PREPARING THE MIXTURE FOR THE KILN. 399 



(1) Normal dry methods; 



(2) Methods with slag-limestone mixtures; 



(3) Blast-furnace methods; 



(4) Wet methods; 



(5) General crushing practice. 



(i) Dry Methods of Preparation. 



In dry methods of preparation the raw materials are almost always 

 dried to remove any natural moisture they may contain. This drying 

 may take place entirely before the grinding has commenced; or, as 

 is more usual, the two raw materials are first crushed and partly re- 

 duced separately, then dried, and finally mixed and pulverized. For 

 convenience, however, the subject of drying will be taken up first. 



Drying the raw materials. With the exception of the marls 

 and clays used in the wet method of manufacture, Portland-cement 

 materials are usually dried before the grinding is commenced. This 

 is necessary because the raw materials as they come from the quarry, 

 pit, or mine will almost invariably carry appreciable, though often 

 very small, percentages of water, which greatly reduces the efficiency 

 of most modern type of grinding-mills and tends to clog the discharge 

 screens. 



Percentage of water in raw materials. The percentage of water thus 

 carried by the crude raw material will depend largely on the character 

 of the material, partly on the method of handling and storing it, and 

 partly on weather conditions. 



)n the case of hard limestones freshly quarried, the water will com- 

 monly range from per cent to 3 per cent, rarely reaching or exceeding 

 the higher figure except in the very wet quarries or during a rainy sea- 

 son. Such limestones, comparatively dry when quarried, are frequently 

 sent to the grinding-mills without artificial drying. 



With the soft, chalky limestones, which absorb water very rapidly, 

 the percentage can usually be kept down to 5 per cent or less in dry 

 weather, while prolonged wet weather may necessitate the handling 

 at the mill of material carrying as high as 15 to 20 per cent of water. 



The clays present a much more complicated case. In addition to 

 the hygroscopic or mechanically held water that they may contain, 

 there is also always present a certain percentage of chemically com- 

 bined water. The amount of hydroscopic water present will depend 

 on the treatment and exposure of the clay, and may vary from 1 pre 

 cent or so in clays which have been stored and air-dried to as high as 



