Some aspects of the effect of pH and acid stress in heap bioleaching

The chemical and physical conditions in sulphide heaps provide a complex environment for micro-organisms, with differences in redox potential, acidity, temperature, oxygen and solution chemistry conditions being experienced both temporally and spatially. One of the most important parameters for successful microbial colonisation and active microbial metabolism is suitable pH conditions in the heap. Typically heaps reach tens of metres high and the pH of irrigation solution travelling through heap changes significantly.In this study, we investigated the effect of pH and acid stress for moderately thermophilic and thermophilic mixed cultures, operating at 50-60 °C in a heap bioleaching environment. Results collected from laboratory scale column reactors packed with the low grade whole ore and irrigated with different pH solutions during a temperature shift from moderately thermophilic conditions to thermophilic conditions are discussed.     

Dynamic microbial populations in heap leaching of zinc sulphide ore

Polymerase chain reaction (PCR) technology was used to monitor the microbial population within two 6-m-high sulphide ore heaps where zinc was bioleached. Genomic DNA was isolated from microorganisms attached to the ore particles. The microbial population varied in diversity at different heap heights and at different stages of the leaching cycle. Nine bioleaching microorganisms were identified, including iron oxidizers, sulphur oxidizers, whether mesophiles, or moderate thermophiles. No bioleaching extreme thermophiles were detected. Moderate thermophiles were present at average heap temperatures of 40 °C or higher. No moderate thermophiles were present in the top 2 m of the heaps. Mesophiles were present throughout the lifetime of the heaps and at all heap depths. Cultures of Acidithiobacilluscaldus and Sulfobacillusthermosulfidooxidans originally introduced to inoculate the heaps did not proliferate but were instead displaced by other species; no benefits were obtained from heap inoculation. The most predominant species were Acidithiobacillus thiooxidans and Acidithiobacillus albertensis. Leptospirillum sp. was not predominant in the heaps, in contrast to the dominant role this organism plays in tank bioleaching operations.     

Determining the effect of acid stress on the persistence and growth of thermophilic microbial species after mesophilic colonisation of low grade ore in a heap leach environment

The microorganisms involved in the bioleaching of sulphidic mineral ores are acidophilic. Generally, a pH in the range of pH 1–2.5 is applied for optimal growth in these systems. In operating heaps, perturbation of conditions could result in changes in the pH outside this “safe” window, so an understanding of the effect of changes in pH on growth and activity of bioleaching microbes is needed. Previous work has shown that some microorganisms e.g. Acidithiobacillus thiooxidans, Leptospirillum ferriphilum and Leptospirillum ferrooxidans are able to adapt to low pH environments (∼pH 0.9). However, most studies on the response of micro-organisms implicated in mineral bioleaching to pH have been conducted under submerged, aerated culture conditions, with limited performance-based studies conducted under conditions mimicking a heap environment. In this study, the effect of acid stress on the persistence of the thermophilic micro-organisms in the ore bed inoculated at mesophilic conditions and their subsequent growth on reaching thermophilic conditions is considered.Following inoculation, five columns loaded with a low grade chalcopyrite ore were irrigated at a feed pH of 1.7 at 25 °C. After a few days, the temperature was sequentially increased from 25 °C through 37 °C to 50 °C, resulting in an Eh above 850 mV across all columns. The irrigation feed pH was then varied across the range pH 1.0 to 1.7 at 50 °C. Eh values greater than 800 mV could be attained in the columns with feed pH values between pH 1.2 and pH 1.7 at 50 °C. The Eh of the column receiving feed solution at a pH of 1.0 at 50 °C dropped to below 700 mV and did not recover. The temperature was then increased gradually to 60 °C. All the columns with feed pH of 1.2 and higher achieved Eh values above 800 mV. Quantitative analyses of the microbial community on selected PLS and ore samples indicated that lower pH affected the persistence of the thermophilic micro-organisms in the ore bed and their subsequent growth on reaching thermophilic conditions. The microbial population detached from the ore sample after 120 days decreased by a factor of 5–15 and 25–100 fold on decreasing the operating pH from 1.5–1.7 to 1.4 and 1.2 respectively. Poor microbial activity was found at pH 1.0, suggesting ineffective growth or persistence of the archaea.     

