Concentration or Dressing of Ore

The ores are found mixed with earthy impurities like sand, clay, lime stone, etc. These unwanted impurities in the ore are called gangue or matrix.

The process of removal of gangue from powdered ore is called concentration or ore dressing. There are several methods for concentrating the ores. The choice of method depends on the nature of the ore.

Gravity Separation (Hydraulic Washing)

In this method, the light (low specific gravity) earthy impurities are removed from the heavier metallic ore particles by washing with water. Therefore, it is used for the concentration of heavier oxide ores, like haematite (Fe₂O₃), tinstone (SnO₂) and gold (Au).

In this method, the powdered ore is agitated with water or washed with a strong current of water. The heavier ore settles down rapidly in the grooves and the lighter sandy and earthy materials (gangue particles) are washed away.

Magnetic separation Method

By this method, those ores can be concentrated which either contain impurities which are magnetic or are themselves magnetic in nature. For example, the tin ore, tinstone (SnO₂) itself is non-magnetic but contains magnetic impurities such as iron tungstate (FeWO₄) and manganese tungstate (MnWO₄).

The finely powdered ore is passed over a conveyer belt moving over two rollers, one of which is fitted with an electromagnet. The magnetic material is attracted by the magnet and falls in a separate heap. In this way, magnetic impurities are separated from non-magnetic material.

Froth Floatation Method

This method is especially applied to sulphide ores, such as galena (PbS), zinc blende (ZnS), or copper pyrites (CuFeS₂). It is based on the different wetting properties of the surface of the ore and gangue particles. The sulphide ore particles are wetted preferentially by oil and gangue particles by water.

In this process, finely powdered ore is mixed with either pine oil or eucalyptus oil. It is then mixed with water. Air is blown through the mixture with a great force. Froth is produced in this process which carries the wet ore upwards with it. Impurities (gangue particles) are left in water and sink to the bottom from which these are drawn off.

Chemical Method

In this method, the ore is treated with a suitable chemical reagent which dissolves the ore leaving behind insoluble impurities. The ore is then recovered from the solution by a suitable chemical method.

This is applied for extraction of aluminium from bauxite (Al₂O₃.2H₂O).