Comparative Analysis of Heavy Metal Leaching from Metallurgical Slag Using Different Types of Leaching Agents: A Kinetic Models Approach

In accordance with the Circular Economy, the metallurgical solid wastes are considered secondary resources of many heavy metals, such as lead, zinc, iron, cadmium, thallium, and others. To separate valuable and toxic metals, numerous processes, among them solid–liquid extraction, have been studied in recent years. In line with this trend, the objective of this study was to investigate the possibility of leaching selected heavy metals (Pb, Zn, Cu, and Fe) from slag generated by the zinc and lead metallurgical industry. The effect of the leaching agent type (HCl, NaOH, and Na2NTA) and concentration, pH, solid-to-liquid (S/L) ratio, and interphase contact time on process efficiency was evaluated experimentally. The slag was comprehensively characterized using atomic absorption spectrometry (AAS), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and BCR (Community Bureau of Reference) sequential extraction to determine the total metal concentration, element and phase composition, as well as chemical speciation of the investigated metals. To determine the kinetic mechanisms the Weber–Morris and shrinking-core models were used. The efficiency of metal separation from slag strongly depended on the process parameters. HCl was highly effective for simultaneous metal leaching, particularly Pb, Zn, and Cu (up to over 80%), while NaOH and Na2NTA allowed for the selective separation of Pb with leaching efficiencies of up to 80% and 90%, respectively. Sequential BCR analysis confirmed the different forms of metal occurrence, with predominating highly mobile Pb fractions and stable, residual Cu and Fe fractions. Comparative analyses using SEM and sequential BCR extraction revealed significant changes in slag morphology and metal speciation following leaching with the HCl solution. The Weber and Morris model indicated the kinetic contribution of the intraparticle diffusion within the slag’s porous structure, especially under HCl-based leaching. An analysis using shrinking-core models revealed different predominant mechanisms for specific metal–leaching agent systems.