| Summary: | 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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