The effect of redox control on the continuous bioleaching of chalcopyrite concentrate

A continuous bioleaching process was developed for the dissolution of chalcopyrite concentrate with electrochemically redox control. Therefore, using a flotation concentrate containing 46% chalcopyrite and 23% pyrite, bioleaching tests were carried out at 47 °C with 15% pulp density under controlled and uncontrolled redox conditions. To increase the copper recovery in contrast to the conventional bioleaching (∼39.62%), the effect of redox potential on the chalcopyrite bioleaching was investigated by electrochemically controlled bioleaching. The results showed that by controlling the redox potential, faster copper leach kinetics could be achieved. At last, reducing the redox potential from high levels to optimum window (420–440 mV SCE) caused an increase in copper recovery from around 39% to higher than 69% (over 25 g/L Cu²⁺).     

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.     

Formation of jarosite during Fe2+ oxidation by Acidithiobacillus ferrooxidans

Jarosite precipitation is a very important phenomenon that is observed in many bacterial cultures. In many applications involving Acidithiobacillus ferrooxidans, like coal desulphurization and bioleaching, it is crucial to minimize jarosite formation in order to increase efficiency. The formation of jarosite during the oxidation of ferrous iron by free suspended cells of A. ferrooxidans was studied. The process was studied as a function of time, pH and temperature. The main parameter affecting the jarosite formation was pH. Several experiments yielded results showing oxidation rates as high as 0.181–0.194 g/L h, with low jarosite precipitation of 0.0125–0.0209 g at conditions of pH 1.6–1.7 with an operating temperature of 35 °C.     

Possibilities for Co(III) dissolution from an oxidized ore through simultaneous bioleaching of pyrite

Solubilisation of Co(III) from a heterogenite met in copper cobaltiferous oxide ore has been realized through reductive leaching using ferrous iron generated via bio-oxidation of pyrite. Biotic and abiotic experiments at various pulp densities and redox potentials have been performed and results compared. Cobalt leaching at elevated redox potential is possible, offering cost reduction benefits due to reduced consumption of ferrous iron. At elevated potential of 625 mV, however, the initial rate of cobalt leaching has been found as 115 mg/(g ore)⋅(24 h), lower than the rate of 865 mg/(g ore)⋅(24 h) registered at 505 mV. Less stochiometric amount of ferrous iron was required when cobalt leaching was coupled to pyrite bioleaching, with 75% of cobalt recovered for 12 h at the optimally found conditions. It could be inferred that the Fe³⁺–Fe²⁺ cycle exists and is efficiently maintained through bacterial presence in the studied system.     

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.     

Iron solubilization during anaerobic growth of acidophilic microorganisms with a polymetallic sulfide ore

Acidithiobacillus ferrooxidans at 30 °C and Sulfobacillus thermosulfidooxidans at 47 °C were selected from a preliminary screening of various acidophiles for their ferric iron reduction capacities during anaerobic, autotrophic growth on sulfur. The selected cultures were used with a polymetallic sulfide ore under anoxic conditions to demonstrate enhanced solubilization of iron during leaching in shaken flasks and enhanced removal of iron from laboratory ore-leaching columns, compared to leaching with continuous aeration. Ore-associated, ferric iron-rich precipitates, which were formed under previously oxidizing conditions, were a potential influence on extraction of target metals and percolation through ore columns and were available as the source of ferric iron for anaerobic sulfur oxidation. Over twice as much iron was removed by moderate thermophiles when anoxic phases were introduced during the leaching. Enhanced removal of iron and some improvement in extraction of base metals from ore fragments were also demonstrated with a selected “Sulfolobus”-like strain during growth and leaching with alternating periods of aeration and anoxic conditions at 70 °C.     

Effects of pH,temperature and solids loading on microbial community structure during batch culture on a polymetallic ore

The bioleaching of an organic-rich polymetallic ore was conducted under conditions intended to probe the boundaries of microbial activity using iron and sulphur oxidising microorganisms and heterotrophs enriched from self-heating pyritic coal. Solution chemistry parameters such as rapidly increased ORP and reduction in pH subsequent to inoculation point to the development of active microbial communities. The ease with which communities adapted to the organic-rich ore and the bioleaching systems indicated that the organic compounds were not present in leachates at toxic levels. Overall, extractions obtained in three series of inoculated tests were at 35 °C: 79–96% Zn, 48–82% Cu, 47–55% Ni and 79–86% Co; at 55 °C: 96–97% Zn, 72–80% Cu, 46–50% Ni and 82–83% Co. T-RFLP provided semi-quantitative estimates of species abundance. The greatest microbial complexity was observed with moderate pH and low solids loading. Microbial complexity was reduced significantly by low pH or increased solids loading. Nevertheless, efficient bioleaching was observed over a relatively wide range of operating conditions. Even under the more extreme conditions, the community profile was dominated by combinations of organisms not typically seen in most commercial operations.     

