GCSE OCR Gateway Chemistry C6.1c Extracting copper using bacteria and solvents

GCSE OCR Gateway Chemistry C6.1c Higher tier

C6.1c To be able to evaluate alternative biological methods of metal extraction: bacterial and phytoextraction.

Extracting copper using low energy processes

Processes other than smelting can be used to extract copper from its ore.  One process uses special solvents and the other uses special bacteria.

Both processes operate at low ambient temperature and therefore require less energy than conventional smelting.

Using these novel processes, copper to be extracted from an entirely different set of ores and mining by–products than is possible by smelting; namely, oxidized materials.

These may be mined copper minerals that are in oxidized form— minerals such as Azurite (2CuCO3 · Cu(OH)₃), Brochantite (CuSO4), Chrysocolla (CuSiO3 · 2H₂O) and Cuprite (Cu₂O), or residual copper in old mine waste dumps whose sulfide minerals have been oxidized by exposure to the air. 

One advantage of these processes is that they can be used to extract copper from where it is found, that is, without removing the material from the waste dump or from the ground.

The net result of the use of these processes is that copper can be produced from sources that in the past would have gone untouched, thus reducing the reliance on conventional ore reserves.

Furthermore, these processes are capable of removing copper from waste materials that would otherwise contaminate the environment.

There is no effluent since all impurities are returned to the site where they originated and the sulfuric acid used is eventually neutralized by the limestone in the waste dump where it is deposited as calcium sulfate (gypsum) - a very insoluble substance.

Solvent extraction process

The solvent process involves leaching the material with a weak acid solution. Copper from the waste dump dissolves in the acid. The resulting solution, known as pregnant liquor, is recovered and then brought into contact with an organic solvent.  The copper is extracted into the organic solvent from the acid solution leaving behind most of the impurities that were present in the leached solution.

The copper-bearing organic solution is then processed into a metal cathode. The copper cathodes produced this way are as pure as or purer than electro–refined cathodes from the smelting process.

The conventional smelting process requires 65 MJ/kg of copper but the solvent extraction process requires between 15 MJ/kg to 36 MJ/kg depending on where the copper originates.

Another advantage of the solvent extraction process is the low capital investment required relative to the smelting process and its ability to be operated economically in a small scale. In China, for example, where copper deposits are not plentiful and tend to be small, there are 40 to 50 "mom & pop" leaching operations.

Bacterial leaching or extraction

Bacterial leaching or bioleaching is a new technology.  It produces environmentally clean copper.  It is used in tandem with the solvent extraction process.

Modern commercial use of bacterial leaching began in the 1950s at Kennecott's Bingham mine near Salt Lake City, Utah. It was noticed that blue copper-containing solutions were running out of waste heaps that contained copper sulfide minerals - a condition that should not have happened in the absence of powerful oxidizing agents and acid.

An investigation found that naturally occurring bacteria were oxidizing iron sulfides and the resulting iron(III)sulfate was acting as an oxidant and leachant for copper sulfides.

The bacteria involved were given the name ferrooxidans for their action in oxidizing iron sulfides.

A second set of bacteria were also found and given the name thiooxidans because they were able to oxidize sulfur to sulfuric acid.

Bacterial leaching, combined with solvent extraction, offers a method of exploiting small ore bodies with a minimum of capital investment.

Most commercial operations leaching copper from ore dumps are located in the Southern Hemisphere in Australia, Chile, Myanmar and Peru.

The ore dump is injected with cultivated strains of appropriate bacteria.  The bacteria are kept alive in the right conditions so that they can operate effectively and propagate themselves.

Air, for instance, is blown into the ore through air–lines situated under the leach pad.

Pilot plant tests are underway to see if bacterial leaching can replace the smelting of copper ores.

The solvent extraction process and the bacterial leaching process have provided the copper industry with a tool that makes the extraction of copper from its ores significantly more environmentally friendly than by the use of the conventional smelting process.