Electrolytes, cadmium purification

Fig. 2. Electrolytic production of cadmium from 2inc electrolyte purification residue (5,6).

Cadmium production is often connected with zinc production. The impure cadmium metal produced in the purification of zinc electrowinning electrolyte is dissolved in 150 g L-1 sulfuric acid solution and electrowon in a process similar to that for zinc. The impure metals that would disturb electrowinning, for example, Cu, Ni, As, are removed by cementation with impure cadmium. The solution is neutralized with lime. The electrolyte contains 140-180 gL-1 Cd, 30-40gL x Zn, some chlorides, and animal glue as additives. The electrolysis is done using... 

Impure SLP electrolyte is treated using cold-hot-polish zinc dust purification. The cold stage cement is treated in the cadmium plant to produce cadmium metal and a copper cake that is fed to the smelter. Hot stage cement is leached to minimize the loss of zinc and is then directed to the smelter. 

The final overflow solution was then advanced to a small zinc dust purification circuit to cement cadmium, and the purified electrolyte was then forwarded to the calcine leaching plant. 

The zinc dust piuification circuit in the oxide leaching plant was originally conunissioned as a purification system for an old oxide cellhouse, decommissioned about 20 years ago when the oxide and calcine leaching plant electrolytes were combined. However, the oxide purification circuit continued to operate with about one gram per liter addition of dry zinc dust, principally to cement cadmium in the oxide electrolyte prior to entering the calcine leaching plant. A portion of the purification residue was then forwarded directly to the cadmium plant. Some arsenic was also removed fix>m solution. 

There were several problems with the oxide electrolyte purification circuit. The arsenic concentration in the zinc fiime fi-om the smelter had increased since the start-up of KIVCET, resulting in an increased tendency for arsenic breakthrough to the oxide electrolyte. This, in turn, had led to an increased potential for arsine generation during zinc dust purification as well as in the storage tank for purification residues in the cadmium plant. In addition, the oxide purification equipment was in very poor working condition. 

Arsenic and antimony are virtually insoluble in the electrolyte and remain in the slimes. Zinc and cadmium are present in very small amounts and will not co-deposit with lead, but can be controlled by a small bleed of electrolyte. Iron normally does not dissolve from the anode to any appreciable extent due to its presence in crude lead bullion as a sulfide or matte rather than metallic iron. Nickel is generally not present in sufficient quantities in lead bullion for there to be a concern with accumulation in the electrolyte. As a result of the above, electrolyte purification requirements are minimal. 

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