US2598777A - Recovering gallium from metallic aluminum - Google Patents

Description

Patented June 3, 1952 RECOVERING GALLIUM FROM METALLIC ALUMINUM Francis 0. Frary, Oakmont, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa.,

a corporation of Pennsylvania No Drawing. 7 Application January 23, 1948,

' Serial No. 4,062

3 Claims.

This invention relates to the production of gallium, and relates particularly to recovering that metal from metallic aluminum.

Electrolytically reduced aluminum normally contains a trace of gallium, in addition to minor amounts of other impurities (chiefly iron and silicon) and any alloying components which may have been added. Apparently the presence of gallium results from the fact that the aluminum is produced from alumina made from bauxite which contains gallium. The gallium is not entirely separated from the aluminum during extraction of the aluminum oxide from the bauxite in the customary Bayer process, nor during subsequent electrolytic reduction of the alumina to aluminum. The metallic aluminum contains only traces of gallium, usually no more than 0.02 per cent. No commercially feasible process has heretofore been available for recovering the gallium from the metallic aluminum. It is the principal object of this invention to provide a practical method of recovering gallium from aluminum.'

As is known in the art and disclosed in such U. S. patents as 673,364, 1,534,317 to 1,534,320, inclusive, and 1,535,458, aluminum containing impurities can be refined electrolytically (as distinguished from being reduced electrolytically) by employing it in the molten metal anode layer of an electrolytic cell in which the anode layer is separated from a cathode by a fused salt bath or electrolyte, and aluminum is electrolytically dissolved from the molten anode .layer and deposited on the cathode, leaving impurities behind in the anode layer. I have found that, in spite of its close chemical resemblance to aluminum, gallium is one of the impurities which remains in the anode layer, and that the amount of gallium in that layer gradually increases as more and more aluminum is refined in the cell.

Consequently, continued operation of the refining cell involving replenishment of the aluminum of the molten anode metal gradually increases the gallium' concentration of the anode metal. The gallium in the anode metal can reach a concentration many times that of gallium in the aluminum introduced into the cell before the other impurities deposited in the anode layer increase to a point where the anode metal is no longer suitable for further use in the cell and must be replaced. In fact, to the best of my knowledge, it is the effect of concentration of other impurities in the anode layer upon the quality of the refined aluminum that is the practical limiting factor upon the concentration of gallium in the anode layer. It will, of course, be understood that replenishment of the aluminum content of the anode layer may be either continuous or intermittent, and for want of a better phrase, I shall refer to such replenishment as successive additions of aluminum. The important fact is that the larger the amount of galliumbearing aluminum refined in the cell by passage of aluminum from the anode layer to the cathode, the greater will be the gallium concentration created in the anode layer. 1

The composition of the resulting anode alloy depends on such factors as the original composition of the molten anode layer, the various impurities or alloying ingredients present in the metal which "has been refined in the cell, the amount of metal that has been refined, and the extent to which aluminum has been electrolyzed out of the alloy. As is brought out in the patents mentioned above, during operation of the cell the composition of the anode layer should be such that that layer will remain below the fused salt layer, and will adequately mobile under the temperatureand current conditions maintained in the cell. The most practical anode layer is an aluminum-copper alloy such as is disclosed in Patent 1,534,317.

During operation of the cell, such impurities as iron and silicon, which are normally present in small amounts in commercial aluminum, are known to accumulate in the anode layer as successive additions of metal are refined in the cell. Consequently, the anode alloy containing the accumulated gallium contains aluminum, impurities separated from the metal refined, and ingredients such as copper which have been employed to provide the anode layer with characteristics desired in that layer during the refining operation. The problem of economically recovering gallium from such an alloy by dissolving it and thereafter extracting gallium from the solution is complicated by the necessity for using a solvent for that purpose which will attack the metal sufficiently rapidly for practical operation and extract the gallium from it, and yet will not at the same time dissolve appreciable amounts of other metals present which would interfere with'subsequent recovery of gallium from the solution.

In carrying out the present invention, gallium is dissolved from the solidified anode alloy of the refining cell by means of an alkali metal hydroxide solution, particularly an aqueous sodium hydroxide or potassium hydroxide 'solution, in spite of the fact that the anode alloy contains substantial'amounts of other metals, such as iron and silicon, which might be expected to form compounds with the gallium that would bestable' against alkali metal hydroxide solutions. Aluminum present in the anode alloy also dissolvesrin the solution, forming alkali metal aluminate, but heavy metals such as copper in the anode alloy do not dissolve readily in the solution. A 5-40 per cent solution of the hydroxide is satisfactory for the purpose, and preterably the solution is kept hot during treatment of the anode alloy. To facilitate extraction of the gallium it is desirable to comminute the solidified anode metal before immersing it in the alkali metal hydroxide solution.

Any convenient procedure for contacting the anode metal with the hydroxide solution can be used. For example, the metal may merely be kept immersed in the solution until the gallium in the alloy is dissolved.- However, as the attack of the solution on the metal slows down, it may be more eflicient to transfer the remaining undissolved metal to a fresh supply of solution; alternatively, the efficiency may be increased by maintaining countercurren-t flow of the anode metal and the alkali metal hydroxide solution.

