Sequential Bioleaching of Pyritic Tailings and Ferric Leaching of Nonferrous Slags as a Method for Metal Recovery from Mining and Metallurgical Wastes

In this work, we proposed a method for biohydrometallurgical processing of mining (old pyritic flotation tailings) and metallurgical (slag) wastes to recover gold and other nonferrous metals. Since this processing allows the removal of toxic metals or at least decreases their content in the solids, this approach may reduce the negative environmental impacts of such waste. The proposed process was based on pyritic tailings’ bioleaching to recover metals and produce leach liquor containing a strong oxidizing agent (ferric sulfate) to dissolve nonferrous metal from slag. This approach also allows us to increase concentrations of nonferrous metals in the pregnant leach solution after pyritic waste bioleaching to allow efficient extraction. The old pyritic tailings were previously leached with 0.25% sulfuric acid for 10 min to remove soluble metal sulfates. As a result, 36% of copper and 35% of zinc were extracted. After 12 days of bioleaching with a microbial consortium containing Leptospirillum spp., Sulfobacillus spp., Ferroplasma spp., and Acidithiobacillus spp. at 35 °C, the total recovery of metals from pyritic tailings reached 68% for copper and 77% for zinc; and subsequent cyanidation allowed 92% recovery of gold. Ferric leaching of two types of slag at 70 °C with the leachate obtained during bioleaching of the tailings and containing 15 g/L of Fe3+ allowed 88.9 and 43.4% recovery of copper and zinc, respectively, from copper slag within 150 min. Meanwhile, 91.5% of copper, 84.1% of nickel, and 70.2% of cobalt were extracted from copper–nickel slag within 120 min under the same conditions.

Distinct Roles of Acidophiles in Complete Oxidation of High-Sulfur Ferric Leach Product of Zinc Sulfide Concentrate

A two-step process, which involved ferric leaching with biologically generated solution and subsequent biooxidation with the microbial community, has been previously proposed for the processing of low-grade zinc sulfide concentrates. In this study, we carried out the process of complete biological oxidation of the product of ferric leaching of the zinc concentrate, which contained 9% of sphalerite, 5% of chalcopyrite, and 29.7% of elemental sulfur. After 21 days of biooxidation at 40 °C, sphalerite and chalcopyrite oxidation reached 99 and 69%, respectively, while the level of elemental sulfur oxidation was 97%. The biooxidation residue could be considered a waste product that is inert under aerobic conditions. The results of this study showed that zinc sulfide concentrate processing using a two-step treatment is efficient and promising. The microbial community, which developed during biooxidation, was dominated by Acidithiobacillus caldus, Leptospirillum ferriphilum, Ferroplasma acidiphilum, Sulfobacillus thermotolerans, S. thermosulfidooxidans, and Cuniculiplasma sp. At the same time, F. acidiphilum and A. caldus played crucial roles in the oxidation of sulfide minerals and elemental sulfur, respectively. The addition of L. ferriphilum to A. caldus during biooxidation of the ferric leach product proved to inhibit elemental sulfur oxidation.

Bioprocess for Leaching of Copper Concentrate

A bioprocess for oxide-sulfide copper flotation concentrates has been proposed. It includes: i) chemical step – leaching with sulfuric acid solution and subsequent high temperature ferric leaching with microbially produced Fe3+-containing solution, and ii) biological step – bioregeneration of ferric iron along with additional biooxidation of the sulfide minerals using moderately thermophilic acidophilic microorganisms. The flotation copper concentrates contained 27.0–37.4% copper as sulfide (digenite, bornite, etc.) and oxide (malachite, azurite, tenorite, etc.) minerals. The acid leaching under batch conditions at 50°C and pH 1.2 during 22 hours led to 40.6% of copper recovery from the concentrate. Subsequent ferric leaching of the acid leach residue at 80°C, pulp density 9%, initial concentration of Fe3+ 30.7 g/L, and pH 1.2–1.3 during 7 hours increased the total copper recovery to 94.5%. Bioregeneration of the Fe3+ was conducted using moderately thermophilic microorganisms including bacteria of the genus Sulfobacillus at 40°C in the presence of 3% leach residue. The average ferrous iron biooxidation rate and total copper recovery within 2 days were 1.0 g/L∙h and 97%, respectively.Leaching of copper under semi-continuous conditions with bioregeneration of Fe3+ at 40°C was studied. It was found that copper recovery achieved 90% within 22 hours and the average oxidation rate of ferrous iron was up to 0.95 g/L·h.