EP1878814A1

EP1878814A1 - Molten salt electrolytic cell and process for producing metal using the same

Info

Publication number: EP1878814A1
Application number: EP06731502A
Authority: EP
Inventors: Yuichi c/o TOHO TITANIUM CO. LTD. ONO; Masanori c/o TOHO TITANIUM CO. LTD. YAMAGUCHI
Original assignee: Toho Titanium Co Ltd
Current assignee: Toho Titanium Co Ltd
Priority date: 2005-04-25
Filing date: 2006-04-10
Publication date: 2008-01-16
Also published as: AU2006240896A1; JPWO2006115027A1; WO2006115027A1; NO20075974L; US20090032405A1; EP1878814A4

Abstract

A molten salt electrolytic cell comprises a vessel filled with a molten salt bath, an anode immersed in the bath, and a cathode immersed in the bath, and the cathode is hollow.

Description

Technical Field

The present invention relates to a molten salt electrolytic cell and relates to a process for producing metal using the same. Specifically, the present invention relates to a technique of molten salt electrolysis used for extraction of calcium chloride containing calcium that may be used as a reductant for producing titanium from titanium compounds.

Background Art

Conventionally, sponge titanium is produced by the Kroll method, and various improvements have been made to reduce the costs of production. However, since the Kroll method is a batch process, efficiency may not be further improved.
As a technique for improving the efficiency of production of titanium by the Kroll method, a technique has been suggested in which titanium oxides are reduced with calcium in a molten salt so as to produce titanium (for example, refer to WO99/064638 ). Moreover, in a technique that is similar to that in the above patent document, a method for refining titanium having three steps has been disclosed (for example, refer to Japanese Unexamined Patent Application Publication No. 2003-129268 ). In the first step of the method for refining titanium, a mixed sample of calcium chloride and calcium oxide is put in a vessel for reduction, and it is heated so as to yield a molten salt. In the second step, the above-mentioned molten salt is electrically decomposed so as to yield a molten salt that has high reducibility and contains calcium ions and electrons in a molten calcium chloride. In the third step, titanium oxides are added to the above molten salt having high reducibility so as to produce titanium by reducing and deoxidizing the titanium oxides with the calcium ions and the electrons. In the technique disclosed in No. 2003-129268 , titanium is produced by reducing titanium oxides with calcium in a molten salt, subsidiarily produced calcium oxide is dissolved in calcium chloride, and the calcium chloride containing the calcium oxide is electrolyzed so as to produce and reuse the calcium as a reductant.
Furthermore, a method for producing titanium is disclosed in Japanese Unexamined Patent Application Publication No. 2003-306725 . In the method, titanium compounds containing titanium halides or titanium oxides are used as a raw material, and the titanium compounds are reduced so as to produce titanium. In this method, the molten salt is electrolyzed in an electrolytic bath containing molten salts of active metals so as to produce a reductant composed of an active metal or an active alloy, and the titanium compounds immersed in the electrolytic bath are reduced by electrons released from the reductant.
In addition, US3226311 discloses a technique in which a complex molten salt having a lower melting point than that of calcium is used, and in which solid calcium is precipitated at cathode. In this technique, the precipitated solid calcium is required to be separated from the electrode.
In each technique disclosed in the above patent documents, calcium is used as a reductant when titanium is produced, and subsidiarily produced calcium oxide is in reduction reaction, which is industrially required to be regenerated into calcium. However, calcium produced by molten salt electrolysis is difficult to separate as calcium itself, because calcium has a solubility for calcium chloride.
In order to solve this problem, Japanese Unexamined Patent Application Publication No. S49-70808 discloses a technique in which a calcium carbide is added to a molten salt, so that the redissolving of the produced calcium in the molten salt can be avoided. However, because carbon in the calcium carbide contaminates the calcium, this technique cannot be used for producing high purity titanium.
It should be noted that, in the aforementioned technique for producing titanium by reducing titanium oxides in a molten salt with calcium, the calcium need not to be a simple substance, and calcium in which a part is dissolved in calcium chloride may be used as a reductant.
Thus, techniques that have been developed are inefficient in producing calcium chloride in which calcium is mixed or is dissolved (hereinafter simply referred to as "calcium chloride containing calcium") by molten salt electrolyzing with calcium chloride.

