Heap bioleaching of chalcopyrite: A review

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Minerals Engineering 21 (2008) 355–365 This article is also available online at: www.elsevier.com/locate/mineng

Heap bioleaching of chalcopyrite: A review N. Pradhan *, K.C. Nathsarma, K. Srinivasa Rao, L.B. Sukla, B.K. Mishra Institute of Minerals and Materials Technology, Bhubaneswar 751013, Orissa, India Received 21 December 2006; accepted 27 October 2007 Available online 20 February 2008

Abstract Bioleaching is an emerging technology with significant potentials to add value to the mining industries so as to deliver attractive environmental and social benefits to all the associates. Chalcopyrite, CuFeS2, is the most important copper-bearing mineral in the world and unlike many other ores it is known to be recalcitrant to hydrometallurgical processing. The main hindrance to the commercial application of biohydrometallurgical processing of chalcopyrite is its slow rate of dissolution. In this piece of review work, the microbiological and other important aspects of chalcopyrite heap bioleaching processes are discussed. The modest nutritional requirements of bioleaching organisms may be provided with the aeration of iron- and/or sulfur-containing mineral suspensions in water or the irrigation of a heap, while working in a large scale. This chemolithotrophic metabolism makes the organisms industrially important. The emphasis is given on the biodiversity of microbial community and the factors affecting heap bioleaching. The cost of bio heap leaching in respect of some existing commercially operating heap bioleaching plants is also included. Application of chalcopyrite bioleaching in heap/dump leach processes can potentially result in lower cost and reduced environmental impact in copper production. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Heap bioleaching; Chemolithotrophic microorganisms; Chalcopyrite; Copper bioleaching; Thermophilic bioleaching bacteria/archea

Contents 1.

2. 3.

4.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Irrigation-based leaching processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Agitation based leaching processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General description of heap bioleaching operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heap bioleaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Salient features of microorganisms involved in bioleaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Microbial diversity in bioheap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors affecting heap bioleaching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Type of ore material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Aeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Irrigation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Some other factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.1. Jarosite formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2. Attachment of microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3. Build up of metal ion/organic matter concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. Tel.: +91 0674 2584091; fax: +91 0674 2581637. E-mail address: [email protected] (N. Pradhan).

0892-6875/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2007.10.018

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5. 6. 7. 8. 9.

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Thermophilic leaching . . . . . . . . . . . . Difficulties in heap leaching processes . Cost of bio heap leaching . . . . . . . . . . Some examples . . . . . . . . . . . . . . . . . Conclusion: present and future of heap References . . . . . . . . . . . . . . . . . . . .

.................... .................... .................... .................... bioleaching of chalcopyrite ....................

1. Introduction Heap and dump leaching offer a number of advantages embracing simple equipment, low investment and operation cost, and reasonable yields over a period of recirculation. The earliest engineering technology used in dump leaching was very basic in nature involving dumping a low-grade (otherwise waste) copper-bearing ore in the form of large rock/boulder into vast mounds and irrigating it with dilute H2SO4 to enhance the growth and activities of mineral-oxidizing acidophiles, i.e., primarily iron-oxidizing microorganisms. Copper was precipitated from the metalrich streams draining out of the dumps by cementation with scrap iron. Later developments on the engineering and hydrometallurgical aspects of biomining have involved the use of thin layer heaps of refractory sulfidic ores (mostly copper, but gold-bearing material) stacked on to water-proof membranes, and the solubilized copper recovered using solvent extraction and electrowinning (SX/EW). The process permits recovery of copper, zinc and other metals using the catalytic activity of several strains bearing ferrous iron and sulfur-oxidizing chemolithotrophic bacteria (Norris, 1990). Generally speaking, industrial-leaching processes operate with the naturally occurring microorganisms in mine waters and in the ore body. Most of experiments carried out on bioleaching of chalcopyrite are so far carried out in shake flask level. Scale up of such studies to an industrial level requires process development through engineering and process modeling. The ore grade and particle size are the controlling factors while making a choice of leaching process. Rawling et al. (2003) categorized the engineering approaches used in biomining in two broad categories as follows: 1.1. Irrigation-based leaching processes It can again be categorized depending on the type of resources to be processed as dump leaching, heap leaching and in situ leaching. In dump leaching, waste rock, lowgrade ore or concentrator tailings (low grade oxides and secondary sulfides) are leached at the place of disposal. This is a mature and widely used technology. Heap leaching deals with the newly mined/run-off-the-mine (ROM) materials (intermediate grade oxides and secondary sul-

