CHEMISTRY

CHEMISTRY

TOPIC: EXTRACTION OF METALS

By Kingsley Idiagbor, B.Sc (Hon.), PGDCs, NCE, MNSM

xtraction is the process by which a metal is obtained from the mineral or its ore. Like the occurrence of metals, the method of extraction adopted depends to a very large extent on the reactivity of the metal. Thus, the very reactive metals, like sodium, potassium, calcium and aluminum are extracted by electrolytic method. Whereas the moderately reactive metals like zinc, copper and iron are extracted by the reaction of their ore by coke or carbon (II) oxide.

Stages of Extraction

The stages of extraction of metals include:

Concentration of the ore;
Reduction (the extraction proper) and
Refining or the purification of the extracted metal

Concentration of the Ore

The metal is more concentrated in the mineral by separating the mineral from the earthy impurities or gangue. Three popular methods by which this may be achieved are Froth floatation, Magnetic Separation and Mechanical method of washing

Froth floatation involves the use of a tank filled with the ore as well as cresol and other oils. Thereafter, the mixture is stirred thoroughly by using special paddles. The mineral containing the metal being lighter than the gangue floats on top of the oils (cresols or creosote) as froth while the denser gangue sinks to the bottom of the vessel. The metal is skimmed off from the vessel by blowing in air.

Magnetic separation is only employed in the concentration of ore with some degree of magnetic property such as iron, copper and nickel. The mineral with a high metal content that has magnetic property are deflected to a stack as the ore is made to pass over a pulley driven belt over magnetic separator. The gangue or earthy impurities are flung off or pass straight on.

The third method of concentration of ore is by mechanical method of washing. This is very useful for ores of tin. The gangue or earthy impurities are washed off in a stream or river while the heavier tin ores are left behind.

Reduction

This, the second stage of extraction of metal, is the extraction proper. As mentioned earlier , the metals that are very reactive are extracted by electrolytic method. This is because it is considerably difficult to reduce their metallic ions by using common reducing agents like coke, and so electrolytic process is preferred.

Less reactive metals however, like iron and zinc, have comparatively lower tendency to exist as positive ions. Therefore, the ores of such metals can readily be reduced by any one of the two methods of chemical reduction or thermal reduction.

Chemical Reduction

Ores of less electropositive metals like Pb, Sn, Fe, and Zn are often obtained by treating their oxides with coke or carbon (II) oxide (also called carbon monoxide). The oxide is heated with coke which combines with the ions of the oxide to give out electrons necessary for the reduction of the metallic ions.

E.g. Oxidation: C + O^2- ---------------> CO + 2 e

Reduction Zn²⁺ + 2e ---------------->Zn

Overall (net) reduction C(s) + Zn²⁺O^2-_(s)--------->Zn_(s) +CO_(g)

Thermal Reduction

There are some metals that can be extracted by heating their ores in air. This is technically known as roasting. For instance, mercury may be obtained by just heating mercury pyrite (or mercuric sulphide) in air.

Refining or Purification of the Metal

The final stage of extraction of some metals is refining or purification. This is done usually by electrolytic method like in the case of copper. A solution containing copper ions (e.g. CuSO₄) is used as the electrolyte while the impure extracted copper is made the sacrificial anode. One may still need to use a pure copper metal and this is usually the cathode. The copper ions in the impure copper will gain the appropriate number of electrons at the cathode where they become the metal. This is subsequently scraped off. The impurities will sink down to the bottom of the coulometer (the vessel used for the refining process) as sludge.