BIOMINING - One of the marvels of Biotechnology of present day for environment-friendly mining and mineral processing industry: By: Partha Das Sharma A. INTRODUCTION: Biohydrometallurgy can be defined as the field of applications resulting from the control of natural (biochemical) processes of interactions between microbes and minerals to recover valuable metals. Many biotechnology-derived processes use microorganisms to help ease the usage of harmful chemicals in various industrial processes. The mining industry uses microorganisms and their natural ability to digest, absorb, and change the quality of different chemicals and metals, to refine ores. Microorganisms have been introduced to various areas of the mining industry with phenomenal success. Advances in biotechnology have permitted the extraction of metals from low-grade ores, improved recovery rates at operations, and reduced operating costs.
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Biomining is an application of biotechnology in recovery of various minerals from ore. Biomining is defined as extracting mineral ores or enhancing the mineral recovery from mines using microorganisms instead of traditional mining methods. Copper was the first metal extracted using microorganisms in the ancient past in the Mediterranean region. Biomining is becoming popular because it is cheap, reliable, efficient, safe, and environmentally friendly, unlike traditional mining methods. The efficiency of biomining can be increased either by finding suitable strains of microorganisms or by genetically modifying existing microorganisms, made possible due to rapid advances in the field of biotechnology and microbiology. Ores of high quality are rapidly being depleted and biomining allows environmentally friendly ways of extracting metals from low-grade ores (ores that have small amounts of valuable metals scattered throughout). Biomining includes two different chemical processes called bioleaching and biooxidation. Thus, biomining is an application of biotechnology and is also known as microbial leaching or alternately, biooxidation. B. MICROORGANISMS IN BIOMINING: There are different types of bacteria present in nature that oxidize metal sulfides and solubilize minerals, thus, helping in their extraction from the ores. It is very important to select suitable microorganisms to ensure the success of biomining, a process which requires knowledge of properties of microorganisms, both physiological and biochemical. Bacteria are found to be the most suitable microorganisms that can be used for extraction of metals in bio-mining. Characteristics of the bacteria used in bio-mining: a. Mineral extraction involves the production of high temperatures so the bacteria should be able to survive the heat, hence, they should be thermophilic. b. Bio-mining involves using strong acids and alkalis, hence, bacteria should be chemophilic. c. Bacteria should produce energy from inorganic compounds, hence, should also be autotrophic (characteristic of an organism capable of making nutritive organic molecules from inorganic sources via photosynthesis i.e., involving light energy or chemosynthesis i.e., involving chemical energy). d. The bacteria should be able to adhere to the solid surfaces or have the ability to form biofilms. Identification of Bacteria Useful for Biomining Operations There are wide varieties of bacteria with varying capabilities existing on earth, Therefore, it is essential to identify precisely the types that can perform biooxidation/bioleaching effectively. Thiobacillus ferrooxidans is a chemophilic, moderately thermophilic bacteria which can produce energy from oxidation of inorganic compounds like sulfur and iron. It is the most commonly used bacteria in biomining. Several other bacteria such as T.thioxidans, Thermothrix thiopara, Sulfolobus acidocaldarius and S. brierleyi are also widely used to extract various minerals.
