A. Ballester

New information on the chalcopyrite bioleaching mechanism at low and high temperature

Abstract The chalcopyrite bioleaching mechanism was studied to determine its direct or indirect nature. At the same time, a study of possible causes that diminish the dissolution rate and inhibit the attack of this sulphide mineral was carried out. An electrochemical study of the mineral was also performed, which included the elimination or dissolution of possible diffusion barriers formed on the mineral surface. Results of these studies showed that the oxidation state of the dissolved iron (Fe3+) was fundamental to chalcopyrite bioleaching because Fe3+ controlled the relative rate of the oxidation reactions. In addition, the attack of chalcopyrite was controlled by elemental sulphur and intermediate, nonstoichiometric, copper sulphides forming on the chalcopyrite surface, which are less reactive than the original sulphide. Intermediate sulphides caused an important barrier effect at low temperature (35 °C). At higher temperature (68 °C), these intermediate sulphides do not constitute a diffusion barrier due to their dissolution. Microbial attachment to the pyritic phase of the copper ore was related to the dissolution rate of the mineral due to the liberation of Fe2+ after the attack of the pyrite by contact bioleaching. It was concluded that the bioleaching of chalcopyrite concentrate is a cooperative effort involving the simultaneous contact bioleaching of the pyritic phase of the mineral, possibly by an indirect mechanism via thiosulphate, and the indirect bioleaching of chalcopyrite, probably by a mechanism by way of polysulphide and elemental sulphur.

Bioremediation of an industrial acid mine water by metal-tolerant sulphate-reducing bacteria

The microbiological diversity associated with mining environments is a very well proven fact. One of the communities appearing in these environments is that formed by anaerobic sulphate-reducing bacteria (SRB) which can be used for the decontamination of acid mine drainage waters. In this work, the potential of a mixed population of SRB, isolated from the bottom of a pyritic tailing pond situated in the Spanish pyritic Belt, has been investigated with the main objective of treating the effluent generated in the same disposal site. The efficiency of the system is based on the presence of an important amount of reducing agents contained in the acid mine drainage received in the pond. Results showed that this option is effective for the precipitation of the dissolved metals (copper and iron), for the reduction and removal of sulphates and even for the alkalising of the waters. SRB were able to remove up to 9,000 ppm of sulphate ion efficiently, to grow in the presence of up to 100 ppm of copper and 30 ppm of iron, and alkalise the medium, provided that this was not extremely acidic (pH>4). Finally, according to the results obtained, the possibility of applying this method to the treatment of a real effluent is discussed.

Silver-catalysed bioleaching of a chalcopyrite concentrate with mixed cultures of moderately thermophilic microorganisms

Abstract Bioleaching of a chalcopyrite concentrate was studied using two mixed cultures of moderately thermophilic microorganisms, one from the drainage of Rio Tinto mines and the other obtained by raising the growth temperature of a mixed culture of mesophilic bacteria. Both cultures were adapted to different concentrations of silver (0.1–0.5 g of silver/kg of concentrate) to test their leaching capacity in the presence of this catalyst. The results showed the important role of silver in the chalcopyrite leaching process carried out at 45°C. Copper yields increased around 3-fold over the control experiment without silver. At 50°C, however, only one of the cultures remained active and the effect of silver was masked by strong jarosite precipitation, which increased with the amount of silver used. The 9K medium played a key role in this respect, and it was clear that more diluted media should be used to avoid this problem. Despite this drawback, the amount of copper extracted using the catalyst was double than that obtained without it.

Electrochemistry of chalcopyrite

Abstract The electrochemical response of a massive chalcopyrite electrode at two different temperatures, 25°C and 68°C, were compared. The electrolyte used in the experiments was an acidic medium (0.4 g ·1 −1 (NH 4 ) 2 SO 4 , 0.5 g ·1 −1 MgSO 4 · 7H 2 O, 0.2 g ·1 −1 K 2 HPO 4 at pH = 2) which is suitable for the growth of the microorganisms involved in the bioleaching process. The chosen temperatures were optimum for the growth of the mesophilic ( Thiobacillus ferrooxidans ) and thermophilic ( Sulfolobus ) microorganisms. The experimental results at both temperatures were similar and confirmed that, during the anodic dissolution of chalcopyrite, a passive film is formed on the surface which restricts the oxidation reactions in the medium by diffusional control of the film. The different responses at the temperatures tested were due to the differing physical structure of the complex films of the electrochemically formed sulphides, polysulphides and elemental sulphur.

