Modelling ion exchange and flow in pulp suspensions

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 fiberdeteriorating 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

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