In the last years microbial processes have gained increasing attention in the mining industry. Bioleaching of heavy metals, biooxidation of gold ores, desulfurization of coal and oil, tertiary recovery of oil and biosorption of metal ions are examples of the wide variety of potential and actual applications of microorganisms in mining and related fields.

The term biomining encompasses two related microbial processes that are useful in the extractive metallurgy of heavy metals: bacterial leaching and biooxidation. Bacterial leaching is a process by which a metal is solubilized from an ore by the oxidative action of bacteria, as in the case of bacterial leaching of copper. On the other hand, biooxidation implies the bacterial oxidation of reduced sulfur species accompanying the metal of interest, as in the biooxidation of refractory gold minerals.

Today bioleaching is being used not only for the recovery of metals from low-grade ores, flotation tailings or waste material, but also as the main process in large-scale operations in copper mining and as an important pretreatment stage in the processing of refractory gold ores.

The main advantages of bacterial leaching of copper and other heavy metals as compared with the traditional pyrometallurgical technologies lies in its simplicity, mild operation conditions, low capital costs, low energy input, and in its friendliness towards the environment. The biooxidation of refractory gold ores presents similar characteristics when compared with alternate processes such as roasting and pressure oxidation.

Bacterial leaching of copper is usually performed in heaps of ground ore or in dumps of waste or spent material. Heaps and dumps are irrigated in closed circuit with an acidic liquor containing part of the bacterial population, the rest being attached to mineral. When the desired metal concentration is attained, the rich liquor is pumped to the recovery section. Heaps and dumps present a number of advantages such as simple equipment and operation, low investment and operation costs and acceptable yields. Nevertheless, it must be realized that their operation present severe limitations: the piled material is very heterogeneous and practically no close process control can be exerted. Moreover, the rates of oxygen and carbon dioxide transfer that can be obtained are limited and the leaching rates are low, so extended periods of operation are required in order to achieve the desired conversions.

From a process engineering standpoint, bioleaching would best be performed in reactors. The use of reactors would allow for a close control of the pertinent variables, resulting in a better performance. Parameters such as volumetric productivity and degree of extraction could be significantly increased. The main limitation in the use of reactors in biomining lies in the large amounts of run-of-mine ore that in most cases is to be treated. This limits their application to the treatment of mineral concentrates or when moderate volumes of ore are to be processed. For instance, over 11,000 tons of gold concentrates are biooxidized in reactors every year.

The selection of the most suitable type of reactor for a biomining process and its design should be based in the physical, chemical and biological characteristics of the system. Adequate attention should be paid to the complex nature of the reacting sludge, composed by liquid, suspended and attached cells, suspended solids, and air bubbles. Most industrial fermentations are run in stirred tank reactors operated batchwise or continuously. Because of the large volumes of material to be processed, bioleaching and biooxidation are best performed in a continuous mode of operation in which volumetric productivity is high and reactor volumes can be kept low. An important feature to consider is the autocatalytic nature of microbial growth and the fact that the affinity of the microbial population towards the mineral species involved is quite low. If a high degree of conversion is desired, a single stirred tank would require a very large volume, so an arrangement of reactors is more suitable. It can be shown that a continuous stirred tank reactor (CSTR) followed by a tubular plug flow reactor (PFR) will give the minimum reaction volume to attain a certain conversion. Because the need of aeration and the presence of solid particles make PFRs unpractical, their performance can be approximated by a series of CSTRs. Other types of reactors that have been studied for their application in biomining are the percolation column, the Pachuca tank, the air-lift column, and the rotary reactor.

Several mass transfer operations occur in biomining processes related to the transport of nutrients, metabolic products and solubilized species. Of especial importance is the supply of oxygen and carbon dioxide from air to the cells. Carbon dioxide is the carbon source for microbial growth, while oxygen acts as the final electron acceptor of the overall oxidation reaction. In reactors these gases are usually supplied by bubbling air into the liquid. In order to be used by the cells, O₂ and CO₂ must dissolve in the liquid, a mass transfer operation that presents a high resistance and can become limiting for the overall process rate. At the usual pulp densities (term that refers to the solids concentration expressed in % weight/volume) of 10 to 18%, the supply of oxygen in a stirred tank or an air-lift column is usually enough to cope with the demand of the cell population, which is not very large due to the low growth rates and low cell concentrations. Nevertheless, at high pulp density oxygen transfer can become limiting, leading to low rates and low productivity. The very low content of CO₂ in air (0.03%) and the low pH of the liquid make carbon dioxide supply a difficult operation that in most cases is limiting. In order to relief this limitation extremely high air flows could be used, air could be enriched with CO₂, or the reactor could be pressurized in order to increase the CO₂ partial pressure. These three alternatives are easily realized at laboratory scale, but up to now they have not been able to be economically scaled-up.

The CSTR is an ideal conception that implies a completely mixed content, so the exit stream has the same composition than the fluid within the reactor. The complex nature of the slurry makes the attainment of homogeneity especially difficult. Agitation has the double purpose of increasing the rate of mass and heat transfer and mixing the reactor content. Under conditions of insufficient agitation the transfer operations become limiting and the overall reaction performance will decline because of the appearance of zones of the fluid with insufficient nutrients and gasses or inadequate temperature and pH. Disk turbines have been used for several decades in tank reactors in the fermentation industry. Although this type of impeller is very efficient in providing high mass transfer coefficients, its mixing characteristics are rather poor and its power requirement is high. Moreover, the high shear stress exerted by the disk turbine on the fluid may also produce metabolic stress and cell growth inhibition. That is why new types of impellers have been developed, and today the hydrofoil impellers are of common use in the bioreactors used in gold mining. This type of impeller achieves a higher degree of slurry homogeneity and requires considerable less power.

Gold is usually obtained from ores by solubilization with a cyanide solution and recovery of the metal from the solution. In refractory ores, small particles of gold are covered by insoluble sulfides impeding the contact between cyanide and gold. In this case, a pretreatment stage must be considered, such as pressure oxidation, chemical oxidation, roasting or bio-oxidation, the latter currently being the alternative of choice. In the biooxidation process, bacteria partially oxidize the sulfide coating covering the gold microparticles in ores and concentrates. In this way, gold recovery from refractory minerals can increase from 15-30% to 85-95%. In the last 15 years at least ten large-scale commercial gold processing units have been established in South Africa, Brazil, Australia, Ghana, Peru and USA, and eight of them use bioreactors.

The future of bioreactors in mining appears promising. Gold biooxidation operations tend to increase in number and size in several countries the world over. The use of reactors will most probably extended to the bioleaching of other metals, such as copper. Currently studies are being carried on for the development of processes for the bioleaching of copper concentrates. The experience gained in the heap leaching of copper and in the biooxidation of gold concentrates is being used in these studies. Bioleaching of chalcopyritic copper concentrates in the next few years will constitute a great breakthrough in biomining. The application of these technologies to the processing of nickel, zinc, and other heavy metals may also become a reality in the near future.