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fides) deposited in the form of a heap on an impervious natural surface or a synthetically prepared pad leached with circulation, percolation, and irrigation of the leaching medium. Primary sulfides like chalcopyrite are suitable for this type of leaching. In situ leaching is employed with abandoned and/or underground mines where the ore deposits cannot be mined by the conventional methods since they are either low grade or of small deposits or both. 1.2. Agitation based leaching processes This is a stirred tank process involving stirred tank bioreactors. The types of resources or raw materials suitable for this kind of leaching range from intermediate to highgrade ore. Chalcopyrite concentrates are taken in a tank and leached using mechanical agitation. In stirred tank processes, highly aerated and continuous-flow reactors placed in series are used to treat the minerals. From a process-engineering standpoint, the complex network of biochemical reactions encompassed in bioleaching would best be performed in reactors that would allow a good control of the pertinent variables resulting in a better performance. 2. General description of heap bioleaching operations Heap reactors are cheaper to construct and operate and are therefore more suited to the treatment of lower grade ores. Commercial bioleaching involving the irrigation of waste ore dumps can take place economically, for which it is considered as a low technology process. The metal extraction process may be made much more efficient by the construction and irrigation of especially designed heaps rather than by the irrigation of an existing dump that has not been designed as per the optimized leaching process. While building a heap, ore is piled onto an impermeable base supplied with an efficient leach liquor distribution and collection system. Acidic leach solution is percolated through the crushed ore, and the microbes growing on the mineral surfaces of the heap produce the ferric iron and acid that result in mineral dissolution and metal solubilization. The microbial population operating in natural leaching processes does not really have the characteristics of a pure culture, although environmental conditions

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357

Fig. 1. Equipment flow diagram for heap bioleaching of chalcopyrite ore.

principally favour the development of acidophilic Acidithiobacilli and Leptospirilli. Aeration in such processes can be passive, with air being drawn into the reactor as a result of the flow of liquids, or active with air blown into the heap through piping installed near the bottom. Metal-bearing leach solutions being drained out from the heap are regularly collected and sent for metal recovery. Furthermore, although one can rely on the natural movement of microbes to inoculate the heap, the initial rates of bioleaching can be improved by the effective heap inoculation; however, this is difficult to achieve. Fig. 1 shows the equipment flow diagram for heap bioleaching of chalcopyrite.

biooxidation of minerals are those responsible for producing ferric iron (Eq. 2) and sulfuric acid (Eq. 3) required for the bioleaching reactions. Ferric sulfate, a powerful oxidizing agent, oxidizes the copper sulfide minerals leading to the in situ leaching of copper by the sulfuric acid generated therein.

3. Heap bioleaching

Another important characteristic is that the microbes grow autotrophically by fixing CO2 from the atmosphere. These are the iron- and sulfur-oxidizing chemolithotrophic bacteria and archea (Johnson, 1998). A further advantageous characteristic of mineral biooxidation operations is that they are not usually subject to contamination by the generated unwanted microorganisms. Another important characteristic of the acidophilic chemolithotrophs is their general tolerance to higher concentrations of metal and other ions. The levels of resistance show a considerable strain variation. Adaptation to high levels of resistance to a metal on exposure is likely to be responsible for much of the variation. The modest nutritional requirements of these organisms are provided by the aeration of an iron and/or sulfur containing mineral suspension in water or irrigation of a heap in a higher scale of operation. This chemolithotrophic metabolism makes the organism industrially important.

In general, the types of microorganisms found in heap leaching processes are similar to those found in stirred tank processes, but the proportion of the microbes may vary depending on the mineral and the conditions under which the heaps or tanks are operated. The most important microorganisms considered for the processes that operate from ambient to 40 °C are to be a consortium of gramnegative bacteria. Among these bacteria, there are species with an extremely limited substrate spectrum. 3.1. Salient features of microorganisms involved in bioleaching Bioleaching microbes have a number of features in common that make their role especially suitable for mineral solubilization. The most important microbes involved in the

CuFeS2 þ 4Hþ þ O2 ! Cuþ2 þ Feþ2 þ 2S þ 2H2 O 4Feþ2 þ 4Hþ þ O2 2S þ 3O2 þ 2H2 O

Iron oxidizing Bacteria

!

Sulfur oxidizing Bacteria

!

