US3274083A - Electrolytic production of titanium - Google Patents

Description

United States Patent 3,274,083 ELECTROLYTIC PRODUCTION OF TITANIUM Linden E. Snyder, Las Vegas, Nev., assignor to Titanium Metals Corporation of America, New York, N.Y., a corporation of Delaware t No Drawing. Filed May 13, 1963, Ser. No. 280,090 7 Claims. (Cl. 20464) This invention relates to the production of metallic titanium and more particularly to an improved method for electrolysis of a titanium halide in a fused salt bath.

The general type of electrolytic process to which this invention relates employs a cell having an anode and a cathode and a fused chloride salt bath as an electrolyte. The anode may be of simple construction, often a bar, plate or rod of graphite. The cathode, however, is particularly characterized by presenting a perforate side facing the anode and such perforate side forming at least part of an enclosure separating the electrolyte-containing portion of the cell into two compartments. The compartment in which the anode is immersed contains anolyte portion of the electrolyte; the portion isolated therefrom by the cathode structure including its perforate side constituting the catholyte. Thus, for example, the cell may comprise a central anode rod surrounded at a distance by a cylindrical cathode of perforate metal. In this case the electrolyte inside the cathode and in which the anode is located is the anolyte and that outside the cathode cylinder is the catholyte. Or an anode plate or plates may be arranged at one side or more sides of the cell and a hollow, rectangular cathode arranged with a perforate side or sides facing the anode or anodes. In this case, the electrotyte outside the hollow cathode and in which the anode or anodes may be located is the anolyte and that inside the hollow cathode is the catholyte. In either case, titanium tetrachloride, for example, is introduced into the catholyte and electrolyzing current passed between anode or anodes and cathode. A concentration of titanium lower chloride, either titanium trichloride, dichloride or both, is maintained in the catholyte and the so-introduced TiCl, is reduced by electrolytic action to metallic titanium which is deposited on a cathode surface. Chlorine is liberated at the anode and the cell interior arranged so that this anode product and the TiCl feed to the cathode are mutually isolated.

During the initial stages of operation of such a cell, titanium metal deposit-s on the perforated cathode surfaces to build up a porous titanium metal structure which prevents leakage of lower titanium chlorides from the catholyte to the anolyte. The anolyte must be free of lower titanium chlorides since if any are present in this portion of the electrolyte, they will combine with chlorine liberated at an anode to reform TiCl which will be lost in the chlorine by-product. The efficiency of the reduction operation will thereby be reduced. After initial formation of the layer on the perforate cathode surface, additional titanium metal deposits on that previously formed and on other receptive portions of the cathode structure. However, the character of the initial deposit appears to appreciably affect the efficiency of the cell operation and the type of deposit obtained during the later stages of the electrolysis cycle. Difiiculty has heretofore been experienced in obtaining the requisite initial dense but porous titanium metal structure which appears to be necessary for most efficient cell operation and production of a compact deposit of product titanium crystals of desired purity.

Summarized briefly, the process of this invention comprises operation of an electrolytic cell as described above for an initial period during a cycle at a very low rate of TiCl, introduction compared to the electric current passed through the cell to provide a high faraday per mol of TiCl, ratio. Subequently, for the remainder of the cycle, TiCl is introduced at a gradually increasing rate until a steady state condition of operation is attained during which the TiCl introduction is more normally related to the reducing power of the current used so that close to of the TiCL, fed is assimilated and converted to titanium metal.

The fused halide salt bath constituting the electrolyte may comprise any alkali or alkaline earth metal halide or combination of such halides. Sodium chloride has been found to be inexpensive and is relatively easy to obtain and store dry but has a rather high melting point. Eutectic compositions of sodium and potassium chlorides as well as combinations including lithium, rubidium, cesium, calcium, strontium, barium or magnesium may be employed, and such combinations may provide lower melting compositions.

