US2898275A - Production of titanium - Google Patents

Description

Aug. 4, 1959 E. w. ANDREWS 2,898,275

PRODUCTION OF TITANIUM Filed Dec. 1'], 1957 INVENTOR. Earl W. Andrews ATTORNEYS I PRODUCTION OF TITANIUM Application December 17, 1957, Serial No. 703,566

4 Claims. (Cl. 204-64) This invention relates to the production of metallic titanium and, more particularly, to the electrolytic deposition of titanium from a fused salt bath.

In the production of metallic titanium by electrolysis of a fused salt bath in an electrolytic cell containing a dissolved titanium salt, it has been considered to be most important that the bath and its adjacent electrolytic cell atmosphere be as free as possible of oxygen and electrolyzable oxygen-containing substances such as water and oxy-salts in order to prevent oxygen contamination, and therefore hardening, of the electrodeposited titanium. Consequently, special precautions are generally taken to free the bath components from moisture before carrying out the electrodeposition of the titanium, and the cell atmosphere is also flushed constantly with an inert gas such as argon while the electrodeposition proceeds. In addition, it has been considered equally important, after electrodeposition has been completed, to harvest the metallic titanium cathode deposit by withdrawing the cathode from the cell bath up into the inert cell atmosphere so that the deposit can cool in this inert atmosphere while the cell and its contents are cooled to a safe opening-temperature substantially below the solidification point of the bath. This latter procedure has been adopted in order to prevent oxidation of the particles of electrodeposited titanium by their exposure to air while hot.

I have now discovered that the titanium deposit can be harvested while hot without damage to the titanium deposit, and that even the inert cell atmosphere is unnecessary, if an alkali chloride cell bath of certain composition is used for carrying the titanium chloride which is electrolyzed to metal. That is, the titanium deposit can be hot-harvested into the ambient atmosphere if the titanium chloride is electrolyzed to titanium metal in a fused bath composed of lithium and potassium chlorides, with or without sodium chloride, provided that the alkali chloride mixture has a melting point not higher than about 450 C.

Thus, the improvement of the present invention is applicable to the electrolytic deposition of metallic titanium on a cathode immersed in a molten mass of chloride salts composed essentially of (a) at least one titanium chloride in which the titanium has a valence of less than four and (b) a diluent salt mixture having a melting point not substantially higher than 450 C. and comprising potassium chloride and lithium chloride, with or without sodium chloride. The improvement comprises hot harvesting the cathode-deposited metallic titanium by using a diluent salt mixture having a composition within the range of compositions in the area defined by lines AB, BC, CD, DE, EF, PG and GA in the accompanying drawing, removing the cathode with its adhering deposit of metallic titanium and entrained molten salt substantially directly from the molten salt mass into the ambient atmosphere while the molten salt mass is maintained at a temperature not more than about 100 C. above its melting point, and thereafter separating the cathode deposit from the cathode.

The practice of the present invention does not require any special cell design. All of the fused bath cells, including cathodes and anodes, presently useful in carrying out the electrodeposition of titanium in an electrowinning or electrorefining operation may be used. Cells provided with a special chamber for cooling the withdrawn titanium-bearing cathode in an inert atmosphere may nevertheless be used, although the special chamber need not be maintained under an inert atmosphere during the hot harvesting pursuant to the invention.

Hot harvesting of a titanium-bearing cathode by the method of my invention can be practiced in both the electrowinning and electrorefining of titanium. In both types of operation, where a fused chloride salt bath is used, the titanium is carried to the cathode in the form of titanous ions derived from a titanium chloride in which the titanium has a valence of less than four. These titanous chlorides, such as titanium dichloride and titanium trichloride, are soluble in a diluent salt bath composed of alkali metal chlorides and are generally formed in situ in the bath during electrolysis. In the case of electrowinning, the titanium component of the bath is provided by introducing titanium tetrachloride into the bath where it reacts with titanium dichloride to form titanium trichloride and the trichloride is converted to the dichlo ride primarily by electrolytic reduction. In the case of electrorefining, a titanium-bearing anode is used and goes into solution in the diluent salt bath ultimately in the form of a lower valent chloride.

The diluent salt bath composition is important in practicing the invention. That is, the diluent bath must be composed of alkali metal chlorides and must have a melting point not substantially higher than 450 C. Thus, the diluent salt baths useful for the practice of the invention comprise a mixture of at least two of the salts sodium chloride, potassium chloride and lithium chloride in proportions falling within the area defined by the lines AB, BC, CD, DE, EF,'FG and GA in the accompanying drawing. The drawing is a ternary diagram of the salt mixture system NaClKCl-LiCl, and the aforementioned lines define the area of compositions of mixtures having melting points of about 450 C. and lower. The compositions within the defined area have sodium chloride contents ranging from O to 20%, potassium chloride contents ranging from 25 to 50% and lithium chloride contents ranging from 45 to each of these ranges being expressed in molar percentages. Thus, the useful mixtures comprise potassium and lithium chlorides, with or without sodium chloride, having proportions falling within the area A-B-CDEFGA in the ternary diagram for the system of these three salts.

