3126 Chapter 25 



as southern red oak, white oak, water oak, black tupelo, sweetgum, ash sp. , and 

 bitter pecan {Carya aquatica (Michx. f.) Nutt.). She concluded that Hquor 

 concentrations above 45 to 50 g/liter do not improve pulp strength and tend to 

 darken the pulps. High digestion temperatures give the best strengths, but 

 produce pulps difficult to bleach. For complete liquor penetration at low tem- 

 peratures, small chips and 2 to 3 hours of retention time are necessary. Of the 

 species studied, only ash was totally unacceptable because of low strength. 



As previously noted, cold soda pulps are characteristically low in brightness. 

 Ahlen et al. (1979) found, however, that Gmelina arborea, a medium-density 

 tropical hardwood, can be used to produce a pulp for newsprint without bleach- 

 ing if sodium sulfite is added to the cold-soda impregnating liquid. They also 

 found that cold-soda hardwood pulps are easily bleached with peroxide as well 

 as hypochlorite, but the latter results in lower yield and higher yellowness. 



ENERGY TO MANUFACTURE MECHANICAL PULP 



At wood temperatures in the range from 300°-350°F, lignin in the middle 

 lamella softens and permits individual wood fibers to be separated from each 

 other with less energy than in cold wood (fig. 25-20). IMP processes exploit this 

 property of wood in a reduction system that involves heating and fiberizing in a 

 revolving-disk mill. Even for heated wood, however, energy required by disk 

 refiners is significantly greater than for other pulping processes. Large disk 

 refiners may be driven by motors totaling 12,000 hp or more. Energy costs for 

 TMP (about $40/air dry ton in 1977) are about 50 percent greater than those for 

 stone groundwood. Chemi-thermomechanical pulp, however, can be fiberized 

 with less energy than that required for stone groundwood. The unbleached kraft 

 pulping process can be almost self sufficient in energy; only in the lime kiln is 

 the use of some fossil fuel necessary. When energy required to manufacture 

 bleaching chemicals is taken into account, energy consumption is somewhat 

 greater for bleached pulp, but is still very low compared with that for mechanical 

 pulping (Poyry 1977). A comparison of energy requirements and yields ot 

 various hardwood pulping processes, from stone groundwood to extensive 

 chemical, are shown in table 25-11. 



The dense southern hardwoods not only have fiber morphology less well 

 suited than preferred softwoods to the manufacture of mechanical pulps, but 

 their density leads to unusually large expenditures of energy for fiberizing. 

 Readers interested in energy aspects of mechanical pulping with disk refiners, 

 and control of large-horsepower thermal mechanical pulping systems will find 

 useful Poyry (1977), Carty (1977), Koenig and Thompson (1978), Loly (1978), 

 Otis (1978), and Kurdin (1979). 



