Mechanisms of Metal Recovery,Bioleaching,Sulphates,Sulphides,Protons,Biooxidation,Arsenopyrite,Thiosulphate,Polysulphide

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Your Ad Here	Mechanisms of Metal Recovery Metal ores are generally found as sulphides, which are insoluble in water, so that they need to be converted into sulphates, which are readily soluble in water and, therefore, can be easily leached for further extraction of the metal. Mainly two strategies are used for the recovery of metal sulphates for further extraction. The first of these strategies, namely bioleaching involves conversion of the insoluble sulphide ores into soluble sulphates (due to action of ferric iron and the protons), so that only those metals can be recovered by this method, whose sulphides are insoluble, but the sulphates are soluble, and the second strategy, namely biooxidation involves decomposition of the mineral matrix to make it available for cyanidation, which is generally used for recovery of gold from recalcitrant arsenopyrite ores.
The two methods that are available for metal extraction through formation of soluble sulphates involve the formation of different intermediate products, which is a thiosulphate in one case (where metal sulphides are acid-insoluble and require attack of ferric iron for oxidation, e.g. FeS2, MnS2, WS2) and a polysulphide in the other (where metal sulphides are acid-soluble, e.g. ZnS, Cu2S, PbS), as shown below: in both these methods, ferric iron (through biooxidation) facilitates metal dissolution, and the resulting ferrous iron (reduced form) may be reoxidized into ferric iron (oxidized form) by iron-oxidizing organism
(e.g., At. caldus, L ferooxidans, L. ferriphilum, Leptosprillum, Sulphobacillus and Acidimicrobium). The main function of the microbes, therefore, is the production of sulphuric acid for the solubilization of metal sulphides, for the proton attack, and to keep the iron in its oxidized ferric state for an oxidative attack on the mineral. FeS₂+6Fe³⁺+3H₂OS₂O₃^2-+7Fe²⁺+6H⁺ thiosulphate S₂O₃^2-+8Fe³⁺+5H₂O2SO₄^2-+8Fe²⁺ 10H⁺
(i) thiosulphate mechanism MS+Fe³⁺+H⁺M²⁺+0.5H₂Sn+Fe²⁺polysulphide 0.5H₂S_n+Fe³⁺0.125S₈+Fe²⁺+H⁺ 0.125S₈+1.5O₂+H₂OSO₄^2-+2H⁺ (ii) polysulphide mechanism 2Fe²⁺+0.5O₂+2H⁺2Fe³⁺+2H₂O (iii) Re-oxidation of ferrous iron to ferric iron]

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