U.S. patent application number 11/576891 was filed with the patent office on 2008-03-06 for method for production of metal by molten-salt electrolysis and method for production of titanium metal.
This patent application is currently assigned to TOHO TITANIUM CO., LTD.. Invention is credited to Masahiko Hori, Susumu Kosemura, Eiji Nishimura, Tadashi Ogasawara, Yuichi Ono, Toru Uenishi, Makoto Yamaguchi, Masanori Yamaguchi.
Application Number | 20080053838 11/576891 |
Document ID | / |
Family ID | 36148274 |
Filed Date | 2008-03-06 |
United States Patent
Application |
20080053838 |
Kind Code |
A1 |
Yamaguchi; Masanori ; et
al. |
March 6, 2008 |
Method for Production of Metal by Molten-Salt Electrolysis and
Method for Production of Titanium Metal
Abstract
A method for production of metal by molten-salt electrolysis is
a method for production of metal by molten-salt electrolysis which
is performed by filling molten salt of a metal chloride in an
electrolysis vessel having an anode and a cathode, and a molten
salt which reduces solubility of the metal in the molten salt is
used.
Inventors: |
Yamaguchi; Masanori;
(Kanagawa, JP) ; Ono; Yuichi; (Kanagawa, JP)
; Kosemura; Susumu; (Kanagawa, JP) ; Nishimura;
Eiji; (Kanagawa, JP) ; Ogasawara; Tadashi;
(Hyogo, JP) ; Yamaguchi; Makoto; (Hyogo, JP)
; Hori; Masahiko; (Hyogo, JP) ; Uenishi; Toru;
(Hyogo, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Assignee: |
TOHO TITANIUM CO., LTD.
3-5,CHIGASAKI, 3-CHOME
CHIGASAKI-SHI, KANAGAWA
JP
253-8510
SUMITOMO TITANIUM CORPORATION
1, HIGASHIHAMA-CHO
AMAGASAKI-SHI, HYOGO
JP
660-8533
|
Family ID: |
36148274 |
Appl. No.: |
11/576891 |
Filed: |
October 5, 2005 |
PCT Filed: |
October 5, 2005 |
PCT NO: |
PCT/JP05/18452 |
371 Date: |
April 9, 2007 |
Current U.S.
Class: |
205/401 ;
205/398 |
Current CPC
Class: |
C22B 34/129 20130101;
C25C 3/02 20130101; C25C 3/28 20130101; C25B 1/26 20130101 |
Class at
Publication: |
205/401 ;
205/398 |
International
Class: |
C25C 3/28 20060101
C25C003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 12, 2004 |
JP |
2004-297873 |
Claims
1. A process for production of a metal by molten-salt electrolysis,
the process comprising a step of filling metal chloride in an
electrolysis vessel having an anode and a cathode, wherein a molten
salt which reduces solubility of the metal in the molten salt is
used.
2. The process for production of a metal by molten-salt
electrolysis according to claim 1, wherein the metal generated by
the electrolysis is recovered alone or as a mixture of the molten
salt and the metal.
3. The process for production of a metal by molten-salt
electrolysis according to claim 1, wherein the molten salt contains
at least one selected from calcium chloride, potassium chloride,
sodium chloride, and calcium fluoride.
4. The process for production of a metal by molten-salt
electrolysis according to claim 1, wherein the molten salt is a
mixed salt of calcium chloride with potassium chloride, sodium
chloride, or calcium fluoride, and a composition of the potassium
chloride, sodium chloride, or calcium fluoride versus the calcium
chloride is a eutectic composition or is not more than the eutectic
composition.
5. The process for production of a metal by molten-salt
electrolysis according to claim 1, wherein the metal is calcium,
potassium, or sodium.
6. The process for production of a metal by molten-salt
electrolysis according to claim 1, wherein the temperature of the
molten salt is not less than the eutectic temperature of a mixed
salt of calcium chloride with potassium chloride, sodium chloride,
or calcium fluoride and is not more than 1000.degree. C., and
wherein the metal generated by the electrolysis is generated alone
or as a mixture of the molten salt and the metal.
7. The process for production of a metal by molten-salt
electrolysis according to claim 6, wherein the solubility of metal
in the molten salt is not more than 3%.
