PBODUCIJSTG SODA PULP FROM ASPEN". 17 



of screened unbleached pulp was identical with that of crude pulp, 

 but for smaller amounts of chemical it rapidly approached zero, 

 while under the same conditions the screenings curve naturally 

 approaches and becomes coincident with the curve for the total 

 crude pulp. In this group of tests the minimum amount of caustic 

 soda for successful cooking, so far as yields alone are concerned, is 

 somewhere between 15 and 20 per cent. 



DURATION OF COOKING. 



The duration of cooking at maximum pressure influenced the 

 yields in very much the same manner as did the amount of chemical. 

 The yield of total crude pulp decreased about 1 per cent for each 

 additional hour of cooking at maximum pressure. However, the 

 curve (fig. 4) seems to approach parallelism with the horizontal axis, 

 thus signifying that beyond a certain point cooking would have had 

 no further effect. 1 The time allowed for these cooks to reach the 

 maximum pressure was one hour, and the extended curve indicates 

 a yield of about 60 per cent for zero hours duration at maximum 

 pressure. This shows that the greater part of the cooking was 

 accomplished during the first hour, or before the maximum pressure 

 was attained, since during that hour about 40 per cent of the wood 

 substance had been dissolved and the dissolving effect during the 

 next 12 hours was only one-fourth as great. 



As determined by the yield curves, the minimum duration for 

 successful cooking under the conditions employed was between one 

 and three hours at maximum pressure. No tests were made between 

 these two points. 



PRESSURE OP COOKING. 



The curve showing the influence of maximum cooking pressure or 

 temperature on yields indicates that all of the tests were made at 

 pressures above the minimum required for successful cooking, under 

 the conditions employed for these tests; hence, no screenings were 

 obtained from any of the cooks, and the curve for screened unbleached 

 pulp coincides with that for total crude pulp. Increases of pressure 

 from 70 to 120 pounds per square inch resulted in decreasing the 

 yields of pulp about 1 per cent for each five pounds, which indicates 

 that the higher pressures increase the thoroughness of cooking, other 

 conditions being constant. 



CONCENTRATION OF CAUSTIC SODA. 



The tests varying the initial concentration of caustic soda in the 

 digester liquors were also made within limits that resulted in thorough 

 cooking for all of the tests. Increasing the concentration under the 



1 Figures 12 and 13 show that the active cooking chemical was consumed at the end of 7 hours at maxi- 

 mum pressure; it is therefore not apparent from these tests what would be the effect of continued cooking 

 in the' presence of available -caustic soda. 



31091°— Bull. -80—14 2 



