Question

Write chemical reactions taking place in the extraction of zinc from zinc blende.

Answer 1

The different steps involved in the extraction of zinc from zinc blende (ZnS) are given below:

(i) Concentration of ore: First, the gangue from zinc blende is removed using the froth flotation method.

(ii) Conversion to oxide (Roasting): Sulphide ore is converted into oxide by the process of roasting. In this process, ZnS is heated in a regular supply of air in a furnace at a temperature which is below the melting point of Zn.

\[\ce{2ZnS + 3O2 -> 2ZnO + 2SO2}\]

(iii) Extraction of zinc from zinc oxide (Reduction): Zinc is extracted from zinc oxide by the process of reduction. The reduction of zinc oxide is carried out by mixing it with powdered coke and then heating it at 673 K.

\[\ce{ZnO + C ->[Coke, 673 K] Zn + CO}\]

iv) Electrolytic Refining: Zinc can be refined by the process of electrolytic refining. In this process, impure zinc is made the anode while a pure copper strip is made the cathode. The electrolyte used is an acidified solution of zinc sulphate (ZnSO₄). Electrolysis results in the transfer of zinc in pure form from the anode to the cathode.

Anode: \[\ce{Zn -> Zn^{2+} + 2e^-}\]

Cathode: \[\ce{Zn^{2+} + 2e- -> Zn}\]

Answer 2

Zinc is mainly extracted from the ore zinc blende. The extraction is carried out in the following steps.

Concentration: In ball mills, the ore is crushed and ground into a fine powder. Levigation, a gravity method, is used to concentrate the finely ground ore, followed by froth flotation. The ore undergoes electromagnetic separation if iron oxide is also present.

Roasting: With too much air present, the concentrated ore is roasted at around 1200 K. Zinc oxide is created from the zinc sulphide found in the ore or zinc sulphate produced by its oxidation.

\[\ce{2ZnS + 3O2 -> 2ZnO + 2SO2}\]

\[\ce{ZnS + 2O2 -> ZnSO4}\]

\[\ce{2ZnSO4 -> 2ZnO + 2SO2 + O2}\]

Calamine ore needs only calcination.

\[\ce{ZnCO3 -> ZnO + CO2}\]

Reduction: The zinc oxide created in the previous stage is reduced using a variety of techniques. The roasted ore is combined with coke or coal powder and cooked in a vertical retort as part of the vertical retort process.

A mixture of fuel, gas, and air is forced to burn inside the jacket that encloses the retort. The retort is heated to roughly 1600 K by the heat generated. Zinc is produced by reducing zinc oxide.

\[\ce{ZnO + C ->[1600 K] Zn + CO}\]

As a result, zinc is vaporised. The CO and zinc vapour are extracted and condensed from an exit close to the retort's top. Zinc spelter is the term for the condensed zinc that is so produced. Carbon monoxide escapes because it does not condense. A screw conveyor installed in the bottom removes the residue once the new charge is introduced through the charging door. The procedure is therefore ongoing.