402 



CEMENTS, LIMES, AND PLASTERS. 



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 dryer. 



The dryer above described is the simplest and is most commonly 

 used. For handling the small percentages of water contained in most 

 cement materials it is very efficient, but for dealing with high percent- 

 ages of water, such as are encountered when marl is to be usd in a dry 

 process, it seems probable that double-heating dryers will be found 

 more economical. 



This type is exemplified by the Ruggles-Coles dryer, a detailed 

 description of which is given in the section on slag cements, p. 649. 

 In this dryer a double cylinder is employed. The wet 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 dryer is employed in almost all of the slag- 

 cement plants in the United States, and is also in use in several Port- 

 land-cement plants. 



When vertical kilns were in use, drying-floors and drying-tunnels 

 were extensively used, but at present they can be found only in a few 

 plants, being everywhere else supplanted by the rotary dryers. 



At the marl-plant of the ill-fated Hecla Portland Cement Company, 

 which is shown in Fig. 82, rotary kilns were actually used as driers, 

 because of the extreme difficulty encountered in properly drying this 

 material in a drier of ordinary type. 



In the Edison plant a stationary vertical tower drier is used for 

 the cement rock and limestone. 



The Edison stack drier shown in Fig. 83 is described as follows 

 in a recent article * in the Iron Age: The chimney surmounting this 

 flue is used only when starting a fire, the gases of combustion ordi- 

 narily passing directly to the dryer stack to rise through the falling 

 stream of rock and thoroughly dry it. The baffle-plate system is such 

 that the fall of a piece of rock from the lowest screen to the bottom 

 of the dryer requires 26 seconds. From above the baffles near the 

 top of the stack the gases are drawn out by an 80-inch exhaust- 

 fan, driven by a 50-horse-power motor, and are passed through a 

 dust-settling chamber on their way to the atmosphere. A 12- 

 inch screw conveyor returns the collected dust to the bottom of the 

 dryer stack and replaces it in the system. The baffle-plates of the 



* The Iron Age, Dec. 24, 1903, p. 5. 



