H.R. Watling

The role of iron-hydroxy precipitates in the passivation of chalcopyrite during bioleaching

Abstract The bioleaching of chalcopyrite in an acidic sulphate nutrient medium was investigated using Sulfobacillus thermosulfidooxidans, a moderately thermophilic iron- and sulphur oxidising bacterium. Copper release to solution was initially rapid but this slowed significantly after about SO hours. The decrease in chalcopyrite dissolution rate coincided with significant precipitation of jarosite on the mineral surface. Cultures of the moderately thermophilic acidophilic bacteria Acidimicrobium ferrooxidans, Sulfobacillus acidophilus and Sulfobacillus thermosulfidooxidans were grown in anaerobic media containing chalcopyrite passivated by jarosite. The moderate thermophiles used the ferric ion in the jarositic surface precipitate as a terminal electron acceptor in place of oxygen in the anoxic environment. Despite extensive bioreduction of the iron-hydroxy precipitates, it was found that the jarosite was not completely removed and that subsequent biooxidation of the treated concentrate achieved no significant increases in copper release compared with concentrate that had not been subjected to prior biooxidation or bioreduction.

Sulphur speciation of leached chalcopyrite surfaces as determined by X-ray photoelectron spectroscopy

Abstract A key factor in improving the bioleaching route for chalcopyrite processing is a better understanding of the surface speciation that exists under chemical leaching conditions that mimic the acid bioleach. The surface sulphur speciation of chalcopyrite under such conditions has been revisited using X-ray photoelectron spectroscopy (XPS). Objectives of the study were to resolve the issue of possible passivation candidates and to understand the relative roles of ferric and ferrous ions in the oxidative leaching process. Neither severely metal-deficient sulphides nor polysulphides were found to be major surface layer components during initial leaching. The primary surface species produced with an acid ferric leach is elemental sulphur. This is largely volatilisable and coats the underlying unleached sulphide mineral but not adjacent minor sulphate domains. It is important to distinguish between the loss of multilayer elemental sulphur and the loss of monolayer or submonolayer quantities of the same species, especially as it impacts on polysulphide identification. The second major leach product on the chalcopyrite surface is disulphide S 2 2− . Although the cation association of the S 2 2− is not known, evidence from the Cu 2p spectra discounts the formation of any CuS 2 type species. Both acidic ferric and ferrous leaches produced the elemental sulphur and disulphide surface, though the more aggressive ferric produced a greater quantity of elemental sulphur. Evidence for polysulphides with a chain length greater than 2 remains an open question. There is some evidence that such polysulphides might form with acidic ferrous leaching, but the prime candidate for any initial leaching inhibition (prior to jarosite formation) is elemental sulphur.

An X-ray photoelectron spectroscopy study of the mechanism of oxidative dissolution of chalcopyrite

Abstract An alternative mechanism based on the application of simple chemical principles involving hydrated (hence realistic) solution species is proposed for the oxidative acid leaching of copper and iron from chalcopyrite. The consistent surface speciation for abiotic and microbial, aerobic and anaerobic conditions fits with the need for a common mechanism. X-ray photoelectron spectroscopy detected the expected sulphur and sulphate surface species, plus a disulphide phase. The sulphate species was found to be a basic ferric sulphate akin to jarosite. A key conclusion is that chalcopyrite dissolves by oxidation of the disulphide phase that forms rapidly on freshly fractured chalcopyrite surfaces and also persists on leached chalcopyrite surfaces. The oxidation of the disulphide phase is thought to occur by electron transfer to ferric ions, via surface protonation and hydration, with the likely consequent production of thiosulphate. The thiosulphate being further oxidized to sulphate producing a basic ferric sulphate phase which acts as a nucleation template for jarosite formation that ultimately leads to chalcopyrite “passivation”. The copper ions thus take no part in the oxidation mechanism and are simply solubilised as the chalcopyrite surface is destroyed. Neither thiosulphate nor polysulphide species were directly detectable as surface intermediates. Synthesis and XPS examination of model polysulphides indicates that they do not play any role in inhibiting chalcopyrite dissolution. For the microbes examined they appeared to play no active role in the oxidation process at the solid solution interface. The same surface speciation for abiotic conditions suggests the microbial role is one of a solution oxidant of the ferrous ions produced. An examination of silver catalysis indicates an entirely different mechanistic pathway.

