The utilization of lime-dried sludge as resource for producing cement

Lime drying is an effective technique to achieve the volume reduction of sewage sludge produced by wastewater treatment plants. It holds the advantages featuring most cost-effective, practical operation with high dryness fraction and good stability. However, the great challenge facing environmental engineering is to solve the outlet for lime-dried sludge (LDS). Since calcium in LDS is a potential raw material for producing cement, the feasibility of combining LDS disposal with cement kiln and the influence on sintering of cement were investigated in this work. The results indicated that adding LDS (<15 wt %) improved the burnability of clinker leading to reduce the temperatures for calcium carbonate (CaCO3) decomposition and liquid phase formation. Furthermore, the presence of phosphorus and other trace elements increased the amount of liquid phase and decreased the viscosity, making it favorable for forming tricalcium silicate (C3S) in cement clinker. The free lime content in the clinkers was below 1.0%; thus, meeting the standard of Chinese current regulatory thresholds of 1.5%. Compressive strength of the clinkers produced with different amount of sludge at 3, 7, and 28 d met cement production standards (GB/T17671–1999). Moreover, when LDS was substituted, it not only provided chemical components but also contributed 2441 J/g of LDS with moisture content less than 50% in cement producing kilns. Overall, this work could effectively achieve the sludge treatment target of environmental protection, solid reduction, and resources utilization.