Bacterial Oxidation Of Ferrous Iron Research Articles

In situ recovery of copper sulfide ores: Alternative process schemes for bioleaching application

There is a growing interest in exploring the application of bioleaching for treating copper sulfide ores of decreasing grade and located at larger depth by in situ recovery (ISR) for its potential savings in ore extraction, crushing, grinding and the avoidance of tailings. As copper sulfide ores are usually present in well consolidated rocks a crucial aspect for ISR to be successful is to increase the ore permeability in order to allow a good access of solution to the valuable metal values. However, in bioleaching an additional critical aspect is to guarantee an adequate supply of oxygen, the final electron acceptor in the reaction chain. Two alternative schemes to solve this issue are here characterized and compared using process simulation.One approach considers pre-treating the rock with hydrofracturing and/or confined blasting in order to create a network of cracks adequate for pumping the pressurized leaching solution into the ore bed in a saturated flow regime. In this scheme oxygen needs to be added in an external bioreactor where ferric iron is generated by bacterial oxidation of ferrous iron. A second approach considers pre-treating the ore by application of sublevel stopping techniques with partial removal of the rock to be able to irrigate the solution into the ore bed under an unsaturated flow regime. In this scheme oxygen can be directly supplied into the ore bed enabling in situ generation of ferric iron by the oxidative action of microorganisms present in the ore.The influence of irrigation rate, leaching solution concentration, bacterial population and ore bed dimensions on copper recovery and bioreactor requirements was assessed for these two process schemes. Some economic aspects of these two alternatives are discussed.

Co-processing of sulfidic mining wastes and metal-rich post-consumer wastes by biohydrometallurgy

The consequence of a strong economic growth in emerging countries combined with the rise of the world population is an increase in the demand for raw materials, leading to growing concern regarding their availability and the global efficiency of the supply chain. These tensions reinforce the need to associate the development of the recycling industry to the identification of new resources which could be used for the recovery of valuable materials. The purpose of this study is to develop a novel biological co-processing approach for the recovery of strategic metals in both sulfidic mining wastes and post-consumer wastes (WEEE). The principle of this treatment is based on two steps: mine wastes are biologically oxidized, resulting in the production of a ferric iron-sulfuric acid lixiviant solution which is used to leach base and other soluble metals contained in e-scraps. Batch tests were carried out using flotation tailings wastes containing 60% of pyrite and grinded Printed Circuit Boards (PCB<750μm) with a solid load of 2.5%. Two series of tests were conducted in order to study the influence of the ferric iron concentration and of the bacterial activity on metals dissolution. Results showed that a higher ferric iron concentration led to an increase in the dissolution rate of copper which is the main metal contained in the PCBs. Moreover, a dissolution yield of 98.3% was reached for copper after 2days when bacterial activity was observed, corresponding to an increase of about 20% compared to the tests without bacterial activity. Finally, this study highlights the importance of the availability of ferric iron and of the bacterial oxidation of ferrous iron for the feasibility of this bioleaching process dealing with the recycling of PCBs.

Isolation, identification and arsenic-resistance of Acidithiobacillus ferrooxidans HX3 producing schwertmannite

Schwertmannite, a ubiquitous mineral present in iron oxyhydroxides formed in iron- and sulfate-rich acid media, favors incorporation of some toxic anions in its structure. We reported an iron-oxidizing bacterial strain HX3 from a municipal sludge that facilitates the formation of pure schwertmannite in cultures. Ferrous iron oxidation by the isolated strain HX3 was optimum at an initial pH of 2.0–3.3 and temperature of 28–35°C. Pure schwertmannite was found through bacterial oxidation of ferrous iron at an initial pH2.8 and temperature 28°C. Following 16S rDNA gene sequence analysis the bacterial strain HX3 was identified as Acidithiobacillus ferrooxidans. The arsenic-resistance A. ferrooxidans HX3 showed the potential of environmental application in arsenic removal from the As(III)- and iron-rich acid sulfate waters directly by As(III) adsorption or the formation of schwertmannite in the environment.

