Effect of Carbon Sources on Pyrite-Arsenopyrite Concentrate Bio-oxidation and Growth of Microbial Population in Stirred Tank Reactors

Biooxidation of a Pyrite-Arsenopyrite Concentrate Under Stressful Conditions

Gold recovery from refractory pyrite-arsenopyrite concentrates using stirred tank reactor biooxidation is widely applied worldwide. Therefore, studies to address the characteristic problem of this technology are urgent. The goal of the present work was to research the possibility of counteracting the negative effects of unfavorable conditions (increasing pulp density and temperature) on the biooxidation of pyrite-arsenopyrite concentrate in laboratory-scale stirred tank reactors using additional carbon supply in the form of CO2. A refractory concentrate containing pyrite (48%) and arsenopyrite (13%) was used in biooxidation experiments. In the control experiment, biooxidation was performed under "normal conditions": temperature 40 °C, pulp density (solid to liquid ratio, S:L) 1:10, residence time 5 days. It was shown that under "normal conditions", additional carbon dioxide supply insignificantly affected the biooxidation rate and composition of the microbial population of biooxidation reactors. In addition, the effect of "stressful conditions" was studied. In this case, either temperature or pulp density were increased (up to 50 °C and S:L 1:5, respectively), which provided unfavorable conditions for biooxidation and led to the decrease in biooxidation rate. Under "stressful conditions", additional carbon dioxide supply affected biooxidation to a greater extent and made it possible to increase both pyrite and arsenopyrite biooxidation rates. The analysis of microbial populations showed that additional carbon dioxide supply also changed their composition.

Shifts in the Microbial Populations of Bioleach Reactors Are Determined by Carbon Sources and Temperature

In the present study, the effect of additional carbon sources (carbon dioxide and molasses) on the bio-oxidation of a pyrite-arsenopyrite concentrate at temperatures of 40-50 °C was studied, and novel data regarding the patterns of the bio-oxidation of gold-bearing sulfide concentrates and the composition of the microbial populations performing these processes were obtained. At 40 °C, additional carbon sources did not affect the bio-oxidation efficiency. At the same time, the application of additional carbon dioxide improved the bio-oxidation performance at temperatures of 45 and 50 °C and made it possible to avoid the inhibition of bio-oxidation due to an increase in the temperature. Therefore, the use of additional carbon dioxide may be proposed to prevent the negative effect of an increase in temperature on the bio-oxidation of sulfide concentrates. 16S rRNA gene profiling revealed archaea of the family Thermoplasmataceae (Acidiplasma, Ferroplasma, Cuniculiplasma, and A-plasma group) and bacteria of the genera Leptospirillum, with Sulfobacillus and Acidithiobacillus among the dominant groups in the community. Temperature influenced the composition of the communities to a greater extent than the additional sources of carbon and the mode of operation of the bioreactor. Elevating the temperature from 40 °C to 50 °C resulted in increases in the shares of Acidiplasma and Sulfobacillus and decreases in the relative abundances of Ferroplasma, Leptospirillum, and Acidithiobacillus, while Cuniculiplasma and A-plasma were more abundant at 45 °C. A metagenomic analysis of the studied population made it possible to characterize novel archaea belonging to an uncultivated, poorly-studied group of Thermoplasmatales which potentially plays an important role in the bio-oxidation process. Based on an analysis of the complete genome, we propose describing the novel species and novel genus as "Candidatus Carboxiplasma ferriphilum" gen. nov., spec. nov.

Options for Increasing the Rate of Bioleaching of Arsenic Containing Copper Concentrate

In the present work the effect of alkaline sulfide leaching (ASL) on the extraction of copper and zinc from low-grade copper concentrate containing chalcopyrite, tennantite, sphalerite, and pyrite during batch and continuous bioleaching experiments was studied. It was demonstrated that ASL and further bioleaching may be a promising approach for treatment of copper–zinc concentrates containing tennantite as this approach allows increasing copper extraction degree in comparison to one-stage bioleaching by 1.6–2.3 times. Thus, ASL was shown to be effective for pretreatment of tennantite containing concentrates to improve bioleaching for copper extraction. At the same time, ASL led to decrease in zinc extraction by 1.4–1.5 times. Therefore, the development of combined hydrometallurgical processes including ASL and bioleaching for effective metal leaching requires further studies to avoid negative effect on zinc extraction.

