Recovery and Marine Clay Stabilization of Heavy Metals Present in Spent Hydrotreating Catalysts

This paper presents the results of an investigation on the treatment of two industrial solid wastes produced by oil refineries in Singapore, namely Ni/Mo/(gamma)Al2O3 and Co/Mo/(gamma)Al2O3. Calcination at 500°C was effective in removing carbon and sulfur in both wastes followed by generation of heavy metal oxides. A two-stage leaching process was successful in removing 44% nickel, 77% cobalt, and 83% molybdenum. However, toxicity characteristic leaching procedure leaching tests for spent catalyst residues of Co/Mo/(gamma)Al2O3 and Ni/Mo/(gamma)Al2O3 after the two-stage leaching process showed that the concentration of Mo, Co, and Ni in the leachate exceeded the allowable limits. Finally, the residues, when subjected to an enhanced binder stabilization treatment with marine clay, produced a commercial-value brick made of up to 30% by weight spent catalysts. It is believed that stabilization can be achieved through either encapsulation whereby the clay forms a matrix around the heavy metals to prevent leaching or incorporation in which the heavy metals actually become part of the stabilizing matrix.