Treatment of copper cyanide solutions with the use of sulfate-reducing bacteria

The study was aimed at developing a technology for regenerating cyanide and precipitating copper from copper cyanide solutions through the sulfate reduction process. To this end, a mixture of strains of anaerobic sulfate-reducing bacteria was used: Desulfonatronum zhilinae, Desulfonatronum cooperativum, and Desulfonatronobacter acetoxidans from the S.N. Vinogradsky Institute of Microbiology of the Russian Academy of Sciences (Moscow). The sulfate reduction was carried out at temperatures of 20–40℃ and at pH>9.5. Ethanol was used as the electron donor, and sulfate ions were used as the acceptor. In order to ascertain the limiting substrate (ethanol or sulfate) and establish the optimal concentrations of the acceptor and the electron donor, a mathematical microbial growth model was used– the Monod equation. The calculation results indicate substrate competition for the right to limit the process. Thus, at concentrations of <0.3 g/dm3, the limiting substrate is sulfate, whereas at sulfate concentrations of 0.5–1.0 g/dm3 and ethanol concentrations of 0.1–0.3 g/dm3, the limiting substrate is ethanol. The co-limitation point of the process was determined; at this point, the concentrations of sulfate and ethanol are 0.8 and 0.3 g/dm3, respectively. In order to ascertain the hydraulic retention time of the liquid phase in a bioreactor, the Monod equation was used, taking inhibition by hydrogen sulfide into account. The presence of 0.1–0.5 g/dm3 hydrogen sulfide in bacterial solution was found to reduce the bacterial growth rate by 27–65%. The hydraulic retention time of the liquid phase in the bioreactor at the co-limitation point, taking the inhibition by hydrogen sulfide into account, should be equal to approximately 90 hours. Laboratory tests show the calculated hydraulic retention time to be sufficient to obtain 0.25–0.27 g/dm3 hydrogen sulfide for 99% copper precipitation and over 99% cyanide regeneration. The obtained copper precipitates contained copper and sulfur (65% and 35%, respectively). Thus, the examined microorganisms allow hydrogen sulfide to be obtained directly in copper cyanide solutions with different copper concentrations, which eliminates the need for a bioreactor and all auxiliary communications for transporting hydrogen sulfide.

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