Describe the froth flotation method for concentrating zinc sulfide ore and explain the necessity and methods of purifying the zinc obtained after reduction.

10) (ii)
The process of concentrating ores by froth flotation is used to separate valuable zinc sulfide ore from unwanted gangue (rock and other impurities).

• The finely ground ore is mixed with water, a frothing agent (e.g., pine oil), and a collector (e.g., xanthate).
• Air is blown through the mixture, creating bubbles. The collector selectively coats the zinc sulfide particles, making them hydrophobic, so they attach to the air bubbles and float to the surface as a froth, which is then skimmed off. The hydrophilic gangue particles sink to the bottom.

10) (iii)
The purification process after the reduction method is necessary because the zinc obtained from the reduction method (e.g., in a blast furnace) is typically impure, containing metals like lead, cadmium, and iron.

• This impure zinc is usually purified by distillation due to zinc's relatively low boiling point ($907^\circ\text{C}$). Zinc vaporizes, leaving behind higher boiling point impurities like lead and iron. Cadmium, with a lower boiling point, can be separated in a fractional distillation column.
• Alternatively, the impure zinc can be further refined using an electrolytic method similar to copper refining, where impure zinc acts as the anode and pure zinc is deposited at the cathode.