A review of purification of ductile iron: kinetic limiting factors and process optimization

Purification of ductile iron is essential for achieving stable graphite spheroidization, improved mechanical properties, and high-quality cast products. This review systematically summarizes the current understanding of impurity removal from ductile iron melts, with emphasis on sulfur, phosphorus, manganese, chromium, and non-metallic inclusions. The thermodynamic principles and kinetic mechanisms governing desulfurization, dephosphorization, demanganization, and inclusion removal are critically analyzed, with particular attention to the roles of mass transfer, interfacial reactions, slag chemistry, and stirring conditions. The transition between diffusion-controlled, mixed-controlled, and reaction-controlled regimes is discussed to clarify the factors limiting purification efficiency under different operating conditions. Industrial purification technologies, including Mg- and CaO-based treatments, KR mechanical stirring, double-slag processes, gas and electromagnetic stirring, and filtration techniques, are compared in terms of removal efficiency, operating conditions, advantages, and limitations. The review further highlights the intrinsic conflict between the reducing conditions required for desulfurization and the oxidizing conditions required for dephosphorization, which remains a major challenge for integrated impurity control. Based on the available literature, key knowledge gaps are identified, including the scarcity of ductile-iron-specific kinetic data, the lack of unified criteria for determining rate-controlling steps, and the need for environmentally friendly refining agents and process routes. Finally, future research directions are proposed to support the development of efficient, sustainable, and intelligent purification technologies for high-performance ductile iron production.