US2893935A - Electrolytic process for producing metallic titanium - Google Patents

Description

aired States ELECTROLYTIC PROCESS FOR PRODUCING METALLIC TITANIUM lral B. Johns, Marblehead, Mass, assignorto Monsanto No Drawing. Application November 18, 1955 Serial No. 547,824

2 Claims. (Cl. 2 04-64) This inventionrelates to an electrolytic process for producing metallic titanium. More particularly, the invention relates to an electrolytic process in which titanium tetrachloride is used as a raw material feed to the electrolysis cell.

It has long been recognized that titanium tetrachloride is a very desirable raw material for production of titanium because it is one of the few titanium compounds which can be readily obtained in highly 'pure'form without con tamination by deleterious impurities such as oxygen, nitrogen, carbon, etc. In the past, considerable efiort has been directed toward the electrolysis of titanium tetrachloride in both aqueous and anhydrous solvents, both organic and inorganic. However, titanium tetrachloride itself is electrically non-conducting, and there has been considerable difiiculty in finding a solvent in which the tetrachloride could be electrolyzed. Attempts to electrolyze titanium tetrachloride in fused inorganic halide salts have been described, but these attempts generally result in electrolysis of alkali or alkaline earth metal from the fused salt bath, with subsequent direct chemical reduction of titanium tetrachloride by the free alkali or alkaline earth metal.

Until the present time, one of the most satisfactory substitutes for electrolysis of titanium tetrachloride has involved the partial reduction of titanium tetrachloride to a reduced titanium chloride, e.g., titanium dichloride and/ or titanium trichloride, both of which are ionic salts soluble in alkali or alkaline earth metal halides (e.g., NaCl, KCl, CaCl and which can be electrolyzed to give deposits of elemental titanium upon the cathode. However, processes involving the electrolysis of lower valence titanium have the very marked disadvantage that unless elaborate precautions are taken to separate the anode and cathode regions (such as by a porous diaphragm), divalent titanium can be oxidized to trivalent titanium at the anode just as fast as trivalent titanium is reduced to divalent titanium at the cathode-so that the net cell efficiency with respect to production of titanium and chlorine may approach or even reach zero. Thus, there is still a great demand for a good electrolytic process utilizing titanium tetrachloride as a raw materialand it is an object of the present invention to provide such a process.

It has now been found that titanium tetrachloride can be dissolved in a molten mixture of alkali and alkaline earth halides which contain at least about 0.6 atom percent of fluoride and that the resulting solution can be readily electrolyzed, without resort to diaphragms or analogous expedients, to produce a metallic deposit of titanium upon the cathode and to evolve gaseous chlorine at the anode. The process is readily adaptable to semicontinuous operation in that (a) titanium tetrachloride easily dissolves in the electrolyte solution merely by bubbling the tetrachloride vapors through the fluoridecontaining bath and (b) titanium and chlorine are produced at the electrodes at rates equivalent to the introduction of titanium tetrachloride so that the bath comatent position remains constant. The process is only kept from calcium, magnesium, barium or strontium). The bath must contain at least about 0.6 atom percent of fluoride, anid preferably more than about 2 atom percent of fluor1 e.

The chloride concentration should be greater than about 10 atom percent, and preferably above 30 atom percent. The result of having too low a chloride ion concentration will have no substantial adverse effect upon deposition of titanium, but will mean that gaseous fluorine (or iodine or bromine if those ions are present in high concentrations) will be evolved at the anode in addition to (or even instead of) gaseous chlorine. However, if the process is operated in a continuous or semicontinuous fashion (as mentioned above) any initial excess of halide other than chloride will correct itself, since the combined effect of gaseous evolution of excess halogen (other than chlorine) and addition of chloride (as titanium tetrachloride) will increase the chloride concentration to a level where only the gaseous chlorine is evolved at the anode.

In general, it is preferred that the foregoing fused electrolyte bath conta'in only chlorides in addition to the necessary fluoride. However, minor proportions of the chlorides can be replaced with bromides or iodides with no substantial deleterious effect. The alkali metal (particularly sodium and potassium) halides are preferred materials, particularly when used in combination with each other.

It should be kept in mind that titanium must be in the bath and adjacent to the cathode before electrolysis of titanium can take place. If electrolysis is started before titanium tetrachloride is introduced to the bath, then the alkali or alkaline earth metal halides of the bath will be electrolyzed, releasing free alkali or alkaline earth metals. Subsequent introduction of titanium tetrachloride will result in chemical reduction of the tetrachloride by the free alkali or alkaline earth metal in the bath with substantially no electrolysis of titanium at the cathode. Thus, in initiating the electrolysis of the present invention it is recommended that introduction of titanium tetrachloride be started and that the concentration of dissolved titanium be 'brought up to at least about 0.1 atom percent before initiating electrolysis. It is also recommended that the concentration of dissolved titanium be maintained at be tween about 0.1 atom percent and about 6 atom percent (preferably between about 0.3 and 4 atom percent) during the electrolysis. During the electrolysis, the relative ratio of tetrachloride addition and current flow must be regulated to maintain the atomic ratio of fluoride to tita nium dissolved in the bath substantially in excess of 6. This ratio is preferably maintained between about 6.5 and about 100. Lower ratios will give substantially decreased solubility of TiCl, in the bath and will result in loss of gaseous TiCl along with the gaseous product chlorine.

