SPAN GH OLOCISt: SLT) 
The coal is usually received in nut size. The dryer is usually about 30 
feet long by 5 feet in diameter. The grinding is done in two stages. 
For the first grinding there may be employed: 
it INOS 
2. Disintegrators. 
3.  Ball-mills. 
The disintegrator appears to be the most efficient machine for this 
purpose. The fine grinding may be accomplished in: 
if Nie cehOmpulverizer: 
2 eX (Grauinin saad, 
2. JAN, eee raat 
The ero pulverizer is a disintegrator in two or three stages, that is, 
with two or three revolving cages or hammers varying in size and 
revolving with great rapidity. The fine coal is blown up a tube and 
passes through a chamber in which the coarse particles are separated 
and drop, being returned to the pulverizer by gravity. In size the ma- 
chine is much smaller than the clay or limestone disintegrators. The 
writer was informed by those using this machine that it is very efficient, 
but has a reputation, justified or not, of being somewhat dangerous 
owing to the great velocity of the rotating parts, and is said to be 
liable to coal dust explosions. This last statement, the writer has been 
unable to verify. This system is illustrated in Fig. 59 on page 318. 
THE GRINDING OF THE CLINKER, 
From the kilns, the hot clinker drops either into iron cars, a hori- 
zontal pan conveyor or a cylinder cooler. The conveyor collects the 
clinker from all the kilns and delivers it to a chain elevator, which again 
conveys it to a horizontal conveyor. ‘The latter takes it to the clinker 
pile. Where vertical cylinder coolers are used, the clinker from two 
kilns is delivered to one cooler. In some mills the clinker is quenched 
by spraying water on it or the pan conveyor may be aranged to run in 
water so that the clinker is cooled without becoming wet. The effect 
of the spraying is beneficial so far as the quality of the clinker is con- 
cerned, inasmuch as clinker with any tendency to dust is improved and 
also it becomes more friable, easier to grind. On the other hand the 
heat of the clinker is thus to a large extent dissipated and it would be 
hardly worth while to make an attempt to save it. 
The cooled clinker is now ready to be ground. At this stage the 
raw gypsum is added either by rough weighing or by guess, so many 
shovels to a barrow load of clinker. The amount usually added is 2 
per cent., and the cost of the plaster is hence a considerable item in the 
manufacture, its cost being from 7 to 10 dollars per ton. This is a 
powerful argument in favor of cement compositions requiring no gypsum 
or but a small amount. 
