Low residual dissolved phosphate in spent medium bioleaching enables rapid and enhanced solubilization of rare earth elements from end-of-life NiMH batteries

Successful heterotrophic bioleaching with high metal yields requires an efficient leaching agent production and minimization of secondary reactions such as precipitation of leached metals with growth medium components. In this study, the role of the secondary reactions on bioleaching of spent nickel-metal-hydride batteries was investigated. Substitution of K2HPO4 by yeast extract (YE) reduced precipitation of both base metals and rare earth elements (REEs). REEs were proportionally more affected by precipitation than base metals. Optimizing the ratio of YE to glucose in the growth medium resulted in glucose to gluconic acid conversion yield of 90% by Gluconobacter oxydans. In one-day leaching with YE medium, 28.8% Mn, 52.8% Fe, 22.9% Co, 12.0% Ni, and 19.5% of total REEs were extracted. The leach liquor obtained with the YE medium resulted in leaching of 1.5 and 11.0 times more of total base metals and REEs, respectively, than with phosphate medium. Experimental results were consistent with geochemical modeling results corroborating the benefit of low phosphate concentrations in leaching systems at neutral to moderately acidic pH. In summary, substitution of K2HPO4 with YE in gluconic acid production phase with G. oxydans reduced subsequent base metal and especially REE precipitation during leaching and, thus, enhanced the overall metal extraction.