Sintered Ceramsites from Heavy Metal-Contaminated Soil and Printing and Dyeing Sludge Fly Ash: Mechanisms of Heavy Metal Stabilization and Optimization of Sintering Conditions
Printing and dyeing sludge (PDS) fly ash is often classified as hazardous waste due to its high content and diversity of heavy metals (HMs). This study co-disposed PDS fly ash and heavy metal-contaminated soil to produce sintered ceramsites, investigating the effects of sintering conditions on physical properties and HM migration/transformation, and elucidating the immobilization mechanisms. The optimal sintering process was identified as preheating at 400 °C for 10 min, followed by sintering at 1150 °C for 10 min. The resulting ceramsites exhibited a 1-h water absorption of 2.7%, a bulk density of 830 kg/m³, HM volatilization rates below 15%, and a residual fraction (F4) proportion exceeding 86%. Characterization revealed that during sintering, HMs were encapsulated by the glassy phase and reacted with amorphous silica-alumina to form stable silico-aluminates, synergistically reducing HM mobility. However, sintering temperatures ≥1200 °C destabilized the ceramsite structure, causing secondary HM release. This research provides an efficient and simple route for the resource utilization of dyeing sludge fly ash and contaminated soil.