Rotary kilns equipped with cyclone preheaters are very commonly used at modern cement plants. The system is excellent in energy efficiency, but has a problem of frequent clogging of the cyclones. With the increasing use of waste tires as alternative energy at the kiln inlet (the cool end), clogging of cyclones has become a crucial issue for stable kiln operation. In order to identify the clogging mechanism, the authors investigated the effects of alternative energy and raw meal used in the kiln inlet on the circulations of sulfur (S), chlorine (Cl) and potassium (K) and on the coating growth. It was found that, when combustible waste containing sulfur was directly fed to the kiln inlet, the volume of gaseous SOx passing through the kiln inlet increased as a result of direct combustion of the waste in addition to vaporization of sulfur from raw meal in a hot kiln due to enhanced reduction. This accelerated formation of low melting point minerals such as sulphate spurrite [2(2CaO・SiO
2)・CaSO
4] and the ellestadite group [Ca
10(SiO
4)
3(SO
4)
3(OH,F,Cl)
2], which led to increased clogging of the cyclones. It was also suggested that direct use of aluminum- containing alternative raw meal at the kiln inlet could be another cause of coating growth through formation of mayenite (Ca
12Al
14O
33 ) and wadalite (Ca
6Al
5Si
2O
16Cl
3).
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