General Principles and Processes of Isolation of Elements

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Minerals: Naturally occurring chemical substances in the earth’s crust obtainable by mining.

Ores: The minerals from which metal can be extracted profitably and conveniently.

Metallurgy: The entire scientific and technological process used for isolation of the metal from its ore.

The concentration of ore: Removal of gangue from the ore is known as concentration or dressing of the ore.

Extraction and isolation of metals:

Calcination: It is the process of heating the ore (like carbonate and hydrated oxide) strongly below its melting point either in the absence of air or in the limited supply.

Roasting: It is the process of heating the ore strongly below the melting point in a sufficient supply of air to bring about its oxidation and remove volatile impurities.

Reduction or conversion of ores to metals: The ore obtained after calcination or roasting is reduced to metal and choice of reducing agent depends upon the nature of the ore.

Carbon or carbon monoxide is used for oxides of Fe, Cu, Zn, Mg, Co, etc. and the process is called smelting.

Depending upon the nature of the ore and the impurities present, different methods are used for the concentration of ore.

Processes of Isolation

Electropositive metals like Na, Al, Mg or hydrogen are used for the reduction of ores of Mn, Cr, Ti, Mo, W, etc.

Water-gas is used for nickel ores.

Auto–reduction process is used for ores of Pb, Hg, Cu, etc.

The electrolytic reduction is used for highly electropositive metals.

Hydrometallurgy or displacement method is used for Ag, Au, etc.

Refining of metals: Purification of crude metals by removing the impurities present in it is called refining.

Different methods are used depending upon the differences in properties of the metal and the impurity.

Thermodynamic and electrochemical

Principles of extraction:

Thermodynamic principles:

Thermodynamics help in understanding the conditions of temperature and selecting a suitable reducing agent in a metallurgical process.

Gibb’s Helmholtz equation is used to check the feasibility of a reduction process.

– DG° =DH° – TDS°, if DG° < 0, the reduction is feasible.

– For a reaction, the enthalpy change is fixed but the temperature factor can be controlled and it can be the deciding factor for the feasibility of that reaction.

Coupled reactions: If reactants and products of two reactions are put together in a system and the net DG of two possible reactions is –ve, the overall reaction will take place. These reactions are called coupled reactions.

Ellingham diagram: It consists of plots of Df G° vs. temperature for the formation of oxides of elements.

It provides a sound idea about selecting a reducing agent in the reduction of oxides.

Such diagrams help in predicting the feasibility of the thermal reduction of ore. DG must be –ve at a given temperature for a reaction to be feasible.

Electrochemical principles:

Electrolysis is used to carry out the reduction of a molten metal salt.

The electrochemical principles of this method can be understood through the equation, DG° = –nFE°

More reactive metals have large negative values of the electrode potentials so, their reduction is difficult.

If the difference of two E ° values corresponds to a positive E° and consequently, negative DG°, then the less reactive metal will come out of the solution and the more reactive metal will go to the solution.

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