a current
density of 0.1-100 A/cm.sup.2 is preferably employed in
electrolysis.
[0019] In an eighth embodiment of the invention, a molten calcium
salt or a mixed salt containing calcium salt in a molten state is
used as an electrolytic bath to make it possible to obtain metallic
calcium or a molten salt containing metallic calcium by applying
the relevant method for producing a metal (inclusive of any of the
above-mentioned embodiments).
[0020] In this case, the metallic calcium can be obtained as a
solid in a ninth embodiment of the invention and, in a tenth
embodiment of the invention, the metallic calcium can be obtained
as a molten substance.
[0021] The method for producing a metal according to the invention
is a method using an yttria-containing porous ceramic body as a
diaphragm and can be utilized in the production of such a metal as
calcium or a rare earth element and, in particular, is suited for
the production of metallic calcium. When the method is applied to
the production of calcium, for instance, metallic calcium can be
produced with much ease and at low cost without consuming enormous
heat energy to be needed in the prior art aluminum reduction
method.
BRIEF DESCRIPTION OF THE DRAWING
[0022] FIG. 1 is a drawing schematically illustrating the
configuration of an example of the apparatus in which the method
for producing a metal according to the invention can be carried
out.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023] In the following, the method for producing a metal according
to the invention is described in detail, referring to the
drawing.
[0024] FIG. 1 is a drawing schematically illustrating the
configuration of an example of the apparatus in which the method
for producing a metal according to the invention can be carried
out. In FIG. 1, the electrolytic cell 1 comprises an anode 2 and a
cathode 3 and retains molten calcium chloride (CaCl.sub.2) as an
electrolytic bath 4. A diaphragm 5 is disposed between the anode 2
and cathode 3, and the inside of the electrolytic cell 1 is divided
thereby into an anode 2 side and a cathode 3 side.
[0025] When the molten calcium chloride is electrolyzed in the
electrolytic cell 1, two electrode reactions occur according to the
equation (1) and equation (2) given below and Cl.sub.2 gas is
generated in the vicinity of the surface of the anode 2 while Ca is
formed in the vicinity of the surface of the cathode 3.
Anode: 2Cl.sup.-.fwdarw.2e.sup.-+Cl.sub.2 (1)
Cathode: Ca.sup.2++2e.sup.-.fwdarw.Ca (2)
[0026] The Cl.sub.2 gas formed rises in the electrolytic bath 4 and
then leaves the bath 4, while the Ca floats to the surface owing to
the difference in specific gravity from the molten calcium chloride
and forms a Ca layer 6 on the liquid surface of the molten calcium
chloride. This Ca layer 6 is extracted and thereby metallic calcium
can be obtained. The Cl.sub.2 gas that has left the bath 4 is
recovered and reused.
[0027] If there is provided no diaphragm 5, the so-called back
reaction, namely the reaction between the Cl.sub.2 gas formed and
part of the Ca in the electrolytic bath 4 to return to CaCl.sub.2,
will occur, the current efficiency will then decrease and the
production of Ca will be markedly obstructed. On the contrary, in
the configuration example shown, neither of the Cl.sub.2 gas nor Ca
can pass through the diaphragm 5 and no back reaction will occur.
The electrolysis sufficiently proceeds since CaCl.sub.2 can pass
through the diaphragm 5 in the form of ions (Ca.sup.2+,
Cl.sup.-).
[0028] The method for producing a metal according to the invention
is an "electrolytic method for producing a metal according to which
an yttria-containing porous ceramic article or body is used as a
diaphragm". That is, the method is characterized in that, in the
apparatus configuration shown in FIG. 1, the diaphragm 5 comprises
an yttria-containing porous ceramic body.
[0029] The content of yttria is not particularly restricted since
insofar as yttria is contained, the resistance to calcium reduction
can be expected. However, a higher content of yttrium gives a
greater resistance to calcium reduction, so that a porous ceramic
body containing a considerable amount of yttria is used in
practice.
[0030] The porosity and pore diameter, among others, of the porous
ceramic body are not particularly restricted, either. The ceramic
body is made of a material obtained via a process including the
step of firing or sintering and, so long as it can be regarded as
porous according to a generally accepted perception, the
electrolytic bath can pass through the ceramic body to thereby
enable electrolysis.
