SLAG CEMENT. REQUISITES AND TREATMENT OF THE SLAG. 649 



Drying the Slag. 



The slag as it is brought to the cement mill from the granulating 

 tanks carries from 15 to over 40 per cent of water absorbed during 

 granulation. As will be noted later attempts have been made to 

 utilize this contained water in the slaking of the lime, but these 

 attempts have hitherto proved unsuccessful. As the manufacture 

 is at present conducted, therefore, the large percentage of water 

 carried by the slag is of no service, and in order to get good results 

 from the grinding machinery the water must be removed as completely 

 as possible before pulverization is attempted. 



Before describing the various types of driers in use, a few words 

 on the general problem may be serviceable. The slag may carry, as 

 above noted, from 15 to over 40 per cent of water, varying with the 

 method of granulation, the fineness of grain, etc. In test runs slag 

 can be thoroughly granulated without the use of more than 10 to 15 

 per cent of water, but in actual practice it will usually be found that 

 the granulated slag carries from 30 to 45 per cent. As the slag must 

 be reduced to extreme fineness it is necessary that this moisture be 

 reduced as much as possible. With a well-conducted rotary drier it 

 is possible to economically reduce the percentage of moisture in the 

 dried product to about one-fourth of one per cent. 



The temperature to which the product is carried in drying is not 

 a matter of serious moment so long as it does not pass the point at 

 which the slag begins to re-fuse. Theoretically, of course, it is neces- 

 sary only to carry the temperature above 212 F., but in practice it is 

 economically impossible to keep it as low as this. It may be carried 

 as high as a dull-red heat without injury to the slag. Indeed, it is 

 probably the case that drying at relatively high temperatures improves 

 the hydraulic properties of the slag, rather than otherwise, as it is well 

 known that the natural puzzolanic materials are improved by roasting. 

 It would not, therefore, be a matter of surprise if drying the slag at a 

 higher temperature than is actually necessary should result in mate- 

 rially accelerating the set of the resulting cement and also in increasing 

 the strength of briquettes made from it. 



The Ruggles-Coles drier (see Fig. 161) consists of two concentric 

 hollow cylinders bolted together and revolving on an axis slightly inclined 

 from the horizontal. The outside cylinder is made of steel plates, the 

 longitudinal seams having butt joints with inside lapping straps. The 

 inner cylinder, which is also made of steel, is connected with the outer 

 cylinder at its middle by heavy cast-iron arms A solidly riveted to 



