Biohydrometallurgy and the Environment Toward the Mining of the 21st Century: Proceedings of the Int

Biohydrometallurgy and the Environment Toward the Mining of the 21st Century: Proceedings of the International Biohydrometallurgy Symposium IBS'99 Held in.
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Hydrometallurgy, Volume 71, Issues , Pages October Ballester Process Metallurgy, Volume 9, Elsevier The International Biohydrometallurgy Symposia IBS conference series are an integral part of the development of knowledge and practical technologies of biohydrometallurgy. This meeting established the forum for presentations in fundamental research and applications of microbial processes for biohydrometallurgy. The IBS series evolved to occur every two years in different countries.

Meetings continue to be based on volunteer organization with session topics designated by the host organizers.

An creation provides a path in quantitative body structure built for undergraduate scholars of Biomedical Engineering at Virginia Commonwealth collage. The textual content covers all of the components of body structure in 9 devices: Antibody Expression and Production: Spain, June , Process Metallurgy. Ballester the foremost topic of the overseas Biohydrometallurgy Symposium IBS 'Biohydrometallurgy and the surroundings towards the mining of the twenty first Century', held in El Escorial Spain from June , is biohydrometallurgy and the surroundings because it is expected that during the arriving century biotechnology will make its maximum contribution during this sector.

From the papers in those volumes it truly is transparent that environmental matters are already of significant curiosity to the biohydrometallurgical group.

Application of bioleaching to copper mining in Chile

Many operating factors have strong influence on the performance of heaps as will be discussed in the next section. Among them, curing and agglomeration are crucial for a satisfactory operation. Curing consists in the addition of concentrated sulphuric acid to the crushed ore to provide an adequate moisture content allowing a fast and efficient conditioning of the gangue.

Agglomeration of the fine solids with the coarser fraction results in a homogeneous heap of high porosity facilitating the down flow of liquid and up flow of gasses Acevedo et al. The fundamentals and recent advances in heap bioleaching have been reviewed by Bustos , Watling , Petersen and Dixon a , Petersen and Dixon b and Brierley Although the performance of heaps is satisfactory, they are not very efficient reactors from a kinetic point of view because of their heterogeneity and difficulties in establishing proper control systems for important variables such as temperature, pH, dissolved oxygen concentration, dissolved carbon dioxide concentration and others Acevedo, Considering this situation, it would be desirable to use other types of reactors for bioleaching that allow for a more stringent control of the reactions; however, this would be difficult because of the large volume of ores involved in copper mining.

The situation in gold mining is different because of the much smaller volumes involved. In fact, continuous stirred tank reactors CSTR are successfully used in the pretreatment of refractory gold concentrates. Since several decades ago, it has been postulated that bioleaching of copper concentrates in CSTR could be possible considering that concentration reduces the volume 25 to 35 times McElroy and Bruynesteyn, ; Groudev, ; Acevedo et al.

The kinetic and engineering fundamentals of the design of CSTRs for bioleaching have been addressed by several authors Gormely and Brannion, ; Greenhalgh and Ritchie, ; Harvey et al. Other reactor configurations such as bubbling columns, airlift columns, percolation columns, Pachuca tanks and rotary reactors have been studied Murr and Brierley, ; Atkins et al. After several years of operation, the project was shut down more for economical than for technical reasons Morales, ; Clark et al.

As said before, bioleaching is an operation where the metal sulphides are extracted from the mineral with the contribution of microbial cells that are chemolithotrophic and grow at pH as low as 1. The most studied microorganism has been Thiobacillus ferrooxidans , today renamed Acidithiobacillus ferrooxidans Kelly and Wood, , but it is well established that many different genus and species participate actively in the bioleaching process Norris, ; Schippers, Table 5 shows some examples of bioleaching microorganisms grouped according to their optimal growth temperature.

Ferric iron oxidizes the mineral sulphide liberating the metal into solution. The reduced sulphur compounds generated during the ferric iron attack are further oxidized by cells to sulphate, helping to keep the low pH required for cell activity and ferric iron action. Finally, the reduced iron generated during mineral attack is re-oxidized by the microorganisms.

Two ways of sulphide ores oxidation chain has been proposed according to the nature of the mineral species Schippers and Sand, ; Sand et al. Species like pyrite, molybdenite and tungstenite are oxidized by the so-called thiosulphate pathway while sphalerite, chalcopyrite, arsenopyrite and galena are oxidized via polysulphide pathway. This indirect mechanism in opposition to an enzymatic direct mechanism of sulphur moiety oxidation of heavy metals sulphides is considered the only relevant mechanism.

