Aspects on metal ion distribution models in pulp washing simulation

The closure of the liquid system in a pulp mill has increased the concentration of metal ions in the process as compared to traditional plants. These ions commonly referred to as non-process elements, NPE, enter the pulp mill with the wood chips, the water supply and the pulping chemicals. The presence of these ions may lead to various problems, e.g. incrust formation, filtration failures and reduced bleaching efficiency and the objective of the pulp washing operation is to remove a major part of these metal ions from the pulp. The objective of this work was to study the influence of sorption of metal ions on the simulation of pulp washing. A process simulator, ASPEN Plus, has been used to describe the different steps in a displacement drum washer. In order to take account of the sorption of metal ions on the pulp fibres blocks with FORTRAN subroutines have been developed and included. The simulations studies show that the washing results are quite similar for all models at high concentration levels. Hence, the adsorption does not contribute significantly except for low concentration cases. The influence of the sorption model and thus also the choice of model is important only in washing stages where the concentration of ions is very low. Comparison with data for an industrial displacement drum washer indicated the usefulness of the simulation model in evaluation of the operation. It was found that the displacement-washing step was far from ideal probably due to uneven liquid distribution and formation problems. For washing of solids other than paper pulp, e.g. lime mud, the effect of sorption metal ions on the washing result is less pronounced and a simpler model can be used with good results