Electromagnetic Separation
- Electromagnetic Separation : When one component either the ore or impurity is magnetic in nature, this method can be used for separation. A magnetic separator consisting a belt moving on two rollers is used. One of which is a strong magnet. The powdered ore is dropped on the belt from one end (non-magnetic) and at the other end (magnetic), the magnetic particles are attracted and fall nearer the roller while non-magnetic particles fall away from the roller. Ferro-magnetic ores are concentrated by this method. For example, wolframite (FeWO4) - a magnetic ore, is separated from the non-magnetic ore, cassiterite SnO2, by this method.
- Froth floatation process : This method is used for the concentration of sulphide ores. The method is based on the preferential wetting properties with the frothing agent and water. The powdered ore is added to water containing pine oil (frothing agent) and sodium ethyl xanthate (collecting agent). A vigorous stream of air is now passed through, which thoroughly agitates the mixture and disposes the oil into colloid sized particles. As a result of this, the sulphide particles of the ore stick to the oil droplets and rise to the surface in the form of froth supported by air bubbles. Water wets the gangue particles which sink to the bottom. With this method, it is possible to concentrate the dense ores such as galena and zinc blende.
Chemical methods
- Calcination : It involves heating of the ore below its fusion temperature in absence of air. This step expels organic matter and moisture from the ores. It can remove moisture from hydrated oxides or carbon dioxide from carbonates. For example :
Al2O3·2H2O
Al2O3 + 2H2O
2Fe2O3·3H2O2Fe2O3 + 6H2O
CuCO3·Cu(OH)22CuO + CO2 + H2O
Calcination makes the ore porous. The step is generally performed in reverberatory furnace. - Roasting : It is also heating of the ore either alone or with some other material usually in presence of air below its fusion temperature. roasting is generally done in a reverberatory furnace or in a blast furnace. In roasting definite chemical changes like oxidation, chlorination, etc., take place while in calcination these occurs only expulsion of organic matter, water, carbon dioxide etc., i.e., it does not involve any major chemical change.
The roasting process may be any one of the following types :
- Oxidising roasting : This is very common type of roasting in metallurgy and is carried out to remove sulphur and arsenic in the form of their volatile oxides such as SO2 and As2O3 respectively. The ores are simultaneously converted into corresponding oxides. This type of roasting is generally applied in ores of lead, zinc, nickel, copper, etc.
S + O2 SO2
4As + 3O22As2O3
2PbS + 3O22PbO + 2SO2
2CuS + 3O22Cu2O + 2SO2
- Partial oxidising or sulphating roasting : This type of roasting is carried out at a temperature below the melting point of the charge and air is admitted. Part of the sulphide ore is oxidised to sulphate and the rest is converted into oxide. For example, the roasting of galena leads to the formation of a mixture of lead oxide and lead sulphate.
2PbS + 3O22PbO + 2SO2
PbS + 2O2PbSO4
- Chlorinating roasting : This type of roasting is done in the case of silver ore. The ore, argentite is mixed with common salt and the mixture is heated in the presence of air. The final product is the chloride of the metal.
Ag2S + 2NaCl2AgCl + Na2S
- Oxidising roasting : This is very common type of roasting in metallurgy and is carried out to remove sulphur and arsenic in the form of their volatile oxides such as SO2 and As2O3 respectively. The ores are simultaneously converted into corresponding oxides. This type of roasting is generally applied in ores of lead, zinc, nickel, copper, etc.
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