Class 12 Chemistry Isolation of Elements Extraction & Isolation of Metals

Extraction & Isolation of Metals

Following are the steps involved in extracting the metal, then isolating the impure metal from the ore and finally purify the metal.

 Class_12_Chemistry_Isolation_Current_Metals_Extarction_Of_Metals

  • Concentration of the Ore: - The first step is to remove the impurities from the ore and it does not involve any chemical process. It is based on the difference between the physical properties of the metal and the unwanted particles.

 Class_12_Chemistry_Isolation_Current_Metals_Metal_With_Impurities

Class_12_Chemistry_Isolation_Current_Metals_Metal_Without_Impurities

Following are the physical processes involved:-

  1. Hydraulic Washing
  2. Magnetic separation
  3. Froth Flotation Method
  4. Leaching

 

  • Isolation of the metal from its concentrated ore: - This process involves of getting impure metal from its concentrated ore. It is a chemical process.

Following are the processes involved:-

  1. Conversion of oxide (Calcination & Roasting) & Reduction to form metal.
  2. Electrochemical Process
  3. Oxidation reduction

 

  • Purification of the metal: - It involves the purification of the metal.

Following are the processes involved:-

  1. Distillation
  2. Liquation
  3. Electrolysis
  4. Zone refining
  5. Vapour phase refining
  6. Chromatographic methods

 

Concentration of ores: -

  1. Benefaction: - Removal of the unwanted materials (e.g., sand, clays, etc.) from the ore is known as concentration, dressing or benefaction. It involves several steps and selection of these steps depends upon the differences in physical properties of the compound of the metal present and that of the gangue.
  2. Hydraulic Washing: - This method is based on the differences in gravities of the ore and the gangue particles. The process by which lighter earthy impurities are removed from the heavier ore particles by washing with water is called levigation. The lighter impurities are washed away. This method is commonly used for oxide ores such as haematite, tin stone and native ores of Au, Ag, etc.

Class_12_Chemistry_Isolation_Current_Metals_Extarction_Of_Metals_Hydraulic_Washing

  1. Magnetic Separation: - This method is based on differences in magnetic properties of the ore components. If either the ore or the gangue (one of these two) is capable of being attracted by a magnetic field, then such separations are carried out (e.g., in case of iron ores). The ground ore is carried on a conveyer belt which passes over a magnetic roller.

Class_12_Chemistry_Isolation_Current_Metals_Extarction_Of_Metals_Magnetic_Separation

  1. Froth Flotation Method: - In this method is use to remove gangue from sulphide ores. In this process, a suspension of the powdered ore is made with water. This method is based on the preferential wetting of ore particles by oil and that of gangue by water .As a result the ore particles become light and rise to the top in the form of froth while the gangue particles become heavy and settle down. Thus adsorption is involved in this method.

Working of Froth Flotation Method

  1. It contains Collectors (e. g., pine oils, fatty acids, xanthates, etc.) which will enhance the non-wettability of the mineral particles and froth stabilisers (e. g., cresols, aniline) as a result it stabilises the froth.
  2. The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates the mixture and draws air in it. As a result, froth is formed which carries the mineral particles. The froth is light and is skimmed off. It is then dried for recovery of the ore particles.
  3. Sometimes, it is possible to separate two sulphide ores by adjusting proportion of oil to water or by using ‘depressants’. For example, in case of an ore containing ZnS and PbS, the depressant used is NaCN. It selectively prevents ZnS from coming to the froth but allows PbS to come with the froth.

Class_12_Chemistry_Isolation_Current_Metals_Extarction_Of_Metals_Froth_Flotation

  1. Leaching: - Leaching is often used if the ore is soluble in some suitable solvent.

Following are the examples to explain the process of leaching.

(a) Leaching of alumina from bauxite :-

The principal ore of aluminium, bauxite, usually contains SiO2, iron oxides and titanium oxide (TiO2) as impurities.

Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at (473 – 523) K and (35 – 36) bar pressure.

This way, Al2O3 is leached out as sodium aluminate (and SiO2 too as sodium silicate) leaving the impurities behind:

Al2O3(s) + 2NaOH (aq) + 3H2O (l) → 2Na [Al (OH) 4]( aq)

The aluminate in solution is neutralised by passing CO2 gas and hydrated Al2O3 is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al2O3 which induces the precipitation

2Na [Al (OH) 4] (aq) + CO2 (g) → Al2O3.xH2O(s) + 2NaHCO3 (aq)

The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al2O3:

Al2O3.xH2O(s) à Al2O3(s) + xH2O (g) (Temp at 1470K)

Class_12_Chemistry_Isolation_Current_Metals_Extarction_Of_Metals_Leaching

 

(b) Other examples

In the metallurgy of silver and that of gold, the respective metal is leached with a dilute solution of NaCN or KCN in the presence of air (for O2) from which the metal is obtained later by replacement:

4M(s) + 8CN (aq) + 2H2O (aq) + O2 (g) → 4[M (CN) 2] (aq) + 4OH (aq)

(M= Ag or Au)

2[M (CN) 2] (aq)] + Zn(s) à [Zn (CN) 4]2-(aq) + 2M(s)

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