52 



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 grind- 

 ing machinery, and is then sent to the grinding mills. 



Theoretical fuel requirements. As a preliminary to a discus- 

 sion of actual practice in the matter of fuel, it will be of interest 

 to determine the heat units and fuel theoretically required in the 

 manufacture of Portland cement from a dry mixture of normal 

 composition. 



In burning such a mixture to a clinker, practically all of the 

 heat consumed in the operation will be that required for the dis- 

 sociation of the lime carbonate present into lime oxide and car- 

 bon dioxide. Driving off the water of combination that is 

 chemically held by the clay or shale, and decomposing any cal- 

 cium sulphate (gypsum) that may be present in the raw mater- 

 ials, will require a small additional amount of heat. The 

 amount required for these purposes is not accurately known, 

 however, but is probably so small that it will be more or less en- 

 tirely offset by the heat which will be liberated during the com- 

 bination of the lime with the silica and alumina. We may, 

 therefore, without sensible error, regard the total heat theoreti- 

 cally required for the production of a barrel of Portland ce- 

 ment as being that which is necessary for the dissociation of 450 

 pounds of lime carbonate. With coal of a thermal value of 

 13,500 B. T. U., burned with only the air supply demanded by 

 theory, this dissociation will require 25^2 pounds of coal per 

 barrel of cement, a fuel consumption of only 6.6 per cent. 



Losses of heat in practice. In practice with the: rotary kiln, 

 however, there are a number of distinct sources of loss of heat, 

 which result in a fuel consumption immensely greater than the 

 theoretical requirements given above. The more important of 

 these sources of loss are the following : 



1. The kiln gases are discharged at a temperature much 

 above that of the atmosphere, ranging from 300 F. to 2,000 F., 

 according to the type of materials used and the length of the 

 kiln. 



2. The clinker is discharged at a temperature varying from 

 3OOF. to 2,5ooF., the range depending, as before, on materials 

 and length of the kiln. 