Biosolubilization of phosphorus from rock phosphate by moderately thermophilic and mesophilic bacteria

Biosolubilization of phosphorus from rock phosphate (RP) by a moderately thermophilic bacterium, Acidithiobacillus caldus and a mesophilic bacterium, Acidithiobacillus thiooxidans were carried out in shake flasks containing elemental sulfur (S⁰) as an energy substrate. The phosphate solubilizing ability of A. caldus was more effective than A. thiooxidans when carried out under pH 2.5, 160 rpm, 45 °C and 30 °C for the moderate thermophile and the mesophile, respectively. Moreover, the phosphorus solubilizing rate from RP by A. caldus was influenced markedly by initial pH, temperature, RP concentration and dosage of S⁰ addition.     

硫化矿高温生物浸出工艺研究和应用进展

高温菌浸矿不仅比中温菌有更快的反应速率,而且还能有效浸出部分难溶原生硫化矿如黄铜矿等。因此,高温生物冶金工艺在湿法冶金中占据越来越重要的位置。对国内外高温菌浸矿的研究进展进行了综述,详细介绍了高温生物冶金工艺在提取金、铜、锌3种金属方面的应用,最后就如何发展和推广高温生物浸矿工艺提出了自己的一些看法。     

In situ investigation and visualisation of microbial attachment and colonisation in a heap bioleach environment: The novel biofilm reactor

In this paper, the development of a novel means of investigating the attachment and subsequent biofilm formation of mineral bioleaching micro-organisms to mineral surfaces in situ is described. The protocol was developed to investigate the interactions of micro-organisms with sulfide minerals and low-grade chalcopyrite ore under conditions resemblant of a bioheap environment. The method makes use of a biofilm reactor in which thin sections of mineral ore are mounted. The reactor is operated as a continuous flow-through system. Attachment of pure and mixed cultures of Acidithiobacillus ferrooxidans and Leptospirillum ferriphilum is assessed. The technique allows for the investigation of microbial ecology with special regard to microbe–mineral attachment, site and mineral specific associations of micro-organisms and spatial organisation of microbial communities present through the use of fluorescent microscopy techniques. Preliminary fluorescent in situ hybridisation (FISH) analysis of the attachment of L. ferriphilum and A. ferrooxidans to massive chalcopyrite sections, as well as to low-grade chalcopyrite containing ore sections is presented. In the case of both low-grade and massive sulfide mineral samples, attachment of mixed micro-colonies was observed in regions where surface defects were prevalent. In low-grade samples, preferential attachment was observed in regions where sulfide minerals were present. The density of the attached micro-colonies increased with an increase in contacting time (from 20, 72 and 96 h) and was indicative of an actively growing mono-layered biofilm.     

Salt-tolerant microorganisms potentially useful for bioleaching operations where fresh water is scarce

Anions and cations can accumulate in process waters due to the source water, to evaporation and because of gangue mineral dissolution. Common salts that can accumulate from mineral gangues are the anions, chloride and sulfate, both of which impact on microorganisms used in bioleaching processes. The search for salt-tolerant acidophilic microorganisms able to tolerate high concentrations of these salts, as well as high concentrations of metals, has been underway for at least 20 years because their application would considerably improve bioleaching process efficiency in areas where fresh water is scarce.A thorough search of microorganisms from saline and acidic drains, lakes and sediments in the South West of Western Australia and a CSIRO culture collection was undertaken to bio-prospect for salt-tolerant bioleaching cultures. Pure strains of common bioleaching acidophiles did not tolerate seawater salinities, however, enrichment cultures of mesophilic acidophilic microorganisms that could tolerate up to 70 g/L sea salts and 350 g/L MgSO₄⋅7H₂O were established. The salt tolerance of acidophiles was less in thermophilic temperature ranges, compared with mesophilic and moderately thermophilic temperature ranges. Tolerance to sulfate salts was greater than chloride with magnesium ions likely limiting maximum sulfate tolerance. Iron oxidising cultures were more sensitive than sulfur oxidising cultures to higher chloride concentrations. The addition of pyrite to enrichment cultures increased salt tolerance. The efficacy of the salt tolerant cultures to extract copper will be determined in bioleaching experiments with chalcopyrite ore and salty process water.     

