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History and capacity of cement mill

Capacity of cement mills

The cement mills on a cement plant are usually sized for a clinker consumption considerably greater than the output of the plant's kilns. This is for two reasons:

The mills are sized to cope with peaks in market demand for cement. In temperate countries, the summer demand for cement is usually much higher than that in winter. Excess clinker produced in winter goes into storage in readiness for summer demand peaks. For this reason, plants with highly seasonal demand usually have very large clinker stores.

Cement milling is the largest user of electric power on a cement plant, and because they can easily be started and stopped, it often pays to operate cement mills only during "off-peak" periods when cheaper power is available. This is also favourable for electricity producers, who can negotiate power prices with major users in order to balance their generating capacity over 24 hours. More sophisticated arrangements such as "power shedding" are often employed. This consists of the cement manufacturer shutting down the plant at short notice when the power supplier expects a critical demand peak, in return for favourable prices. Clearly, plenty of excess cement milling capacity is needed in order to "catch up" after such interruptions.

Cement mill History

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.

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