Zinc (Zn) is utilized in many industrial applications, such as batteries, cosmetics, pharmaceuticals, and metal production. Due to urbanization and the depletion of high-grade ore deposits, efficient resource management is required from both primary and secondary resources. In terms of the latter, the most widely used recycling method of Zn-containing scrap is the Waelz process, particularly for electric arc furnace dust (EAFD). The Waelz process is a pyrometallurgical technique where the Zn scrap is loaded into a rotary kiln with a carbon-containing reducing agent at 1200-1300 ^oC to extract Zn [1]. Zn subsequently vaporizes and oxidizes in the gas stream to form particulate ZnO, which is then collected on bag filters.

In Europe, approximately 250,000 tons/year of Zn is recovered via the Waelz process. However, the process also generates nearly 800,000 tons/year of slag. Utilization of the “Waelz Slag” is hindered due to the lack of environmental compatibility [2], mainly because of the complex chemical and mineralogical composition. As a result, Waelz slag is largely landfilled, even though the iron content exceeds that of high-grade iron ores (~25% iron). 

Numerous studies have investigated the recycling potential of Waelz slag by different methods, such as in a vertical retort [1], in a top-blown rotary converter, and as a charge to an electric arc steelmaking furnace. However, these studies were either theoretical, or in the early stages of development. Nevertheless, the main component of Waelz slag is iron (Fe), followed by Zn, manganese (Mn), and lead (Pb). Of these, Fe and Zn represent the target elements for downstream utilization. 

The project’s primary goals are to generate pig iron, slag (ideal for the building materials sector), and Zn-rich fly ash (for Zn recovery). Information from a number of analytical techniques, including X-ray fluorescence (XRF), X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and mineral liberation analysis (MLA), were employed to augment parameters for simulation of one-kilogram experiments conducted in an induction furnace using FactSage 8.2.

The study employs an iterative approach where the result of each experiment serves as a guide for the subsequent experiments. The highest total iron recovery is 83.18%. A combination of the FactSage and Einstein-Roscoe viscosity models was used to determine slag viscosity, implying that viscosity depends more on composition than temperature. Addition of 16% SiO₂ and 3% Al₂O₃ shows a high slag viscosity and delayed Mn reduction, possibly due to insufficient Si dissolved in the metal phase and the system being furnace cooled, giving time for nucleation of Mn-containing phases. The calculated actual oxygen partial pressure on all experiments ranges from 10^-8 to 10^-21. XRD analysis of dust recovery filter paper confirmed the presence of Zn. The slag produced in this study has similar compositions to those studied by Grudinsky et al. that can be used as concrete material to enhance its properties [3]. Overall, the study opens the way for holistic valorization of Waelz slag, resulting in more sustainable Zn resource management.

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.
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].
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.
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].