PREPARING THE MIXTURE FOR THE KILN. 413 



they are for this use. The slags utilized will generally run from 30 to 

 40 per cent lime. The presence of over 3 per cent or so of magnesia 

 in a slag is, of course, enough to render its use as a Portland-cement 

 material inadvisable; and on this account slags from furnaces using 

 dolomite (magnesian limestone) as a flux are unsuited for cement- 

 manufacture. The presence of any notable percentage of sulphur is 

 also a drawback, though, as will be later noted, part of the sulphur 

 in the slag will be removed during the processes of manufacture. 



Granulation of slag. If slag be allowed to cool slowly, it solidifies 

 into a dense, tough material, 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. 

 This granulated slag or "slag sand" is much more readily pulverized 

 than a slowly cooled slag; its sudden cooling has also intensified the 

 chemical activity of its constituents so as to give it hydraulic properties, 

 while part of the sulphur contained in the original slag has been 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. 



In practice the granulation of the slag is effected by directing the 

 stream of molten slag direct from the furnace into a sheet-iron trough. 

 A small stream of water flows along this trough, the quantity and rate 

 of flow of the water being regulated so as to give complete granulation 

 of the slag without using an excessive amount of water. The trough 

 may be so directed as to 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 slag. As above noted, the granulated slag may carry 

 from 20 to 40 per cent of water. This is ren^erol by treating the slag 

 in rotary driers. In practice such driers give an evaporation of 

 8 to 10 pounds of water per pound of coal. The practice of slag-dry- 

 ing is very fully described in Part VII of this volume, pages 649-652, 

 where figures and descriptions of various driers are also given, with 

 data on their evaporative efficiency. As noted earlier in this article, 

 one of the methods of manufacturing Portland cement from slag 

 puts off the drying of the slag until after it has been mixed with the 

 limestone, and then accomplishes the drying by utilizing waste heat 

 from the kilns. Kiln gases could, of course, be used anyway in the 

 slag-driers, but it so happens that they have not been so used except 

 in plants following the method in question. 



