Zinc

Overview of Industrial Zinc Production Process

Since ancient times, zinc ores were converted into zinc compounds having healing properties. Metallic zinc was first produced in India in the 14th century and then in China in the 17th century.

Ten million tons of zinc is now produced annually around the world; 80 per cent of the mines exist underground, 12 per cent are a combination of underground and open pits and the remaining 8 per cent are solely open pits.

They contribute 64, 21 and 15 per cent to the overall zinc production volumes respectively. In all cases, the mined ore contains copper, iron, lead, silver and other mineral impurities, and the zinc content is only around 10 to 15 per cent.

The ore is crushed and ground at the mining site to increase its zinc content to 55 per cent. It forms a concentrate, which is the raw material used in industrial zinc production processes.

Extraction of Metallic Zinc from the Concentrate:

The zinc concentrate contains up to 30 per cent of sulfur with traces of lead, iron and silver. There are two main processes to recover metallic zinc from this concentrate, as described below.

1. Hydrometallurgical or Electrolytic Process:

  • The zinc concentrate is roasted or sintered at 900 degrees Celsius in the presence of air to get rid of the sulfur content. Zinc sulfide converts to zinc oxide, while sulfur and oxygen react to create sulfur dioxide gas; sulfuric acid is the main byproduct of the commercial zinc production process.
  • The impure zinc oxide is called calcine; it is powdered and mixed with diluted sulfuric acid through a process called leaching to separate zinc from iron. Zinc oxide is quite active and contains dissolved metallic zinc. Iron precipitates while lead and silver do not dissolve; hence these substances can be easily removed.
  • Cadmium, cobalt, copper and nickel dissolve in the solution, so further purification of the dissolved solution is carried out by adding zinc powder. Zinc is a reactive substance; it oxidizes and dissolves while the other impurities lie below zinc in the electrochemical series and hence convert back to metallic form; they are separated through precipitation.
  • The purified zinc sulfate solution is electrolyzed between aluminum cathodes and lead alloy anodes with a difference of 3.5 Volts. The electric current passing through the electrolytic solution for about 48 hours causes pure zinc to get deposited on the cathode.
  • The metallic zinc can be easily stripped off from the aluminum cathodes. Once it dries, it is melted and cast into high grade (HG) or special high grade (SHG) zinc ingots of 99.95 and 99.99 per cent purity respectively.

2. Pyrometallurgical or Imperial Smelting Process:

This is another process used in 10 per cent of the zinc production cases. It produces both metallic zinc and lead in a 2:1 ratio.

  • The concentrates are roasted on a slow-moving grate in a special imperial melting furnace. Air is blown over the grate to burn the sulfur present in the concentrate and produce zinc oxide and lead oxide.
  • The presence of carbon in the furnace reduces these oxides to metallic zinc and lead respectively.

Imperial smelting is a high energy-consuming thermal process, making it too expensive commercially; its use is restricted to Japan, China, Poland and India. Also, the zinc produced through this process contains some amounts of copper, tin, lead, cadmium and iron; it has a lower purity than the final product of the electrolytic process. The electrolytic process of producing zinc is the most common and is used 90 per cent of the time.