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Thermodynamic model and data on the partitioning of nonprocess elements between kraft fibres and water in a pulp suspension Bygrave, Geoffrey Scott


The partitioning of non-process elements in kraft fibre suspensions has been modeled using the Donnan equilibrium framework with activity coefficients introduced to account for the non-ideal nature of the electrolyte solutions. Activity coefficients have been obtained using Pitzer's model. Total anionic charge, dissociation constants, and mass of water within the fibres are used as input to the model and were determined by independent experiments. Partitioning data were also obtained for a post brown stock washer kraft pulp, from an interior BC mill, for the metal ions of Na⁺, Ca⁺² , and Mg⁺² over the pH range 2-13. Such data above a pH of 11 were not previously available in the literature. Two models were used to obtain the charge on the fibres at any pH; a two dissociation constant model (corresponding roughly to the carboxyl and phenolic hydroxyl) and an "exact" charge model. Results indicate that below a pH of 7 both models do a good job of representing the experimental data obtained in this work and data available in the literature. Above a pH of 7 the two dissociation constant model under-predicts the partitioning data while the exact charge model follows the experimental data over the entire pH range. This indicates Donnan equilibrium is an excellent framework for non-process element partitioning prediction as long as the fibre charge is known accurately. Experiments with elevated Mg and Ca contents were also performed, resulting in precipitation within the fibres at pHs above 10. The model does not account for precipitation phenomena that may occur when concentrations of Ca and Mg are increased above those naturally occurring in post brown stock pulps.

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