Inhibition of bacterial oxidation of ferrous iron by lead nitrate in sulfate-rich systems

Inhibition of bacterial oxidation of ferrous iron (Fe(II)) by Pb(NO3)2 was investigated with a mixed culture of Acidithiobacillus ferrooxidans. The culture was incubated at 30°C in ferrous-sulfate medium amended with 0–24.2mM Pb(II) added as Pb(NO3)2. Anglesite (PbSO4) precipitated immediately upon Pb addition and was the only solid phase detected in the abiotic controls. Both anglesite and jarosite (KFe3(SO4)2(OH)6) were detected in inoculated cultures. Precipitation of anglesite maintained dissolved Pb concentrations at 16.9–17.6μM regardless of the concentrations of Pb(NO3)2 added. Fe(II) oxidation was suppressed by 24.2mM Pb(NO3)2 addition even when anglesite was removed before inoculation. Experiments with 0–48mM KNO3 demonstrated that bacterial Fe(II) oxidation decreased as nitrate concentration increased. Therefore, inhibition of Fe(II) oxidation at 24.2mM Pb(NO3)2 addition resulted from nitrate toxicity instead of Pb addition. Geochemical modeling that considered the initial precipitation of anglesite to equilibrium followed by progressive oxidation of Fe(II) and the precipitation of jarosite and an amorphous iron hydroxide phase, without allowing plumbojarosite to precipitate were consistent with the experimental time-series data on Fe(II) oxidation under biotic conditions. Anglesite precipitation in mine tailings and other sulfate-rich systems maintains dissolved Pb concentrations below the toxicity threshold of A. ferrooxidans.

Bacterial Oxidation of Ferrous Iron and RISCs at Low Temperatures: Their Effect on Acid Mine Drainage and Bioleaching of Sulphide Minerals

Chilean copper production has been growing in the last 20 years reaching an annual production of 5,557,000 tons of Cu in 2007. For each ton of copper produced, about 200 tons of sterile and low grade ore and 100 tons of tailings are discharged in the environment. Most of these wastes contain significant amounts of sulphide minerals, mainly pyrite, and once submitted to weathering, may produce acid mine drainage. On the other hand, the high price of copper raised the interest for processing by leaching the low grade ore deposited in large dumps. An important part of these mining wastes and low grade ores is located in the Andes, where the mean temperature is usually ~5°C or less. The rate at which bioleaching reactions occur is directly related to the temperature at which the microorganisms (bacteria and archaea) develop. A temperature decrease causes both a decrease of the rate of the involved chemical reactions and a decrease or inhibition of microbial growth. In this work we present the results of the isolation of microorganisms from an old tailing deposit, exposed at low temperatures (5oC) during most of the winter. The isolated microorganisms initially showed a low capacity to oxidize 3g/L Fe(II) sulfate at pH 1.6, and tetrathionate 0.01 M, with an initial pH 4 both at 5oC. However, after successive cultures, microorganisms showed a slow capacity to oxidize both substrates, as well as the sulphide contained in the tailings samples. The terminal Restriction Fragment Length Polymorphism (tRFLP) of the isolated cells grown in basal medium containing Fe(II) showed a nearly pure culture of Acidithiobacillus ferrooxidans. The present study indicates that, even at very low temperatures, microorganisms play an important role in the generation of acid mine drainage and in the oxidation and leaching of sulphide ores.