DISCLOSURE OF THE INVENTION

In view of the above circumstances, an object of the present invention is to provide an electrolytic cell for extraction of calcium chloride containing calcium used for reducing an oxide or chloride of titanium. Specifically, the present invention provides a molten salt electrolytic cell in which calcium chloride is efficiently extracted by molten salt electrolysis.
The inventors have performed research on a molten salt electrolytic cell in view of the above circumstances. As a result, the inventors found that calcium chloride can be efficiently extracted by hollowing out a cathode. Moreover, the inventors found that calcium chloride is more efficiently extracted and collected when an anode and a hollow cathode are immersed and are disposed in a molten salt bath and a fin member is arranged and is disposed on the surface of the cathode. That is, the calcium chloride containing calcium that is produced at the cathode is efficiently introduced into the hollow space of the cathode with minimum influence of the flow in the bath and difference in specific gravity with respect to the surroundings. Then, the calcium chloride is extracted through the hollow portion by a pressure reducing device provided outside. Thus, the inventors have completed the invention as described below.
That is, the present invention provides a molten salt electrolytic cell comprising a vessel filled with a molten salt bath, and an anode and a cathode immersed and disposed in the molten salt bath, and the cathode is hollow. According to a preferred embodiment of the present invention, in the above molten salt electrolytic cell, the vessel filled with the molten salt bath comprises a lid, and the anode and the cathode are immersed and are disposed in the molten salt bath passing through the lid from above the vessel. Moreover, the above molten salt electrolytic cell preferably comprises a pressure reducing device connected to the above cathode, a nozzle for introducing a gas from the outside of the vessel into the vessel, a nozzle for discharging the gas from the inside of the vessel to the outside thereof, a nozzle for feeding a molten salt from the outside of the vessel into the vessel, a fin member provided on the outer surface of the cathode, and a through hole provided to the cathode surface directly above the connected portion of the fin member.
The inventors have researched regarding production efficiency of calcium at the cathode as an important subject. As a result, the inventors have found that calcium chloride produced at the cathode can be efficiently extracted and collected when the cathode is entirely immersed and is disposed in the molten salt bath so as to minimize the bath flow influences owing to the specific gravity with respect to the surroundings. Thus, the inventors have completed the invention as described below.
That is, according to a preferred embodiment of the present invention, in the above molten salt electrolytic cell, the vessel filled with the molten salt bath comprises a lid, the anode is immersed and is disposed in the molten salt bath passing through the lid from above the vessel, and the cathode is immersed and is disposed under the anode in the molten salt bath and is upwardly widely open. Moreover, the molten salt electrolytic cell preferably comprises a tube for discharging molten salt bath, which is connected to the cathode and is extended to the outside of the vessel, a pressure reducing device connected to the tube for discharging molten salt bath at the outside of the vessel, a nozzle for introducing a gas from the outside of the vessel into the vessel, a nozzle for discharging the gas from the inside of the vessel to the outside thereof, and a nozzle for feeding a molten salt from the outside of the vessel into the vessel.
In the present invention, a process for producing metal uses the above molten salt electrolytic cell. According to the process for producing metal, molten calcium or molten magnesium is obtained.
According to the present invention, the cathode is hollow, or the cathode is hollow and is provided with a fin member by connecting and a through hole is provided directly above the connected portion. Therefore, the calcium or calcium chloride partially containing calcium is efficiently collected and is extracted to the outside before calcium that has precipitated on the surface of the cathode dissolves or diffuses into the entire calcium chloride bath. Moreover, according to the present invention, in addition to the above effects, the cathode is entirely immersed and is dispersed under the anode, whereby chlorine gas produced at the anode and calcium produced at the cathode will not recombine. As a result, current efficiency is improved.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a sectional side view showing a preferred molten salt electrolytic cell of the present invention.
Fig. 2 is a sectional side view showing another preferred molten salt electrolytic cell of the present invention.
Fig. 3 is a sectional side view showing another preferred molten salt electrolytic cell of the present invention.
Fig. 4 is a sectional side view showing an improved cathode that is used in the electrolytic cell shown in Figs. 2 and 3.
Fig. 5 shows a schematic view of longitudinal grooves on the anode of an embodiment of the present invention.

