The refining of lead and zinc

In the case of the direct oxidation, the oxygen partial pressure must be greater than that at the Pb/PbO equilibrium, while in the process involving sodium-based salts, the oxygen pressure is less than this. The two equilibrium constants for the refining reactions 

The removal of silver from lead is accomplished by the addition of zinc to the molten lead, and slowly cooling to a temperature just above the melting point of lead (600 K). A crust of zinc containing the silver can be separated from the liquid, and the zinc can be removed from this product by distillation. The residual zinc in the lead can be removed either by distillation of the zinc, or by pumping chlorine through the metal to form a zinc-lead chloride slag. 

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