Stabilities and Transformations of Manganese Incorporated Magnesium–Chromium Spinels

The rapid advancement in stainless steel manufacturing has resulted in the substantial accumulation of stainless steel slags. In those slags, a significant proportion of trivalent chromium is susceptible to oxidation, forming highly toxic hexavalent chromium, particularly under oxidizing atmosphere. Therefore, prior to landfilling or resource recycling, specialized modification of phase transitions is essential. This study is based on the strategy of converting chromium into spinel structure to achieve detoxification. Initially, the thermodynamic calculations were conducted to analyze the formation and transformation behaviors of spinels in an alkaline environment, with particular emphasis on changes in the valence states of manganese. The synthesis and investigation of typical Mg_1−xMn_xCr₂O₄ spinels commonly encountered in stainless steel slags treatment process were subsequently conducted. Through various characterization techniques and leaching tests, it was found that the stability of Mn²⁺ incorporated MgCr₂O₄ was diminished. However, with an increase in MnO content from 5 to 20 pct (in mole), the stability of Mg_1−xMn_xCr₂O₄ spinels showed an upward trend, though remaining lower than that of pure MgCr₂O₄. The subsequent introduction of CaO for high-temperature calcination revealed that the disruption of tetrahedral sites in the spinels primarily originates from Mn²⁺, whereas Mg²⁺ exhibited a greater stability in occupying tetrahedral sites. A comprehensive understanding of the properties of Mg_1−xMn_xCr₂O₄ spinels could contribute to the target modification and application of stainless steel slags.