Controlling magnetite heterogeneous nucleation for enhanced iron removal from zinc leachate via magnetic separation

The separation of Fe from zinc leachate is a critical challenge in zinc hydrometallurgy. While current Fe removal methods (e.g., jarosite and goethite precipitation) generate hazardous low-iron wastes, this study presents a novel approach through magnetite precipitation, which offers high Fe content (∼72 %) and strong ferromagnetism for effective impurity separation. We systematically evaluated three magnetite precipitation methods, analyzing their iron-removal products, magnetic separation performance, and crystallization mechanisms. Our investigation revealed critical limitations of two conventional approaches: partial oxidation yields large, strongly magnetic magnetite particles but suffers from high nucleation barriers (∼15,000 k_BT) that favor goethite impurities, while coprecipitation exhibits lower nucleation barriers (∼1000 k_BT) but produces ultrafine, weakly magnetic particles that compromise separation efficiency. Building upon our previously established slow-dropping methodology, we demonstrated that this nucleation-controlled approach enables in situ formation of magnetite seeds, shifting the nucleation mechanism from homogeneous to heterogeneous. This innovative approach achieves dual benefits: reduced nucleation barriers and promoted growth of micrometer-sized magnetite mesocrystals. The resulting product demonstrates remarkable improvements, reaching 104.12 emu/g, substantially higher than that achieved by partial oxidation (77.75 emu/g) and coprecipitation (25.12 emu/g). The impurity removal efficiency reached 70 % for Pb and 81 % for Ca, significantly outperforming conventional methods. These findings extend the application of our slow-dropping protocol to investigate solid–solid separation between magnetite and impurities, thereby demonstrating its broader potential for sustainable resource recovery applications.