WOOD PULP AND PAPER 49 



dissociate the valuable fibrous portion of the plant from the resinous and 

 non-fibrous portion. As a result of this boiling the wood loses about one- 

 half of its weight. 



The digesters are of various styles and shapes and may be either 

 spherical, cylindrical, or egg-shaped, being constructed to revolve at a 

 slow rate of speed, or they may be fixed permanently in an upright posi- 

 tion. Digesters of the spherical type are usually about 9 or 10 ft. in 

 diameter and the cylindrical digesters are from 40 to 50 ft. high and from 

 12 to 15 ft. in diameter. These digesters vary in size from 3 to 20 tons 

 capacity. 



The inside of these digesters which are used in this alkaline process 

 do not have to be Hned with brick as do the digesters used in the acid 

 process. 



The mixture in the digesters is heated by means of steam at a pres- 

 sure of from 80 to 100 lb. per square inch. This steam may bje 

 blown directly into the digester or may pass through a large coil at the 

 bottom of the digester. Each of these systems has its advantages and 

 disadvantages, as in the former the steam is condensed by the material 

 in the digester and so increases its volume while, in the latter, it is drawn 

 off from the coil. 



In the manufacture of soda pulp, revolving digesters are most com- 

 monly used and are found to produce the best results. Here a pressure 

 of from 60 to 80 lb. is also found to produce the best results. 



Cooking. 



The cooking and the manufacture of the cooking liquor in this 

 process are not nearly so complicated as in the sulphite process. Here 

 the wood chips are emptied into the digesters and are covered with a 

 6 to 9 per cent solution of sodium hydroxide (caustic soda — NaOH) and 

 this is cooked at a temperature of about 240° F. and a pressure of from 

 60 to 80 lb., for a period of from eight to nine hours. 



When the cook is completed, the valve at the bottom of the digester 

 is opened and the semi-liquid solution passes out as a result of the pressure 

 in the retort. This is called " blowing " and the material passes into a 

 large wooden tank called a " blow-pit." Here the steam which escapes is 

 passed into the open air through a large pipe running from the top of the 

 tank. 



Washing. 



The next step is to wash the pulp free from the spent cooking liquor 

 and soluble portions which it contains. As the caustic soda is recovered 