REFERENCE NUMERALS

"B" denotes a molten salt bath, 10 denotes a vessel, 10a denotes a lid, 11 denotes an anode, 12 denotes a cathode, 13 denotes a pressure reducing device, 14 denotes a nozzle for introducing a gas, 15 denotes a nozzle for discharging the gas, 16 denotes a nozzle for feeding a molten salt, 17 denotes a fin member, 18 denotes a through hole, 28 denotes a flow guide, and 51 denotes a longitudinal groove.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments are explained with reference to the figures hereinafter. The embodiments relate to a molten salt electrolytic cell and to a process for producing metal using the same of the present invention.
Fig. 1 is a sectional side view showing a preferred molten salt electrolytic cell of the present invention. As shown in Fig. 1, the electrolytic cell comprises a vessel 10 filled with a molten salt bath "B" and provided with a lid 10a on the top thereof, an anode 11 and a cathode 12 immersed and disposed in the molten salt bath "B" passing through the lid 10a from above the vessel 10, and a pressure reducing device 13 connected to the cathode 12. The lid 10a of the electrolytic cell is provided with a nozzle 14 for introducing a gas from the outside of the vessel 10 into the vessel 10, a nozzle 15 for discharging the gas from the inside of the vessel 10 to the outside thereof, and a nozzle 16 for feeding a molten salt from the outside of the vessel 10 into the vessel 10.
In the molten salt electrolytic cell, the cathode 12 is hollow and is connected to a fin member 17 at the outer surface. Moreover, a through hole 18 is provided at the cathode 12 directly above the connected portion to which the fin member 17 is connected.
The cathode 12 is provided with a through hole 18, whereby molten calcium, which is produced on the surface of the cathode 12, is efficiently collected into the hollow portion of the cathode 12 before dissolving or diffusing into the molten salt bath "B".
When the molten salt electrolytic cell shown in Fig. 1 is used, an anode 11 and a cathode 12 are immersed in the molten salt bath "B", which is held in the vessel 10, and the vessel 10 is tightly closed with a lid 10a. Then, it is preferable that inert gas be introduced into the space in the vessel 10 from a nozzle 14 for introducing a gas, which is connected to a lid 10a, and that the inert gas be discharged to the outside from a nozzle 15 for discharging the gas.
Thus, the inert gas-flow through the space in the vessel 10 effectively reduced the invasion of air into the space in the vessel 10, and chlorine gas produced at the anode 11 is effectively discharged to the outside. If air invades into the space in the vessel 10, calcium chloride composing the molten salt bath "B" is oxidized, and the molten salt electrolysis reaction may be reduced.
The anode 11 is preferably composed of a material such as carbon. Thus, the anode 11 will not be corroded by chlorine gas produced thereat, and electrolysis may be stably continued. On the other hand, the cathode 12 at which calcium is produced is preferably composed of a stainless steel or a titanium material, because such materials are not easily corroded by the calcium produced at the surface of the cathode 12.
As described above, the cathode 12 used in the electrolytic cell shown in Fig. 1 is hollow. The cathode 12 is preferably provided with and is connected to a fin member 17 at the surface that is to be immersed into the molten salt. The fin member 17 is placed in upwardly widely open condition, and the connected portion of the fin member 17 on the cathode 12 is preferably provided with at lest one through hole 18 which leads to the inside of the cathode 12. An oblique angle of the fin composing the fin member 17 is preferably selected from a range of 30° to 45°.
In the electrolytic cell of the present invention shown in Fig. 1, the two to five fin members 17 are preferably arranged. The through holes 18, which perforate the surface of the cathode 12 at even intervals, preferably have diameters in a range of 10 to 30% of inner diameter of the cathode 12. The cathode 12 is perforated by the through holes 18 having such diameters, whereby calcium chloride containing calcium, which is produced at the surface of the cathode 12, is efficiently extracted to the outside.
The inventors performed various experiments before deciding on a molten salt electrolytic cell as shown in Fig. 1. As a result, when a cathode without a fin member on the outer surface and an anode were immersed and were electrolyzed in a calcium chloride bath, molten salts of calcium (hereinafter simply called "metal fog") diffused from the periphery of the cathode to the entire molten salt bath "B" in a short time.
