. Extraction of crude metal The extraction of crude metals from the concentrated ores is carried out in two steps namely, (i) conversion of the ore into oxides of the metal of interest and (ii) reduction of the metal oxides to elemental metals. In the concentrated ore, the metal exists in positive oxidation state and hence it is to be reduced to its elemental state. We can infer from the principles of thermodynamics, that the reduction of oxide is easier when compared to reduction of other compounds of metal and hence, before reduction, the ore is first converted into the oxide of metal of interest.
Let us discuss some of the common methods used to convert the concentrated ore into the oxides of the metal of interest. . . Conversion of ores into oxides Roasting Roasting is the method, usually applied for the conversion of sulphide ores into their oxides.
In this method, the concentrated ore is oxidised by heating it with excess of oxygen in a suitable furnace below the melting point of the metal. Powdered ore Magnetic Wheel Magnetic ore Non-Magnetic ore Moving belt wdered ore Magnetic Figure . Magnetic separation XII U1 Metallurgy - XII U1 Metallurgy - - - - - 2PbS + 3O 2PbO + 2SO 2ZnS + 3O 2ZnO + 2SO 2Cu S + 3O 2Cu O + 2SO ∆ ∆ ∆ Roasting also removes impurities such as arsenic, sulphur, phosphorous by converting them into their volatile oxides. For example 4As + 3O 2As O S + 8O 8SO P + 5O P O The sulphur dioxide produced during roasting process is harmful to the environment.
In modern metallurgical factories, this by product is trapped and converted into sulphuric acid to avoid air pollution. Calcination Calcination is the process in which the concentrated ore is strongly heated in the absence of air. During this process, the water of crystallisation present in the hydrated oxide escapes as moisture. Any organic matter (if present) also get expelled leaving behind a porous ore.
This method can also be carried out with a limited supply of air. For examples, During calcination of carbonate ore, carbon dioxide is expelled PbCO PbO + CO CaCO CaO + CO ZnCO ZnO + CO MgCO .CaCO MgO + CaO + 2CO