Zinc - Extraction
Zinc• Atomic Number → 30• Melting Point → 420oC• Boiling Point → 908oCOccurrenceZinc does not occur in free state. In combined form, it exist as sulfides, oxides, carbonates etc. Some of the ores of zinc are;→ Zinc Blende : Zns→ Calamite : ZnCO3→ Zincite or Red Zinc Oxide : ZnO→ Frank Linite : ZnO.Fe2O3→ Willemite : Zn2SiO4 / 2ZnO.SiO2Extraction of ZincZinc is the main ore of zinc. Zinc Oxide obtained after concentration and roasting of zinc blende is heated with coke to get zinc vapour that gives molten zin on codensation which further on purification by fractional distillation and electrolytic refining gives pure Zinc.ZnS + O2 ------------------->ZnO + SO2 ↑ZnO + C -------------------> Zn + CO ↑
During this process, some of the zinc sulphide is also concentrated into sulphate. 
To avoid this, the roasting is done at above 900oC temperature. At this temperature, sulphate is decomposed to oxide again. 2ZnSO4-----------------> ZnO + 2SO2 +O2 ↑ In this roasting process, volatile impurities are also removed as their oxide. 
4. Reduction Zinc Oxide obtained by roasting is heated with carbon in the furnace to reduce the oxide into metallic form. 
• Vertical Retort ProcessThe powdered oxide form the ore(ZnO) obtained from roasting process is mixed with the coke(C) in a ratio 2:1 to make small briquets which are fed into the retort furnance through charging door. This feeding is a continous process. The retort is heated externally by burning the producer gas (CO + N2) at around 1400-1500oC. In this temperature, zinc oxide gets reduced to zinc by coke. 
The vapour of zinc and carbon monoxide gas produced in this process are carried into condenser by a wild current of producer gas passed through the bottom of furnance. The zinc vapour is condensed and zinc thus obtained is zinc spelter which is 97% pure.5. Purificationa. Fractional Distillation→ Zinc Spelter is subjected for distillation at 1000oC. This temperature is greater than the boiling point of Zinc and cadmium. So, Zn and Cd vapouriz and Fe & Pb remain in the distillation vessel. The vapour of Zn and Cd are again heated at 800oC, cadmium is distilled over leaving zinc which is 99% pure.b. Electrolysis→ An electrolytic cell is made with acidified ZnSO4 solution as electrolyte, anode of zinc spelter and cathode of pure zinc. Pure Zinc is deposited at cathode and impurities are left at the bottom of anode as anode mud after passing electric current. At anode: Zn -------------> Zn++ + 2e- At cathode: Zn++ + 2e- -------------> Zn
Steps in the metallurgy
1. Crushing and Pulverization→ The ore is first crushed by a jaw crusher into small pieces and then powdered or pulverized in ball mills. This crushing and pulverization process removes rocky material from the ore.2. Concentration→ The Sulphide ore is concentrated by froth floatation process. In this process, powdered ore is dropped into water containing some pine oil in a big drum and stirred by means of blast of air. The particles of ore come to the surface along and leaving the froth behind and impurities at the bottom of the tank.
During this process, some of the zinc sulphide is also concentrated into sulphate. 
To avoid this, the roasting is done at above 900oC temperature. At this temperature, sulphate is decomposed to oxide again. 2ZnSO4-----------------> ZnO + 2SO2 +O2 ↑ In this roasting process, volatile impurities are also removed as their oxide. 
4. Reduction Zinc Oxide obtained by roasting is heated with carbon in the furnace to reduce the oxide into metallic form. 
• Vertical Retort ProcessThe powdered oxide form the ore(ZnO) obtained from roasting process is mixed with the coke(C) in a ratio 2:1 to make small briquets which are fed into the retort furnance through charging door. This feeding is a continous process. The retort is heated externally by burning the producer gas (CO + N2) at around 1400-1500oC. In this temperature, zinc oxide gets reduced to zinc by coke. 
The vapour of zinc and carbon monoxide gas produced in this process are carried into condenser by a wild current of producer gas passed through the bottom of furnance. The zinc vapour is condensed and zinc thus obtained is zinc spelter which is 97% pure.5. Purificationa. Fractional Distillation→ Zinc Spelter is subjected for distillation at 1000oC. This temperature is greater than the boiling point of Zinc and cadmium. So, Zn and Cd vapouriz and Fe & Pb remain in the distillation vessel. The vapour of Zn and Cd are again heated at 800oC, cadmium is distilled over leaving zinc which is 99% pure.b. Electrolysis→ An electrolytic cell is made with acidified ZnSO4 solution as electrolyte, anode of zinc spelter and cathode of pure zinc. Pure Zinc is deposited at cathode and impurities are left at the bottom of anode as anode mud after passing electric current. At anode: Zn -------------> Zn++ + 2e- At cathode: Zn++ + 2e- -------------> Zn