The role of iron-oxidizing bacteria in stimulation or inhibition of chalcopyrite bioleaching

A series of bacterial and chemical leaching experiments were conducted to clarify contradictory reports in the literature regarding the role of bacteria in the bioleaching of chalcopyrite. Tests containing a high bacterial concentration showed inhibited leaching, even lower than non-inoculated controls. However, when bacterial cells were washed before inoculation, it was apparent that it was not the bacterial cells but rather the chemical species introduced with them that influenced the leaching rate. In addition, the results of comparative tests with 0.1 M ferrous sulphate or ferric sulphate showed that copper was leached from the ore 2.7 times faster in leach solutions containing ferrous ion, suggesting that ferric ions inhibit chalcopyrite dissolution. The results indicated that the chalcopyrite dissolution rate is strongly dependent on the reduction potential (Eh) in solution, and that this parameter is far more influential than the number or activity of bacterial cells. These results imply that the role of bacteria may only be stimulatory when the prevailing electrochemical conditions are also favourable.

Enrichment and characterisation of thermophilic acidophiles for the bioleaching of mineral sulphides

Abstract Thermophilic acidophilic Archaea were enriched from samples collected from geothermally active sites in Papua New Guinea. Pure cultures (JP2 and JP3) were obtained from mixed culture enrichments and were characterised and tested for their bioleaching ability. All cultures possessed Sulfolobus -like morphology, and the presence of distinctive cyclized tetraether lipids. The two pure cultures were identified by their 16S rRNA gene sequences as being most closely related to Sulfolobus solfataricus . Each isolate was able to oxidise both Fe 2+ and sulphur, and grow on both pyrite and chalcopyrite under autotrophic conditions. Leaching experiments showed that the isolates were capable of rapidly leaching a chalcopyrite concentrate (up to 91% Cu release in 108 h). Optimal temperatures for growth and chalcopyrite leaching were determined for each strain. Chalcopyrite dissolution rates for JP2 at different temperatures were determined using a previously described kinetic model. An Arrhenius plot to investigate the relationship between dissolution rate and temperature, showed that for JP2, an increase in temperature from 70 to 83 °C resulted in a 6.6-fold rate increase. Studies with both mixed and pure cultures showed that these cultures were capable of rapidly leaching a chalcopyrite concentrate at very high temperatures (up to 90 °C), but also were capable of bioleaching at 50 °C. These thermophilic acidophiles possess the ability to bioleach over a wide range of temperatures. They are potentially well suited to industrial leaching applications where considerable temperature fluctuations limit the growth of other non-thermophilic bioleaching microorganisms.

The resilience and versatility of acidophiles that contribute to the bio‐assisted extraction of metals from mineral sulphides

In this paper, a brief outline is presented on acidic ferric ion oxidation of mineral sulphides for the extraction of metals in both stirred tank reactors for mineral concentrates and heaps for low‐grade ores. The identities and capabilities of the relatively few acidophiles that assist the oxidative processes are summarized and their responses to selected extremes in their growth environments described. Individually, the organisms adapt to the presence of high concentrations of heavy metals and other elements in the bioleaching environment, tolerate a wide range of acidities and can recover from prolonged exposure to temperatures significantly above their preferred temperatures for growth. However, the presence of chloride in their acidic environment presents a significant physiological challenge. Species that exhibit a chemotactic response and attachment to sulphide surfaces, where they can create their own micro‐environments, would be favoured in both heap bioreactors with low availability of energy substrates and physically aggressive, agitated continuous stirred‐tank reactor environments treating concentrates.In this paper, a brief outline is presented on acidic ferric ion oxidation of mineral sulphides for the extraction of metals in both stirred tank reactors for mineral concentrates and heaps for low-grade ores. The identities and capabilities of the relatively few acidophiles that assist the oxidative processes are summarized and their responses to selected extremes in their growth environments described. Individually, the organisms adapt to the presence of high concentrations of heavy metals and other elements in the bioleaching environment, tolerate a wide range of acidities and can recover from prolonged exposure to temperatures significantly above their preferred temperatures for growth. However, the presence of chloride in their acidic environment presents a significant physiological challenge. Species that exhibit a chemotactic response and attachment to sulphide surfaces, where they can create their own micro-environments, would be favoured in both heap bioreactors with low availability of energy substrates and physically aggressive, agitated continuous stirred-tank reactor environments treating concentrates.