The metal fog is affected by the convection flows accompanied by the generation of chlorine gas at the anode, or it is affected by differences in specific gravity between the metal fog and the surroundings of the cathode. Therefore, the metal fog tends to diffuse to the entire bath in a short time. However, at first, when calcium is produced at the surface of the cathode 12, a part of the calcium tends to precipitate in the molten salt bath "B" and accumulate at the bottom of the vessel 10. Therefore, in the present invention, it is preferable that the fin member 17 be connected to the cathode 12 in a condition in which it is upwardly widely open and that the fin member 17 be placed at the lower end of the cathode 12 as low as possible. The fin member 17 is disposed in this way, whereby calcium that is produced on the entire surface of the cathode 12 immersed in the molten salt bath "B" is efficiently collected.
According to the difference in specific gravity between calcium and the surrounding thereof, the calcium produced on the surface of the cathode 12 is efficiently introduced into the hollow portion of the cathode 12 via a through hole 18. The through hole 18 is formed along the fin member 17 which is upwardly placed on the surface of the cathode 12. Thus, the calcium produced on the surface of the cathode 12 is efficiently introduced into the hollow portion, whereby it will not contact and will not react with chlorine gas produced at the anode 11. As a result, the decrease of the current efficiency can be avoided.
Calcium chloride containing calcium, which is introduced into the hollow portion of the cathode 12, is easily extracted to the outside of the vessel 10 by connecting another end of the cathode 12, which is opposite to the lower end, to a pressure reducing device 13. The calcium produced on the surface of the cathode 12 is dissolved into the molten salt bath "B", and it is introduced to the hollow portion of the cathode 12 via the through hole 18 which is provided at the surface of the cathode 12. Then, the calcium is drawn to the upper side of the cathode 12 by the pressure reducing device 13, and it is discharged in a tank (not shown in the figure) provided at the middle of a pipe of the pressure reducing device.
When the above operation is carried out, the liquid level of the molten salt bath is decreased according to the amount of the molten salt bath "B" that is discharged from the vessel 10 to the outside. According to the amount of the molten salt that is discharged, a new molten calcium chloride is fed into the vessel 10 from a nozzle 16 which feeds a molten salt and is mounted on the lid 10a, whereby the molten salt bath "B" is maintained at a constant liquid level. Thus, calcium chloride can be continuously extracted.
As a calcium chloride to be fed to the vessel 10, a new calcium chloride may be used. In addition, calcium chloride which is produced by the following method may be used. In the method, calcium chloride containing subsidiarily produced calcium oxide when titanium oxides are reduced with calcium (refer to the section of the background art), is chlorinated in order to change the total amount thereof into calcium chloride.
For example, the calcium chloride containing calcium, which is extracted via the hollow portion of the cathode 12, may be used as a reductant used for reducing titanium oxides to titanium (refer to the section of the background art). In this case, the calcium chloride containing calcium, which is extracted via the hollow portion of the cathode 12, may be cooled down to near the melting point of calcium, so that a part of calcium dissolved in the calcium chloride is precipitated.
The temperature of the molten salt bath "B" is preferably maintained at least at a temperature of the melting point of calcium chloride. Moreover, the temperature is preferably maintained at a temperature which is not higher than the melting point of calcium by 100°C. The reason for this is that, when the temperature of the molten salt bath "B" is higher than the melting point of calcium by 100°C, the molten salt bath "B" evaporates rapidly, and the rate of the electrolysis reaction is undesirably decreased.
The melting point of the molten salt bath "B" may be lowered by adding potassium chloride into the calcium chloride composing the molten salt bath "B". Thus, the melting point of the molten salt bath "B" is lowered, whereby the operating temperature of electrolysis can be extended. Potassium chloride is preferably added to calcium chloride in a range from 18wt% to 67wt%. The potassium chloride in such a range is added, whereby the melting point of the molten salt bath "B" is lowered to be from 600°C to 760°C, and the operating temperature of the molten salt bath "B" is stably lowered. As a result, the amount of calcium, which is produced at the cathode and which dissolves in calcium chloride, may be reduced.
Fig. 2 is a sectional side view showing another preferred molten salt electrolytic cell of the present invention. As shown in Fig. 2, the electrolytic cell comprises a vessel 20 which holds a molten salt bath "B" and is provided with a lid 20a on the top thereof, an anode 21 immersed and disposed in the molten salt bath "B" passing through the lid 20a from above the vessel 20, a cathode 22 which is immersed and disposed under the anode 21 in the molten salt bath "B" and which is upwardly widely open, a tube 23 for discharging the bath, which extends to the outside of the vessel 20 and is connected to the cathode 22, and a pressure reducing device 24 connected to the tube 23 for discharging the bath to the outside of the vessel 20. The lid 20a of the electrolytic cell is provided with a nozzle 25 for introducing a gas from the outside of the vessel 20 into the vessel 20, a nozzle 26 for discharging the gas from the inside of the vessel 20 to the outside thereof, and a nozzle 27 for feeding a molten salt from the outside of the vessel 20 into the vessel 20.
