Question

Extraction of zinc from zinc blende is achieved by:
A. Electrolytic reduction
B. Roasting followed by reduction with carbon
C. Roasting followed by reduction with another metal
D. Roasting followed by self-reduction

Answer 1

Roasting is a process of heating of a sulfide ore to a high temperature in the presence of air or oxygen. It is an intermediate step of the processing of certain ores. More specifically, roasting is a metallurgical process which involves the gas and solid reactions at high temperatures with the aim of purifying the metal component(s).

Complete step by step answer:
The zinc is extracted from its ore which is the zinc blende ($ZnS$ ). The ore is concentrated by the froth floatation process by the use of pine oil. The concentrated ore is then roasted in the presence of air at a high temperature of $1200K$ . When the roasting is carried out at a temperature below than this temperature, zinc sulfate ($ZnS{O_4}$ ) is formed from a zinc blender. This zinc sulfate is converted back to zinc sulfide when the roasted ore is reduced by carbon. The stepwise reactions that take place is as follows:
(i) Roasting of zinc blende: The above reaction is for the roasting of zinc blende in the presence of oxygen.
$ZnS + \frac{3}{2}{O_2}\xrightarrow{{ \geqslant 1200K}}ZnO + S{O_2} \uparrow$
If the temperature is below $1200K$ , formation of zinc sulfate takes place.
$ZnS + 2{O_2}\xrightarrow{{ < 1200K}}ZnS{O_4}$
(ii) Reduction of zinc sulfate using carbon: The zinc sulfate is reduced by the carbon to give zinc sulfide (during smelting).
$ZnS{O_4} + 4C \to ZnS + 4CO \uparrow$

So, the correct answer is Option B.

Note:
During smelting, the coke that is used for the reduction is taken in excess to prevent the formation of carbon dioxide. The carbon dioxide has the tendency to reoxidize the metal to its oxide. If carbon dioxide is formed, it again forms the carbon monoxide in the presence of carbon.