In electrolytic refining, the lead bullion is cast at the smelter into plates weighing several hundred kilograms, which serve as anodes. Thin sheets of pure lead are inserted into the electrolytic tanks to act as the cathodes. The continuous application of electricity in the solution dissolves the anodes while the pure lead is deposited on the cathodes. Impurities settle in the tank. The refined lead, up to 99.999 per cent pure, is then cast into slabs, called pigs, ready for the industrial market.
Pyrometallurgical refining is carried out in a series of large open kettles.
The molten lead is transferred from one kettle to the other by pumping or by ladle and crane, and each impurity or precious metal is progressively removed from the lead. The techniques of pyrometallurgical refining vary with the types of impurities and by-product metals contained in the molten lead. Selective reagents are added to the lead under carefully controlled temperatures, and the different drosses that collect on the surface are skimmed off. When all impurities have been removed, the lead is cast into pigs. Primary and Secondary Lead Lead differs from other common metals in that in addition to the production of metallic lead from mined ore, a large amount of metal is produced from recycled scrap, mostly from old car batteries.