Bioleaching of GaAs metal hydroxide sludge: a biohydrometallurgical alternative to conventional acid leaching

The economically important metal gallium (Ga) is considered to be a critical raw material with a potentially high supply risk. The industrial semiconductor production process generates multiple waste streams that contain Ga in economically significant amounts. This study explores bioleaching for Ga recycling from a waste metal hydroxide sludge originating from industrial GaAs-wafer production. Previous studies have demonstrated that hydrometallurgical recycling of this sludge provides an effective approach for gallium recovery. Furthermore, it was shown that reducing the ferric iron in the leaching solution has a beneficial effect on downstream processing, enabling less consumption of chemicals. Based on these results, the present study investigates the potential of bioleaching using Acidithiobacillus thiooxidans with the addition of elemental sulfur for the recovery of gallium and the reduction of ferric iron from the metal hydroxide sludge. Shake flask experiments showed that 60% of the total iron in the leachate was present as ferrous iron after processing 3% (w/v) sludge after 8 days. Subsequent reactor experiments at 3% (w/v) solid load resulted in only 30% iron reduction after 14 days, while element recoveries reached 80% for Fe, 100% for Ga, and 99% for As. A further bioreactor experiment with a gradual addition of the sludge up to 5% (w/v) led to a 70% reduction of ferric iron after 14 days. The experiments revealed that microbially mediated ferric iron reduction decreased at pH values below 1.0. Fluoride was found to inhibit microbial activity during the experiments, which was mitigated by the addition of aluminum at an Al:Fe ratio of 1.4:1. Overall, this study demonstrates that bioleaching is a possible approach for the recycling of gallium-containing metal hydroxide sludge.