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Grinding Raw materials are normally ground to finenesses between 10% and 15% residue on a 90 micron sieve To attain a sufficiently large passage area for drying air, hall mills for raw grinding generally have a greater diameter and are shorter than cement mills. The specific power consumption for grinding raw materials is in the region of 10-20 kWh/t when using ball mills. The use of the vertical mill normally reduces total power consumption by 10-20%. However, in cases where the moisture content is low, the difference may be lower.  Drying Most cement raw materials have a moisture content of 3-8%, but in some cases it may exceed 20%. In order to grind effectively it is necessary to remove most of the moisture prior to the grinding process. The raw meal must be dried to about 1% residual moisture, to ensure an easy flow of materials. For this reason FLS raw mills of the ball mill type are designed with a drying compartment. Normal 4-stage preheater Uns will produce exhaust gas of sufficient heat content to dry about 10% moisture in the raw materials. With a mill run factor of about 85% compared to the Kiln, the average drying capacity of the Kiln gas is reduced to about 8% water in the raw materials.  If the raw materials contain more than about 8% moisture, extra heat must be supplied. This could come from an auidliary furnace, or from other sources of waste heat - for instance the grate cooler. The relative drying capacity of the mill is reduced considerably with increasing mill size The gas velocity is the restricting factor. The diagram is a guide to the drying capacity of various types of mills, based on the average grindability of the raw materials. The maximum permissible moisture content of the various types is shown in relation to the capacities of mill and ldln and based on a mill run factor of 80% compared to the Kiln and maximum use of Kiln gas. Furthermore, the diagram is based on grinding raw materials of average grindability to a fineness of 10% residue on 0.09 mni. Harder raw materials necessitate bigger mills, but normally contain less moisture, while softer feed will normally contain more water to be evaporated. The smaller mill required for the grinding process in the latter case will reduce the relative drying capacity of the mill. In case the moisture content for a ball mill is higher than shown in the diagram a flash drier can be installed at the mill inlet. The increase in drying capacity is highly dependent on material but normally 3-5% higher moisture content can be accepted. 
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