Some quantitative data on bacterial attachment to pyrite

Bacterial attachment and biofilm formation are important for microbial bioleaching of metal sulfides, however, many details of the role played by bacteria are still unknown. Attachment as the first step in biofilm formation is critical. Our investigations are focused on these processes to control bacterial bioleaching processes.In moderately thermophilic pure cultures of Acidithiobacilluscaldus and Leptospirillumferriphilum, only the latter one showed significant attachment and biofilm formation to metal sulfides. In pure cultures of the mesophilic strains Acidithiobacillusspp., Leptospirillumferrooxidans, Ferrimicrobiumacidiphilum and a novel undescribed leaching organism designated SPIII/3, attachment was significant by L. ferrooxidans, At. thiooxidans and SPIII/3.In general, interaction of different species in mixed cultures resulted in increased attachment, increased production of extracellular polymeric substances (EPS) and leaching. Bacteria like Leptospirillumspp. enhanced the attachment of other species in mixed biofilms. Another finding is that large areas of the surface of minerals remain uncolonized, whereas at some places bacteria attach in clusters.Based on these findings we conclude that attachment and thereby bacterial leaching can be improved by the production of EPS by several strains especially in combination with Leptospirillumspp.     

The generation of toxic reactive oxygen species (ROS) from mechanically activated sulphide concentrates and its effect on thermophilic bioleaching

Two types of laboratory mills, planetary and vibratory, were used to activate sulphide mineral concentrates mechanically before thermophilic (bio)leaching. These samples were analysed in terms of particle size, surface area, density, SEM, XRD line profile analysis and reactivity. The product particle size distributions indicated different particle breakage mechanisms of the two mills. The surface area for pyrite milled with the planetary mill was three fold that milled in the vibratory mill for the same length of time. Planetary milled samples showed lower densities, up to 4% less for pyrite samples, compared to vibratory milled samples. Particle surface oxidation, observed by SEM, occurred post milling. Surface oxidation products were more prevalent with planetary milled sulphide samples. XRD line profile analysis showed more line broadening effects with the planetary mill. This indicated that more bulk particle-related structural defects were present in the planetary milled samples. The reactivity in acidic solution was measured in terms of the generation of toxic reactive oxygen species (ROS): hydrogen peroxide and hydroxyl radicals. The ROS generation from milled sulphides, normalised to constant surface area loading, increased with increased mechanical activation. The planetary milled samples generated greater ROS per sample surface area than vibratory milled samples, more than 4-fold for pyrite after 60 min of milling. Increased ROS generation was postulated to result from increased surface area defects, solubilisation of iron oxidation products and bulk particle-related defects.The effect of mechanical activation on performance on thermophilic leaching and bioleaching tests was investigated using milled samples at 2% (w/v) pulp density. Short mill times improved leach rates from both mills, up to 7-fold cf. unactivated feed leach rates. Poor bioleaching performance resulted following long periods of mechanical activation (20-60 min). Pyrite and chalcopyrite bioleaching performance decreased dramatically above surface area loadings of 25 and 125 m²/L respectively. Planetary milled samples were less amenable to bioleaching. For pyrite milled for 20 and 60 min and chalcopyrite milled for 40 min, no viable cells were observed following inoculation via fluorescence microscopy, suggesting culture death supported by compromised ferrous iron oxidation. The generation of ROS was postulated to cause poor bioleaching performance under these conditions.     