The initial period of the production cycle constitutes a small part, preferably between 5 and 10% of the total cycle time from the start of introduction of TiCL, into the catholyte to the final shutting off of TiCl feed preparatory to shutting off the current supply and recovery of titanium metal from the cathode. During the initial period, TiCl is fed to the cell, and electric current passed so that the average current to TiCl, ratio is high, preferably between 20 and 40 faradays per mol of TiCl and at all times greater than about 9 faradays per mol and preferably greater than 14 faradays per mol. Thus, the current supplied during this period is at. all times substantially greater than and preferably at least 3 /2 times the theoretical 4 faradays per mol required to reduce Ti++++ to Ti metal and at least double the more practical operating ratio (considering cell losses, etc.) of between 4.5 and 7 faradays per mol. The average ratio over the initial period of between 20 to 40 faradays per mol of TiCl permits a period when no TiCl at all is being introduced into the catholyte. I have found that it is advantageous to feed TiCl for the first part amounting to 50 to 70% of the initial period at a rate of about 14 to 30 faradays per mol and during the latter part operate with no TiCl being fedgthe relative times for feeding and not feeding TiCl, and the rate of feed being adjusted so that the average rate over the entire initial period is between 20 and 40 faradays per mol of TiCl The length of the initial period should preferably be 5 to 10% of the total TiCL, introduction cycle time since this provides sufiicient time to deposit the required titanium metal on the perforated cathode surface Without extending the initial period too long and thereby reducing the overall time cycle efiiciency of the cycle. In like manner, the current to TiCl, ratios specified for the initial period will provide sufiicient TiCl, input to produce the metal deposit required and in a reasonable time without causing loss of Ti values through recombination at the anode.

After completion of the initial period, the rate of TiCl, addition is gradually increased and the addition rate can be controlled by reference to TiCl in the chlorine by-product from the cell. Any TiCl, in the chlorine byproduct shows that titanium values, lower chlorides, are lea-king from the catholyte and combining with chlorine to reform TiCl Feed rate of TiCL, is therefore adjusted so that no appreciable amount of TiCl is detectable in the by-product chlorine and preferably just below the rate which will result in this indication. It will be found that the rate of TiCL, feed can gradually be increased to the point where a current to TiCL; feed rate ratio of between 4.5 and 7 faradays per mol of TiCL, can be continually maintained without TiCl, showing in the chlorine by-product. This is considered steady state operation during which the bulk of the titanium metal will be deposited on a cathode surface as desirable type crystals of excellent purity.

After a period of steady state operation, it will be found that the TiCl feed rate must be reduced to prevent TiCL; from showing in the chlorine byproduct. This indicates that the cathodic deposit has increased in bulk and density to a point where steady state assimilation of TiCL; cannot be maintained. At this point, if desired, the TiCL, feed may be discontinued and the cycle ended or operation may be continued for a reasonable time with lowered TiCl input. The TiCl feed rate can be correspondingly gradually reduced but still maintaining a rate just below the amount causing the presence of TiCl in the by product chlorine. When the TiCL; feed .rate has been reduced, following the assimilation capability, and the current to TiCh, ratio thereby increased to between 9 and 14 faradays per mol of TiCl or about half the rate at steady state operation, the TiCl feed is discontinued. Current passage may be continued, if desired, for a short period to strip any residual reduced Ti values from the catholyte and then the current is shut off and the cathode removed from the cell for recovery of the titanium metal product.

It will be noted that cell operation according to this invention has been described as a procedure in which varying TiCl tfeed rates are employed with the amount of current passed being substantially constant to provide the critical current to TiCl feed ratios recited. Except for a period when no TiCl is being fed, as in the preferred embodiment, it will be obvious that the amount of current used may be varied, with varied or constant TiCl feed rates, to obtain the required ratios. As a practical matter, however, with normal limitations of power supply control and for best utilization of the current supplied, it has been found that it is more convenient and efiicient to maintain a constant ampere current input to the cell and to vary the TiCL; feed rate.

It will be apparent also, to those skilled in the art, that the amounts of current and TiCl supplied, the length of the production cycle, the amount of titanium metal produced per cycle and other factors while interdependent among themselves, will be in actual values dependent principally on the design and size of the cell.

Operation during the initial period at extremely high faraday per mol ratios results in deposition on the perforate cathode surface of a dense but porous titanium crystal structure. While such structure is not preferred for the bulk of the titanium product, its formation at the beginning of the cycle provides the required porous deposit over the cathode perforations which effectively confines titanium lower chlorides in the catholyte and at the same time permits passage of chloride ions to the anode where they are released as gaseous chlorine. It is to be stressed that during the initial period the current to TiCL, feed ratio is at all times at least double the theoretical amount for TiCL; reduction to metal, thus preventing during this time the presence of any appreciable reduced titanium values such as TiCl or TiCl in the catholyte. This is important since the perforations in the cathode will not at this stage yet have been scaled and loss of lower chlorides by migration to the anolyte could readily occur. Additionally the high current ratio apparently causes formation of the desirable type of dense but porous crystal structure needed. Indeed, the current ratios are so high that it is suspected that the mechanism may involve electrolysis of the electrolyte to produce a reactive reducing metal, such as sodium from a sodium chloride electrolyte salt, and this may react chemically with TiCl to produce the dense spongy metal obtained.

