US3203881A - Production of metallic halides - Google Patents

Description

Aug. 31, 1965 R. w. ANCRUM ETAL 03,881

PRODUCTION OF METALLIC HALIDES Filed Jan. 15, 1958 United States Patent 3,203,881 PRODUCTIGN 'OF METALLIC HALIDES Robert William Ancrum, Saint-Germain-en-Laye, Paris,

France, and Arthur Wallace Evans, 'Nunthorpe, Middlesbrough, England, assignors to British Titan Products 'Company Limited, 'Billingham, Co. Durham, England, a corporation of the United Kingdom Filed Jan. 15, 1958, Ser. No. 709,155 Claims priority, application Great Britain, July 16, 1952, 17,971/52; Nov. 27, 1957, 37,034/57 7 Claims. (Cl. 204-611) This application is a continuation-in-part of application Serial No. 366,611, filed July 7, 1953 Serial No. 700,638, filed December 4, 1957, both now forfeited, and Serial No. 700,556, filed December 4, 1957.

This invention comprises improvements in or relating to the production of metallic halides, particularly the halides of tetravalent metals. The expression tetravalent metals is to be understood to include silicon. References herein to halides and halogenation are to be understood to exclude fluorides and fluorination.

The invention is more particularly concerned with the production of the metallic halides by an electrolytic process using as starting materials corresponding metalliferous materials. In the electrolytic process the metallic halide is produced on the anode, which comprises the metalliferous material. The electrolyte serves as a source of halogen for the halogenation of the metalliferous material, and is a metal halide in a molten state, especially a halide of an alkali metal and/or an alkaline earth metal, suitable metal halides being lithium chloride, sodium chloride, potassium chloride, calcium chloride, strontium chloride, barium chloride, beryllium chloride and magnesium chloride. It will be appreciated that the metal of the electrolyte is deposited on the cathode of the electrolysis apparatus, and in some cases this is of interest as a method of producing such metal, especially for example when a magnesium halide is used as the electrolyte, whereby magnesium is produced on the cathode.

The invention is of particular importance in respect of the halogenation of siliceous materials, e.g. silica as such, and also compounds of combinations of silicon dioxide with other oxidic bodies, e.g. zircon and bauxitic clays or residues resulting from the extraction of the aluminium constituents from the latter. It will be understood however that the invention applies also to the production of halides from metalliferous materials other than siliceous materials, e.g. baddeleyite which consists mainly of zirconium dioxide.

The invention of application Serial No. 366,611 relates to a method or" preparing titanium tetrahalide which comprises electrolysing a fused bath consisting of a member of the group consisting of alkali metal halides and alkaline earth metal halides in which the halide component is that of the titanium compound desired with an anode comprising a mixture of carbon with a material containing at least 90 percent of TiO the amount of carbon being at least stoichiometrically equivalent to the amount of "H0 present in said material, maintaining the temperature of the bath 700 to 1400 C., maintaining a neutral atmosphere over the electrodes, whereby titanium tetrahalide is formed and vapourising and removing the titanium tetrahalide from the bath.

The invention will now be described with particular reference to the problems involved in the halogenation, especially chlorination, of siliceous materials, and especially silicon dioxide.

2 Silicon. tetrahalides, and especially silicon tetrachloride, are becoming of increasing importance in commerce. They have economic advantages, especially from the aspect of purification, i.e. on account of their comparatively low boiling points and ease of distillation, and are of special interest as intermediates in the preparation of various chemicals which may be silico-organic, or even in the preparation of special forms of high grade silica, particularly in the preparation of fine silica as described and claimed in application Serial No. 598,913 filed July 19, 1956, now abandoned.

The chlorination of metal oxides, including oxides of metalloids such as silicon, has been the subject of considerable research 'over many years but the chlorination of silica in particular has proved to be very difiicult. For example Green, Richardson & Clews, Trans. Brit.

I Ceram Society, 41, 196, 1942, indicate the conditions for chlorination of a number of oxides, and have demonstrated that silica is extremely difiicult to chlorinate under normal conventional procedures, i.e., when admixed with an oxygen acceptor such as carbon. According to conventional practice a mixture of silica and carbon will only chlorinate with difficulty at temperatures of the order of 1150 C. and for efficient operation much higher temperatures would be required.

Similar problems have arisen in the chlorination of certain other siliceous materials, for instance compounds of silicon dioxide, e.g. zircon, bauxitic clays, or residues following extraction of the aluminium constituents from the latter.