Study of Bacterial Attachment During the Bioleaching of Pyrite, Chalcopyrite, and Sphalerite

In this study the degree of bacterial attachment of the main microorganisms involved in the bioleaching of metal sulfides and their influence on the dissolution rate and final metal extractions were determined. Three different mineral sulfides were bioleached: chalcopyrite (CuFeS 2 ), sphalerite (ZnS) and pyrite (FeS 2 ). A mixed culture of mesophilic bacteria ( Acidithiobacillus and Leptospirillum spp.) was tested at 35°C and thermophilic bacteria ( Sulfolobus spp.) were tested at 68°C. The results confirmed that a relationship exists between attachment and mineral dissolution rates. The bioleaching process can be divided into three stages. An initial stage with extensive bacterial attachment to the pyritic phase, which is of major importance in order to obtain high dissolution rates (since attached cells liberate Fe 2+ by contact bioleaching and oxidize Fe 2+ to Fe 3+ avoiding iron accumulation at the surface). In a second stage the bacterial attachment diminishes due to the saturation of the surface by the attached cells. This limitation, together with the presence of Fe 2+ in solution, produces an increase in the concentration of free cells, which use Fe 2+ as an energy source. Finally, in a third stage, a balance between free and attached cells is reached, giving rise to a cooperative mechanism. In this case, the attached cells attack the pyrite phase of the mineral generating Fe 2+ . This in turn is oxidized by the free cells in solution, regenerating the oxidizing agent (Fe 3+ ) for the indirect bioleaching of the mineral.

Bioleaching of a Spanish complex sulphide ore bulk concentrate

Abstract In the present work the applicability of bioleaching using a mixed culture of mesophilic microorganisms (Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirilum ferrooxidans) on a bulk concentrate of a Spanish complex sulphide ore was studied. The bulk concentrate mainly consisted of by chalcopyrite, sphalerite and pyrite. Effects of nutrient medium, stirring, pulp density, temperature and the addition of CO 2 (1% v/v) to the air flow were also studied. The highest leaching rates and recoveries were obtained with mechanically stirred reactors at 5% pulp density and 9K medium. However, by using 9K medium higher jarosite precipitation was observed. Results showed that the optimum temperature for copper bioleaching was 30°C, whereas zinc dissolution increased with a rise in the temperature.

Effect of the presence of lead on the biosorption of copper, cadmium and zinc by activated sludge

The effect of the presence of Pb on the biosorption efficiency of Cu, Cd and Zn by activated sludge was investigated. To evaluate the two-metal sorption system efficiency, simple isotherm curves had to be replaced by three-dimensional sorption isotherm surfaces. In order to describe the isotherm surfaces mathematically, three Langmuir-type models were evaluated. The isotherms indicated a competitive uptake of the different metals with Pb that was preferentially adsorbed.

A study of the bioleaching of a Spanish uranium ore. Part I: A review of the bacterial leaching in the treatment of uranium ores

The rapid growth of the nuclear power industry has led to the progressive exhaustion of high grade reserves of uranium ores and new extraction techniques have had to be developed for use with low grade ores. The expense of these techniques is, of course, important and one of the solutions proposed has been percolation leaching with the active participation of micro-organisms. This paper intends to give a general overview of the phenomenon of uranium ore bioleaching, in which a wide variety of bacterial species intervene in symbiotic association. Microbiological activity inside the heap is conditioned by its mineral composition and environmental and operational factors must also be taken into account. The industrial application of these techniques, particularly in Spain, is studied.

Study by SEM and EDS of chalcopyrite bioleaching using a new thermophilic bacteria

Abstract The main aim of this work is to provide new evidence regarding the interaction of chalcopyrite with an unknown thermophilic bacteria isolated from Rio Tinto mines (Huelva, Spain). This new microorganism gives very high copper recoveries (more than 80%) when a chalcopyrite concentrate is bioleached. Study of the solid massive samples after bioleaching by scanning electron microscopy (SEM) and non-dispersive X-ray microanalysis (EDS) confirmed its ability to attack ore. Chalcopyrite leached in the presence of this bacteria underwent a very strong superficial transformation in comparison with the sample attacked either in the absence of bacteria or in the presence of Fe 3+ . The attack started in cracks and genetic defects and finally reached the whole chalcopyrite surface. As a consequence of this, the ore showed deep cracks and a very rough surface. The presence of Fe + to increase the bacteria activity did not improve chalcopyrite bioleaching. These results were similar to those obtained in the absence of bacteria but in the presence of ferric ions. Therefore, the very good capacity o f this new thermophilic microorganism for bacterial leaching and its advantages over conventional chemical leaching were demonstrated.

Leaching capacity of a new extremely thermophilic microorganism, Sulfolobus rivotincti

Abstract The leaching capacity of a new extremely thermophilic microorganism ( Sulfolobus rivotincti ) was tested on different metal concentrates. The results using three complex sulphide concentrates demonstrated, in all cases, the cultures capacity to recover high percentages of copper and zinc with slight differences according to the composition of the concentrate used. The bioleaching of chalcopyrite concentrates was studied in the presence of different compounds (chlorides and metallic ions) in an attempt to improve the efficiency of bioleaching. The results confirm the high toxicity of the chloride ion and the negative effect of silver. Of the other ions studied, only bismuth had a positive effect, increasing the rate of copper leaching, although the final percentage of metal in solution was very similar to that obtained with experiments not involving bismuth.