[0031] While, in the configuration example shown in FIG. 1, the
diaphragm 5 is positioned approximately in the middle of the
electrolytic cell 1 and divides the cell into the anode 2 side and
cathode 3 side, the position of the diaphragm is not restricted to
such position but may be much closer to the anode side or cathode
side. Further, it may be disposed so as to surround the anode to
prevent the calcium and chlorine from contacting with each
other.
[0032] The first embodiment of the invention is directed to a
production method which comprises using a porous ceramic article or
body made of an yttria-based material as a diaphragm. The term
"yttria-based" means that the yttria content is not less than 50
mass %.
[0033] There is no rigid reason why the yttria content should be
not less than 50 mass %. When yttria accounts for at least half of
the material constitution, however, the porous ceramic body
obtained by firing the material exhibits the characteristics of
yttria to a greater extent and the calcium reduction resistance
thereof is insured to be good and stable.
[0034] The second embodiment of the invention is directed to a
production method comprising using a metal halide as the
electrolytic bath. In the production of metallic calcium, calcium
chloride (CaCl.sub.2) and calcium fluoride (CaF.sub.2) are suitable
as the "metal halide". In the production of rare earth metals, the
respective metal chlorides or fluorides are preferably used.
[0035] The third embodiment of the invention is directed to a
production method according to which the porous ceramic body to be
used as the diaphragm has a porosity of 1% or more. The "porosity"
so referred to herein is the porosity measured by mercury
porosimetry. When this porosity is less than 1%, the electrolytic
bath will fail to pass through the diaphragm to a sufficient
extent, so that the resistance on the occasion of electrolysis will
increase, possibly rendering the operation difficult.
[0036] The upper limit to the porosity is fixed of its own accord
under the restrictions as to the constitution (strength in
particular) of the diaphragm. For efficient and smooth progress of
the electrolysis, however, the porosity of the ceramic body is
desirably 10-40%, more desirably 20-30%.
[0037] The fourth embodiment of the invention is directed to a
production method according to which the diaphragm to be used has a
thickness of 0.05-50 mm. The "diaphragm thickness" defined herein
is the thickness of the diaphragm when a single diaphragm is
disposed as shown in FIG. 1 and, when two or more diaphragms are
disposed, it corresponds to the sum of the respective thicknesses
of the diaphragms.
[0038] When the diaphragm thickness is less than 0.05 mm, the
diaphragm is so thin that the strength thereof cannot be secured
and the selective permeation function which is inherently held by
the diaphragm cannot be fully performed. On the other hand, when
the thickness exceeds 50 mm, the passage of the electrolytic bath
becomes difficult and the resistance on the occasion of
electrolysis is great, leading to a failure in smooth operation.
The diaphragm thickness is desirably 2-10 mm.
[0039] The fifth embodiment of the invention is directed to a
production method according to which a porous ceramic body having a
pore diameter of 20 .mu.m or less is used as the diaphragm. The
"pore diameter of porous ceramic body" so referred to herein is the
pore diameter measured by mercury porosimetry. When this pore
diameter is 20 .mu.m or less, the Ca formed in the vicinity of the
cathode surface is effectively prevented from passing through the
diaphragm. The lower limit to the pore diameter is not particularly
set since an excessively small pore diameter results in an increase
in electric resistance, which in turn makes the operation
difficult, and, in this manner, the lower limit is fixed of its own
accord.
[0040] For efficiently performing the electrolysis while inhibiting
the passage of Ca through the diaphragm, it is desirable that the
ceramic body have a pore diameter of 0.1-10 .mu.m.
[0041] The sixth embodiment of the invention is directed to a
production method which comprises using a porous ceramic body made
of a material comprising yttria at a purity of 90 mass % or more as
the diaphragm. The diaphragm used in this mode of embodiment is
constituted of a ceramic body mostly made of yttria and therefore
is highly resistant to calcium reduction. The yttria purity is
desirably 99% or more.
[0042] The seventh embodiment of the invention is directed to a
production method wherein the current density on the occasion of
electrolysis is 0.1-100 A/cm.sup.2.
[0043] The eighth embodiment of the invention is directed to a
method for producing a metal wherein a calcium salt or a mixed salt
containing calcium salt in the molten state is used as the
electrolytic bath and metallic calcium or a metallic
calcium-containing molten salt is obtained by applying the method
for producing a metal according to the invention (including the
above-mentioned first to seventh embodiment).
