Zinc extraction

Zinc extraction

Zinc extraction can be done from the following ores.

Calamine – Zinc carbonate

Zinc blende which is zinc sulphide which is mainly found mixed with galena which is lead (II) sulphide

 

First the ore is concentrated by froth floatation

Then roasted in air to form its oxide

ZnCO3(s)  → ZnO (s) +   CO2 (g)

(Calamine)

2ZnS(s) +  3O2 (g)  → ZnO (s) +   2SO2 (g)

(Zinc blende)

The impurity lead (II) oxide is also oxidised to lead (II) oxide

2PbS(s)  +   3O2 (g)  → PbO (s) +   2SO2 (g)

Zinc extraction is then done by either reduction or electrolysis

Electrolysis

Zinc oxide is reacted with sulphuric (VI) acid to form zinc sulphate

ZnO (s) + H2SO4 (aq)  → ZnSO4 (aq)    +H2O (l)

Lead oxide (impurity) also reacts with dilute sulphuric (VI) acid forming insoluble lead (II) sulphate which is precipitated and filtered off

PbO (s)  + H2SO4 (aq) → PbSO4 (s) + H2O (l)




The zinc sulphate is then dissolved in water and the solution electrolysed

Cathode is lead containing 1% silver

Anode is made of aluminium sheets

Cathode

Zinc ions are discharged

Zn2+ (aq) +   2e  → Zn (s)

The zinc obtained is 99.5% pure

If zinc electrodes were used hydrogen would be produced instead of zinc

Anode

Hydroxide ions are discharged forming oxygen gas

4OH (aq) → 4H2O (l)       + O2 (g)    + 4e

Reduction

The zinc oxide from the roaster is mixed coke and limestone and heated in a blast furnace where it is reduced

ZnO (s)  + C(s) → Zn (g)   + CO (g)

ZnO (s)  + CO (g) → Zn (g)   + CO (g)

Limestone decomposes to calcium oxide and carbon (IV) oxide

CaCO3 (s)  → CaO (s)     + CO2 (g)  

Carbon (IV) oxide is then reduced to carbon (II) oxide

CO2 (g) + C (s)  → 2CO (g)

Coke and carbon (II) oxide are the reducing agents

At furnace temperature (above 10000C) zinc is in gas (vapour) form.

It is cooled rapidly to about 6000C by mixing it with a spray of molten lead

The spray of lead condenses zinc and prevents it from being re – oxidised back to zinc oxide

Molten zinc separates and settles above the molten lead since it is less dense than lead and is run off

Lead produced in the process is a liquid and trickles at the bottomed from where it is tapped off

Calcium oxide combines with silica to form slag