Bioleaching of Copper

Biological copper leaching is practiced in many countries including Australia, Canada, Chile, Mexico, Peru, Russia, United States of America etc. Copper recovery from bioleaching accounts for about 20% of the world copper production.

Thiobacillus ferrooxidans was isolated from coalmine water and studies revealed its presence in copper-leaching operations. Thiobacillus bioleaching is significant in commercially important recovery of copper from iron and sulphur containing ores such as chalcopyrite- CuFeS₂, chalcocite- Cu₂S and Covellite-CuS.

The majority of copper minerals in the ore are sulfides, with chalcopyrite (CuFeS2) being the most abundant and thus economically the most important.

Sulfides are insoluble in water or acid solutions unless they are first oxidized. Thiobacillus ferrooxidans catalyzes the oxidation of iron whereas and Thiobacillus thiooxidans  catalyzes the oxidation of sulfur.

There are two mechanisms involved in bioleaching. The first is the direct microbial action on the sulfide mineral in the ore, and enhanced rate of oxidation of the mineral directly. This is known as the direct mechanism. Indirect process involves the microbial oxidation of ferrous to ferric ions followed by the chemical oxidation of the sulfide mineral by the ferric ion. This is known as the indirect mechanism.

Regardless of the mechanism, it is the enhanced oxidation of the mineral created by the microorganism that makes it  soluble in the acid for leaching and recovery

 

1)      Direct oxidation of sulfide ores such as CuS (covellite)

CuS (covellite) + 20₂ è CuS04 

 

2)      Indirect oxidation via ferric-ferrous cycle

Microorganisms catalyze the oxidation of iron sulfides to create ferric sulfate. Ferric sulfate, which is a powerful oxidizing agent, then oxidizes the copper sulfide minerals to liberate copper sulphate and elemental sulphur.

CuFeS₂ (chalcopyrite) + 2Fe₂(S0₄)₃ (Ferric sulphate) è CuS0₄ + 5FeS0₄ (Ferrous sulphide) + 2S

Elemental sulphur generated by indirect leaching can be converted to sulphuric acid by Thiobacillus ferrooxidans:

2S + 30₂ + 2H₂Oè2H₂S0₄

Copper is then leached by the sulfuric acid formed.The sulphuric acid maintains the pH at levels favourable to the growth of bacteria. 

It can also be of environmental concern as acid mine drainage.

Direct oxidation can also occur for Chalcopyrite, as shown with Covellite.

.Bioleaching operations worldwide for copper follow the same pattern.

Ø  copper ore mined from open pits is segregated

Ø  higher-grade material is concentrated for smelting, while the lower-grade ore is subjected to leaching

Ø  The ore is piled on an impermeable surface as a dump 

Ø  The top is levelled

Ø  Leach solution (water or water with dilute sulphuric acid) is flooded or sprayed onto the dump

Ø  Leach solutions enriched with copper exit at the base of the dump and are recovered

Ø  Copper recovery from leachate/leach solutions is either by solvent partitioning or reaction with scrap iron.

 

(Electrowinning- electroextraction, is the electrodeposition of metals from their ores)

Practically, 50-70% copper can be recovered from a low grade ore by biomining. Costs are about ½ or 1/3^rd of direct smelting. However, it is a slow process and may need many years for reasonable recovery of the metal.