The next step in processing is to extract the metal from the ore. The oldest and most widely used method is pyrometallurgy, the use of heat. Sulfide metal ores are heated, or roasted, in air. This roasting converts the sulfide into metal oxide, while the sulfur is vented away as a gas. The ore is now ready for smelting, the process of separating impurities from the ore by melting it.
Heat is provided by burning the coke, while the flux lowers the melting point of the gangue.
The gangue and flux then combine and float to the top of the furnace, and the molten iron flows out of the base. Zinc, lead, and other metals are also extracted by this method. To make iron and steel production more economical, metallurgists have developed other separation methods, called direct reduction methods, that require less energy. Many other metals are extracted by smelting. When copper is smelted it produces matte, a molten liquid containing sulfides and other impurities.