(Chemistry Ch-6) 2. Extraction of Crude Metal from Concentrated Ore & Thermodynamic Principles of Metallurgy

Extraction of Crude Metal from Concentrated Ore

  • Two steps −

  • Conversion of concentrated ore into oxide

  • Reduction of the oxide to metal

  • Ores are converted into oxides because it is easier to reduce oxides

  • Conversion into oxide

  • Calcination

It involves the conversion of hydroxide and carbonate ores into oxides by heating the ores either in the absence or in a limited supply of air, at a temperature below the melting point of the metal.

This process causes the volatile matter to escape, thereby leaving behind the metal oxide.

  • Roasting

It involves the conversion of sulphide ores into oxides by heating the ores in a regular supply of air, at a temperature below the melting point of the metal.

If the ore contains iron, then it slags off as iron silicate.

 

  • Reduction of the oxide to metal

  • It involves heating the oxide with reducing agents such as C, CO and some metals.

  • Some metal oxides require heating as they are difficult to be reduced.

Thermodynamic Principles of Metallurgy

  • ΔG = ΔH − TΔS

Where,

ΔG = Change in Gibbs energy

ΔH = Change in enthalpy

T = Temperature

ΔS = Change in entropy

Where,

= Change in Gibbs energy in standard condition

R = Gas constant

= Temperature

K = Equilibrium constant

  • Reaction will proceed only when ΔG = − ve, i.e., ΔG < 0

When ΔS > 0 and T is large, TΔS > ΔH

And then, ΔH − TΔS < 0

⇒ ΔG < 0

Ellingham Diagram

  • Graphical representation of Gibbs energy

  • Useful for predicting the feasibility of thermal reduction of ores

  • Limitations:

  • It predicts whether a reaction is feasible or not.

It does not give an idea about the kinetics of the reactions

  • The interpretation of is based on (). It is presumed that the reactants and the products are in equilibrium. But this is not always true as the reactant or the product may be solid.

  • When a metal oxide is reduced, the oxygen lost by it is taken away by the reducing agent and as a result, the reducing agent gets converted into its oxide as follows:

These reactions describe the actual reduction of the metal oxide MxO. The values for these reactions can be calculated fromvalues, and from this, it can be predicted whether a reaction is feasible or not. A selection of suitable reducing agent and appropriate temperature is required for the reduction of a metal oxide.

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