8. A process for production of titanium metal comprising a step of
using the metal produced in the method according to claim 1, as a
reducing agent of titanium tetrachloride.
Description
TECHNICAL FIELD
[0001] The present invention relates to the recovery of metal from
a chloride thereof, and in particular, relates to a method for
production of metal by molten-salt electrolysis. Furthermore, the
present invention relates to a method for production of titanium
metal using the metal produced by the method.
BACKGROUND ART
[0002] Conventionally, titanium metal, which is a simple substance,
is produced by the Kroll method, in which titanium tetrachloride is
reduced by molten magnesium to obtain sponge titanium, and various
kinds of improvements have been made to reduce the cost of
production. However, since the Kroll method is a batch process in
which a set of operations is repeated noncontinuously, there is a
limitation to its efficiency.
[0003] To overcome this problem, a method in which titanium oxide
is reduced by calcium metal in molten salt to obtain titanium metal
directly (see WO99/064638 and Japanese Unexamined Patent
Application Publication No. 2003-129268), one in which an EMR
method in which a reducing agent containing an active metal such as
calcium or an active metal alloy is prepared, and one in which a
titanium compound is reduced by electrons from the reducing agent
to yield titanium metal (see Japanese Unexamined Patent Application
Publication No. 2003-306725) have been proposed. In these methods,
calcium oxide, which is a by-product of the electrolytic reaction,
is dissolved in calcium chloride, and molten-salt electrolysis is
performed to recover and reuse calcium metal. However, since the
calcium metal generated during the electrolytic reaction is in a
liquid state and is highly soluble in calcium chloride, it
dissolves easily in the calcium chloride, and there has been a
problem in that the yield of the metal is reduced.
[0004] As explained above, there has been a problem in that it has
been difficult to recover metal such as calcium metal efficiently
by a conventional method.
DISCLOSURE OF THE INVENTION
[0005] The present invention has been completed in view of the
above circumstances, and an object of the present invention is to
provide a method for production of metal by molten-salt
electrolysis, in which metal used for reducing, such as an oxide or
chloride of titanium metal, is efficiently recovered, and another
object of the present invention is to provide a method for
production of titanium metal in which the metal produced by the
method is used.
[0006] The method for production of metal by molten-salt
electrolysis of the present invention is a method for production of
metal by molten-salt electrolysis which is performed by filling
molten salt of a metal chloride in an electrolysis vessel having an
anode and a cathode, and a molten salt which reduces solubility of
the metal in the molten salt is used.
[0007] In the method for production of titanium metal of the
present invention, the metal produced in the above-mentioned method
is used as a reducing agent of titanium tetrachloride.
[0008] By the method for production of metal by molten-salt
electrolysis of the present invention, since the solubility of the
metal in the molten salt is reduced, the metal that is deposited is
difficult to dissolve in the molten salt. Therefore, the metal can
be effectively recovered.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a conceptual cross sectional diagram showing the
electrolysis vessel used in the molten salt electrolysis of the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0010] Embodiments of the present invention are explained below
with reference to the drawings. Here, a case in which the metal is
calcium metal, the metal chloride is calcium chloride, and the
chloride added to reduce the melting point of the electrolysis bath
of the molten salt of the present invention is potassium chloride,
is explained.
[0011] FIG. 1 shows a desirable embodiment of the apparatus
structure to perform the present invention. In FIG. 1, reference
numeral 1 indicates an electrolysis vessel, and an electrolysis
bath 2 mainly containing calcium chloride is filled in the vessel.
The electrolysis bath 2 is heated to a temperature above the
melting point of calcium chloride by a heater, which is not shown,
so as to be maintained in a melted condition. As the electrolysis
bath 2, a bath of a mixture of calcium chloride and potassium
chloride is used. Not only can the melting point of the
electrolysis bath 2 be reduced by adding potassium chloride to
calcium chloride, but the solubility of calcium metal in the
electrolysis bath 2 can also be reduced.
[0012] Reference numeral 3 indicates an anode and reference numeral
4 indicates a cathode, and they are immersed in the electrolysis
bath 2. Between the anode 3 and the cathode 4, for example, a
dividing wall 5 made of graphite is arranged.