ð1Þ

4Feþ3 þ 2H2 O

ð2Þ

þ 2SO 4 þ 4H

ð3Þ

CuFeS2 þ 4Feþ3 ! Cuþ2 þ 2S þ 5Feþ2

ð4Þ

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3.2. Microbial diversity in bioheap A wide variety of microorganisms consisting mainly of bacteria and archea are found in natural leaching environments such as acid mine drainage. The majority of known acidophilic microorganisms have been isolated from such natural environments. These microorganisms are employed for the leaching of metals from ores in an industrial scale. The only difference is that in some cases they have been selected for rapid growth on the ore or concerned concentrate and in the plant operating conditions. In commercial processes of bacterial leaching, a wide variety of microorganisms living in symbiotic associations take part. These microorganisms, whose role can be considered similar to that of catalysts, may be mesophilic, thermophilic, autotrophic or heterotrophic in nature. Studies had revealed that microbial bioleaching communities composed of a vast variety of microorganisms result in complex microbial interactions and nutrient patterns (Erlich, 1999; Johnson, 1998; Edwards et al., 2000). Various methods are used for studying microbial diversity in ecological samples as well as in industrial bioheap leaching plants. There is still widespread uses of enriched cultures with ferrous iron, sulphur and pyrite as substrates for isolating acidophiles. Again in some cases this may be useful, but the enrichment process may be selective for a relatively narrow range of acidophiles that grow better under the imposed culture conditions to give a false impression of the relative importance of a particular bacterium in situ. Recently, detailed investigations based on molecular methods such as DNA–DNA hybridization, 16S rRNA sequencing, PCR-based methods with primers derived from rRNA sequencing, fluorescence in situ hybridization (FISH), or immunological techniques are used for assessing biodiversity of leaching community. Fig. 2 shows some of the identified microorganisms involved in the bioleaching processes. Understanding the microbiology of a bioheap is important for advancement in commercial bioheap applications. Such knowledge will increase the applications to various types of ores as well as to the diversity of mineral deposits that can be processed by bioheap technology. It will also enable the better control of conditions to improve upon the leaching rates, metal recoveries and cost of production. A limited comprehension is available of what actually occurs in a full-scale microbiologically operated bioheap, despite the commercial achievement in the copper ore bioheap leaching. The chemical and physical conditions within the bioheap change drastically from the time of stacking, inoculation and completion of bioleaching. Redox conditions, acidity, temperature, oxygen supply and solution chemistry conditions vary widely during the oxidation period. Such conditions do likely be selective for microorganisms or may affect a succession of organisms in different portions of the Bioheap. Bioheap solutions are recycled and the building up constituents over the time period also affects the microbiology. Heterotrophic microorganisms build up during the period may play some role in bioheap leaching.

Copper recovery from the chalcopyrite concentrates was greater when the native isolates were employed compared with the reference strains as the former adapts readily to metal sulphide ores and leaching conditions (Keeling et al., 2005). Brierley and Brierley (2001) described different chemical, physical and microbiological practices in monitoring commercial bioheaps. He considered different conditions that control microbial populations in bioheaps in addition to the type of ore deposits that could be bioleached. The microorganisms responsible for setting in proper physico-chemical changes like Eh, pH, temperature and concentration of metals and metalloids in to the system leading to mineral oxidation and dissolution are of particular significance, while the other organisms associated with the original ore are of minor, or of no importance in the process of mineral dissolution. A culture-independent approach based on PCR amplification and denaturing gradient gel electrophoresis (DGGE) and sequencing of 16S rRNA gene fragments from both bacteria and archea were used to analyze the microbial community inhabiting a low-grade copper sulfide run-off-mine (ROM) test heap of a project in Chile (Demergasso et al., 2005) for one year. Phylogenetic analyses of 16S rRNA fragments revealed that the retrieved sequences clustered together with Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum, Ferroplasma acidiphilum and environmental clones related to them. In addition, some sequences were distantly related (<95% similarity in the 16S rRNA gene fragment analyzed) to cultured microorganisms from the Sulfurisphaera and Sulfobacillus genera. Thus, the prokaryotic assemblage might be mainly composed of sulfur- and iron-oxidizing microorganisms. The temporal distribution of microbial 16S rRNA gene sequences could be divided in three periods. In the first stage of bioleaching cycle, the A. ferrooxidans and Sulfurisphaera-like archea were dominant within each respective phylogenetic domain. In the second stage of operation from 255 to 338 days, Leptospirillum and Ferroplasma groups were mainly detected, respectively. Finally, in the third period of operation from 598 to 749 days, Sulfobacillus-like microorganisms became predominant, while Ferroplasma was the only archea detected. Using the analysis of 16 S rDNA amplification products of total DNA, the sulphur-oxidizing bacterium A. thiooxidans (Kelly and Wood, 2000) and the iron-oxidizing bacterium L. ferrooxidans have been found as the dominant populations in bioleaching tanks that contain copper ores leached at low ferrous iron concentration and in a continuous flow biooxidation tank for the treatment of gold bearing arsenopyrite concentrates. At temperatures higher than 40 oC, the moderately thermophilic A. caldus (Kelly and Wood, 2000) was also present (Espejo and Romero, 1997; Lawson, 1997; Pizarro et al., 1996; Rawlings and Silver, 1995). A. ferrooxidans and A. thiooxidans play an important role in the bioleaching process. Experiments showed that copper extraction in a mixed culture composed of A. ferro-