Reductive Action of Activated Carbon on Ferric Iron Interferes on the Determination of the Oxidative Activity of &lt;i&gt;Acidithiobacillus Ferrooxidans &lt;/i&gt;on Ferrous Iron

Ferrous iron oxidation studies in the presence of activated carbon were conducted at 30 °C in basal medium at pH 1.6 with a pure strain of Acidithiobacillus ferrooxidans. Two-chamber modified shake flasks were used in these experiments, which prevented direct contact between the microorganisms and the carbon contained in the flasks. This design permitted an accurate determination of bacterial population during the experiment and enabled the involvement of ferric iron reduction with carbon to be evidenced. Notably, iron was initially added as ferric iron in a concentration of 3 g/L. It could be observed that bacteria could grow in this condition evidencing that bacteria was in fact oxidizing ferrous ion produced from reduction of ferric by carbon. From complementary experiments in which activated carbon was contacted with abiotic solutions containing ferric ion in the concentration range 0.1 – 1.2 g/l, the chemical reductive action of carbon of ferric iron was confirmed and a kinetic expression for this reaction was determined. A mathematical model was developed which incorporated expressions for the kinetic of bacterial oxidation of ferrous ion and the chemical reduction of ferric ion. This model enabled the prediction of the rate of bacterial growth and ferrous ion oxidation in a bioreactor as a function of the initial concentrations of iron, activated carbon and bacterial population. Results in this work imply that the observed variations in activity observed by other authors during bacterial oxidation of ferrous iron with A. ferrooxidans adsorbed on carbon can be in fact related to bacterial utilization of supplementary ferrous iron produced by the chemical action of carbon, phenomenon which is not explicitly accounted for.

Bacterial oxidation of ferrous iron at low temperatures

This study comprises the first report of ferrous iron oxidation by psychrotolerant, acidophilic iron-oxidizing bacteria capable of growing at 5 degrees C. Samples of mine drainage-impacted surface soils and sediments from the Norilsk mining region (Taimyr, Siberia) and Kristineberg (Skellefte district, Sweden) were inoculated into acidic ferrous sulfate media and incubated at 5 degrees C. Iron oxidation was preceded by an approximately 3-month lag period that was reduced in subsequent cultures. Three enrichment cultures were chosen for further work and one culture designated as isolate SS3 was purified by colony isolation from a Norilsk enrichment culture for determining the kinetics of iron oxidation. The 16S rRNA based phylogeny of SS3 and two other psychrotolerant cultures, SS5 from Norilsk and SK5 from Northern Sweden, was determined. Comparative analysis of amplified 16S rRNA gene sequences showed that the psychrotolerant cultures aligned within Acidithiobacillus ferrooxidans. The rate constant of iron oxidation by growing cultures of SS3 was in the range of 0.0162-0.0104 h(-1) depending on the initial pH. The oxidation kinetics followed an exponential pattern, consistent with a first order rate expression. Parallel iron oxidation by a mesophilic reference culture of Acidithiobacillus ferrooxidans was extremely slow and linear. Precipitates harvested from the 5 degrees C culture were identified by X-ray diffraction as mixtures of schwertmannite (ideal formula Fe(8)O(8)(OH)(6)SO(4)) and jarosite (KFe(3)(SO(4))(2)(OH)(6)). Jarosite was much more dominant in precipitates produced at 30 degrees C.

Factors limiting bacterial iron oxidation in biodesulphurization system

Batch studies have been conducted to examine the effect of various factors (which are suspected to limit the coal biodesulphurization process rate) on bacterial oxidation of ferrous iron. An attempt has been made to find whether this reaction is inhibited due to the presence of solid particles and/or the release of inorganic components from solids during coal biodesulphurization. Three types of suspended particulates (pyrite, coal and activated charcoal) and their filtrates were used in this study. The presence of pyrite and coal as suspended particulates inhibited the bacterial iron oxidation to a greater extent as compared to the inert solid (activated charcoal powder). The inhibition at 2% pulp density in case of pyrite, coal and activated charcoal was 96%, 33% and 10%, respectively. The filtrates of pyrite and coal also caused significant inhibition while activated charcoal filtrates were non-inhibitory. At higher pulp densities (5–10%) of coal particulates, the contribution of soluble components to the total inhibition of iron oxidation was more pronounced. The inhibitory concentrations of nickel (1.0 g/l), lead (0.3 g/l), zinc (9.5 g/l), aluminum (2.5 g/l) and silicon (0.2 g/l) for bacterial iron oxidation were worked out and compared with those present in the actual leachates (produced during coal biodesulphurization). Based on the results, silicon turned out to be the main suspect responsible for retarding the central step of iron oxidation during coal biodesulphurization. Consequently, operational strategies, which tend to minimize the concentration of toxic inorganic components in the leachate, could be better option than conventional batch process for enhanced biodesulphurization.