The melting point of the diluent salt mixture is important because it determines the temperature of the cathode deposit as the cathode is withdrawn from the bath for harvesting. It is the presently preferred practice in fused salt electrodeposition of titanium to carry out the electrolysis at a bath temperature about C. higher than the melting point of the bath. Thus, for baths having w a melting point as high as about 450 C., the bath temperature is generally maintained at about 550 C. during electrolysis. It has been found that at temperatures as high as about 550 C., but not substantially in excess of this temperature, a titanium metal deposit on a cathode can be exposed to the ambient atmosphere without significant oxidation. Accordingly, hot harvesting of the cathode deposit pursuant to my invention is carried out at temperatures up to about 550 C. and is made possible by the use of a main bath salt mixture having the aforementioned range of composition and therefore having a melting point not substantially higher than 450 C. By maintaining a difference of about 100 C. between the melting point of the bath and the temperature of the Patented Aug. 4, 1959 bath at the time the cathode is withdrawn, there are obtained not only the usual advantages in cell operation but there is further achieved the desired drainage of molten salt from the hot harvested titanium deposit.

The cathode deposit is thus harvested pursuant to my invention, by drawing the cathode out of the bath and directly into the ambient atmosphere. This can be done either by withdrawing the hot cathode and its deposit completely out of the cell or by withdrawing it only into the upper part of the cell which is above the bath level and which is furthermore open to the ambient atmosphere. In either procedure, the molten salt entrained in the cathode deposit drains away from the deposit so as to leave only a relatively small amount of salt in the cathode deposit. The deposit is advantageously separated from the cathode while the residual entrained salt is still molten, the advantage in this procedure being that the deposit is more readily removed from the oathode while the residual salt is molten than after the residual salt has solidified. The cathode is then returned to the cell and electrolysis is resumed. It has been found that the practice of the invention makes possible sustained electrolytic production of metallic titanium in an electrorefining operation, interrupted only for hot harvesting of the cathode deposit and without maintenance of any protective atmosphere above the molten salt bath, and that it makes possible a similarly sustained operation in the case of electrowinning except that it is advantageous to flush the cell with argon or other inert gas before resuming the electrolysis.

The following example shows how the practice of the method of my invention, characterized by hot harvesting, produces a grade of metallic titanium of at least as high quality as that produced by prior art cold harvesting of the titanium-bearing cathode deposit:

An electrowinning run was made in an 11 inch diameter electrolytic cell using a diluent salt bath consisting of a mixture of 55.0 mol. percent LiCl, 40.0 mol. percent KCl and 5.0 mol. percent NaCl. During the electrolysis, which was conducted at 600 C., the cell was supplied with a continuous feed of titanium tetrachloride. At the end of the electrolysis, after discontinuing the titanium tetrachloride feed and stripping the titanium in the melt by means of continued application of electrolyzing current, the cell was cooled to room temperature with the cathode and its deposit remaining in the melt. An argon atmosphere was maintained above the melt throughout the entire operation.

The cooled melt, with the cathode and its deposit, was removed and leached in cold water to remove the soluble salts. The separated titanium metal granules were dried at 50 C. The dried granules were screened and the +35 mesh fraction was arc-melted in an argon atmosphere. f the total metal in the deposit, 73.8% was +35 mesh. The Brinell hardness of the ingot produced from this fraction was 111.

A second run was then made under conditions identical with those in the first run except that after the discontinuance of the titanium tetrachloride feed and after stripping of the melt and cooling of the melt to 450 C., the cathode with its adhering deposit wasremoved from the cell and was allowed to cool in the ambient air to room temperature.

The cathode deposit from the second run was leached in cold water to remove soluble salts and the titanium metal granules were dried at 50 C. The dried granules were screened and the +35 mesh fraction was arc-melted in an argon atmosphere. Of the total metal in the deposit,.76.5% was +35 mesh. The Brinell hardness of the ingot produced from this fraction was 107.

I claim:

1. In the electrolytic deposition of metallic titanium on a cathode immersed in a molten mass of chloride salts composed essentially of (a) at leastone titanium chloride in which the titanium has a valence less than four and (b) a diluent salt mixture having a melting point not substantially higher than 450 C. and comprising at least two salts of the group consisting of sodium chloride, potassium chloride and lithium chloride, the improvement which comprises hot harvesting the cathode-deposited metallic titanium in the absence of a protective atmosphere by (1) using a diluent salt mixture having a composition within the range of compositions in the area defined by the lines AB, BC, CD, DE, EF, PG and GA in the accompanying drawing, (2) removing the cathode with its adhering deposit of metallic titanium and entrained molten salt substantially directly from the molten salt mass into the ambient atmosphere while the molten salt mass is maintained at a temperature not more than about C. above its melting point, and (3) thereafter separating the cathode deposit from the cathode.