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359

Common Ancestor Eukaryota

Archebacteria

Crenarchaeota Korarchaeota

Nanoarchaeota

Bacteria

Euryarchrota

Sulfobales S.acidocaldarius

Ferroplasma acidophilum

S.sulfotaricus

Ferroplasma acidarmanus

Solfolobus

Metallosphaera

M.sedula A.brierleyi

Acidianus A.infernus Sulfurisphaera

Proteobacteria

γ Proteobacteria

Gram Negative

βProteobacteria

Acidithiobacillus ferrooxidans Acidithiobacillus thiooxidans Acidithiobacillus caldus

Thiobacillus

Leptospirillum Sulfobacillus

Firmicutes

Gram Positive Acidimicrobium

Actinobacteria

Ferromicrobium Fig. 2. Identified microorganisms involved in bioleaching.

oxidans and A. thiooxidans is higher than that in a pure culture (Qiu et al., 2005). An important potential of A. thiooxidans to the leaching of chalcopyrite was indicated in which jarosite accumulation on the substrate is prevented to allow further dissolution of copper through the action of ferric ion. The selection of a suitable pH in a leaching solution would be significant. In agreement with other reports, Rawlings et al. (1999) concluded that high Fe3+/Fe2+ ratio (high redox potential), low pH, and high temperature often favor L. ferrooxidans over A. ferrooxidans in commercial leaching operations carried out either in column- or heap-type, or continuous-flow tank reactors. Operational constraints like the desirability of decreasing pulp residence times in stirred tank bioreactors can result in the selection of more efficient bioleaching populations (Dew et al., 1997). The presence of anaerobic micro-

organisms in the lower parts of the heaps is another real possibility, since the zones of low or zero concentrations of oxygen are expected to exist. These conditions as well as the presence of a reducing agent such as organic matter, anaerobic bacteria (Desulfovibrio desulfuricuns), in the system are capable of reducing the sulphate ions to sulphide ion with the consequent precipitation of insoluble metallic compounds which may reduce the metal recovery. 4. Factors affecting heap bioleaching Bacterial leaching, like any other process involving living beings, is influenced by environmental, biological and physico-chemical factors, which affect the yield of metal extraction (Torma, 1977; Lundgren and Silver, 1980). Optimal conditions of humidity, pH, temperature, energy

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sources and nutrients have to exist along with the absence of possible inhibitors for the growth of microorganisms involved in the process. These conditions are prerequisite for the growth of microorganisms to favorably affect copper solubilization (Table 1). Correct chemical and physical conditions must exist for the percolation leaching system to function: a suitable ore particle size, access of oxygen and humidity to the mineral surfaces, reduced acid consumption, the presence of sulphides susceptible to bacterial oxidation, and the greatest possible elimination of precipitated basic ferric salts, which might block the percolation channels. In addition, the geometry of heap may also affect the bioleaching process and rate of recovery. For these leaching systems to function, the medium must be kept in such conditions that the bacteria requirements are adequately covered. When the environment can maintain these optimal conditions, an adequate yield of copper can be obtained.

The effectiveness of bioleaching process depends largely on the nature of ore material (Munoz et al., 1995) in which the metal exists. When the medium is alkaline, it is probable that precipitates will be formed that will hinder the natural percolation of leach solution through the heap. Porosity of ore material allows the solution to penetrate more into the ore body. Another important aspect of the mineral/ore is to provide sufficient quantity of nutrients to the microorganisms for growth. Quartzic and garnitic ore show variable susceptibility to microbial leaching.

aerated, is prone to anaerobic conditions. Aeration of bioheaps can accelerate biooxidation reactions by reducing the time of leach cycle for which the supply of oxygen is very important for viability and activity of leaching microorganisms. Air may be delivered to the system via a network of pipes installed in a gravel layer at the base of heaps. Air distribution networks may typically include 500 mm headers and 50 mm diameter laterals at 2 m spacing. Holes are drilled at the bottom of the 50 mm diameter air distribution pipes (Brierley and Briggs, 2002). The density of holes is dependent on the amount of sulfide–sulfur to be oxidized and the oxidation rate. Air is injected into the heap using a set of low-pressure high volume fans or blowers. Bioleaching determines the oxygen concentration profile with respect to the height of the aerated heap. At the bottom of the pile, where air is forced into the heap, oxygen is close to saturation, but as the air flows upwards through the void spaces, the bacteria catalyzing the oxidation of sulphide consumes oxygen and as a result, the degree of oxygen depletion near the top of the heap prevails. Appreciable oxygen concentration exists as a gradient with depth. Heap oxidative capacity is considered a better indicator of leaching. Copper leaching is directly related to oxygen consumption in the heap. Oxygen consumption, in turn, is related to bacterial activity and the rate of forced aeration. Increasing the rate of aeration, may improve copper leaching. This may be the case when the heap is in a state of oxygen depletion. As oxygen is required for oxidative metabolism, its depletion has rate limiting effect. If oxygen is sufficiently present at all the points of the heap, increase in aeration does not increase the leaching rate.