Bacterial oxidation of ferrous iron by Acidithiobacillus ferrooxidans in the pH range 2.5–7.0

The oxidation of ferrous iron by Acidithiobacillus ferrooxidans in the pH range 2.5–7.0 was characterized. In order to measure the rate of bacterial oxidation of ferrous iron with A. ferrooxidans in this high pH range, a novel experimental methodology was developed. The results showed that the inhibition of ferrous iron oxidation activity by A. ferrooxidans observed at pH values above 3.0 is partially linked to the formation of ferric iron precipitates, which apparently hinder transport processes on the cell surface. By carefully controlling the amount of iron precipitates formed during the oxidation of ferrous iron in this pH range, enhanced bacterial activity was obtained.

Modeling the bacterial oxidation of ferrous iron with Acidithiobacillus ferrooxidans using kriging interpolation

A modeling approach of the bacterial oxidation of ferrous iron in batch culture is presented. It is based on the spatial interpolation of experimental data known as kriging interpolation. It is shown that, using the proposed method, the prediction error between model and true system variables is optimal in the sense of minimum variance. The results obtained improve the prediction error associated to the use of the current “knowledge-based” models. Results on modeling the kinetics of the bacterium Acidithiobacillus ferrooxidans, previously called Thiobacillus ferrooxidans, growing in ferrous iron as energy source in a batch airlift reactor are presented. The prediction error bound associated with the model is deduced theoretically and calculated for the application case.

Solid–liquid mass transfer limitation of ferrous iron in the chemical oxidation of FeS 2 at high redox potential

The chemical oxidation rate of pyrite with ferric iron increases with increasing ferric-to-ferrous iron concentration ratios in the bulk-phase of the solution. In batch experiments, these high ratios were due to the presence of Leptospirillum ferrooxidans bacteria. It was proposed that the observed maximum chemical pyrite oxidation rate (mole pyrite oxidised/mole pyrite s), is determined by the mass transfer rate of chemically produced ferrous iron from the pyrite surface to the bulk-phase. The overall kinetics of chemical pyrite oxidation at pseudo-steady-states in these batch experiments are appropriately described by rate equations for the three independent sub-processes involved: (i) a linear equation that describes the chemical pyrite oxidation kinetics at the pyrite surface (i.e. production rate of ferrous iron) in terms of the ferric iron concentration in the bulk-phase, and the surface concentration of ferrous iron, (ii) mass transfer of ferrous iron from the pyrite surface to the bulk of the solution, where the solid–liquid mass transfer coefficient of ferrous iron, k L(m/s), was estimated using a Sherwood correlation, and (iii) bacterial oxidation of ferrous iron by L. ferrooxidans.

Bacterial ferrous iron oxidation of acid mine drainage as pre-treatment for subsequent metal recovery