Bioleaching strategies for the treatment of nickel-copper sulphide concentrates

Different process options for the bioleaching of nickel-copper containing sulphide concentrates were evaluated. Tests in continuously operated reactor systems showed >98% nickel extractions over a range of temperatures, whereas for copper occurring as chalcopyrite, extractions of >95% could only be achieved at 70 °C. During further optimisation of the process the effects of process parameters such as grind size, residence time, redox potential and feed solids concentration on the leach kinetics, metal extractions and performance of both moderate thermophile and thermophile cultures were addressed. Redox control in the first-stage reactor of thermophilic systems resulted in considerably faster copper leach rates and extractions of up to 95% could be achieved at increased feed concentrations and coarser grind sizes. High redox levels in the secondary reactors ensured >98% nickel recoveries. These operating conditions and control strategies have the potential to increase the rates of nickel and copper extractions to the extent that leaching times could be considerably reduced.     

Synchrotron X-ray photoelectron spectroscopic study of the chalcopyrite leached by moderate thermophiles and mesophiles

SXPS (Synchrotron X-ray Photoelectron Spectroscopy) and NEXAFS (Near Edge X-ray Absorption Fine Structures) have been applied to study the surface chemical species of chalcopyrite leached by a moderate thermophilic consortia, Leptospirillum ferrooxidans and a mesophilic mixed culture of L. ferrooxidans, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. A sulfur-rich layer dominated by S_n²⁻ developed with time, which was found to control the rate of bioleaching. Fe L_2,3-edge NEXAFS and Fe 2p spectra indicate the formation of jarosite during bioleaching. Thermophiles significantly enhanced the leaching efficiency, in which 1.34 g/L copper was dissolved in 25 days, while less than 0.3 g/L copper was released in 30 °C bioleaching. This was mostly caused by the increased abiotic reaction rate. The solution copper concentration in presence of L. ferrooxidans was higher than that with mesophilic mixed culture, which suggests the synergistic effect of mixed microorganisms did not play a comparably important role as temperature under the conditions used in this study. Explicit evidence of elemental sulfur was only found in samples leached by L. ferrooxidans by Raman spectroscopy. However, the formation of elemental sulfur does not significantly hinder the leach rate.     

Composition and properties of bio-reagent to intensify leaching of nonferrous metals from sulfide ores and concentrates

New information is obtained on composition and properties of a bio-reagent–oxidizer generated by mesophilic aerobic chemo-tropholytic bacteria Acidithiobaccilus ferrooxidans under oxidation of iron (II) ions in sulfuric acid solution. The composition and properties of the bio-reagent are compared with iron (III) sulfate used to intensify agitation and heap leaching of metals from sulfide ores and concentrates. The research with IR spectroscopy, mass spectrometry, Moessbauer spectrometry and potentiometry has revealed distinctive features of the bio-reagent and explained the experimentally observed increase in its oxidative activity when interacting with minerals.     

Bio hydrometallurgical recovery of metals from Fine Shredder Residues

The leaching step of an integrated hydrometallurgical process for the selective recovery of metals from polymetallic concentrates has been investigated. This concentrate has been produced by physical treatment of Fine Shredder Residues derived from a shredding plant processing a mixed feed of metallic scraps, waste electric equipments and end-of-life vehicles. Bacterially assisted leaching experiments have been carried out using a copper-adapted consortium of mesophilic bacterial strains. The influence of various operating conditions such as stirring speed, temperature (25-50 °C), pulp solids density (5-20%) and initial iron concentration (0-15 g/L) has been studied. Temperature and stirring speed have proved to be the most influential parameter regarding copper dissolution kinetics, while pulp solids density and initial iron concentration have been found to have a subordinate importance. In optimum conditions, 95% extractions of zinc and copper were achieved within 48 h. Bacterial presence has been found beneficial in terms of catalysing copper dissolution.     

Bio-dissolution of zinc sulfide concentrate in 160 l 4-stage continuous bioreactor

This paper reports the bioleaching of zinc sulfide concentrates by mesophilic chemolithotrophic bacteria. The leaching studies were carried out in 160 l 4-stage bioreactor in optimized conditions based on 40 l batch scale studies. Extraction of zinc up to 83.4% could be obtained in continuous operation.     