4.2. Aeration

4.3. Irrigation

As most of the metal leaching bacteria are aerobic and chemolithotrophic in nature, aeration takes care of the supply of both O2 and CO2 to the leaching system. Sufficient carbon dioxide in air serves as a source of carbon needed for biomass generation. Interior of heap, if not properly

Regarding the type of irrigation, continuous or discontinuous, there are different trends, although it is generally thought that discontinuous irrigation favours metal dissolution. In such cases, the attack solution is intermittently sprayed onto the surface of the heap and is allowed to percolate before a new solution is applied, thus setting up an inverse capillary effect, which permits the leaching of coarse ores. During irrigation, the capillary forces draw the liquid into the mass of the ore. When irrigation ceases, the liquid drains out from the capillary and remains on the surface and a new irrigation carries with it the dissolved metal ion and the process begins again with the introduction of a fresh liquid into the capillary. In this way, discontinuous irrigation may be more effective for coarse ores than the continuous one, since the alternate draining and drying of the capillaries is considerably faster than the simple ionic diffusion through a static capillary full of fluid. Thus, alternate irrigation and drying helps to leach coarse particles and drain out soluble salts from their surfaces as well as increasing the diffusion of oxygen and carbon dioxide to the ore surface where active bacteria are located. However, the frequency of irrigation is an important factor to be

4.1. Type of ore material

Table 1 Factors and parameters affecting heap bioleaching and metal recovery (Brandl, 2001) Factor

Parameters affecting bioleaching

Physical and chemical parameters

Temperature, pH, redox potential, CO2 and O2 content, nutrient availability, oxygen availability, homogenous mass transfer, Fe (III) concentration, presence of inhibitors, etc Microbial diversity, population density, microbial activities, metal tolerance, spatial distribution of microorganisms, attachment to ore particles, adaptation abilities of microorganisms, and inoculum Composition, mineral type, acid consumption, grain size, mineral dissemination, surface area, porosity, hydrophobic galvanic interactions, and formation of secondary minerals

Biological parameters

Ore characteristics

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considered. Several studies (Aslam and Aslam, 1970) have demonstrated that daily irrigation in the presence of bacteria is more effective than the irrigation once in a week. Industrially, the irrigation frequency (cycle) is determined by the rate of evaporation and the concentration of the metal in the exiting liquid phase (Bruynesteyn, 1983). 4.4. Temperature Microorganisms are classified in terms of the temperature range in which they can survive: within optimum temperatures of 30–40 °C for mesophiles, around 50 °C for moderate thermophiles and above 65 °C for extreme thermophiles. Below the optimum temperature the microbes become inactive, but at temperatures above it, they are rapidly destroyed. Biooxidation of sulfide minerals, being an exothermic process, produces significant heating in stirred tank reactors and heaps. Controlling the temperature in heaps is difficult. Heap height is an important factor for the rise in temperature, which increases with the square of heap height (Ritchie, 1997). In industrial operations, the temperature inside the heap can reach up to 50 °C for which the biological and chemical oxidation reactions cannot be regulated, and the attainment of an excessively high temperature will obviously inhibit mesophilic bacterial activity. The heaps get heated up due to the growth of mesophilic microorganisms and the oxidation of sulphides. As temperature increases more than 40 °C, mesophiles are displaced by the moderately thermophilic iron- and sulfuroxidizers (Murr and Brierley, 1978). Extreme thermophiles may displace moderate thermophiles where temperatures increase above 60 °C. Changes of temperature within a heap, as a result of seasonal changes, can cause modifications in the microbial population while favoring the development of mesophilic or thermophilic bacteria. Thus, the heap temperature, due to the exothermic reactions, can be a controlling factor in some commercial operations. Temperature inside the heap is determined by various factors, as Petersen and Dixon (2002), through their heat generation modeling study on chalcopyrite concentrates, have established a correlation between temperature, aeration and the rates of irrigation. According to them, the temperature profile within a heap is governed by the generation of heat due to chemical reaction, lowering of heat due to the flowing down of solution, increase of heat due to upflow movement of humid air, boundary effects due to solar radiation and evaporation, and to a lesser extent by the conduction through rock. Thus, Dixon (2000) has formulated a comprehensive heap-heat conservation model to show that the ratio of irrigation and aeration rates is critical in determining the proper distribution of temperature all over the heap. 4.5. Some other factors Metal oxidation mediated by acidophilic microorganisms can be inhibited by a variety of factors such as organic

