Question

Zn is to be recovered from an ore containing ZnS. The ore is first roasted with oxygen to produce ZnO, which is leached with aqueous H2SO4 to produce water - soluble ZnSO4 and an insoluble, worthless residue called gangue. The decanted sludge of 20,000 kg/h contains 5 wt% water, 10 wt% ZnSO4, and balance as gangue. This sludge is to be washed with water in a continuous, countercurrent washing system to produce an extract, called a strong solution, of 10 wt% ZnSO4 in water, with a 98% recovery of ZnSO4. Assume that underflow from each washing stage contains, by weight, two parts of water (sulfate - free basis) per part of gangue. Determine the stages required.

Answer 1

The stages required for purification of Zinc from its ore ZnS is,

Roasting

ZnS ore is heated in presence of air above 900⁰c on the reberveratory furnace. During roasting, Zinc sulphide is converted to Zinc Oxide as shown below.

2ZnS + 3O₂ -------> 2ZnO + 2SO₂

Electrolysis process

Leaching

The basic reaction lead to leaching of Zinc is

2ZnS + O₂ + 2H₂SO₄ → 2ZnSO₄ + 2H₂O + 2S

ZnO is leached with aqueous H₂SO₄ to produce water - soluble ZnSO4 and an insoluble, worthless residue called gangue. The decanted sludge of 20,000 kg/h contains 5 wt% water, 10 wt% ZnSO4, and balance as gangue. This sludge is to be washed with water in a continuous, countercurrent washing system to produce an extract, called a strong solution, of 10 wt% ZnSO4 in water, with a 98% recovery of ZnSO4.

ZnO + SO₃ ------> ZnSO₄

Purification

Zinc of higher purity can be obtained by electrolysis. Pure zinc rod is used as cathode while a block of impure zinc is used as anode. A mixture of ZnSO₄ and dill H₂SO₄ is used as electrolyte. On passing current impure zinc dissolves and equivalent amount of pure zinc is deposited at cathode.