After the initial period and when the TiCl is being fed up to its assimilation limit, the titanium deposit on the perforated cathode surface will become more established and efficient as indicated by increasing TiCL as similation capability until steady state operation is obtained. During this period a concentration of titanium lower chlorides, principally TiCl is being built up in the catholyte and effectively confined to the catholyte by the dense but porous titanium metal deposited on the perforated cathode surface. The titanium dichloride concentration in the cathode may reach 1 to 10 mol percent during steady state operation, this amount apparently being necessary for its function as an intermediate electrolyzable compound or a solubility promoter for TiCL; or whatever it actually does; the precise nature of its tunetion being unknown so far as I am aware.

The following is an illustrative example of the practice of this invention.

Example 1 An electrolytic cell was employed having a hollow box-like cathode with two opposite sides perforated. Additional surfaces were provided inside the cathode box for metal deposition. Flat graphite anodes were arranged in the cell facing the perforated sides of the cathode. A TiCL, feed pipe led into the interior of the cathode box. Sufficient molten sodium chloride was placed in the cell to immerse the cathode and anode assemblies.

Electric power at about 7 volts and 11,500 amperes was turned on to pass current between anode and cathode and TiCl was fed through feed pipe into the interior of the cathode box, that is, into the catholyte. TiCl, feed for the first hour was at 8 pounds/hour and for the next two hours was 10 pounds/hour. At the end of the third hour, TiCl feed was shut off entirely for two hours. This five hours constituted the initial period of the production cycle.

Then the TiCl feed was started again at a rate of 10 pounds/hour and this rate slowly increased as checks of the by-product chlorine for TiCl showed that it was being substantially all assimilated. At the end of the 26th hour (after first start of TiCL, feed) the feed rate was 29 pounds/hour and steady state operation had been attained.

Steady state operation was continued until the 60th hour at which time TiCL started to show in the byproduct chlorine, indicating that assimilation capability or reduction efiiciency was becoming reduced. The TiCL; feed rate was decreased gradually over the following 3 hours at which time it had dropped to about 15 pounds/hour. TiCL; feed was then shut off while power was kept on for a short while to strip any residual lower chlorides from the catholyte. Power was then shut off and the cathode box removed and cooled in an inert atmosphere. The titanium metal product largely in the form of pure, 'well-formed crystals was recovered from the cathode box.

The run described above resulted in production of 240 pounds of titanium metal of +20 mesh size having Brinell hardness of 134. The assimilation and utilization of the TiCl fed to the cell was 97.8%, considered an exceptionally favorable efficiency.

The power input remained substantially constant during the entire cycle at between 11,500 and 12,000 amperes. Calculating the power into a faraday per mol of TiCl base, it is seen that during the initial period, TiCL, was fed to provide ratios of 23 for the first hour, 18 for the next two hours and an infinitely high ratio for the next two hours when the TiCL; feed was shut off. The average for the five-hour initial period which amounted to 7.99 of the total 63-hour cycle, was 33 faradays per mol and the lowest ratio was during the second and third hours at 18 faradays per mol.

After the initial period TiCl feed was started at a ratio of 18 until steady state operation at 6.4 faradays per mol was attained and continued during this period. At the end of the 63-hour cycle, the ratio had increased to 12 faradays per mol at which point the TiCl feed was shut off.

