Consider the following metallurgical processes :
(i) Heating impure metal $$CO$$  and distilling the resulting volatile carbonyl $$\left( {b.p.\,{{43}^ \circ }C} \right)$$   and finally decomposition at $${150^ \circ } - {200^ \circ }C$$    to get the pure metal
(ii) Heating the sulphide ore in air until a part is converted to oxide and then furher heating in the absence of air to let the oxide react with unchanged metal sulphide
(iii) Electrolysis of the molten electrolyte containing approximately equal amounts of the metal chloride and $$NaCl$$  to obtained the metal
The processes used for obtaning magnesium, nickel and copper are respectively :

Solution :
\[\begin{align} & \left( \text{i} \right)\underset{\text{(impure)}}{\mathop{Ni}}\,+4CO\xrightarrow{{{50}^{\circ }}C}\left[ \underset{\text{(volite)}}{\mathop{Ni{{\left( CO \right)}_{4}}}}\, \right]\xrightarrow{{{230}^{\circ }}C}\underset{\text{(pure)}}{\mathop{Ni}}\,+4CO\uparrow \\ & \left( \text{ii} \right)C{{u}_{2}}S+\frac{3}{2}{{O}_{2}}\xrightarrow{\Delta }C{{u}_{2}}O+S{{O}_{2}} \\ & C{{u}_{2}}S+2C{{u}_{2}}O\xrightarrow[\text{temp}\text{.}]{\text{high}}6Cu+S{{O}_{2}} \\ & \left( \text{iii} \right)MgC{{l}_{2}}\left( s \right)\xrightarrow{\text{electrolysis}}M{{g}^{2+}}\left( l \right)+2C{{l}^{-}}\left( l \right) \\ & \text{At cathode :} \\ & M{{g}^{2+}}2{{e}^{-}}\to Mg\left( s \right) \\ & \text{At anode :} \\ & 2C{{l}^{-}}\left( l \right)\to C{{l}_{2}}\left( g \right)+2{{e}^{-}} \\ \end{align}\]