The production of silicon tetrachloride by chlorination of silica has posed a serious problem economically. It will be obvious that there are difficulties in respect of the design and construction of plant and particularly in the selection of materials of construction, since, at temperatures of the order mentioned above, there are very few types of containers which will withstand the action of chlorine, and which will be sufficiently gas-tight to warrant their use, bearing in mind especially the danger to health involved by gas-leaks. Thus in the production of silicon tetrahalides, and particularly in the production of silicon tetrachloride, there is a long-felt want in respect of a suitable and commercially satisfactory method of production. 7

We have now developed a method of chlorinating silica and other siliceous bodies such as zircon, by means of which chlorination can be eifected at relatively low temperatures, i.e. below 1200 C., at which chlorination would otherwise be inoperable and the problems enumerated above can be overcome in respect of the construction of plant and of eifecting economies in production.

Accordingly, there is provided, by an embodiment of the invention, a method for the production of silicon tetrahalide, which method comprises electrolysing a fused metal halide salt, using an anode comprising a mixture of carbon and siliceous material, the halogen liberated at are so low as to be comparatively unimportant. When mixtures of halides are used, the properties of the mixtures as opposed to those of the individual constituents are relevant. Into this bath is inserted the anode, which may be constructed in a variety of ways and which is more particularly described below, but which essentially comprises a conducting rod e.g. carbon on which is built or moulded, or which is surrounded or otherwise closely associated with, the mixture of siliceous material with an adequate proportion of carbon. The cathode consists of a conducting rod which maybe any of a group of materials, e.g. carbon, iron, tantalum and tungsten. In some instances, at least, tungsten is the preferred material for use as an electrode, e.g. when calcium metal is deposited on a carbon cathode, it is found that carbides are liable to form. The electrodes in the bath are separated, at least in so far as the upper portion of the bath and the vapour space above are concerned, i.e. there is a barrier between the two which extends below the surface of the bath to a level to be determined according to choice of one skilled in the art.

An important feature in the operation of this invention is the construction of the anode. This may be accomplished in a variety of ways, as is apparent when a selection of such procedures is described. The siliceous starting material may be a prepared substance, or a natural mineral such as quartz, cristobalite or tridymite, and may be a silicate such as zircon. The siliceous material may be used in a sand-like form and mixed with carbon of a similar size, or the two starting materials may be initially of somewhat large dimensions and be suitably comminuted together. The carbon employed may be selected from a number of well-known sources, which may include charcoal and petroleum coke or may even be derived in situ from carboniferous material, e.g. coal. The term carbon is to be understood to include graphite, amorphous carbon and mixtures thereof. The siliciferous material and carbon, suitably mixed, may be for instance packed into a porous pot with a carbon rod inserted therein. Alternatively, the siliceous-carbon mixture with the aid of a bonding agent such as gum, pitch or glue may be compressed, and if necessary fired, and an electrode, preferably of carbon, inserted into the compressed mass. In a further alternative, the carbon and siliceous material may be made into a paste with glycerine or dehydrated castor oil with or without the presence of water, and this paste may be shaped, dried and fired, preferably in a non-oxidising atmosphere, with or without a core of carbon, at 900l400 C. to produce a hard electrically-conducting mass. A further method may consist of packing either the siliceous material 'or carbon in the form of loose lumps into a porous pot with an electrode inserted therein, or in certain cases loosely packed lumps or compressed powder, as described above, may be inserted into a hollow porous electrode, from which the silicon tetrahalide subsequently generated may be withdrawn. If it is desired to make the process operate continuously, a mixture, preferably of finely-divided siliceous material and carbon with or without a bonding agent, may be fed in the form of a thick paste onto the top surface of a slowly decaying anode which is periodically or continuously lowered into the bath as it becomes eaten away; the paste so fed to the anode will, on account of the heat transmitted through the submerged part of the anode, and the heat derived by convection and radiation from the anode chamber, dry, cake and ultimately bind and cement together in the form of a hard adherent mass and so become suitable to function as an anode.

As has already been mentioned, the following salts are suitable for use in the bath, either separately or in admixture, lithium chloride, sodium chloride, potassium chloride, calcium chloride, strontium chloride, barium chloride, beryllium chloride and magnesium chloride. These salts should be used in anhydrous form.

