Science and Technology for UPSC Examinations [Chemistry] Part 13- Basics of Metallurgy

The process of extracting metals from their ores followed by refining is known as metallurgy. The four steps usually employed in metallurgy are :

• Concentration of ore (or enrichment of ore)
• Conversion of concentrated ore into metal oxide.
• Reduction of metal oxide to metal
• Refining of impure metal.

Concentration of ores

Ores are usually associated with unwanted earthly matter called gangue (sand, clay etc.,) and the removal of this unwanted impurity is called concentration. Concentration of ores is generally carried out by any one of the following processes

Gravity separation

The method is generally used for the concentration of oxide ores. The ore is thoroughly crushed, sieved and levigated in a stream of water. The heavier ore particles stay back while the lighter earthly impurities are washed away.

Froth floatation process

The process is commonly employed for the concentration of sulphide ores, which are preferentially wetted by oils. The well powdered ore is added to a mixture of pine or eucalyptus oil and water and agitated by flowing air into the mixture. The ore which is wetted by oil comes to the surface with the froth while the impurities go to the water layer below. For example, Zinc sulphide and Galena are concentrated by this method.

Electromagnetic separation

If one of the impurities present is magnetic in nature, it can be removed by this method. Thus, tinstone, an ore of tin, contains Wolframite as an impurity, which is paramagnetic (i.e., attracted by a magnet). To remove wolframite the powdered tinstone ore is dropped over a travelling belt passing over electromagnetic rollers. Wolframite being paramagnetic is attracted and collected in a heap near the magnets while tinstone is dropped away from the roller and forms another separate heap.

Conversion of concentrated ore into metal oxide

Calcination

It is the process of heating the concentrated ore in the controlled supply of air at a temperature sufficient to melt the ore.

During calcination process, the moisture and volatile impurities are removed from the concentrated ore and the mass becomes porous. Decomposition of the ores may also take place. Thus, in the case of oxide ores water is lost from the ores and Carbonate ores undergo decomposition with the evolution of carbon dioxide leaving behind a porous oxide ore.

Roasting

Roasting is the name of process of heating the concentrated ore strongly in the presence of excess air . Usually, sulphide ores are subjected to roasting. For example, zinc sulphide gives zinc oxide on roasting.

Reduction of metallic oxide to the metal

Reduction is carried out either by electrolysis or by smelting.

Electrolysis

Electrolysis is the process employed for highly electropositive metals which cannot be reduced by reducing agents such as carbon, carbon monoxide, hydrogen etc., Electrolysis is carried out on fused metallic salts (halides or oxides) resulting in deposition of metal ions at cathode. Oxides of potassium, sodium, calcium, magnesium and aluminium are reduced to metals by electrolysis of their fused metallic salts.

Smelting

The calcined or roasted ore is reduced to the metallic form. The high temperature reduction process in which the metal is usually obtained in a molten state is called smelting. The smelting operations are usually carried out in the presence of a flux.

Metallic oxides are reduced to metals by coke, carbon monoxide or hydrogen. Zinc oxide is reduced by coke. Oxides of Iron, lead and copper are reduced by carbon, carbon monoxide or hydrogen. Oxides of Mercury and silver are reduced by thermal decomposition.

Refining of metals

The metals obtained by any of the methods described above need further purification as they may contain other metals, dissolved oxides, carbon, phosphorous etc. The following methods are employed for refining.

Distillation

This is employed for purifying volatile metals like zinc and mercury. On heating, pure metal is vaporised, condensed and gets collected and non-volatile impurities remain behind.

Liquation

It is used for refining easily fusible metals having low melting point like tin. Impure metal is placed on the inclined bed of a furnace and heated. When the metal melts, it flows down leaving the non-fusible impurities behind.

Oxidation

If the impurities present in a metal can be easily oxidised, then the metal is refined by stirring the molten mass thoroughly in the presence of air. During this process, the impurities come to the surface in the form of a scum and can be skimmed off.

Electrolytic refining

The impure metal to be refined is made as anode and the cathode consists of a piece of pure metal in an electrolyte which is a suitable salt of the impure metal. Pure metal gradually passes from the anode to the cathode while the impurities settle to the bottom. Copper, tin, lead, aluminium etc., are purified by this method.

Some basic terms related to Metallurgy

Gangue or matrix

The ore mined from the earth’s crust contains some unwanted substances or impurities such as sand, rocky or clay materials. These substances are called gangue or matrix. The gangue has to be removed before the extraction of metals.

Flux

A flux is a substance that is added to the furnace charge (roasted or calcined ore and coke) during the process of smelting to remove the non-fusible impurities present in the ore.

Slag

Flux combines with non-fusible impurities to convert them into fusible substances known as slag. It is being light, floats over the molten metal and is removed from there. Impurities present in metal oxides may be acidic or basic. For acidic impurities, such as silica or phosphorus pentoxide (SiO2 or P2O5), calcium oxide is added as a flux to the mixture during smelting. If basic impurities such as manganese oxide are present, silica is added as a flux.