3138 Chapter 25 



easier to bleach hardwood pulps. Kraft hardwood pulps normally have perman- 

 ganate or Kappa numbers (numbers proportional to lignin content) of one-half 

 or less that of comparable softwood pulps. As an example, hardwood pulps are 

 often compared at a permanganate number of 12 ml. (25 ml. basis), whereas 

 softwood pulps are compared at a permanganate number of 25 ml. (40 ml. 

 basis). 



Kraft pulp yield (pulp weight as a percent of ovendry wood weight) from 

 hardwoods is higher than from softwoods even though their pulp retains less 

 lignin and screening rejects. Pine-site southern hardwoods have 1.1 to 9.6 

 percent extractives and these lower pulp yields. Since southern pines have 

 slightly more extractives (3 to 9 percent), their yield is reduced proportionately. 

 The higher hardwood yield stems not only from their lower extractive content, 

 but from their 1 to 2 percent higher average content of cellulose and their higher 

 content of xylan hemicellulose retained in kraft pulping. Yields of bleachable 

 pulp from southern pine are typically 45 to 46 percent as compared to about 48 to 

 49 percent for oak, sweetgum, and tupelo sp. (ovendry-weight basis). 



Pulping costs for hardwoods are less than those for southern pine because 

 kraft pulping conditions are less drastic for hardwoods than for softwoods; 

 depending upon species and type of pulp sought, either the time can be reduced 

 or the temperature lowered or both modified. Whereas southern pines are often 

 pulped at 170-175°C for 75 to 90 minutes, most hardwoods are adequately 

 treated at 160-170°C in 45 to 75 minutes. 



Hardwoods can be pulped with a lower liquor-to- wood ratio, a more concen- 

 trated liquor solution, and a more effective use of chemical than is possible with 

 southern pine. The amount of active (or effective) alkali required and the sulfi- 

 dity can be reduced several percentage points. For most hardwoods an active 

 alkali content of 15 percent (13.5 percent effective alkali) and a sulfidity of 20 

 percent will suffice to give a full chemical kraft pulp with acceptable rejects. 



Disadvantage of kraft pulps from pine-site hardwoods include lack of the 

 valuable byproducts (rosin and turpentine) typical of southern pine, and resis- 

 tance of hardwood pulps to diffuser washing; they are, however, well suited to 

 filter washing. 



Beating and bleaching. — Hardwood chemical pulps require less energy for 

 beating than comparable softwood pulps, probably because of their high hemi- 

 cellulose (specifically xylan) content. The presence of polar groups, especially 

 ionizable carboxyl groups, such as the uronic acids attached to xylan, has a 

 favorable influence on beatability of pulp. The availability of these uronic acids 

 account for most of the differences between sulfite and kraft pulps, and between 

 hardwood and softwood kraft pulps. The power requirement for beating pulps to 

 a given degree of hydration is usually 25 to 40 percent less for hardwood than for 

 softwood pulps. Hardwood pulps, however, must be beaten to much lower 

 freeness values than softwood pulps to obtain anywhere near equivalent values 

 for tensile and burst strength. Commercially, hardwood pulps are not usually 

 beaten to these freeness values to avoid slow drainage on the paper machine due 

 to fragmentation of thin- walled vessels and parenchyma. 