A study of atmospheric acid leaching of a South African nickel laterite

Nickel and cobalt acid leaching from a low-grade South African saprolitic laterite using sulphuric acid was studied. Ore characterisation was performed by XRD and XRF. Batch agitation leaching tests were conducted at atmospheric pressure investigating main parameters: particle size and percent solids at 25 °C and 90 °C. Ore characterisation showed that the ore is a saprolitic laterite with nickel present in lizardite. Leaching tests showed that nickel and cobalt could be leached from the ore at atmospheric pressure. Nickel was found to be more leachable from the coarser −106 + 75 μm fraction, with 98% Ni being extracted at 90 °C after 480 min. Cobalt was not favoured by variation in particle size and increased percent solids. Increasing ore percent solids improved nickel extraction at 25 °C however at 90 °C extraction decreased due to a diffusion layer build-up as a result of amorphous colloidal silica. The co-dissolution of magnesium and iron was elucidated. Nickel leaching data at increased temperature and percent solids fit the shrinking core model equation, k_dt = 1−2/3x − (1 − x)^2/3 showing that nickel leaching reaction was diffusion controlled under the set conditions.     

Bioleaching of six nickel sulphide ores with differing mineralogies in stirred-tank reactors at 30 °C

A bioleaching study was conducted with six nickel sulphide ores from different geographical locations across Canada. Mineralogical and chemical examination revealed considerable variability between the samples, particularly in the silicate phases. The ores contain 0.3–1% nickel, primarily in pentlandite and secondarily in pyrrhotite. Copper is present primarily in chalcopyrite, and cobalt in pentlandite. The ores were subjected to the same crushing and grinding procedure, and bioleached under the same conditions for 3 weeks with a mixed culture of iron- and sulphur-oxidizing bacteria. Stirred-tank experiments with finely ground ore (−147 μm) at 30 °C were conducted to assess the effect of pH (2–5) and the impact of the bacteria. Nickel extraction from pentlandite and pyrrhotite during bioleaching at pH 2 and 3 was generally good (49–86% after 3 weeks), and cobalt extraction tracked nickel extraction over most conditions. All six ores showed a similar response to a change in pH; an increase in pH from 2 to 3 resulted in approximately the same nickel and cobalt extraction (within statistical error), and a statistically significant reduction in sulphuric acid consumption, dissolved iron, and magnesium extraction.     

Bioleaching of anilite using pure and mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus

Anilite oxidation was evaluated with two acidophilic thiobacilli that are important in bioleaching processes. The experiments were carried out in shake flasks in the absence and presence of energy sources such as 2 g/L powdered sulphur and 10 g/L Fe²⁺ (as ferrous sulphate) at pH 2.0, 150 rpm, 35 °C. Tests showed that copper extraction in a mixed culture of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus was higher than in pure cultures with added sulphur in the presence of anilite. The effect of supplemental iron clearly improved Cu leaching by the A. ferrooxidans culture and the mixed culture. The oxidation of anilite by A. caldus was negligible and this bacterium seemed to have no ability to initiate anilite solubilization. On the other hand, an important potential of A. caldus to leaching anilite was indicated. It can decrease pH of the medium and supply a suitable bioleaching environment.     

Cooperative effect of chalcopyrite and bornite interactions during bioleaching by mixed moderately thermophilic culture

The cooperative interactions between chalcopyrite and bornite during bioleaching by mixed moderately thermophilic culture were investigated mainly by bioleaching experiments and electrochemical experiments. Bioleaching results showed that a cooperative effect existed between chalcopyrite and bornite. When the mass ratio of chalcopyrite to bornite was 3:1, an extremely high copper extraction of more than 88% was achieved after bioleaching for 27 days. One of the major reasons for the cooperative effect was that a certain redox potential range (370–450 mV vs. Ag/AgCl) could be maintained for a long period of time during bioleaching due to the mixture of chalcopyrite and bornite. Electrochemical measurements revealed that chalcopyrite was much easier to be reduced than oxidized, while bornite was prone to be directly oxidized. Hence, galvanic effect between chalcopyrite and bornite enhanced the reduction of chalcopyrite to secondary copper-iron species and promoted the oxidative dissolution of bornite. Therefore, redox potential controlling and galvanic effect both contributed to the cooperative bioleaching of chalcopyrite and bornite.     

