eckel.] METHODS OF MANUFACTURE. 49 
notable percentage of sulphur is also a drawback, though part of the* 
sulphur in the slag will be removed during the processes of manu- 
facture. 
Granulation of slag. — If slag be allowed to cool slowly, it solidities 
into a dense, tough material, without hydraulic properties, which is 
not readily reduced to the requisite fineness for a cement mixture. If 
it be cooled suddenly, however, as by bringing the stream of molten 
slag into contact with cold water, the slag is "granulated" — i. e., it 
breaks up into small porous particles — and is much more readily pul- 
verized than a slowly cooled slag. Sudden cooling intensities the 
chemical activity of its constituents so as to give it hydraulic properties, 
while part of the sulphur contained in the original slag is removed. 
The sole disadvantage of the process of granulating slag is that the 
product contains 20 to 40 per cent of water, which must be driven off 
before the granulated slag is sent to the grinding machinery. 
The granulation of the slag is effected b}^ running the stream of 
molten slag from the furnace into a sheet-iron trough, in which flows 
a small stream of water w r hose rate of flow is so regulated as to give 
complete granulation of the slag without using an excessive amount 
of water. The trough may discharge the granulated slag into tanks 
or into box cars, which are usually perforated at intervals along the 
sides so as to allow part of the water to drain off. 
Drying the dag.— As above noted, the granulated slag may carry 
from 20 to -10 per cent of water. This is removed by treating the 
slag in rotary driers. In practice such driers give an evaporation of 
6 to 10 pounds of water per pound of coal. The practice of slag drying 
lis very fully described in volume 10 of the Mineral Industry, pages 
84-95, where figures and descriptions of various driers are also given, 
with data on their evaporative efficiency. In one of the methods the 
slag is dried by waste heat from the kilns after it has been mixed with 
the limestone. Kiln gases could of course be used in the slag driers, 
but they have not been utilized except in plants following the method 
described. 
Grinding the slag. — Slag can be crushed with considerable ease to 
ibout 50 mesh, but notwithstanding its apparent brittleness it is diffi- 
ult to grind it finer. Until the introduction of the tube mill it was 
ilmost impossible to reduce this material to the fineness necessary for 
j,j |i cement mixture, and the proper grinding of the slag is still an 
,( 8 Expensive part of the process as compared with the grinding of lime- 
H ptone, shales, or clay. 
(In Composition of the limestone. — As the slag carries all the silica and 
J ilumina necessary for the cement mixture, the limestone to be added 
^ o it should be simply a pure lime carbonate. The limestone used for 
I J lux at the furnace which supplies the slag will usually be found to be 
aD j >f suitable composition for use in making up the cement mixture. 
Bull. 243—05 4 
