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Extraction Of Crude Metal

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Extraction Of Crude Metal - Lesson Summary

Ores can be concentrated by using any of the four methods - hydraulic washing, magnetic separation, froth flotation and leaching.

Crude metal extraction concentrated ore:

The concentrated ore must be converted into an oxide, which is more suitable for reduction.

The isolation or extraction of a metal from concentrated ore involves two major steps:

a) Converting ore into its metal oxide

b) Reducing the metal oxide into the metal

Concentrated ore can be converted into its metal oxide either by calcination (or) by roasting.

Calcination is the process of converting an ore into its oxide, by heating it strongly below its melting point, in the presence of a limited supply of air. It is usually carried out in a reverberatory furnace. This method is commonly used to convert carbonate ores, such as calamine and dolomite into their oxides. Also, the hydrated ores lose water of crystallisation during calcination.

When calamine, is subjected to calcination, zinc oxide is formed and carbon dioxide is liberated.

                              Δ
  ZnCO3(s)        →               ZnO(s)         +        CO2
Calamine     Calcination     Zinc Oxide           Carbon Dioxide

Similarly, when dolomite is calcined, it gives calcium oxide, magnesium oxide and carbon dioxide gas.

When hematite is calcined, it loses its water of crystallisation.


                                     Δ
FeO3.xH2O(s)            →                Fe2O3(s) +  xH2O(g)
Hematite               Calcination         Iron(III)Oxide

Roasting is also used to convert an ore into its metal oxide. It is the process of heating an ore in a furnace with a regular supply of excess air, at a temperature below the melting point of the metal. This process is commonly employed for sulphide ores.

Ex: Zinc sulphide is heated at about 850 °C to give zinc oxide and sulphur dioxide.

    
                                     850°C            
2ZnS(s)      +      3O2   → 2ZnO(s)  +   2SO2 (g)
Zinc Sulphide                  Zinc Oxide      Sulphur Dioxide

Lead sulphide is roasted at about 600 °C to give lead oxide and sulphur dioxide.

The iron oxide present as an impurity in the form of gangue is removed by adding flux silica.

     Gangue                                        Slag
         ↑                                                  ↑
      FeO       +      SiO2             →        FeSiO3
Iron Oxide         Flux Silica             Iron Silicate

Iron oxide reacts with silica and slags off as iron silicate. The slag can be separated more easily than gangue.

Roasting (or) calcination converts an ore into metal oxide and makes it porous. The metal oxide is porous because the volatile impurities are expelled and the moisture is removed during roasting (or) calcination.

The metal oxide is reduced to metal by using a suitable reducing agent.

Ex: When carbon is used as the reducing agent, it combines with the oxygen of the metal oxide to give metal and carbon monoxide (or) carbon dioxide.


                Reduction
MO + C        →               M    +       CO ↑
                                                  Carbon Monoxide

Carbon is used to reduce zinc oxide to metallic zinc. In this reaction, carbon monoxide is released.

                                  Reduction
ZnO            +       C          →             Zn   +     CO ↑
Zinc Oxide          Carbon                             Carbon Monoxide

Generally, heating is required for a metal oxide to be reduced to its metal.

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