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Extraction of Metals, the separation of metals in a pure or relatively pure state from the minerals in which they occur naturally. Metals are found in the Earth either as compounds or, in the case of very unreactive metals, as the crust of the uncombined element. The rock that contains the metal is called an ore. Common metal ores include bauxite (aluminium oxide), haematite (iron oxide), zinc blende (zinc sulphide), galena (lead sulphide), and cinnabar (mercury sulphide).

The method used to extract a metal from its ore industrially depends upon the position of the metal in the reactivity series. A metal high in the reactivity series (for example, aluminium) is extracted by electrolysis. A metal in the middle of the reactivity series, such as iron, is extracted by reduction (see Redox Reactions). A metal low in the reactivity series, such as mercury, is either found uncombined or extracted by heating alone. These three examples are discussed below.

Extraction of Aluminium

Bauxite, the main ore of aluminium, is first purified using sodium hydroxide solution to produce alumina (aluminium oxide). Then electricity is passed through a solution of aluminium oxide in molten cryolite (sodium aluminium fluoride). The cathode is the carbon lining of the cell and the anodes too are made of carbon.

During the electrolysis, aluminium ions (Al³⁺) combine with electrons (e^-) at the cathode to produce aluminium:

Al³⁺ + 3e^- → Al

At the anodes oxygen ions (O^2-) give up electrons and are converted to oxygen:

2O^2- → O₂ + 4e^-

During the process, molten aluminium is tapped off from the bottom of the furnace. The carbon anodes have to be replaced frequently as they burn away in the oxygen produced.

Extraction of Iron

Iron is extracted from iron ore by reduction in a blast furnace. The furnace is loaded with iron ore (which consists of various minerals, including magnetite, Fe₃O₄, and haematite, Fe₂O₃), coke (which consists almost wholly of carbon, C), and limestone (calcium carbonate, CaCO₃). Blasts of hot air are blown into the furnace. The following reactions take place:

C + O₂ → CO₂

CO₂ + C → 2CO

These reactions are exothermic, or heat-yielding, and they heat the furnace to temperatures over 1000° C (1830° F). The products are carbon dioxide (CO₂) and carbon monoxide (CO). Carbon monoxide is the reducing agent that reduces iron oxide to iron, Fe. In the case of haematite:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

The limestone is added to the furnace to remove impurities, mainly sandstone (silicon(IV) oxide), from the ore. Limestone is decomposed in the furnace:

CaCO₃ → CaO + CO₂

Calcium oxide (CaO, a basic oxide) reacts with silicon(IV) oxide (an acidic oxide) to form calcium silicate (a salt), which floats on the liquid as slag. Molten iron and molten slag are tapped off at the bottom of the furnace.

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