Comparative Study of Hydrometallurgical and Pyrometallurgical ZnO Recovery from Mixed Metallurgical Sludge

Abstract

The recovery of zinc from mixed steelmaking and blast furnace sludge was investigated through a direct comparison of the hydrometallurgical acetic acid leaching process and pyrometallurgical processing using coke dust as a reductive agent, with particular emphasis on the structure and functional properties of the resulting ZnO products. The mixed sludge contained 9.71 wt.% ZnO, with zinc occurring predominantly in the form of stable ferritic phases, mainly franklinite ((ZnFeII)Fe2O4). Hydrometallurgical treatment using 1 mol·dm−3 acetic acid resulted in only a limited relative decrease in Zn content (16.8%) based on solid-phase concentrations, as mainly readily soluble zinc phases were dissolved, while ferrite-bound zinc remained largely unaffected. In contrast, pyrometallurgical treatment of the sludge–coke dust mixture at 1200 °C enabled highly effective zinc volatilization, reducing the residual Zn content below 0.05 wt.% and resulting in a relative decrease in Zn content of 99.5% based on solid-phase concentrations. Characterization by XRD, FTIR, SEM, and EDS revealed substantial differences between the products obtained by the two processing routes. The pyrometallurgically prepared ZnO-rich product exhibited higher crystallinity, well-developed rod-like morphology, and substantially lower Fe and Pb contamination than the hydrometallurgically prepared material. In contrast, the hydrometallurgical product showed lower crystallinity and elevated Fe and Pb contents and therefore represented a highly contaminated ZnO-rich oxide mixture. Photocatalytic tests using Rhodamine B showed measurable but low activity of the pyrometallurgical product, with approximately 18% degradation after 120 min of UV irradiation, whereas the hydrometallurgical product was practically inactive. Overall, carbothermic treatment provided substantially more effective Zn removal than acetic acid leaching, although the recovered ZnO-rich product exhibited only limited photocatalytic activity.