Formation kinetics and reaction behavior of pentavalent chromium formed in the cement kiln co-processing of solid waste

Cement kiln co-processing is becoming the main strategy to dispose of hazardous waste containing Cr. A newly-discovered pentavalent Cr compound, which was proved to be formed during cement kiln co-processing of solid waste, is partly responsible for the water-soluble Cr released from the cement. However, the formation characteristics and the solubility of Cr(V) are still unclear to date. In this study, the reaction kinetics and further redox reactions of Cr(V) at high temperature were examined, and its crystal structure and solubility were also explored. At the temperature range of 1000–1200 °C, the formation rate of Ca5(CrO4)3O0.5 reached over 90 % within 10 min, and then slowly increased to near 100 % from 10 min to 10 h. shows that Ca5(CrO4)3O0.5 is formed by interface reaction at an early period, and by diffusion at a later period. The kinetic analysis indicates that Ca5(CrO4)3O0.5 is initially formed through an interface reaction and subsequently through diffusion. Ca5(CrO4)3O0.5 was identified and assigned as hexagonal crystal group (P63/m). Approximately 0.55 g and 0.15 g of Ca5(CrO4)3O0.5 dissolve in neutral water at 100 °C and 50 °C, and the concentrations of Cr(V) in water reach 550 and 150 mg/L, respectively. Additionally, this study finds that at the temperature range of 400–700 °C Ca5(CrO4)3O0.5 can be oxidized into CaCrO4, and at the temperature higher than 1400 °C, it can be further converted into Ca3(CrO4)2 and reduced into CaCr2O4. This study gives a deep insight into Cr oxidation-reduction reaction during thermal treatment of solid waste. These insights provide a comprehensive understanding of Cr oxidation-reduction reactions during the thermal treatment of solid waste, offering valuable guidance for waste management strategies.