The Donnan theory applied to pulp washing - Experimental studies on the removal of anionic substances from an assumed fiber lumen volume and from the fiber wall

• We have studied how anionic materials, i.e. methylorange, IndulinAT, and water soluble lignin residues from mill sampled pulps, are distributed between a restricted fiber wall liquor volume and the suspension liquor, as a function of the suspension liquor composition. The influence of time, temperature, and molecular size on the leaching of lignin residues from pulp fibers was also studied, as was the exchange of the residues between an assumed fiber lumen volume and the surrounding suspension liquor.It was shown that all the liquor up to a consistency of 35% can be considered as one homogeneous suspension liquor in all practical mill and laboratory situations. A gentle mechanical mixing or liquor displacement is sufficient to exchange the dissolved lignin residues between the assumed lumen volume and the surrounding suspension liquor. It was also shown that the level of mobile anionic material in the fiber wall liquor volume is significantly depleted at low salt concentrations and that the Donnan theory, which describes the fiber ion exchange properties, could be used to estimate the leaching of lignin residues from pulp fibers. Relatively low concentrations of magnesium, i.e. below 50 mM, or high concentrations of sodium ions, i.e. above 400 mM, seem to cause precipitation of these residues within the fiber wall, and reduce the amount of leachable lignin. This phenomenon is not accounted for by the Donnan theory. The leaching of high molecular weight lignin residues was found to be more sensitive to salt and temperature effects than low molecular weight residues.

Nyckelord

donnan equilibrium; fiber wall; leaching; lignin residues; precipitation; pressing; pulp washing

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