As described above, in the component portions of the electrolytic cell shown in Fig. 2, the vessel 20, the anode 21, the nozzle 25 for introducing a gas, the nozzle 26 for discharging the gas, and the nozzle 27 for feeding a molten salt correspond to each component portions of the electrolytic cell shown in Fig. 1. On the other hand, in the component portions of the electrolytic cell shown in Fig. 2, the cathode 22, which is entirely immersed in the molten salt bath "B", and the tube 23 for discharging the bath, which is disposed between the cathode 22 and the pressure reducing device 24, differ from the electrolytic cell shown in Fig. 1 in structure.
As shown in Fig. 2, the cathode 22 is entirely immersed under the anode 21 in the molten salt bath "B", so that chlorine gas produced on the surface of the anode 21 will not contact calcium produced on the surface of the cathode 22. Therefore, most of the calcium that is produced is introduced to the tube 23 for discharging the bath with minimum influence of the specific gravity of the surroundings. Accordingly, the molten salt bath "B" may be efficiently extracted to the outside via the tube 23 for discharging the bath.
The cathode 22 need not be precisely disposed at the extended line under the anode 21 as shown in Fig. 2, and it may be disposed under the anode 21 without being centered. In the composition shown in Fig. 2, the cathode 22 is upwardly widely open, and the oblique angle facing upward with respect to the horizontal surface may be selected from 30° to 45°. Such a cathode 22, which is widely open, has the same effect as the fin member 17 that is a component portion of the electrolytic cell shown in Fig. 1. That is, the cathode 22 minimizes the influence of differences in specific gravity between the calcium and the surroundings of the calcium, and the calcium that is produced can be efficiently extracted to the outside.
The back surface of the cathode 22 (in Fig. 2, the surface of the cathode 22 which does not face the anode 21) and the surface of the tube 23 for discharging the bath are preferably coated by thermal spraying with silica or a ceramic such as alumina, which has a high electrical insulation characteristics. The above insulating treatment is performed so that calcium is produced on the inner surface of the cathode 22, and so that the inner surface of the tube 23 for discharging the bath is efficiently used as a surface for precipitating calcium.
The tube 23 for discharging the bath is connected to the pressure reducing device 24 at the end portion opposite to the cathode 22. The above end portion opposite to the cathode 22 and the pressure reducing device 24 are preferably connected to a tank (not shown in the figure), whereby calcium is more efficiently extracted to the outside. The tube 23 for discharging the bath may be disposed by extending to the outside of the vessel 20 passing through the lid 20a so that the molten salt bath "B" is upwardly extracted as shown in Fig. 2, or it may be disposed by extending to the outside of the vessel 20 passing through a through hole provided at the bottom or the side of the vessel 20. Thus, the tube 23 for discharging the bath is disposed, whereby chlorine gas produced on the anode 21 will not contact with calcium produced at the cathode 22.
Fig. 3 shows a sectional side view showing another preferred molten salt electrolytic cell of the present invention. The electrolytic cell shown in Fig. 3 is an improvement example of the electrolytic cell shown in Fig. 2, and explanations of the same components as those of the electrolytic cell shown in Fig. 2 are omitted. The electrolytic cell shown in Fig. 3 differs from the electrolytic cell shown in Fig. 2 in that a flow guide 28 is provided by extending to an intermediate portion between the anode 21 and the cathode 22 from the inside wall of the vessel 20.
As shown in Fig. 3, the flow guide 28 is preferably disposed by being upwardly inclined with respect to the horizontal surface. The oblique angle of the flow guide 28 with respect to the horizontal surface is preferably selected from 10° to 45°, whereby upward flow of the molten salt bath "B" that occurred near the cathode 22 is efficiently decreased.
The edge portion of the flow guide 28 is preferably provided with a bent portion facing downward. According to the bent portion, the upward flow of the molten salt bath "B" containing calcium may be directed to the central portion of the cathode 22.
When chlorine gas is produced at the surface of the anode 21, the chlorine gas rises in the molten salt bath "B". Therefore, in the molten salt bath "B" under the anode 21, the flow of the bath is from the bottom side to the upper side, as shown in Fig. 3. Accordingly, chlorine gas produced on the anode 21 and calcium produced on the surface of the cathode 22 may contact and react with each other.
