Biotechnologies in extractive metallurgy - Microbiology and metal extraction

The use of biotechnologies in extractive metallurgy has become an irreversible industrial reality. It is the fruit of the discovery of major phenomena relating to the transformation of mineral materials at the interface between biology and geology, and the relatively rapid transfer of the exploitation of these discoveries to an industrial sector undergoing accelerated change.

Knowledge of the properties of micro-organisms, which use the oxidation of reduced forms of iron and sulphur as sources of energy for their metabolism, has led to the development of processes for extracting copper, uranium, gold and other metals of significant economic interest, from the sulphide ores that contain most of the resources of these metals. These processes have been put into practice almost as a matter of course, due to their apparent simplicity. In fact, the practice existed long before the role of micro-organisms was even known: it's called bioleaching.

The most widespread form of treatment is heap bioleaching. It consists of breaking up the ore into more or less coarse fragments and percolating acidic water, which acts as a growth medium for micro-organisms that accelerate the dissolution of sulfide minerals.

When the size of the sulfide minerals to be dissolved is so small as to require extensive fragmentation for an aqueous solution to gain access, bioleaching is carried out in agitated, aerated tanks. In this case, the process is somewhat more complex, but does not require excessively advanced technical skills.

You don't need to be a microbiologist, or even a biologist, to operate a bioleaching plant. On the other hand, the recovery of metals from the aqueous solution generated by the bioleaching process into one or more commercial products, and the management of liquid and solid flows and discharges, involving quantities ranging from thousands to hundreds of thousands of tons and cubic meters per day under optimal environmental conditions, represent a challenge in know-how typical of the mineral resources field.

Another form of application at the interface between microbiology and metallurgy, which has led to commercial processes, is the use of a process that is exactly the reverse of bioleaching. This involves using the reduction of oxidized forms of sulfur to produce sulfide, which chemically combines with metals to extract them from an aqueous solution by precipitation.

This article begins by describing bioleaching processes for sulfide ores, the main applications in this field of treatment. The main elements of the reaction systems involved are discussed, followed by the various forms of implementation. Finally, other applications of biohydrometallurgy are reviewed....