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compounds, surface-active agents, solvents, or specific metals. 4.5.1. Jarosite formation The jarosite formation is problematic in bacterial leaching. Once the jarosite is formed, it precipitates on the mineral surfaces and decreases the effectiveness of reagent and mineral surface interaction, and the inhibition of metal oxidation mediated by acidophilic microorganisms is one such phenomenon. One of the problems in the leaching of chalcopyrite needs to be overcome is the formation of passivating layer. The inhibition in bioleaching is caused by the formation of an intermediate sulfide passivation layer. It is believed that the passivation layer is less reactive than the original chalcopyrite and may inhibit the flow of electrons and oxidants to and from the chalcopyrite. The exact nature of this passivation layer is complex and it is reported to have come from one or both the sources: (1) Fe(OH)3 tends to form the jarosite [KFe3(SO4)2(OH)6] which coats the unreacted material to form a passivating layer and/ or, (2) the elemental sulfur formed during the process tends to coat the surface. The formation of ferric iron precipitate and jarosite are highly pH dependent and precipitation of jarosite is more pronounced within the pH 1.9–2.2. Use of thermophilic microorganisms raises the temperature to 60 °C or higher to destabilize the passivating layer on chalcopyrite in a high temperature heap bioleaching process (US Patent No. 6110253, 2000). 4.5.2. Attachment of microorganisms Attachment of microorganism to the ore particles has been well proven (Sand et al., 1995). Numerous studies have demonstrated the presence of bacteria attached to the ores and/or present in the pregnant leach solutions (PLS) in commercial operations. Sand et al. (1995) proposed a typical model for the bacterial leaching, which essentially proceeds by ferric ions complexation by the secreted extra cellular polymeric substances (EPS). This model, originally conceived for A. ferrooxidans, has been applied to L. ferrooxidans, because both iron-oxidizing bacteria share the quality of EPS enabling the attachment to solid substrates (Gehrke et al., 1995; Schippers et al., 1996; Gehrke et al., 1998; Rojas-Chapana and Tributsch, 2004). Addition of ferric ion alone did not induce any form of local corrosion on pyrite, which indicates that the reactions taking place between the attached bacteria and the underlying pyrite surface are responsible for the leaching leading to corrosion (Rojas-Chapana and Tributsch, 2004). 4.5.3. Build up of metal ion/organic matter concentration During bioleaching of sulphide mineral concentrates, heavy metals such as copper, zinc, arsenic and iron accumulate in the leach liquor, and beyond certain concentrations they become toxic to the microorganism, thereby deleteriously affecting the rates of biooxidation. The synergism of toxic effects due to the presence of multi-metal ions in the medium on oxidation of ferrous iron by A. ferrooxidans

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have been demonstrated by Das et al. (1997). Single metal ion studies show that among Fe3+, Cu2+ and Zn2+, Fe3+ is the most toxic whereas Zn2+ is the least. The influence of different concentrations of base metal ions such as Cu2+, Zn2+ and Fe3+, when present alone or in different possible binary and ternary combinations in a 9 K medium, was studied on the ferrous ion oxidation ability of A. ferrooxidans. The reports indicate that it is technically possible to induce a change from a chemical leaching process with high ionic strength to a bacterial leaching process, by dilution of the irrigation solution (Escobar and Lazo, 2003). Another frequent problem in the operation of heap bioleaching, according to the reports, is the high concentration of sulfate and other ions in the re-circulating irrigation solution on the heaps, after copper has been removed by solvent extraction. Additionally, the organic compounds used in copper removal might be introduced into the raffinate, generating adverse conditions to bacteria (Rusin et al., 1995). 5. Thermophilic leaching Chalcopyrite, the most important ore of copper, is not successfully bioleached with mesophiles due to passivation after partial extraction of copper for which Thermophilic bio-heap leaching may be a feasible low-cost low maintenance alternative, provided the necessary temperature conditions for thermophiles can be achieved and maintained within the heap. The connection between microbial activity and heat production in copper sulfide ore heaps and dumps (Olson et al., 2003) suggested the importance of thermophiles in bioheap leaching. Use of thermophiles was found to improve metal sulfide biooxidation in at least two ways (Brierley and Brierley, 1986); firstly, the reaction rates increased with increasing temperature and secondly there was increase in the quantum of metal extraction from certain minerals by the elevated temperature, and most notably this is the case with bioleaching of copper from chalcopyrite. At higher temperatures the chemistry of mineral solubilization is much faster and in the case of chalcopyrite, temperatures within 75–80 °C are required to achieve copper extraction at an economically viable rate. Dixon (2000) has shown with the aid of his heap-heat conservation model that it is possible to harness the heat of reaction in a sulphide heap to achieve optimal temperature distribution within. During the past few years, it has been detected that the moderately thermophilic microorganisms are capable of growing at temperatures of 45–50 °C attained in the center of heaps (Hanies et al., 1988). Since 1977, many such organisms have been detected and the genus Sulfobacillus is one of them. To these organisms must be added the extremely thermophilic bacteria capable of oxidizing sulphur, both autotrophically and heterotrophically, and which can grow at 60–80 °C (temperatures reached inside the heap in some industrial operations (Rossi, 1990) and Sulfolobus is one of these bacteria.

