L = Ligand
This process moves the copper ions from the water, where they are at a low concentration, to the organic solvent, where they are at a high concentration. After the copper ions have been removed the remaining Fe2+ ions and Fe3+ ions are oxidised in an open pond with the T. ferro-oxidans catalysing this reaction. The recharged leeching solution is then pumped back to the beginning of the cycle to be used again.
The copper is extracted as sheets through an electro-winning process. An electric current is passed through the copper ion solution. The copper ions are oxidised and collect at the negative electrode as pure copper. This is the half equation occurring at the negative electrode:
Cu2+ (aq) + 2e- → Cu (s)
Gold, Au, is found as microscopic particles encapsulated in a mineral matrix. Common minerals containing gold particles are arsenopyrite (FeAsS), iron pyrites (FeS2) and chalcopyrite (FeCuS2). Froth floatation is used to separate these refractory minerals from any unwanted oxide ores and non-metallic minerals present. We are now left with gold particles encapsulated in these minerals.
If we take arsenopyrite (FeAsS) as the example the mineral has to be oxidised into soluble compounds of iron, arsenic and sulphur. The overall reaction is as follows:
2FeAsS + 7O2 + 4H+ + 2H2O → 2Fe3+ + 2H3AsO4 + 2HSO4-
In this reaction, the bacterial oxidation occurs in two stages. Firstly, the bacteria act as a catalyst in the formation of soluble compounds of iron(II), arsenic(III) and sulphur(VI), this is shown in the equation below.
FeAsS Fe(II) + As(III) +S(VI)
Secondly, iron(II) and arsenic(III) are oxidised by the bacteria.
Fe(II) Fe(III)
As(III) As(V)
During the whole bacterial oxidation of arsenopyrite, no gases are produced. The main products are iron(III), arsenic(V) and sulphuric acid(VI), which are all water-soluble.
The gold has now been liberated from its ore. It now needs to be removed from the solution. This is done by cyanidation. Sodium cyanide is added to the solution. The gold and cyanide react to give complex gold cyanide. The gold is then precipitated from the aurocyanide by the addition of zinc dust, a more reactive metal. Pure gold is produced and is removed by smelting.
By using bacterial leaching to extract copper from its ore rather than using traditional methods produces both advantages and disadvantages. The main advantages are that bacterial leaching does not produce any SO2 compared to smelting, which produces two tonnes of SO2 per one tonne of copper produced. Smelting is much more expensive than bacterial leaching, $130-$200 per tonne compared to $70 per tonne. At the present moment bacterial leaching does not require new mines to be dug as old mines can be converted. This saves money and preserves the environment. Bacterial leaching can be used on low-grade ores to extract copper where smelting would not be cost effective. Tailing pools are not produced. Requires little energy as bacteria “feed” off iron ions and sulphur ions. The main disadvantages are that bacterial leaching takes a lot longer than smelting to extract the same amount of copper. It could take bacterial leaching decades to recover most of the copper from a deposit, whereas smelting can recover it in a matter of months. In addition, the bacteria require a source of nutrients containing phosphorus and nitrogen.
The advantages and disadvantages of using bacterial oxidation to liberate gold from its ore are as follows. The advantages are that after bacterial oxidisation of the mineral encapsulating the gold, 100% of the gold can be recovered by cyanidation as oppose to 10% using cyanidation alone. When this method is used on arsenopyrite, no gases are produced. Whereas, traditional methods produce sulphur dioxide and arsenic(III) oxide, which both have undesirable environmental and economic implications. This method of gold extraction can also lead to the production of base metals such as copper, nickel, cobalt and zinc. The disadvantages are that tailing pools are still produced, arsenic is produced and existing technology would have to be adapted.
The reason that bacterial leaching is used as a primary extraction process for gold and as secondary extraction process for copper is that when this method is applied to copper extraction it takes a lot longer than the traditional method to produce the same amount of copper, so the delay in cash flow stops this being a viable solution to the mining companies needs. Using this method to extract gold from its ore is a primary process because the bacteria do nothing to hinder the process. They do not increase the time significantly; all they do is greatly increase the amount of gold produced.
The development stages that a new mining process must pass before it can operate commercially are that it must be able to meet demand, the development must be fast enough to produce the metal so that the process is cost effective. It must be safe and not damage the environment.
The main points of my report are as follows:
- Detail the processes involved in using bacteria to liberate copper and gold from their ores
- Outline the advantages and disadvantages of using bacterial leaching to extract copper and gold from their ores rather than traditional methods
- Summarise the development stages a new mine must pass to operate commercially