CHAPTER XXXIIL 

 THE ROTARY KILN. 



IN the early days of the Portland-cement industry a simple vertical 

 kiln, much like that used for burning lime and natural cement, was 

 used for burning the Portland-cement mixture. These kilns, while 

 fairly efficient so far as fuel consumption was concerned, were expen- 

 sive in labor, and their daily output was small. In France and Ger- 

 many they were soon supplanted by improved types, but still stationary 

 and vertical, which gave very much lower fuel consumption. Kilns 

 of these types have been discussed in the preceding chapter. In America, 

 however, where labor is expensive while fuel is comparatively cheap, 

 an entirely different style of kiln has been evolved. This is the rotary 

 kiln. With the exception of a very few of the older plants, which have 

 retained vertical kilns, all American Portland-cement plants are now 

 equipped with rotary kilns. 



FIG. 115. Exterior view of rotary kiln. (Bonnot & Co.) 



The rotary kiln as at first used in cement-manufacture was adapted 

 to dry materials only, while gas or oil were used as fuel. A long series 

 of experiments and improvements have perfected a burning process 

 in which finely pulverized coal is used as fuel, while wet mixtures can 

 now be fed directly to the kiln. The present condition, in which the 

 rotary kiln is adapted to the use of several different types of fuel, and 

 to all kinds of Portland-cement mixtures, has been attained only through 

 long and earnest effort on the part of American cement-manufacturers. 



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