Influence of pectic acids on aggregation and deposition of colloidal pitch

The effect of pectic acids on deposition of pitch extracted from thermomechanical pulp (TMP) and the deposition mechanisms in the presence of Ca2+-ions was investigated. A deposition tester with steel collector surfaces was applied to quantify the amount of deposits. The effect of pH on deposition was also studied. Model pectic acids with different molar masses (8,000 and 12,000 Da) prepared from a commercial pectin, and pectic acids in water from bleached TMP were tested. Destabilization and aggregation of colloidal pitch resulted in its accumulation on the surface and also in deposition. Addition of peroxide-bleached TMP water, rich in pectic acids, to a pitch dispersion in the presence of calcium caused an extensive aggregation. Addition of model pectic acids inhibited the aggregation to some extent. The effect was stronger with pectic acids of 12,000 Da. The deposition tendency of pitch was decreased to some extent by both TMP pectic acids and model pectic acids. The deposition was inhibited more efficiently at pH 8 than at pH 5, probably due to higher surface charge of the pitch droplets, leading to increased adsorption of pectic acid aggregates onto pitch droplets.