Inorganic ions in wood chips- Leaching prior to Kraft pulping and effect of calcium on Kraft delignification

Doctoral thesis, 2009

Wood is the main source of Non-Process Elements (NPEs), such as, calcium, potassium, manganese and chloride in Kraft pulp mills. Due to the recirculation of cooking chemicals and water, process disturbances related to the accumulation of NPEs in the process streams are inevitable. One option to decrease the problems caused by NPEs would be to include a chip leaching operation prior to cooking. This thesis examines the leaching of metal ions from wood material. The goal was to develop a model for the simulation and evaluation of different leaching options. The work also aimed to increase understanding of the impact of the removal of metal ions on Kraft delignification. Wood chips from Scandinavian softwood, birch and eucalyptus were leached. Birch and softwood were shown to have the same general leaching pattern for calcium and potassium. Eucalyptus showed the same leaching trend for potassium, but it was harder to remove calcium from eucalyptus than from softwood and birch. Further investigations of the eucalyptus wood material showed a high concentration of oxalate, which most likely partly explains why calcium was hard to remove. It was also noted that chloride is relatively easy to remove from eucalyptus chips. A model that describes the leaching of softwood chips, taking into account cracks, rough surfaces and the size distribution of the wood chips, was developed. The diffusion coefficient for calcium was determined from leaching experiments with hand-sawn pieces of spruce wood. The diffusion coefficient and the chip model were used together to develop models of a leaching process that can be used to simulate and evaluate different leaching designs. Findings from the Kraft delignification trials show that calcium in birch and aspen chips decreases the delignification rate when carbonate-free white liquor is used. The addition of carbonate or DTPA to the cooking liquor eliminates the retarding effect of calcium. These results imply that non-chelated soluble calcium retards the solubilisation of lignin. No major calcium effect on delignification could be seen in eucalyptus. A proposed explanation for this is that substances with calcium-chelating properties are released from eucalyptus during Kraft cooking.