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>Knowledge Center>Grinding Machine>Milling and Grinding Technology
Milling and Grinding Technology
The importance of milling and grinding
Firstly we have to consider why the grinding and milling processes are so important in cement manufacturing? In raw grinding it is vital that the raw materials are presented to the kiln in a finely ground and intimately mixed state. In converting the raw materials into clinker we are relying on bringing together a mixture of different minerals containing lime, CaO, silica, SiO2, alumina, Al2O3, and iron oxide, Fe2O3 and sintering those raw minerals to form an entirely new range of clinker minerals. The tri-calcium silicate, C3S, di-calcium silicate, C2S, tricalcium aluminate, C3A, and tetra-calcium aluminoferrite, C4AF, of the cement clinker minerals. This conversion is a ‘sintering’ reaction because the majority of the raw reactants (the CaO and SiO2) and the majority of the products (the C3S and the C2S) remain in a solid state throughout the reaction process. Solid state reactions will only take place at a sufficiently rapid rate if the raw minerals are in a finely divided (ground) state and intimately mixed to bring the different minerals into close proximity with one-another in the kiln feed.
In finish or cement grinding we are again focusing on making sure that the product is in the optimum, finely ground, condition to undergo chemical reactions at the required rate. In this case the hydraulic reactions of the cementitious clinker minerals when the cement is mixed with aggregates and water to form a mortar or concrete. The rate of hydration of these hydraulic minerals is closely related to the fineness of grinding as the surface area they present to the mixing water determines how quickly the reactions take place. In turn this determines the heat of hydration, setting characteristics and strength growth of the mortar or concrete.
Areas of milling and grinding to explore
So the importance of grinding and milling in cement manufacture is to ensure that the kiln feed and the finished cement are in the optimum, finely ground, condition for the required chemical reactions to take place at the desired rate. How are we to define and monitor that finely ground condition to ensure it is at the optimum, and also to control the grinding and milling process? The residue retained on a particular size of sieve mesh? Or the surface area? Or the particle size distribution as represented by the Tromp curve or the Rosin-Rammler distribution? What do all these mean? How are they inter-related? And when is it appropriate to use which measure? How is the attainment of that desired fineness to be controlled? Most milling systems deployed in cement manufacture include integral separators within the circuits which reject oversize particles and return these within the system for further grinding. How do operators monitor the efficiency of those separators? The CemNet course will certainly have to explore all these questions.
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