Extraction of Zinc

The important ore of Zinc is Zinc blende (ZnS). So it is extracted from a zinc blend by the carbon reduction process. The following are the steps involved in the extraction process of zinc.

a. Crushing and Pulverization:

The big lumps of ore are first crushed by a jaw crusher and powdered in ball mills.

b. Concentration:

The crushed and pulverized ore is concentrated by the froth flotation process. This process is used for the concentration of sulphide ore. The powered ore is suspended in water and after adding pine, it is stirred by passing the compressed air. The particles of the sulphide ore which are wetted by the oil come to the surface along the froth (fizzy substance) which is taken out and the gangue particles remains at the bottom and are then removed.

c. Roasting:

The concentrated ore is roasted in the excess air at a temperature of 900°C in the reverberatoryfurnace. Here, the ore reacts with oxygen to form Zinc Sulphide (ZnS) and Sulphur dioxide (SO₂) which is given by the reaction below.

2ZnS + 3O₂ → 2ZnO + SO₂

Reverberatory furnace

During this process, some more of the ZnS is also converted into zinc sulphate.

ZnS + 2O₂ → ZnSO₄

Above 900°C, this zinc sulphate decomposes to give Zinc Oxide.

ZnSO₄^- → 2ZnO + SO₂ + O₂

In this roasting process, the volatile impurities are also removed as their oxides.

C + O₂ → CO₂

S + O₂ → SO₂

P₄ + 5O₂ → 2P₂O₅

2As + 3O₂ → 2As₂O₃

d. Reduction:

The roasted ore is mixed with coke and heated strongly so that the zinc and oxides get converted into Zinc. For reduction, either the Belgian process or the vertical retort process is used. But vertical retort is the modern and continuous method.

Vertical retort process

It is one of the most important, continuous and commercial techniques for the reduction of zinc. Here, the zinc ore is mixed with the coke in a ratio of 2:1. It is passed through the charging door. The retort is heated by the producer gas (CO + N₂ ). Zinc vapour along with the carbon monoxide is led to the condenser by applying suction and forcing producer gas from below. The following reaction takes place during reduction.

ZnO + C → Zn + CO

Molten Zinc is taken out from the condenser from time to time. The crude metal obtained is called zinc spelter which contains about 3% of other impurities.

e. Purification:

The spelter zinc contains Fe, Pb, As, Sb, and Cd as impurities. It is further purified by the electrolysis process.

Electrolysis Process:

Pure zinc is made of an anode and pure aluminium is an anode. Zinc sulphate solution is acidified by sulphuric acid and is taken as an electrolyte. When electricity is passed through the electrolytic solution, pure zinc gets deposited at the cathode whereas impurities remain as anode mud. The following electrolytic reaction occurs here:

At anode: Zn – 2e → Zn⁺⁺

At cathode: Zn⁺⁺ + 2e → Zn