Roasting (Metallurgy)

Roasting, in metallurgy, is a pyrometallurgical process that heats an ore, most often a sulfide mineral, in the presence of air to convert it chemically before further processing such as smelting. The reaction is exothermic and typically converts metal sulfides into metal oxides, releasing sulfur dioxide gas, and the ore is usually concentrated beforehand, for example by froth flotation, to make the process more efficient. Before the early twentieth century, roasting was commonly carried out by burning wood on top of piles of ore, and early industrial sulfide roasting used open-hearth roasters that workers manually stirred with rake-like tools, a practice called rabbling, to keep exposing unroasted ore to oxygen. Historical roasting operations released heavy sulfur dioxide pollution, and some roast bed sites remained largely lifeless for sixty to eighty years afterward. Modern metallurgy recognizes several distinct roasting variants, including oxidative, volatilizing, chloridizing, sulfating, magnetic, reduction and sinter roasting, chosen to suit the ore being processed.