Strategies for efficient start-up of continuous biooxidation process for refractory gold ores

Strategies for efficient start-up of a continuous process for biooxidation of refractory gold ore and concentrate obtained from Hutti, Gold Mines Limited (HGML), India are discussed in this work. The biooxidation of the concentrate at high pulp density (10%) with wild strain of Thiobacillus ferrooxidans isolated from HGML mines is characterized by significant lag phase (20 days) and incomplete oxidation (35%) even after prolonged operation (60 days). Two strategies, biooxidation with concentrate adapted cells and a step leaching strategy, in which the pulp density is progressively increased from 2% to 10% were considered and the latter resulted in efficient biooxidation of concentrate. Conversion of such a process from batch to continuous operation is shown to result in complete biooxidation of the concentrate and gold extraction efficiency in excess of 90%. (C) 2002 Elsevier Science Ltd. All rights reserved.

Adhesion of Acidithiobacillus ferrooxidans to mineral surfaces

Direct contact mechanism in bioleaching implies prior mineral adhesion of Acidithiobacillus ferrooxidans and subsequent enzymatic attack.Prior bacterial adaptation to sulfide mineral substrates influences bacterial ferrous ion oxidation rates. It is highly beneficial to understand major biooxidation mechanisms with reference to solution- and mineral-grown cells in order to optimize bioleaching reactions. For A. ferrooxidans grown in the presence of solid substrates such as sulfur, pyrite and chalcopyrite, bacterial adhesion is required for its enzymatic machinery to come into close contact for mineral dissolution.But when grown in solution substrate such as ferrous ions and thiosulfate, such an adhesion machinery is not required for substrate utilization. Proteinaceous compounds were observed on the surface of sulfur-grown cells. Such an induction of relatively hydrophobic proteins and down regulation of exposed polysaccharides leads to changes in cell surface chemistry. Sulfur-grown and pyrite- and chalcopyrite-grown bacterial cells were found to be more efficient in the bioleaching of chalcopyrite than those grown in the presence of ferrous ions and thiosulfate. (C) 2010 Elsevier B.V. All rights reserved.

Growth and development of Thiobacillus ferrooxidans for engineering applications

A bioprocessing approach for the extraction of base, nuclear and precious metals from refractory and lean grade ores has been reviewed in this paper. Characteristic morphological features of Thiobacillus ferrooxidans, the organism which has been extensively used for biooxidation of sulphide ores have been discussed. Mechanisms of chemoautotrophy and mineral oxidation have been illustrated. The current engineering applications of this microorganism have also been brought out. Various methods for accelerating the growth of Thiobacillus ferrooxidans for faster biooxidation and genetic manipulation for development of desired strains have been outlined.

Immobilized cells of Acidithiobacillus ferrooxidans in PVC strands and sultite removal in a pilot-scale bioreactor

The biooxidation of ferrous ion into ferric ion by Acidithiobacillus ferrooxidans can be potentially used for the removal of H2S from industrial gases. In this work, Fe3+ ions were obtained through the oxidation of Fe2+ using the LR strain of At. ferrooxidans immobilized in PVC stands in a pilot-scale bioreactor, while H2S was removed in an absorption tower equipped with Rasching rings. At. ferrooxidans LR strain cells were immobilized by inoculating the bacterium in a Fe2+-mineral medium and percolating it through the support. After complete Fe2+ oxidation, which took around 90 h, the reactor was washed several times with sulfuric acid (pH 1.7) before a new cycle was started. Four additional cycles using fresh Fe2+ mineral medium were then run. During these colonization cycles, the time required for complete iron oxidation decreased, dropping to about 60 h in the last cycle. The batch experiments in the H2S gas removal trials resulted in a gas removal rate of about 98-99% under the operational conditions employed. In the continuous experiments with the bioreactor coupled to the gas absorption column, a gas removal efficiency of almost 100% was reached after 500 min. Precipitate containing mainly sulfur formed during the experimental trial was identified by EDX. (c) 2005 Elsevier B.V. All rights reserved.

