Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals.
Pyrometallurgical treatment may produce saleable products such as pure metals, or intermediate compounds or alloys, suitable as feed for further processing. Examples of elements extracted by pyrometallurgical processes include the oxides of less reactive elements like Fe, Cu, Zn, Chromium, Tin, Manganese.
The most important pyrometallurgical operation is the reduction of iron. Iron occurs in many different minerals, but most important sources are two iron oxide minerals are hematite (Fe2O3) and magnetite (Fe3O4). The reduction of iron oxides can be accomplished in a blast furnace. A blast furnace is essentially a huge chemical reactor capable of continuous operation.
The furnace is charged at the top with a mixture of iron ore, coke, and limestone. A Coke is coal that has been heated in the absence of air to drive off volatile components. It is about 85% to 90% carbon. Coke serves as the fuel, producing heat as it is burned in the lower part of the furnace. Through reactions with oxygen and water, coke also serves as the source of the reducing gases CO and H2- Limestone (CaCO3) serves as the source of the basic oxide CaO, which reacts with silicates and other components of the ore to form slag.
Air, which entered the blast furnace at the bottom after preheating, is also an important raw material; it is required for combustion of the coke. Production of 1 kg of crude iron, called pig iron, requires about 2 kg of ore, 1 kg of coke, 0.3 kg of limestone, and 1.5 kg of air.
In the furnace, oxygen reacts with the carbon in the coke to form carbon monoxide. Water vapor present in the air also reacts with carbon, forming both carbon monoxide and hydrogen. Limestone is calcined in upper part of furnace :
CaCO3 (s) === CaO (s) + CO2
Iron oxides reduced by CO2 and H2 in upper part of furnace :
Fe3O4 (s) + 4CO (g) === 3Fe (s) + 4CO2 (g) enthalphy (H) = -15 kJ
Fe3O4 (s) + 4H2 (g) === 3Fe (s) + 4H2O (g) enthalphy (H) = +150 kJ
Iron in form of a spongy mass moves down and its temperature reaches the melting point at the bottom regions of the furnace where it melts and accumulates.
The gangue, ash and other fractions of ore and coke are mixed by fluxes, forming slag which is capable to absorb sulphure and other impurities. The furnace is periodically tapped and the melt (pig iron) is poured into ladles, which are transferred to steel making furnaces. Pig iron usually contains 3-4% of carbon, 2-4% of silicon, 1-2% of manganese and 1-1.2% of phosphorous.