Thermophilic archea/bacteria, with an optimal growth temperature between 60 and 80 °C, have been the subject of several recent studies because of their capacity to solubilise a number of metallic sulphides. The results obtained demonstrate a greater efficiency than that obtained using the mesophilic microorganisms for similar processing times. Thermophilic archea/bacteria possess a series of characteristics that make them especially suitable for bioleaching. They increase the speed of dissolution of metal sulphides as a consequence of a higher reaction temperature and in addition, they are capable of dissolving ores such as molybdenite and chalcopyrite that are difficult to attack using mesophilic microorganisms. Thermophilic bio-heap leaching, on the other hand, may be a feasible low-cost low maintenance alternative, provided the necessary temperature conditions can be achieved and maintained within the heap. Bioleaching processes for demonstration plants operated within a temperature range of ambient to 80 °C have been achieved. As would be expected, the types of iron- and sulfur-oxidizing microbes present differ depending on the temperature range. The types of microbes found in the processes that operate from ambient to 40 °C tend to be similar irrespective of the mineral, as are those within the temperature ranges of 45–55 °C and 75–80 °C. Newmont mining company’s commercial biooxidation operation in Nevada has measured temperatures up to 81 °C in heaps containing 1.4–1.8% sulfide–sulfur (Tempel, 2003). Temperatures as high as 66 °C were noted in a 960,000 tons test heap of low grade run-off-mine chalcopyrite ore containing 4% pyrite at Kennecott Utah Copper (Ream and Schlitt, 1997). Moderate thermophiles are cultured from hot heaps (Brierley, 1997; Ream and Schlitt, 1997), but extremely thermophilic archea are much less commonly detected. However, large populations of extremely thermophilic archea can be maintained in heaps inoculated with these organisms (Tempel, 2003). 6. Difficulties in heap leaching processes Heaps and dumps present a number of advantages such as simple equipment and operation, low investment and operation costs, and acceptable yields. On the other hand, it must be realized that the operation suffers from some severe limitations: the piled material is heterogeneous and practically no close process control can be exerted, except for intermittent pH adjustment and the addition of some nutrients. Compared with tank reactors, heap reactors are more difficult to aerate efficiently and the undesirable formation of gradients of pH and nutrient levels as well as liquor channeling are difficult to manage. 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, 1989).

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7. Cost of bio heap leaching Copper is recovered from the bio heap leach liquor by solvent-extraction and electrowinning to produce highgrade copper cathode. There are four major processing components (Fig. 1):  A crushing plant reduces ore size.  A heap-leach operation dissolves copper by using the chemical and bacteriological agents.  A solvent extraction plant concentrates and purifies the dissolved copper in leach solutions.  An electrowinning plant produces high-grade, highquality cathode copper. Generally, the capital cost of a bioleaching operation is considerably less than that of a conventional smelting/ refining operation. Quoted operating costs (2002), based on the current technology for leaching dumps and in-place ore, and is between US $ 0.18 and US $ 0.22 per pound of cathode copper, which are competitive with the unit costs of smelting/refining. Heap leaching has the additional cost of mining and transporting to the leach pad. In some operations additional costs are incurred wherein the ROM is crushed and mixed with acid for agglomeration prior to being deposited in the heap. Because of this, direct production costs vary with the grade of the material being leached. Whether by bioleaching or conventional acid leaching, the operating costs for heap leaching for the currently operating or planned projects range between US $ 0.34 and US $ 0.60. (Dresher, 2004). 8. Some examples Copper metal is recovered in the largest quantity by means of heap reactors, although comparisons are difficult as data are presented in different ways. The bioleaching of copper sulfide ores in heaps is a technology widely developed in Chile, with more than 85,000 tons of ore processed per day (Brierley, 1999). Examples of large copper leaching operations are those by Sociedad Contractual Minera El Abra and the Codelco Division Radimiro Tomic in Chile producing 225,000 and 180,000 tons of Cu per annum, respectively. An excellent example of a current commercial bioleach application is the Quebrada Blanca operation in northern Chile (Brierley and Brierley, 2001) located on the Alti Plano at an elevation of 4400 m, in spite of the thinking that the leaching bacteria cannot function under the cold temperatures and low oxygen partial pressure of high altitudes. At Quebrada Blanca 17,300 t/day of sulfide ore are 100% crushed to 9 mm size, agglomerated with sulfuric acid and hot water, and stacked to form 6 to 6.5 m high heaps. Bacterial activity is facilitated by aeration using an array of airlines installed beneath the heap and low-pressure fans. Bacterial process monitoring includes the on-site measurements of respiration. The Quebrada Blanca bio-