Mining activities at Falu copper mine in Sweden started around the year 1080 AD and continued until 1993. During all these centuries, the acid mine drainage has caused low pH values and high metal levels in the nearby Falu river and it's surroundings. This work is part of a plan to achieve a long-term solution to this environmental problem, where the aim is to treat the mine water overflow in such a way that a neutral pH and an essentially metal free aqueous effluent is obtained. The idea is to first oxidise the ferrous iron with bacteria, in order to be able to make a selective ferric iron precipitation at a pH of around 3. One possible process option is to use the precipitated ferric hydroxide for the production of red pigment. Thereafter, zinc will be recovered in such a way that it will be suitable for recycling. In this paper, only the initial stage of bacterial ferrous iron oxidation is discussed. The ferrous iron oxidation was initially studied in laboratory scale using batch cultures of mesophilic (35°C), moderate thermophilic (45°C) and extreme thermophilic (65°C) microorganisms. The ferrous iron oxidation kinetics was determined with two different concentrations of ferrous iron. The moderate thermophilic culture did not grow well on ferrous iron only. Since the mesophilic and extreme thermophilic microorganisms showed approximately the same oxidation kinetics, the mesophilic bacteria was selected for further studies in pilot scale, due to their lower operating temperature which reduces the heating cost. A pilot plant was erected at the mine site with three 500 l reactors in series with a treatment capacity of up to approximately 500 l h −1. The reactors were filled with plastic bodies in order to support the formation of a permanent biofilm to avoid bacterial washout. The pilot plant has been operating continuously for several months and the influence of flow rate, ferrous concentration, pH and temperature was investigated. The feasibility for a biological ferrous iron oxidation step was successfully demonstrated. With a ferrous concentration of 3.5 g l −1, 35°C, pH 1.8 and a flow rate of 330 l h −1 a ferrous iron oxidation rate of 750 mg l −1 h −1 was achieved. The results obtained will be used to calculate the capital and operating cost for a full scale plant, capable of treating 25 m 3 of acid mine drainage per hour. Before the final decision, regarding the ferrous iron oxidation step is made, alternative processes such as hydrogen peroxide and ozone oxidation will be evaluated.

Dissolution and structural alteration of phlogopite mediated by proton attack and bacterial oxidation of ferrous iron

Microbiological weathering of a research-grade mica mineral, phlogopite, was studied using ferrous sulfate media that were inoculated with an acidophilic iron-oxidizing bacterium, Thiobacillus ferrooxidans. Weathering due to dissolution was monitored by analysis of Si, Al, Fe, K, Na, Mg, and Ca in the leach solutions and in chemical controls at pH 1.0, 1.5 and 2.0. Structural alterations of phlogopite were analyzed by X-ray diffraction. At pH 2, the oxidation of Fe(II) by T. ferrooxidans was accompanied by the formation of jarosite within 7 days of incubation at 22°C. The precipitation of jarosite was coupled with partial alteration of phlogopite to vermiculite and an interstratified (mixed-layer) phlogopite/vermiculite. Similar results were obtained with chemical controls containing 120 mM ferric sulfate. The data suggested that K incorporated into jarosite was released from interlayer positions in phlogopite; thus, jarosite constituted a sink for K. The formation of jarosite and expansible layer silicate phases was pH-dependent. At pH<1.5, jarosite was not formed and phlogopite weathering was due to chemical dissolution without detectable structural alteration.

The effect of temperature on the continuous ferrous-iron oxidation kinetics of a predominantly Leptospirillum ferrooxidans culture.

The ferrous-iron oxidation kinetics of a bacterial culture consisting predominantly of Leptospirillum ferrooxidans were studied in continuous-flow bioreactors. The bacterial culture was fed with a salts solution containing 12 g/L ferrous-iron, at dilution rates ranging from 0.01 to 0.06 l/h, and temperatures ranging from 30 to 40 degrees C, at a pH of 1.75. The growth rate, and the oxygen and ferrous-iron utilization rates of the bacteria, were monitored by means of off-gas analysis and redox-potential measurement. The degree-of-reduction balance was used to compare the theoretical and experimental values of r(CO(2)), -r(O(2)) and -r(Fe(+2)), and the correlation found to be good. The maximum bacterial yield on ferrous-iron and the maintenance coefficient on ferrous-iron, were determined using the Pirt equation. An increase in the temperature from 30 to 40 degrees C did not appear to have an effect on either the maximum yield or maintenance coefficient on ferrous-iron. The average maximum bacterial yield and maintenance coefficient on ferrous-iron were found to be 0.0059 mmol C/mmol Fe(2+) and 0.7970 mmol Fe(2+)/mmol C)/h, respectively. The maximum specific growth rate was found to be 0.077 l/h. The maximum specific ferrous-iron utilization rate increased from 8.65 to 13.58 mmol Fe(2+)/mmol C/h across the range from 30 to 40 degrees C, and could be described using the Arrhenius equation. The kinetic constant in bacterial ferrous-iron oxidation increased linearly with increasing temperature. The ferrous-iron kinetics could be accurately described in terms of the ferric/ferrous-iron ratio by means of a Michaelis-Menten-based model modified to account for the effect of temperature. A threshold ferrous-iron level, below which no further ferrous-iron utilization occurred, was found at a ferric/ferrous-iron ratio of about 2500. At an overall iron concentration of 12 g/L, this value corresponds to a threshold ferrous-iron concentration of 78.5 x10(-3) mM.