In operation of the process generally with respect to the simple type of cell described above using calcium chloride as the metal halide salt byway of example, the calcium chloride is heated to a temperature in the neighbourhood of 1000 C. The vapour space immediately above the liquid in both the anode and the cathode compartments is swept with an inert gas, so as to avoid the presence of reactive gases, such as oxygen. The flow of such inert gases may then either be arrested for the duration of the electrolysis, or according to the discretion of the operator, may be continued so as to assist in removing the product vapours generated during the electrolysis. The electrodes are connected to a source of electric energy and electrolysis ensues, the chlorine being liberated at the anode, and immediately reacting with the mixture of siliceous material and carbon so as to form silicon tetrachloride and a carbonaceous gas, e.g. carbon monoxide and/or carbon dioxide. The silicon tetrachloride and carbonaceous gas generated at the anode and appearing in the vapour space above the calcium chloride are led, assisted if necessary by a flow of inert gas, to a suitable condensing apparatus for the condensation of the silicon tetrachloride. The carbonaceous gas is then either discharged to the atmosphere, or utilised as desired. Simultaneously, at the cathode there is liberated the calcium, i.e. the metal corresponding to the fused salt of which the bath is composed.

Depending on the properties of the metal of which the salt is composed at the temperature under which the electrolysis takes place, the metal may be deposited and collected at the cathode in a variety of ways. The metal may, for instance, be deposited on the cathode in the form of a solid which may adhere to the cathode and be removed by periodic or continuous withdrawal of the cathode. Alternatively, the metal may be deposited at the cathode and subsequently fall away again as a mud into the cathode bath from which it may be removed by subsequent purging of the bath and extraction by wellknown means. The metal, if liquid at the temperature of the bath, may be collected as a liquid pool which may be periodically removed from the bottom or from the surface of the bath, depending on the relative densities of the metal and the fused salt. The metal may be removed as such or in the form of a solution. Alternatively the metal may be deposited onto a liquid cathode which again may be periodically purged for the removal and refinement of the deposited metal contained therein. On the other hand the metal deposited at the cathode may at the temperature of the bath be in the form of a vapour, which may be removed from the cathode compartment by suit able methods, including the assistance, if necessary, of inert gas flowing therethrough, followed by condensation of the metal vapour, or any other treatment which may be particularly applicable.

The invention is more particularly described with reference to the accompanying diagrammatic drawing, which shows a view in elevation of an apparatus suitable for carrying out the process of the invention.

A container 1, which may consist of metal, for instance iron, or some suitable ceramic, e.g. silica, is filled with a molten salt 2, in this instance magnesium chloride,

- to a level 3. The container is heated externally by consilica tube 9, the lower end 12 of which is left open and immersed in the bath, its precise position being variable, as desired. The carbon rod 21 is afiixed by means of a bung 10 through the top of the silica tube 9, and a lead 11 from the carbon rod 21 is connected to the positive pole of the DC. electric power supply. The silica tube 9 has a side conduit 13, which is positioned immediately above the level of salt 2 in the bath, and through which may be fed inert gas or any suitable carrier gas as described below. According to requirements, this gas is discharged from the top of the silica tube 9 via a duct 14 to a condenser or other equipment appropriate for removal of the silicon tetrahalide produced at the anode.

In the operation of this apparatus, the bath is heated to about 1000C., the anode and the cathode compartments are swept with inert gas, and the current is passed through the cell so that, a has already been explained, electrolysis is established; the chlorine liberated at the anode reacts with the silicon dioxide and carbon to pro duce silicon tetrachloride and carbonaceous gases which, assisted, if necessary, by continuing the flow of inert gas, are removed from the anode compartment for the silicon tetrachloride to be collected by condensation; simultaneously, magnesium, i.e. the metallic element of the halide salt used, is deposited at the cathode, and forms as a pool on the bottom of the salt bath, being removed by periodic purging. During the passage of current through the cell, heat is generated therein and this may augment the heating of the bath and may suffice to maintain it at the required temperature. It will be obvious that this latter effect will depend in large measure on the dimensions of the cell and upon the construction, especially the degree of insulation provided.

The silicon tetrahalide formed may, if necessary or desirable, be subjected to suitable purification treatment, e.g. fractional distillation or any other known means, and may be used as an intermediate for the production of pigments, for instance by vapour phase reactions, either with steam or by direct oxidation, to yield pigments of a very high degree of whiteness and with a controlled particle size of the order of, for instance, 0001-0005;, such products having exceptional value in for instance the rubber industry. The silicon tetrahalides may also be used in connection with the production of organic materials to produce the so-called silico-organic compounds, well known in the art.