Solid state reduction of Panzhihua titanomagnetite concentrates with pulverized coal

Titanomagnetite concentrates and pre-oxidized titanomagnetite concentrates were reduced isothermally at 1000-1200 °C by pulverized coal. XRD, SEM and EDS were used to characterize the samples. The influence of reduction conditions on the metallization degree of the samples, such as reduction temperature, C/Fe molar ratio, pre-oxidation and additives were studied. It was found that the pre-oxidation and additives played a considerable role in the metallization degree of titanomagnetite concentrates. During the pre-oxidation of titanomagnetite concentrates in air, the magnetite was oxidized to hematite and ilmenite was oxidized to hematite-ilmenite solid solution, hematite and TiO₂. The pre-oxidation destroyed the crystal lattice of raw titanomagnetite concentrates and formed pores in the particles, which accelerated the reduction process. The effect of different additives on the metallization degree of pre-oxidized titanomagnetite concentrates was also discussed. The results show that when 1.5 wt.% Na₂CO₃ is used as an additive, the metallization degree of pre-oxidized titanomagnetite concentrates reaches 96.4%, after heating for 1 h at 1200 °C. The final reduced sample was composed of metallic iron, pseudobrookite solid solution and TiO₂.     

The effect of potential heap construction methods on column bioleaching of copper flotation tailings containing high levels of fines by mixed cultures

It is known that excess fines may reduce heap permeability and block channelings of leachate flow in heap bioleaching operation, and further cause low metal recovery. The purpose of this investigation was to compare the effects of three potential heap construction methods including layered heap construction method (Method A), agglomerate heap construction method (Method B) and pelletized sintering heap construction method (Method C) of copper flotation tailings on column bioleaching behaviors. In the three heap construction methods, the tailings copper extractions achieved 54.61%, 60.09% and 43.93%, respectively, in Method A, B and C on day 83. Copper extraction of Method B reached maximum. In addition, compositions and structures of microbial communities were examined using Illumina Miseq sequencing technology based on 16S rRNA amplification. Acidithiobacillus, Leptospirillum and Ferroplasma were main microorganisms in three heap construction methods. Detrended correspondence analysis showed that Method C had little effect on microbial communities. These studies revealed the influence of different heap construction methods on leaching behaviors and microbial dynamics, which will facilitate the bioleaching of fine-grained ores.     

Reductive dissolution of ferric iron minerals: A new approach for bio-processing nickel laterites

Nickel laterites represent the major ore reserves of this base metal present in the lithosphere. However, processing these ores by conventional technologies involves considerable energy or reagent expenditure and consequently is less cost-effective than extracting nickel from sulfide ores. Biological options, using metal-complexing organic acids and mineral acids generated by fungi and bacteria, have been investigated but generally found to be ineffective in terms of extraction dynamics or yields. We have examined the possibility of using bacteria that can bring about the reductive dissolution of ferric iron minerals and thereby facilitate the extraction of nickel from a lateritic ore at relatively low (<30-45 °C) temperatures.Four species of iron-reducing acidophilic bacteria were screened for their abilities to solubilise nickel from a limonitic laterite ore in which the major iron mineral present was goethite. One of these (Acidithiobacillus ferrooxidans) was selected for further study only the basis of it being able to use a cost-effective energy source (elemental sulfur) to mediate the dissolution of goethite at mildly acidic conditions (pH < 2). Cultures were set up in 2 L bioreactors, maintained at pH 1.8 (±0.1) and 30 °C, and initially aerated (to promote growth of the bacteria on sulfur) and then switched to anaerobic conditions when nickel laterite ore (crushed to <6 mm, with a nickel grade of 0.5%) was added. Over 70% of the nickel present in the ore was solubilised within 14 days, and solubilised metals remained in solution due to the low pH of the leachate. In contrast, only 10% of the nickel was solubilised (by non-reductive acid dissolution) when the cultures were continuously aerated. The results suggest that biological processing of limonitic nickel laterites is technically feasible and, more generically, that reductive dissolution can be used to bioprocess ferric oxide mineral ores.     

