# Pyrometallurgical valorization of waelz, fayalite, and linz-donawitz slag mixtures

## TL;DR

This paper shows how industrial metal slags can be recycled using pyrometallurgy to produce useful materials and pig iron, reducing waste and supporting circular economy goals.

## Contribution

The study introduces a method for valorizing mixed slags without added fluxes, linking slag properties to performance using a ternary phase diagram and XCT analysis.

## Key findings

- Mixtures of Waelz, Linz-Donawitz, and Fayalite slags can be directly valorized into pig iron and construction-grade slag.
- XCT analysis revealed microstructural correlations between viscosity, interfacial tension, and slag performance.
- The roasted slag is suitable for ceramic applications, expanding its commercial use.

## Abstract

Waelz, Linz-Donawitz, and Fayalite slags are one of the largest industrial byproducts. They are presently under-utilized, and ending up in landfill. This work demonstrates that mixtures of these slags can be directly valorized, given that the combined composition has basicity (CaO/SiO₂ = 0.9–1.0) without the addition of fresh fluxes. Furthermore, the current work contributes to a deeper understanding of slag behavior by linking basicity and viscosity to interfacial tension and by constructing an isoviscosity-ternary phase diagram of CaO-SiO2-Al2O3. The current methodology gives a more comprehensive approach for designing self-fluxing slag mixtures that is feasible for smelting. X-ray computed tomography (XCT) measurements of the smelted products show the internal micro-structure, which provided insights into the correlations between viscosity, interfacial-tension, and the performance of each mixture. The metal product qualifies as vanadium-bearing pig iron based on the elemental analysis while the slag met the criteria for non-hazardous, construction-grade material according to the specifications of the United States, United Kingdom and Germany. Additionally, the roasted slag contains phases suitable for ceramic and porcelain applications, broadening its commercial application. The study illustrates a practical implementation of circular-economy principles for the metal industry and for metallurgical waste management by demonstrating electrified pyrometallurgy based on the induction furnace.

The online version contains supplementary material available at 10.1038/s41598-026-44763-3.