Kinetic model for the cyanidation of silver ammonium jarosite in NaOH medium

A synthesis of silver ammonium jarosite has been carried out obtaining a single-phase product with the formula: [(NH₄)_0.71(H₃O)_0.25Ag_0.040]Fe_2.85(SO₄)₂(OH)_5.50. The product consists on compact spherical aggregates of rhombohedral crystals. The nature and kinetics of alkaline decomposition and also of cyanidation have been determined. In both processes an induction period followed by a conversion period have been observed. During decomposition, the inverse of the induction period is proportional to [OH⁻]^0.75 and an apparent activation energy of 80 kJ mol^− 1 was obtained; during the conversion period, the process is of 0.6 order (OH⁻ concentration) and an activation energy of 60 kJ mol^− 1 was obtained. During cyanidation, the inverse of the induction period is proportional to [CN⁻]^0.5 and an apparent activation energy of 54 kJ mol^− 1 was obtained; during the conversion period the process is of 0 order (CN⁻ concentration) and an activation energy of 52 kJ mol^− 1 was obtained. Results obtained are consistent with the spherical particle model with decreasing core and chemical control, in the experimental conditions employed. For both processes and in the basis of the behaviour described, two mathematical models, including the induction and conversion periods, were established, that fits well with the experimental results obtained. Cyanidation rate of different jarosite materials in NaOH media have also been established: this reaction rate at 50 °C is very high for potassium jarosite, high and similar for argentojarosite and ammonium jarosite, lower for industrial ammonium jarosite and negligible for natural arsenical potassium jarosite and beudantite. These results confirm that the reaction rate of cyanidation decreases when the substitution level in the jarosite lattice increases.     

Comparative study of inorganic arsenic resistance of several strains of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans

The growth characteristics of several strains of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans were studied in the presence of soluble inorganic arsenic(III) and (V) with regard to media pH changes, total bacterial populations and sulfur oxidation rates. Most of these bacteria could reach large populations and have strong sulfur oxidation activity in the absence of arsenic. However, in the presence of up to 120 mM arsenite or arsenate, different strains showed different inorganic arsenic resistance. A. thiooxidans LYS and A. ferrooxidans BY-3 were two of the best performers which showed high arsenite resistance: up to 80 mM and 60 mM, respectively. On the other hand, A. thiooxidans JY and A. ferrooxidans TKY-2 could adapt up to 120 mM and 100 mM arsenate, respectively. These bacteria strains may play key roles in the bioleaching of arsenopyrite or in the bio-oxidation pretreatment of arsenic-bearing refractory gold sulfide ores and concentrates.     

Leaching of chalcopyrite with ferric ion. Part IV: The role of redox potential in the presence of mesophilic and thermophilic bacteria

This paper reports a study on the effect of redox potential in chalcopyrite bioleaching in the presence of iron- and sulphur-oxidizing bacteria. Bioleaching tests were carried out in stirred Erlenmeyer flasks at 180 rpm, with 0.5 g of chalcopyrite mineral, 99 ml of a sulphate solution of Fe³⁺/Fe²⁺ (with the redox potential ranging between 300 and 600 mV Ag/AgCl) at pH 1.8 and 1 ml of a mesophilic (35 °C) or thermophilic (68 °C) culture. The overoxidation of the leaching solution, due to the activity of iron-oxidizing microorganisms (Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans and Sulfolobus BC), favoured the precipitation of jarosite on chalcopyrite surfaces followed by passivation. Iron- and sulphur-oxidizing microorganisms, such as A. ferrooxidans and Sulfolobus BC adapted for 4 months to elemental sulphur as the sole energy source, recovered their iron-oxidizing ability after being in contact with Fe²⁺.     

