The Exchange and Removal of the Metal Cations in Pulps

Metal cations modify the optical and mechanical properties of pulps and in pulp mills can have adverse effects on such parameters as corrosion rate and recovery furnace temperature. With today’s emphasis on recycling effluents and on tighter control of all operations, there is a need for quantitative theory predicting the uptake of cations by pulps and the buildup of cations in process liquors. A variation of the Donnan Equilibrium Theory was recently developed to describe the partitioning of cations between the fibres and the liquor in a pulp suspension [1]. In this report we extend use of the theory to examine the ion-exchange of pulps from saturation with one cation to saturation with another. Theoretical predictions are closely duplicated by experiments using sodium, magnesium and lanthanum as model cations of different valencies. Pulps readily exchange cations even with very dilute solutions, preferentially taking up and most tenaciously holding the higher valency cations. Consequently, the replacement of high valency cations by monovalent cations requires higher concentrations of the replacement ion than for the reverse exchange. The differing tenacities by which cations are held are also shown during acid washing of pulps. Upon progressively lowering the pH of a suspension of pulp in mixed ionic form, monovalent, divalent and trivalent cations are successively released and replaced by hydrogen ions. The implication of these results for the “acid washing” of pulps is discussed.