The advantages of the process of this invention in which the feed rate of TiCL; is programmed during the initial period and subsequently during the remainder of the operating cycle will be appreciated by comparison of the results obtained in Example 1 with another experiment conducted in the same equipment and in which the TiCli, feed was started at an 8-pound/hour rate and then increased gradually as rapidly as possible. The electric current input started at about 11,000 amperes but during the greater part of the run was within the same range as in Example 1, that is between 11,500 and 12,000 amperes. The fed rate of TiCl was increased from the starting rate of 8 pounds/hour until at the end of 24 hours as assimilation of 28 to 29 pounds/hour was attained. However, this steady state operation could only be maintained for 10 hours before it was necessary to start reducing the TiCL; feed rate to maintain assimilation. Later in the run the feed rate was gradually increased from the 20 pounds/hour level to 29 pounds/ hour but this could not be maintained because appreciable TiCl evolution from the cell started and the feed rate had to be reduced to 18 to 20 pounds/hour. The run was continued at a lower feed rate to a point when only about pounds/hour was being assimilated. The total time to reach the same total amount of TiCl fed in the run of Example '1 required 75.5 hours of T iCl feed time whereas only 63 hours was required in the run of Example 1. The assimilation was only 89% of the feed compared to almost 100% in the run of Example 1. The yield of mesh metal was 240 pounds or 55.5% of the theoretical, and its average Brinell hardness was 16 5. Thus, it will be seen that with a TiCl feed program following the normal assimilation capability of the cell, the assimilation efiiciency was lower, the yield efficiency was lower, and the purity of the metal indicated by Brinell hardness was lower in the test run described in Example 1 and conducted according to this invention.

To ascertain the assimilation of TiCl in the cell it is most convenient to determine the presence of TiCL; in the by-product chlorine as previously described. This may readily be accomplished and with sufficient accuracy for the purpose by a simple condensation and weighing procedure. The chlorine by-product will, when it comes from the cell, be hot, being generated in the fused halide electrolyte. Its TiCl content, if any, will be as vapor mixed with gaseous chlorine. The chlorine is transferred from the cell directly to a cooled container connected by flexible lines and placed on a weigh scale. TiCl, will condense at 136 C. and remains as liquid in the container while the chlorine passes through to be collected for other use, or neutralized and discarded, or disposed of otherwise. The contents of the cooled container can be weighed periodically to determine the amount of TiCL; collected.

The TiCl feed rate control during the feed increase and steady state periods is arranged so that no appreciable amount of TiCl is found in the by-product chlorine. Clearly, once TiCl is detected in the by-product chlorine, then the TiCl feed rate is greater than the ideal maximum. However, in practical operation, the ideal maximum is not generally attainable and therefore most convenient control is obtained when some insignificant amount of TiCl; is present in the by-product chlorine but this amount is not appreciable and by this is meant not enough to significantly affect the cell operation and TiCL, efficiency and percentwise, not more than a percent or two and ordinarily less than three percent of the TiCl being fed. Once the operating characteristics of a particular cell have been established as by pilot runs based on careful control by reference to TiCl in by-product chlorine, then for following runs a program for TiCL; feed in pounds/ hour based on the pilot runs may be followed to insure continuing operation according to this invention and realization of the beneficial results therefrom.

The process of this invention is characterized by a relatively long period of eflicient steady state operation and a relatively rapid drop in assimilation capability at the end of a cycle. This contrasts sharply with operation in which TiCl is continually fed at an increasing rate from the beginning of the cycle and which results in only a short period of steady state operation and a relatively long period of reducing or reduced assimilation capability and inefficient operation.

Since the decrease in assimilation capability is relatively rapid once steady state operation cannot be maintained, it may be desirable, for operating economics and cell turn around, to discontinue TiCl addition and end the cycle at this point. Additional production can, however, be obtained by following TiCl feed down with the reducing assimilation capability until a current to TiCl feed rate of about 9 to 14 faradays per mol. of TiCl is reached. It will not generally be found efficient or economical to continue TiCl addition beyond this point since the general efficiency is appreciably reduced and the time required for deposition of additional metal weight is inordinately long.

I claim:

1. In a process in which titanium metal is produced by electrolysis of a titanium halide in a cell having an anode and a cathode characterized by a perforated surface facing said anode, the said perforated cathode surface separating a fused halide salt electrolyte in said cell into a catholyte and an anolyte, the improvements comprising;

(a) passing electric current through said cell between said anode and said cathode, meanwhile,

(b) during an initial period introducing TiCl into said catholyte at a rate to provide a current to TiCl ratio at all times greater than 9 faradays per mol of TiCl and for a time suificient to deposit a porous titanium structure on said cathode, and,

(c) subsequently gradually increasing the rate of introduction of TiCl into said catholyte while maintaining a rate just below a rate which would cause the presence of an appreciable amount of TiCl in the chlorine by-product from said cell, until a steady state condition of operation is attained during which TiCL; is introduced at a rate to provide a current to TiCl ratio of from 4.5 to 7 faradays per mol of TiCl 2. In a process in which titanium metal is produced by electrolysis of a titanium halide in a cell having an anode and a cathode characterized by a perforated surface facing said anode, the said perforated cathode surface separating a fused halide salt electrolyte in said cell into a catholyte and an anolyte, the improvements comprising:

