Sustainable conversion of acid mine drainage sludge into high-quality iron-based pigments through thermal processing

This study investigates the calcination of Acid Mine Drainage (AMD) sludge to produce stable iron-based pigments, providing a sustainable solution for mining waste management. Thermal processing at temperatures ranging from 100 °C to 1200 °C led to critical mineralogical transformations: dehydration of gypsum to anhydrite at 400 °C, conversion of schwertmannite to hematite at 800 °C, and formation of magnetite at 1200 °C, as confirmed by XRD and TG-DSC analyses. These transformations enhanced pigment stability and optical properties, with ΔE* ab values demonstrating colour stability comparable to commercial standards. Pigments exhibited vibrant red hues at 800 °C and transitioned to dark-grey tones at 1200 °C, aligning with industrial requirements. A key innovation of this study is the introduction of a predictive polynomial model for ΔE * ab variation that was developed to optimize pigment production processes. This work highlights the potential of AMD sludge valorisation to promote circular economy principles, reducing environmental risks and creating high-value materials for industrial applications.