Bioleaching of gold from waste printed circuit boards by Chromobacterium violaceum

Chromobacterium violaceum, a cyanide-generating bacterium has been used to leach out gold from the waste printed circuit boards in culture medium with yeast extract, peptone and glycine. And gold leaching efficiency is affected by many factors, such as dissolved oxygen, base metals, particle size and nutriment, especially several metal ions, which can serve as the catalyst in the metabolism. The dissolved oxygen concentration in every solution decreased to a minimal level after 24 h without oxygen supplement, which was consumed by the bacterial respiration and the reaction of gold and cyanide. pH had a little increase as the generation of OH−. In order to supply the oxygen, a homemade oxygenator was used to offer the sterile oxygen for bacterial respiration, and gold leaching efficiency increased drastically. Pretreated by At. ferrooxidans at optimum conditions (selected by orthogonal experiment), above 80 % copper and other base metals can be removed and increased the gold/copper ratio in the residual solid. Bioleaching the biooxidized e-waste significantly improved gold leaching efficiency and 200 mesh is the optimum particle size. The addition of nutritive salts (NaCl and MgSO4·7H2O) strengthened the leaching efficiency, respectively, where the optimum amount was 4 × 10−3 mol/L and 1.7 × 10−1 mol/L, and MgSO4·7H2O enhanced gold leaching efficiency further. All the promoting effect can also be seen from the total cyanide amount generated, which corresponds to the optimum leaching efficiency. The combination of oxygen supplement, nutrients addition and the pretreatment by biooxidation made the gold bioleaching efficiency reach 70.6 %, which was higher than the previous reports.