438 CEMENTS, LIMES, AND PLASTERS. 



power figure seems unusually high, for at other plants the dry-pan 

 doing similar work is usually estimated as taking about 15 H.P. 



A dry-pan used on fairly hard shale at another wet-process cement- 

 plant takes fragments up to 4"X8" in size and reduces them to about 

 8-mesh. It requires about 20 H.P. and gives a product of about 6 tons 

 per hour. 



FIG. 92. Dry-pan. (Allis-Chalmers Co.) 



Class 6. Centrifugal Grinders. 



In the more typical examples of this class of mills the material 

 is crushed between a horizontal die ring and one or more vertically 

 hung rollers which are held against the ring by centrifugal force. 

 Such typical examples among cement-grinding machinery are the 

 Huntingdon and Griffin mills. A somewhat different arrangement 

 exists in the Kent mill, which is included here for convenience. In 

 this mill the ring is set vertically and the three rolls horizontally. Only 

 one of the rolls is positively driven, the other two rolls and the ring 

 taking motion from the driven roll. 



The mills of this class have been used as one-stage reducers at a 

 number of cement-plants, taking raw material or clinker in ^- to 1-inch 

 sizes and reducing it to 85 to 95 per cent through a 100-mesh sieve, 

 and they have accomplished this task with a fair degree of efficiency. 

 Of late years, however, the tendency has been very markedly in favor 

 of lightening this task by putting in tube mills for the final reduction. 



Huntingdon mill. The Huntingdon mill as used in the manufac- 

 ture of Portland cement is a slightly modified form of the mill of the 

 same name used in gold-ore treatment. The rights of the Huntingdon 



