Biomining-A useful approach toward metal extraction Abstract: Biomining is the extraction of specific metals from their ores through .. Rawlings, D.E., Biomining [Douglas E. Rawlings, D. Barrie Johnson] on *FREE* shipping on qualifying offers. Biomining uses microorganisms to recover metals, . Editorial Reviews. From the Back Cover. Biomining is the biotechnology that uses Biomining – Kindle edition by Douglas E. Rawlings, D. Barrie Johnson.
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The use of reactors in biomining processes. Abstract Microbial processes applied to mining operations are gaining increasing interest in the last years. Potential and current applications include the mining of gold, copper and other heavy metals, desulfurization of rawlijgs and oil, tertiary recovery of oil and biosorption of metal ions. Rawliings, bacterial leaching of copper and biooxidation of refractory gold concentrates are well-established large-scale rawlinfs that are carried on using heaps and tank reactors.
Heap operation is simple and adequate to handle large volumes of minerals, but their productivity and yields are limited because of the severe difficulties in exerting an adequate process control. On the other hand, reactors can economically handle moderate volumes of material, but they allow for a close control of the variables involved, rendering significantly better performances.
This paper reviews rawlihgs basis of reactor selection and design for bioleaching processes. Special attention is given to the influence of oxygen and carbon dioxide mass transfer, process stoichiometry, solids suspension and slurry homogeneity, and the use of bioreactors in gold mining.
It is concluded that the future of reactors in biomining is promising and that new applications, such as the bioleaching of copper concentrates, will soon be a reality. Microbial processes are gaining increasing interest 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 Karavaiko, ; Kelley and Tuovinen, ; Lawrence and Poulin, ; Rawlings, ; Brierley and Brierley, Currently, bacterial leaching and biooxidation are large-scale processes that are being successfully used in copper and gold processing Acevedo et al.
The term biomining have been coined to refer to the use of microorganisms in mining processes. Biomining rawlinga two related microbial processes that are useful in the extractive metallurgy of heavy metals: Leaching is the solubilization of one or more rawoings of a complex solid by contact with a liquid phase. In bacterial leaching, the solubilization is mediated by bacteria. So bacterial leaching is a process by which the metal of interest is extracted from the ore by bacterial action, 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. For many years bioleaching rawligs thought as a technology for the recovery of metals from low-grade ores, flotation tailings or waste material Torma, ; Gentina and Acevedo, Today bioleaching is being applied 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 pyrometallurgy lie in its relative 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 roasting and pressure oxidation Gentina and Acevedo, ; Rzwlings, 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 that contains a fraction of the bacterial population, the rest being attached to mineral. When the desired metal concentration is attained, the rich liquor is pumped to the solvent extraction SE section and then sent to electrowinning EWwhere the fine metal is recovered. The raffinate from the SE section is recycled to the heap or dump and the spent liquor of the EW section is recycled to the SE operation Montealegre et al.
Heaps and dumps present a number of advantages such as simple equipment and operation, low investment and operation biojining and acceptable yields. On the other hand it must be rawlngs that the operation suffers from some severe limitations: Moreover, the rates of oxygen and carbon dioxide transfer that can be obtained are low, and extended periods of operation are required in order to achieve sufficient conversions Acevedo and Gentina, From a process engineering standpoint, the complex network of biochemical reactions encompassed in bioleaching would best be performed in reactors.
The use of reactors would allow a good control of the pertinent variables, resulting in a better performance. Parameters such as volumetric productivity and degree of extraction can be significantly increased Pinches et al. The main limitation in the use of reactors in biomining is the very large amounts of run-of-mine ore that in most cases is to be treated.
The Chuquicamata copper mine in Chile producedtons of fine copper in The production of that amount of metal implied the treatment of around 6 million tons of run-of-mine. If such amount would to be treated in bioreactors, the required equipment volume would of the order of 30 million cubic meters, an unthinkable figure. This limits their application to the treatment of rawllngs concentrates or when moderate volumes of ore are to be processed.
For instance, over 11, tons of gold concentrates are biooxidized in reactors every year. The discussion that follows will center on the use of bioreactors in biomining, with emphasis in oxygen and carbon dioxide transfer, the maintenance of an adequate solids suspensions and the application of bioreactors to commercial mining operations.
The selection of a suitable 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 an aqueous liquid, suspended and attached cells, suspended solids, and air bubbles Gormely and Brannion, Because of the very 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.
Considering the kinetic characteristics of microbial growth, a continuous stirred tank reactor, CSTR, appears as the first choice. An important consideration rawlngs selecting a suitable reactor refers to the autocatalytic nature of microbial growth.
Copper Mining Using Acidothiobacillus
This fact is common to all fermentation operations, but in bioleaching there is an important difference. In industrial fermentations the nutrients are chosen by their high affinity with the microbial population, bjomining in biomining the mineral species involved are usually recalcitrant to microbial action, implying that the affinity is quite low.
The substrate-microorganism affinity is related to Monod’s saturation constant, K S Monod, High affinities are reflected in low K S values, of the order of a few milligrams per liter, as in the case of most sugars. This situation affects the selection of the reactor.
