Bioleaching of organic carbon rich polymetallic black shale

Abstract

The present study describes the extraction of metals from organic-carbon rich Kyrk Tåsjö (Sweden) polymetallic black shale using mixed cultures of acidophilic iron- and sulfur-oxidizing microorganisms. Quartz, illite, microcline, calcite, dolomite and pyrite minerals were present in shale matrix. Black shale contained 10.77% organic carbon as kerogen and 1.16% inorganic carbon (graphite). The leaching experiments were performed in shake flasks and stirred tank reactors with and without acidophilic Fe- and S-oxidizing psychrotolerant, mesophile and moderate thermophile strains at 6, 30 and 45°C. Biological oxidation of pyrite generated sulfuric acid and ferric sulfate in leach solutions during leaching process. Microbial leaching solubilized 80–90% of the total metals (U, Cu, Ni, Mn, Mo, Y and Zn) after 15–20days of bioleaching at 30 and 45°C; whereas metal solubilization was slower with acidophilic psychrotolerant bacteria at 6°C. The biodegradation of kerogen released tetradecane (CH3(CH2)12CH3), a long-chain aliphatic hydrocarbon compound and several other un-identified hydrocarbons in leach solutions during bioleaching of black shale. The addition of PO43− and NH4+ in the growth medium during bioleaching had no effect or decreased the metal solubilization, suggesting that the microorganisms obtained these nutrients from the minerals and kerogen (C100H112O9N2S5), a nitrogenous hydrocarbon compound present in the shale matrix. Metal dissolution from black shale was mainly attributed to the acid concentration in leach solution and temperature. The leaching data demonstrate the feasibility of extracting metals from the black shale without additional nutrient supply that constitute a cost saving for commercial scale application of bioleaching process. The bioleaching approach does not appear warranted to view of the low concentrations, albeit relatively high recoveries of valuable metals from the black shale. The leaching data indicate that exposed black shale occurrences, being subject to ambient weather conditions, constitute a long term environmental challenge.