Investigation of sulfate on the migration/solidification mechanism and control of cadmium during sintering in lightweight aggregates

Abstract

• The effect of sulfate on the migration of cadmium in LWAs was studied. • Sulfate contributes to the volatilization of cadmium in the form of Cd(g) and CdO(g). • The formation of CdFe 2 O 4 promotes the solidification rate of cadmium in LWAs up to 99.9%. • A reasonable heating rate and CdFe 2 O 4 production are conducive to the retention of cadmium. • An environmental risk assessment of cadmium leaching in LWAs was carried out. The effective solidification of cadmium is crucial for the clean production of lightweight aggregates (LWAs) from cadmium-contaminated soil (CCS), and the promotion of cadmium volatilization by sulfate is not negligible during sintering. This work methodically investigated the effect of CaSO 4 and Na 2 SO 4 with 0.5–2.5% sulfur content on the migration and solidification of cadmium in LWAs prepared from CCS and lithium slag (LS). The results showed that the two sulfates were irrelevant to the volatilization form of cadmium, including Cd(g) and CdO(g), in LWAs. The effect of Na 2 SO 4 was mainly reflected in the two aspects of generating CdO and lowering the formation temperature of CdO, while the role of CaSO 4 was just the latter. The cadmium retained in the LWAs was effectively solidified in the form of CdFe 2 O 4 , and its solidification rate was up to 99.99%. We found that the formation of CdO was reduced by rapid heating above 360 °C, which could achieve the long-term retention of cadmium. In addition, generating more CdFe 2 O 4 during sintering was beneficial for the solidification of cadmium. This study will help improve the understanding of the migration and solidification mechanism of cadmium in the presence of sulfur during the sintering process of LWAs and realize the clean production of LWAs and the harmless utilization of CCS and LS.