Comparison of the oxidation kinetics of different pyrites in the presence of Thiobacillus ferrooxidans or Leptospirillum ferrooxidans

The question whether the overall bacterial oxidation rate of a pyrite is determined by both the chemical reactivity of the pyrite as well as the affinity of a bacterial strain to the ferrous to ferric iron ratio, was examined. Bioleaching studies were carried out with a pure culture of Thiobacillus ferrooxidans and an enrichment culture of Leptospirillum bacteria on a framboidal pyrite from Germany and a euhedral pyrite from Prieska, South Africa. It was found that the Leptospirillum bacteria were able to oxidise euhedral pyrite and framboidal pyrite, whereas T. ferrooxidans was able to oxidise framboidal pyrite but not euhedral pyrite. The German, framboidal pyrite has a granular and irregular surface structure, which is probably more chemically reactive than the highly crystalline surface structure of euhedral, Prieska pyrite. In previous work it has been shown that the oxidation of pyrite by Leptospirillum bacteria is determined by a mechanism which consists of two sub-processes: (i) the chemical oxidation of pyrite with ferric iron to ferrous iron and sulphate, and (ii) the bacterial oxidation of ferrous iron. By using roughly estimated kinetic constants from a batch culture of T. ferrooxidans on framboidal pyrite, and the independently determined ferrous iron oxidation kinetics of T. ferrooxidans, it was possible to explain why this strain is not able to oxidise the refractory euhedral pyrite. The results are explained in terms of the relative affinities of T. ferrooxidans and the Leptospirillum bacteria for the ferrous/ferric-iron ratio, and susceptibility of the two types of pyrite to ferric leaching.

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in batch cultures

The ferrous iron oxidation kinetics of Thiobacillus ferrooxidans in batch cultures was examined, using on-line off-gas analyses to measure the oxygen and carbon dioxide consumption rates continuously. A cell suspension from continuous cultures at steady state was used as the inoculum. It was observed that a dynamic phase occurred in the initial phase of the experiment. In this phase the bacterial ferrous iron oxidation and growth were uncoupled. After about 16 h the bacteria were adapted and achieved a pseudo-steady state, in which the specific growth rate and oxygen consumption rate were coupled and their relationship was described by the Pirt equation. In pseudo-steady state, the growth and oxidation kinetics were accurately described by the rate equation for competitive product inhibition. Bacterial substrate consumption is regarded as the primary process, which is described by the equation for competitive product inhibition. Subsequently the kinetic equation for the specific growth rate, μ, is derived by applying the Pirt equation for bacterial substrate consumption and growth. The maximum specific growth rate, μmax, measured in the batch culture agrees with the dilution rate at which washout occurs in continuous cultures. The maximum oxygen consumption rate, qO2,max, of the cell suspension in the batch culture was determined by respiration measurements in a biological oxygen monitor at excess ferrous iron, and showed changes of up to 20% during the course of the experiment. The kinetic constants determined in the batch culture slightly differ from those in continuous cultures, such that, at equal ferric to ferrous iron concentration ratios, biomass-specific rates are up to 1.3 times higher in continuous cultures.