(a) passing electric current through said cell between said anode and said cathode, meanwhile,

(h) during an initial period introducing TiCL; into said catholyte at a rate to provide a current to TiCL; ratio at all times greater than 14 faradays per mol of TiCl and for a time sufficient to deposit a porous titanium structure on said cathode, and,

(c) subsequently gradually increasing the rate of introduction of TiCl into said catholyte while maintaining 21 rate just below a rate which would cause the presence of an appreciable amount of TiCl in the chlorine by-produ-ct from said cell, until a steady state condition of operation is attained during which TiCl is introduced at a rate to provide a current to TiCl ratio of from 4.5 to 7 faradays per mol of TiCl 3. In a process in which titanium metal is produced by electrolysis of a titanium halide in a cell having an anode and a cathode characterized by a perforated surface facing said anode, the said perforated cathode surface separating a fused halide salt electrolyte in. said cell into a catholyte and an anolyte, the improvements comprising;

(a) passing electric current through said cell between said anode and said cathode, meanwhile,

(b) during an initial period constituting between and of the total cycle time of TiCL, feed to said cell, introducing TiCl into said catholyte at a rate to provide a current to TiCl, ratio at all times greater than 9 faradays per mol of TiCl and,

(c) subsequently gradually increasing the rate of 1ntroduction of TiCL, into said catholyte while maintaining a rate just below a rate which would cause the presence of an appreciable amount of TiCl in the chlorine byproduct from said cell, until a steady state condition of operation is attained during which TiCl is introduced at a rate to provide a current to TiCL, ratio of from 4.5 to 7 faradays per mol of TiCl 4. In a process in which titanium metal is produced by electrolysis of a titanium halide in a cell having an anode and a cathode characterized by a perforated surface facing said anode, the said perforated cathode surface separating a fused halide salt electrolyte in said cell into a catholyte and an anolyte, the improvements comprising;

(a) passing electric current through said cell between said anode and said cathode, meanwhile,

(b) during an initial period constituting between 5 and 10% of the total cycle time of TiCl feed to said cell, introducing TiCl into said catholyte at a rate to provide an average current to TiCl ratio of between and faradays per mol of TiCl and at all times greater than 9 faradays per mol of TiCl and,

(c) subsequently gradually increasing the rate of introduction of TiCL, into said catholyte while maintaining a rate just below a rate which would cause the presence of an appreciable amount of TiCl in the chlorine by-product from said cell, until a steady state condition of operation is attained during which TiCL; is introduced at a rate to provide a current to TiCL, ratio of from 4.5 to 7 faradays per mol of TiCl 5. In a process in which titanium metal is produced by electrolysis of a titanium halide in a cell having an anode and a cathode characterized by a perforated surface facing said anode, the said perforated cathode surface separating a fused halide salt electrolyte in said cell into a catholyte and an anolyte, the improvements comprising;

(a) passing electric current through said cell between said anode and said cathode, meanwhile,

(b) during an initial period constituting between 5 and 10% of the total cycle time of TiCL, feed to said cell, introducing TiCL; into said catholyte at at a rate to provide a current to TiCl ratio of between 14 and 30 faradays per mol of TiCl for the first 50 to 70% of said initial period and discontinuing introduction of TiCl during the remainder of said initial period, and,

(c) subsequently gradually increasing the rate of introduction of TiCl into said catholyte while maintaining a rate just below a rate which would cause the presence of an appreciable amount of TiCl in the chlorine by-product from said cell, until a steady state condition of operation is attained during which TiCL; is introduced at a rate to provide a current to TiCl ratio of from 4.5 to 7 faradays per mol of TiCl 6. In a process in which titanium metal is produced by electrolysis of a titanium halide in a cell having an anode and a cathode characterized by a perforated surface facing said anode, the said perforated cathode surface separating a fused halide salt electrolyte in said cell into a catholyte and an anolyte, the improvements comprising;

(a) passing electric current through said cell between said anode and said cathode, meanwhile,

(b) during an initial period introducing TiCl into said catholyte at a rate to provide a current to TiCL, ratio at all times greater than 9 faradays per mol of TiCl and for a time sufficient to deposit a porous titanium structure on said cathode,