In contrast, when the flow guide 28 is provided as shown in Fig. 3, after a flow of the bath containing a part of calcium produced on the cathode 22 reaches the flow guide 28, the direction of the flow of the bath is changed and is directed to the central portion of the vessel 20. Then, each flow affects the other, and the flow of the bath is directed downward, which is represented by arrows shown in Fig. 3. As a result, the molten salt bath "B" containing calcium is introduced to the central portion of the cathode 22, and it is efficiently extracted to the outside of the vessel 20 via a tube 23 for discharging bath.
Fig. 4 is a sectional side view showing a cathode 30, which is an improved cathode 22, used in the electrolytic cell shown in Figs. 2 and 3. In the cathode 30, portions 31 and 32, which are upwardly widely open, compose a 2-step structure, and a through hole 33 is provided between the portions 31 and 32 that are upwardly widely open. According to the cathode 30 having such a structure, a greater amount of calcium is extracted compared to the case of using a cathode which is formed with a fin member (corresponding to the reference numeral 17 in Fig. 1) and a through hole (corresponding to the reference numeral 18 in Fig. 1) shown in Fig. 3. As a result, productivity of calcium is further improved compared to the productivity of calcium of the electrolytic cells shown in Figs. 2 and 3.
In the above electrolytic cell shown in Figs. 1 to 3, since chlorine gas is produced at the anodes 11 and 21, the anodes 11 and 21 are preferably made from graphite in view of corrosion resistance. Moreover, in the example shown in Fig. 1, the lower end of the anode 11 is preferably formed in the shape of hemisphere or a pencil with a sharp point. The lower end of the anodes 11 and 21 are formed in such a shape, whereby bubbles will not grow to large sizes as the chlorine gas accumulates. As a result, convection flow of the molten salt bath "B" between the anode 11 and the cathode 12 or between the anode 21 and the cathode 22 may be minimized. As shown in Fig. 5, the lower end of the anodes 11 and 21 are preferably formed with a longitudinal groove 51 in a longitudinal direction. According to such a longitudinal groove, chlorine gas produced at the surfaces of the anodes 11 and 21 rises more smoothly.
The longitudinal groove 51 is provided at the lower end of the anodes 11 and 21, and it is disposed at intervals such that the periphery of the anodes 11 and 21 are divided into from 4 to 10 equal parts. The groove preferably has a width and a depth which are selected from a range of from 5% to 20% of the diameter of the anodes 11 and 21. For example, when the diameter of the anode 12 is 15 mm, the width and the depth of the longitudinal groove are preferably selected from a range of from 1 mm to 3 mm. Thus, the width and the depth of the longitudinal groove are selected to have larger diameters than the bubble diameter of chlorine gas that is produced at the surface of the anodes 11 and 21. As a result, chlorine gas that is produced on the lower portion of the anodes 11 and 21 rises smoothly.
Since the calcium that is produced is reducible, the fin member 17 shown in Fig. 1 and the cathodes 22 and 30 shown in Figs. 2 to 4 may be composed of a carbon steel or a stainless steel. Note that since the spaces of the vessels 10 and 20 hold chlorine gas produced at the anodes 11 and 21, the surfaces of the cathode 12 shown in Fig. 1 and the tube 23 for discharging the bath shown in Fig. 2 are preferably coated with a ceramic that can resist chlorine gas.
The calcium, which is produced in the present invention, may be used for a process in which titanium oxides are reduced with calcium in a molten salt so as to produce titanium. In order to produce titanium with high purity, the cathodes 12, 22, and 30 shown in Figs. 1 to 4 are preferably made from a carbon steel having a low amount of nickel and chromium. The reason for this is that the metals easily dissolve into calcium and molten salt bath, and they may contaminate the titanium produced by reducing titanium oxides with calcium.
The present invention may be used for the case in which the molten salt bath "B" is composed of magnesium chloride so as to electrolyze magnesium chloride in a molten salt. In this case, molten magnesium is produced at the cathode, which has little solubility with respect to magnesium chloride, unlike the case of the calcium chloride. Therefore, the magnesium, which is extracted accompanied by magnesium chloride through a tube 23 for discharging the bath, is statically maintained so as to separate them, whereby simple magnesium is collected. As a result, titanium tetrachloride is efficiently reduced.
As described above, by using the molten salt electrolytic cell of the present invention, calcium chloride containing calcium is efficiently extracted to the outside, whereby high current efficiency is obtained.

