18 



BULLETIN 80, IT. S. DEPARTMENT OF AGRICULTURE. 



WO 



50 



»' 



















a 



*7~ 



?i 



i 



s . 











*—i 





.10 



■20 .30 .40 



POUNDS NaOH PER POUND OF WOOD 



100 



50 



100 



TR 



JiSU-jS 



2 4 6 8 10 12 



DURATION AT MAX. PRESSURE-HOURS 



conditions employed resulted in decreasing the yields of pulp about 

 1 per cent for each 13 grams per liter increase in concentration. It 

 is thus evident that with a given amount of chemical the greater 

 cooking effect is secured by means of the more concentrated solutions. 

 A practical limit of course exists at the point where the volume of 

 the digester liquor becomes too small to afford favorable cooking 

 conditions. 1 



PROPERTIES OF UNBLEACHED PULPS. 



NATURAL COLOR. 



Curves indicating the effects of the conditions of cooking on the 

 natural color of the unbleached pulps are shown in figure 5. 



The larger the amount of 



caustic soda used per pound 

 of wood the lighter in color 

 was the pulp, as indicated by 

 the "parts black" color rat- 

 ing, but the curve approaches 

 parallelism with the horizontal 

 axis as the amounts of caustic 

 increase. White pulps or those 

 with zero " parts black" would 

 not be obtained even if exceed- 

 ingly large amounts of chemi- 

 cal were used. 



Longer periods of cooking 

 produced lighter-colored pulps 

 up to the point where the 

 maximum yield of screened 

 pulp was obtahied. Beyond 

 this point there was a tendency 

 for the pulp to become slightly 

 darker as the duration of cook- 

 ing was increased. This was probably due to the pulp fibers absorbing 

 and retaining coloring matters from the "black liquors." It is gen- 

 erally believed that as the cooking becomes more thorough the 

 cellulose of the fibers gradually becomes more gelatinous or hydrated, 

 and would therefore tend to retain coloring matter during the subse- 

 quent leaching and washing treatments. 



The pressure (temperature) of cooking seems to have had compar- 

 atively little effect on the color of the pulp within the range investi- 

 gated. 



1 As the initial concentrations increased, the volumes of digester liquors at the start of cook decreased 

 (see fig. 17), since the amount of caustic soda was held constant. Hence, increasing concentrations would 

 eventually result in a volume of digester liquor so small that the whole charge of chips would not be covered 

 until late in the cooking period after the liquor had been sufficiently diluted by the condensed steam used 

 in cooking. In this case part of the chips would receive very severe treatment, while the remainder would 

 more or less escape the cooking effect. The resulting pulp would represent a composite of the two con- 

 ditions. 



« 50 



-2°U 



60 



i 



70 80 90 100 110 120 

 MAX. PRESSURE-PDS. PER SQ. IN. 



100 



50 



__, 



1 







S* 







<— 





2 



as' 





r *** 



40 50 60 70 80 90 100 110 

 CONCENTRATION NaOH -GRAMS PER LITER 



Fig. 5.— Effects of cooking conditions on the color 

 ("parts black") of pulp. 



