Temperature control of cement mill manufacturing

Heat generated in the grinding process of cement mill causes gypsum  to lose water, forming bassanite or γ-anhydrite . The latter minerals are rapidly soluble inside cement mill, and about 2% of these in cement grinded by cement mill is needed to control tricalcium aluminate hydration. If more than this amount forms, crystallization of gypsum inside cement mill on their re-hydration causes "false set" - a sudden thickening of the cement mix a few minutes after mixing in cement mill, which thins out on re-mixing inside cement mill. High milling temperature of cement mill would cause this. On the other hand, if milling temperature of cement mill is too low, insufficient rapidly-soluble sulfate is available and this causes "flash set" - an irreversible stiffening of the mix inside cement mill. Obtaining the optimum amount of rapidly-soluble sulfate requires milling with a cement mill exit temperature within a few degrees of 115°C . Where the milling system of cement mill is too hot, some manufacturers use 2.5% gypsum and the remaining calcium sulfate as natural α-anhydrite (CaSO4) inside cement mill. Complete dehydration of this mixture yields the optimum 2% γ-anhydrite. In the case of some efficient modern cement mill, insufficient heat is generated. This is corrected by recirculating part of the hot exhaust air to the cement mill inlet.