Kraft pulp swelling

Black liquor from the kraft pulping of mixtures of Papua New Guinea hardwoods has poor swelling properties and sometimes high viscosities that could lead to burning difficulties in commercial recovery furnaces (130, 139-141). 

Figure 9.2 Comparison of the effect of repeated recycling of Kraft and thermomechanical pulp (TMP) fibres on tensile strength and fibre swelling. 

Black liquor from kraft or soda mills pulping hardwoods may present problems during its evaporation and combustion by conventional methods. These problems have been encountered with some eucalypt species, particularly with wood from trees of old age, and special measures may be needed to overcome the problems caused by extractives. Apart from the formation of deposits and scale, which reduces heat transfer in the evaporators, the extractives increase the viscosity (so that the evaporation process is affected) and reduce the swelling properties when incinerated, resulting in incomplete combustion (130). These difficulties with black liquor from the soda process can be avoided with the use of a wet oxidation process conducted under high pressure (97). 

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