Source of metal: Metals occur in earth's crust and in sea water, in the form of ores. Earth's crust is the major source of metal. Sea water contains many salts, such as sodium chloride, magnesium chloride, etc.
Extraction of metal from an ore depends on the place it has in the reactivity series. Following points explain this.
Metals found at the bottom of reactivity series are least reactive and they are often found in nature in free-state, such as gold, silver, copper, etc. Copper and silver are also found in the form of sulphide and oxide ores.
Metals found in the middle of reactivity series, such as Zn, Fe, Pb, etc. are usually found in the form of oxides, sulphides or carbonates.
Metals found at the top of the reactivity series are never found in free-state as they are very reactive, e.g. K, Na, Ca, Mg and Al, etc.
Many metals are found in the form of oxides because oxygen is abundant in nature and is very reactive.
Metals can be categorized into three parts on the basis of their reactivity: most reactive, medium reactive and least reactive. Following flow chart shows various methods applied to extract metals at different levels of reactivity.
Different methods of extraction are shown in this flow chart. You can see, that metals of high reactivity, electrolysis of molten ore is done to obtain pure metal. For metals of medium reactivity, cacination and roasting are applied. For metals of least reactivity, roasting is applied. Finally, purification of metal is done.
Concentration of ores: Removal of impurities, such as soil, sand, stone, silicates, etc. from mined ore is known as Concentration of Ores.
Ores which are mined often contain many impurities. These impurities are called gangue. First of all, concentration is done to remove impurities from ores. Concentration of ores is also known as enrichment of ores. Process of concentration depends upon physical and chemical properties of ores. Gravity separation, electromagnetic separation, froth flotation process, etc. are some examples of the processes which are applied for concentration of ores. Gravity separation involves separating impurities with the help of gravity. This can be done by sieving. It more or less similar to sieving the flour in order to separate husk and other impurities from it. Magnetic separation helps in separating non-magnetic materials from magnetic materials. This method can be applied to separate impurities from iron ore because iron is a magnetic material. Froth floatation process involves producing froth in the gangue so that impurities can start floating on the surface.
It is easy to obtain metals from their oxides. So, ores found in the form of sulphide and carbonates are first converted to their oxides by the process of roasting and calcination. Oxides of metals so obtained are converted into metals by the process of reduction.
Roasting: Heating of sulphide ores in the presence of excess air to convert them into oxides is known as ROASTING. This process is applied to treat sulphide ores. Sulphide of metal changes to oxide of metal in this process.
Calcination: Heating of carbonate ores in limited supply of air to convert them into oxides is known as CALCINATION.
The word calcination come from its most prominent use. This process involves removing carbon from limestone through combustion to yield calcium oxide (quicklime). The reaction for this process can be given by following equation.
CaCO3(s) → CaO(s) + CO2(g)
Reduction: Heating of oxides of metals to turn them into metal is known as REDUCTION.
Purification: Metal, so obtained is refined using various methods.
|Minerals and Ores||A naturally occuring substance with uniform chemical composition is called mineral. An ore is a mineral from which a metal can be extracted in profitable way.|
|Concentration of ores||It inovlves removing impurities from mineral.|
|Roasting||Heating of sulphide ores in presence of excess air.|
|Calcination||Heating of carbonate ores in presence of limited air. Both roasting and calcination give oxides of metals.|
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