FIRST EXAMPLE

The present invention is explained in detail by way of examples hereinafter.
Electrolysis of calcium chloride was tested under the following conditions by using the electrolytic cell shown in Fig. 1.

1) Molten salt bath
Molten salt:anhydrous calcium chloride
Weight: 1 kg
2) Anode
Material:graphite
Size:diameter 15 (mm) x length 600 (mm)
3) Cathode
Material:stainless steel
Size:inner diameter 6 (mm) x outer diameter 9 (mm) x length 600 (mm)
Fin members:circular cone shape, three steps of fin members
Through holes:six through holes having a diameter of 3 mm and formed on the cathode directly above each connected portion on which a fin member having a circular cone shape is connected
4) Container
Material:transparent quartz
Size:diameter 70 (mm) x length 500 (mm)
5) Tube for discharging bath
Material:transparent quartz
Size:diameter 6 (mm) x length 200 (mm)
6) Heating furnace
Output power:1 kW (rating)
Heater:nichrome wire

Anhydrous calcium chloride was supplied to a vessel 10 shown in Fig. 1, and an anode 11 and a cathode 12 were immersed and disposed in the anhydrous calcium chloride. Then, the entire vessel 10 was sealed with a lid 10a, and argon gas was gradually supplied thereto through a nozzle 14 which introduces the gas and is connected to the lid 10a. Simultaneously, the argon gas was discharged through a nozzle 15 which discharges the gas and is connected to the lid 10a, and a space over the anhydrous calcium chloride, which was supplied, was maintained in a slightly pressurized state. A heater, which was disposed around the vessel 10 (not shown in Fig. 1), was supplied with electricity, and anhydrous calcium chloride was heated and was maintained at 800°C ± 5°C.
Next, direct-current voltage was applied between the anode 11 and the cathode 12, and anhydrous calcium chloride was electrolyzed. The applied voltage was adjusted so that current density at the cathode 12 was from 0.2 A/cm² to 0.5 A/cm². Chlorine gas, which was subsidiarily produced by the electrolytic reaction, was discharged to the outside from the nozzle 15 which discharges the gas and is connected to the lid 10a.
After direct-current voltage was applied, according to the formation of calcium, areas near the surface of the cathode 12 were colored. Then, calcium chloride with dissolved calcium was extracted to the outside through the lower end of the cathode 12 by operating a pressure reducing device 13. In this case, in the vessel 10, the calcium produced at the cathode 12 did not diffuse to the anode 11. In addition, the calcium that was produced did not accumulate under the cathode 12.
The above test was repeated 5 times. After each test was completed, the amount of calcium in the calcium chloride extracted from the vessel 10 was analyzed, and current efficiency was calculated on the basis of the quantity of electric current which was applied. In the present invention, the current efficiency was high as shown in Table 1. Table 1

(unit: %)

test No. 1 2 3 4 5 Average

current efficiency 75 80 70 73 77 75

SECOND EXAMPLE

Calcium chloride was electrolyzed in a molten salt under the same condition as in the first example, and the current efficiency was calculated. In this case, note that a cathode 22, which was upwardly widely open as shown in Fig. 2, was used instead of the cathode 12 used in the first example. As a result, high current efficiency as shown in Table 2 was obtained. Table 2

