LIME AND CEMENT INDUSTRIES 707 



times of masonry, at others of wood, the usual diameter being 

 20 feet and the depth 6 feet to 7 feet. In the center of the tank 

 is a masonry pier, which supports a revolving vertical shaft, 

 which carries the wooden frame with the scrapers. This is driven 

 by means of a rack and pinion. Such a mill makes about 20 

 revolutions a minute, and sometimes the materials are put through 

 twice in succession to insure a thorough mixing, enough water 

 being added to keep the whole in suspension. If the water has 

 to be evaporated, then it is best to use as little as possible. Water 

 is continually added during the stirring, and, as it overflows, it 

 passes off through chutes to the backs. These chutes also serve 

 the purpose of arresting any sand that the material may contain. 



One of the disadvantages is that, owing to differences in spe- 

 cific gravity of the clay and chalk, they may settle with different 

 rapidity. This can be guarded against by repeated testings. It 

 is claimed by some^ that the wet process does not mix the mate- 

 rials as thoroughly as the dry method, specially where limestone 

 is used. 



Another disadvantage is the time required to dry, and also the 

 floor space needed. 



Dry method. In this method the raw materials are ground 

 dry separately, after w^hich they are mixed, and then wet up to 

 a paste known as slurry, which is molded into slabs or bricks to 

 facilitate charging it into the kiln. This method of preparation 

 may be used in connection with any type of kiln except the rotary 

 one, for in this case there is no need of forming the slurry into 

 cakes of any kind. At some works a dry press instead of a stiff 

 mud machine is used to mold the bricks. 



Burning". After the raw materials have been thoroughly mixed 

 they are burned to a condition of incipient vitrification. Accord- 

 ing to the type of kiln used, the mixture is charged in either the 

 wet or dry condition. The changes which take place in burning 



are of great importance, for on their proper manipulation depends 



* 



IGary, M. Trans. Am. soc. civ. eng. 1893. 30: 3. 



