In general, rock strata contain metals in their sulfide form. These metal sulfide ores are the principal targets during the process of leaching. In the leaching process, sulfide ores undergo the process of weathering or decaying by physical, chemical, biological means or their combinations. Microbes especially, the bacteria are involved in leaching process which is also termed as bioleaching or mining. It is natural but a very slow process. Bacteria mine or leach or extract the metal in its elemental form from its ore for the simple and sole purpose: to get energy. Natural leaching mechanism is actually a set of biochemical reaction happening at the mercy of dynamic environmental conditions. The first step is formation of protons by oxidation of mineral sulfides. Inorganic acid like sulfuric acid (H2SO4) is produced which dissolve metal oxides. Sometimes, organic acids are formed leading to acidolysis of ore constituents and formation of chelate complexes like succinate, oxalate, citrate or malonate which is known as chelation. Chelation is responsible for excretion of complexing agents like ligands resulting in the surface concentration of metal ions by ligand complex. During bioleaching, oxidation and reduction reactions occur simultaneously at acidic pH. The presence of ferric oxide (Fe+3) is prerequisite for mineral degradation. Both indigenous bacterial flora and/or laboratory cultured bacteria have been used for extraction of various metals from their ores. In this article, the words mining, leaching and extraction have been used synonymously.
Copper leaching is principally carried out by bacteria Thiobacillus ferroxidans and Thiobacillus thiooxidans. Actual process occurs at bacterial cell membrane. Copper minerals like chalcocite, chalcopyrite, bornite and cuprite are broken down to an elemental form of higher oxidation state by sulfur oxidation process triggered by these bacteria. This is achieved with simultaneous reduction of oxygen (O2) by hydrogen sulfide (H2S) and Fe+3; Fe+3 dissolves into soluble ferrous form (Fe+2). The reduction reaction leaves behind precipitate of Cu+2. This process is carried out by cyclic biochemical reactions. Copper precipitate so obtained is then dissolved in kerosene solution in the laboratory. Copper in the aqueous form can be obtained by acidification and which is later on subjected to electrowinning. Copper in its elemental form (Cu) is collected at negative cathode or by iron displacement in aqueous solution.
Ferromanganese, manganocalcite, rhodochrosite, manganese silicate, manganiferrous limestone and psilomelane are some of the important manganese ores which are prone to degradation by Manganese (Mn) leaching bacteria. Acidithiobacillus ferroxidans is known to be a potent Mn leacher but other bacterial genera Carnobacter, Pseudomonas, Shewanella and Bacillus are also actively involved in bioleaching of manganese. In A. ferroxidans, Mn is extracted by oxidation of mineral salts of copper like pyrite and chalcopyrite to H2SO4 via production of thiosulfate and polysulfide respectively as intermediates. Reducing power is generated from metabolic oxidation of glucose (as carbon and energy source) which is an essential step for reduction of Mn (IV) to Mn (II) state.
Aerobic sulfate reducing bacteria are also employed as functional gold extracting bacteria. They catalyze refractory sulfidic gold ore or gold trapped in base-metal sulfides like pyrite. Dissolved sulfate waste produced during this process is converted into dissolved bisulfide by sulfate reducing bacteria which then neutralizes acidic pH of waste. Gold is dissolved in this bisulfide solution and recovered with activated charcoal or zinc dust. The described process of gold leaching is in fact natural sulfur cycle and it has been postulated that sulfur cycle is absolute solution to the recovery of this precious metal.
Silver leaching is side reaction of copper leaching and can be performed simultaneously with copper mining. Silver is thus actually extracted from copper ores, chalcocite and chalcopyrite. Sulfate reduction of minerals is followed by cyanide treatment of aqueous solution to concentrate silver. Extraction of both these noble metals that is gold and silver is carried out by Sulfate reducing bacteria like Thiobacillus thiooxidans, T. ferroxidans, T. thermophilica, Leptospirillum ferroxidans, Sulfolobus acidocaldarius and Thermothrix thioparus; bacteria from genus Bacillus (B. megaterium, B. licheniformis, B. polymyxa and B. luteus) and Pseudomonas (P. cepacia and P. fluorescens).
Importance of bacterial leaching process: Richer mineral deposits are being depleted for the extraction of heavy metals from low grade ores with low investment and operating costs. Metal leaching by bacteria is ecofriendly metallurgical process. Only these bacteria are able to extract all the metal present in the respective ore at very low concentration or in presence of other impurities. Comparatively, traditional physicochemical extractions processes like heap leaching, roast smelting and molal extraction can be utilized in mining industries only when ore consist of sufficient concentration of metal. This is very important consideration, when the richer mineral deposits worldwide are being exploited by abiotic leaching processes. One day, such exploitation will sure result in formation of low grade ores which would have depleted mineral content. That time no option other than bioleaching would be available for extraction of metals and it will be an important process for the recovery of minerals from their ores. Bioleaching has always been economical and very efficient method of choice preferred by industries to extract important metals like copper, manganese and noble metals gold and silver. Bacterial leaching of gold and silver is extremely economical as this is only way by which complete recovery of all the metal present in the ore is possible.
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