High-temperature thermal treatment for the whitening of secondary aluminum dross towards high-value utilization: Overcoming carbon-iron coloration and regulating mineral phase reconstruction

Although the high alumina content in secondary aluminum dross (SAD) presents excellent resource recovery potential, its conversion into high-value materials is severely restricted by low whiteness caused by iron and carbon. However, the coupled roles of residual carbon and iron during high temperature mineral phase reconstruction remain insufficiently understood, while existing whitening strategies mainly rely on the removal of chromogenic impurities rather than controlling iron valence states and its incorporation into newly formed mineral phases. In this study, coloration mechanisms and phase reconstruction behaviors during calcination are investigated using a pure reagent simulated system and real SAD. Results indicate that residual carbon creates a localized reducing atmosphere, forcing the reduction of Fe(III) into highly chromogenic Fe(II), which is subsequently irreversibly incorporated into complex silicate or aluminosilicate lattices. To disrupt this pathway, rather than using conventional subtractive iron-removal methods, a novel “phase reconstruction and solid-solution inhibition” strategy is proposed via a two-step aerobic thermal treatment coupled with MgO regulation. This strategy exploits the thermodynamic stability zone of Fe 2 O 3 at medium temperatures to preferentially eliminate carbon sources, and this avoids high temperature iron reduction and lattice fixation. Additionally, introducing MgO promotes the formation of high whiteness magnesium aluminum spinel and competitively consumes silicon based chromogenic carriers. Under optimal conditions of two-step calcination in air with 3% MgO, product whiteness increases from 23.9% to 54.1%, alongside the deep denitrification of hazardous AlN, dechlorination and highly efficient fluorine immobilization. This study elucidates the evolution mechanisms of chromogenic impurities, providing an innovative, acid- and chlorine-free theoretical foundation for the safe disposal and utilization of SAD.