The Extraction of Metals and Its Impact on the Environment (PART TWO)

ZINC EXTRACTION PROCESSES



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The most important raw material for production of Zinc :
Zinc Sulphide (ZnS) (Sphalerite)
Zinc Carbonates (Calamine/Smithsonite)
Zinc Silicate


A. Pyrometallurgical Processes:
i) Horizontal Retort Process;
ii) Vertical Retort Process;
iii) Electro Thermal Process;
These three processes have become outdated due to high power consumption and low recovery.

iv) Imperial Smelting Process
This process is similar to blast furnace processes except that it is operated with hot top whereby preventing reoxidation of zinc vapours. The process consists of basic two operations namely:
1. Sintering
2. Blast furnace smelting of sintered lumps to extract lead and zinc simultaneously

B. Hydrometallurgical Processes:

i) Roast Leach Electrowin Process

• To convert it into a product, this is amenable to further treatment through hydrometallurgical process for extraction of zinc.
• To fix the sulphide contents into sulphur dioxide gases for subsequent economical recovery as sulphuric acid.
• The principal reaction:
  2 ZNS + 3O2 = 2ZnO + 2SO2
• These rich gases are cleaned and cooled to recover dust content as zinc calcine and to remove the various harmful impurities such as Hg, Se, F, Cl, As, etc… for manufacture of sulphuric acid.
• The zinc calcine is leached with recycled electrolyte to extract zinc content.
• The enriched zinc sulphate solution is further subjected to purification with zinc dust to eliminate impurities like copper, cadmium, cobalt, nickel etc. before being subjected to elecrolysis.

ii) Pressure Leaching Process

• In this process zinc sulphide or zinc concentrates are oxidized under oxygen overpressures of 1200 kpa abs. at a temperature of 1500C in sulphuric acid medium to produce zinc sulphate solution directly.
• The sulphide content is precipitated as elemental sulphur according to the following overall reaction:
  ZnS + H2S04 + 0.5 02 = ZnSO4 + H20 + Sº

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