Cement mill

A cement mill (or finish mill in North American usage) is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement. Most cement is currently ground in ball mills and also vertical roller mills which are more effective than ball mills. A cement mill (or finish mill in North American usage) is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement. Most cement is currently ground in ball mills and also vertical roller mills which are more effective than ball mills. Early hydraulic cements, such as those of James Parker, James Frost and Joseph Aspdin were relatively soft and readily ground by the primitive technology of the day, using flat millstones. The emergence of Portland cement in the 1840s made grinding considerably more difficult, because the clinker produced by the kiln is often as hard as the millstone material. Because of this, cement continued to be ground very coarsely (typically 20% over 100 μm particle diameter) until better grinding technology became available. Besides producing un-reactive cement with slow strength growth, this exacerbated the problem of unsoundness. This late, disruptive expansion is caused by hydration of large particles of calcium oxide. Fine grinding lessens this effect, and early cements had to be stored for several months to give the calcium oxide time to hydrate before it was fit for sale. From 1885 onward, the development of specialized steel led to the development of new forms of grinding equipment, and from this point onward, the typical fineness of cement began a steady rise. The progressive reduction in the proportion of larger, un-reactive cement particles has been partially responsible for the fourfold increase in the strength of Portland cement during the twentieth century. The recent history of the technology has been mainly concerned with reducing the energy consumption of the grinding process. Portland clinker is the main constituent of most cements. In Portland cement, a little calcium sulfate (typically 3-10%) is added in order to retard the hydration of tricalcium aluminate. The calcium sulfate may consist of natural gypsum, anhydrite, or synthetic wastes such as flue-gas desulfurization gypsum. In addition, up to 5% calcium carbonate and up to 1% of other minerals may be added. It is normal to add a certain amount of water, and small quantities of organic grinding aids and performance enhancers. 'Blended cements' and Masonry cements may include large additions (up to 40%) of natural pozzolans, fly ash, limestone, silica fume or metakaolin. Blastfurnace slag cement may include up to 70% ground granulated blast furnace slag. See cement. Gypsum and calcium carbonate are relatively soft minerals, and rapidly grind to ultra-fine particles. Grinding aids are typically chemicals added at a rate of 0.01-0.03% that coat the newly formed surfaces of broken mineral particles and prevent re-agglomeration. They include 1,2-propanediol, acetic acid, triethanolamine and lignosulfonates. Heat generated in the grinding process causes gypsum (CaSO4.2H2O) to lose water, forming bassanite (CaSO4.0.2-0.7H2O) or γ-anhydrite (CaSO4.~0.05H2O). The latter minerals are rapidly soluble, and about 2% of these in cement is needed to control tricalcium aluminate hydration. If more than this amount forms, crystallization of gypsum on their re-hydration causes 'false set' - a sudden thickening of the cement mix a few minutes after mixing, which thins out on re-mixing. High milling temperature causes this. On the other hand, if milling temperature is too low, insufficient rapidly soluble sulfate is available and this causes 'flash set' - an irreversible stiffening of the mix. Obtaining the optimum amount of rapidly soluble sulfate requires milling with a mill exit temperature within a few degrees of 115 °C. Where the milling system is too hot, some manufacturers use 2.5% gypsum and the remaining calcium sulfate as natural α-anhydrite (CaSO4). Complete dehydration of this mixture yields the optimum 2% γ-anhydrite. In the case of some efficient modern mills, insufficient heat is generated. This is corrected by recirculating part of the hot exhaust air to the mill inlet. A Ball mill is a horizontal cylinder partly filled with steel balls (or occasionally other shapes) that rotates on its axis, imparting a tumbling and cascading action to the balls. Material fed through the mill is crushed by impact and ground by attrition between the balls. The grinding media are usually made of high-chromium steel. The smaller grades are occasionally cylindrical ('pebs') rather than spherical. There exists a speed of rotation (the 'critical speed') at which the contents of the mill would simply ride over the roof of the mill due to centrifugal action. The critical speed (rpm) is given by:nC = 42.29/√d, where d is the internal diameter in metres.Ball mills are normally operated at around 75% of critical speed, so a mill with diameter 5 metres will turn at around 14 rpm.