(c) subsequently gradually increasing the rate of introduction of TiCl into said catholyte while maintaining a rate just below a rate which would cause the presence of an appreciable amount of TiCll, in the chlorine by-product from said cell, until a steady state condition of operation is attained during which TiCl is introduced at a rate to provide a current to TiCl ratio of from 4.5 to 7 faradays per mol of TiCl and,

(d) continuing introduction of TiCL, into said catholyte at said steady state condition of operation until assimilation capability of TiCL; is reduced causing the presence of an appreciable amount of TiCL, in the chlorine by-product from said cell, then,

(e) discontinuing TiCL, introduction into said catholyte.

7. In a process in which titanium metal is produced by electrolysis of a titanium halide in a cell having an anode and a cathode characterized by a perforated surface facing said anode, the said perforated cathode surface separating a fused halide salt electrolyte in said cell into a catholyte and an anolyte, the improvements comprising;

(a) passing electric current through said cell between said anode and said cathode, meanwhile,

(b) during an initial period introducing TiCL, into said catholyte at a rate to provide a current to TiCL; ratio at all times greater than 9 f-aradays per mol of TiCl and for a time sufficient to deposit a porous titanium structure on said cathode,

(c) subsequently gradually increasing the rate of introduction of TiC1 into said catholyte while maintaining a rate just below a rate which would cause the presence of an appreciable amount of TiCl in the chlorine by-product from said cell, until a steady state condition of operation is attained during which TiCl is introduced at a rate to provide a current to TiCL, ratio of from 4.5 to 7 faradays per mol of TiCl and,

(d) continuing introduction of TiCl into said catholyte at said steady state condition of operation until assimilation capability of TiCL, is reduced causing the presence of an appreciable amount of TiCL; in the chlorine by-product from said cell, then,

(e) gradually reducing the rate of TiCl introduction into said catholyte while maintaining a rate just below a rate which would cause the presence of an appreciable amount of TiCl in the chlorine byproduct from said cell until the current to TiCl ratio has been increased to between 9 and 14 fara days per mol of TiCl and then,

(f) discontinuing introduction of TiCl into said catholyte.

Swanstrorn, et a1. 20464 Reimert et al. 204-64 Opie et a1. 204--64 Barnett 20464 3/1961 Reimert et al. 204--64 3/1963 Reimert LT 20464 FOREIGN PATENTS 6/1959 Canada.

JOHN H. MACK, Primary Examiner.

G. IQXPLAN, Assistant Examiner.

Claims (1)

1. IN A PROCESS IN WHICH TITANIUM METAL IS PRODUCED BY ELECTROLYSIS OF A TITANIUM HAIDE IN A CELL HAING AN ANODE AND A CATHODE CHARACTERIZED BY A PERFORATED SURFACE FACING SAID ANODE, THE SAID PERFORATED CATHODE SURFACE SEPARATING A FUSED HALIDE SALT ELECTROLYTE IN SAID CELL INTO A CATHOLYTE AND AN ANOLYTE, THE IMPROVEMENTS COMPRISING; (A) PASSING ELECTRIC CURRENT THROUGH SAID CELL BETWEEN SAID ANODE AND SAID CATHODE, MEANWHILE, (B) DURING AN INITIAL PERIOD INTRODUCING TICL4 INTO SAID CATHOLYTE AT A RATE TO PROVIDE A CURRENT TO TICL4 RATIO AT ALL TIMES GREATER THAN 9 FARADAYS PER MOL OF TICL4, AND FOR A TIME SUFFICIENT TO DEPOSIT A POROUS TITANIUM STRUCTURE ON SAID CATHODE, AND, (C) SUBSEQUENTLY GRADUALLY INCREASING THE RATE OF INTRODUCTION OF TICL4 INTO SAID CATHOLYTE WHILE MAINTAINING A RATE JUST BELOW A RATE WHICH WOULD CAUSE THE PRESENCE OF AN APPRECIABLE AMOUNT OF TICL4 IN THE CHLORINE BY-PRODUCT FROM SAID CELL, UNTIL A STEADY STATE CONDDITION OF OPERATION IS ATTAINED DURING WHICH TICL4 IS INTRODUCED AT A RATE TO PROVIDE A CURRENT TO TICL4 RATIO OF FROM 4.5 TO 7 FARADAYS PER MOL OF TICL4.