MANUFACTURE OF NATURAL CEMENTS. 



225 



Types of kiln used. With but few exceptions the kilns used in the 

 American natural-cement industry are of the vertical continuous mixed- 

 feed type.* That is to say, the rock and fuel are fed to the kiln either 

 mixed together or in alternate layers. Kilns of the separate-feed type, 

 in which the fuel does not come in contact with the rock but is burned 

 in a separate furnace, are rarely used in the cement industry. In the 

 lime industry separate-feed kilns are rapidly supplanting the mixed- 

 feed kilns, but the reasons for using them in burning lime do not hold 

 in burning cement. The principal advantage of the separate-feed kiln 

 is that it gives a clean white product unmixed with fuel ashes. This 

 is, of course, a distinct advantage to a lime manufacturer, but it would 

 be of little importance in the cement industry. On the other hand, 

 the mixed-feed kilns are apparently more economical of fuel than the 

 others. 



The attention of American natural-cement manufacturers might be 

 profitably turned to consideration of some of the improved types of 

 continuous kilns. The Aalborg kiln, for example, described on pages 

 101-103, is used in hydraulic-lime manufacture in Europe, and gives 

 remarkably good results as to fuel consumption and quality of product. 



The Campbell kiln is not, properly speaking, a new type of kiln, but 

 rather a kiln containing a new type of "pot". In the ordinary lime 



* On pp. 99 to 108 will be found descriptions of the various types of kilns 

 used in the lime industry. These descriptions may be referred to as being of in- 

 terest in the present connection. 



