III.

Occurrence

Lead is widely distributed all over the world in the form of its sulfide, the ore galena. Lead ranks about 36th in natural abundance among elements in Earth's crust. Ores of secondary importance are cerussite and anglesite. The principal method of extracting lead from galena is to roast the ore—that is, convert it to the oxide, and reduce the oxide with coke in a blast furnace (see Metallurgy). Another method is to roast the ore in a reverberatory furnace until part of the lead sulfide is converted to lead oxide and lead sulfate. The air supply to the furnace is then cut off and the temperature raised; then the original lead sulfide combines with the lead sulfate and lead oxide to form metallic lead and sulfur dioxide.

Waste material, such as battery scrap, recovered from various industrial processes, is also smelted and constitutes an important source of lead. Because galena often has other minerals associated with it, the crude lead, or pig lead, that is obtained from the smelting processes contains metals such as copper, zinc, silver, and gold as impurities. The recovery of precious metals from lead ores is often as important economically as the production of lead itself. Silver and gold are recovered by the Parkes process, whereby a small amount of zinc stirred into molten lead dissolves the precious metals. This molten alloy then rises to the surface of the lead as an easily removed scum, and the zinc is removed from the silver or gold by distillation. Pig lead is often purified by stirring molten lead in the presence of air. The oxides of the metallic impurities rise to the top and are skimmed off. The purest grades of lead are refined electrolytically.