Raw Material Preparation
When raw materials arrive at the cement plant, they are stored in dry sheds or silos. These materials are then carefully proportioned accounting for combustion fuel ash and other additives to create cement with a specific chemical composition. Two different methods, dry-process and wet-process, are used to grind raw materials. In the dry-process, raw materials are proportioned, ground to a powder, blended together and fed to the kiln in a dry state. In the wet-process, a slurry is formed by adding water to the properly proportioned raw materials. The grinding and blending operations are then completed with the materials in slurry form. This significantly lowers dust levels but requires additional energy to later remove the water from the raw meal.
Limestone, -0.375 inch to -0.75 inch in size, is blended with other raw materials (Appendix A Table A.3) and fed into grinding mills where it is reduced in size until approximately 70% to 85% passes a 200-mesh screen (0.0027 inch, 0.069 mm screen opening). Raw meal grinding equipment includes: ball mills, tube mills, compound mills, ring roll mills, and impact mills. Raw meal grinding has a significant effect on pyroprocessing and clinker quality. The raw meal quality (fineness, the amount and nature of oversized particles), and flowability (kiln circulating and separation patterns, and conductive energy transfer) affect the final clinker quality.
Raw material grinding consumes about 2% of the on-site energy associated with cement manufacturing. Grinding and milling are electrically driven processes. In addition to the electricity consumed on-site, electric processes have large tacit energy requirements associated with the generation and transmission of electricity. Grinding raw materials for cement manufacturing consumes approximately 93,885 (273,235t) Btu/tonne of cement (Appendix A Table A.11). Grinding energy is a function of numerous factors including how the material fractures (under slow pressure, under impact), compressive strength, coefficient of elasticity, hardness, starting and finish size, desired particle distribution, etc. The exact amount of work required for grinding is difficult to calculate. Grinding efficiency ranges from 6% to 25% based on the thermal measurements.20 Since grinding consumes electricity, a costly energy source, cement manufacturers work to improve grinding efficiency. On-site grinding efficiency improvements provide significant offsite energy savings related to electric generation and transmission losses.
Last update: December 30, 2011
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