The copper ions are transferred from the water where they are of a high concentration to the ligand-solvent solution where they are of low concentration. The ligand is then mixed with a small volume of sulphuric acid to reverse the process and further increase the concentration of copper ions. An electro-winning process is then carried out, whereby a current is run through the solution containing the Cu2+ ions, and pure copper atoms collect on the cathodeε.
The extraction process for gold is different, initially froth floatation occurs to separate the tailings from the tailings and bioleaching is then carried out on the slurry of concentrate. Also the bacteria used to break down the mineral matrix surrounding the gold particlesη can be different, for example sulpholobus acidocalderius was used during research by King’s College, London, researchersη. Following this cyanidation is carried out, during this process aerated sodium cyanide is blended with mixture A(see figure 2 below) and the gold atoms dissolve in the cyanide and the oxygen then causes the formation of sodium cyanoaurite and sodium hydroxideθ which are solids and then separated from them through absorption by carbon.
The extraction process for gold is different, initially froth floatation occurs to separate the tailings from the tailings and bioleaching is then carried out on the slurry of concentrate. Also the bacteria used to break down the mineral matrix surrounding the gold particlesη can be different, for example sulpholobus acidocalderius was used during research by King’s College
Advantages and disadvantages of using bacterial leaching over traditional methods
There are many advantages of using bacterial leaching which are associated with both gold and copper, for instance tailings from traditional methods and very low grade ore can be used which increases the mining company’s profit and decreases the detrimental effects of mining on environmental appearance. Environmental advantages also include that the bacteria used can be recycled for reuse, decreasing expenditure. In some cases the use of bacterial leaching means that excavation need not occur because the bacteria can be pumped into holes in the ground, left to oxidise the ores and then pumped out through other holes. There are also economical advantages associated with bacterial leaching, including decreased labour costs as less specialist knowledge is required by employeesζ.
Some environmental disadvantages are associated with bacterial leaching including that heavy ions such as iron, zinc and arsenic, can leak out of the mine during draining, forming an ionic precipitate when diluted by fresh waterζ. Economical disadvantages include that the process is lengthy in comparison to traditional methods, causing cash flow problems in new plants.
Some specific advantages and disadvantages apply, for example the greatest advantage of using bacterial leaching to extract copper over smelting is that no sulphur dioxide is released into the atmosphere, traditionally 2 tonnes of gas are produced for every tonne of copperα. Another advantage is that ligands allow concentration of the copper ions before electro-winningγ, decreasing the volume of waste solution at the end of the process. The major disadvantage of using this method is that it can take decades for an ore deposit to be exhausted rather than a few months or years, as occurs with smelting.
The advantages of using bioleaching to extract gold are that the proportion of gold recovered increases dramatically because the bacteria breakdown the mineral matrix which surrounds the gold, allowing more effective cyanidation. Additionally, even with no gold recovered, other metals that are recovered such as copper, nickel, cobalt and zinc produce an economically competitive mineη. Bacterial leaching is also effective over a wide range of temperatures and pHsη, meaning that it can be used throughout the world, this also applies to the financial ease of production. There are no specific disadvantages of using bioleaching to extract gold.
I suggest that the reason that bioleaching is the primary extraction method of gold and only a secondary method for copper is that copper mining companies are reluctant to take into account the long production time and gold mining companies are attracted by the significantly increased recovery.
Developmental stages of new mining processes prior to commercial use
In order for a new mining process to become commercial, funding must be found for essential research to ensure that the process is economical for the companyη. Pilot plants must be tested in locations where the mine is likely to be placed and a thorough environmental impact assessment carried out to ensure that the processes do not cause undue environmental damageι. Finally governmental permission must be obtained before commercial activity can take placeι.
References
α – Article 1 from examination pack: Mining with microbes, John Merson.
β – How hydrometallurgy and the SX/EW process made copper the “green” metal, William
Dresher.
γ – Article 2 from examination pack: Extracting copper from leaching solutions.
δ – Columbia electronic encyclopaedia.
ε – www.allrefer.com/reference/encyclopaedia
η – Article 3 from examination pack: A golden opportunity, Jack Barrett and Martin Hughes.
θ – www.gaiaguys.net
ζ – Wikipedia, the free online encyclopaedia.
ι – www.deh.gov.au/industry
