Preparation of CFB fly ash/sewage sludge ceramsite and the morphological transformation and release properties of sulfur

Abstract

In order to improve the resource utilization of circulating fluidized bed (CFB) fly ash and sewage sludge, light-weight and high-strength CFB fly ash/sewage sludge ceramsite (CFBS ceramsite) was prepared by using CFB fly ash and original wet sludge as the main raw materials. The effects of dosages of CFB fly ash, sewage sludge, fluxing agent, and foaming agent and sintering temperature on the physical and mechanical properties of CFBS ceramsite were studied by single factor experiment method. Using SEM, XRD, sulfur analyzer, TG-DSC, flue gas analyzer, TG-MS and XPS analyses, the micro morphology and mineral composition of CFBS ceramsite were studied, and the morphological transformation and release properties of sulfur were also revealed. The results show that the optimal preparation conditions are: the mass ratio [CFB fly ash: sludge] = [65 %∼80 %: 35 %∼20 %], addition of 10 % glass powder, addition of 2.5 %∼10 % calcium carbonate, and sintering at 1200 ∼ 1250℃ for 15 ∼ 20 min. The resulting CFBS ceramsite can be obtained with bulk density of 750 ∼ 850 kg/m3, water absorption of 0.27 %∼0.94 % and cylinder compressive strength of 5.11 ∼ 7.86 MPa. Sulfur in the CFBS ceramsite is mainly derived from organic sulfur in the sewage sludge and inorganic sulfur in the CFB fly ash, and it exists in the form of sulfate and sulfide when the sintering temperature exceeds 400℃. The sulfur content of the final prepared CFBS ceramsite is 0.0217 wt%. Decomposition of SO2 from the anhydrite in CFB fly ash is the main reason for the swelling and foaming of the CFBS ceramsite. However, calcium carbonate, derived from either sulfur-fixing agent in CFB fly ash or auxiliary foaming agent, cannot play the role of foaming agent, while the CaO decomposed from calcium carbonate increases the content of fluxing component in the ceramsite and finally affects the properties of CFBS ceramsite.