Other operating details with respect to such things as voltage, current and temperature will depend upon factors such as cell construction, the size, shape and spacing of electrodes, electrolyte composition, etc., and can be varied in accordance with principles well known in the electrolysis art. In general, however, the voltage externally impressed across the cell will be considerably in excess of 1.7 volts, the approximate deposition voltage 3 of titanium in this electrolyte system. A suitable cathode can be made from metallic titanium-a suitable anode from graphite.

The temperature at which the cell should be operated according to the present invention will depend to a large extent upon the particular combination of electrolyte bath components utilized. As a minimum, the temperature must be high enough to maintain the electrolyte bath in the molten phase. The solubility of titanium tetrachlo ride in the electrolyte bath decreases with increasing temperature, so it will generally not be advantageous to operate the cell at a temperature more than about one or two hundred degrees C. higher thannecessary. to keep the electrolyte molten.

Since metallic titanium is so susceptible to contamination 'by impurities such as oxygen and nitrogen, it isim portant that these elements or compounds thereof be excluded from the electrolyte bath and the cathode when either one are at elevated temperatures. This precaution is conveniently eflected by carrying out the process in a closed cell in which the electrolyte and electrodes are shielded by an inert atmosphere of, for example, argon,

neon, krypton, or the like. When the cathode is removed from the cell for recovery of metallic titanium, the titanium should bev kept shielded from oxygen, nitrogen,

water vapor, etc., until it has been cooled to a temperature' at which titanium is not so highly reactive to these impurities.

The following example is set forth as illustrative of the practice of the present invention.

Example A mixture of about 465 grams of potassium fluoride and 772 grams of sodium chloride was charged to a graphite crucible in an enclosed electrolysis cell. After evacuating the cell and refilling it with an atmosphere of argon, it was heated to 750 C. When the salt mixture through the cell. Gaseous chlorine was evolved at the anode during the electrolysis. After the electrolysis was completed, the cell was opened and a granular adherent deposit of metallic titanium was found on the cathode.

I claim:

1. A method for electrolytically producing Ti from TiCl which method comprises passing TiCL; as the sole gaseous reactant into a molten salt bath comprising a mixture of salts selected from the group consisting of alkali metal and alkaline earth metal halides containing at least about 10 atom percent of chloride and at least 0.6 atom percent of fluoride while passing a direct current between an anode and a, cathode immersed in said bath, the relative rates of current and TiCl introduction being controlled .to maintain the dissolved titanium concentration between about 0.1 and about 6 atom percent and to maintain the atomic ratio of fluoride to titanium dissolved in said bath substantially in excess of 6.

2. A method for electrolytically producing Ti from TiCl which method comprises continuously passing TiCl as the sole gaseous reactant into a molten salt bath comprising a mixture of salts selected from the group consisting of alkali metal and alkaline earth metal halides containnig at least about 30 atom percent of chloride and at least about 2 atom percent of fluoride while passing a direct current between an anode and a cathode immersed :in, said bath, the relative rates of current and TiCL, in-

troducion being controlled to maintain the dissolved titanium concentration between about 0.3 and about 4 atom percent and to maintain the atomic ratio of fluoride to titanium dissolved in said bath substantially in excess of 6.

References Cited in the file of this patent UNITED STATES PATENTS 2,722,510 Schaefer Nov. 1, 1955 2,731,404 Wainer Jan. 17, 1956 6 2,735,855 Buck et a1 Feb. 14, 1956 2,741,588 Alpert et a1 Apr. 10, 1956 FOREIGN PATENTS 3,859 Japan Sept. 26, 1952 698,151 Great Britain Oct. 7, 1953

Claims (1)

1. A METHOD FOR ELECTROLYTICALLY PRODUCING TI FROM TICL4 WHICH METHOD COMPRISES PASSING TICL4 AS THE SOLE GASEOUS REACTANT INTO MOLTEN SALT BATH COMPRISING A MIXTURE OF SALTS SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AND ALKALINE EARTH METAL HALIDES CONTAINING AT LEAST ABOUT 10 ATOM PERCENT OF CHLORIDE AND AT LEAST 0.6 ATOM PERCENT OF FLUORIDE WHILE PASSING A DIRECT CURRENT BETWEEN AN ANODE AND A CATHODE IMMERSED IN SAID BATH, THE RELATIVE RATES OF CURRENT AND TICL4 INTRODUCTION BEING CONTROLLED TO MAINTAIN THE DISSOLVED TITANIUM CONCONTRATION BETWEEN ABOUT 0.1 AND ABOUT 6 ATOM PERCENT AND TO MAINTAIN THE ATOMIC RATIO OF FLUORIDE TO TITANIUM DISSOLVED IN SAID BATH SUBSTANTIALLY IN EXCESS OF 6.