Manufacture Of Cement
Calcareous and argillaceous raw materials are used in the manufacture of Portland cement. The calcareous materials used are cement rock, limestone, marl, chalk and marine shell. The argillaceous materials consist of silicates of alumina in the form of clay, shale, slate and blast furnace slag.
From the above materials, others like lime, silica, alumina, iron oxide and small quantities of other chemicals are obtained. Cement can be manufactured either by dry process or wet process.
The dry process is adopted when the raw materials are quite hard. The process is slow and the product is costly. Limestone and clay are ground to fine powder separately and are mixed. Water is added to make a thick paste. The cakes of this paste, which contain about 14 per cent of moisture, are dried and are charged into rotary kiln (Fig. 23). The product obtained after calcination in rotary kiln is called clinker. The clinker is obtained as a result of incipient fusion and sintering at a temperature of about 1400 o - 1500 o C. Because ferric oxide has lower melting point than the other oxides, it acts as a flux. Aeration of cement clinker, which is commonly practised to slake free lime, also causes an absorption of some moisture and carbon dioxide. Absorption of moisture tends to decrease the setting whereas that of carbon dioxide accelerates setting. The clinker is cooled rapidly to preserve the metastable compounds and their solid solutions - dispersion of one solid in another - which are made as the clinker is heated. Clinker is then cooled and ground in tube mills (Fig. 24),
where 2-3% of gypsum is added. Generally, cement is stored in bags of 50 kg. A flow diagram of dry process is shown in Fig. 25. The purpose of adding gypsum is to coat the cement particles by interfering with the process of hydration of the cement particles. This retards the setting of cement.
operations in the wet process of cement manufacture mixing, burning and grinding. The crushed raw erials are fed into ball mill (Fig. 26) and a little water dded. On operating the ball mill, the steel balls in it verize the raw materials which form a slurry with er. This slurry is passed to silos (storage tanks), where proportioning of the compounds is adjusted to ensure red chemical composition. The corrected slurry ing about 40 per cent moisture content, is then fed rotary kiln (Fig. 24) where it loses moisture and ms into lumps or nodules. These are finally burned at 0-1600 o C. The nodules change to clinker at this perature. Clinker is cooled and then ground in tube s. While grinding the clinker, about 3 per cent gypsum dded. The cement is then stored in silos from where supplied. A flow diagram of manufacturing cement wet process is shown in Fig. 27.
Comparison of Wet and Dry Process: The chief advantages of the wet process are the low cost of excavating and grinding raw materials, the accurate control of composition and homogeneity of the slurry, and the economical utilization of fuel through the elimination of separated drying operations. On the other hand the longer kilns, essential in the wet process, cost more and are less responsive to a variable clinker demand than the short kilns which can be used in the dry process.
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