(752e) Bioleaching of Metals From Spent Refinery Catalysts Using Acidithiobacillus Thiooxidans and Chelating Agent

Bioleaching of metals from hazardous spent hydro-processing catalyst was attempted in the second stage after growing the bacteria with sulfur in the first stage. The first stage involved transformation of elemental sulfur particles to sulfuric acid through an oxidation process by acidophilic bacteria (Acidithiobacillus thiooxidans). In the second stage, the acidic medium was utilized for the leaching process. Nickel and Molybdenum containing spent catalyst were chelated with EDTA and leached from the solid materials to liquid medium by the action of sulfuric acid that was produced by acidophilic leaching bacteria and compared with chemical leaching. Involved microorganisms were sulfur oxidizing bacteria. Experiments were conducted varying the reaction time, amount of spent catalysts, amount of elemental sulfur and temperature. At 50 g/L spent catalyst concentration and 20 g/L elemental sulfur, 84.3% Ni, 46.3% Mo, were recovered after 7 hrs of heating at 100⁰C. Chemical leaching with commercial sulfuric acid of the similar amount that produced by bacteria was compared. Thermodynamic parameters were calculated and the nature of reaction was found to be exothermic. Leaching kinetics of the metals was represented by different reaction kinetic equations, however, only diffusion controlled model showed the best correlation here. During the whole process Mo showed low dissolution because of substantiate precipitation with leach residues as MoO3. Bioleach residues were characterized by EDX and XRD.