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leach process illustrates the successful evolution of biohydrometallurgy in the mining industry. The plant design at Quebrada Blanca and other similar operations incorporates the bacterial requirements in the process. Research findings on improving bacterial activity are now applied in commercial operations. A year-long pilot trial at Copper Mines of Australia’s Mt. Lyell operation in Tasmania (Rhodes and Deeplaul, 1998) has demonstrated the technical and commercial viability of using moderately thermophilic bacteria to leach a finely ground concentrate and recover the solubilized copper with solvent extraction-electrowinning. BHP Billiton Ltd., has developed BioCOP Process, which is an agitated tank oxidation and leaching of copper sulfides. This process is proposed to be employed by BHP Billiton and Codelco, in a joint venture, Alliance Copper Ltd., in a demonstration plant at Chuquicamata, in northern Chile, proposed to produce 20,000 tons of cathode copper a year. Similarly, BacTech/Mintek Process is an agitated tank oxidation and leaching of copper sulfides developed by BacTech Enviromet. GeoBiotics, LLC has developed and patented several technologies for biooxidizing or bioleaching of sulfide ores and concentrates in an engineered heap environment. The two principal technologies are the GEOCOATÒ and GEOLEACH processes. Both technologies incorporate the patented Hot HeapTM control philosophy to ensure optimum biological performance. In the GEOCOATÒ process, sulfide flotation or gravity concentrate slurry is coated onto crushed and sized support rock which may be barren or may contain sulfide or oxide mineral values. The coated material is stacked on a lined pad for biooxidation. The process is applicable to the biooxidation of refractory sulfide gold concentrates and to the bioleaching of copper, nickel, cobalt, zinc, and polymetallic base metal concentrates. Mesophilic or thermophilic biological systems are used to catalyze the sulfide oxidation reactions. In the processing of chalcopyrite concentrates, the higher temperatures associated with the use of thermophilic microorganisms have proven highly beneficial in increasing the rate and extent of copper leaching. The GEOLEACH technology is applicable for the whole ore systems where the metals occur as sulfides. The driving force behind the GEOLEACH process is that most sulfide whole ore leaching systems have enough energy present in the sulfides to allow the heap to obtain very high temperatures, but poor heat management prevents significant temperature rise. Without significant increase in temperatures beyond ambient within the heap, sulfide-leaching kinetics are extremely slow and in the case of chalcopyrite, extraction is limited by passivation. The GEOLEACH technology is designed to maximize heat conservation through careful control of aeration and irrigation rates. Both the processes are simple, robust, and ideally suited to operation in remote locations. Nicico and Mintek are working in Iran, for treatment of Sarcheshmeh ore. This is taking place at the Sarcheshmeh copper complex in southern Iran, about 160 km southeast TM

TM

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of Kerman, which produces some 170,000 tones of copper a year. Mintek has indicated that it is possible to recover as much as 60% copper in the heap material. A test run for this was done in a six-meter column leach using Sarcheshmeh ore, which is predominantly chalcopyrite, but also contains chalcocite and other copper bearing minerals. Agreement includes treating highly refractory copper ores in which the copper occurs predominantly as the copper sulphide material, chalcopyrite. This type of copper ore at Sarcheshmeh accounts for 80% of copper reserves worldwide. 9. Conclusion: present and future of heap bioleaching of chalcopyrite The current scenario of bioleaching in developing countries is quite encouraging. Examination of the current large-scale bioleaching operations reveals that an important number of plants are located in developing countries. This is not purely accidental but is due to two important factors. Firstly, many developing countries have significant mineral reserves and mining constitutes one of the main sources of income and secondly, bioleaching technique, because of its simplicity and low capital cost, is suitable for developing countries (DaSilva, 1981; Gentina and Acevedo, 1985; Acevedo et al., 1993). This is the case for countries like Chile, Indonesia, Mexico, Peru and Zambia. These developing countries share over 50% of the world copper production. Developing countries should increase their efforts in research and development in bioleaching technology, as comparatively they have competitive advantages in this area. If not utilized, stacks of low-grade ore generated during mining, potentially endanger the environment, as the metals they contain may be released to the environment in hazardous forms due to weathering. This leads to acid mine drainage and pollution of drinking water sources like surface/underground/river water. The application of biohydrometallurgy to metal recovery is likely to grow because with suitable mineral deposit, it offers advantages of low capital and operating cost, operational simplicity, and shorter construction time that no other alternative process can provide. Increased concern regarding the effect of mining on the environment is likely to improve the competitive advantage in favor of microorganism based metal recovery processes (Rawlings, 2004). It is expected that in coming years several new commercial bioleaching plants will be installed. The use of thermophilic bacteria and archea will get more importance for increasing the leaching rates and metal recoveries and allowing for the treatment of recalcitrant chalcopyrite ores. International cooperation is also important in the establishment of new operations that can significantly contribute to the social and economic developments of developing countries. It is very likely that heap leaching will continue to be the choice for low-grade ores and tailings in future, and at the end it is concluded that the next few years would bring in exciting prospects in copper bioleaching industry for the treatment

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