Control of Product Quality
In addition to control of temperature (mentioned above), the main requirement is to obtain a consistent fineness of the product. From the earliest times, fineness was measured by sieving the cement. As cements have become finer, the use of sieves is less applicable, but the amount retained on a 45 μm sieve is still measured, usually by air-jet sieving or wet-sieving. The amount passing this sieve (typically 95% in modern general-purpose cements) is related to the overall strength-development potential of the cement, because the larger particles are essentially unreactive.
The main measure of fineness today is specific surface. Because cement particles react with water at their surface, the specific surface area is directly related to the cement's initial reactivity. By adjusting the fineness of grind, the manufacture can produce a range of products from a single clinker. Tight control of fineness is necessary in order to obtain cement with the desired consistent day-to-day performance, so round-the-clock measurements are made on the cement as it is produced, and mill feed-rates and separator settings are adjusted to maintain constant specific surface.
A more comprehensive picture of fineness is given by particle size analysis, yielding a measure of the amount of each size range present, from sub-micrometer upwards. This used to be mainly a research tool, but with the advent of cheap, industrialized laser-diffraction analyzers, its use for routine control is becoming more frequent. This may take the form of a desk-top analyzer fed with automatically gathered samples in a robotized laboratory, or, increasingly commonly, instruments attached directly to the output ducts of the mill. In either case, the results can be fed directly into the mill control system, allowing complete automation of fineness control.
In addition to fineness, added materials in the cement must be controlled. In the case of gypsum addition, the material used is frequently of variable quality, and it is normal practise to measure the sulfate content of the cement regularly, typically by x-ray fluorescence, using the results to adjust the gypsum feed rate. Again, this process is often completely automated. Similar measurement and control protocols are applied to other materials added, such as limestone, slag and fly-ash.
Read more about this topic: Cement Mill
Famous quotes containing the words control of, control, product and/or quality:
“In Vietnam, some of us lost control of our lives. I want my life back. I almost feel like I’ve been missing in action for twenty-two years.”
—Wanda Sparks, U.S. nurse. As quoted in the New York Times Magazine, p. 72 (November 7, 1993)
“The poets were not alone in sanctioning myths, for long before the poets the states and the lawmakers had sanctioned them as a useful expedient.... They needed to control the people by superstitious fears, and these cannot be aroused without myths and marvels.”
—Strabo (c. 58 B.C.–c. 24 A.D., Greek geographer. Geographia, bk. 1, sct. 2, subsct. 8.
“A product of the untalented, sold by the unprincipled to the utterly bewildered.”
—Al Capp (1909–1979)
“The past is interesting not only for the beauty which the artists for whom it was the present were able to extract from it, but also as past, for its historical value. The same goes for the present. The pleasure which we derive from the representation of the present is due not only to the beauty in which it may be clothed, but also from its essential quality of being present.”
—Charles Baudelaire (1821–1867)