2. In the electrolytic deposition of metallic titanium on a cathode immersed in a molten mass of chloride salts composed essentially of (a) at least one titanium chloride in which the titanium has a valence less than four and (b) a diluent salt mixture having a melting point not substantially higher than 450 C. and comprising at least two salts of the group consisting of sodium chloride, potassium chloride and lithium chloride, the improvement which comprises hot harvesting the cathode-deposited metallic titanium in the absence of a protective atmosphere by (1) using a diluent salt mixture having a composition within the range of compositions in the area defined by the lines AB, BC, CD, DE, EF, PG and GA in the accompanying drawing, (2) removing the cathode with its adhering deposit of metallic titanium and entrained molten salt substantially directly from the molten salt mass into the ambient atmosphere while the molten salt mass is maintained at a temperature not more than about 100 C. above its melting point, and (3) thereafter separating the cathode deposit from the cathode while the entrained salt component of the deposit is still molten.

3. In the electrolytic deposition of metallic titanium on a cathode immersed in a molten mass of chloride salts composed essentially of (a) at least one titanium chloride in which the titanium has a valence less than four and (b) a diluent salt mixture having a melting point not substantially higher than 450 C. and comprising at least two salts of the group consisting of sodium chloride, potassium chloride and lithium chloride, the improvement which comprises not harvesting the cathode-deposited metallic titanium in the absence of a protective atmosphere by (1) using a diluent salt mixture having a composition within the range of compositions in the area defined by the lines AB, BC, CD, DE, EF, PG and GA in the accompanying drawing, (2) removing the cathode with its adhering deposit of metallic titanium and entrained molten salt substantially directly from the molten salt mass into the ambient atmosphere while the molten salt mass is maintained at a temperature not more than about 100 C. above its melting point, (3) thereafter separating the cathode deposit from the cathode, returning the depositfree cathode to the electrolysis operation, and (4) resuming the electrolysis of the bath while exposed to the ambient atmosphere.

4. In the electrolytic deposition of metallic titanium on a cathode immersed in a molten mass of chloride salts composed essentially of titanium dichloride and titanium trichloride in solution in a diluent salt mixture having a melting point not higher than about 450 C. and comprising at least two salts of the group consisting of sodium chloride, potassium chloride and lithium chloride, the improvement which comprises hot harvesting the cathodedeposited metallic titanium in the absence of a protective atmosphere by (1) using a diluent salt mixture having a composition Within the range of compositions in the area defined by the lines AB, BC, CD, DE, EF, FG and GA in the accompanying drawing, (2) removing the cathode with its adhering deposit of metallic titanium and entrained molten salt substantially directly from the molten salt mass into the ambient atmosphere while the molten salt mass is maintained at a temperature not more than about 100 C. above its melting point, and (3) thereafter separating the cathode deposit from the cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,745,802 Schmidt May 15, 1956 6 2,780,593 Snow et a1. -Q. Feb. 5, 1957 2,786,808 Raney Mar. 26, 1957 2,789,943 Kittelberger Apr. 23, 1957 OTHER REFERENCES Canadian Institute of Mining and Metallurgy, Transactions, vol. 49, 1946, pages 521 and 522.

Claims (1)

1. IN THE ELECTROLTIC DEPOSITION OF METALLIC TITANIUM ON A CATHODE IMMERSED IN A MOLTEN MASS OF CHLORIDE SALTS COMPOSED ESSENTIALLY OF (A) AT LEAST ONE TITANIUM CHLORIDE IN WHICH THE TITANIUM HAS A VALENCE LESS THAN FOUR AND (B) A DILUENT SALT MIXTURE HAVING A MELTING POINT NOT SUBSTANTIALLY HIGHER THAN 450* C. AND COMPRISING AT LEAST TWO SALTS OF THE GROUP CONSISTING OF SODIUM CHLORIDE, POTASSIUM CHLORIDE AND LITHIUM CHLORIDE, THE IMPROVEMENT WHICH COMPRISES HOT HARVESTING THE CATHODE-DEPOSITED METALLIC TITANIUM IN THE ABSENCE OF A PROTECTIVE ATMOSPHERE BY (1) USING A DILUENT SALT MIXTURE HAVING A COMPOSITION WITHIN THE RANGE OF COMPOSITIONS IN THE AREA DEFINED BY THE LINES AB, BC, CD, DE, EF, FG AND GA IN THE ACCOMPANYING DRAWING, (2) REMOVING THE CATHODE WITH ITS ADHERING DEPOSIT OF METALLIC TITANIUM AND ENTRAINED MOLETN SALT SUBSTANTIALLY DIRECTLY FROM THE MOLTEN SALT MASS INTO THE AMBIENT ATMOSPHERE WHILE THE MOLTEN SALT MASS IS MAINTAINED AT A TEMPERATURE NOT MORE THAN ABOUT 100* C. ABOVE ITS MELTING POINT, AND (3) THEREAFTER SEPARATING THE CATHODE DEPOSIT FROM THE CATHODE.

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