164 ANNUAL REPORT 
proaches red heat. However, the whole scheme, as has been said before, 
needs to be tested on a large scale before it can be pronounced commercial- 
ly feasible. 
Intermediate Grinding. —From the dryer the slag is conveyed to 
iron cooling bins, and then to the intermediate grinding machinery, that 
is, machines which grind sufficiently fine so that the bulk of the material 
passes the 20 mesh sieve. These machines may be ball-mills, disintegra- 
tors, Kent mills, rolls, dry-pans or buhr mills. We cannot discuss the 
relative merits of these machines here, since it will be necessary to study 
them more in detail in the chapter devoted to Portland cement, but it 
may be said that the machines more commonly employed are the ball-mill, 
the disintegrator and the Kent mill. ach of these machines will do the 
work satisfactorily. Although they may differ in efficiency, these dif- 
ferences are not very pronounced with material like granulated slag. 
The roll crusher, as far as cheapness of operation is concerned, is 
more efficient than any of the three preceding machines, but, probably 
owing to its aptness to choke when overcrowded, the unequal wear and the 
small amount of fine material it turns out, it has not found as general 
application as this system really deserves. 
The drypan, although a machine of great capacity, occupies much 
room and does not do as uniform grinding. 
As to the buhr mill, it must be classified as a wasteful machine, in 
regard to power consumption, besides requiring constant attention owing 
to the dressing of the stones. A higher claim of efficiency can be made 
for the emery mills in which the latter item is reduced considerably. 
The Lime.—After leaving the intermediate grinding machine, the 
slag is taken to a bin in order to be delivered to the fine grinding ma- 
chines. At this stage the hydrated lime is added. 
The amount of lime to be added varies, and must be determined for 
each individual slag, by grinding the dried granulated slag together with 
varying amounts of dry, slaked lime, and applying the various cement 
tests. In the majority of cases the addition of lime necessary will fluctu- 
ate between 5 and 15 per cent., being usually closer to 5 than to 15 per 
cent. The lime is ordinarily bought as quicklime, and is slaked at the 
cement works, though of course a considerable saving might be effected 
by burning it at the plant, since limestone is invariably shipped to blast 
furnaces to be used as flux, and never comes a great distance. As to the 
burning, it could be accomplished in an ordinary limekiln, operated con- 
tinuously, using coal or blast furnace gas as fuel. The lime should be 
as low in clay and free silica as possible in order that it may slake freely 
and not give rise to hard lumps which slake slowly and give trouble when 
eround with cement. Such slowly slaking lime, though finely ground, will 
invariably cause difficulties owing to its increase in volume when the 