The following examples are given for the purpose of illustrating the invention; all parts and percentages are by weight.

The apparatus used in the examples comprises, with reference to the figure illustrated in the accompanying drawing, a silica pot 1, of 4" diameter and 5" height into which is inserted a fused silica chamber 9, which is open at the bottom, and is sealed at the top by a plug with a A" diameter anode 21 passing through the plug and hav ing bonded on its lower end, i.e. that which is immersed within the bath, a moulded mass 8 formed as follows:

Approximately one part of non-graphitising carbon, such as 82.4% C. Northumbrian coal or 83.1% C. Yorkshire coal or sugar charcoal, and two parts of the siliceous material to be chlorinated, both constituents ground to pass a standard 200 mesh sieve, were intimately mixed with about one tenth of a part of a gum, such as gum acacia, or gum arabic or gum tragacanth, and made into a stiff paste with a few drops of dehydrated castor oil. This paste is moulded round the lower end of the anode 21 to form a rough cylinder approximately 4 in diameter and 2 /2" in length. The molded electrode is then placed in an atmosphere of coal gas and heated to 1000 C. for three hours. On removal, the electrode is ready for use. The amount of carbon present in the fired electrode is about by weight.

Also inserted in the bath to a depth of about 2 /2" is a cathode 5, which consists of a tungsten rod /8" in diameter.

Example 1 Using the apparatus described above, a melt of fused calcium chloride was prepared by external heating of the silica pot to a temperature of 1030 C., as measured by the thermocouple 4. The moulded portion of the anode was made from a carbon/silica sand mixture bonded with gum acacia. The atmosphere in contact with the top of the anode 11, i.e. in the chamber 9, was swept with inert gas by passing nitrogen through conduit 13 and discharging through 14. Thereafter the port 13 was closed, and conduit 14 connected to a condensing and collecting system for the silicon tetrachloride and other product gases. The anode and cathode were thereafter respectively connected to a DC. circuit which produced a voltage of 4.8 across the electrodes and a current of 5.8 amps. This is equivalent to a current density of 15 amps./dm. at the anode, and of 75 amps./dm. at the cathode. With the bath maintained by external heating at a temperature of 1030 C., these conditions were maintained for 3 /2 hours, during which period the gases, which consisted in the main of carbonaceous gas, silicon tetrachloride and some chlorine which escaped attack at the anode, were removed via the port 14. In all, 10 litres of gas, as measured at room temperature, were collected and were found to contain 1590 cc. of CO and 3900 cc. of CO. It was found that of the chlorine liberated had been converted to silicon tetrachloride.

Example 2 The apparatus described above was again used, but the bath consisted of magnesium chloride fused and maintained at a temperature of 1025 C. The same procedure was followed, and, on connecting with the DC. current, the voltage across the electrodes was 4.2 and the current 6 amps. The current density at the anode was 16 amps./dm. and the current density at the cathode was 72 amps./dm. The operation was conducted for 3 /2 hours and 10 litres of gas were collected. This gas, as measured at room temperature, was found to contain 1480 cc. of CO and 3520 cc. of CO, and 70% of the chlorine generated at the anode had been converted to silicon tetrachloride.

Example 3 The apparatus and cathode were the same as above, but the melt consisted of 50% sodium chloride and 50% potassium chloride maintained at a temperature of 980 C. The moulded portion of the anode was prepared from a zircon sand/ carbon mixture. The voltage across the electrodes was 4.5 and the current 6 amps, the currenty density at the anode being 12 amps./dm. and at the cathode 70 amps./dm. The total volume of gas collected from the anode over a period of 2 hours was 10 litres, and this was found to contain 420 cc. of CO and 2900 cc. CO. The conversion of chlorine liberated at the anode was 71%. The product gas contained both silicon tetrachloride and zirconium tetrachloride, zirconium tetrachloride being primary condensed before the silicon tetrachloride.