Microbial depyritisation of three different coals by Acidithiobacillus ferrooxidans in a batch stirred tank reactor

Abstract

The present study aimed to investigate the depyritisation potential of Acidithiobacillus ferrooxidans on two different types of coals, namely lignite and anthracite collected from three different countries (Korea, China and Indonesia). The experimental work was conducted on a batch mode in a stirred tank reactor. All the batch biooxidation of pyrite in the different coal samples were conducted in a pH controlled condition (pH?=?1·5). The growth medium employed for the batch biooxidation of pyritic coal was free from iron supplement. At. ferrooxidans oxidised mineral pyrite of Korean anthracite at a greater rate (98%) compared to 96 and 92% of pyrite oxidation for Indonesian and Chinese lignite respectively. The ratio of bioleach residue to the feed was reasonably good with range of 8·56–9·06 stating the net mass loss of 9–14. Coal depyritisation was carried out by the available Fe³⁺ ion in the inoculum producing Fe²⁺ ion as a product and this Fe²⁺ ion was further oxidised to Fe³⁺ ion by At. ferrooxidans. This Fe³⁺ ion produced by At. ferrooxidans continued the oxidation of the residual pyrite in the coal until all pyrite content was oxidised completely. The three different coals were found to be feasible for biological depyritisation of the coal could be scaled up for further studies in a continuous stirred tank bioreactor.

L’étude présente avait pour but d’examiner le potentiel d’enlèvement de la pyrite par Acidithiobacillus ferrooxidans dans deux types de charbons, soit le lignite et l’anthracite, récoltés dans trois pays, soit la Corée, la Chine et l’Indonésie. On a effectué le travail expérimental dans un mode en vrac, dans un réacteur agité. On a effectué toute la bio-oxydation en vrac de la pyrite des échantillons de charbon à un pH contrôlé de 1·5. Le médium de croissance utilisé pour la bio-oxydation en vrac du charbon pyritique ne contenait pas de supplément de fer. At. ferrooxydans oxydait le minerai de pyrite de l’anthracite coréen en plus grande proportion (98%) que l’oxydation de la pyrite du lignite indonésien (96%) ou chinois (92%), respectivement. Le rapport de résidu de biolixiviation à l’alimentation était raisonnablement bon, avec une gamme de 8·56 à 9·06, établissant la perte de masse nette de 9 à 14. L’enlèvement de la pyrite du charbon était effectué par l’ion Fe³⁺ disponible dans l’inoculum, donnant lieu à l’ion Fe²⁺ comme produit et cet ion Fe²⁺ était davantage oxydé en ion Fe³⁺ par At. ferrooxidans. Cet ion Fe³⁺ produit par At. ferrooxidans continuait l’oxydation de la pyrite résiduelle dans le charbon jusqu’à ce que toute la pyrite soit complètement oxydée. On a trouvé qu’il était possible d’effectuer l’enlèvement biologique de la pyrite des trois charbons et d’augmenter l’échelle pour des études futures dans un bioréacteur en continu agité.     

(Bio)chemistry of bacterial leaching—direct vs. indirect bioleaching

Bioleaching of metal sulfides is effected by bacteria, like Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus/Acidianus, etc., via the (re)generation of iron(III) ions and sulfuric acid.According to the new integral model for bioleaching presented here, metal sulfides are degraded by a chemical attack of iron(III) ions and/or protons on the crystal lattice. The primary iron(III) ions are supplied by the bacterial extracellular polymeric substances, where they are complexed to glucuronic acid residues. The mechanism and chemistry of the degradation is determined by the mineral structure.The disulfides pyrite (FeS₂), molybdenite (MoS₂), and tungstenite (WS₂) are degraded via the main intermediate thiosulfate. Exclusively iron(III) ions are the oxidizing agents for the dissolution. Thiosulfate is, consequently, degraded in a cyclic process to sulfate, with elemental sulfur being a side product. This explains, why only iron(II) ion-oxidizing bacteria are able to oxidize these metal sulfides.The metal sulfides galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS₂), hauerite (MnS₂), orpiment (As₂S₃), and realgar (As₄S₄) are degradable by iron(III) ion and proton attack. Consequently, the main intermediates are polysulfides and elemental sulfur (thiosulfate is only a by-product of further degradation steps). The dissolution proceeds via a H₂S*⁺-radical and polysulfides to elemental sulfur. Thus, these metal sulfides are degradable by all bacteria able to oxidize sulfur compounds (like T. thiooxidans, etc.). The kinetics of these processes are dependent on the concentration of the iron(III) ions and, in the latter case, on the solubility product of the metal sulfide.     

