Rapid and efficient extraction of Zn from wasted Zn-rich paint residue by indirect bioleaching and successive production of high-purity ZnCO3/ZnO by precipitation.

Waste zinc-rich paint residue (WZPR) represents a typical hazardous waste containing both toxic organic substances and heavy metals. The extraction of Zn from WZPR by traditional direct bioleaching has been attracting attention owing to its eco-friendliness, energy conservation and low cost. However, a long bioleaching time and a low Zn release cast a shadow on the reputed bioleaching. To shorten the bioleaching time, the spent medium (SM) process was first used to free Zn from WZPR in this study. The results showed that the SM process had a much higher performance in Zn extraction. Zn removals of 100% and 44.2% (8.6 g/L and 15.2 g/L in the released concentration) were gained within 24 h under pulp densities of 2.0% and 8.0%, respectively, being over 1000 times of the release performance of Zn by previously reported direct bioleaching. On the one hand, the biogenic H+ in SM attacks ZnO to liberate Zn (Ⅱ) via quick acid dissolution. On the other hand, the biogenic Fe3+ not only highly oxidizes Zn0 in WZPR to generate and release Zn2+ but also intensely hydrolyzes to produce H+ to attack ZnO for further dissolution of Zn2+. Both biogenic H+ and Fe3+ contribute to over 90% of Zn extraction as the leading indirect bioleaching mechanism. Due to the high concentration of released Zn2+ and fewer impurity, the bioleachate was used to successfully produce high-purity ZnCO3/ZnO using a simple precipitation, thus achieving the high-value recycling of Zn in WZPR.