US1347189A - Recovery of zinc by electrolysis - Google Patents

Description

R. H. STEVENS.

RECOVERY OF ZINC BY ELECTROLYSIS. APPLICATION FILED NOV. 11. 1919.

1 ,347, 18 9 Patented July 20, 1920.

INVE/VT'OR Roy/2L5 HILL-MAN STEVE/v.5

UNITED STATES PATENT OFFICE.

ROYALE HILLMAN STEVENS, 0F RISDON ROAD, HOBART, TASMANIA, AUSTRALIA, ASSIGNOR T0 ELECTROLYTIC ZINC COMPANY OF AUSTRALASIA PROPRIETARY LIMITED, OF MELBOURNE, VICTORIA, AUSTRALIA.

RECOVERY OF ZINC BY ELECTROLYSIS.

Specification of Letters Patent.

Patented July 20, 1920.

Application filed November 11, 1919. Serial No. 337,320.

To all whom it may concern:

Be it known that I, ROYALE HILLMAN S'rnvENs, a citizen of the United States of America, residin at care of Electrolytic Zinc Company of Australasia Proprietary Limited, of ltisdon Road, Hobart, in the State of Tasmania, Commonwealth of Australia, have invented certain new and useful Improvements in the Recovery of Zinc by Electrolysis, of which the following is a specification.

This invention relates to certain improvements in the recovery of zinc by electrolysis and refers more especially to the electrodeposition of zinc from zinc sulfate solutions obtained by leaching ores, concentrates or other zinc bearing materials with sulfuric acid (or spent electrolyte.)

In the recovery of zinc by electrolysis the zinc is deposited on cathodes preferably of aluminium. The deposition is allowed to continue for certain periods,- when the metallic zinc is stripped at required intervals.

These cathodes usually consist of sheets of aluminium which are suspended in the electrolytic cells from copper supporting bars to which the aluminium sheets are riveted, the said copper bars forming electrical contact with the busbars through which the electric current is led to the cells.

These copper protecting bars are liable to corrosion from the acid sprays arising from the cells causing also contamination of the cell solution and are therefore as a rule protected by being covered with sheet lead where they overhang the cell.

The cathode sheets are immersed in the solution within the cells leaving a space of exposed aluminium between the surface of the solution and the supporting bars which surface is found to be subject to corrosion both on account of the creeping upward of the acid solution by capillarity and by the action of the acid sprays arising from the cells during electrolysis sov that the alu minium is found in practice to corrode most rapidly at this part.

The object of this invention is to provide means whereby these exposed surfaces of" the cathodes may be protected from corrosion and thereby prolong their life and usefulness in the electrodeposition of zinc.

I accomplish this object by providing arranged in such a manner that the ordinary deposit obtained from the cell itself is easily stripped along the line of the said solution level from time to time as required.

For the purpose of more clearly illustrating the application of this invention reference may be made to the accompanying sheet of drawings which shows diagrammatically a cathode A suspended in a cell B and supported from bars A. The cathode A is preferably of,sheet aluminium and the supporting bars A are usually of copper adapted to rest upon the busbars B. These supporting bars A are usually sheathed in lead, (as shown) for the purpose of protecting the copper from acid spray and avoiding contamination of the solution with copper salts. The line a-b represents the normal solution level of the cell. In the adaptation of this invention to the practice of the electrodeposition of zinc a protective coating of zinc is applied to the cathode sheets from about the line a--b to just below the edge of the lead sheathing around the supporting bars A-say on the line ef.

In carrying this invention into effect the cathode sheets are first thoroughly cleansed, and scrubbed with sandstone or wire brushes so as to slightly roughen the surface of the upper part of the cathode where the protecting deposit is required to adhere firmly. Two Wooden strips about three quarters of an inch wide are then placed across the face of the sheet and held in place by rubber bands or clips the upper edge of the said wooden strips coinciding approximately with the normal solution level of the cell (line a-b). The cathode plates with their wooden strips as indicated are then placed in a special cell in which the purified solution is at a higher level than in the ordinary cells being just below the lead covering of the supporting bars A.

The deposition of zinc is then proceeded with for the requisite period, say 24 hours at 25 to 30 amperes per square foot current density, or until a sufficient deposit has been formed to provide the required protective coating. The cathodes are then lifted from the cell, cleaned down and the wooden strips removed, and after drying, any exposed parts of the aluminium (particularly just below the lead covering of the supporting bars) are covered with a protective covering of pitch and tar (applied hot) varnish or other material of a like nature.

The wooden strips prevent the deposition of the protecting coating of zinc over the area thus covered so that after the cathode plate has been protectively covered and subsequently used for deposition of zinc in the ordinary way there will be a line of weakness along the line ab where the subsequent deposit of zinc meets this protecting deposit. Consequently when the stripping is elfected later the zinc deposit will part, readily along this line a-b and peel away from the cathode without removing or disturbing the protecting deposit of zinc in the upper exposed part of the cathode above the line w-b.

The cathodes A with their protective coating are then used for the electrodeposition ofthe zinc in the ordinary way.

In practice it is sometimes found that the protective coating breaks away slightly when stripping, at or about the line ab and for the purpose of restablishing the coating it is sometimes desirable to aflord means for the deposition of zinc above the line of the -normal solution level -(ab) and for this purpose a temporary bafile or the like may be inserted in the effluent lip of the cell with a view to. raising the'solution level-for the time being to about the line cd. The electrodeposition is proceeded with for a few hours at this higher level until the protective coating is reestablished when the level is again dropped to the normal line a-b by removing the baffle or'obstruction from the efliuent lip of the cell and the electrodeposition proceeded with in the ordinary way.

The protecting of the cathode plate in this way affords a deposit upon the surfacecomplete protection to the exposed parts and furthermore does not introduce into the solution any foreign material which would interfere with the electrolytic process.

I claim 1. In the electrolytic process for the recovery of zinc providing a protective coating of zinc on the surface of the cathodes above the solution level whereby the cor-.

roding action of the acid sprays is substantially eliminated.

2. In the electrolytic process for the recovery of zinc providing a protective coating of zinc on the surface of the cathodes by first electrolytically depositing a thin strips affixed at or about the normal solution level which prevent the deposition of zinc under them, and then removing the said. strips and proceeding with the electrolysis in the ordinary Way in cells with the solution at normal level whereby the subsequently deposited zinc strips along the lineof weakness at or about the solution level; substantially as described.

4. In the electrolytic process for the recovery of zinc providing a protective coating above the normal solution level substantially as claimed in claim 3 and reestablishing the said coating at or about the solution level by temporarily raising the solution level and proceeding with electrolysis and then loweringthe level to the normal line and proceeding with electrolysis in the ordinary way; substantially as described.

In testimony whereof I have signed my name to this specification in the Presence of two subscribing witnesses.

ROYALE HILLMAN STEVENS.

Witnesses:

.ALLAN- MCINTY E, V. J. CHAMBnRs.

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