Bioleaching

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

The mining industry has recently started using a new technique to extract minerals such as gold and copper from mines using bioleaching technology. Traditionally, the extraction would involve many expensive stages such as roasting in a smelting process.

The process

There are two bacteria involved in the process, [Thiobacillus ferro-oxidans]? and Thiobacillus thio-oxidans. In stage 1, bacteria catalyse (accelerate without being used up) the break-up of the mineral arsenopyrite (FeAsS) by oxidising (when the ions lose electrons) the sulfur and metal (in this case arsenic) ions to higher oxidation states whilst reducing dioxygen H₂ and Fe₃⁺. This allows the soluble products to dissolve in the solution.

  FeAsS(s) -> Fe2+(aq) + As3+(aq) + S6+(aq)

This process occurs actually at the cell membrane of the bacteria. The electrons pass in to their 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⁺ in 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, disolved 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.

Details of metal extraction from mixture

Copper

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 smaller molecules 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. By adding concentrated acid it 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 purity. This is where an electric current is passed through the resulting the 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 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 as it accepts electrons, iron is the reducing agent as it loses them.

Gold

Left in the original solution there may be traces of precious metals such as gold. Treating the mixture with [sodium cyanide]? with free oxygen present dissolves the gold. The gold is removed from the solution by adsorbing (taking it up on the surface) to charcoal?.

Advantages/disadvantages of bacterial leaching compared to traditional mining processes

Advantages of bioleaching

• It is more environmentally friendly than smelting. This also translates in to profit, as the companies were having to spend money finding ways of limiting their SO₂ emission, now this is not a problem.

• Less landscape damage has to occur, as the habitat of the bacteria grow naturally. The bacteria can operate, with good efficiency, at temperature ranges of -22'C to 55'C. Other parts of the process are also natural such as air and water, and the bacteria can be recycled.

• Compared to smelting, where the industry was having to smelt greater amounts of less concentrated ore to produce the same amount of copper, in biohydrometallurgy the concentration is not a problem, as the bacteria do all the extraction and ignore the low-concentrate waste which surrounds the precious metals. The company simply collects the ions out of solution when they have been extracted.

• The bacteria breed on their own, it does not cost more to get more bacteria. Also with 92% extraction in some cases, more metal extracted for the same amount of ore. Without needing to pay for the temperatures and chemicals needed in smelting plants the companies were able to drop the price from $130-$200 per kilo to $70 per kilo. The companies were having to smelt ores with lower concentrations of copper.

• It is simpler and therefore cheaper to operate and maintain, as no specialists need to be involved operating complex chemical plants.

Disadvantages

• The bacterial leaching process is very slow compared to smelting. Not only would this bring in less profit annually, but their would be a significant delay in cash flow when the system was first installed making companies less likely to want to adopt it.

• Dangerous chemicals are produced in the leaching process. The [sulfuric acid]? and H⁺ ions formed can leak in to the ground and surface water turning it acidic - spoiling habitats for animals. Also heavy ions such as iron/zinc/arsenic leak with the 'acid mine drainage', when the pH of this solution rises as a result of dilution by fresh water these ions precipitate out forming 'Yellow Boy' pollution. Fe₃⁺ is highly corrosive and can damage pipes.

Choice of extraction methods

For copper, at the moment, it is more economical to smelt the ores as they generally have quite a high concentration of the metal. This means that the profit from more copper being produced faster, is more than the lower cost and higher yield efficiency of bacteria make up for in terms of profit. Whereas because the concentration of gold in ore is so low the cost to smelt it, despite the fast rate equaling more gold, would be so high compares to the slower rate but cheaper cost of bacterial leaching.

Development stages that a new mining process must pass through before it can operate commercially

First research has to be done proving that this method is economically viable. When additional funding is gained, the research is continued to try and further improve the efficiency of the process, varying factors such as temperature, pressure, pH, (and in bacterial oxidation) strain of bacteria. Then the experiments start to scale the process up to the real size, using more realistic conditions and amounts. This enables researchers to anticipate problems and further refine the process. Finally a plant is built using this technique.

Further Reading

• Billiton - http://www.imm.org.uk/gilbertsonpaper.htm

• Bactech Company involved in bioleaching http://www.bactech.com

• T.A Fowler and F.K Crundwell Leaching of zinc sulphide with Thiobacillus ferrooxidans