Bioleaching (Biomining or Microbial Leaching) Microbial leaching is extraction of metals from ores using microorganisms. Recently, bacterial activity has been implicated in the weathering, leaching and deposition of mineral ores. The new discipline created by the merging of biotechnology and metallurgy is known as biohydrometallurgy or bioleaching or biomining. biomining. Conventional metallurgy involves smelting of ores at high temperatures. This involves high energy cost and also leads to pollution. That copper could be leached from its ores by the activity of a bacterium Acidithiobacillus ferroxidans was discovered in 1947. Microbes useful for metal recovery depend upon the temperature of the recovery process. Gram-negative bacteria belonging to the genus Acidithiobacillus and Lactospirillum are important at 40°C. e.g. Atferrooxidans - oxidizing Fe and S At.thiooxidans} At. caldus}-oxidizing S L ferroxidans}-oxidizing Fe
L ferriphilum } Microbial technology is helpful in recovery of ores which cannot be economically processed with chemical methods, since they contain only low-grade ores. During separation of high-grade ores, large quantities of low-grade ore are produced, and are discarded into waste heaps from where they reach the environment. Microbial leaching plays its role here, to retrieve Pb, Ni and Zn ores which are there in significant amounts.
Large-scale chemical processing causes environmental problem when the dump is not managed properly.
Leach fluid containing large quantity of metals at very low pH seeps into nearby natural water supplies and ground water. Biomining is thus an economically sound hydrometallurgical process with lesser environmental problem than conventional chemical processing.
Biomining: Using microbial gold-diggers to recover precious metals Biomining uses microbial agents to recover precious metals from low-grade ores and mine tailings. The same approach may also achieve remediation of metal-contaminated soil and water. What is biomining? Biomining is the extraction of metals especially from low-grade ores using microorganisms. High-grade ores are fast getting depleted and so there in the need for innovative solutions to recovery of precious metals from lowgrade deposits. Ores with low metal content are not economical for smelting. Traditional methods are very expensive and also harmful to the environment. Hence biomining is a cost-effective and more environment-friendly alternative to conventional techniques. Usually microbial mining is done for copper, gold, and silver from low-grade, sulfide-rich minerals. Biomining is also known as microbial mining or biohydrometallurgy. The crux of this technology is the application of bioleaching using specific types of bacteria-microbial athletes which work under highly acidic conditions. Bioleaching is the microbially-mediated conversion of an insoluble metal compound into a water soluble form. Microbial agents The most common agent is Thiobacillus ferrooxidans(now called Acidithiobacillus ferrooxidans), one of the bacteria which oxidizes iron and sulfur. Acidithiobacillus thiooxidans and other such bacteria are so-called mesophilic bacteria that work under lower temperatures, 300 to 500 C. Some thermophilic archaebacteria-bacteria considered ancient and that thrive in strange environments such as hot springs,
hydrothermal vents, salt lakes, and swamps(microbial equivalents of ‘supermen’)-also participate in bioleaching. Some examples are Sulfolobus spp. The most active player is Acidithiobacillus ferrooxidans which obtains its energy from oxidation of inorganic compounds such as fe and S compounds. Conversion of Fe2+ to Fe3+ and S to So4 causes bioleaching of metals from their oxide and sulfide ores. The basic reaction is: MS + 2O2 ? MSO4, where M is the metal (say copper) and the reaction is catalyzed by a bioleaching microbe (say Thiobacillus ferrooxidans).. Background Biomining has been around for a long time, though recognition of the role of microbes came only recently. Copper was extracted by man even in 1000 BCE from metal rich waters which passed through copper deposits. Thiobacillus ferrooxidans was isolated by Colmer and Hinkle in 1947. Subsequently several microbes involved in bioleaching have been identified. As of 1989, more than 30% of Cu and U mining in the USA is microbial. Types of bioleaching Bioleaching may be direct or indirect. If the microbe directly oxidizes the sulfide of copper to leach it it is direct bioleaching. Cu2S(chalcocite) is converted to CuS(covellite) by microbial oxidation. In indirect bioleaching Copper sulfide (chalcocite) is oxidized by ferric sulphate and later the Ferrous sulfate generated is reoxidized to ferric form by the bioleaching microorganism. 2Cu2S + 2O2 + 4H+ ? 2CuS + 2Cu2+ + 2H2O Copper ions are recovered from solution using scrap iron: Cu2+ + Fe0 ? Fe2+ + Cu The major type of bioleaching techniques used for copper mining are: • Heap bioleaching • Dump bioleaching • Vat bioleaching • In situ bioleaching In heap bioleaching the ore is crushed and piled into a heap over an impermeable base. Acidic leaching solution is percolated through the heap and microbes growing in it produce the ferric iron and acid that result in metal solubilization. Metal-containing leachates are collected and sent for metal recovery.
Dump leaching involves uncrushed waste rock with very low copper content(0.1-0.5%) over which the leach liquor is percolated through. And the leachate is collected for metal recovery as in heap bioleaching. Vat leaching involves dissolution of crushed ores in a tank with continuous stirring. The recovery rate is higher with this technique. In situ bioleaching involves pumping of solution and air into a mine. The resulting metal-rich solution is recovered through wells drilled below the body of the ore. This method causes less harm to the environment but has the disadvantage that the permeability of the rock may be low and the drilled passages may not allow adequate supply of nutrients and oxygen to the ore. Bioleaching of other precious metals Bioleaching has been developed for gold and uranium also. The details of biomining of these metals will be discussed in a forthcoming column. Problems of biomining Production of acid (sulfuric acid) is a major problem associated with bioleaching. The possible solutions are addition of CaCO3 to neutralize the acid but it generates solid wastes which must be disposed properly. Sulfate reducing bacteria may also be added to counteract the acid produced by the bioleaching microbes. The slow growth of mesophilic bioleaching organisms such as Thiobacillus ferrooxidans is also a problem and also when the temperature in the ores rises they will not survive. In such cases bioleaching is better achieved with thermphilic microorganisms such as Sulfolobus spp. Coda Bacterial leaching of ores is a revolutionary technique useful for recovery of precious metals from low-grade and lean ores. This is also less capital-intensive and more environment-friendly. The same approach may be used for recovery of base metals. As the world’s high-grade ores get increasingly depleted, the world will increasingly use the invisible microbial friends for recovery of precious metals from low-grade ores and mine tailings in both the developed and the developing world. *The author teaches Biochemistry in Manipur University and researches on various aspects of Microbial Diversity and Microbial Biotechnology relevant to Manipur.
