CHAPTER XXX. 

 PREPARING THE MIXTURE FOR THE KILN. 



THE preparation of the raw materials for burning involves the re- 

 duction of both ingredients to a very fine powder and their intimate 

 mixture in proper proportions. In practice the two operations grind- 

 ing and mixing are usually carried on to some extent simultaneously; 

 the raw materials are commonly crushed and ground more or less finely 

 and then well mixed, during and after which mixing the final reduction 

 to powder takes place. 



Two general methods of preparation for the kiln are in use at differ- 

 ent plants the dry and the wet, the dry method being almost invariably 

 followed unless the limey constituent of the mixture is a marl already 

 full of water. This dry method consists merely in keeping the mate- 

 rials in as dry a condition as possible throughout the entire process of 

 grinding and mixing, any small percentage of moisture they may nat- 

 urally contain being removed by driers early in the process. In the 

 wet method, on the other hand, the materials are powdered and mixed 

 while in a very fluid state, containing usually 60 per cent or so of water. 



In addition to these two main methods noted, it will be necessary 

 to describe separately and briefly certain peculiar modifications of the 

 dry method. Some of these modifications are practiced when slag is 

 used as one of the raw materials, certain changes in the process being 

 introduced which are of both technologic and commercial interest. 

 The other modification of the dry process is an interesting type in which 

 the materials are actually fused in a furnace. This requires descrip- 

 tion because of the eminence of the engineers who have devised it. 



Certain features in regard to crushing and grinding are common 

 to all methods of preparing the mix, and these general principles and 

 features will be described in the last section of this chapter, while crush- 

 ing machinery in general will be described in Chapter XXXI. 



In the present chapter the various methods of preparation will be 

 taken up in the following order. 



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