54 CEMENT MATERIALS AND INDUSTRY. [hill. 243. 
At the different American cement plants the process of burning i 
rapidly approaching- uniformity, though differences in materials, etc 
will always make variations necessary. The kiln in which the materia 
is burned is now almost invariably of the rotary type, the rotar 
process, which is essentially American in its development, being base 
upon the substitution of machines for hand labor wherever possible 
A brief summary of the process will first be given, after which certai 
subjects of interest will be taken up in more detail. 
SUMMARY OF BURNING TROCESS. 
The rotary kiln is a steel cylinder about 6 feet in diameter and, fo 
dry materials, 60 or 80 feet long. For wet mixtures a kiln 80 to 10( 
feet long, or even longer, is frequently employed. This cylinder i 
set in a slightly inclined position, the inclination being approximately 
one-half inch to the foot. The kiln is lined, except near the uppei 
end, with very resistant fire brick, to withstand both the high tern 
perature to which its inner surface is subjected and the destructiv 
action of the molten clinker. 
The cement mixture is fed in at the upper end of the kiln, while f u 
(which may be either powdered coal, oil, or gas) is injected at its lowe 
end. The kiln, which rests upon geared bearings, is slowl} T revolved 
This revolution, in connection with the inclination at which the cjdind 
is set, gradually carries the cement mixture to the lower end of th 
kiln. The intense heat generated by the burning fuel first drives o 
the water and carbon dioxide from the mixture and then causes thl 
lime, silica, alumina, and iron to combine chemically to form the pa 
tially fused mass known as "cement clinker." This clinker drops o 
of the lower end of the kiln, is cooled so as to prevent injury to th 
grinding machinery, and is then sent to the grinding mills. 
THEORETICAL FUEL REQUIREMENTS. 
As a preliminary to a discussion of actual practice in the matter ol 
fuel, it will be of interest to determine the heat units and fuel the! 
retically required in the manufacture of Portland cement from a dry 
mixture of normal composition. 
In burning such a mixture to a clinker the heat needed will be tlnf 1 
amount required for the dissociation of the lime carbonate into liini 
oxide and carbon dioxide. A small additional amount of heat will b] f® 
required to drive off the water that is chemically held by the clay o 
shale and to decompose any calcium sulphate (gypsum) that may bj 
present. The amount required for these purposes is not accuratelj 
known, however, but is probably so small that it will be more or les 
entirely offset by the heat which will be liberated during the combin 
(ion of the lime with the silica and alumina. We may, therefore, witho 
sensible error, regard the total heat theoretically required for the pJ 