If a high degree of conversion is desired, a single agitated tank will require a very large volume, so an arrangement of reactors will be more suitable Dew, It can be shown that a CSTR followed by a tubular plug flow reactor, PFR, gives the minimum reaction volume to attain a certain conversion Levenspiel, 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 some special designs such as rotary reactors Atkins and Pooley, ; Atkins et al.
Several mass transfer operations occur in a biomining operation. Nutrients have to reach the attached and suspended cells, metabolic products have to migrate from the cells to the liquid and solubilized species must be transported from the surface of the mineral particles to the liquid. In addition, two other important transport processes are to be considered: Carbon dioxide is demanded raslings the cell population as carbon source, while oxygen is needed as the final electron acceptor of the overall oxidation process.
In reactors these gases are usually supplied by bubbling air into the liquid. In order to be used by the cells, oxygen and carbon dioxide must dissolve in the liquid, a mass transfer operation that presents a high resistance and can become rzwlings for the overall process rate.
A gas mass balance around the bioreactor gives Wang and Humphrey, The gas supply,must equal the gas demand in order to biomibing growth limitation, so. The gas demands can be calculated as. The bioleaching process can be represented by a stoichiometric biominlng Acevedo, ; Acevedo and Gentina, In the case of a leaching organism such as Thiobacillus ferrooxidans growing in a simple defined culture media with ferrous iron as the energy source, the following equation can be written: C 5 H 7 O 2 N represents the biomass with an elemental composition of Elemental mass balances on C, H, O and N, together with the experimental value of the ferrous ion cell yield of 0.
The oxygen and carbon dioxide cell yields can be calculated from equation : In an actual bioleaching operation, a similar stoichiometric representation can be made. For instance, for the biooxidation of enargite Cu 3 AsS 4 from a refractory gold concentrate the following equation applies, considering an experimental value of 0.
Biomining : Theory, Microbes and Industrial Processes
In this case the oxygen and carbon dioxide cell yields are: Rawilngs could be expected, the carbon dioxide yield, related only with cell growth, is fawlings same in the defined soluble medium and in the bioleaching of a mineral. Equations , , , ,  and  can be used to estimate the required mass transfer coefficients, as shown in Table 1.
When not enough experimental data are available, the required coefficient for oxygen can be estimated from the stoichiometry equation of the main oxidation reaction.
The required k L a’s for oxygen are of the same order of magnitude or less than those that have been obtained experimentally in bioreactors Acevedo et al.
This situation may change at higher pulp densities Bailey and Hansford, ; Hansford gawlings Bailey, ; Loi et al. In fermentation technology it is usual to correlate k L a with agitation power per unit volume and gas superficial velocity Wang and Humphrey, ; Boon and Heijnen, ; Harvey et al.
In leaching bioreactors, the transfer coefficient may be influenced by the presence rawlins solids Mills et al. Table 2 shows some correlations of this type. Because of its very low concentration in air, 0. Carbon dioxide limitation has been demonstrated by several authors Torma et al. The CSTR is an ideal conception that implies a completely mixed content that presents no gradients, so the value of each variable is the same at every point within the liquid.
That being the case, the exit stream has the same composition as the fluid within the reactor. As stated previously, tank bioleaching is a three-phase system composed by the incoming air and the outlet gas, the acidic aqueous liquor, and the microbial cells and mineral particles. The complex nature of this slurry makes the attainment of homogeneity especially difficult Brucato and Brucato, Agitation has a double purpose: Under conditions of insufficient agitation the transfer operations may become limiting and the overall reaction performance will decline because of the appearance of zones of the fluid with insufficient nutrients or inadequate temperature or pH Namdev et al.
For several decades the use of disk turbines or Rushton turbines have been common in industrial fermentors. Back in the fifties investigators were mainly looking for impellers that specifically enhanced oxygen transfer, but they neglected other important factors such as mixing, impeller gas flooding and power biomihing, which are all negative assets for the disk turbine Humphrey, The high shear stress exerted by the disk turbine on the fluid may also produce metabolic stress and cell growth inhibition Toma et al.
When mixing is specially important, axial flow impellers such as the hydrofoils become an advantageous alternative Nienow, ; Junker et al. This is the case of bioleaching, where the oxygen demands are modest but the presence of fine solid particles impose an additional difficulty in obtaining homogeneous slurries.
Biomining : Douglas E. Rawlings :
Table 3 lists the most commonly used type of impellers. It can be seen that the rawlinfs required by disk turbines is very high compared with the requirements of other rawlingw. Some of the hydrofoil designs developed in the mid-eighties Lally, present convenient characteristics for their use in bioleaching reactors.
Their power requirement is low, the mixing and solids suspension capabilities are good and oxygen and carbon dioxide transfer coefficients are comparable with those of the disk turbine Kubera and Oldshue, ; Kaufman et al.
The problem of solids suspension in agitated vessels has been addressed by several investigators.
An important early work was that of Zwieteringwho studied the minimum required stirrer speed referred afterwards as critical speed and the stirrer dimensions for the complete suspension of solids. In this work the main objective was to avoid solids deposition on the bottom of the tank, but the homogeneity of the slurry was not of special concern. Different expressions for the critical speed have been proposed since then Oldshue, ; Tatterson, In this respect, some early work by Oldshuecan be cited.