Pulp and Paper 3 1 25 



Kraft black liquor has also been used to pretreat birch chips prior to thermo- 

 mechanical pulping; Kossoi et al. (1976) found that the black liquor plasticizes 

 the chips and neutralizes acids formed during the cook, thus reducing hydrolysis 

 of hemicelluloses and degradation of the fibers. 



Also, Eucalyptus sp., is commercially pulped in Tasmania and elsewhere by 

 chemimechanical processes (Higgins et al. 1977, 1978; Gavelin and Lunden 

 1980). 



Northern red oak and five other hardwood species of varying morphology 

 were pretreated by Marton et al. (1979) with sodium hydroxide and sodium 

 sulfite prior to fiberization, or in the disk refiner with hydrogen peroxide in 

 conjunction with second-stage refining. They found that the pretreatment im- 

 proved strength properties at the expense of yield, brightness, and opacity; these 

 shortcomings were eliminated by in-refiner hydrogen peroxide bleaching which, 

 in addition, allowed savings in refiner energy. 



COLD SODA PULP 



In their review of mechanical and semichemical pulping of hardwoods grow- 

 ing on southern pine sites McGovem and Auchter (1976) noted that chemime- 

 chanical pulping of hardwoods (yields of 85-95 percent) of a wide range of 

 densities started in the mid- 1950's and grew to operations in about 20 mills. The 

 process has tended to be superseded by other processes as operation economics 

 and fiber use requirements have changed. Although the development was mostly 

 applied in the North, two cold soda chemimechanical installations were made 

 in the South. One of these making a pulp for bleached paperboard was discontin- 

 ued because of a paper-grade change (Paper Trade Journal 1974). The other, 

 making a brightened news-grade pulp from southern hardwoods, primarily oak, 

 continues successfully; the chemimechanical pulp makes up about 15 percent of 

 the newsprint pulp furnish, and the spent pulping liquor is recovered in the 

 plant's kraft mill system (McGovern and Auchter 1976). 



Brown (1958) estimated the following requirements for the manufacture of 

 one ton of cold soda pulp for newsprint: 



0.7 cord of oak 86 pounds of lime for bleaching 



155 pounds of caustic soda 72 pounds of chlorine for bleaching 



40 hp days of energy for fiberizing 



The cold soda process, as developed by the U.S. Forest Products Laboratory 

 consists of steeping hardwood chips in a sodium hydroxide solution at ambient 

 temperature and atmospheric pressure, draining the liquor, and fiberizing the 

 softened chips in a disk refiner. It is a simple process, with high yield, and is 

 applicable to many hardwoods otherwise difficult to pulp. The pulps are charac- 

 teristically low in brightness, and if the wood is particularly dense incomplete 

 penetration of solution results in a nonuniform pulp — chips being softened on 

 the outside but uncooked at the center. These disadvantages often outweigh the 

 advantages of cold soda and have limited use of the process (Cabella 1963). 



Readers interested in the cold soda process as applied to water oak, willow 

 oak {Quercus phellos L.), and sweetgum will find Brown's (1959) work useful. 

 Cabella (1963) investigated cold soda pulping of such dense southern hardwoods 