[0013] Starting the electrolysis of the electrolysis bath 2 by
connecting the anode 3 and cathode 4 to a direct current power
supply, which is not shown, chloride ions in the electrolysis bath
2 are attracted to the anode 3 and donate electrons, forming
chlorine gas 6, which is expelled from the system. Calcium ions are
attracted to the cathode 4 and accept the electrons, forming
calcium metal 7, which is deposited on the surface of the cathode
4.
[0014] It is desirable that the temperature of the electrolysis
bath 2 be not less than 650.degree. C. which is a eutectic
temperature of calcium chloride and potassium chloride, and that it
be not more than 1000.degree. C. In the case in which the target
calcium metal is required to be recovered in a solid state, the
temperature of the electrolysis bath is maintained at not less than
the eutectic temperature of calcium chloride and potassium chloride
and at not more than the melting point of calcium metal
(845.degree. C.). In the case in which calcium metal is recovered
in a melted state, the temperature of the electrolysis bath 2 is
maintained at not less than the melting point of calcium metal.
[0015] The temperature of the electrolysis bath is different
depending on whether the target calcium metal is to be recovered in
a solid state or a melted state, as explained above; however, the
bases for improving recovery efficiency are the same. The upper
limit is set at 1000.degree. C.; however, in the case in which the
present invention is performed at a temperature not less than the
melting point of calcium metal, recovery becomes difficult if
solubility of calcium which dissolves in the molten salt is
increased. In addition, the vapor pressure of calcium metal
increases above 1000.degree. C., and it becomes difficult to
recover the calcium metal that is generated. Therefore, in the
present invention, the upper limit of the temperature of the
electrolysis bath 2 is desirably not more than 1000.degree. C.
[0016] It is believed that the range of temperature of the
electrolysis bath 2 is desirably from 650.degree. C. to 850.degree.
C. If the temperature of the electrolysis bath 2 is less than
650.degree. C., the electrolysis bath 2 will solidify, as mentioned
above. If the temperature of the electrolysis bath 2 is 650.degree.
C. or more, it is possible for an electrolysis bath containing a
sufficient calcium source to be prepared, and the rate of
generation of calcium will be high. In addition, if the temperature
is 850.degree. C. or less, the rate of dissolution of calcium in
the electrolysis bath 2 will be low, and deterioration of material
used for the electrolysis vessel or the like will be low; this
temperature range is therefore desirable for practicing the present
invention.
[0017] The eutectic composition of the electrolysis bath 2
mentioned above is 25 mol % as a ratio of addition of potassium
chloride to calcium chloride. Therefore, it is desirable that
potassium chloride in the electrolysis bath 2 also be selected to
be not more than 25%. It is desirable that the amount of potassium
chloride in the electrolysis bath 2 be low; however, from the
viewpoint of reducing the melting point of the electrolysis bath 2,
it is desirable that the amount be higher. Therefore, the ratio of
the addition of potassium chloride to calcium chloride should be
determined while considering the tradeoffs.
[0018] In the case in which the present invention is performed at a
temperature not less than the melting point of the electrolysis
bath 2 and that not more than 845.degree. C. (not more than the
melting point of calcium metal), it is possible for the calcium
metal to be deposited near the electrode and to be recovered in a
solid state. In the case in which the metal is not deposited, the
metal is dispersed in the bath as metal particles, and since the
specific gravity thereof is less than that of the bath, the
particles float up to the surface of the bath around the cathode.
In the case in which the metallic particles are recovered, it is
possible to recover them in a mixed condition with the electrolysis
bath, and as an embodiment of the present invention, a mixture of
the electrolysis bath and solid metal or the metal alone can be
recovered.
[0019] On the other hand, also in the case in which the
electrolysis is performed at a temperature not less than
845.degree. C. and not more than 1000.degree. C., the solubility of
calcium metal in the electrolysis bath 2 can be reduced by
controlling the concentration of chlorides added to the
electrolysis bath 2. As a result, calcium metal in a solid state is
partially deposited at the surface of an electrode and is dispersed
in the bath. On the other hand, since the specific gravity of
calcium metal partially generated in a melted state is lower than
that of the bath, it will ultimately float up near the cathode as a
melted metal.
