Modelling the distribution of "free", complexed and precipitated metal ions in a pulp suspension using Donnan equilibria

Mill kraft pulps from oxygen delignification (O) and alkaline hydrogen pero xide (P) stages were ana analyzed for ion exchange capacity and metal conte nt, and the O-stage pulp was manipulated to simulate a chelating treatment stage (Q). The anionic charge of the O-stage pulp fibres was balanced by Na + ions, while the divalent metal ions were present as solid carbonates. On acidification, these carbonates gradually dissolve and the divalent cations predominantly replace Na+ in the fibre wall. Since the amount of cations e xceed the fibre ion-exchange capacity, a significant portion of the metal i ons end up in the suspension liquor. Addition of strong chelants forms anio nic complexes with Mn2+, e.g. Mn(DTPA)(3) and these anions are expelled fro m the fibre wall. On pulp re-alkalization, as in a P stage, Mg2+ and a smal l portion of the Ca2+ are re-precipitated. The metal ion distribution can, in all of these cases, be quantitatively modelled by the Donnan theory if a lso precipitated and complexed forms of the metal ions are considered.