[0044] Suited for use as the above-mentioned calcium salt are
calcium chloride and calcium fluoride (CaF.sub.2). The mixed salt
containing calcium salt is a mixture resulting from addition, to a
calcium salt, of such a chloride as potassium chloride (KCl),
lithium chloride (LiCl) or barium chloride (BaCl.sub.2) or another
salt (but only the salt being higher in decomposition voltage than
the calcium salt) for the purpose of lowering the melting point
and/or adjusting the viscosity.
[0045] The configuration example shown in FIG. 1 corresponds to a
case where calcium chloride is used as the electrolytic bath. The
calcium formed by electrolysis may be separated as a solid or a
molten substance; either mode of practice can be employed. The
calcium formed, either in the form of a solid or in the form of a
molten substance, is lower in specific gravity than molten calcium
chloride and forms a Ca layer 6 on the liquid surface of the
electrolytic bath 4.
[0046] The ninth embodiment of the invention is directed to a
method for producing a metal wherein metallic calcium is obtained
as a solid in the above-mentioned eighth mode of embodiment. When,
for example, the electrolytic bath temperature is maintained at a
temperature higher than the melting point of the calcium salt or
the mixed salt containing calcium salt, which constitutes the bath,
but lower than the melting point of calcium, the metallic calcium
formed by electrolysis is obtained as a solid matter.
[0047] Therefore, the metallic calcium that has floated to the
surface can be extracted from the electrolytic cell in the form of
solid metallic calcium or together with the molten salt in the form
of a molten salt containing solid metallic calcium.
[0048] The tenth embodiment of the invention is directed to a
method for producing a metal wherein metallic calcium is obtained
as a molten substance in the above-mentioned eighth mode of
embodiment. In this case, the electrolytic bath temperature is
maintained at a temperature higher than the melting point of
calcium. The metallic calcium formed by electrolysis floats, as a
molten substance, to the surface of the electrolytic bath 4, so
that it can be extracted from the electrolytic cell either as a
molten substance of metallic calcium or together with the molten
salt.
[0049] As mentioned above, the method for producing a metal
according to the invention is characterized in that the diaphragm
used on the occasion of electrolysis is constituted of an
yttria-containing porous ceramic article or body. For producing
this porous ceramic body that constitutes the diaphragm, it is
desirable that an yttria powder having a predetermined purity be
pressure-molded and the molded body be sintered by burning at a
temperature of 1600.degree. C. or higher for 0.5-10 hours.
[0050] The yttria powder desirably has a particle diameter within
the range of 0.1-500 .mu.m. When the content of particles having a
diameter exceeding this range is high, it will become difficult for
the body after molding to maintain the shape thereof, or the
ceramic body after sintering will be low in strength and the use
thereof as a diaphragm will be sometimes hindered. When the content
of particles having a diameter smaller than the above range is
high, the desired porosity cannot be obtained in some
instances.
[0051] For the pressure-molding, an appropriate amount of water is
added to the above-mentioned yttria powder, and the mixture is
placed in a mold and molded by applying a pressure of about 0.2-20
MPa. At a pressure lower than 0.2 MPa, it is difficult to maintain
the shape of the molded body after removal from the mold while
application of a pressure exceeding 20 MPa will result only in a
slight increase in strength after molding, hence it is not
necessary to further increase the pressure to be applied.
[0052] The sintering or firing is carried out in a fairly high
temperature range, namely 1600.degree. C. or above, because yttria
is sintered without adding any sintering auxiliary. At a
temperature lower than 1600.degree. C., the sintering will not be
effected to a sufficient extent. The upper limit to the firing
temperature is fixed of its own accord from the viewpoint of plant
capacity and of reduction in energy required for sintering, hence
it is not particularly specified, but it is considered to be about
1800.degree. C. The firing time may be properly adjusted within the
range mentioned above according to the firing temperature, taking
into consideration the thickness of the ceramic body and the
desired porosity, among others.
[0053] The porosity and pore diameter of the sintering product
(ceramic body) can be controlled by properly combining the firing
conditions (firing temperature and time) with the particle diameter
of the yttria powder.
[0054] The thus-obtained porous ceramic body contains no added
sintering auxiliary, is made of a material based on yttria highly
resistant to calcium reduction, has a necessary level of mechanical
strength, and can be satisfactorily used as the diaphragm in the
method (electrolytic method) for producing a metal according to the
invention.
[0055] The cathode to be used on the occasion of electrolysis is
desirably made of a material which will not form an alloy with the
metal to be produced. In the case of production of Ca, metallic
titanium or pure iron, for instance, is preferably used. As the
anode, graphite is generally used.