Acid leaching and rheological behaviour of a siliceous goethitic nickel laterite ore: Influence of particle size and temperature

This study investigates the isothermal, batch, H₂SO₄ acid leaching behaviour of siliceous goethitic (SG) nickel (Ni) laterite ore and its links to pulp rheology. Specifically, the effect of feed ore particle size (−0.2 vs −2.0 mm), leaching temperature (70 vs 95 °C) and pulp rheology on Ni and pay metal, cobalt (Co) extraction kinetics and yield was studied for 4 h on 40 wt.% solid dispersions at pH 1. The leaching behaviour was distinctly incongruent, reflecting the disproportionate proliferation of major gangue mineral’s constituent elements (e.g., Fe, Al, Mg, Na, Si) alongside Ni and Co in the pregnant leach solution. At 70 °C, Ni/Co extraction rates were notably lower (<20%) in contrast with 95 °C where a significant increase in Ni/Co extraction to 78/77% and 74/77%, respectively, for the −0.2 and −2.0 mm feeds occurred. The slurries displayed a non-Newtonian, shear thinning Bingham plastic rheological behaviour of which the viscosity and shear yield stress increased markedly in the course of 4 h leaching. The pulp viscosity and shear yield stress were greater at lower temperature than at higher temperature and they were also greater in slurries with finer than coarser feed particles. The dynamic pulp rheology, however, had no marked effect on the overall Ni/Co extraction rates. Whilst the feed ore particle size had no remarkable impact on overall Ni/Co extraction, it led to noticeably higher acid consumption and enhanced slurry rheology in the finer sized ore. The mechanism of leaching the SG ore followed a two-stage, first order chemical reaction-controlled shrinking core model, the kinetics of which gave higher rate constants and lower activation energies for the release of Ni, Co, Fe and Mg in the first stage. A faster leaching process involving more reactive minerals during the first 30 min is envisaged to be followed by leaching of the more refractory minerals.     

Effect of mechano-chemical activation on bioleaching of Indian Ocean nodules by a fungus

The effect of mechano-chemical activation of Indian sea nodules, while recording the zeta potential, particle size distribution and surface area, on the bio-dissolution of metals by Aspergillus niger has been investigated. Activation is a term used to indicate what takes place when increasing grinding time does not result in significant change in particle size but rather results in the accumulation of energy that may lead to the development of lattice defects within the particles that can aid biological attack. It was observed that the mechano-chemical activation improved the bio-dissolution of metals such as copper, nickel and cobalt from the sea nodules at initial pH in the range 4.0–5.0. With 10 min milling of particles of ?75 μm size, 86% material was reduced to ?10 μm size with a change in zeta potential from ?18 to ?34 mV. Above 95% copper, nickel and cobalt each was leached out in 15 days time from the nodules activated for 10 min at 5% (w/v) PD and 35 °C temperature with initial pH of 4.5; the biorecovery being almost similar when the material was activated for 30 min. In the case of nodules without activation, ?89% metal bioleaching was achieved in 25 days time at an initial pH of 4.5 under this condition. The mechano-chemical activation of sea nodules has thus influenced the bio-dissolution process, while providing a wider pH range available for processing of nodules with the involvement of organic acids such as oxalic and citric generated from the fungus.     

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.     

Review of rate constants for thiosulphate leaching of gold from ores,concentrates and flat surfaces: Effect of host minerals and pH

A review of literature data for different types of sulphide concentrates and gold ores has been carried out to examine the impact of host minerals and pH upon gold leaching. Analysis of initial rate data over the first 30–60 min of gold leaching from sulphide concentrates or silicate ores over a range of ammonia, thiosulphate, and copper(II) concentrations, pH (9–10.5) and temperatures up to 70 °C shows the applicability of a shrinking sphere kinetic model with an apparent rate constant of the order k_ss = 10⁻⁶–10⁻³ s⁻¹. The dependence of apparent rate constant on pH and initial concentrations of copper(II) and thiosulphate is used to determine a rate constant k_Au(ρr)⁻¹ of the order 1.0 × 10⁻⁴–7.4 × 10⁻⁴ s⁻¹ for the leaching of gold over the temperature range 25–50 °C (ρ = molar density of gold, r = particle radius). These values are in reasonable agreement with rate constants based on electrochemical and chemical dissolution of flat gold surfaces: k_Au = 1.7 × 10⁻⁴–4.2 × 10⁻⁴ mol m⁻² s⁻¹ over the temperature range 25–30 °C. The discrepancies reflect differences in surface roughness, particle size and the effect of host minerals.     

