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.