Metals tolerance in moderately thermophilic isolates from a spent copper sulfide heap, closely related to Acidithiobacillus caldus, Acidimicrobium ferrooxidans and Sulfobacillus thermosulfidooxidans

Selective enrichments enabled the recovery of moderately thermophilic isolates with copper bioleaching ability from a spent copper sulfide heap. Phylogenetic and physiological characterization revealed that the isolates were closely related to Sulfobacillus thermosulfidooxidans, Acidithiobacillus caldus and Acidimicrobium ferrooxidans. While isolates exhibited similar physiological characteristics to their corresponding type strains, in general they displayed similar or greater tolerance of high copper, zinc, nickel and cobalt concentrations. Considerable variation was found between species and between several strains related to S. thermosulfidooxidans. It is concluded that adaptation to metals present in the bioleaching heap from which they were isolated contributed to but did not entirely explain high metals tolerances. Higher metals tolerance did not confer stronger bioleaching performance, suggesting that a physical, mineralogical or chemical process is rate limiting for a specific ore or concentrate.

Gibbsite crystallization inhibition: 1. Effects of sodium gluconate on nucleation, agglomeration and growth

Abstract Conditions that facilitate the study of gibbsite secondary nucleation, agglomeration and growth as separate crystallization processes have been described. The effects of sodium gluconate, one of the strongest inhibitors of gibbsite crystallization from alkaline aluminate solutions, have been studied under the selected conditions. Sodium gluconate was found to promote secondary nucleation, relative to a control, when present at very low concentrations (1–3 mmol L −1 ); at slightly higher concentrations (4–6 mmol L −1 ) it suppressed the release of secondary nuclei. Gluconate caused an increase in the induction period, delayed but did not prevent agglomeration, and reduced ordered growth rates significantly. The observed small changes in crystal morphologies, induced by 4–6 mmol L −1 gluconate in solution, should assist in understanding the mechanisms of gibbsite crystallization inhibition because they reflect different deposition rates on specific crystal faces.

Thiocyanate removal from saline CIP process water by a rotating biological contactor, with reuse of the water for bioleaching

Abstract Two strains of bacteria that were known to be capable of degrading thiocyanate were isolated from the Youanmi gold mine in Western Australia. Both strains demonstrated the ability to utilise thiocyanate as their sole energy and nitrogen source. Physiological characterisation indicated that both strains had the potential to tolerate the variable conditions encountered in Youanmi tailings water. Phylogenetic analysis showed that the strains were either members of the genus Thiobacillus or Halomonas but could not be accommodated within any existing described species. Phosphate was the only additional nutrient required. Both strains were inoculated into a laboratory-scale rotating biological contactor, where degradation took place in saline, low-nutrient water. The biomass supported on the reactor surface (20 m 2 ) was capable of degrading approximately 2800 mg thiocyanate L −1 to less than 1 mg L −1 at a flow rate of 30 mL min −1 and a hydraulic retention time of 11.1 h. When this degraded water was used as the basal medium for testing the bacterial oxidation of ferrous ion and of arsenopyrite and pyrite, the rates obtained were similar to those obtained with thiocyanate-free medium. This showed that degradation was successful, that the by-products of the reaction, ammonium and sulphate ions and carbon dioxide, were not toxic to iron- and sulphide-oxidising bacteria, and that water could be recycled to a biological oxidation plant after passage through the thiocyanate-degrading reactor.

Spectroscopy of Concentrated Sodium Aluminate Solutions

Sodium aluminate solutions have been investigated by infrared absorption and Raman scattering in a broad concentration range in order to detect possible changes in aluminate ion species and distribution. Concentration-dependent changes in spectra include shifts in frequency at maximum band intensity, a loss of proportionality in band intensities, and the appearance of new vibrational bands or the increased asymmetry of existing ones. Changes in infrared spectra correlate well with those observed for Raman spectra and are consistent with the presence of more than one aluminate ion species in equilibrium in concentrated aluminate solutions. Assuming that the observed effects are due to changed solution speciation, it is found that the maximum concentration of Al(OH)4 occurs in solutions of 4–5 M aluminum, and that other species become relatively more abundant as the aluminum concentration is increased. Comparison of solution spectra with spectra of crystalline aluminates indicates that new species are likely to be oligomeric and polymeric anions in which aluminum is mainly 4-coordinate, but the possibility of low concentrations of aluminate species containing 6-coordinate aluminum cannot be excluded.