Jarosite pseudomorph formation from arsenopyrite oxidation using Acidithiobacillus ferrooxidans

In this work, the oxidizing action of a native strain type A. ferrooxidans on a sulphide containing a predominance of arsenopyrite and pyrite has been evaluated. Incubation of the A. ferrooxidans strain in flasks containing 200 mL of T&K medium with the ore (particle size of 106 mu m) at pulp density 8% (w/v) at 35 degrees C on a rotary shaker at 200 rpm resulted in preferential oxidation of the arsenopyrite and the mobilization of 88% of the arsenic in 25 days. Mineralogical characterization of the residue after biooxidation was carried out with FTIR. XRD and SEM/XEDS techniques. An in situ oxidation of the arsenopyrite is suggested on the basis of the frequent appearance of jarosite pseudomorph replacing arsenopyrite, in which the transformations Fe(2+) -> Fe(3+), S(-2) -> S(+6) and As(-1) -> As(+3) -> As(+5) occur for the most part without formation of soluble intermediates, resulting in a type of jarosite that typically contains high concentrations of arsenic (type A-jarosite). However, during pyrite oxidation, dissolution of the constituent Fe and S predominates, which is evidenced by corrosion of pyrite particles with formation of pits, generating a type of jarosite with high quantities of K (type B-jarosite). Lastly, a third type of jarosite (type C-jarosite) also precipitated forming a thin film that covered the grains of pyrite principally. (C) 2010 Elsevier B.V. All rights reserved.

Effect of sulfide concentration on autotrophic denitrification from nitrate and nitrite in vertical fixed-bed reactors

Autotrophic denitrification coupled with sulfide oxidation represents an interesting alternative for the simultaneous removal of nitrate/nitrite and sulfide from wastewaters. The applicability of such bioprocess is especially advantageous for the post treatment of effluents from anaerobic reactors, since they usually produce sulfides, which can be used as endogenous electron donor for autotrophic denitrification. This study evaluated the effect of sulfide concentration on this bioprocess using nitrate and nitrite as electron acceptors in vertical fixed-bed reactors. The results showed that intermediary sulfur compounds were mainly produced when excess of electron donor was applied, which was more evident when nitrate was used. Visual evidences suggested that elemental sulfur was the intermediary compound produced. There was also evidence that the elemental sulfur previously formed was being used when sulfide was applied in stoichiometric concentration relative to nitrate/nitrite. Nitrite was more readily consumed than nitrate. For both electron acceptors and sulfide concentrations tested, autotrophic denitrification was not affected by residual heterotrophic denitrification via endogenic activity, occurring as a minor additional nitrogen removal process. (C) 2012 Elsevier Ltd. All rights reserved.

Archaeal diversity in two thermophilic chalcopyrite bioleaching reactors

This study used a culture-independent molecular approach to investigate the archaeal community composition of thermophilic bioleaching reactors. Two culture samples, MTC-A and MTC-B, grown with different concentrations of chalcopyrite (CuFeS2), a copper sulfidic ore, at a temperature of 78 degrees C and pH 1.6 were studied. Phylogenetic analysis of the 16S rRNA genes revealed that both cultures consisted of Archaea belonging to the Sulfolobales. The 16S rRNA gene clone library of MTC-A grown with 4% (w/v) chalcopyrite was dominated by a unique phylotype related to Sulfolobus shibatae (69% of total clones). The remaining clones were affiliated with Stygiolobus azoricus (11%), Metallosphaera sp. J1 (8%), Acidianus infernus (2%), and a novel phylotype related to Sulfurisphaera ohwakuensis (10%). In contrast, the clones from MTC-B grown with 12% (w/v) chalcopyrite did not appear to contain Sulfolobus shibatae-like organisms. Instead the bioleaching consortium was dominated by clones related to Sulfurisphaera ohwakuensis (73.9% of total clones). The remaining microorganisms detected were similar to those found in MTC-A.