54 



as equivalent to a ton of coal. This estimate is, however, based 

 upon too little data to be as close as those above given for oil 

 or coal. 



Effect of composition on burning. The differences in com- 

 position between Portland cement mixtures are very slight if 

 compared, for example, to the differences between various nat- 

 ural cement rocks. But even such slight differences as do ex- 

 ist exercise a very appreciable effect on the burning of the mix- 

 ture. Other things being equal, any increase in the percentage 

 of lime in the mixture will necessitate 1 a higher temperature in 

 order to get an equally sound cement. A mixture which will 

 give a cement carrying 59 per cent, of lime, for example, will 

 require much less thorough burning than would a mixture de- 

 signed to give a cement with 64 per cent, of lime. 



With equal lime percentages, the cement carrying high silica 

 and low alumina and iron will require a higher temperature 

 than if it were lower in silica and higher in alumina and iron. 

 But, on the other hand, if the alumina and iron are carried too 

 high, the clinker will ball up in the kiln, forming sticky and un- 

 manageable masses. 



Character of kiln coal. The fuel most commonly used in 

 modern rotary kiln practice is bituminous coal, pulverized very 

 finely. Coal for this purpose should be high in volatile matter, 

 and! as low in ash and sulphur as possible. Russell gives the 

 following analyses of West Virginia and Pennsylvania coals 

 used at present at various cement plants in Michigan. 



Analyses of kiln coals. 



1234 



Fixed carbon 56.15 56.33 55.82 51.69 



Volatile matter 35.41 35.26 39.37 39.52 



Ash 6.36 7.06 3.81 6.13 



Moisture 2.08 1.35 1.00 1.40 



Sulphur 1.30 1.34 0.42 1.46 



The coal as usually bought is either "slack" or "run: of mine." 

 In the latter case it is necessary to crush the lumps before pro- 

 ceeding further with the preparation of the coal, but with slack 

 this preliminary crushing is not necessary, and th? material can 

 go directly to the dryer. 



