how to preparation cement raw material and Additives ?
Raw materials and fuel preparation
Energy used in preparing the raw material consists of preblending (prehomogenization and proportioning), crushing, grinding and drying (if necessary) the raw meal which is mostly limestone. All materials are then homogenized before entering the kiln. Solid fuels input to the kiln must also be crushed, ground, and dried.
Best practice for raw materials preparation is based on the use of a longitudinal preblending store with either bridge scraper or bucket wheel reclaimer or a circular preblending store with bridge scraper reclaimer for preblending (prehomogenization and proportioning) at 0.5 kWh/t raw meal, a gyratory crusher at 0.38 kWh/t raw meal, an integrated vertical roller mill system with four grinding rollers and a high efficiency separator at 11.45 kWh/t raw meal for grinding, and a gravity (multi-outlet silo) dry system at 0.10 kWh/t raw meal for homogenization.
Based on the above values, the overall best practice value for raw materials preparation is 12.05 kWh/t raw material. Ideally this value should take into account the differences in moisture content of the raw materials as well as the hardness of the limestone. Higher moisture content requires more energy for drying and harder limestone requires more crushing and grinding energy. If drying is required, best practice is to install a preheater to dry the raw materials, which decreases the efficiency of the kiln. For BEST Cement, it is assumed that pre-heating of wet raw materials is negligible and does not decrease the efficiency of the kiln.
Solid fuel preparation also depends on the moisture content of the fuel. It is assumed that only coal needs to be dried and ground and that the energy required for drying or grinding of other materials is insignificant or unnecessary. Best practice is to use the waste heat from the kiln system, for example, the clinker cooler (if available) to dry the coal. Best practice using an MPS vertical roller mill is 10-36 kWh/t anthracite, 6-12 kWh/t pit coal, 8-19 kWh/t lignite, and 7-17 kWh/t petcoke or using a bowl mill is 10-18 kWh/t product. Based on the above,it is assumed that best practice for solid fuel preparation is 10 kWh/t product.
In addition to clinker, some plants use additives in the final cement product. While this reduces the most energy intensive stage of production (clinker making), as well as the carbonation process which produces additional CO2 as a product of the reaction, some additives require additional electricity for blending and grinding (such as fly ash, slags and pozzolans) and/or additional fuel for drying (such as blast furnace and other slags).
Additional requirements from use of additives are based on the differences between blending and grinding Portland cement (5% additives) and other types of cement (up to 65% additives). Portland Cement typically requires about 55 kWh/t for clinker grinding, while fly ash cement (with 25% fly ash) typically requires 60 kWh/t and blast furnace slag cement (with 65% slag) 80 kWh/t (these are typical grinding numbers only used to determine the additional grinding energy required by additives, not best practice; for best practice refer to data below in cement grinding section).
It is assumed that only fly ash, blast furnace and other slags and natural pozzolans need additional energy. Based on the data above, fly ash will require an additional 20 kWh/t of fly ash and slags will require an additional 38 kWh/t of slag. It is assumed that natural pozzolans have requirements similar to fly ash. These data are used to calculate cement grinding requirements. For additives which are dried, best practice requires 0.75 GJ/t (26 kgce/t) of additive. Generally, only blast furnace and other slags are dried. Those additives that need to be dried (the default is all slags, although the user can enter this data as well in the production input sheet) best practice requires an additional 0.75 GJ/t (26 kgce/t) of additive.
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