[0020] By recovering the melted metal, the present invention can be
performed in the temperature range. During the recovery, since it
would take a long time to separate calcium metal dispersed in the
bath and the electrolysis bath 2, it is desirable that the melted
calcium and the electrolysis bath 2 be recovered in a mixed state.
Apart from these recovery methods, it is possible for the molten
salt and calcium to be entirely recovered in a solid state. In the
case in which the recovery method is performed, it is possible to
use the entire range of the temperature of the present
invention.
[0021] Calcium metal deposited on the surface of the cathode 4 is
partially dissolved in the electrolysis bath 2, and calcium metal
partially floats up to the surface of the electrolysis bath. The
calcium metal which floated up to the surface of the electrolysis
bath may flow to near the anode and will be blocked by the dividing
wall 5 to efficiently reduce the back reaction with chlorine gas
generated at the anode 3.
[0022] Since calcium metal is soluble in calcium chloride, in the
case in which a conventional electrolysis bath consisting of
calcium chloride alone is used, the calcium metal deposited will be
dissolved in the electrolysis bath. However, in the present
invention, since the above-mentioned chloride is added to calcium
chloride to reduce the solubility of calcium chloride in the bath,
calcium metal alone or the electrolysis bath in which calcium metal
is precipitated can be efficiently recovered.
[0023] In addition, by determining the solubility of calcium in the
electrolysis bath at not more than 3%, calcium metal generated by
electrolysis or a bath containing a large amount of calcium metal
can be efficiently recovered. The solubility of calcium metal in
the electrolysis bath is more desirably not more than 1.5%, and by
selecting the solubility, the recovery efficiency of calcium metal
generated by electrolysis can be improved further.
[0024] As a method for reducing the solubility of calcium metal in
the electrolysis bath, two methods may be considered. One is a
method in which the content of calcium chloride is decreased and
the content of potassium chloride, sodium chloride or calcium
fluoride is increased to reduce the solubility of calcium metal,
and the other is a method in which the temperature of the
electrolysis bath 2 is reduced. By each of these methods, the
solubility of the calcium metal in the electrolysis bath can be
efficiently reduced. It should be noted that the solubility of
calcium metal can be efficiently reduced if the temperature of the
electrolysis bath is near the melting point of calcium chloride in
the case of the bath of calcium chloride alone.
[0025] Calcium metal or the electrolysis bath 2, in which calcium
metal is precipitated and recovered in this way, can be used in
direct reduction of titanium oxide, for example.
[0026] In the case in which potassium chloride is added to calcium
chloride at 5 mol % to 50 mol %, solubility of calcium versus
calcium chloride can be reduced to a level of 0.1 % to 0.3%, in a
temperature range of 650.degree. C. to 800.degree. C. in the
electrolysis bath 2.
[0027] In addition, by adding the above-mentioned chlorides, not
only can the solubility of the calcium metal in calcium chloride be
reduced, but the melting point of the electrolysis bath can also be
reduced. Since the melting point of calcium chloride is 780.degree.
C. and the melting point of calcium metal is 845.degree. C.,
calcium metal in a solid state can be deposited on the cathode 4 in
the case in which the temperature of the conventional electrolysis
bath consisting of calcium chloride alone is set at 800.degree. C.
In this case, the difference between the temperature of the
electrolysis bath and the melting point of the electrolysis bath
(780.degree. C.) is only 20.degree. C., and since the electrolysis
bath would solidify if the temperature were to go below the melting
point, it is necessary that the temperature of the electrolysis
bath be controlled precisely.
[0028] However, in the present invention, since the melting point
of the electrolysis bath 2 is reduced by mixing the above-mentioned
chlorides in the electrolysis bath 2, precise control of
temperature is no longer required, and molten-salt electrolysis can
be performed reliably. For example, since the electrolysis bath 2
does not solidify even if the temperature of the electrolysis bath
2 is set at around 750.degree. C., calcium metal can be deposited
in a solid state on the cathode 4. Practically, by adding potassium
chloride to calcium chloride at 5 to 50 mol %, electrolysis can be
performed in the electrolysis bath having a temperature about 30 to
140.degree. C. lower than in the case of the bath of calcium
chloride alone.
