Modelling ion exchange and flow in pulp suspensions

Abstract

This thesis summarizes the results of four papers dealing with modelling of ion exchange between cellulosic fibers and their surrounding aqueous solution.

Transition metals, in particular manganese, iron and copper, induce fiber-deteriorating radical reactions in oxygen-chemical-based bleaching, while magnesium inhibits such reactions. Thus the removal of the harmful transition metals from a pulp is required while the beneficial magnesium should be retained in a pulp. Selective control of metal contents of pulps is a key factor in the control of bleaching chemistry. Thus, ability to predict the extent of ion exchange and amounts of metals are of great practical interest.

An ion exchange model to describe interaction of ionic species with kraft pulps has been developed in the present work. The model can be applied in pulp washing and chelation. The present model takes into account overall ionic composition of pulp suspensions, i.e. the cations and simple anions as well as the ligands and the complexes. The model facilitates calculation of pH, complexation and distribution of ionic species between the aqueous phase confined to fibers and the external solution. The present model also accounts for the ion exchange kinetics and flow of water through the immobile fiber network, i.e. the "pulp bed".

The present model is based on the formerly developed ion exchange model, which is based on so-called Donnan equilibrium theory. The development of the new model required the inclusion of the different anionic species to model and the combination of the Donnan model with a complexation model. The ion exchange kinetics is described as a two-step phenomenon. The first rate-limiting step can be due to the dissolution of solid metal compounds in the fiber phase as well as the diffusion of ions in the fiber wall, while the second step is the transfer of the ionic species between the fiber phase and the external solution, which is governed by the concentration gradient between the phases. In the model, the transfer of ions between the phases is directly related to the Donnan theory.

In the testing of the elements of the ion exchange model, it was found that the combination of the Donnan model with a complexation model quantitatively predicts the pH, the ionic distribution as well as the complexation equilibria in batch-type systems when the time to achieve full equilibrium is not limited. In the pulp beds subject to flow the ion exchange kinetics reduces the metal removal efficiency and selectivity. This can be considered to be due to the repulsion of anionic species (simple anions, ligands and complexes) from the negatively charged fiber phase. It can also be considered that the rate of change of fiber charge determines the overall ion exchange kinetics.

It was found in the experiments that a low pH of displacement solution favors rapid metal removal from pulp beds. However, a low pH reduces the metal removal selectivity, i.e. harmful manganese and the beneficial magnesium is also removed from the pulp. The metal removal process in laboratory or in industrial scale could be optimized in terms of chemical concentrations, pH, reaction time and wash flow conditions. The present model facilitates realistic modelling of these conditions.