Evaluation of spray-dried sludge from drinking water treatment plants as a prime material for clinker manufacture

This study evaluates the suitability of a water treatment plant spray-dried (atomized) sludge as a prime material for clinker manufacture. A study of the thermal behaviour of the atomized sludge was followed by a comparison of the burnability of two raw mixes, one (control) made with industrial prime materials (limestone, clay, sand) and the other with the same components except that the clay was replaced by sludge. The materials were ground to the same fineness in both mixes. The composition of the two raw mixes was: silica modulus (Ms = SiO2/(Al2O3 + Fe2O3) = 2.30), the lime saturation factor (LSF = CaO/(2.8SiO2 + 1.18Al2O3 + 0.65Fe2O3) = 0.98) and flux modulus (Mf = Al2O3/Fe2O3 = 1.50). The free lime in the two raw mixes was measured after clinkerization at 1400, 1450 and 1500 °C for 30 min. The clinkers obtained at 1450 °C were characterized with XRF, XRD, FTIR, OM and SEM/EDX. Atomized sludge can be used as a prime material in cement manufacture, as a substitute for clay (and also partially for limestone). Its organic fraction of the spray-dried sludge was observed to burn at around 300 °C, releasing 678 J/g sludge. This combustion temperature is high enough to guarantee the absence of spontaneous combustion and explosions in the raw meal mill. The control clinker contained less free CaO than sludge clinker at all three temperatures. When mix with atomized sludge was studied with the Miller procedure, the reactivity of the sludge, as a component of the raw mix, was found to be high. Both clinkers contained high proportions of alite (>70%) and their microstructure was similar in terms of alite and belite crystal size and composition, while the distribution of the liquid phase was somewhat more uniform in the control.