34 CEMENTS, LIMES, AND PLASTERS. 



Grinding gypsum and plaster. In American plants using the kettle- 

 calcining process the gypsum is finely pulverized before calcination. 

 This pulverizing is usually accomplished in three stages, though when 

 gypsum earth is used instead of rock gypsum the coarse crushers are 

 dispensed with. The three stages are: 



(1) The lump gypsum, as quarried, is crushed to 2- to 4-inch size 

 in a Blake, Gates, or other coarse crusher. 



FIG. 5. Nipper for coarse crushing of gypsum. (Butterworth & Lowe.) 



(2) The product of the coarse crushers is fed to reducers of the 

 coffee-mill type, which crush it to about $ inch or so. 



(3) The final pulverizing is accomplished in either buhrstone mills, 

 Sturtevant rock-emery mills, or Stedman disintegrators. These reduce 

 the gypsum so that from 55 to 65 per cent will pass a 100-mesh sieve, 

 and it is then ready to be fed to the kettles. 



A typical series of gypsum-grinding machinery is shown in Figs. 

 5-8. Fig. 5 shows a "nipper", used for the first coarse reduction. 

 It is a heavy crusher of the jaw type, and when used for gypsum- 

 crushing is usually equipped with corrugated jaws, in order to pre- 

 vent clogging. The machine shown in the illustration has a jaw- 

 opening of 16i"X25f", and a shipping-weight of 10,200 Ibs. A smaller 



