Red-hot clinker tumbles from the kiln onto a grate and is cooled by recirculating air. The hot air recovered from this cooling process is recycled back to the kiln or preheater system to recover its thermal energy. After energy is recovered from the process gases, the gases are routed to pollution control devices. Electrostatic precipitators or fabric filters remove particulate matter (dust) from the gases before they are discharged to the atmosphere. Particulate removal efficiencies of 99.7 % are common throughout the industry.
Cooled clinker is combined with approximately 3% to 6% gypsum and ground/milled into an extremely fine gray powder. This fine gray powder is cement. Gypsum is used to control the setting time of concrete.
Finish milling usually uses tube or ball mill equipment. Clinker is milled so finely that nearly all of it passes through a 200 mesh (0.0027 inch, 0.069 mm) screen that will hold water. The final powder has a surface area of 3,000 to 5,000 cm2/g and about 85% to 95% of the powder will pass through a 325-mesh screen (0.0017 inch, 0.043 mm). The fineness of the cement is an important property and affects the rate of hydration. The finer the milled cement, the more surface area is available for hydration. The greater area for hydration causes greater early strength and more rapid generation of heat. Cements are classified by their fineness with Type III containing more finely milled cement than Type I cement.
Clinker milling uses the same general type of equipment as raw meal grinding. However, the final product is much finer and requires almost three times the energy 270,610 (778,043t) Btu/tonne of cement as raw meal grinding requires (Appendix A Table A.11). Clinker milling is not energy efficient and typical cement milling systems can consume nearly twenty times their theoretical energy requirement. After milling, Portland cement is ready to be bagged or shipped in bulk by truck or rail to concrete producers.
Last update: December 30, 2011
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