It can happen in contact and non-contact ways Sand et al. The first one is accomplished by planktonic cells and the latter by cells attached to the mineral surface. The extracellular polymeric substances EPS play an important role in cell attachment Sand et al. Copper bioleaching is influenced by a number of factors that, in addition to the operation conditions, play an important role: All these elements are relevant to assess the technical and economical feasibility of a bioleaching process for treating copper sulphides.

Both mineral species and ore composition and structure affect the rate and extension of the copper extraction by bioleaching from reduced sulphur compounds. For instance, the secondary copper minerals are easier to bioleach than the primary ones. In fact, the industrial bioleaching processes of copper ores are currently focused almost exclusively on secondary type minerals.

Table 6 lists the more common primary and secondary copper minerals. Being the primary copper sulphides by far the most abundant in nature, to find the economical way of processing these minerals through bioleaching has became a challenge to researchers, both at the scientific and technological levels. Some of these materials can interfere with the process inhibiting the microbial activity or modifying the required acidity of the leach liquor.

Particle ores can differ also in their component distribution, being probable in some cases that the mineral species becomes mainly exposed at the particle surface after milling facilitating bioleaching, or, in other cases, the mineral grains may allocate inside the particle making the contact with the leaching solution difficult.

These situations directly affect the kinetics and yield of copper extraction. Particle size is a key factor since smallest particles contribute directly to improve bioleaching kinetics, but at the same time grinding is a highly energy demanding operation having a strong impact in the economy of the whole process. Nevertheless, the range of particle size is defined by the bioleaching operation mode.

Dump leaching, one of the earliest way of bioleaching, utilizes run-of-mine lumps of several inches in size. Heap leaching, the operation mode used as a standard technology for bioleaching, uses particles of controlled size in the range of half inch. Use of smaller particle sizes is not recommended because bed permeability decreases impairing leach liquor circulation and oxygen and carbon dioxide transfer from air to the liquid phase.


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As a result, dump leaching operation can take 4 or more years, heap leaching operation times are in the range of 5 to 10 months and less than a month is required for stirred tank reactor Domic-Mihovilovic, ; Bustos, Of course, operation time is not only determined by particle size but also by different factors related to the prevailing environmental conditions that improve from dump to tank mode of bioleaching operation.

Copper solubilization and microbe activity are strongly influenced by these operation conditions and not necessarily in the same way. Sulphide oxidation rate and microbial activity increases with temperature. However, in the latter case temperature cannot be increased over the optimal microbial growth temperature. Beyond optimal temperature, microbial activity decreases and consequently bioleaching kinetics.

In this sense, thermophilic and hyperthermophilic microorganisms are preferred because they can withstand the highest operation temperatures favouring the chemical steps of copper solubilization. This has been demonstrated in several studies at laboratory level, especially in the case of copper extraction from chalcopyrite, where it has been possible to increase significantly both kinetic parameters and percentage of copper extraction. However, care must be taken with respect to oxygen and carbon dioxide transfer from gas to leach liquor.

Besides increasing the consumption rate of both gases, as a consequence of a higher bioleaching activity, the driving force acting on the mass transfer rate tends to decrease because the equilibrium saturation concentrations of gases decrease while temperature increases Hougen et al. During bioleaching there are several heat flows that induce changes in the system temperature Acevedo and Gentina, , being the principals those derived from microbial activity and from chemical oxidation of reduced sulphur minerals.

These temperature changes are difficult to control, especially in static mode of operation dump, in-situ , in-place and heap leaching where wide temperature profiles inside the ore beds are generated affecting in a non-homogeneous way the performance of the bioleaching operation.

At microbial level, temperature changes induce certain populations to increase their activity and others to diminish it according to their own range of growth temperature. Microorganisms involved in bioleaching are acidophilic, being mostly active in the pH range from 1. From the point of view of the process, operating pH over 2. However, pH cannot be allowed to drop down to 1. The former case contributes to raise pH while the latter contributes to lower it.

A more complex situation occurs when copper is inserted in a basic gangue ore, which causes important proton consumption that in turn requires the addition of sulphuric acid to the bioleaching system. Again, as in the case of temperature, depending upon the operation mode it will be possible to correct deviations from convenient bioleaching pH. Static bed operations will develop pH profiles affecting in different ways both cell activity and copper extraction.