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Thermothrix thiopara is an extremely thermophilic bacteria that can survive very high temperatures between 60-75C and is used in extraction of sulfur. Techniques like genetic engineering and conjugation are used to produce bacteria with desired characteristics to increase the rate of biooxidation thus increasing the mineral yield through biomining. It is also important to identify biomining bacteria present in colonies of other bacteria. Techniques developed for this purpose include: (a) Immunoflourescence, (b) Dot immunoassay, and (c) Dot-blot hybridization. (a) Immunofluorescence - This technique is generally used to identify specific antibodies or antigens present in biological fluids. Fluorescent antibodies are used to identify biomining bacteria. (b) Dot Immunoassay - This technique is used to identify ore-adhering bacteria like T.ferrooxidans and T.thiooxidans. The bacteria are applied in the form of dots on a nitrocellulose film. Antigen-antibody reaction is carried out on the film and then treated with a secondary antibody to make the reaction visible by producing a color. The sample can be approximated by comparison of the test sample with that of a known sample. (c) Dot-blot Hybridization- This is a DNA based technique to identify biomining bacteria such as T.ferrooxidans. The bacteria are isolated from samples of ores and soil treated with sodium dodecyl sulfate (SDS). The cells are disrupted to extract DNA and the extracted DNA is then purified. The DNA obtained from ore sample is fixed on nitrocellulose membrane using southern blotting technique. Genetic probes are used to identify and distinguish various biomining bacteria used in this procedure. The DNA fragments on the membrane are treated with standard probes. C. BIOMINING RECOVERY: Minerals are recovered from ores by the microorganisms mainly by two mechanisms: (a) Oxidation and (b) Reduction. (a) Oxidation The microorganisms like T.ferroxidans and T.thioxidans are used to release iron and sulfur respectively. T.ferroxidans oxidize ferrous ion to ferric ion. 4Fe++ + O2 + 4H+ → Fe+++ + 2H2O The bacteria attach to the surface of the ore and oxidize by a direct and indirect method. Direct Method In this method the ore is oxidized by the microorganisms due to the direct contact with the compound. 2FeS2 + 7O2 + 2H2O → 2FeSO4 + 2H2SO4 Indirect Method In this method the mineral is indirectly oxidized by an agent that is produced by direct oxidation. For example, the ferric ion produced by the above reaction is a powerful oxidizing agent and can release sulfur from the metal sulfides. Thus production of ferric
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ion indirectly causes oxidation of metal sulfide resulting in the breaking of the crystal lattice of the heavy metal sulfide and separating the heavy metal and sulfur. CuS + Fe+++ → Cu+ + S + Fe++ (b) Reduction Bacteria like Desulfovibro desulfuricans play an active role in reduction of sulfates which results in the formation of hydrogen sulfides. 4H2 + H2SO4 → H2S + 4H2O D. TYPES OF BIOMINING: 1. Stirred Tank Biomining - This method is used for leaching from substrates with high mineral concentration. Since the method is expensive and time consuming, substrates with lower concentration are not used for leaching. Copper and refractory gold ores are well suited for this type of method. Special types of stirred tank bioreactors lined with rubber or corrosion resistant steel and insulated with cooling pipes or cooling jackets are used for this purpose. Thiobacillus is the commonly used bacteria. Since it is aerobic the bioreactor is provided with an abundant supply of oxygen throughout the process provided by aerators, pumps and blowers. This is a multi-step process consisting of large numbers of bioreactors connected to each other. The substrate moves from one reactor to another and in the final stage it is washed with water and treated with a variety of chemicals to recover the mineral. The name is fairly self-explanatory, as the process requires constructing large aerated tanks that are generally arranged in a series, so that runoff from one tank serves as raw material for the next. In this way, the reactor can operate in continuous flow mode, with fresh ore being added to the first tank while the runoff from the final tank is removed and treated. The ore to be processed is generally crushed to a very small particle size, to ensure that the solids remain suspended in the liquid medium. Mineral nutrients in the form of (NH4)2SO4 and KH2PO4 are also added to the tanks to ensure maximal microbial density is maintained.
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Due to the extremely high cost of stirred tank reactors, they are only used for highly valuable materials. For gold extraction for example, this technique is usually used when the ore body contains high concentrations of arsenopyrite (AsFeS). 2. Bioheaps - Bioheaps are large amounts of low grade ore and effluents from extraction processes that contain trace amounts of minerals. Such effluents are usually stacked in large open space heaps and treated with microorganisms to extract the minerals. Bioheaps are also called biopiles, biomounds and biocells. They are also used for biodegradation of petroleum and chemical wastes. The low grade ores like refractory sulfide gold ore and chalocite ore (copper) are crushed first to reduce the size then treated with acid to promote growth and multiplication of chemophilic bacteria. The crushed and acid-treated ore is then agglomerated so that the finer particles get attached to the coarser ones, and then treated with water or other effluent liquid. This is done to optimize moisture content in the ore bacteria that is inoculated along with the liquid. The ore is then stacked in large heaps of 2-10 feet high with aerating tubes to provide air supply to the bacteria thus promoting biooxidation. Advantages of using bioheaps are that they are: (a) cost effective, (b) of simple design and easy to implement, and (c) very effective in extracting from low concentration ores. Disadvantages of using bioheaps are that they: (a) are time consuming (takes about 6-24 months), (b) have a very low yield of mineral, require a large open area for treatment, have no process control, (c) are at high risk of contamination, and (d) have inconsistent yields because bacteria may not grow uniformly in the heap. 3. In-situ Bioleaching - In this method the mineral is extracted directly from the mine instead of collecting the ore and transferring to an extracting facility away from the site of the mine. In-situ bio-mining is usually done to extract trace amounts of minerals present in the ores after a conventional extraction process is completed. The mine is blasted to reduce the ore size and to increase permeability and is then treated with water and acid solution with bacterial inoculum. Air supply is provided using pipes or shafts. Biooxidation takes place in-situ due to growing bacteria and results in the extraction of mineral from the ore. E. FACTORS EFFECTING BIOMINING: Success of biomining and efficiency in recovery of minerals depends on various factors some of which are discussed below. (a) Choice of Bacteria - This is the most important factor that determines the success of bioleaching. Suitable bacteria that can survive at high temperatures, acid concentrations, high concentrations of heavy metals, remaining active under such circumstances, are to be selected to ensure successful bioleaching.
