400 



CEMENTS, LIMES, AND PLASTERS. 



30 per cent in fresh clays. The chemically combined water will depend 

 largely on the composition of the clay, and may vary from 5 to 12 per 

 cent. The hygroscopic or mechanically held water of clays can be 

 driven off at a temperature of 212 F., while the chemically combined 

 water is lost only at a low red heat. The total water, therefore, to be 

 driven off from clays may range from 6 to 42 per cent, depending on 

 the weather, the drainage of the clay-pit, and the care taken in pre- 

 venting unnecessary exposure to moisture of the excavated clay. The 

 average total amount of moisture will probably be about 15 per cent. 



In dealing with shales, the mechanically held water will rarely rise 

 above 10 per cent, and can commonly be kept well below that limit. 

 An additional 2 to 7 per cent of water will be carried by any shale 

 in a state of chemical combination. 



At a few plants marl is used with clay in a dry process. As 

 noted elsewhere the marls as excavated carry usually about 50 per 

 cent of water. Marl presents a more difficult problem than do the other 

 raw materials, because the vegetable matter usually present in marls 

 is extremely retentive of water. 



It will be seen, therefore, that cement materials may carry from 

 1 per cent to 50 per cent of water when they reach the mill. The aver- 

 age throughout the country would probably fall close to 5 per cent 

 if the marls are excluded. In a dry process it is necessary to remove 

 practically all of this water before commencing the grinding of the 

 materials. One reason for this is that fine pulverizing cannot be eco- 

 nomically or satisfactorily accomplished unless absolutely dry material 

 is fed to the grinding machinery. Another reason, which is one of con- 

 venience rather than of necessity, is that the presence of water in the 

 raw materials complicates the control of the cement mixture. 



Methods and costs of drying. The type of dryer used at most cement- 

 plants is a cylinder approximately 5 feet in diameter and 40 feet or so 

 in length, set at a slight inclination to the horizontal and rotating on 

 bearings. The wet raw material is fed in at the upper end of the cylinder, 

 and it moves gradually toward the lower end, under the influence of 

 gravity, as the cylinder revolves. In many dryers angle irons are bolted 

 to the interior in such a way as to lift and drop the raw material alter- 

 nately, thus exposing it more completely to the action of the heated 

 gases and materially assisting in the drying process. The dried raw 

 material falls from the lower end of the cylinder into an elevator boot 

 and is then carried to the grinding-mills. 



The drying-cylinder is heated either by a separate furnace or by 

 waste gases from the cement-kilns. In either case the products of 