After the gallium has been dissolved from; the anode alloy by means of the alkali metal hydroxide solution, metallic gallium canbe directly precipitated from the resultant solution by electrolysis. The fact that the gallium can be precipitated directly in metallic form from the solution is important in reducing the cost of the metal. Prior to such precipitation of gallium,

undissolved materials present'may be removed from the solution, and if, desired, dissolved constituents which mightcontaminate the gallium produced, or interfere with precipitation of the gallium (such as lead and zinc), may be precipitated from'thesolution. H

Ordinarily the anode alloy from the aluminumrefining ,cell contains a very much larger prop rtionof aluminum thanof galliu -m at the time of the alkali metal hydroxide solution treatment. Thealuminum also dissolvesreadily in; the alkali metal hydroxide solution, forming alkali 'metal aluminate and thus making a; large part of the alkali metal hydroxide unavailable for dissolving gallium, or for holding galliumin'solution during subsequent evaporation to increase the concentration of gallium in the solution.- Consequently, prior to precipitating gallium from the solution it may in some instances be desirable to precipitate aluminum from the solution as' calcium aluminate by adding to the solution a calcium compound, such as hydrated lime or quick lime, prior to precipitating metallic gallium from the solution. 'That procedure is described in detail in my U. S. patem p ucaugn serial: No, 739,538, filed April 5, 1947, now" Patent No. 2,582,376, issued January 15,; 1-952, and entitled Process'for Producing Meta Little or no g'alliuinis precipitated from the solution along with the calcium aluminate. Thereafter the concentration of gallium in the solution can beincreased to a much greater extent than would otherwise be the 7 case, either by using the solution to dissolve solutio'n'thlls'fb'rmed. "I

As an example of the operation of the invention, metallic aluminum containing small amounts of iron and silicon and about 0.015 per cent of gallium was refined by periodically introducing many successive batches of such metal into the molten anode alloy in an electrolytic refining cell of the general type described in U. S. Patent 1,534,320, in which the anode alloy employed initially contained almost 60 per cent of aluminum, 40 per cent of copper, and a total of less than 0.5 per cent of silicon, iron and other impurities. The fused salt bath layer floating on theanode alloy of the cell was like that described and claimed in U. S. Patent 1,534,317. Ultimately the anode alloy resulting from operation of the cell in the customary manner, in the refining of successive additions of aluminum, contained 40.44- per cent of aluminum, 46.04 per cent of copper, 5.51 per cent of iron, 5.33 per cent of silicon, 0.685 per cent of gallium, and minute amounts of lead, zinc, manganese, titanium, and chromium. The anode alloy was then tapped from the cell and allowed to solidify.

A portion of it having the composition stated above, and ground to pass a 60 mesh screen, was added to a 10 per cent aqueous solution of commercial flake sodium hydroxide containing sodium hydroxide equal in weight to the portion of alloy used. The mixture was" boiled for one hour, after which the solution was filtered to remove undissolved metal remaining. The resultant solution contained about 26 grams per liter of dissolved aluminum, and 0.43 gram per liter of dissolved gallium, which was about 63 per cent of the gallium in the anode alloy used. The solutionwas then evaporated to about 30 per cent of its original volume; and placed in a glass containerprovided with two spaced nickel electrodes extending 'into the solution. This electrolytic cell was then operated at a cathode current density of about 0.51 ampere per square inch and 4 volts, while the electrolyte was main tained at about F. Substantially all of the gallium in the electrolyte was deposited on the cathode as metallic gallium in the course of 28 hours.

I claim: V v

1. The method or recovering gallium from metallic aluminum, comprising adding aluminum containing gallium to a molten metal anode of an electrolytic cell, electrodepositing' aluminum from the said anode on a cathode of the said cell through a molten salt bath layer, whereby gallium in the aluminum added to the said anode accumulates in the anode metal, subsequently dissolving gallium from the resultant anode metal by means of an alkali metal hydroxide; solution after'the gallium content of the anodeme'tal has become higher in such operation of the electrolytic cell than the gallium content of the said aluminum added to the said anode, and thereafter pre- 'after the gallium content of the anode metal has become higher insuch operation ofthe elec- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,534,320 Hoopes Apr. 21, 1925 1,855,455 McCutcheon Apr. 26, 1932 OTHER REFERENCES A Treatise on Chemistry, by H. E. Roscoe and Schorlemmer, vol. 2 (The Metals) 1913, pages The Journal of the American Chemical Society, vol. 40 (1918), July-December, page 1536,

Metals and Metallic Compounds, vol. IV, by U. R. Evans, 1923, pages 195, 196, 206-208.

Information Circular No. 6401 of the U. S. Bureau of Mines, Dept. of Commerce, Nov. 1930, page 4.

Handbook of Chemistry and Physics, 26th edition, pages 380, 381, Chemical Rubber Publish- 15 ing Co. 1942, 1943,

Claims (1)

1. THE METHOD OF RECOVERING GALLIUM FROM METALLIC ALUMINUM, COMPRISING ADDING ALUMINUM CONTAINING GALLIUM TO A MOLTEN METAL ANODE OF AN ELECTROLYTIC CELL, ELECTRODEPOSITING ALUMINUM FROM THE SAID ANODE ON A CATHODE OF THE SAID CELL THROUGH A MOLTEN SALT BATH LAYER, WHEREBY GALLIUM IN THE ALUMINUM ADDED TO THE SAID ANODE ACCUMULATES IN THE ANODE METAL, SUBSEQUENTLY DISSOLVING GALLIUM FROM THE RESULTANT ANODE METAL BY MEANS OF AN ALKALI METAL HYDROXIDE SOLUTION AFTER THE GALLIUM CONTENT OF THE ANODE METAL HAS BECOME HIGHER IN SUCH OPERATION OF THE ELECTROLYTIC CELL THAN THE GALLIUM CONTENT OF THE SAID ALUMINUM ADDED TO THE SAID ANODE, AND THEREAFTER PRECIPITATING SUCH DISSOLVED GALLIUM FROM THE RESULTANT SOLUTION.