CALCIUM-INDUCED AGGREGATION OF DISSOLVED AND COLLOIDAL SUBSTANCES IN MECHANICAL PULP SUSPENSIONS

The calcium-induced aggregation of dissolved and colloidal substances (DCS) obtained from Norway spruce TMP suspensions was studied. At conc entrations exceeding 10 mmol/L added CaCl2, colloidal substances from unbleached TMP were only partially aggregated, whereas colloidal subst ances from alkaline peroxide bleached TMP were completely aggregated. Aggregation was caused by interactions between calcium and polygalactu ronic acids. Depolymerization of the polygalacturonic acids. Depolymer ization of the polygalacturonic acids by pectinase treatment of bleach ed TMP did not prevent complete calcium-induced aggregation of colloid al substances from occurring in the bleached samples. However, it did decrease the rate of aggregation. The alkaline conditions during perox ide bleaching probably caused the loss of colloidal stability observed upon addition of CaCl2 to DCS from peroxide-bleached TMP. The results also suggest that dissolved galactoglu-comannans have an affinity for colloidal lipophilic extractives and may therefore contribute to the steric stabilization of colloidal substances from unbleached TMP.