51 



The design, construction and operation of the vertical sta- 

 tionary kilns of various types is discussed in many reports in 

 Portland cement, the most satisfactory single paper being prob- 

 ably that referred to below*. As the subject is, in America, at 

 least, a matter of simply historical interest, no description of 

 these kilns or their operation will be given in the present bulle- 

 tin. 



At present, practice in burning at the different American ce- 

 ment plants is rapidly approaching uniformity, though differ- 

 ences in materials, etc., will always prevent absolute uniformity 

 from being reached. The kiln in which the material is burned 

 is now almost invariably of the rotary type, the rotary process, 

 which is essentially American in its development, being based 

 upon the substitution of machines for hand labor wherever pos- 

 sible. A brief summary of the process will first be given, after 

 \N hich certain subjects of interest will be taken up in more de- 

 tail. 



Summary of burning process. As at present used, the rotary 

 kiln is a steel cylinder about 6 feet in diameter ; its length, for 

 dry materials, is usually 60 or 80 feet, while for wet mixtures an 

 8o-foot, or even longer, kiln is frequently employed. 



This cylinder is set in a slightly inclined position, the inclina- 

 tion being approximately one-half inch to the foot. The kiln is 

 lined, except near the upper end, with very resistant fire brick, 

 to withstand both the high temperature to which its inner sur- 

 face is subjected and also the destructive action of the molten 

 clinker. 



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 carbon dioxide 

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



*Stanger, W. H., and Blount, B., Gilbert, W., and Candlot, E., 

 (Discussion of the value, design and results obtained from various 

 types of fixed kilns). Proc. Institution Civil Engineers, vol. 145, pp. 

 44, 48, 81, 82, 99, 100; 1901. 