(unit:%)

test No. 1 2 3 4 5 Average

current efficiency 80 85 78 78 79 80

THIRD EXAMPLE

Calcium chloride was electrolyzed in a molten salt under the same condition as in the second example, and current efficiency was calculated. In this case, note that the cathode 22, which was used in the second example, was coated by thermal spraying with silica. As a result, high current efficiency as shown in Table 3 was obtained. Table 3

(unit: %)

test No. 1 2 3 4 5 Average

current efficiency 83 88 85 84 85 85

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, a cathode is hollow, or in addition, a fin member is connected thereto and a through hole is provided directly above the connected portion. Therefore, calcium containing calcium chloride precipitated at the surface of the cathode may be efficiently extracted to the outside. Specifically, the present invention may be used for extraction of calcium containing calcium chloride that is used as a reductant for producing titanium from titanium compounds. Accordingly, the present invention is superior.

Claims

A molten salt electrolytic cell comprising:
a vessel filled with a molten salt bath;

an anode immersed in the bath; and

a cathode immersed in the bath,
wherein the cathode is hollow.
The molten salt electrolytic cell according to claim 1, wherein the cell comprises a vessel with a lid holding the molten salt bath, the anode and the cathode immersed and disposed in the molten salt bath passing through the lid from above the vessel, and the cell further comprises:
a pressure reducing device connected to the cathode;

a nozzle for introducing a gas from the outside of the vessel into the vessel;

a nozzle for discharging the gas from the inside of the vessel to the outside thereof;

a nozzle for feeding a molten salt from the outside of the vessel into the vessel;

a fin member provided on the outer surface of the cathode; and

a through hole provided at the cathode directly above the connected portion to which the fin member is connected.
The molten salt electrolytic cell according to claim 2, wherein one end of the cathode immersed in the molten salt bath is closed, and another end of the cathode, which is not immersed in the molten salt, is connected to the pressure reducing device.
The molten salt electrolytic cell according to claim 1, wherein the cell comprises a vessel with a lid holding the molten salt bath, the anode immersed and disposed in the molten salt bath passing through the lid from above the vessel, and the cathode immersed and disposed under the anode in the molten salt bath with being upwardly widely open, and the cell further comprises:
a tube for discharging the bath, which is connected to the cathode and is extended to the outside of the vessel;

a pressure reducing device connected to the tube for discharging the bath at the outside of the vessel;

a nozzle for introducing a gas from the outside of the vessel into the vessel;

a nozzle for discharging the gas from the inside of the vessel to the outside thereof; and

a nozzle for feeding a molten salt from the outside of the vessel into the vessel.
The molten salt electrolytic cell according to claim 4, wherein insulating layers are provided at the cathode at the surface which does not face the anode and to the outer surface of the tube for discharging the bath.
The molten salt electrolytic cell according to claims 4 or 5, wherein a flow guide is upwardly extended from the inside wall of the vessel to an intermediate portion between the cathode and the anode, which guide is upwardly inclined with respect to the horizontal surface.
The molten salt electrolytic cell according to one of claims 4 to 6, wherein the tube for discharging the bath is extended to the outside of the vessel passing through the lid of the vessel.
The molten salt electrolytic cell according to one of claims 4 to 6, wherein the tube for discharging the bath is extended to the outside of the vessel passing through one of the bottom of the vessel and the side of the vessel.
The molten salt electrolytic cell according to one of claims 1 to 8, wherein the anode is composed of a graphite, and the cathode is composed of a carbon steel or a stainless steel.
The molten salt electrolytic cell according to claim 9, wherein longitudinal grooves are formed on the surface of the anode, which groove width is larger than the diameter of bubble of chlorine gas produced on the anode.
The molten salt electrolytic cell according to one of claims 1 to 10, wherein the molten salt is composed of calcium chloride, magnesium chloride, or a mixture of calcium chloride and potassium chloride.
A process for producing metal using the molten salt electrolytic cell according to claim 1.
A process for producing metal using the molten salt electrolytic cell according to claim 2.
A process for producing metal using the molten salt electrolytic cell according to claim 4.
The process for producing metal according to one of claims 11 to 14, wherein molten calcium or molten magnesium is produced as the metal.