Changes in an Australian laterite ore in the process of heat treatment

This paper examines an Australian garnieritic-type ore and changes in phase composition and morphology caused by heating in argon at 400–1000 °C using XRF, XRD, DTA/TG, SEM/EDS and BET analyses. The mineral phases detected by XRD in the original ore include chlorite, talc, hematite and quartz. Traces of iron silicate, Fe–Cr spinel and monoxide phase (predominantly manganese oxide) were observed by EDS. Nickel was detected in chlorite, talc, iron silicate and monoxide phase. Heat treatment at 400–500 °C did not change XRD patterns. At 600 °C, dehydroxylation of the brucitic phase of chlorite occurred. Chlorite was converted into olivine (forsterite) and enstatite at 600–800 °C. Upon heating to 900–1000 °C, talc was also converted into olivine and enstatite. Ni-bearing phases after heat treatment at 800–850 °C were forsterite, enstatite, talc, iron silicate and monoxide.     

Bioleaching of djurleite using Acidithiobacillus ferrooxidans

The bioleaching of djurleite using Acidithiobacillus ferrooxidans (LD-1) was investigated in this paper. Experiments were carried out in shake flasks at pH 2.0, 160 r/min and 30 °C. The leaching residues were analyzed using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The total copper extraction of djurleite under optimal condition reached 95.12%. The XRD analysis indicated the residues mainly consisted of ammoniojarosites and S₈. It was observed by the SEM image that the djurleite was heavily etched. The XPS results confirmed the intermediate product formed during djurleite leaching was CuS. The result indicates the reaction pathway is: Cu₃₁S₁₆ → CuS → tCu²⁺ and S⁰.     

Statistical analysis of bioleaching copper,cobalt and nickel from polymetalic concentrate originating from Kamoya deposit in the Democratic Republic of Congo

The effects of five parameters, temperature, pH, leaching duration, stirring speed and pulp density on the bioleaching of copper, cobalt and nickel from a polymetallic flotation concentrate were investigated. The leaching was carried out according to the L₂₅ (5⁵) orthogonal design. The optimal values of the parameters were determined using a Taguchi method through signal-to-noise analysis. ANOVA was applied to verify the individual contribution of each parameter and their degree of significance. It was found out that pulp density was the most influential factor on the bioleaching yield of the three metals altogether, followed by pH and temperature. For the copper bioleach, the following optimal parameters were determined: temperature – 37.5 °C, pH 1.6, leaching duration – 20 days, stirring speed – 350 rpm and pulp density – 7.5%. Verification experiments conducted according to these optimal parameters brought copper yield of 72.6%. For the cobalt bioleach, SEM observations of pure carrolite indicated a progressive bacterial colonization of mineral surface with time.     

Effect of physico-chemical and operating conditions on the growth and activity of Acidithiobacillus ferrooxidans in a simulated heap bioleaching environment

Recent understanding of microbial retention within heap bioleaching systems has highlighted the importance of quantifying microbial growth and activity in both the bulk flowing solution and in the ore-associated phases. Typically, industrial heap bioleaching operations report variations in process conditions such as inoculum preparation and concentration and elevated copper concentrations in the recycled irrigation solution. In this paper, a mini-column reactor system containing pre-constructed and agglomerated, low-grade ore samples representing grab samples from a larger heap, were used to investigate the effect of a selection of physico-chemical and operating conditions on microbial growth, colonisation and substrate utilisation kinetics, considering both the planktonic and sessile populations of Acidithiobacillus ferrooxidans. The factors studied included inoculum size, inoculum cultivation conditions, availability of ferrous iron in the bulk flowing solution and copper concentration in the bulk flowing solution. The microbial population in the interstitial phase, i.e. associated with, but not bound to, the ore, remained the most abundant within the heap under all physico-chemical conditions considered. A comparison of the tests with different inoculum sizes found that a smaller inoculum size resulted in an increased delay in microbial growth and ferrous iron oxidation, but similar apparent maximum specific growth rates and iron oxidation rates. In contrast to the microbial culture grown on pyrite, a delay in microbial activity was observed for the culture grown on ferrous iron. However, greater microbial cell densities were reached, in the interstitial and attached phases compared with the pyrite-grown culture. The introduction of 6 g L⁻¹ cupric ions into the feed solution containing 0.2 g L⁻¹ ferric iron resulted in decreased microbial growth rate in the interstitial phase but not in the attached phase. Where the pyrite culture was pre-exposed to cupric ion, the microbial growth rate in the interstitial and attached phases was significantly enhanced. Nevertheless, the presence of cupric ion in the irrigation solution resulted in a decrease in microbial ferrous iron oxidation rate, irrespective of pre-culture conditioning. This study emphasises the important role played by the stagnant interstitial phase during the colonisation of a low-grade heap, particularly under adverse conditions for microbial growth and activity. It also highlights the role of inoculum culture conditions on the potential trade-off between increased heap colonisation and increased lag periods in microbial activity during heap start-up.