Continuous Bioleaching of Arsenic-Containing Copper-Zinc Concentrate and Shift of Microbial Population under Various Conditions

The goal of this work was to study the bioleaching of arsenic-containing polymetallic concentrate that contained 6.2% Cu, 7.3% Zn and 1.7% As, depending on different temperatures and in the presence of CO2 and molasses in the medium, as well as the difference in the composition of microbial population formed under various conditions. A mixed population of moderately thermophilic and thermotolerant acidophilic microorganisms formed during the continuous bioleaching of copper concentrate was used as an inoculum. The experiments were carried out in a continuous mode in laboratory scale reactors, with a temperature range of 40 °C to 60 °C. To assess the effect of CO2 and molasses on metal leaching and microbial population composition, the experiments were carried out in three reactors: CO2 (~0.01 L/min) was supplied into the first reactor; 0.02% molasses were added to the pulp of the second reactor; and no additional carbon sources were supplied into the control reactor. The highest copper recovery (27%) was achieved at 50°C in the experiment with molasses, while the highest zinc recovery (82.1%) was reached at 45°C in the control experiment. Additional carbon sources affected the extraction of non-ferrous metals only at 60 °C and increased the extraction of copper and zinc by 12.6% and 24.2%, respectively. Both the temperature and carbon source used affected the microbial population composition. The main microbial genera revealed in the populations by next generation sequencing (NGS) were bacteria of the genera Sulfobacillus and Acidithiobacillus, as well as archaea of the genera Ferroplasma, Acidiplasma, and Cuniculiplasma. At low temperatures (40 and 45 °C), Acidithiobacillus, Sulfobacillus, and Ferroplasma predominated, while at temperatures 50–55 °C, the decrease in relative abundance of these genera occurred, and the predominance of Acidiplasma archaea was observed. The usage of both CO2 and molasses led to the increase in Sulfobacillus and Acidiplasma in relative abundance.

Non-Ferrous Metals and PGM Recovery from Low-Grade Copper–Nickel Concentrate by Bioleaching and Further Cyanidation

The aim of the present work was to perform copper, nickel, and platinum group metals (PGMs) recovery from low-grade copper–nickel concentrate containing pyrrhotite, pentlandite, and chalcopyrite by bioleaching in stirred tank reactors in batch mode and subsequent cyanidation. The concentrate contained (%) Fe 32.7, Cu 0.7, Ni 2.3, Stotal 20.9, Ssulfide 17, 0.1 g/t Pt, and 1.35 g/t Pd. The bioleaching was performed at 30 and 40 °C using two different microbial consortia. At 30 °C, bioleaching was performed using mixed culture including Acidithiobacillus ferrivorans strains isolated from the sample of acid mine drainage from copper–nickel deposit. At 40 °C, bioleaching was performed using microbial population formed during long-term bioleaching of copper-zinc concentrate at 40 °C. Bioleaching was performed for 40 days at pulp density of 10% (solid to liquid ratio 1:10). At 30 °C, 70% Ni and 14% Cu were leached, while 72% Ni and 34% Cu were recovered in the solution at 40 °C. PGM were extracted from the concentrate and bioleaching residue obtained at 40 °C by cyanidation. Cyanidation made it possible to extract 5.5% Pt and 17.3% Pd from the concentrate and 37.8% Pt and 87.8% Pd from the bioleaching residue. Thus, it was shown that the concentrate studied might be processed using bioleaching and subsequent cyanidation to extract both non-ferrous metals and PGM.

Carbon Sources as a Factor Determining the Activity of Microbial Oxidation of Sulfide Concentrate at Elevated Temperature

The goal of the present work was to evaluate the possibility of improving the efficiency of the stirred tank reactor biooxidation of sulfide gold-bearing concentrate by means of addition of carbon sources required for the constructive metabolism of microorganisms. Biooxidation experiments were performed on gold-bearing pyrite-arsenopyrite concentrate in continuous mode at 45 °C to determine the influence of additional carbon sources (carbon dioxide and molasses) on sulfide mineral oxidation. The use of CO2 allowed increasing the efficiency of the biooxidation and the extents of sulfide sulfur (Ss) oxidation and gold recovery were 79% and 84%, respectively. Biooxidation in a control experiment (without additional carbon sources) and when using molasses allowed achieving 39% and 66% oxidation of Ss as well as 73% and 81% of gold recovery. Analysis of the microbial populations formed in biooxidation reactors using NGS methods demonstrated that CO2 application led to an increase in the relative abundance of the genus Sulfobacillus. Thus, it was determined that application of additional carbon source makes it possible to manage the biooxidation process, affecting both sulfide mineral oxidation and microbial population composition.