The Phases of Metal Extraction

Metal must be extracted from the earth before it can be refined into a usable alloy.

Metal extraction involves the removal of a metal from its surroundings and may occur above or below ground. Different phases may be required for extraction and are determined by factors such as the location of the metal, its concentration and the chemical characteristics of the surrounding ore.

Physical removal

The first phase of metal extraction involves the physical removal of the metal ore from the ground. The ore may be obtained from the surface of the earth or it may be mined underground. In surface mining, the surrounding overburden, such as plants and dirt, is cleared away from the metal ore, which is then removed using a surface drill or explosives. Underground removal is often more expensive and requires construction of tunnels and the use of heavy machinery, including production drills.

Physical extraction

Once ore is removed from the ground, the desired metal must be separated from the other metals in the ore. Sometimes, the ore can be manipulated through its physical characteristics. For example, magnetic separation involves the creation of a magnetic field to separate magnetic metals, such as iron, from other surrounding metals. Another physical extraction method is froth flotation, which involves crushing an ore and treating it to cause the desired metal to become hydrophobic, or repelled by water. Water is then introduced to the ore, forcing the hydrophobic pure metal away from the impurities. Froth flotation is used in the extraction of copper.

Carbon reduction

When carbon is introduced to metal oxides such as iron or aluminum, it acts as a reducing agent by removing the oxygen and producing the pure metal. Carbon reduction is a preferred method of extraction because carbon sources, such as coal, are inexpensive. In addition, the carbon provides energy to support the reducation reaction. However, carbon reduction sometimes leaves carbon behind as an impurity and removal of that impurity requires an additional extraction phase.

Electrolysis

Electrolysis directly introduces electricity to the metal ore. This phase of metal extraction will only be used for reactive metals. The electricity provides free electrons that reduce the ore in a manner similar to carbon reduction. While electrolysis is capable of producing pure metals, it is not always feasible because of the cost of electricity.

References

• EPA; Extraction and beneficiation of ores and minerals, Volume 3 Iron
• Jim Clark; The extraction of metals - an introduction