[0029] As explained, in the present invention, since calcium
chloride can be deposited in a solid state, dissolution of calcium
metal in the electrolysis bath 2 is reduced, and the yield of
calcium metal can be effectively improved.
[0030] In the case in which calcium metal is deposited in a solid
state, after a certain amount of calcium metal is deposited, supply
of electric power to the anode 3 and cathode 4 is stopped, the
cathode 4 is pulled out of the electrolysis bath 2, and the calcium
metal is scraped off to be recovered. Alternatively, the cathode is
transported to a recovery vessel, which is prepared in advance and
which is not shown, and calcium metal deposited on the cathode is
melted and recovered by heating the recovery vessel to a
temperature not less than the melting point of calcium metal.
[0031] It should be noted that the mixed salt in which sodium
chloride or calcium fluoride is added, instead of the potassium
chloride mentioned above, can be used as the electrolysis bath 2.
The eutectic temperature of the mixed bath in which sodium chloride
is added to calcium chloride is 500.degree. C. Furthermore, the
eutectic temperature of the mixed bath in which calcium fluoride is
added to calcium chloride is 670.degree. C. In each case, the
temperature of the electrolysis bath 2 can be effectively reduced
compared to the case of the melting point of calcium chloride
(780.degree. C.) alone. In addition, the temperature of the
electrolysis can also be reduced, and as a result, dissolution loss
of calcium metal generated in the electrolysis reaction of the
electrolysis bath 2 can also be efficiently reduced.
[0032] While the electrolysis of the molten salt is performed using
the electrolysis bath in which potassium chloride is added to
calcium chloride, it is desirable that the voltage of the
electrolysis be selected so as not to cause deposition of potassium
metal. Since the theoretical decomposition voltage of calcium
chloride is 3.2 V and the theoretical decomposition voltage of
potassium chloride is 3.4 V, a range of from 3.2 V to 3.4 V is
desirable. However, if the electrolysis is performed at a
decomposition voltage of not less than 3.4 V, potassium metal that
is produced will react with calcium chloride to produce calcium
metal. Therefore, it may not cause a substantial problem even if
the decomposition voltage is high.
[0033] If the voltage applied to the anode and cathode is
increased, the amount of electricity supplied to the electrolysis
vessel 1 and rate of deposition of metal can be increased. However,
according to the increase of the voltage applied, both surfaces of
the dividing wall 5 will be polarized. Metal is deposited on the
anode-side of the dividing wall 5 and chlorine gas is generated on
the cathode-side of the dividing wall 5 when the voltage applied
reaches twice the theoretical decomposition voltage. The chlorine
gas generated on the cathode-side of the dividing wall 5 could
bring the back reaction with calcium metal generated at the cathode
4, reducing the yield of calcium metal. Therefore, the voltage
applied to the anode 3 and cathode 4 is desirably an electrolysis
voltage which does not produce the polarization of the dividing
wall 5. Such a range of voltages is not less than the theoretical
decomposition voltage of calcium chloride and is less than twice
thereof. Practically, it is from 3.2 V to 6.4 V.
[0034] The anode used in the present invention is required to be
made from a material which is durable when exposed to chlorine gas
at high temperature. As such a material, graphite is desirable. Not
only is graphite durable when exposed to chlorine gas at high
temperature, but it is also durable in electrolysis baths at high
temperature, and it has appropriate conductivity. It is desirable
that the anode be arranged penetrating an upper lid of the
electrolysis vessel 1, which is not shown, while being immersed in
the electrolysis bath 2. The surface of the anode 3 consisting of
graphite and penetrating the upper lid can be coated with a ceramic
material. Such a structure can minimize a corrosion of the
graphite.
[0035] Since chlorine gas is not generated from the cathode, the
cathode, at least, can be made of a material durable to molten salt
at high temperature, such as a conventional carbon steel. In the
cathode, since there is a possibility of generating carbide when
metal is generated, a steel material having a low concentration of
carbon is desirable. This carbon steel is desirable since it is
durable to molten salt and calcium metal at high temperatures. In
addition, it is practical since it is inexpensive and durable.