Column bioleaching of a polymetallic ore: Effects of pH and temperature on metal extraction and microbial community structure

A complex, polymetallic ore with approximately 6 wt% organic carbon and 3 wt% inorganic carbon (calcite) was bioleached using mixed cultures of mesophilic and moderately thermophilic microorganisms enriched from materials obtained from an auto-heating coal mine. The microorganisms adapted readily to the ore despite the organic carbon content. Two characteristics of the pyrite-rich ore, a porous organic-sericite phase that allowed the lixiviant and oxidant to penetrate particles and contact sulfide grains and a quartz-feldspar phase that partly occluded the acid-consuming carbonate phase, facilitated rapid and efficient metal (Co, Ni, Cu and Zn) extraction. Metal extractions showed little temperature or acid dependence. Extractions of Co, Ni, Cu and Zn in inoculated tests were higher than in uninoculated tests under the same conditions, indicating at least partial association of the metals with sulfide minerals and highlighting the roles of microorganisms in enhancing bioleaching. Approximately 40–60 wt% of the metals were acid soluble. The oxidation of pyrite with concomitant acid generation was a key parameter in lessening acid consumption during leaching. On the basis of solution monitoring data and leached residue analyses of the carbonate contents and sulfur speciation, it is concluded that leachate free acidity is also an important parameter in metal extraction. Both parameters relate directly to acid use in bioleaching systems and impact on the economics of proposed processes. The microbial communities in the inocula and bioleaching samples contained three archaeal and five bacterial species. The archaeal species detected were related to Ferroplasma (Fp.) acidiphilum, Thermogymnomonas (T.) acidicola and Metallosphaera (M.) hakonensis. The bacterial species detected were related to Leptospirillum (L.) ferriphilum, Acidithiobacillus (At.) thiooxidans, Ferrimicrobium (Fm.) acidiphilum, At. caldus and Acidimicrobium (Am.) ferrooxidans. The Ferrimicrobium acidiphilum-related species was the only species present in the 35 °C and possibly the 50 °C columns thought to be native to the ore.     

Bioleaching of gold and copper from waste mobile phone PCBs by using a cyanogenic bacterium

Chromobacterium violaceum (C. violaceum), a cyanide generating bacterium has been used to leach out gold and copper from the waste mobile phone printed circuit boards (PCBs) containing ∼34.5% Cu and 0.025% Au in YP (yeast extract and polypeptone with glycine) medium. The bioleaching was carried out in an incubator shaker (150 rpm) at 30 °C and 15 g/L pulp density in the pH range 8-11. Dissolution of gold and copper increased from 7.78% (0.225 ppm) to 10.8% (0.46 ppm) and 4.9% (419 ppm) to 11.4% (879 ppm) in 8 days with increase in pH from 8 to 11 and 8 to 10 respectively. Supplementing oxygen with 0.004% (v/v) H₂O₂ increased the copper leaching to 24.6% (1743 ppm) at pH 10 in 8 days whereas improvement in gold leaching was insignificant with the recovery of 11.31% Au at pH 11.0. The waste PCBs can thus be recycled in environmental friendly manner.     

Effect of pulp density on planktonic and attached community dynamics during bioleaching of chalcopyrite by a moderately thermophilic microbial culture under uncontrolled conditions

An enriched and adapted moderately thermophilic culture showed good performance during bioleaching of chalcopyrite under uncontrolled conditions. The copper extractions were up to 85.0%, 77.3% and 56.3% at pulp densities of 10%, 15% and 20% (w/v) within 22 days, respectively. The real-time quantitative PCR was employed to monitor planktonic and attached community dynamics during bioleaching of chalcopyrite by the moderately thermophilic microbial culture. Only three species, including Acidithiobacillus caldus, Sulfobacillus acidophilus and Ferroplasma thermophilum, were detected during the whole bioleaching process. The results show that pulp density had significant effects on planktonic and attached microbial community dynamics. The succession of attached cells was different from community dynamics of their planktonic counterparts. F. thermophilum and A. caldus preferred to attach to mineral surface, especially at pulp densities of 15% and 20%.