166 ANNUAL REPORT 
of hot air, produced by passing the air through an iron coil heated by a 
furnace. ‘The dried lime is then run through a disintegrator and again 
converted into powder. ‘This method, being continuous, evidently must | 
result in a much greater capacity than could be attained by either of the 
other methods. 
The Addition of the Lime. 
its preparation, is added to the granulated slag coming from the inter- 
The lime hydrate, whatever may be 
mediate grinding machine and now stored in a bin above the fine grinder. 
The mixture may be accomplished by a mechanical feeder, consisting of an 
iron disc revolving beneath both the slag and the lime bin, so that both 
materials are fed upon it, the slag through an aperture whose area is 
to the area of the lime feed hole as the proportion of slag is to that of the 
lime necesary. It is evident, of course, that both of the apertures must 
be adjustable. As the disc revolves, a scraper turns off the two materials 
into the screw conveyor, feeding the fine grinding machines. Another 
device observed at an Ohio slag cement plant was a flat slider similar to 
an engine slide valve having a reciprocating motion and moving beneath 
the slag and the lime bin. It had two sets of holes, large ones for the slag, 
and smaller ones for the lime. With each stroke of the slide, a quantity of 
slag and lime was allowed to fall through these holes for a short interval 
of time. In some plants the slag and lime are weighed out by means of 
scale cars and dumped into a small bin feeding the fine grinders. 
‘Fine Grinding.—As fine grinders, there are two machines to be con- 
sidered principally; the tube mill and the Griffin mill. The former is a 
machine resembling a tube, from 18 to 22 feet in length and about 5 feet 
in diameter, into which the slag is fed and from which it is discharged 
continually. The grinding is done by means of flint pebbles filling the 
machine to slightly above the axis. Beside the grinding action proper, 
this machine affords a splendid blending of materials approaching almost 
ideal mixing. At the same time it eliminates the use of all screens, the re- 
quired fineness being obtained by the rate of feeding. On the other 
hand it consumes much power. 
The Griffin mill might be compared with a mortar and pestle, since 
it consists of a revolving conical pestle weighing about 100 pounds, and 
about 18 inches in circumference, suspended from a universal bearing. 
When in motion the pestle is pressed against a steel ring by centrifugal 
force and thus effects the grinding. ‘This ring is about 6 inches high and 
a little over 30 inches in diameter. The pestle itself revolves about its 
own axis, thus rolling against the surface of the ring. By means of fan 
blades, above and below the cone the fine particles produced are blown 
through screens surrounding the grinding space. 
