Pulp and Paper 3 1 29 



Table 25-13. — NSSC and GLSC^ pulping of southern hardwoods and properties of 

 corrugating board made from the pulps (Dawson 1974) 



Statistic NSSC GLSC 



Chemical, percent of ovendry weight of wood 



Na2S03 



Na2C03 



Na2S . ^ 



NaOH 



Total 



Yield, percent of ovendry weight of wood 



Caliper, mils 



Concora, pounds^ 



IPC runnability index 



'Neutral sulfite semichemical process 

 ^Green liquor semichemical process 

 ■'Flat crush strength 



NEUTRAL SULFITE SEMICHEMICAL PULPING' 



Neutral sulfite semichemical (NSSC) pulping was first developed in 1925 for 

 utilization of tannin-extracted chestnut chips in corrugating medium. After 

 World War II, NSSC pulping for corrugating medium was extended to southern 

 hardwoods; this application continues to be commercially viable. 



In neutral sulfite semichemical pulping, a solution of sodium sulfite and 

 bicarbonate is used, usually in the ratios from four to six parts of sulfite to one of 

 carbonate. Pulping with a slightly alkaline solution prevents acid hydrolysis, 

 without removing a major portion of the hemicellulose. Swartz (1962) described 

 batch cooking conditions for hardwoods as 1 to 3 hours at 170°C, depending on 

 end-product requirement. 



In a continuous cooking process for 9-point corrugating medium (fig. 25-22), 

 hardwood chips may be presteamed, impregnated with hot NSSC liquor and 

 cooked to about 80-perent yield in a continuous digester, and fiberized in a 

 double disk refiner (see Kato (1961) for more complete process diagram). In this 

 mill, duplicate production lines allow dense hardwoods to be pulped separately 

 from low-density hardwoods. 



Cooking chemical requirements are 10 to 15 percent, expressed as sodium 

 carbonate to wood, but the relationships of amount and ratio of chemicals, pH, 

 temperature, and cooking time are not simple. Low-density hardwoods general- 

 ly produce better pulps than denser woods. For the manufacture of newsprint, 

 NSSC pulps may be produced whose characteristics are between those of regular 

 groundwood and the long-fibered part of the furnish. To produce maximum 

 strength, mechanical treatment is necessary, which reduces opacity and results 

 in "tinniness"; the degree of treatment must be adjusted according to the other 

 components in the furnish to produce the desired result (Swartz 1962). Southern 

 hardwood pulp yields by the process average near 75 percent of weight of 

 ovendry wood (Schroeder 1976). 



^With some additions related to process description, text under this heading is taken from 

 McGovem and Auchter n976^. with nermission of the authors. 



