WOOD PULP AND PAPER 43 



under a pressure of about 80 Ib. and a temperature of about 340 F. Read- 

 ings are taken hourly and reported. 



Cooking spruce, balsam and hemlock usually requires about eight 

 hours, but this may vary widely, according to size of digester, strength 

 of the acid and freeness of the vent. If the vent becomes clogged it may 

 require much longer to cook. In one case it took thirty-one hours to 

 cook a 3-ton digester of hemlock, because of a clogged vent. Instances are 

 known where the packing has been blown out of the top of a digester, 

 from this cause. The vapors which pass out through this vent are 

 piped into the acid storage tank where they deposit the acid which they 

 contain and w T arm up the acid in storage. 



When the cook is finished the steam is turned off and the blow valve 

 at the bottom is opened. The pressure in the retort forces the semi- 

 liquid mass out through the large pipe and into a large wooden tank 

 called a blow-tank or blow-pit. The excess steam which is freed in this 

 process passes out of the tank through a chimney into the open air. 



Care must be taken in manufacturing the acid, as acid too weak does 

 not thoroughly disintegrate the wood and produces a so-called hard stock 

 which is full of small slivers. Acid that is too strong will dissolve the 

 wood. 



Washing. 



After the pulp has sufficiently cooled so that the blow-pits can be 

 opened, it is washed thoroughly with water to remove all of the liquid 

 which it contains. As soon as the stock is washed, it is pumped into the 

 feed tank from which it passes onto the screens as needed. 



The blow pits are simply large wooden tanks which catch the pulp 

 and liquor as it rushes out of the retort, and allows the steam to escape 

 at the top. These tanks are made large enough to accommodate at least 

 three digesters full of pulp. This is done so that the digesters can con- 

 tinue to run, even if the pulp mill should close down because of breaks or 

 any other reason. 



Screening. 



The screens used in this process are of the flat plate type, each plate 

 fitted with a vibrator which aids the fibers in passing through the 

 V-shaped slits. These screens are arranged in four lines, with a slant 

 from head to foot so that the pulp-laden water will flow freely over the 

 plates. 