Example 4 The apparatus and cathode were the same as above, but the moulded portion of the anode was prepared from a carbon and ground silica sand/ 10% ground zircon sand mixture. The bath consisted of fused calcium chloride maintained at a temperature of 960 C. The voltage across the electrodes was 5.1 and the current 5.8 amps, the current densities being 14 amps./dm. for the anode and 72 amps./dm. for the cathode. The duration of the operation was 45 minutes, during which time 5 litres of gas were collected, the gas containing 450 cc. of CO and 300 cc. of CO. The conversion of chlorine liberated at the anode was 63%. As with the product obtained from Example 3, zirconium tetrachloride and silicon tetrachloride were obtained.

Example 5 The apparatus and the cathode were as described above, but the moulded portion of the anode was prepared from a carbon/spent bauxitic stone mixture. The spent bauxitic stone had a composition of 7.6% TiO 6.7% A1 78.4% SiO 0.65% Fe. The bath consisted of fused magnesium chloride maintained at 1020 C. The voltage across the electrodes was 4.8 and the current 5.9 amps, the current density at the anode being 12 amps./ dmF, and the current density at the cathode being 70 amps./dm. The duration of operation was 3 hours, and the gas collected via the port 14 had a volume of 9 litres as measured at room temperature, containing 1010 cc. CO and 3370 cc. CO. Of the chlorine liberated at the anode, 69% was converted to silicon tetrachloride which was recovered by condensation.

Although the invention has been more particularly described with relation to the formation of silicontetrachloride, as has already been mentioned the invention applies also to the halogenation of other metalliferous materials to form the corresponding metallic halides. When forming a halide other than a chloride, the electrolyte will of course comprise the appropriate halide of a suitable metal. Whenusing other starting materials than have been particularly described above, the necessary conditions of operation, e.g. the temperature, may be varied as desired for the particular material used. Generally the temperature will lie between 700 C. and 1400 C., preferably between 900 C. and 1100 C.

Example 6 A graphite rod was covered with paste consisting of coal and TiO in dehydrated castor oil, and the whole was dried and fired. The resulting electrode was inserted in a porous pot inside a cell containing a mixture of sodium and potassium chloride in equal quantities heated to 900 to 1000 C. The melt was electrolysed with an anode current density of 11 amps. per square decimeter and under these conditions, titanium tetrachloride was formed at the anode.

Example 7 A molten bath was employed containing 50% potassium chloride and 50% sodium chloride and maintained at 820 C., using an anode prepared by mixing titanium oxide and coal and pre-fired in a non-oxidising atmosphere, and a cathode consisting of a carbon rod, the anode department being sealed to permit only the extraction of gases. The cell was operated with an anode density of amps. per square decirneter and at the anode titanium tetrachloride was liberated admixed with carbon dioxide and some carbon monoxide, and the titanium tetrachloride was subsequently recovered by condensation.

Example 8 The cell contained a molten mixture consisting of 50% potassium chloride and 50% sodium chloride. The anode comprised a rod of sintered titanium carbide and the cathode was a carbon rod. The anode compartment was sealed to permit only the extraction of gases.

With the cell maintained at 700 C. and an anode current density of 10 amps. per square decimeter, titanium tetrachloride was liberated in the anode compartment and the titanium tetrachloride vapours were recovered by condensation.

What is claimed is:

1. A process for producing silicon tetrachloride, comprising electrolysing a fused bath consisting of at least one member of the group consisting of alkali .metal chlorides and alkaline earth metal chlorides with an anode comprising a mixture of carbon and siliceous material containing at least 90% by Weight of silicon dioxide, the amount of carbon being at least stoichimetrically equivalent to the amount of silicon dioxide present in the said siliceous material, maintaining a neutral atmosphere over the electrodes, preventing metal evolved at the cathode from contacting the products of the anode, maintaining the temperature of the bath at between 700 to 1400 C. whereby silicon tetrachloride is formed, and vapourising and removing the silicon tetrachloride from the bath.

2. A process for producing silicon tetrahalide which comprises electrolyzing a fused bath of at least one member of the group consisting of alkali metal halides and alkaline earth metal halides, said bath being substantially inert to the elemental halogen of said halide, with an anode comprising a mixture of silicon dioxide and carbon at a temperature of 700 to 1400 C., contacting the halogen liberated from the bath pursuant to electroylsis with said mixture while maintaining the temperature of the bath at between 700 to 1400 C. and thereby producing silicon tetrahalide, and vaporizing and removing the silicon tetrahalide from the bath.

