404 



CEMENTS, LIMES, AND PLASTERS. 



unsuccessful installation for marl-drying, reference should be made to 

 the paper cited below.* 



Grinding and Mixing. 



Part at least of the reduction is usually accomplished before the 

 materials are dried, but for convenience the subjects have been separated 

 in the present chapter. 



General methods. Usually the limestone or cement rock is passed 

 through a crusher at the quarry or mill before being sent to the drier; 

 and occasionally one or both of the raw materials is still further reduced 

 before grinding, but the principal part of the grinding process always 

 takes place after the material has been dried. 



After drying, the two raw materials may either be mixed imme- 

 diately or each may be separately reduced before mixing. Automatic 

 mixers, of which many slightly different types are in use, give a mix- 

 ture in the proportions determined upon by the chemist. 



The further reduction of the mixture is usually carried on in two 

 stages, the material being ground to, say, 30-mesh in a ball mill, kom- 

 minuter, Griffin mill, etc., and finally reduced in a tube mill. At a 

 few plants, however, single-stage reduction is practiced in Griffin or 

 Huntington mills, while at the Edison plant at Stewartsville, N. J., 

 the reduction is accomplished in a series of rolls. 



The majority of plants use either the Griffin mill and tube mill or 

 the ball mill and tube mill; and there is probably little difference 

 in the total cost of operating these two combinations. The former 

 combination (Griffin + tube) is commonly considered to require less 

 power, but more repairs than the latter; but even this can hardly be 

 regarded as an established fact. The ball mill has never been quite as 

 much of a success as its companion, the tube mill, and has been replaced 

 at a number of plants by the kominuter. 



Plans of actual plants. Plans of several actual plants have been 

 inserted for the purpose of illustrating the brief statement made above. 



The plant of the Lawrence Cement Company, of Siegfried, Pa., 

 published by courtesy of Messrs. Lathbury and Spackman, is given 

 in Fig. 85. The materials used here are cement rock and limestone. 

 These are separately crushed in Gates crushers and dried in rotary 

 driers, after which they are mixed and reduced in Williams mills and 

 tube mills. 



* Plant and buildings of the Hecla Portland Cement and Coal Co. Engineering 

 News, vol. 51, pp. 243-245. 1904. 



