<p>This study explores a sustainable approach to pyro-metallurgical recovery of metallic raw materials from mixed sulfidic fine-grained waste streams, named as Theisenschlamm [1, 2]. As part of the FINEST project (https://finest-project.de/), Subproject 3 "FINEST Disperse Metals," our focus is on optimizing the secure blending of fine and ultra-fine-grained material flows to recover valuable metals through a multi-stage pyro-metallurgical recycling process. Specifically, we investigate the utilization of calcium- and zinc-rich industrial residues as alternative feeds for the pyro-metallurgical metal recovery process.<br>Using FactSage™ 8.2 software, we simulate and evaluate the behavior of the slag systems throughout both the oxidation and reduction stages of the process. Ternary phase diagrams are constructed for the key components of the slag systems, providing insights into phase equilibria, solidification behavior, and the stability of various phases under different thermal conditions [3].<br>A significant aspect of this work involves calculating the viscosity of the slag during the high-temperature processing stages, as this property is critical for ensuring efficient metal separation and refining [4]. Viscosity calculations are performed using the Einstein-Roscoe model, integrated with the Quasi-chemical model from FactSage™ platform, to predict the flow behavior of the slag in relation to its composition and temperature. These findings offer a deeper understanding of the impact of alternative flux materials on slag characteristics, contributing to process optimization.<br>This detailed modeling-driven approach not only facilitates the refinement of metal recovery processes from complex waste streams but also promotes sustainable circular economy practices by reducing dependence on traditional flux materials and enhancing resource efficiency in pyro-metallurgical recycling [5].</p>