US2757135A

US2757135A - Electrolytic manufacture of titanium

Info

Publication number: US2757135A
Application number: US318518A
Authority: US
Inventors: Gleave Wilfrid Wilson; Quin James Paterson
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1951-11-23
Filing date: 1952-11-03
Publication date: 1956-07-31
Anticipated expiration: 1973-07-31

Description

United States Patent ELECTROLYTIC MANUFACTURE OF. TI AN M Wilfrid Wilson Gleave and James Paterson Quin, Widues, England, assignors to Imperial; Chemical Industries Limited, a corporation of Great Britain Claims priority, application Great Britain November 23, 1951 6 Claims- (cl. 20441.1)

This inv n n relat s to the manu act e o me l and more particularly to an electrolytic process for'the manufacture of metallic titanium.

It is known that metallic titanium may be produced by the electrolysis of titanium dioxide either in sohp tion in a fused salt such as anhydrous. calcium chloride or as a suspension therein. By this method the titanium so produced is usually in the form of a fine powder wln'ch is disseminated throughout. the fused salt melt and cannot be removed by a continuous Operation. The electrolysis is therefore carried out batchwise and when a sufiicient quantity of the titanium has been produced in the melt the usual procedure is to stop the electrolysis and allow the contents of the cell to cool and solidify. The titani m is thereafter recovered by leaching with water whereby the soluble salt. is. extracted and the titanium powder is isolated.

We have now found that when metallic titanium is produced by the electrolysis of a titanium compound in a fused salt the process may be carried out continuoulsy if there is provided in contact with the said fused salt a liquid metal in which the titanium is soluble at the temperature of electrolysis. f

According to the present invention, therefore, a process for the manufacture of titanium comprises electrolysing a titanium compound dissolved or suspended in a fused salt in the presence of a molten metal in which the titanium is soluble at the temperature of electrolysis, withdrawing the alloy of titanium and said molten metal so formed during the electrolysis and thereafter recovering the titanium from the said alloy.

Advantageously the molten metal that is introduced to dissolve the titanium as it is formed is itself used as the cathode of the electrolytic cell.

The range of temperature within which the electrolysis is carried out will depend on a number of factors such as the freezing and boiling points of the fused salt, the melting and boiling points of the cathode metal and the solubility of titanium in the cathode metal. Thus the temperature must be such that the fused salt, the cathode metal and the alloy formed by the titanium and the cathode metal during the electrolysis are all in a molten condition, while at the same time the temperature must be low enough to avoid undue volatilisation of the salt and the cathode metal. Moreover the temperature must be high enough to ensure that the titanium has an appreciable solubility in the cathode metal.

In the case of the electrolysis of titanium dioxide in calcium chloride using a molten zinc cathode we prefer to conduct the operation at a temperature of approximately 850 C. while if lead is employed as the molten cathode using the same materials in the electrolyte a temperature of approximately 1000 C. is preferred.

In general it is desirable to operate at as low a temperature as is consistent with satisfactory electrolysis in order to avoid the excessive repairs and maintenance earth titanates or 2,757,135 Patented July 31, 1956 to the cell which are invariably involved in operating electrolytic processes at a high temperature.

A number of compounds of titanium may be employed as for example, titanium halides, or alkali or alkaline mixtures of these but advantageously we use titanium s ubchlorides or titanium dioxide, primarily on account of their cheapness and their solubility in many fused salts.

Chemically pure titanium dioxide is preferred. Alternatively a selected high grade natural rutile, with a titanium oxide content of over 98% and substantially free from silica and other objectionable impurities may be employed. Rutile of this quality may be obtained from certain t ally wa r n beach s nd b known physical separation methods.

The preferred fused salt in the cell may consist of any of the alkaline earth metal halides or mixtures of the same, and one suitable halide for this purpose is calcium chloride, but a number of other salts may be employed such as, for example, alkali metal fluotitanates and salt mixtures containing them. The preferred melt, however, which must be anhydrous, is one in which the titanium compound is soluble but it must be understood that our invention is not limited to such melts. Fused salts may be employed which on electrolysis produce a metal which will reduce to metallic titanium a suspension of a titanium compound, such as suspended titanium dioxide.

The liquid metal cathode may consist of any of the metals, zinc, cadmium and lead or mixtures of two or more ofthem but we prefer to use zinc on account of its relative cheapness and this metal is easily separable from the. dissolved titanium.

