Extractive Metallurgy

Extractive metallurgy is the art of obtaining valuable metals from an ore or metallic concentrate and refining the initial products into a purer form. In order to convert a metallic oxide or sulfide to a pure metal, the ore must pass through several mechanical, thermic or chemical treatments so that their physiochemical properties can be improved.

Metals can be found on the earth in several chemical forms such as natives (gold, copper), oxides, sulphides, sulphates, carbonates, nitrates, and others. With some exceptions such as iron minerals, limestone, and quartz, mostly are disseminated or mixed with other components that can have or not metallurgical and economical importance. In this case can be necessary a previous treatment of concentration.

In a first step, is eliminated the major quantity of material no useful (gangue) by employing any concentration process such as flotation, magnetic separation or other process. This step determines an important part of the economical aspect to be considered until get a pure metal, and sometimes, it could limit the purity of the metal.

Most of metals are reducers and rarely are in native state. Then, it is necessary to extract a metal form a mixture. This can be done employing several chemical reactions and changes of physical state. All these operations need energy and are practiced many times to high temperatures.

The concentrates obtained are transformed to metal in one or several steps. For example, direct reduction employing an appropriate reducing agent (e.g. to reduce iron oxides by carbon). Other option is to transform the chemical compounds in soluble or reducible species (e.g. by roasting, sulphides are transformed in sulphates, which can be leached later).

Prior to the reduction stage, it is often necessary to separate metal compounds to exclude reduction of different metals and contamination of the final product. There is a great variety of separation processes: roasting, oxidative smelting, converting, and leaching. The final step usually involves an electrolytic process (e.g. copper, lead, zinc).

The extractive metallurgy comprises mineral processing, pyrometallurgy, hydrometallurgy, and electrometallurgy. Mineral processing involves crushing, grinding, and concentration. Pyrometallurgy involves the treatment of ores at high temperature to convert ore minerals to raw metals, or intermediate compounds for further refining. Roasting, smelting and converting are the most common pyrometallurgical processes. (e.g. treatment of lead, copper and zinc concentrates). Hydrometallurgy involves the use of aqueous solutions (leaching agent) to extract metals or compounds from their ores (e.g. gold cyanidation, leaching of copper oxides by sulfuric acid, alkaline uranium leaching). Some of the hydrometallurgical processes include leaching, precipitation of insoluble compounds, pressure reduction. Electrometallurgy involves passing a large current through a molten metal oxide or an aqueous solution of the metal's salt (e.g. copper and lead cathodes obtained from electrolytic cells).

Other classification divides extractive metallurgy in two fields: ferrous metallurgy and non-ferrous metallurgy. Ferrous metallurgy involves the treatment of iron ores until produce a specific type of steel (e.g. steels of low, medium and high carbon, stainless steel). And non-ferrous metals involve the treatment all other metals: copper, lead, zinc, gold, silver, antimony, bismuth, tin, aluminium, magnesium, etc.