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 temperature that the continuous operation of the kilns will result in a clinker that is neither underburned nor overburned.
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
