A good ligand for Copper is dissolved in an organic solvent i.e. Kerosene, then shaken with water containing the Cu ions, causing this reaction:
Cu2+(aq) + 2HL(organic) CuL2(organic) + 2H+(aq)
(Where L represents the ligand) Equation 1
This removes the low concentration of copper ions from the water and transfers them to the organic solvent, where their concentration is higher. This reaction (Equation 1) is reversible by mixing the organic solution with concentrated acid; pushing the Cu2+ ions back into the acidic solution, further increasing the concentration. Both these processes depend on the pH.
The remaining leaching solution flows into an open pond, where oxidation of Fe2+ and Fe3+ is catalysed by T. ferro-oxidans, recharging the leaching solution, ready to restart the cycle.
Copper is extracted as sheets by ‘electro-winning’, where a current is passed through the copper ion solution, causing the metal to collect at the cathode. This isn’t energy efficient. Bioabsorbtion filters such as algae are being researched, to collect the copper.
Biohydrometallurgy could also prevent the environmental damage caused by conventional underground mining. In Arizona, a mine consists of 5 holes drilled into a fractured ore deposit. An acidic leaching solution, containing bacteria is pumped down the central hole. Pumped from the other holes is the resulting solution, rich in copper ions.
The miners are reluctant to use biological processes due to their slowness. They have finally been applied to low-grade ores where traditional methods aren’t cost-effective. If metals could be extracted from lower grade ores, tailings sites could be transformed into sources of raw materials instead of pollution. Research on biological techniques of metal extraction is rare, but experts see environmental regulations as key to encouraging research.
This is rejected by the mining industry, arguing that it has always been advancing research barriers, searching for new techniques e.g. they funded the biohydrometallurgy research.
If society wants the mining industry to protect the environment, they must accept higher prices.
Gold mining is risky, after finding deposits, the gold’s extraction is also difficult. Roughly 30% of the world’s gold reserves occur as microscopic particles of gold encapsulated in a mineral matrix. The first stages of obtaining gold is to use froth floatation to separate these refractory minerals from any unwanted substances (see diagram 2). The gold is liberated by a sulphide concentrate and then extracted by treating this mixture with aerated sodium cyanide. This process, called cyanidation, is difficult because the mineral matrix acts as a shield, protecting the gold particles from the cyanide ions. Also, only a fraction of the gold is yielded. Roasting has undesirable environmental implications.
Undergraduates investigated ways alternative of converting the gold in the concentrate into soluble compounds, using acidic solutions of thiourea and different oxidants. Researchers at Queen Elizabeth College treated the refractory sulphide concentrate with Sulpholobus acidocalderius. These bacteria catalysed the oxidation of the sulphide minerals. Cyanidation of the resulting extract led to an increase in gold recovery to 100%, but the team lacked the funding to continue researching. Richard Ratcliffe managed to attract support in the UK and Australian businesses set up a company, BacTech, to exploit the process in Australia. These funds allowed an extension of the studies to bacterial oxidation to a variety of gold-bearing mineral concentrates. Ali Nobar succeeded in isolating a mixed culture of thermophilic bacteria. This culture was robust and adaptable, effective over a wide range of conditions. It worked best temperatures of 46°C and in solutions with pH 0.5-1.5.
In London, they discovered the overall mechanism of the bacterial oxidation, shown below:
2FeAsS + 7O2 + 4H+ + 2H2O 2Fe3+ + 2H3AsO4 + 2HSO4-
Equation 2
1st stage: reactions occur at the interface between the bacteria cell surface and the arsenopyrite:
FeAsS Fe(II) + As(III) + S(VI)
Equation 3
2nd stage: separate oxidation reactions:
Fe(II) Fe(III)
Equation 4
As(III) As(V)
Equation 5
Wastewater is treated with limestone, neutralising the sulphuric(VI) acid, causing the precipitation of iron(III) arsenates, FeO(OH) and CaSO4.2H2O. Possibly, the precipitated material and dissolve arsenic(V) into water, but the highest concentration in such rivers doesn’t reach the US limit.
They couldn’t progress beyond the 1dm3 reaction vessel before the technology was sold to BacTech, who progressed to a 450 dm3 laboratory plant, then to a 32 m3 transportable plant which was used for test runs around the world, giving excellent results under varied conditions.
11 years later, the first full-scale plant using this process was commissioned. The mine operated as an open-cut until 1992, when accessible oxide layers became uneconomic. In 1993, underground mining allowed access to the deeper layers. The mining process can be seen in diagram 4. The cost of running this plant is low because they use locally Quarried calcrete to neutralise effluent and the oxidised concentrate. Other reagents are bacterial nutrients, mainly in the form of ammonium phosphate. Recent work shows that bacterial recovery of base metals is feasible and economically competitive.
Diagram 2