Effect of pH on ferrous-iron oxidation kinetics of Leptospirillum ferrooxidans in continuous culture

The ferrous-iron oxidation kinetics of Leptospirillum ferrooxidans were studied at dilution rates ranging from 0.01 to 0.10 h −1, and pH values ranging from pH 1.10 to 1.70. The growth rate, and the oxygen and ferrous-iron utilisation rates of the bacteria, were monitored by means of off-gas analysis and redox potential measurement. The degree-of-reduction balance was used to compare the theoretical and experimental values of − r 2+ Fe, − r O 2 and r CO 2 , and the correlation found to be good. The bacterial culture maintained at pH 1.30 achieved the greatest measured maximum growth rate. An increase in the pH from pH 1.10 to pH 1.70 did not affect the maximum yield and maintenance coefficients, or the maximum specific ferrous-iron and oxygen utilisation rates. However, the kinetic constants in bacterial ferrous-iron oxidation, viz. K Fe 2+ and K O 2 , increased linearly with increasing pH, across the range from pH 1.10 to 1.70. The kinetics could be described in terms of the ferric/ferrous-iron ratio using a Michaelis–Menten based model modified to account for the effect of pH on K Fe 2+ and K O 2 . Furthermore, the threshold concentration of ferrous-iron increased with a decrease in the pH of the solution.

Continuous ferrous iron biooxidation in flooded packed bed reactors

In this paper the biological ferrous iron oxidation in flooded packed bed reactors is studied. Bacterial oxidation of ferrous iron is a potential industrial process for the treatment of acid mine drainage and in the regeneration of ferric iron as a leaching agent in hydrometallurgical processes. The aim of this work is the development of a reactor capable of attaining the oxidation rates demanded by industrial processes, which are higher than those shown in the literature. The bioreactor consists of a polymethylmetacrylate column randomly packed with siliceous stone particles with inlets for fresh medium and air at the bottom from where they flood the reactor. The performance of ferrous iron biooxidation reactors is strongly improved by applying this bioreactor design.

Studies on the mechanism and kinetics of bioleaching

The use of off gas analysis and redox potential measurement has shown that bioleaching involves at least three important sub-processes. The primary attack of the sulphide mineral is a chemical ferric leach. The role of the bacteria is to convert the iron from the ferrous to the ferric form, thereby maintaining a high redox potential. The kinetics of bacterial ferrous iron oxidation by Thiobacillus ferrooxidans and a Leptospirillum-like bacterium, and the chemical ferric leach kinetics of pyrite have both been described as functions of the ferric/ferrous-iron ratio. Thus, the chemical ferric leach of the mineral and the bacterial oxidation of the ferrous iron are linked by the redox potential, and are in equilibrium when the rate of iron turnover between the mineral and the bacteria is balanced. These rate equations have been used to predict the steady state redox potential and sulfide mineral conversion in a continuous bioleach reactor. The model successfully predicts laboratory data and is being tested against data from pilot-plant and full-scale bioleach systems. Furthermore, the model predicts which bacterial species will predominate and which mineral will be preferentially leached under specific operating conditions. Enzyme restriction analysis has shown that in pyrite-arsenopyrite bioleach reactors the dominant iron oxidizer is L. ferrooxidans, which is in agreement with the predictions of the model.

The use of on-line off-gas analyses and stoichiometry in the bio-oxidation kinetics of sulphide minerals

New theoretical and experimental methods have been developed to examine the chemical and bacterial oxidation kinetics in bio-oxidation of sulphide minerals, which are determined by an `indirect' mechanism. On-line off-gas analyses to measure the oxygen and carbon dioxide consumption rate in batch and continuous bio-reactors, together with the application of elemental balances, provides a useful method for the accurate and continuous determination of biomass and mineral specific rates, which must form the basis for reliable kinetic equations. The ferric to ferrous iron concentration ratio is a key parameter in the bacterial oxidation of ferrous iron as well as in the chemical oxidation of pyrite. In bio-oxidation experiments with pyrite extremely high [Fe 3+]/[Fe 2+] ratios were measured using on-line redox potential measurements. Results are illustrated for Thiobacillus ferrooxidans in continuous and batch experiments with ferrous iron, and for batch experiments with Leptospirillum bacteria with staged addition of pyrite.