516 CEMENTS, LIMES, AND PLASTERS. 



Coal-pulverizing is usually carried on in two stages, the material 

 being first crushed to 20- or 30-mesh in a Williams mill or ball mill and 

 finally reduced in a tube mill. At many plants, however, the entire 

 reduction takes place in one stage, Griffin or Huntingdon mills being 

 used. 



Descriptions of the Williams, Griffin, and Huntingdon mills, and 

 of several makes of ball mills and tube mills, will be found in Chapter 

 XXXV. The Smidth ball mill, however, was not described in that 

 chapter, and as it is recommended by its manufacturers for use as an inter- 

 mediate reducer on coal, its general make-up can properly be noted here. 



The following description of the Smidth ball mill is given by its 

 makers : 



"The illustration (Fig. 129) shows the internal arrangement of the 

 type A ball mill. This machine has a through-shaft, with journals 

 running in bearings at both ends. 



"The machine consists of a drum with two strong end-plates c, 

 between which the curved drum-plates d are fixed. As will be seen 

 these drum-plates do not form a cylinder, but one end of each is set 

 a few inches toward the center, forming steps. The balls and materials 

 tumble over these steps when the drum revolves, and by this the pound- 

 ing action of the balls is considerably increased, the steps at the same 

 time allowing the residue from the sieve to be caught and re-enter the 

 drum. The curved drum-plates are protected on the inside by thick 

 steel plates e which are divided in several sections, so that each sec- 

 tion can be separately renewed. The lining plates are fixed by means 

 of bolts. The end-plates c are also lined with thick plates cl. 



"The grinding-plates have rows of perforations / through which 

 the crushed material constantly falls on the slotted-steel screen-plates 

 g. Whatever is fine enough to pass these screen-plates falls on the 

 inner sieves I, which are coarse and strong sieves, and that which 

 passes through these falls on the finishing-sieves k. The material 

 passing the finishing-sieves is collected in the lower hopper-shaped part 

 of the dust-casing m surrounding the drum, and from this it either 

 falls to a conveyor or elevator, or may be dropped direct into the con- 

 tainers. The outlet of the dust-casing is provided with a slide-gate. 



"The residue from the sieves, coarse and fine, is carried up with the 

 mill until it falls through large holes in the curved part / of the slotted- 

 steel screen-plates, and, together with the residue from the slotted-steel 

 screen-plates themselves, falls into the drum again through the steps. 



"In one of the end-plates a manhole is fitted and in the dust-cas- 

 ing a door corresponding to the manhole, giving access to the interior 



