282 ANNUAL REPORT 
Usually tube-mills 18 feet by 5 feet are employed, although with 
marls full of shells a 22 foot mill would be none too large. Extreme 
care should be taken to feed the slurry at a uniform and constant rate, 
as otherwise all benefits of grinding are thrown away. With fine grained 
material the capacity of a tube-mill is quite large. At one plant with 
which the writer is familiar 88 cubic yards of slurry, one cubic yard being 
equal to from 134 to 2 barrels of cement, were ground by an 18 by 5 mill in 
3 hours and 15 minutes, using 42 horsepower. . The slurry left a residue of 
4 per cent. on the hundred mesh sieve. It is important to make frequent 
sieve tests so as to control the grinding closely. 
From the mills the slurry 1s pumped to large tanks holding about 
So cubic yards. ‘These are kept agitated by means of revolving gates, 
screw propellers or compressed air. The first are the most wasteful of 
power, while the second are much more efficient. There should be three 
propellers, one near the bottom, one in the middle and one near the top. 
The lowest and the middle propeller should work upward, the top one 
downward. Compressed air at a pressure of from 40 to 60 pounds is said 
to be the cheapest mode of agitation. i 
There are two auxiliary tanks which do not contain the regular charge ; 
one holds a slurry high in clay, the other high in lime. If now the slurry 
in one of the large tanks is found on analysis to be lacking in lime or con- 
taining an excess of it, some of the high clay or high lime correction 
slurry is pumped into the tank, so that after an hour’s agitation the 
corrected slurry is ready for the kiln and the chemist is absolutely sure 
of his composition. By knowing the volume of the slurry in the regular 
tank, its composition and the amount of water it contains, and the same 
facts in regard to the correction slurry, the volume to be added is 
readily determined. By means of correction tables the amount can be 
read off at once. No other method of preparation offers such a complete 
control of the composition. From the storage tanks, the slurry is either 
fed through a pipe, by gravity, to the kiln, or it is allowed to flow into 
a pit kept agitated, from which it is pumped by means of a piston pump 
with adjustable stroke into the rotary. 
G?NERAL CONSIDERATIONS OF GRINDING, 
There is a general tendency to simplify and cheapen the grinding 
process by the adoption of simpler and less power-consuming ma- 
chines, like the rolls, and to effect the separation of the coarse par- 
ticles by means of centrifugal separators or air blasts. ‘This was car- 
ried out at the works of the Edison Portland Cement Co., New Village, 
N. J., on a most elaborate scale. No grinding machines are used but 
rolls and the fine material is removed by means of an air blast, all the 
coarser particles being returned to the rolls. It is readily seen that this 
