40 



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 preventing 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 car- 

 riedi, 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. This case presents a more difficult prob- 

 lem than do the other raw materials, because the vegetable mat- 

 ter usually present in marls is eixtremely retentive of water. 



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

 from i per cent, to 50 per cent, of water when they reach the 

 mill. 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 can not be economic- 

 ally or satisfactorily accomplisheid unless absolutely dry mate- 

 rial is fed to the grinding machinery. 



Another reason, which is one of convenience rather than of 

 necessity, is that the presence of water in the raw materials com- 

 plicates the calculation of the cement mixture. 



Methods and cost of drying. The type of dryer commonly 

 used in cement plants is a cylinder approximately 5 feet in di- 

 ameter and 40 feet or so in length, set at a slight inclination to 

 the horizontal, and rotating on bearings. The wet raw mater- 

 ial is fed in at the upper end af 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 mater- 

 ial alternately, thus exposing it more completely to the action 

 of the heated gases, and materially assisting in the drying pro- 

 cess. The dried raw material falls from the lower end of the 

 cylinder into an elevator boot, and is then carried to the grind- 

 ing mills. 



The drying cylinder is heated either by a separate furnace or 

 by waste gases from the cement kiln. In either case the pro- 