3. A process for producing silicon tetrachloride which comprises electrolyzing a fused bath of at least one member of the group consisting of alkali metal chlorides and alkaline earth metal chlorides, said bath being substantially inert to chlorine, with an anode comprising a mixture of SiO and carbon at a temperature of 700 to 1400 C., contacting the chlorine liberated from the bath pursuant to the electrolysis with said SiO while maintaining the temperature of the bath at between 700 to 1400 C. and thereby producing silicon tetrachloride, and vaporizing and removing silicon tetrachloride from the bath while preventing the metal evolved at the cathode from contacting the evolved silicon tetrachloride.

4. The process of claim 3 wherein the anode comprises a mixture of carbon and silica sand, the amount of carbon being at least stoichiometrically equivalent to the SiO of said sand.

5. A process for producing silicon tetrachloride which comprises electrolyzing a fused bath of at least one member of the group consisting of alkali metal chlorides and alkaline earth metal chlorides, said bath being substantially inert to chlorine, with an anode comprising a mixture of zircon and carbon at a temperature of 700 to 1400 C., contacting the chlorine liberated from the bath pursuant to the electrolysis with said zircon while maintaining the temperature of the bath at between 700 to 1400 C. and thereby producing silicon tetrachloride, and vaporizing and removing silicon tetrachloride from the bath While preventing the metal evolved at the cathode from contacting the evolved silicon tetrachloride.

6. A process for producing a volatile halide of a tetravalent metal from the group consisting of silicon and zirconium which comprises electrolyzing a fused bath of at least one member of the group consisting of alkali metal halide and alkaline earth metal halide, said bath being substantially inert to said metal halide, with an anode comprising a mixture of carbon and an oxide of a tetravalent metal of the group consisting of silicon and zirconium, contacting halogen liberated from the bath pursuant to the electrolysis with said tetravalent metal oxide while maintaining temperature of the bath at between 700 C. to 1400 C. and thereby producing volatile metal halide, and vaporizing and removing metal halide from the bath while preventing the metal evolved at the cathode from the contacting the evolved metal halide vapor.

7. A process for producing a volatile halide of a tetravalent metal which comprises electrolyzing a fused bath of at least one member of the group consisting of alkali metal halide and alkaline earth metal halide, said bath being substantially inert to said metal halide, with an I anode comprising a mixture of carbon and zircon, contacting the halogen liberated from the bath pursuant to the electrolysis with said mixture while maintaining the temperature of the bath at between 700 C to 1400 C. and thereby producing volatile metal halide, and vaporizing and removing tetravalent metal halide from the FOREIGN PATENTS bath While preventing the metal evolved at the cathode 1 134 073 11/56 France from contacting the evolved metal halide vapor. 4/50 Great Britain References Cited by the Examiner 5 gfig 5 23 UNITED STATES PATENTS 5 231 9 9 Blackman 2o4 1 WINSTON A. DOUGLAS, Primary Examiner.

2,734,855 2/56 Buck et a1. 20461 JOHN R. SPECK, Examiner. 2,870,071 1/59 Juda et a1. 20461

Claims (1)

1. 6. A PROCESS FOR PRODUCING A VOLATILE HALIDE OF A TETRAVALENT METAL FROM THE GROUP CONSISTING OF SILICON AND ZIRCONIUM WHICH COMPRISES ELECTROLYZING A FUSED BATH OF AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF SILICON AND ZIRCONIUM WHICH COMPRISES ELECTROLYZING A FUSED BATH OF AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF ALKALI METAL HALIDE AND ALKALINE EARTH METAL HALIDE, SAID BATH BEING SUBSTANTIALLY INERT TO SAID METAL HALIDE, WITH AN ANODE COMPRISING A MIXTURE OF CARBON AND AN OXIDE OF A TETRAVALENT METAL OF THE GROUP CONSISTING OF SILICON AND ZIRCONIUM, CONTACTING HALOGEN LIBERATED FROM THE BATH PURSUANT TO THE ELECTROLYSIS WITH SAID TETRAVALENT METAL OXIDE WHILE MAINTAINING TEMPERATURE OF THE BATH AT BETWEEN 700*C. TO 1400*C. AND THEREBY PRODUCING VOLATILE METAL HALIDE, AND VAPORIZING AND REMOVING METAL HALIDE FROM THE BATH WHILE PREVENTING THE METAL EVOLVED AT THE CATHODE FROM THE CONTACTING THE EVOLVED METAL HALIDE VAPOR.

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