Pulp washing: Sorption equilibria of metal ions on kraft pulps

A kraft pulp mill with a high degree of system closure has an increased accumulation of trace elements, and since the alternative bleaching methods used, such as peroxide and ozone bleaching, are sensitive to transition metals, there is a need for more efficient separation operations. An important aspect to establish is the influence of metal ion equilibria in pulp washing. In this investigation, the sorption equilibria of Na⁺, Mg²⁺, Ca²⁺ and Mn²⁺ were studied, both for each ion and for combinations of ions, including temperature dependence, at a constant pH and with no chelant added. It was found that the Langmuir isotherm gave a good correlation of the sorption of single ions. A model for the multi-ion sorption, based on the Langmuir model, was found to give an acceptable correlation. The ion charge was accounted for in different ways in the summation of the concentration: linear dependence on the charge in the liquor and no dependence in the fibre gave the best correlation, although the more plausible case with quadratic dependence in the liquor and linear in the fibre was nearly as good. The separation factor, the quota between two ion equilibrium ratios, showed that Na⁺ had a weaker bond to the fibres than Mg²⁺ and Ca²⁺, and that Mn²⁺ bound more strongly than Mg²⁺. A slight temperature dependence of the equilibria was observed.