Direct versus indirect bioleaching

The dissolution of metal sulfides is controlled by their solubility product and thus, the [H⁺] concentration of the solution, and further enhanced by several chemical mechanisms which lead to a disruption of sulfide chemical bonds. They include extraction of electrons and bond breaking by [Fe³⁺], extraction of sulfur by polysulfide and iron complexes forming reactants [Y⁺] and electrochemical dissolution by polarization of the sulfide [high Fe³⁺ concentration]. All these mechanisms have been exploited by sulfide and iron-oxidizing bacteria. Basically, the bacterial action is a catalytic one during which [H⁺], [Fe³⁺] and [Y⁺] are breaking chemical bonds and are recycled by the bacterial metabolism. While the cyclic bacterial oxidative action via [H⁺] and [Fe³⁺] can be called indirect, bacteria had difficulties harvesting chemical energy from an abundant sulfide such as FeS₂, the electron exchange properties of which are governed by coordination chemical mechanisms (extraction of electrons does not lead to a disruption of chemical bonds but to an increase of the oxidation state of interfacial iron). Here, bacteria have evolved alternative strategies which require an extracellular polymeric layer for appropriately conditioned contact with the sulfide. Thiobacillus ferrooxidans cycles [Y⁺] across such a layer to disrupt FeS₂ and Leptospirillum ferrooxidans accumulates [Fe³⁺] in it to depolarize FeS₂ to a potential where electrochemical oxidation to sulfate occurs. Corrosion pits and high resolution electron microscopy leave no doubt that these mechanisms are strictly localized and depend on specific conditions which bacteria create. Nevertheless, they cannot be called ‘direct’ because the definition would require an enzymatic interaction between the bacterial membrane and the cell. Therefore, the term ‘contact’ leaching is proposed for this situation. In practice, multiple patterns of bacterial leaching coexist, including indirect leaching, contact leaching and a recently discovered cooperative (symbiotic) leaching where ‘contact’ leaching bacteria are feeding so wastefully that soluble and particulate sulfide species are supplied to bacteria in the surrounding electrolyte.     

Electrochemical noise analysis of bioleaching of bornite (Cu5FeS4) by Acidithiobacillus ferrooxidans

Electrochemical noise (EN) is a generic term describing the phenomenon of spontaneous fluctuations of potential or current noise of electrochemical systems. Since this technique provides a non-destructive condition for investigating corrosion processes, it can be useful to study the electrochemical oxidation of mineral sulfides by microorganisms, a process known as bacterial leaching of metals. This technique was utilized to investigate the dissolution of a bornite electrode in the absence (first 79 h) and after the addition of Acidithiobacillus ferrooxidans (next 113 h) in salts mineral medium at pH 1.8, without addition of the energy source (Fe²⁺ ions) for this chemolithotrophic bacterium. Potential and current noise data have been determined simultaneously with two identical working bornite electrodes which were linked by a zero resistance ammeter (ZRA). The mean potential, E_coup, coupling current, I_coup, standard deviations of potential and current noise fluctuations and noise resistance, R_n, have been obtained for coupled bornite electrodes. Noise measurements were recorded twice a day in an unstirred solution at 30 °C. Significant changes in these parameters were observed when the A. ferrooxidans suspension was added, related with bacterial activity on reduced species present in the sulfide moisture (Fe²⁺, S²⁻). ENA was a suitable tool for monitoring the changes of the corrosion behavior of bornite due to the presence of bacterium.     

