bckel.] METHODS OF MANUFACTFKK. 45 
In dealing with shales the mechanically held water rarely amounts 
to more than about 10 per cent, and is commonly well below that 
limit. An additional '1 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 mail- as excavated carry usually about 50 per cent of 
water. These present a more difficult problem than the other raw 
materials, because the vegetable matter usually present in marls i- 
extremely retentive of water. 
It will be seen, therefore, that cement materials may carry from 1 to 
50 per cent of water when the} T reach the mill. In a dry process it is 
necessary to remove practically all of this water before commencing 
the grinding. One reason for this is that line pulverizing can not 
be economically or satisfactorily accomplished unless absolutely dry 
materia] is fed to the grinding machinery. Another reason, which is 
one of convenience rather than of necessity, i- that the presence of 
water in the raw material- complicates the calculation of the cement 
mixture. 
METHODS AND COSTS OF DRYING. 
With the exception of the marls and clays used in the wet method 
of manufacture, Portland-cement materials are usually dried before 
the grinding is commenced. This is necessary because the raw 
matt rial-, a- they come from the quarry, pit, or mine, will almost 
invariably carry appreciable, though often very small, percentages of 
water, which greatly reduces the efficiency of most modern types of 
grinding mills and tend- to clog the discharge screen-. 
The type of drier commonly used in cement plants is a cylinder, 
approximately ."> feet in diameter and about 40 feet in length, set at a 
slight inclination to the horizontal and rotating on bearings. The wet 
raw material i- fed in at the upper end of the cylinder, and it moves 
gradually toward the lower end under the influence of gravity a- the 
cylinder revolves. In many driers angle iron- are bolted to the 
interior in such a way as to lift and drop the raw material alternately. 
thus exposing it more completely to the action of the heated gases and 
materially assisting in the drying process. The dried raw material 
fall- 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 kiln.-. In either case the product- of 
combustion are introduced into the cylinder at its lower end. are 
drawn through it. and escape up a stack set at the upper end of the 
drier. 
The drier above described is the simplest and i- most commonly 
For handling the small percentage- of water contained in most 