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(b) Crystal Lattice Energy - This determines the mechanical stability and degree of solubility of the sulfides. The sulfide ores with lower crystal lattice energy have higher solubility, hence, are easily extracted into solution by the action of bacteria. (c) Surface Area - Rate of oxidation by the bacteria depends on the particle size of the ore. The rate increases with reduction in size of the ore and vice-versa. (d) Ore Composition - Composition of ore such as concentration of sulfides, amount of mineral present, and the extent of contamination, has direct effect on the rate of biooxidation. (e) Acidity - Biooxidation requires a pH of 2.5-3 for maximum results. The rate of biooxidation decreases significantly if the Ph is not in this range since the activity of acidophilic bacteria is reduced. (f) Temperature - The bacteria used in biomining are either mesophilic or thermophilic. Optimum temperature is required for biooxidation to proceed at a fast rate. Optimum temperature range for a given bacteria is between 25-35° C depending on the type of ore being selected. The rate of biooxidation is reduced significantly if the temperature is above or below the optimum temperature. (g) Aeration - The bacteria used in biomining are aerobic thus require an abundant supply of oxygen for survival and growth. Oxygen can be provided by aerators and pipes. Mechanical agitation is also an effective method to provide continuous air supply uniformly and also to mix the contents. (h) Solid-liquid Ratio - The ratio of ore/sulfide to the leach solution (water + acid solution + bacteria inoculum) should be maintained at optimum level to ensure that biooxidation proceeds at maximum speed. The leach solution containing leached minerals should be removed periodically and replaced with new solution. (i) Surfactants - Adding small amounts of surfactants like Tween 20 to the leaching process increases the rate of biooxidation of minerals from sulfide ores. The surfactants decrease the surface tension of the leach solution, thus, wetting the ore and resulting in increased bacterial contact which ultimately increases the rate of biooxidation. F. EXAMPLES OF BIOMINING: (a) Biomining of Copper - Copper was the first metal extracted by bioleaching. It is the metal most commonly extracted from oxide ores by this method. In the United States, alone, about 11% of copper is produced from low grade ores by bioleaching technique every year. Copper is available in mines across the world in more than 350 types of ores, but it is mainly present along with sulfur. Copper from low-grade ores like copper sulfide minerals is most commonly extracted by biooxidation since it is not economically viable to use conventional metallurgical techniques.
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(b) Biomining of Gold - Biooxidation of refractory gold ores to extract gold is carried out by a commercial procedure called BIOX developed by GENCOR S.A Ltd Johannesburg South Africa in an effort to replace existing procedures which posed severe pollution problems. The BIOX process had several advantages over existing procedures including lower cost. (c) Microbially Enhanced Oil Recovery (MEOR) - Recent technological developments have helped to make possible the recovery of oil. Using microorganisms is one such technique to improve the recovery process hence called “microbially enhanced oil recovery” (MEOR). It was discovered in 1926 that microorganisms can be used in the petroleum industry to enhance oil recovery, but the concept became popular only after the 1950s. Microbes can enhance the recovery of petroleum products directly or indirectly. G. FUTURE TREND OF BIOMINING FOR PROCESSING OF MINERALS : Although mining is one of humankind's oldest activities, the techniques used to extract minerals haven't changed substantially for centuries. Ores are dug from the earth, crushed, then minerals such as copper and gold are extracted by extreme heat or toxic chemicals. The environmental and health effects of traditional mining technologies have been deleterious. In the past few years, the mining industry has been turning to a more efficient and environmentally salubrious method for extracting minerals from ores: microorganisms 7