[0036] The dividing wall of the present invention must be made from
a material that is durable to calcium chloride and chlorine gas at
high temperature, similar to the case of the anode. Practically,
graphite is desirable. The dividing wall itself can be constructed
of graphite, or alternatively, an inner part may be constructed of
a ceramic and the outer part may be constructed of graphite, and
the strength thereof at high temperatures can be maintained for
long periods.
[0037] The dividing wall is required to be dense as possible as
can; however, some porosities in the wall, which do not allow
penetration and migration of calcium metal generated in the cathode
4 to the anode side, do not pose problems in conducting the present
invention. Furthermore, it is not necessary for the lower edge of
the dividing wall to reach the bottom part of the electrolysis
vessel, and it is sufficient for the dividing wall to have a
sufficient length so as not to allow calcium metal generated at the
cathode 4 or a calcium chloride layer having precipitated calcium
metal to migrate to the anode.
[0038] Chlorine gas is recovered from the system, and for example,
it can be used in a chlorination reaction of titanium ore.
Furthermore, calcium metal can be used in a reduction reaction of
titanium oxide or titanium chloride using molten salt to produce
titanium metal. For example, it can be used as the reducing agent
of titanium tetrachloride disclosed in Japanese Unexamined Patent
Application Publication No. 2005-068540, to produce ingots of
titanium metal. Alternatively, it can be used as the reducing agent
of titanium metal in the FFC method in which titanium oxide is used
as a raw material disclosed in Japanese Application Laid Open No.
2002-517613.
[0039] By using the mixed salt explained above as the electrolysis
bath, the melting point of the electrolysis bath can be reduced,
which brings to the reduction of the electrolysis temperature, and
as a result, the solubility of calcium metal in calcium chloride
can be reduced. Furthermore, since the ratio of calcium chloride in
the electrolysis bath is decreased by using the mixed salt, the
amount of the calcium metal dissolved into the electrolysis bath
can be reduced compared to the case in which calcium chloride alone
is used as the electrolysis bath.
[0040] It should be noted that sodium chloride or calcium fluoride
can be used instead of the potassium chloride mentioned above. In
this case, the eutectic composition of sodium chloride to calcium
chloride is 54%. Furthermore, the eutectic composition of calcium
fluoride to calcium chloride is 20%. Therefore, in the case of
using any of the chlorides, the electrolysis bath 2 having the
above-mentioned eutectic composition, or a composition not more
than that, is desirable.
[0041] In this way, by practicing the present invention, the
melting point of the electrolysis bath can be reduced, and the
solubility of calcium metal in the electrolysis bath can be
reduced. As a result, the calcium metal generated according to the
present invention can be efficiently recovered compared to the
conventional methods.
EXAMPLES
Example 1
[0042] Using the electrolysis vessel shown in FIG. 1, while
maintaining the temperature of the electrolysis bath consisting of
calcium chloride at 75 mol % and potassium chloride at 25 mol % at
650.degree. C., and applying a voltage of 4.5 V between an anode 3
made of carbon and the cathode 4 made of carbon steel, the
electrolysis of the molten salt of calcium chloride is started.
Accompanied by the electrolysis of the molten salt, calcium metal
is deposited on the cathode in a solid state. After depositing a
predetermined amount of calcium metal on the cathode in a solid
state, electric power supply to the positive and cathodes is
stopped. After that, the cathode, having deposited calcium metal on
its surface, is transferred to a recovery vessel which is heated to
a temperature not less than the melting point of calcium metal, and
the calcium metal deposited on the surface of the cathode is melted
so that it can be recovered. The ratio of the amount of calcium
metal actually recovered to the amount of calcium metal generated,
calculated from the electric power applied to the electrolysis
bath, was 85%. It was confirmed that an electrolysis reaction
having high efficiency could be performed.
Example 2
[0043] Using the electrolysis vessel shown in FIG. 1, while
maintaining the temperature of the electrolysis bath consisting of
calcium chloride at 85 mol % and potassium chloride at 15 mol % at
730.degree. C., and applying a voltage of 5.0 V between an anode 3
made of carbon and the cathode 4 madc of low-carbon steel, the
electrolysis of the molten salt of calcium chloride was started.
Accompanied by the electrolysis of the molten salt, calcium metal
in a solid state floated up to the bath surface around the cathode.