EXAMPLES
[0056] An electrolytic cell was prepared applying metallic titanium
as the cathode and graphite as the anode, calcium chloride mixed
with 25 mole percent of potassium chloride was used as the
electrolytic bath, and the bath temperature was adjusted to
700-750.degree. C. (in certain tests, 850.degree. C.).
[0057] A diaphragm was disposed between the cathode and anode, and
electrolysis was carried out for 300 minutes. The diaphragm was
constituted with a porous ceramic body obtained by firing an
yttria-containing material under varied conditions. The purity of
yttria, the porosity, pore diameter and thickness of the ceramic
body obtained, and the current density on the occasion of
electrolysis are shown in Table 1.
[0058] As the yttria-containing material, employed was high-purity
yttria (.gtoreq.99.9 mass %) whose content of impurities
(Fe.sub.2O.sub.3, SiO.sub.2, etc.) resulting from the production
process was less than 0.1 mass %. For investigating the influence
of the yttria purity, materials being low in yttria purity were
used in some test runs.
[0059] The porosity and pore diameter of each porous ceramic body
were measured by mercury porosimetry using a Micromeritics model
AutoPore III 9400 mercury porosimeter. The pore diameter and
porosity measured by this method are respectively represented by
the average pore diameter and porosity defined as follows:
Average Pore Diameter (D):
[0060] The value (D=4V/A) obtained by dividing the total pore
volume (V=nD.sup.2L/4) by the total pore surface area (A=.pi.DL) on
the assumption that each pore is cylindrical. Here, V is the total
volume of all pores, and L is the average pore depth.
Porosity:
[0061] The ratio of the total volume of open/through-wall pores
(the pores stretching from one side of the ceramic body to the
opposite side) to the volume of the ceramic body. The volume of
closed pores is not included.
[0062] For comparison, electrolysis was carried out in the same
manner for the cases where a ceramic body obtained by firing
alumina, magnesia, silicon nitride or zirconia (each being a
high-purity material) was used as the diaphragm.
[0063] Table 1
TABLE-US-00001 TABLE 1 Diaphragm Purity of Pore Current Current
Test Y.sub.2O.sub.3 Porosity diameter Thickness density efficiency
No. (mass %) (%) (.mu.m) (mm) (A/cm.sup.2) (%) Remarks 1 25 19 5 5
1.0 61 2 50 20 6 5 1.0 77 3 90 20 6 5 1.0 83 4 .gtoreq.99.9 18 5 5
1.0 100 5 .gtoreq.99.9 3 4 5 1.0 80 6 .gtoreq.99.9 8 5 5 1.0 85 7
.gtoreq.99.9 13 5 5 1.0 97 8 .gtoreq.99.9 21 6 5 1.0 100 9
.gtoreq.99.9 42 7 5 1.0 96 10 .gtoreq.99.9 55 8 5 1.0 91 11
.gtoreq.99.9 17 0.5 5 1.0 100 Increase in electrolytic resistance
12 .gtoreq.99.9 18 5 5 1.0 100 13 .gtoreq.99.9 20 11 5 1.0 98 14
.gtoreq.99.9 19 18 5 1.0 97 15 .gtoreq.99.9 20 24 5 1.0 96 16
.gtoreq.99.9 20 5 0.3 1.0 87 17 .gtoreq.99.9 18 4 1.5 1.0 92 18
.gtoreq.99.9 20 5 5 1.0 100 19 .gtoreq.99.9 19 5 20 1.0 100
Increase in electrolytic resistance 20 .gtoreq.99.9 20 5 5 0.2 100
21 .gtoreq.99.9 20 6 5 1.0 100 22 .gtoreq.99.9 18 7 5 10 100 23
.gtoreq.99.9 20 5 5 20 98 24 .gtoreq.99.9 19 5 5 0.2 100 25
.gtoreq.99.9 20 6 5 1.0 100 26 .gtoreq.99.9 19 6 5 10 100 27
.gtoreq.99.9 20 5 5 20 100 28 Al.sub.2O.sub.3 20 6 5 1.0 --
Unusable as a diaphragm because of being reduced by Ca 29 MgO 19 5
5 1.0 -- Unusable as a diaphragm because of being reduced by Ca 30
Si.sub.3N.sub.4 20 5 5 1.0 -- Unusable as a diaphragm because of
being reduced by Ca 31 ZrO.sub.2 20 5 5 1.0 -- Unusable as a
diaphragm because of being reduced by Ca
[0064] The current efficiency data obtained in the electrolysis
tests are also shown in Table 1. Each current efficiency value was
calculated from the yield of calcium (theoretical precipitate
amount) calculated from the quantity of electricity as supplied and
the sum of the amount of calcium actually formed in the vicinity of
the cathode and found adhering to the cathode and the amount of
calcium actually formed and retained by the electrolytic bath (on
the bath surface and within the bath). The above-mentioned total
amount of calcium was determined by causing the portion of calcium
adhering to the cathode and the portion of calcium retained by the
electrolytic bath to react with H.sub.2O, determining the amount of
H.sub.2 thus produced and converting this amount to the amount of
Ca.
