Pulp and Paper 



313 



THE KRAFT PULPING CYCLE 



|turpentine| I water /condensate I 



WOOD- 



WATER- 



FUEL- 



MAKE-UP 

 CaC03 



MUD 



WASHER 



FILTER 



WOOD CHIPS. 

 ROOM 



Co CO -J 



I 



z 



H ^'^J^'^^-^t rTi ^ '^'^'^^-'^^ | -»'rrPAl!-»BLEACH 



[CLARIFIER 



CaC03 



^NoOH 

 Na2S 

 ^Na2C03\ 

 ^CaC03 / 



LIME 

 KILN 



T 



^o^ 



STEAM 



X 



FINISHED 

 PULP 



- | EVAPORATOR 



CAUSTICIZER TURBINE 



WATER 



X 



SMELTER 



CLARIFIER W- 



Na2C03 



z 



Na2Sl 



DISSOLVER 



DREGS I WEAK GREEN 

 WASHER 



— DREGS — ▼ 



GRITS — DREGS - 



LIQUOR 

 ELECTRICAL POWER 



•SAPONIFIED 

 ROSIN 



CONDENSATE 

 STRIP a REUSE 



MAKE-UP 

 Na2S04 



Figure 25-23. — Kraft pulping cycle. Turpentine and rosin yielded during southern pine 

 digestion, are not a product of hardwood kraft pulping. (Drawing after B. K. Mayer; 

 see text footnote 1 5.) 



Bleached NSSC pulps. — Bleached semichemical hardwood pulps were pro- 

 duced in two southern mills in 1960. These and a number of northern mills made 

 pulps from a wide range of hardwoods at higher yields, and about the same or 

 higher pulp strength, than bleached kraft pulps (tables 25-15 and 25-16). These 

 pulps were used in fine papers. The high chemical requirements for pulping and 

 bleaching made the process uneconomical and the last such mill in the United 

 States converted in the 1970s to bleached alkaline-type pulping. The two south- 

 ern bleached NSSC mills converted to bleached kraft mills. 



GREEN LIQUOR SEMICHEMICAL PULPING^° 



The use of kraft green liquor (see figure 25-23 for source of green liquor) in 

 semichemical pulping was practiced as long ago as 1930 in at least one southern 

 mill. Green liquor semichemical pulping (GLSC), utilizing green liquor alone as 

 the pulping agent or as a buffer for sodium sulfite (Dawson 1974), is increasing. 

 Chemical requirements of the NSSC and GLSC processes, and properties of 

 corrugating board made from the pulps are compared in table 25-13. Board 

 property differences are insignificant. The advantage in chemical recovery in 

 GLSC pulping is substantial, especially because it adds less load to recovery 

 systems than NSSC pulping. 



'^Text under this heading is condensed from McGovern and Auchter (1976). 



