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 degreesC was effe ctive in removing carbon and sulfur in both wastes followed by generation o f heavy metal oxides. A two-stage leaching process was successful in removi ng 44% nickel, 77% cobalt, and 83% molybdenum. However, toxicity characteri stic leaching procedure leaching tests for spent catalyst residues of Co/Mo /gamma Al2O3 and Ni/ Mo/gamma Al2O3 after the two-stage leaching process sh owed that the concentration of Mo, Co, and Ni in the leachate exceeded the allowable limits. Finally, the residues, when subjected to an enhanced bind er stabilization treatment with marine clay, produced a commercial-value br ick made of up to 30% by weight spent catalysts. It is believed that stabil ization can be achieved through either encapsulation whereby the clay forms a matrix around the heavy metals to prevent leaching or incorporation in w hich the heavy metals actually become part of the stabilizing matrix.