Concentration : When zinc blende is used, the powdered ore isconcentrated by froth-floatation process. Roasting : The concentrated ore is heated in excess of oxygen at about 900°C. Zinc sulphide is oxidised to zinc oxide. If some of the ore is oxidisedto zinc sulphate, it also decomposes at 900°C into ZnO. $2\,\mathrm{ZnS} + 3\,\mathrm{O}_{2} \xrightarrow{900^{\circ}\mathrm{C}} 2\,\mathrm{ZnO} + 2\,\mathrm{SO}_{2}$$\mathrm{ZnS} + 2\,\mathrm{O}_{2} \xrightarrow{900^{\circ}\mathrm{C}} \mathrm{ZnSO}_{4}$For roasting, a reverberatory furnace may be used.Reduction : The principal reaction that takes place during reduction is theconversion of the oxide into the metal with the help of carbon.$\mathrm{ZnO} + \mathrm{C} \xrightarrow{1673\,\mathrm{K}} \mathrm{Zn} + \mathrm{CO}$Electrolytic refining : Purification of zinc is done by electrolytic refiningusing pure Zn as cathode and impure Zn as anode. The electrolyte is $\mathrm{ZnSO}_{4}$.Reaction at cathode: $\mathrm{Zn}^{2+}_{(aq)} + 2e^{-} \rightarrow \mathrm{Zn}_{(s)}$Reaction at anode : $2\,\mathrm{H}_{2}\mathrm{O}_{(l)} \rightarrow \mathrm{O}_{2(g)} + 4\,\mathrm{H}^{+}_{(aq)} + 4e^{-}$$\mathrm{ZnSO}_{4}$ electrolyte is added from time to time.