Occurrence and Extraction of Metals: Extraction of Metals: Concentration or Dressing of the Ore (For CBSE, ICSE, IAS, NET, NRA 2022)

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Extraction of Metals

Concentration or Dressing of the Ore

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

Some methods are:

Gravity Separation (Hydraulic Washing)

  • This method are used to remove the light (low specific gravity) earthy impurities from the heavier metallic ore particles by washing with water.
  • Heavier oxide ores like Haematite , tinstone and gold are concentrated by this method.
  • The powdered ore is agitated with water or washed with a strong current of water as shown in fig.
Gravity Separation Method

The heavier ore settle down rapidly in the grooves and the lighter sandy and earthy materials (gangue particles) are washed away.

Magnetic Separation Method

  • Those ores which either contain impurities which are magnetic or are themselves magnetic in nature can be concentrated by this method .
  • For example, the tin ore, tin stone itself is non-magnetic but contains magnetic impurities such as iron tungstate and manganese tungstate .
  • The finely powdered ore is passed over a conveyer belt moving over two rollers, one of which is fitted with an electromagnet (Fig)
Magnetic Separation Method

The magnetic material is attracted by the magnet and falls in a separate heap.

Froth Flotation Method

  • It is different wetting properties of the surface of the ore and gangue particles.
  • This method is especially used Sulphide ores, such as galena zinc blende , or copper pyrites .
  • Finely powdered ore is mixed with either pine oil or eucalyptus oil and then mixed with water.
  • Air is blown through the mixture with a great force. Froth is produced in this process which carries the weted ore upwards with it.
  • Impurities (gangue particles) are left in water and sink to the bottom from which these are drown off (fig) .
Froth Flotation Method

Chemical Method

  • In this method ore is treated with a suitable chemical reagent which dissolve the ore leaving behind insoluble impurities.
  • The ore is then recovered from the solution by suitable chemical method.
  • For example, Extraction of aluminium from bauxite .
  • Bauxite is contaminated with iron (III) oxide , titanium (IV) oxide and silica .
  • These impurities are removed by digesting the powdered ore with aqueous solution of sodium hydroxide at under pressure.
  • Aluminium oxide dissolves in sodium hydroxide, whereas, iron (III) oxide, silica and titanium (IV) oxide remain insoluble and are removed by filtration.

  • Sodium aluminate is diluted with water to obtain precipitate of aluminium hydroxide.
  • It is filtered and ignited to obtain pure alumina.

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