Zinc is commonly used metal for making protective coating (galvanizing) because it forms an impervious ZnCO3 layer. Zinc protects irons by being more electro positive. It is also used widely as Cu-Zn brasses because Zn has a low melting point, high structural strength, good dimensional stability. It is ideal for die casting. It can also be rolled into zinc plates that are used in dry cell batteries. It is also used in paints and pigments
Ores of zinc such as Sphalerite (ZnS), Zincite or Zinc calcine (ZnO) and others are low-grade ores that need concentration. ZnS melts at 15000C. However, concentrate (55 % Zn) can be roasted at 8000 C, then ground, agglomerated, sintered to provide feed for retorts. ZnO is reduced at about 12000 C. Finally, Zinc distils off for collection in condensers.
More than 80% of world’s total zinc output is produced by the hydrometallurgical route.
Concentration: Froth floatation
This step is mainly used to remove gangue from Sphalerite which is a sulphide ore of zinc. NaCN is used as a depressant to separate ZnS and PbS. NaCN reacts with ZnS to form a complex compound while PbS selectively releases in the froth. The forth is light so it can be skimmed off.
4NaCN + ZnS Na2[Zn(CN)4] + Na2S
Since Zn is at mid-range of there activity series the electrolytic solution must be free of Cd, Pb, Cu, Fe, Ag and Ge or else Zn deposited on cathode will be contaminated. Hence purification is necessary prior to electrolysis.
Roasting converts the sulphide ore into oxide, zinc calcine with the help of heat and air in the furnace below the melting point of Zinc.
ZnS +1.5O2ZnO + SO2
Calcination is a process to convert carbonate ore, Calamine to oxide in the presence of heat.
ZnCO3 (s)ZnO(s) +CO2(g)
Reduction using carbon
Since zinc is mid ranged in the reactivity series, carbon can be used to reduce Zinc Oxide to free metal. Coke is used in the furnace at about 1673K to ensure the reduction of the oxide.
ZnO + CO Zn + CO2
C + CO2 2CO
It should be noted that the partial pressure of CO in the retort gases must be maintained.
About 10% of the zinc sulphide reacts with iron impurities to form zinc ferrite. Dead roasted zinc calcine undergoes leaching with the recycled electrolyte of ZnSO4
The pressure leaching process is used under oxygen over pressure (1200 kPa) at 150°C in sulphuric acid as a medium. Roasted zinc concentrates are leached in dilute acid.
ZnS + H2SO4 + +0.5O2 ZnSO4 + H2O + S
ZnO + H2SO4 + +0.5O2 ZnSO4 + H2O
The metal follows a distillation chamber where it is allowed rapid chilling.
Fig 1: Zinc extraction flow chart.
For 99.99% purity, Zinc is electro refined where the anode is impure Zinc and the cathode is pure zinc strips. Purified ZnSO4 and dilute H2SO4 is used as an electrolyte.
The net result is the transfer of zinc in pure form from the cathode to the anode.
At the anode: 4OH- 2H2O +O2 + 4e-
At the cathode: Zn2+ + 2e– Zn