MANUFACTURE OF PLASTERS. 



47 



crushed to pass a 1-inch ring is carried by elevator E and delivered 

 into bin F. From this bin the crushed rock is fed mechanically by 

 feeder H into the Cummer rotary calciner G. In this machine most of 

 the free moisture is eliminated, and the process of calcining also carried 

 forward toward completion. The material delivered from the rotary 

 calciner is steaming and heated to from 350 to 400 F., the exact tem- 



FIG. 14. Plan of plaster-mill equipped on Cummer rotary calcining system, 

 perature depending upon the nature and density of the rock. Elevator 

 J carries the product from the calciner to the Cummer calcining-bins K, 

 where it is allowed to remain about thirty-six hours. During this time 

 the resident heat in the material completes the process of calcination, 

 and the material is cooled, ready for the mills. The now calcined mate- 

 rial is mechanically discharged from the bins into elevator M, which 

 carries it into the small bins situated over the mills 0. From the mills 

 the conveyor R delivers the pulverized material into screen S. The 

 finished material is sacked at T. The tailings from the screen are spouted 

 into elevator M and returned to the mills. 



The calcining-bins noted above are an integral part of the Cummer 

 process. These bins are built preferably of brick and are lined with 

 paving brick, so that they will not absorb the moisture given off from 

 the gypsum rock during calcination. Three bins are required for each 

 plant, and the capacity of each bin is equal to the daily output of the 

 plant. By the use of three bins a continuous process is obtained. One 

 bin is being discharged of its cooled calcined material while the process 



