Metals and Non-Metals - Part VIII

6. Metallurgy : extraction of ores and refining of ores
Extraction of metals from below the surface of the earth forms a major industrial endeavor. Since most metals are reactive, most of them are not found as free elements under the earth’s surface. Relatively unreactive metals like silver (Ag), gold (Au), Platinum (Pt) are found as pure elements. Copper (Cu) is found in free state as well as in combined state. Naturally occurring materials or compounds which are a combination of metals with other elements are called minerals. Minerals are found amongst rocks and stones inside the earth’s crust. Such rocks and stones are called mineral ores. Ores have to be mined, crushed, and metals have to be extracted from them by various chemical processes. These processes form a separate branch of engineering called metallurgy. Thus metallurgy deals with extraction and refining or ores.

There are various types of ores. Metals combine with non-metals to form stable mineral deposits. These are as follows :

These ores have to be extracted and converted into free metals, purified and refined for their use as metals for industrial applications. The various steps taken in metallurgy for obtaining metals in pure form are :

1. Concentration of ore : Ore is purified and concentrated. Unwanted rocks and grit are eliminated in this process. Hydraulic washing, froth floatation process, magnetic separation and chemical separation are some of the techniques used for concentrating or purification of ores.
2. Conversion into metal oxide : After an ore is removed of all unwanted rocks and grit, the ore is chemically converted into an oxide. This is because it is easy to reduce a metal-oxide to metal by various known processes. A carbonate ore is made into a metal-oxide by calcination. A sulphide ore is converted into a metal-oxide by heating the ore in air.
3. Reduction of metal-oxide : Metal oxides are reduced by processes such as electrolytic reduction. Metal is obtained, but this may not be pure metal as there may be mixtures of metals
4. Refining of impure metal : In a given ore, there may be more than one metal. To obtain a certain metal, the combination of metals obtained from reducing processes has to be refined. The refining and purification process eliminates unwanted metal impurities. There are many methods of purification. They are electrolysis, distillation, chemical methods, etc.

Metals that are very reactive, such as sodium (Na) and potassium (K), are extracted by electrolysis of chlorides. Calcium in the form of fluorspar (CaF₂) is also extracted by electrolysis. Al is sometimes extracted from bauxite by electrolysis.

In the following sections, we will discuss in details the various steps taken in metallurgy to purify and refine mineral ores.

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