Autotrophic acidophilic bacteria Acidithiobacillus ferrooxidans is a model species for studying metal bioleaching from low-grade sulfide ores and concentrates. Arsenopyrite gold-bearing concentrates are refractory and often processed using biohydrometallurgical approaches; therefore, it is important to develop methods to improve arsenopyrite bioleaching. In the present work, we have studied the possibility of improving arsenopyrite concentrate bioleaching by the strain of A. ferrooxidans. For this purpose, we have analyzed the genome of the strain A. ferrooxidans TFBk to reveal the genes potentially important in the bioleaching process. Genes determining resistance to arsenic, as well genes involved in the utilization of C1-compounds and resistance to oxidative stress, were revealed. Therefore, the possibility of increasing the rate of arsenopyrite concentrate bioleaching using C1-compounds (methanol and formate) was studied. Formate was able to increase both the biomass yield of the strain A. ferrooxidans TFBk as well as the bioleaching rate. In addition, the effect of redox potential increase by means of the addition of sodium persulfate in the medium on arsenopyrite concentrate bioleaching was studied. It was shown that the addition of 0.1% sodium persulfate stimulated strain growth, while a higher concentration inhibited it. Despite this, the rate of concentrate bioleaching increased in the presence of 0.5–1.0% of persulfate, which may be explained by the interactions of added oxidizer with concentrate components.
Hydrometallurgical production of valuable and non-ferrous metals is traditionally accompanied with acid waste effluents/acid mine drainage leading to acidification of the mining areas. The traditional cause of this pollution is the well-known technology based on the recovery of metals with acid solutions and the application of strong acidophilic leaching bacteria for the oxidation of sulfide ores. In our experiments, we used neutrophilic autotrophic bacteria (NAB) stimulated with formic acid or coupled with acidophilic bacteria. The first approach was based on using formic acid as an energetic substrate by autotrophic bacteria. In the second case, the NAB provided initial biogenic acidification for the following growth of the inoculated acidophilic bacteria. Our experiments resulted in increased nickel recovery from the low-grade sulfide ores, which was provided by the NAB in a medium supplemented with formic acid. Bioleaching resulted in 1116 mg Ni/L (69.75%) in the medium with formate and only 35.4 mg Ni/L without formate in 43 days. As a whole, our bench scale experiments showed that the stimulated NAB can be effective at pH 7–5. Partially replacing sulfuric acid with formic acid could also give benefits via the following natural degradation of acid wastes. As a whole, this approach is more environmentally friendly than conventional bioleaching techniques.
Acid metal bioleaching is common and classical for nickel recovery from the sulfide refractory ores: various microorganisms can oxidize sulfides as energetic substrates. Silicate nickel ores are widespread in the world but their bioleaching is more problematic because silicates cannot serve as energetic substrates. Meanwhile iron in the silicate nickel ores presents a significant part and can be used by some acidophilic autotrophic microorganisms for the ore destruction. In model experiments, we studied application of acidophilic autotrophic sulfur-/ iron-oxidizing bacteria Acidithiobacillus ferrooxidans VKM B-3655 for the nickel recovery from the nickel-bearing silicate ore with high content of iron. The strain was selected by its ability of iron oxidation and resistance to arsenic which also presented in the ore. We also evaluated possibility to stimulate the bioleaching with formate as additional energetic substrates or with persulfate for increasing the medium redox. It was shown that low concentrations of sodium formate (0.3%) and persulfate (0.1%) stimulated growth of A. ferrooxidans while higher persulfate concentration (1.0%) stimulated the ore bioleaching.
Hydrometallurgical mining of valuable and non-ferrous metals is traditionally accompanied by a large-scale pollution of the territories with sulfuric acid. This pollution is the global problem and requires the significant remediation costs. There are known some attempts to replace sulfuric and other strong inorganic acids with organic acids which could be easier utilized under natural conditions. However, this approach proved to be ineffective due to the weak leaching effect of organic acids. In our experiments on chemical leaching, we investigated leaching of nickel from low-grade silicate ores with a mixture of organic acids and persulfate. Organic acids ensured both the acidic reaction of the leaching solution while persulfate provided a short-term formation of persulfuric (peroxysulphuric) acid, which is the stronger leaching agent than sulfuric acid. In whole, experimental results showed that metal leaching can be intensified with application of organic acids.
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