The slag is then tapped out at the end, and the bullion produced in the reduction zone flows back into the oxidation zone to join the lead bullion being tapped out there. Lead bullion is refined to remove remaining impurities and traces of other metallic elements including precious metals, either by the electrolytic process or by the pyrometallurgical process. 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.