Optimization of thermal pre-treatment for simultaneous and efficient release of both Co and Mo from used Co–Mo catalyst by bioleaching and their mechanisms

There are a number of attempts to release and recover the valuable metals from spent refinery catalysts by bioleaching, and the heat pretreatment is widely used to remove the oil-like compounds for better bioleaching performance. However, the irrational selection of high calcinations temperature of 600–800 °C not only resulted in high energy consumption but also might be cause lower bioleaching efficiencies of metals. In this work, Plackett-Burman design was used to optimize the heat treatment process and recognize the significant control factor. Especially, the mechanisms why low calcinations temperature of 400 °C had the best enhancement on the bioleaching performance of spent catalyst were expounded via microcosmic phase change analysis, biochemical leaching process analysis and bioleaching kinetics analysis. The results showed that the calcinations temperature greatly affected the bioleaching performance and the calcinations of 400 °C witnessed the maximum release efficiencies of 94% for Co and 100% for Mo, respectively; whereas poor bioleaching occurred with 200 or 600 °C. Further studies demonstrated the 400 °C-treatment removed the toxic oil-like compounds, excited the growth of cells, transformed refractory Co/Mo sulfides into tractable Co₃O₄/MoO₃, reduced the percent of residual from, enabled more exposure of Co/Mo oxides at catalyst surface, which caused the best bioleaching performance.