The electrolysis bath and calcium metal were drawn off and
recovered from the bath surface around the cathode. The recovered
calcium content in the electrolysis bath was measured to be 50%.
The amount of calcium metal generated was measured from the
recovered amount and the concentration, and a ratio was calculated
with a theoretical generated amount calculated from the time of
electric power supply. As a result, it was confirmed that not less
than 75% of calcium metal was recovered. This operation was
repeated, and the efficiency was improved.
Example 3
[0044] Using the electrolysis vessel shown in FIG. 1, while
maintaining the temperature of the electrolysis bath consisting of
calcium chloride at 85 mol % and potassium chloride at 15 mol % at
950.degree. C., and applying a voltage of 5.0 V between an anode 3
made of carbon and a cathode 4 made of low-carbon steel, the
electrolysis of the molten salt of calcium chloride was started.
Accompanied by molten-salt electrolysis, calcium metal in a melted
state floated up to the bath surface around the cathode. The
electrolysis bath and melted calcium metal were drawn off and
recovered from the bath surface around the cathode. Melted calcium
was recovered and the concentration of calcium in the electrolysis
bath which was recovered was measured and was 30%. The amount of
calcium metal generated was measured from the recovered amount and
the concentration, and a ratio with a theoretical generated amount
calculated from the time of electric power supply was calculated.
As a result, it was confirmed that not less than 60% of calcium
metal was recovered. This operation was repeated, and the
efficiency was improved. As an additional experiment, the
electrolysis bath consisting of calcium chloride at 85% and
potassium chloride at 15% was maintained at 950.degree. C. and
solubility of calcium in a saturated state was measured, and it was
2.8%.
Example 4
[0045] Except that 20 mol % of calcium fluoride was added to
calcium chloride instead of potassium chloride, electrolysis tests
were performed under the same conditions as those of Example 3.
Calcium metal recovered in this Example 4 was 70% of the
theoretical value.
Example 5
[0046] A molten salt in which the added ratio of potassium chloride
to calcium chloride was 25 mol % was prepared, and calcium metal
corresponding to 10 wt % of the total of all the molten salts was
added to the molten salt to perform heating and melting testing. In
the testing, the heating temperature was set at several levels to
determine the effects on the recovery ratio of calcium metal. As a
result, as shown in Table 1, there was a tendency for the recovery
ratio of calcium metal to continuously decreased with increasing
temperature in a range of heating temperature of 800.degree. C. to
1000.degree. C. However, when the heating temperature was above
1000.degree. C., a strong tendency for the recovery ratio of
calcium metal to decrease was observed. The reason for this is
estimated to be that both the evaporation loss of calcium metal and
the solubility of calcium metal in the molten salt are increased by
increasing the bath temperature. Furthermore, similar testing was
performed in the cases of combinations of sodium chloride and
calcium chloride, and combinations of calcium fluoride and calcium
chloride, and results similar to those in the case of potassium
chloride were obtained. TABLE-US-00001 TABLE 1 Unit: wt %
Temperature Mixed salt 800.degree. C. 900.degree. C. 1000.degree.
C. 1010.degree. C. 1050.degree. C. CaCl.sub.2--KCl (25) 95 70 60 45
30 CaCl.sub.2--NaCl 97 75 65 50 40 (54) CaCl.sub.2--CaF.sub.2 (20)
92 66 55 40 25 *Values in parentheses are eutectic
compositions.
Comparative Example 1
[0047] An electrolysis bath consisting of calcium chloride alone
was maintained at 900.degree. C., a voltage of 4.5 V was applied to
an anode made of carbon and a cathode made of carbon steel, so as
to begin an electrolysis of a molten salt of calcium chloride. At
this time, little melted calcium metal was observed at the surface
of electrolysis bath. The electrolysis bath around the surface was
drawn off to analyze the concentration of calcium metal, and the
concentration of the calcium metal was 1%. In addition to the
electrolysis examination, the solubility of calcium in a saturated
state in calcium chloride at 900.degree. C. was measured, and it
was 3.2%.
[0048] As explained above, metal used for reduction of oxides or
chlorides of titanium can be efficiently recovered by the present
invention.
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