[0065] In Table 1, Test Nos. 1-4 each is the case where the yttria
purity was varied (the porosity, pore diameter, thickness and
current density were retained at almost constant levels). A higher
current efficiency was obtained at a higher purity and, at a purity
of 99.9 mass % or more, the current efficiency was 100%.
[0066] The decrease in current efficiency with the decrease in
yttria purity is the result of reduction of alumina by the metallic
calcium formed and the passage of calcium through the diaphragm to
cause the back reaction.
[0067] Test Nos. 5-10 each is the case where the porosity of the
ceramic body was varied while the other conditions were maintained
almost constant. In Test Nos. 7 and 8, where the porosity was in
the above-mentioned desirable range (10-40%), the current efficient
was 100% or close thereto.
[0068] Test Nos. 11-15 each is the case where the pore diameter of
the ceramic body was varied. There was found a tendency for the
current efficiency to increase with the decrease in pore diameter.
When the pore diameter was 0.5 .mu.m, however, the resistance on
the occasion of electrolysis increased to some extent.
[0069] Test Nos. 16-19 each is the case where the diaphragm
thickness was varied. The current efficiency increased with the
increase in diaphragm thickness. When the thickness was 20 mm,
however, the resistance in electrolysis increased. When the
thickness was 0.3 mm (Test No. 16), the current efficiency
decreased to some extent and this was presumably due to occurrence
of the back reaction as a result of the passage of a very small
proportion of the formed calcium through the diaphragm.
[0070] Test Nos. 20-23 each is the case where the electrolytic bath
temperature was raised (to 850.degree. C.) and the metallic calcium
was obtained as a molten substance. The current efficiency was
good, namely 100% or close thereto. Among the tests, in Test No. 23
in which the current density was 20 A/cm.sup.2, the current
efficiency decreased, although to a slight extent. Since the
metallic calcium was in a molten state, it was easy for the same to
pass through the diaphragm accompanying the passage of large
quantities of the electrolytic bath (Ca.sup.2+, Cl.sup.-) through
the diaphragm and the decrease in current density was presumably
due to the actual passage of a very slight portion of the molten
calcium through the diaphragm to cause the back reaction.
[0071] Test Nos. 24-27 each is the case where the electrolytic bath
temperature was maintained at 700-750.degree. C. to obtain the
metallic calcium as a solid matter, and the current efficiency was
100% in all the cases. In these cases, the metallic calcium was a
solid and presumably no passage of metallic calcium through the
diaphragm occurred even at increased current density levels.
[0072] Test Nos. 28-31 are comparative examples in which the
diaphragm to be used was constituted of a sintered alumina
(Al.sub.2O.sub.3), magnesia (MgO), silicon nitride
(Si.sub.3N.sub.4) or zirconia (ZrO.sub.3). In each case, the
diaphragm suffered reduction by the calcium formed by electrolysis
and allowed the passage of calcium through the same, so that the
back reaction occurred and the current efficiency fell too far and,
thus being impossible. to be employed as the diaphragm.
[0073] As is evident from the results shown above, the method for
producing a metal according to the invention which comprises using
an yttria-containing porous ceramic body as the diaphragm makes it
possible to produce metallic calcium with a high current efficiency
not less than 80%.
INDUSTRIAL APPLICABILITY
[0074] The method for producing a metal according to the invention
is the one which uses an yttria-containing porous ceramic body as
the diaphragm and can be utilized in producing such a metal as
calcium or a rare earth element, in particular. When it is applied
to the production of calcium, for instance, it is possible to
produce metallic calcium in an easy and simple manner and at low
cost without the need for enormous heat energy, and the method can
be expected to contribute greatly to promoted utilization of
calcium, in particular, as a reducing agent.
* * * * *