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

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 ((ZnFe II )Fe2 O4 ). 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 product.