Bioleaching of Molybdenum from a Coal Liquefaction Catalyst Residue

bioleaching, molybdenum disulfide, catalyst residue Molybdenum disulfide, MoSz, is used as a catalyst in coal-oil coprocessing. For most processes using molybdenum catalysts to be economical, the molybdenum must be recovered and recycled. Chemical recovery schemes have been devised, but typically recover only a fraction of the molybdenum. There are reports in the literature that bacteria can oxidatively solubilize MoSz (molybdenite) ore. This paper reports the initial results of experiments where Thiobacillus ferrooxidans are shown to microbially solubilize MoSZ present in coal liquefaction catalyst residues. Small-scale shake-flask experiments show that after leaching of the catalyst residue by ferrooxidans at pH -2 f o r 6-7 weeks, as much as 66-71% o f the Ma can be solubilized. Analyses of the microbially-leached products and mass-balance calculations can account for 78-103% of the Mo initially present in the coal liquefaction residue. INTRODUCTION Molybdenum disulfide is used as a catalyst in coal-oil coprocessing. It is effective at concentrations as low as ZOO ppm Mo by weight, based on coal (ref. 1). Molybdenum is added to the coal in the form of aqueous solutions of ammonium heptamolybdate, (NH,,) 6M0702 + , or ammonium tetrathiomolybdate (NH!)zMoS*. The MoSz catalyst i s then formed in situ at coal liquefaction conditions at about 350°C, in the presence of hydrogen. Even though MoSz is used at low concentrations, it is necessary to recover the molybdenum so that it can be recycled. A chemical recovery scheme has been developed (ref. 2). The coal liquefaction residue is first roasted (calcined). Under these conditions, the mineral matter in the residue and the molybdenum from the MoSz form compounds that are only partially soluble in the aqueous NH, used to leach the roasted material. This results in only a partial recovery of the molybdenum from the MoSz present in the coal-oil coprocessing 1 iquef action residue. There are reports in the literature (refs. 3-7) that bacteria can oxidatively solubilize MoSz (molybdenite) ore. . This paper reports on the use of Thiobacillus ferrooxidans to aerobically solubilize MoSz present in a coal liquefaction (coprocessing) catalyst residue to facilitate recovery of the molybdenum. EXPERIMENTAL Catalyst Residue: Two gallons of liquids produced from reaction of Illinois No. 6 hvB bituminous coal and Maya atmospneric tower bottoms under coprocessing conditions (ref. 1) were processed as shown in Figure 1. Approximately 100 grams of catalyst residue that contained MoSz, obtained as the tetrahydrofuraninsolubles, Yere dried in a vacuum oven at lOOOC for 4 hours and then ground to