Pulp and Paper 3121 



when severely pretreated; the bursting and tensile strength of such red oak pulps 

 were reduced by bleaching, but not sufficiently to lower usability in good quality 

 paper. 



Schafer (1961), in a review of wood factors affecting groundwood pulp 

 quality, noted that the strength of hardwood groundwood can be improved by 

 treating the pulp with a hot dilute caustic soda solution. Like chemigroundwood 

 pretreatment, such caustic treatment of the pulp generally reduces bulk, opacity, 

 and brightness of the paper. 



Gavelin's (1966, p. 221) text on mechanical pulp manufacture describes two 

 processes by which a few European groundwood mills have been able to utilize 

 hardwoods. In the first of these processes, the logs are peeled, air dried to 30 

 percent moisture content, cut to 4-foot lengths, and then subjected to a vacuum- 

 pressure cycle of 4 to 6 hours while being impregnated with hot liquor containing 

 about 15 g of sodium sulfite/liter, buffered with sodium carbonate to a pH of 9.4. 

 During the cook, 10 to 12 percent chemicals (based on ovendry weight of wood) 

 are absorbed by the wood and the pH of the liquor drops to 8.2. At the end of the 

 cook the liquor is withdrawn and fortified for re-use. Logs, discharged from the 

 digester, are quenched in cold water and ground on conventional stone grinders. 

 The specific power required is only about half that for ordinary grinding, and 

 pulp yield is 88 to 93 percent. At a freeness of 200 ml such pulps had 40 percent 

 higher bursting strength than hardwood groundwood of 1 50 ml freeness made 

 conventionally. Gavelin noted thai Poplus sp. and Betula sp. responded well to 

 this treatment, while Fagus sp. gave less satisfactory results. Brecht (1958) 

 provided additional data on the process. 



In another process (ALB Semicell) used by some European groundwood 

 mills, hardwood logs are treated at 260° to 280°F in horizontal digesters with 

 sodium sulfite solution buffered with sodium carbonate to a pH of 7.5 to 9.4, 

 depending on species. After the digestion cycle, all waste liquor is reclaimed, 

 fortified, and reused. When stoneground, the treated logs yield pulp for use in 

 newsprint and as a replacement for conventional softwood groundwood in print- 

 ing papers (Barker 1962). 



These approaches applied to sweetgum and oak, although shown promising in 

 some laboratory experiments, have not been commercially viable; chemical 

 penetration throughout a whole log is unpredictable causing nonuniformity of 

 the groundwood in woods with specific gravity in excess of 0.50 to 0.55 (Swartz 

 1962). 



PRESSURIZED STONE GRINDING 



Because stone grinding requires only about two-thirds the energy per ton pulp 

 required by thermomechanical pulping (TMP) of chips in disk refiners, research- 

 ers seek ways to improve the strength of stoneground pulp to more nearly equal 

 that of TMP. One of the possibilities is to steam pressurize the stone-grinding 



