Q.How zinc is extracted from its ore ?

Zinc is obtained from zinc blende through froth-flotation concentration, oxidative roasting to zinc oxide, carbon (coke) reduction of the oxide to metal, and final purification by distillation or electrolysis.

Principal ore: Zinc blende (sphalerite), $ZnS$ (also calamine, $ZnCO_3$, and zincite, $ZnO$, in smaller amounts).

1. Concentration: The crushed and powdered sulphide ore is concentrated by the froth flotation process — the finely powdered ore is agitated with water, pine oil (frother) and a collector in a flotation cell; air is blown through, and the sulphide mineral particles, rendered water-repellent, adhere to the froth/bubbles and are skimmed off, leaving the gangue (silica, etc.) to settle.

2. Roasting: The concentrated $ZnS$ is roasted in a reverberatory furnace/roaster in a controlled supply of air at about 1200 K, converting it to zinc oxide and driving off sulphur dioxide (which is used to make sulphuric acid):

$2ZnS + 3O_2 \xrightarrow{\Delta} 2ZnO + 2SO_2\uparrow$

(If the ore is calamine, $ZnCO_3$, it is instead calcined: $ZnCO_3 \xrightarrow{\Delta} ZnO + CO_2$.)

3. Reduction: Zinc oxide is a relatively easily reduced oxide. It is mixed with powdered coke (carbon) and heated strongly (~1673 K, i.e. ~1400 °C) in a fire-clay retort or in a vertical (Imperial Smelting) furnace:

$ZnO + C \rightarrow Zn + CO$

(also $ZnO + CO \rightarrow Zn + CO_2$). Because the boiling point of zinc (1180 K) is well below the reduction temperature, the metal is obtained directly as vapour, which is led off and rapidly condensed (often quenched in molten lead or condensed as a spray) to give crude/spelter zinc, minimising re-oxidation.

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