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POPULAR SCIENCE MONTHLY. 



rate of one revolution in from one to three minutes. Fig. 7 shows the 

 arrangement of apparatus for injecting at the center of the kiln an 

 air blast which carries with it the powdered coal, received from the 

 hopper shown on the right. The rotation of the kiln keeps the '^mix' in 

 constant motion as it passes through the kiln, when it is first dried, 

 then deprived of its carbonic acid and then vitrified or partially fused 

 in such manner as to insure the proper chemical reaction between the 

 basic lime and the acid silica, alumina and iron. Only that skill that 

 is determined by experience can direct the burning at such a tempera- 

 ture that the continuous operation of the kilns will result in a clinker 

 that is neither underburned nor overburned. 



Fic. 7. Apparatvs foi! Fkiodinc; Coal to Kii.xs. 



So far as the chemistry of cement burning is understood, it appears 

 that at a red heat the water is expelled from the clay; the carbonic 

 acid is then driven from the lime, and it escapes. The silica, alumina 

 and iron of the clay then combine with the lime, first forming fusible 

 glasses and then taking on more lime ; at length the tri-calcium silicate 

 informed with the alumina and iron as calcium alumino-ferrite. 



Properly burned clinker is in hard rounded grains about the size of 

 dried peas and of a greenish-black color. If it is underburned, it is 

 light colored and soft. If it is overburned it becomes like slag. If it 

 is burned too long, it falls to powder on cooling. Uneven burning is 

 more common in vertical than in rotary kilns, hence the product of 

 rotary kilns is more uniform, ensuring a better cement as the burning 



