3 1 34 Chapter 25 



SPECIES PROPORTIONS FOR KRAFT PULPING 



Unbleached kraft linerboard. — Schroeder (1976) found that the 21 southern 

 mills that incorporated some hardwood pulp in their manufacture of unbleached 

 kraft linerboard daily produced 16,900 tons of pulp, of which 1,515 tons (9 

 percent) was from hardwood. One mill used as little as 2 percent and others up to 

 15 percent. Three mills used 4 to 6 percent hardwood pulp in sack or bag paper. 



Schroeder's (1976) survey indicated a definite trend to increase the amount of 

 hardwood pulp in unbleached kraft linerboard. Fifteen percent was considered 

 the maximum practical hardwood content, the balance being southern pine. (See 

 sect. 25-4 for a discussion of press drying, a technology that may increase 

 maximum acceptable proportions.) Almost all mills pulped hardwoods together 

 with southern pine primarily because they have only one or two continuous 

 digesters for pulping unbleached kraft linerboard. The few mills that have batch 

 digesters apparently pulp hardwoods and pine separately and blend resulting 

 pulps. Schroeder found that the average yield for unbleached kraft linerboard 

 was 53 to 54 percent with range from 45 to 56 percent. 



Pilot scale tests suggest that hardwood content of kraft linerboard furnish 

 could be considerably increased without impairing product quality. Fahey and 

 Setterholm (1960) found that linerboard containing up to 25 percent of sweet- 

 gum kraft pulp could meet standard requirements. Bormett et al. (1981), testing 

 linerboards incorporating 25, 50, and 75 percent red oak kraft pulp blended with 

 southern pine kraft, found that with up to 25 percent hardwood, all carrier 

 strength requirements were met. Linerboards with 50 percent hardwood furnish 

 provided boxes with satisfactory compression strength. 



Bleached kraft pulp. — One mill surveyed by Schroeder (1976) produced 

 1 ,000 tons of hardwood kraft pulp daily at 32 percent yield — all for dissolving 

 pulp. Thirty-five mills daily produced 15,132 tons of bleached hardwood kraft 

 pulp at yields of about 47 or 48 percent; reported yields ranged from 42 to 63 

 percent. These pulps were generally bleached to brightness values between 79 

 and 90-1- GE brightness. The bleaching sequences used most often are CEH, 

 CEHD, CEHED, CEDED, and CEHDED'^ The more stages used, the higher 

 the final GE brightness value, and the more efficient the use of bleaching 

 chemicals. 



Schroeder ( 1 976) found that the products produced from bleached hardwood 

 pulps and the percent of hardwood pulp in these products varied considerably. 

 Market pulp usually had the most hardwood fiber; output from a few mills was 

 100 percent hardwood. Other products usually were in one of three categories 

 depending upon which properties of hardwood pulp were to be exploited. Prod- 

 ucts such as tissues utilize the softness and absorbency of these pulps. Fine 

 papers utilize the good surface texture and high opacity attainable with the pulps. 

 A considerable amount of the hardwood pulp, however, is used primarily as an 



'"^C is chlorine stage, E is alkaline extraction stage, H is hypochlorite stage, and D is chlorine 

 dioxide stage. 



