A thermodynamics-based model and data for Ca, Mg, and Na ion partitioning in kraft pulp fibre suspensions

Ions of Ca, Mg and Na are among the metal ions which originate from wood, water, and make up chemicals and are found in kraft pulp fibre suspensions. In closed-cycle mill operation, which is motivated by environmental concerns, such metal ions accumulate and create various operational and product quality problems. Partitioning data and a model that will predict the distribution of metal ions between the fibres and the surrounding liquor in a pulp suspension are needed for effective design of closed-cycle mill configurations. In this work, partitioning data for the ions of sodium, calcium and magnesium in a post brown stock washer pulp are presented. Such data above a pH of 11 were previously not available. A model based on nonideal Donnan equilibrium is also presented. The model was able to calculate the partitioning of Ca and Na ions. The magnesium data showed considerable scatter that prevented an effective comparison with the model-calculated values.