How is zinc extracted ?

Explanation

Occurrence. Zinc does not occur in the free state. In the combined state, it occurs in the following ores:
(i) Zincite (red zinc ore) ZnO
(ii) Zinc blende Zns
(iii) Calamine (zinc spar) `ZnCO_3`
In India, zinc is found in Zawar mines in Udaipur in Rajasthan and in certain parts of Kashmir.
Extraction
Carbon-reduction Process. It involves the following steps for extraction of zinc from zinc blende and calamine ores.
1. Concentration. The ore is first dressed to remove the rocky material sticking to it. It is then finely powdered. The main impurity present in the ore is that of iron which is removed by electromagnetic separation method.
It is the concentrated by the froth floatation process which removes earthy and siliceous impurities. (it is applied to sulphide ore only).
2. Roasting. (a) In the case of the carbonate ore i.e.. calamine, the concentrated ore is calcined to remove moisture and carbon dioxide.
`ZnCO_3 to ZnO + CO_2`
(b) The sulphide ore is roasted at about 1200 K in excess of air by external heating. The following reactions take place.
`2ZnS + 3O_2 to 2ZnO + 2SO_2`
During roasting some zinc sulphate is also formed.
`ZnS + 2O_2 to ZnSO_4`
But at high temperature the sulphate ore is decomposed
`2ZnSO_4 to 2ZnO + 2SO_2 + O_2`
3. Reduction of zinc oxide to zinc metal. Zno obtained above is reduced to zinc metal by any of the following methods.
Belgian Process. The oxide obtained above is mixed with crushed coke and heated to about 1670 K in fire clay retorts. Zinc being volatile distils over and is received in an earthware pot where it condenses. The crude metal obtained is called zinc spelter.
4. Refining of zinc. It may be carried out by the following methods :
(i) By distillation. Spelter contains lead, iron, cadmium and arsenic as the impurities. These are separated by utilizing the difference in their boiling points. When the spelter is distilled at 1225-1275 K, only zinc and cadmium distil over while iron and lead remain behind. Zinc obtained in this way, is collected in a chamber maintained at about 1075 K where zinc remains but cadmium, being more volatile, escapes out.
(ii) By electrolysis. An electrolytic cell containing acidified solution of zinc sulphate is used. The anode is made of spelter and cathode of the pure zinc wire. On passing the current, pure zinc settles at the cathode and impurities are left at the anode as the anode mud.
Zinc dust. To prepare zinc dust, the metal is melted and then atomised by a blast of air.
Granulated Zinc. It is prepared by pouring molten zinc into cold water.