Pulp and Paper 3137 



Reference Subject 



Perkins (1958) Kraft pulping and bleaching of southern hardwoods 



Schroeder (1976) Review of techniques, properties, and markets 



McGovem (1977) Oak kraft pulp in offset printing papers 



Rushton and Howe (1978) Wood utilization by the southern pulp and paper industry 



Worster (1978) Hardwood potential for kraft linerboard 



Hart (1980) Potential for expanded hardwood use 



The following subsection, which is condensed from Schroeder's (1976) re- 

 view, contrasts kraft pulping of southern hardwoods to the pulping of southern 

 pine, and compares resulting pulps. 



HARDWOOD KRAFT PULPING VERSUS SOFTWOOD 

 KRAFT PULPING 



Pulping. — The advantages tend to be with the hardwoods, with a few notable 

 exceptions. An advantage of high density southern hardwoods is that digesters 

 filled with dense wood have greater output capacity than if filled with low- 

 density wood. Digesters charged with oaks will produce more pulp than if 

 charged with southern pine. Outputs from digesters charged with sweetgum and 

 tupelo sp. are about equal to, while those with yellow-poplar are less than, those 

 charged with southern pine. 



Energy required by a digester filled with dense wood is not greatly different 

 than that needed to pulp one filled with low-density wood; thus dense wood 

 requires less energy per ton of pulp than low-density wood. 



Required liquor-to-wood ratio during digestion is lower for dense than for 

 low-density woods. Chemical requirement is based on wood weight and a lower 

 liquid-to-wood ratio permits use of a more concentrated solution, which uses the 

 chemical more effectively, thereby reducing overall chemical requirement. 

 With a lower liquid-to-wood ratio the solids content of the spent liquor is higher, 

 and the energy required to recover the contained chemicals is lower. 



As with dense latewood of pines, exceptionally dense hardwoods, such as live 

 oaks, present a complex diffusion problem, involving penetration of the chemi- 

 cals into the chip and the cell wall, localized depletion of alkali and outward 

 diffusion of cell wall constituents, especially solubilized lignin. As shorter 

 pulping cycles are adopted, this problem may become critical because reject 

 proportion increases as the pulping cycle is shortened. 



The differences in chemical constitution of hardwoods and softwoods give the 

 hardwoods a very distinct advantage in chemical pulping processes, especially 

 in alkaline processes. There is less lignin in hardwoods, it occurs in the cell wall, 

 is less highly condensed than that in southern pine, and it has less tendency to 

 condense under alkaline pulping conditions. Lessened condensation of hard- 

 wood lignin is the primary reason why hardwoods are more easily pulped than 

 softwoods, and why it is possible to produce acceptable pulps from hardwoods 

 by the hot soda process but not from softwoods. This is also why it is possible to 

 delignify hardwoods further in chemical pulping than softwoods and why it is 



