BIOLEACHING

Bioleaching is the extraction of specific metals from their ores through the use of bacteria.

Bioleaching is a new technique used by the mining industry to extract minerals such as gold and copper from their ores. Traditional extractions involve many expensive steps such as roasting and smelting, which requires sufficient concentrations of elements in ores. Low concentrations are not a problem for bacteria because they simply ignore the waste which surrounds the metals, attaining extraction yields of over 90% in some cases. These microorganisms actually gain energy by breaking down minerals into their constituent elements. The company simply collects the ions out of solution after the bacteria have finished.

Some advantages associated with bioleaching are:

• economical: bioleaching is generally simpler and therefore cheaper to operate and maintain than traditional processes, since fewer specialists are needed to operate complex chemical plants.

• environmental: The process is more environmentally friendly than traditional extraction methods. For the company this can translate into profit, since the necessary limiting of sulphur dioxide emissions during smelting is expensive. Less landscape damage occurs, since the bacteria involved grow naturally, and the mine and surrounding area can be left relatively untouched. As the bacteria breed in the conditions of the mine, they are easily cultivated and recycled.

Some disadvantages associated with bioleaching are:

• not economical: the bacterial leaching process is very slow compared to smelting. This brings in less profit as well as introducing a significant delay in cash flow for new plants.

• not environmental: Toxic chemicals are sometimes produced in the process. Sulfuric acid and H⁺ ions formed can leak into the ground and surface water turning it acidic, causing environmental damage. Heavy ions such as iron, zinc, and arsenic leak during acid mine drainage. When the pH of this solution rises, as a result of dilution by fresh water, these ions precipitate, forming "Yellow Boy" pollution. For these reasons, setup of bioleaching must be carefully planned, since the process can lead to biosafety failure.

Currently it is more economical to smelt copper ore rather than to use bioleaching, since the concentration of copper in its ore is generally quite high. The profit obtained from the speed and yield of smelting justifies its cost. However, the concentration of gold in its ore is generally very low. The cheaper cost of bacterial leaching in this case outweighs the time it takes to extract the metal.

The process

The extraction of copper from its ore involves two bacteria, Thiobacillus ferro-oxidans and Thiobacillus thio-oxidans. In stage 1, bacteria catalyse the breakdown of the mineral arsenopyrite (FeAsS) by oxidising the sulfur and metal (in this case arsenic ions) to higher oxidation states whilst reducing dioxygen by H₂ and Fe₃⁺. This allows the soluble products to dissolve.

FeAsS_(s) -> Fe₂⁺_(aq) + As₃⁺_(aq) + S₆⁺_(aq)

This process actually occurs at the cell membrane of the bacteria. The electrons pass into the cells and are used in biochemical processes to produce energy for the bacteria to reduce oxygen molecules to water.

In stage 2, bacteria then oxidise Fe₂⁺ to Fe₃⁺ (whilst reducing O₂).

Fe₂⁺ -> Fe₃⁺

They then oxidise the metal to a higher positive oxidation state. With the electrons gained from that, they reduce Fe₃⁺ to Fe₂⁺ to continue the cycle.

M₃⁺ -> M₅⁺

The gold is now separated from the ore and in solution.

The process for copper is very similar. The mineral chalcopyrite (CuFeS₂) follows the two stages of being dissolved and then further oxidised, with copper²⁺ ions being left.

Extraction from mixture

Copper (Cu²⁺) ions are removed from the solution by ligand exchange solvent extraction which leaves other ions in the solution. The copper is removed by bonding to a ligand, which is a large molecule consisting of a number of smaller groups each possessing a lone pair. The ligand is dissolved in an organic solvent such as kerosene and shaken with the solution producing this reaction:

Cu²⁺_(aq) + 2LH(organic) -> CuL₂(organic) + 2H⁺_(aq)

The ligand donates electrons to the copper, producing a complex - a central metal atom (copper) bonded to 2 molecules of the ligand. Because this complex has no charge, it is no longer attracted to polar water molecules and dissolves in the kerosene, which is then easily separated from the solution. Because the initial reaction is reversible, and therefore not a displacement reaction, it is determined by pH. Adding concentrated acid reverses the equation, and the copper ions go back into an aqueous solution.

Then the copper is passed through an electro-winning process to increase its purity: an electric current is passed through the resulting solution of copper ions. Because copper ions have a 2+ charge, they are attracted to the negative cathodes and collect there.

The copper can also be concentrated and separated by displacing the copper with Fe from scrap iron:

Cu²⁺_(aq) + Fe_(s) -> Cu_(s) + Fe²⁺_(aq)

The electrons lost by the iron are taken up by the copper. Copper is the oxidising agent (it accepts electrons), and iron is the reducing agent (it loses electrons).

Traces of precious metals such as gold may be left in the original solution. Treating the mixture with sodium cyanide in the presence of free oxygen dissolves the gold. The gold is removed from the solution by adsorbing (taking it up on the surface) to charcoal.

Further Reading

• Billiton - http://www.imm.org.uk/gilbertsonpaper.htm
• Bactech - http://www.bactech.com
• T. A. Fowler and F. K. Crundwell - 'Leaching of zinc sulphide with Thiobacillus ferrooxidans'