46 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
cement materials it is very efficient, but for dealing with high per- 
centages of water, such as are encountered when marl is to be used in 
a dry process, double-heating driers probably will be found more eco-, 
nomical. This type is exemplified by the Ruggles-Coles drier, a 
detailed description of which is given in the Mineral Industry, volume 
1 1 K pages 84-95. In this drier a double cylinder is employed. The wetSi 
raw material is fed into the space between the inner and outer cylinders, 
while the heated gases pass first through the inner cylinder, and then, 
in a reverse direction, through the space between the inner and outer; 
cylinders. This double-heating type of drier is employed in almostfj 
all of the slag cement plants in the United States, and is also in use inil 
several Portland-cement plants. 
When vertical kilns were in use drying floors and drying tunnels 
were extensively used, but at present they can be found in only a fewv 
plants, being everywhere else supplanted by the rotary driers. 
The cost of drying will depend on the cost of fuel, the percentage* 
of water in the wet material, and the type of drier. Even under thee 
most unfavorable conditions it may be expected that 5 pounds off 
water will be evaporated per pound of coal used, while a good drieri 
will usually evaporate 7 or 8 pounds of water per pound of coal. 
GRINDING AND MIXING. 
DRY METHODS. 
Part at least of the grinding is usually accomplished before the dry- 
ing, but for convenience the subjects have been separated in the present I 
paper. Usually the limestone is sent through a crusher at the quarry* 
or mill, and occasionally the raw material is further reduced in a Wil- 
liams mill, etc., before drying, but the principal part of the reduction 
alwa}^s takes place after the material has been dried. 
After the two raw materials have been separately dried they may be .! 
mixed immediately, or each may be further reduced separately beforei 
mixing. Automatic mixers, of which many types are on the market r 
give a mixture in proportions determined from analysis of th«' 
materials. 
The further reduction of the mixture is usually carried on in tw< 
stages, the material being ground to 30 mesh in a ball mill, com mi 
nuter, Griffin mill, etc., and finally reduced in a tube mill. At a fev* 
plants, however, single-stage reduction is practiced in Griffin or Hunt 
ington mills, while at the Edison plant at Stewartsville, N. J., thw 
reduction is accomplished in a series of rolls. 
The majority of plants use either the Griffin mill and tube mill o 
the ball and tube mills, and there is probably little difference in tfoi 
cost of operating these two combinations. The ball mill has neve 
been quite so successful as its companion, the tube mill, and has been 
replaced at several plants by the comminuter. 
