482 



CEMENTS, LIMES, AND PLASTERS. 



The cement mixture is fed in at the upper end of the kiln, while fuel 

 (which may be either powdered coal, oil, or gas) is injected at its lower 

 end. The kiln ; which rests upon geared bearings, is slowly revolved 

 about its axis. This revolution, in connection with the inclination 

 at which the cylinder is set, gradually carries the cement mixture to 

 the lower end of the kiln. In the course of this journey the intense 

 heat generated by the burning fuel first drives off the water and car- 

 bon dioxide from the mixture and then causes the lime, silica, alumina, 

 and iron to combine chemically to form the partially fused mass known 

 as " cement clinker". This clinker drops out of the lower end of the 

 kiln, is cooled so as to prevent injury to the grinding machinery, and 

 is then sent to the grinding mills. 



Shape and size. The rotary kilns in use at various plants differ 

 considerably in both shape and size. 



*0 Teeth 2 "P. 

 5"Face, 2'l#P:D, 



- 8ft- f I 



1 . 



FIG. 117. Driving mechanism of rotary kiln. (Engineering News.) 



As to shape, the simplest and commonest form is that of a cylin- 

 der a straight tube of equal diameter throughout. At many plants, 

 however, the kilns are wider at the lower or discharge end than at the 

 stack end. This is usually accomplished by means of a reducing sec- 

 tion near the middle of the kiln, so that a kiln of this type would con- 

 sist really of a lower section 6 feet in diameter and about 30 feet long, 

 an upper section 5 feet in diameter and 30 feet long, and an intermediate 

 reducing section in the shape of a frustum of a cone. The theory on 

 which this arrangement is nominally based is that the gases, cooling 

 as they near the upper end, should be confined in smaller space to keep 

 up their efficiency. I am informed by Mr. Seaman, however, that the 

 first kilns built in this way were made at the Atlas plant, during the 

 early experiments, merely as a makeshift to utilize two 30-foot kilns 

 of different diameters which happened to be on hand. 



