Although thermodynamically feasible, in practice, magnesium metal is not used for the reduction of alumina in the metallurgy of aluminium. Why ?

Explanation

(a) Inspection of Ellingham diagram (Fig.) shows that `DeltaG `vs T curves for `Al_2 O_3` and MgO intersect at a point corresponding to very high temperature of the order of 2000 K. This means above this temperature, AG for the reaction :
`Al_2 O_3 + 3Mg to 2Al + 3MgO`
Would become negative and hence reduction will be feasible . However, this temperature is very high so that the process is uneconomical and technologically difficult.
(b) Zinc is stronger reducing agent `(E^@ =-0.76 V)` and more electropositive than copper `(E^@ = + 0.34 V)`. Therefore, zinc is used for recovery of Ag from `[Ag(CN_2)]^(-)` complex.
During leaching process, gold (Au) is first oxidised by `O_2` of the air to `Au^(+)` which then combines with `CN^(-)` ions to form the soluble complex, dicyanidoaurate (I).
`4Au (s) + underset("Soluble complex")underset("Sodium dicyanidoaurate(I))underset(4Na[Au(CN)_2] (aq))(8NaCN(aq)+ O_2 (g) )+2H_2 O (l) to 4NaOH (aq)`
Gold is then extracted from this complex by displacement method by using a more electropositive zinc metal. In this method, zinc acts as a reducing agent and it reduces `Au^(+)` to Au. Zinc itself gets oxidised to `Zn^(2+)` ions which combine with `CN^(-)` ions to form soluble complex, sodium tetracyanidozincete (II) .
`2Na[Au(CN)_2 ] (aq) + Zn(s) to 2Au(s) + underset("(Soluble complex)")underset("Sodium tetracyanidozincate (II)")(Na_2 [Zn(CN)_4])`
Thus, extraction of gold by leaching with NaCN involves both oxidation and reduction.
(d) Haematite is an ore of iron and contains silica `(SiO_2)` as the main impurity . The purpose of limestone is to remove `SiO_2` as calcium silicate `(CaSiO_3)` slag.
`underset("Limestone")(CaCO_3) to CaO + CO_2`
`underset("Basic flux")(CaO) + SiO_2 to CaSi O_3`