Adhesion to metal sulfide surfaces by cells of Acidithiobacillus ferrooxidans,Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans

Attachment of four strains of Acidithiobacillus ferrooxidans to pyrite, chalcopyrite, galena, sphalerite or quartz was found to be mineral-selective. The bacterial extracellular polymeric substances (EPS) are responsible for mediating this process. Attachment of cells of A. ferrooxidans as well as of Acidithiobacillus thiooxidans was diminished, when depleted of their EPS. After 5 days of cultivation cells of A. ferrooxidans cover mineral surfaces with a dense biofilm, as visualised by fluorescence microscopy and AFM. Primary attachment was restricted to surface sites with visible defects.Chemical analyses of EPS of A. ferrooxidans, A. thiooxidans and Leptospirillum ferrooxidans indicated neutral sugars, fatty acids and uronic acids. The composition differed with the strain and the growth substrate. IronIII ions were only detectable in EPS of ironII ion- and pyrite-grown cells, but not in EPS of sulfur grown cells. Pyrite oxidation rates correlated with the amount of EPS-complexed ironIII ions in the case of A. ferrooxidans and L. ferrooxidans. Furthermore, pyrite oxidation rates of L. ferrooxidans were correlated with the genetic affiliation of the strains. The data for A. ferrooxidans seem to indicate a similar correlation, however, the results were not as clear-cut as those obtained for L. ferrooxidans. Sulfur oxidation rates of A. thiooxidans did not require EPS complexed ironIII ions.     

Silicate mineral dissolution in the presence of acidophilic microorganisms: Implications for heap bioleaching

Silicate minerals are found with sulfide minerals and therefore, can be present during heap bioleaching for metal extraction. The weathering of silicate minerals by chemical and biological means is variable depending on the conditions and microorganisms tested. In low pH metal rich environments their dissolution can influence the solution chemistry by increasing pH, releasing toxic trace elements, and thickening of the leach liquor. The amenity of five silicate minerals to chemical and biological dissolution was tested in the presence of either ‘Ferroplasma acidarmanus’ Fer1 or Acidithiobacillus ferrooxidans with olivine and hornblende being the most and least amenable, respectively. A number of the silicates caused the pH of the leach liquor to increase including augite, biotite, hornblende, and olivine. For the silicate mineral olivine, the factors affecting magnesium dissolution included addition of microorganisms and Fe²⁺. XRD analysis identified secondary minerals in several of the experiments including jarosite from augite and hornblende when the medium contained Fe²⁺. Despite acidophiles preferentially attaching to sulfide minerals, the increase in iron coupled with very low Fe²⁺ concentrations present at the end of leaching during dissolution of biotite, olivine, hornblende, and microcline suggested that these minerals supported growth. Weathering of the tested silicates would affect heap bioleaching by increasing the pH with olivine, fluoride release from biotite, and production of jarosite during augite and hornblende dissolution that may have caused passivation. These data have increased knowledge of silicate weathering under bioleaching conditions and provided insights into the effects on solution chemistry during heap bioleaching.     

Does aporusticyanin mediate the adhesion of Thiobacillus ferrooxidans to pyrite?

The adhesion of Thiobacillus ferrooxidans to pyrite was quantified by electrical impedance measurements. Cells grown on soluble iron adhered specifically and with high affinity to pyrite, exhibiting an equilibrium dissociation constant of 5×10⁻¹⁵ M cells. Purposeful manipulation of individual cells using optical trapping techniques revealed that 92% of the iron-grown cells adhered to pyrite with a force greater than 5.2 pN, the maximum force exerted by the trap. In contrast, cells grown on sulfur adhered to pyrite with lower affinity, and 91% of sulfur-grown cells were dissociated from pyrite with an average force of 3.6 pN. Purified recombinant aporusticyanin and intact cells of T. ferrooxidans showed an identical pattern of adhesion to the same minerals. The addition of ferrous ions or organic chelators to the binding mixture prevented the binding of either aporusticyanin or intact cells to pyrite. Preincubation of either the pyrite alone or both the pyrite and the cells with exogenous aporusticyanin inhibited the adhesion of cells to pyrite by 41% and 60%, respectively. A His85Ala mutant apoprotein bound much less tightly to pyrite than did the wild type aporusticyanin. These observations are consistent with a model where aporusticyanin located on the surface of the bacterial cell acts as a mineral-specific receptor for the initial adhesion of T. ferrooxidans to pyrite. Binding of the apoprotein to solid pyrite is accomplished in part by coordination of the unoccupied copper ligands with an iron atom at the exposed edge of the pyrite crystal lattice.