MANUFACTURE OF PLASTERS. 49 



direction opposite to that taken by the wet material. Pyrometers for 

 control of the temperature may be placed in the holes H in the masonry. 



Mannheim system. A German plaster-plant using a rotary calcining 

 system has recently been described * in considerable detail, and seems 

 worthy of attention as presenting certain interesting differences to the 

 system discussed above. The mill in question is that of the Rhenish 

 Gypsum Company, located at Mannheim, in Rhenish Prussia. 



The crude gypsum is passed through crushers and nippers, but is 

 not finely powdered previous to calcining. "When the material comes 

 from the crushers and nippers it varies in size from the finest powder 

 to fragments as large as an ordinary hickory-nut. Varying thus in 

 size, the material goes directly to the calciner." 



"The calciner consists of a fire-box with an automatic stoker, which 

 is placed in front of and connected with a chamber containing a rotating 

 cylinder. Above this cylinder is a chamber called the 'forewarmer', 

 through which a spiral conveyor passes from end to end. A pipe leads 

 from the rotating cylinder to the forewarmer and connects at the other 

 end with the chimney. Connected with the fire-box is a fan by which 

 a forced draft is secured. The fire-box is heated to a high temperature, 

 and the fuel gases, forced by the fan, pass through the rotating cylinder 

 and then through the forewarmer. The crude gypsum is carried by 

 bucket elevators from the crushers to a bin above the calciner and 

 thence it flows by gravity into the forewarmer, through which it is 

 carried by the spiral conveyor. It then falls directly into the rotating 

 calciner below. Shelves or buckets on the inside of this cylinder pick 

 up the material and elevate it as the cylinder rotates. When the material 

 nears the top the slant of the shelves is so great that it falls again 

 to the bottom. The strong draft of hot air passing through the cylinder 

 from the fire-box strikes the gypsum as it falls and moves the fragments 

 toward the rear with a velocity directly f proportional to their size. 

 The coarser material moves much more deliberately and thus is exposed 

 to the heat longer than the finer and more readily calcined particles. 

 In this way, though the material entering the rotating cylinder varies 

 greatly in fineness, the coarser material is sufficiently calcined and 

 the finer is not overburned. All of the heat has not been exhausted 

 in passing through the rotary cylinder and this is for the most part 

 saved by forcing the air, after it leaves the cylinder, through the fore- 

 warmer. In this process the heat is so completely utilized that the 

 air and furnace gases pass to the chimney with a temperature of only 



* Wilder, F. A. Vol. 12, Iowa Geological Survey, pp. 213-216. 

 f Evident misprint for "inversely". E. C. E. 



