Lead-silver-calcium anodes

Lead contamination of the cathodes in zinc electrowinning is minimized by the formation of a stable protective coating of Pb02 on the surface of the anode. The formation of the stable Pb02 layer is a slow process on the normally utilized lead-silver anodes. Lead-calcium-silver alloys, investigated and applied in the past, also showed a very slow rate of formation of the protective Pb02 layer. 

As anodes have become larger, it has become much more difficult to cast the anodes to the preferred shape. Rolled lead-silver or rolled lead-calcium-silver sheets have been utilized alternatively in some tankhouses to form these jumbo anodes. The surfaces of the rolled anodes are much smoother than those of the cast anodes generating fewer places for the oxides to adhere. In addition, the texture is oriented in the rolling direction, producing a grain stmcture even more difficult to corrode and to form an adherent layer of Pb02... 

Lead-calcium-silver anodes, 74 777 Lead-calcium-tin alloys, 74 775-776 Lead carbonates, 74 794-795 Lead chalcogenides, 79 157 Lead chloride, 74 785 Lead chromate... 

K. Fischbeck and E. Einecke found that the cathodic polarization of ferrous, cuprous, calcium, and magnesium chromites produces chromic acid, whilst the other chromites are unaffected, and natural chrome ironstone behaves in a like manner, but other commercial chromites are reduced on cathodic polarization, and yield chromic acid on anodic polarization. Chromites behave as an intermediate electrode. 0. Unverdorben observed that chromyl fluoride, prepared by heating a mixture of fluorspar, lead chromate, and sulphuric acid, when passed into water, furnishes an aq. soln. of this oxide. The soln. was treated with silver nitrate, and the washed precipitate of silver chromate decomposed by hydrochloric acid. A. Mans said that anhydrous sulphuric acid or fuming sulphuric acid is not suited for the preparation because of its volatilization with the chromyl fluoride. 

The traditional anodes for base metal electrowinning from sulfate solutions have been lead alloys containing calcium-tin, silver, or similar combinations. One potential concern with the lead anodes is the incorporation of lead corrosion products lowering the cathode purity. 

Of the elements commonly found in lead alloys, zinc and bismuth aggravate corrosion in most circumstances, while additions of copper, tellurium, antimony, nickel, silver, tin, arsenic and calcium may reduce corrosion resistance only slightly, or even improve it depending on the service conditions. Alloying elements that are of increasing importance are calcium, especially in maintenance-free battery alloys and selenium, or sulphur combined with copper as nucleants in low antimony battery alloys. Other elements of interest are indium in anodes , aluminium in batteries and selenium in chemical lead as a grain refiner... 

---