At microenvironment level the pH situation can still be worse since in fixed bed configurations the mass transfer rates are low mainly because of the laminar characteristic of the hydrodynamic regimes. The incoming solutions to the ore body may have the right pH but this does not secure that the pH of the leach liquor film around the ore particles will be the right one.

Eh is a physicochemical condition that changes during a bioleaching operation as a consequence of cell activity. During copper extraction, Eh level is highly dependent on the ferric-ferrous ion ratio, but also depends on other galvanic pairs. Normally at the beginning of a bioleaching operation the leaching solution Eh is around mV and the copper extraction rate becomes important once Eh values get over mV.

In this sense, Eh is an important variable to quantify during bioleaching in order to use it as an index to assess the process behaviour. Lately, Eh has also been pointed out as an important factor, since there are experimental evidence that extraction of copper from chalcopyrite is better achieved using low Eh than the usual values obtained in normal bioleaching operations where Eh reach values around of mV Gericke et al.

Microorganisms involved in bioleaching operations need molecular oxygen and carbon dioxide to grow and keep viable. The first is used by cells as final electron acceptor of their energetic metabolic pathway. The second is used by cells as unique carbon source. Having both gases a low solubility in water and for extension in leach solution, it is crucial to the process transferring both of them at least at the same rate they are demanded by the microorganisms and, in the case of oxygen, also by the oxidation reactions of sulphide minerals.

A shortage of any of them will slow down the whole process and reduce the effectiveness of copper extraction Cautivo and Gentina, The microbial rate of gas consumption depends directly on the number of viable cells and their level of activity.

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More active cells mean greater demand. Another important concern related to these gases is the relatively low proportion of the carbon dioxide present in the air with respect to oxygen. In other words, when using air as source of both gases it is very likely to run into a situation where the growth and activity level of the bioleaching microorganisms become limited by the availability of the carbon source Acevedo and Gentina, This is so because bioleaching systems are most of the time in a condition where transfer rates are sufficient to satisfy the oxygen demand, but not the carbon dioxide demand.

Normally, because of their huge size where it is impossible to control aseptic conditions, commercial heap copper bioleaching operations are carried out using the natural micro flora existing at the ore site, which is advantageous because those cells are supposed to be already adapted to that particular copper ore. However, by doing so, the start-up period may become excessively long because the initial number of microorganisms is not enough to generate significant changes in the system. One practice at industrial level is to use the leach solution of a previous operation containing an acceptable microbial count to irrigate a starting operation, so reducing the start-up period.

An important fraction of cells is adhered to the ore particles and a low count of them remains in the liquid phase. The microbial population is diverse and a large number of bioleaching microorganisms coexist inside the heap including bacteria, archea, mesophiles, thermophiles, etc. In discontinuous operations it has been observed that the relative stratification of the microbial populations change with time, induced by environmental changes as operation proceeds Demergasso et al.

Unfortunately, it is not an easy task to identify the predominant populations Johnson and Hallberg, Molecular biology techniques have proven to be more effective than traditional microbiological protocols, but still they are an expensive and complex solution. Another important drawback is the absence of a methodology to determine the biomass present in the bioleaching system, an extremely useful variable to monitor and control the bioprocess performance.

To count cells present in the leach liquor is easily done, but the quantification of the biomass adhered to the ore particles in a huge commercial operation is far from possible.

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Also not possible is to infer total biomass starting from the known number of planktonic cells. At the present time some industrial operations are using inoculation of specially propagated bioleaching microorganisms with the object of increasing rapidly their number at the start of the bioleaching operation Du Plessis et al. This strategy is currently being studied in detail considering the propagation of highly active strains or mixed populations to enrich the natural flora. Two inoculation moments have been proposed: Both have advantages and disadvantages.

In the first case inoculation is homogeneous, but no culture conditions may exist for a very long period, probably affecting the viability of the cells. In the second case, the inoculated cells immediately find an adequate environment but their distribution is not homogeneous leaving zones inside the heap with none or low cell counts. This phenomenon occurs mainly because of the marked tendency of cells to adhere to ore surfaces decreasing rapidly the cell concentration in the leach solution stream and also because the uneven way the leaching solution percolates through the ore heap.

Bioleaching microorganisms need an energy source, a carbon source and sources of several others elements in order to keep viable and active. Most of their nutritional requirements are found in the mine environment, but some of them, like nitrogen and phosphorous sources, are not frequently found in the amounts required, especially in arid zones.