264 



CHEMICAL MANUEES. 



heat them for some time on the roof of a dryer. The roasting may 

 easily l)e done in a closed vessel into which superheated air or 

 combustion gases are injected. Horn is steamed in a digester. A 

 digester is nothing but a horizontal or vertical boiler heated by 

 steam (Fig. 53). In Fig. 53, a is the steam pipe fitted with a valve 

 b. On the top is a steam escape valve c, safety valve d, and a 

 short piece e intended for a steam gauge ; gg is a double bottom of 

 wrought-iron. The shape and the size of the digester vary con- 

 siderably and depend on the size of the factory. However, it is 

 well to keep to certain rules. Thus a wide somewhat low digester 



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7i 



d 



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k 



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Fig. 53. — Digester foi Treating Horn, Leatlier, etc. 



generally runs cheaper, and works more economically than a tall 

 narrow one, because it condenses less steam on its surface. 



The digester being charged with horn through the manhole h 

 is closed hermetically, the steam valve h is opened and steam of 

 two to three atmospheres injected. It condenses at first until the 

 whole mass is heated. The heating lasts two hours for horn in 

 large pieces. When the operation is finished the valve h is closed 

 and the apparatus left to itself for some time, and finally the 

 steam valve c is opened. AYhen the pressure in the digester has 

 fallen to almost the atmospheric pressure, the digester is emptied. 



