224 cox. 



up and carried aiiitil it would fall almost directly across the flame to the 

 bottom, to be carried up again. It was found that the finely divided 

 cement material was carried by the draft completely out of the kiln, 

 and that it had to be intercepted at the exit end and returned to the 

 feeding hopper. The use of producer-gas, \i-ithout a regenerator (an 

 apparatus for preheating the air to be mixed with the gas) failed to 

 yield the requisite, steady temperature for the process of calcination, 

 and when a regenerator using the waste gases was added, it sj:)eedily 

 became clogged with the cement powder carried into it by the draft. 

 Later inventors have overcome the greater portion of the difficulties and 

 the mechanical defects of Eansome's apparatus have been removed by 

 modifications and improvements, but the consumption of producer-gas 

 does not seem as yet to have been brought to such a state of perfection 

 that it competes with coal as a fuel for firing cement kilns. Both oil 

 and natural gas have been and are still used as fuels. 



The chief combustible for firing cement kilns is powdered coal, al- 

 though ordinary gas coke has been used to a considerable extent. The 

 class of coal to be employed is of special importance. Bituminous 

 coal has been found to be most suitable. It must contain the proper 

 quantity of volatile combustible matter to render it easy of ignition at 

 the proper height in the cylinder in order to produce the maximum 

 temperature at the right point. Anthracite and coke are more difficult 

 to ignite, and therefore this maximmn, when the latter are employed, 

 occurs higher in the cylinder; the result is increased difficulty in watch- 

 ing the vitrification of the clinker and a less even distribution of the 

 heat throughout the length of the cylinder. 



The coal must be thoroughly dried, usually by rotary driers heated 

 by hot air from the clinker coolers, before it can be ground to a sufficient 

 degree of fineness. Care must be exercised in this operation that the 

 temperature does not rise sufficiently to expel volatile combustible con- 

 stituents of the coal. 



The estimated coal consumption, based on the barrel as a unit, varies 

 as much as from 30 to 100 kilograms because so many factors enter into 

 the calculator. Theoretically, the amount of fuel needed in the kiln 

 must be sufficient to expel any moisture, to drive off the carbon dioxide 

 from the calcirmi carbonate and to produce the chemical combination 

 of the silica and lime, and the silica and alumina : practically the amount 

 is much larger than that required by theory because the ordinary rotary 

 kiln has never been made to work economically. The calorific value of 

 the coal, the burning temperature required, the heat lost by the radia- 

 tion from the kiln walls, by the discharge of the hot clinlcer and the 

 escaping gases, the completeness of combustion of the fuel, the excess 

 of air supply, the moisture in the raw materials and the experience of 



