Formation of Mill Scale in Portland Cement Production and Its Processing into Coagulants

Mill scale, a by-product of steel hot rolling operations, represents a significant iron-rich waste stream with considerable potential for valorization. While traditionally utilized as a raw material in Portland cement clinker production, recent advances have demonstrated the feasibility of converting mill scale into valuable iron-based coagulants for water and wastewater treatment. This review examines the formation mechanisms of mill scale during steel production, its current utilization in cement manufacturing, and innovative pathways for processing mill scale into ferric chloride, ferric sulfate, and polyferric sulfate coagulants. The synthesis methods, characterization techniques, and application performance of these recovered coagulants are systematically analyzed. Comparative assessment reveals that mill scale-derived coagulants exhibit treatment efficiencies comparable to commercial products while offering significant economic and environmental advantages. This review provides a comprehensive framework for transitioning from linear disposal practices to circular economy approaches in heavy industry waste management.