that leach them out. Using a bacterium such as Thiobacillus ferooxidans to leach copper from mine tailings has improved recovery rates and reduced operating costs. Moreover, it permits extraction from low grade ores - an important consideration in the face of the depletion of high grade ores. Thiobacillus ferooxidans, which is naturally present in certain sulfur-containing materials, gets energy by oxidizing inorganic materials, such as copper sulfide minerals. This process releases acid and an oxidizing solution of ferric ions, which can wash out metals from crude ore. Poor quality copper ore, which is bound up in a sulfide matrix, is dumped outside a mine and treated with sulfuric acid to encourage the growth of T. ferooxidans. As the bacteria chew up the ore, copper is released and collected in solution. The sulfuric acid is recycled. Currently 25% of all copper worldwide, worth more than $1 billion annually, is produced through bioprocessing. This ranks it as one of the most important applications of biotechnology today. Bioprocessing is also being used to economically extract gold from very low grade, sulfidic gold ores, once thought to be worthless. To increase the efficiency of biomining, the search is on for bacterial strains that are better suited to large-scale operations. Bioprocessing releases a great deal of heat, and this can slow down or kill the bacteria currently being used. Researchers are turning to heat-loving thermophilic bacteria found in hot springs and around oceanic vents to solve this problem. These bacteria thrive in temperatures up to 100 degrees Celsius or higher and could function in a high temperature oxidative environment. Another effort is underway to find - or genetically engineer - bacterial strains that can stand up to heavy metals such as mercury, cadmium, and arsenic, which poison microbes and slow the bioprocessing. Some microbes have enzymes that protect their basic activities from heavy metals or pump them out. If genes that protect microbes from heavy metals can be identified, resistant strains might be engineered. In any event, biomining is now at the top of mining technology, and future development of the technology appears promising. H. SUMMARY AND CONCLUSION: Biomining is the sustainable, biotechnological process utilizing microorganisms to remove metals from sulfide mineral ores and concentrates. The development of biomining has progressed from poorly designed dumps to highly engineered heaps and stirred tank reactors in an industrially important biotechnological process. The release of metals from sulfide minerals is catalyzed by iron oxidizing acidophilic (optimum pH for growth <3) microorganisms that act in consortia with heterotrophic and sulfur oxidizing acidophiles in a mixed culture. The microorganisms catalyze metal release by regenerating ferric iron that oxidizes the mineral sulfide bond to produce metals and reduced inorganic sulfur compounds. The other microorganisms in the mixed culture may oxidize the reduced inorganic sulfur compounds to sulfuric acid that is the source of the acid in these environments.
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Now, tremendous improvements in biomining are expected with continued research in identifying bacterial strains better suited for individual applications and large-scale operations as well as in the genetic engineering of bacterial strains that can stand up to high temperature processes and heavy metals such as arsenic, mercury, or cadmium. Biomining has become one of the premier mining technologies, and the future appears bright. The potential applications of biotechnology to mining and processing are countless. Some examples of past projects in biotechnology include a biologically assisted in situ mining program, biodegradation methods, passive bioremediation of acid rock drainage, and bioleaching of ores and concentrates. Research often results in technology implementation for greater efficiency and productivity or novel solutions to complex problems. Additional capabilities include the bioleaching of metals from sulfide materials, phosphate ore bioprocessing, and the bioconcentration of metals from solutions. One project recently under investigation is the use of biological methods for the reduction of sulfur in coal-cleaning applications. From in situ-mining to mineral processing and treatment technology, biotechnology provides innovative and costeffective industry solutions. References: 1. http://knol.google.com/k/partha-das-sharma/biomining/oml631csgjs7/8 2. www.ebookee.com.cn/Biomining_142912.html 3. Davis Jr., R.A., Welty, A.T., Borrego, J., Morales, J.A., Pendon, J.G. and J.G. Ryan. 2000. Rio Tinto estuary (Spain): 5000 years of pollution. Environmental Geology. 39: 1107-1116. 4. Brierley, C.L. and J.A. Brierley. 1997. Microbiology for the Metal Mining Industry. in Manual of Environmental Microbiology. (Ed.) C.J. Hurst. ASM Press, Washington D.C. 5. Brierley, C.L. 1995. Bacterial oxidation. Engineering and Mining Journal. 196:42-44. 6. Acevedo, F. 2000. The use of reactors in biomining processes. Electronic Journal of Biotechnology. 7. Rawlings, D.E. 2002. Heavy metal mining using microbes. Annual Review of Microbiology. 56:65-91. ***
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