Bioleaching of spent hydrotreating catalyst by thermophilic and mesophilic acidophiles: Effect of decoking

Abstract

Spent hydrotreating catalyst is a major solid waste from the petroleum industry. Although bioleaching of these wastes has been reported, research to date has focused on mesophiles. Bioleaching of industrial wastes by thermophiles has not been investigated. The bioleaching ability of thermophilic archaea Acidianus brierleyi was examined and compared with the commonly used mesophile Acidithiobacillus thiooxidans to establish the potential for high temperature leaching of spent catalyst. Spent Ni-Mo catalyst embedded in an alumina matrix containing substantial amounts of coke (i.e. carbonaceous deposits and volatile foulants) was used in this study. Decoking (i.e. removal of these deposits) as a pre-treatment and its effect on the leaching characteristics were examined. Spent catalyst is considered a hazardous waste since the toxic metal concentration exceeded the regulatory limit for land disposal and failed the toxicity characteristic leaching procedure (TCLP) test. Bioleaching with A. brierleyi and At. thiooxidans reduced the heavy metal leachate concentration to below regulated levels for safe waste disposal. Leaching efficiencies of 31% Al, 65% Ni and 44% Mo were achieved for coked catalyst using spent medium from At. thiooxidans. With decoked catalyst and spent medium from A. brierleyi, higher leaching efficiencies of 76% Al, 98% Ni and 93% Mo were attained. These results demonstrate the feasibility and the higher efficacy of spent medium leaching of decoked catalyst by acidophilic thermophiles. © (2013) Trans Tech Publicutions, Switzerland.