Metallurgical operations mainly involve four steps:
Enrichment of Ores:
Ores are found mixed with many impurities like earthy materials, rocky material, sand, limestone, mica etc.
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These impurities are known as gangue. Removal of gangue is based on the differences between physical or chemical properties of ore and the gangue.
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The separation of gangue from the ore of a metal is called concentration of ore. Following methods may be used for this purpose.
However, all these processes are not used for all metals. Depending on the chemical composition of the ore and impurities associated with it, suitable methods may be chosen:
Hydraulic Washing:
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In hydraulic washing, the advantage of differences in density is taken into account. The lighter gangue particles are washed away in a stream of water while the heavier minerals stay behind.
Ores of tin and lead are very heavy so that they are concentrated by this method.
Magnetic Separation:
Some ores are attracted by a magnet whereas gangue is not attracted. The magnetic ores like iron pyrites, FeS and magnetite, Fe304 are concentrated by this method.
The crushed ore is allowed to pass through electromagnetic belts. The mineral particles are retained and gangue particles are thrown away.
Froth Flotation Process:
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This method is employed for the concentration of sulphide ores. The finely powdered ore is added to water in a tank containing pine or eucalypts oil. The mixture is agitated by blowing a blast of air.
The ore particles preferentially wetted by oil come to the surface with froth while impurities which are wetted with water become heavier and settle at the bottom. The oil froth containing mineral particles which floats on the surface of the water can be skimmed off easily.
Thus, the concentration of copper pyrite and galena etc., are done by this method.
Chemical Separation:
In this method, use is made of differences between chemical properties of minerals and gangue. For example, ore of aluminium (bauxite) can be separated from its gangue by this method.
Bauxite (A1203) is treated with hot sodium hydroxide solution. Sodium aluminate formed is soluble in water and can be easily separated from gangue by simple filtration. Aluminium hydroxide is then precipitated on acidifying the filtrate. A1203 is obtained from aluminium hydroxide on heating.
Conversion of Concentrated Ore into Oxide:
It is easier to obtain metals from their oxides than from sulphides or carbonates. Therefore, sulphide and carbonate ores are converted into oxides as follows:
Calcinations:
It is the process of heating the concentrated ore in the absence of air. By this process, volatile impurities are removed. Carbonate and hydrated oxide ores are calcined as shown.
Roasting:
It is the process of heating the ore in the presence of excess of air. This method is used in case of sulphide ores. The sulphides are converted into oxides by roasting and S02 is evolved. For example,
Reduction of Metal Oxide to Metal:
There are three ways of reducing metal compounds mainly metal oxides to metals.
Reduction on Heating:
Metals low in the activity series can be reduced by heating their compounds.
For example, mercury can be obtained by heating HgS (cinnabar) in air. This process is also called roasting of ore. However, metals higher up in the activity series give oxides on roasting.
Reduction by Metals:
Metals in the middle of activity series, e.g., iron, zinc, nickel, tin etc., can be reduced by heating with carbon.
The process of reducing the oxide with coke (carbon) is called smelting. For example, iron oxide is reduced to metal by heating with carbon.
Sometimes displacement reactions are used. The highly reactive metals such as sodium, potassium, calcium, aluminium etc., can be used as reducing agents because these can displace metals of lower reactivity from their compounds.
These reactions are highly exothermic. The heat evolved is enough to give the reduced metal in the molten form. This process is known as termite reaction.
Electrolytic Reduction:
The oxides of more reactive metals like sodium, potassium and aluminium are very stable and cannot be reduced by chemical reduction.
Such metals are extracted by the electrolysis of their oxides, hydroxides or chlorides in the molten state.
For example, aluminium metal is extracted by the electrolysis of molten aluminium oxide and sodium by electrolysis of fused sodium chloride.
Refining or Purification of the Impure Metal
The metals produced by various reduction processes are not very pure and contain a lot of impurities. The most widely used method for refining impure metals is electrolytic refining.
Electrolytic refining: Many metals like copper, nickel, silver etc., are refined by this method. The impure copper is refined in the following way.
The impure copper metal is made the anode of an electrolytic cell the cathode is made of a thin plate of pure copper.
The electrolyte is a solution of copper salt in water, e.g., copper sulphate. On passing electric current, copper is dissolved from the anode and deposited on the cathode.
The impurities collect at the bottom below the anode which is known as anode mud.