(570a) Bioleaching of Nickel From Spent Catalyst Using Acidophilic Bacteria

Bioleaching of Nickel from spent catalyst of hydro-processing catalyst was attempted in the second stage after growing the bacteria with sulfur in the first stage. The bacteria utilise elemental sulfur (So) for growth and produce sulfuric acid as key metabolite. When grown on elemental sulfur, Acidithiobacilli produce several intermediate sulfur species with high reducing power including thiosulfate and sulfite. The sulfite so formed can be oxidized by oxygen action catalysed by bacteria. Such bacteria can thrive on low pH and tolerate harsh conditions. The effect of time, spent catalyst concentration, amount of elemental sulfur and reaction temperature were studied in detail for two step process. Chemical leaching with sulfuric acid was compared with bacterially produced acid solution, and suitability of sulfur oxidizing Acidithiobacilli for leaching of Nickel from spent catalyst was studied. Process involved transformation of elemental sulfur particles to sulfuric acid through an oxidation process by Acidithiobacilli bacteria. The growth of bacteria was carried out using 10g/L elemental sulfur as the substrate. The initial pH of the medium decreased with increase of incubation period. The cell number attained stationary state after one week of incubation. The oxidation of colloidal sulfur particles decreased the pH of the solution (pH= 0.9-1.0) due to formation of sulfate (SO4 2- ) ion. In the second stage, the acidic medium was utilized for the leaching process. Spent catalyst containing Nickel as NiO in 20%, CaO in 5%, and Al2O3 75% was obtained from petroleum refinery. Experiments were conducted at varying reaction time, solid/liquid ratio, amount of sulfur and temperature. At 1:20 spent catalyst concentration and 1:40 solid liquid ratio elemental sulfur, 88.3% Ni was recovered after 4 hrs of stirring at 320 C.