Biohydrometallurgy is a technique by which microorganisms are used to recover certain metals from ores. The technique was first used over 300 years ago to extract copper from low-grade ores. In recent years, its use has been extended to the recovery of uranium and gold, and scientist believe that it will eventually be applied to the recovery of other metals such as lead , nickel , and zinc.
In most cases, biohydrometallurgy is employed when conventional mining procedures are too expensive or ineffective in recovering a metal. For example, dumps of unwanted waste materials are created when copper is mined by traditional methods. These wastes consist primarily of rock, gravel, sand, and other materials that are removed in order to reach the metal ore itself. But the wastes also contain very low concentrations (less than 0.5%) of copper ore.
Until recently, the concentrations of copper ore in a dump were too low to have any economic value. The cost of collecting the ore was much greater than the value of the copper extracted. But, as richer sources of copper ore are used up, low grade reserves (like dumps) become more attractive to mining companies. At this point, biohydrometallurgy can be used to leach out the very small quantities of ore remaining in waste materials.
The extraction of copper by means of biohydrometallurgy involves two types of reactions. In the first, microorganisms operate directly on compounds of copper. In the second, microorganisms operate on metallic compounds other than those of copper. These metallic compounds are then converted into forms which can, in turn, react with copper ores.
The use of biohydrometallurgical techniques on a copper ore waste dump typically begins by spraying the dump with dilute sulfuric acid . As the acid seeps into the dump, it creates an environment favorable to the growth of acid-loving microorganisms that attack copper ores. As the microorganisms metabolize the ores, they convert copper from an insoluble to a soluble form. Soluble copper is then leached out of the dump with sulfuric acid. It is recovered when the solution is pumped out to a recovery tank.
A second reaction occurs within the dump. Microorganisms also convert ferrous iron (Fe2+)in ores such as pyrite (FeS2) to ferric iron (Fe3+). The ferric iron, in turn, oxidizes copper in the dump from an insoluble to a soluble form.
The mechanism described here is a highly efficient one. As microorganisms act on copper and iron compounds, they produce sulfuric acid as a by-product, thus enriching the environment in which they live. Ferric iron reduces and oxidizes copper at the same time, making the copper available for attack by microorganisms once again.
A number of microorganisms have been used in biohydrometallurgy. One of the most effective for the leaching of copper is Thiobacillus ferrooxidans. Research is now being conducted on the development of genetically engineered microorganisms that can be used in the recovery of copper and other metals.
The two other metals for which biohydrometallurgy seems to be most useful are uranium and gold. Waste dumps in South Africa and Canada have been treated to convert insoluble forms of uranium to soluble forms, allowing recovery by a method similar to that used with copper. In the treatment of gold ores, biohydrometallurgy is used in a pretreatment step prior to the conventional conversion of the metal to a cyanide-complex. The first commercial plants for the biohydrometallurgical treatment of gold ores are now in operations in South Africa and Zimbabwe.
[David E. Newton ]
Rossi, G. Biohydrometallurgy. New York: McGraw-Hill, 1990.