In cases where the titanium is formed by reduction of the titanium dioxide with a metal which is a primary product .of the electrolysis and the metallic oxide thus formed is itself insoluble in the melt, it will then be necessary to circulate the melt withdrawing a portion of it from the cell, removing oxide either physically, as by filtration or chemically, as by reaction with chlorine and then returning the treated electrolyte to the cell. If chlorine is evolved in the cell, as is the case Where the melt consists .of an alkaline earth .chloride, this may be employed to reconvert the metallic oxide to the chloride. In those cases in which the metallic oxide is soluble in the melt, rechlorination to some extent is likely to occur in the cell but it may still be necessary to complete the process by circulation of the melt for further reaction with chlorine.

In carrying out the method of our invention electrolysis is continued until the concentration of titanium in the liquid metal cathode has reached a predetermined level and provision may therefore be made for samples of the liquid metal cathode to be withdrawn from the cell from time to time and an analysis made of their titanium concentration. Advantageously we arrange that the concentration is so limited that no solid complex alloys of titanium are formed in the cell and to ensure this We prefer when employing zinc as the liquid cathode to commence removal of the zinc/titanium alloy from the cell when the concentration of titanium therein has attained a value of 23% by weight of the mixture. At this stage the alloy may be tapped ofl? from the cell through a liquid metal seal in order to avoid contamination by the melt or suspended dioxide and the liquid may be filtered to remove suspended impurities such as the dioxide, carbide or nitride of titanium.

The alloy of titanium and the cathode metal in the liquid of solid state may then be distilled in vacuo at a temperature which volatilises the cathode metal and at the same time sinters the titanium residue so that the latter is not pyrophoric in air. The volatilised metal permitted may conveniently be returned to the cell to take further part in the electrolysis. When zinc is employed as the cathode metal we have found that temperatures of about 800 C. are satisfactory.

Alternatively the liquid alloy may be cooled to a temperature below that at which the cell operates but the temperature to which it is cooled must be sufliciently above the melting point of the cathode metal to avoid solidification; at the same time it must be sufficiently low to ensure precipitation of substantial amounts of the dissolved titanium either as pure metal or as a complex with the cathode metal. The precipitated solid phase, rich in titanurn, may be separated from the cathode metal by filtration and the filtrate returned to the cell for further enrichment by electrolysis. The filtered material may then be distilled in vacuo at temperatures which volatilise the cathode metal and at the same time sinter the titanium residue so that the latter is not pyrophoric in air.

The following example illustrates but does not limit the invention:

Example The electrolysis was carried out in a cell which consisted of a mild steel shell lined with alumina, with a bath of molten anhydrous calcium chloride. The cathode was formed by a pool of molten zinc on the cell bottom, and facilities were provided for tapping ofl this metal from time to time as required. An adjustable graphite rod, set directly above the cathode pool, was

used as anode. During the electrolysis, which was carried out at a temperature of about 800 C., pure titanium dioxide in the form of a fine dry powder was added slowly and continuously to the bath and titanium metal was deposited in the molten zinc cathode which was I stirred from time to time. When the titanium content of the cathode metal had reached about 2.5%, the enriched alloy was tapped off from the cell from time to time and the cathode pool replenished with a further addition of pure zinc. The tapped metal alloy was to cool under a protective coating of calcium chloride and then distilled under reduced pressure to recover the zinc and the titanium and the recovered zinc was returned to the cell when required.

The titanium metal obtained after the removal of the r zinc from the alloy by distillation consisted of a sintered sponge which after arc melting was found to have a hardness of 250 V. P. N. and contained less than 0.01% zinc. The titanium content exceeded 99.5%.

What we claim is:

1. A process for the manufacture of titanium which comprises electrolysing titanium dioxide in a fused salt selected from the group consisting of halogen compounds of alkaline earth and alkali metals and in the presence of a molten metal as the sole cathode material in which the titanium is soluble at the temperature of electrolysis, withdrawing the alloy of titanium and said molten metal so formed during the electrolysis and thereafter recovering the titanium from the said alloy.

2. A process according to claim 1 in which the solvent molten metal is selected from the group consisting of zinc, cadmium and lead and mixtures thereof.

3. A process according to claim 1 in which electrolysis is carried out at a temperature above the melting point of the solution of titanium in the solvent molten metal.

4. A process according to claim 1 in which the electrolysis is continued until the concentration of titanium in the molten metal attains a predetermined level which is such that no solid complex alloys of titanium are formed.

5. A process according to claim 1 in which the solvent molten metal is zinc and the electrolysis is continued until the concentration of titanium in the molten zinc has attained a value of from 2% to 3% by weight of the titanium-zinc alloy.

6. A process according to claim 5 in which electrolysis is carried out at a temperature above the melting point of the solution of titanium in the solvent molten metal.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Journal of Applied Chemistry (U. S. S. R.), vol. 13, (1940) pages 51 thru 55, article by Sklarenko et a1.

Chemical Engineering, December 1951, vol. 58, pages 103 and 104.