bio leaching

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1. Bio leaching Group members Mehwish nazeer, Kanza saleem, Nazish mehmood m.Ishaq javaid m.adil 2. Introduction Bioleaching is the simple and effective technology for metal extraction from low grade ores and mineral concentrate by the use of micro organisms. Commonly used microorganisms are: Mesophiles Moderately thermophilic bacteria extremophiles 3. History Copper recovery from mine waters in the Mediterranean area 3000 years ago. The role of bacteria in bioleaching was shown in 1947. In 1950s copper dump leaching. In 1960s the first industrial copper heap leaching operation. First industrial gold bioleaching plant in 1980s Nowadays about 40 plants in industrial use for copper, gold, zinc, cobalt, uranium. 4. Features of organisms involved Single celled organisms Chemosynthetic metabolism Derive carbon dioxide, oxygen from atmosphere Requires acidic PH 5. Procedure Bacteria perform the key reaction of regenerating the major ore oxidizer, mostly ferric ion. This reaction takes place in the cell membrane of bacteria. In the first step, disulfide is spontaneously oxidized to thiosulfate by ferric iron (Fe3+ ), which in turn is reduced to give ferrous iron (Fe2+ ): FeS2+6 Fe3+ +3 H2O 7 Fe2+ +S2O3 2- +6 H+ spontaneous In second step Microorganisms catalyze the oxidation of ferrous iron and sulphur, to produce ferric iron and sulphuric acid: Fe2+ + 1/4O2 + H+ ---> Fe3+ + 1/2 H2O S + 3/2O2 + H2O ---> H2SO4 6. Thiosulfate is also oxidized by bacteria to give sulfate: S2O3 2- +2O2+H2O 2 SO4 2- +2 H+ (sulfur oxidizers) The ferric iron produced in reaction (2) oxidized more sulfide as in reaction (1), closing the cycle and given the net reaction 2 FeS2+7O2+2 H2O 2 Fe2+ +4SO4 2 +4H+ The net products of the reaction are soluble ferrous sulfate and sulfuric acid. 7. Mechanism involves in bioleaching Two processes are used in bioleaching: Direct bioleaching Indirect bioleaching 8. In direct bioleaching In indirect method of bioleaching of minerals bacteria produce strong oxidizing agent which reacts with metals and extract them from the ores. 9. In direct bioleaching minerals which are susceptible to oxidation undergoes direct enzymatic attack by the microorganisms. Direct bioleaching 10. A: In direct mechanism B: Direct mechanism 11. Commercial process of bioleaching Naturally occur bioleaching process is very slow. For commercial extraction of metal by bioleaching the process is optimized by controlling the PH, temperature, humidity, o2 and co2 concentrations. These processes are: Slope leaching In-situ leaching Heap leaching 12. Slope leaching In slope leaching the ore is finely ground and kept in large pile in a slope which is subjected to continuous sprinkling of aqueous solution of microorganisms. The leach liquor collected at the bottom of the ore is processed further for metal recovery. In situ leaching In in situ leaching ore is subjected to bioleaching in its natural occurrence, aqueous solution of microorganisms is pumped through drilled passages with in the ore. The leach liquid collected at the bottom of the ore used for metal extraction. Heap leaching In heap leaching ore is arranged in heap and goes through the same procedure as in slope leaching. The aqueous solution containing microorganism works on the heap of ore and produces the leach liquor. The leach liquor is used for metal recovery. 13. Main factors affecting bioleaching FACTOR Physicochemical Temperature pH and to keep ferric Oxygen reactions Microbiological Microbial diversity culture Population density Metal tolerance EFFECT affects leaching rate, microbial composition and activity needs to be low to obtain the fastest leaching rates and to keep ferric iron and metals in solution electron acceptor needed in chemical and biological oxidation mixed cultures tend to be more robust and efficient than pure high population density tends to increase the leaching rate high metal concentrations may be toxic to meta 14. Benefits of bioleaching Simple Inexpensive Employed for collecting metals from waste and drainages Use to extract refines and expensive metals which is not possible by other chemical processes no poisonous sulfur dioxide emissions as in smelters no need for hi pressure or temperature ideal for low-grade sulfide ores Environment friendly process 15. Disadvantages Time consuming (takes about 6-24 months or longer) Have a very low yield of mineral Requires a large open area for treatment May have no process control High risk of contamination Inconsistent yield because bacteria cannot grow uniformly