Extraction of copper from its ore by using bacterial leaching

impermeable.Then an acidic leaching solution containing T. ferro-oxidans and T.thio-oxidans is sprayed over ore and the tailings. Because these bacteria need only asupply of S- ions or Fe-ions, carbon dioxide and oxygen to live, they don’tneed any organic material on which to feed. The bacteria than convert theinsoluble sulphide minerals into a solution containing Cu , Fe , Fe and SO ions.It is easy to remove the solution which contains the copper ions, because ofthe impermeable layer. To get out of this solution only the copper ionsanother process is required, the ligand exchange solvent extraction. A good ligandfor copper ions is dissolved in an organic solvent ( e.g. kerosene), but itmust be immiscible with water.Picture of kerosene structure:After shaking this solution with the water containing the copper ions, theseare removed from there where they are in low concentration ( in water) andtransferred to the organic solvent ( high concentration).Equation: (some words to the reaction!)Ligand structure:Two molecules of the ligand react with one copper ion to form a planarcomplex, which has no overall charge and dissolves readily in the kerosene but notin water. The rest of the leaching solution goes in an open pond, where the bacteriacatalyse oxidation of the remaining Fe ions to Fe ions, which would rechargesthe solution. It is then pumped back to the low-grade ore so that the cyclecan start again.In an ‘electro-winning’ process an electric current is passed through thecopper ion solution, which has been removed from the leaching solution. On thenegative electrodes the pure copper collects.Extraction of gold from its ore by bacterial leaching Between 15% and 30% of the world’s gold reserves are in form of refractoryminerals ( microscopic particles of gold encapsulated in a mineral matrix)(1.1), e.g. arsenopyrite (picture), iron pyrites and chalcopyrite. (pictures)To separate these from any unwanted oxide ores and non-metallic mineralsusually froth flotation is used. The product is a sulphide concentrate, whichis then roasted to liberate the gold which is extracted by treating theresulting mixture with an aerated solution of sodium cyanide. The process ofcyanidation is very difficult and also the roasting causes a lot of environmentalproblems because it converts for example any sulphur in the refractoryminerals to sulpherdioxide. Therefore researchers investigated various ways ofconverting the gold in the concentrate into soluble compounds. One of these wereusing the thermophilic bacterium sulpholobus acidocalderius, which catalysethe oxidation of the encapsulating sulphide minerals by dioxygen under aqueousconditions at 70 degree. Cyanidation of the resulting extract increased thegold recovery from 10% to 100%.Bacterial oxidation has two stages:1. Reaction between the surface of the bacterial cell and the arsenopyrite,where the bacteria catalyse the formation of soluble compounds of iron(II),arsenic(III) and sulphur(VI).(diagram)2. Separate reactions in which iron (II) and arsenic(III) are oxidised.(Diagram)Main products are iron(III), arsenic(V) and sulphuric(VI) acid, all in watersoluble. Waste water is treated with hydrated calcium hydroxide or crushedlimestone in the form of a slurry with water, which neutralises thesulphuric(VI) acid and causes the precipitation of a gelatinous mixture of iron(III)arsenates and iron oxohydroxide (FeO(OH)), together with gypsum (CaSO 2H O).The stable arsenic compound arising from the mixture is iron(III)arsenate(V) dehydrate. The overall oxidation can be shown in this equation:Much better results are produced with an isolation of a mixed culture ofmoderately thermophilic bacteria. This one has its best working temperature at46 degree and can greatly enhancing the percentages of gold extracted.Diagram Advantages and DisadvantagesAdvantages: it is cheaper and less polluting, you can use the bacteria in acycle ( leaching solution is recycled), it isn’t so energy intensive.Researches are made to make the process completely biological, because of othermethods, resources became exhaustedDisadvantage: takes a long time, still the process of electro-winning iscostly, These bacteria were given the name - ferrooxidans for their action in oxidizing iron sulfides. A second set of bacteria were also identified and given the name - thiooxidans for their action in oxidizing sulfur to yield sulfuric acid. Most commercial operations leaching copper from ore dumps are located in the Southern Hemisphere in Australia, Chile, Myanmar and Peru. .