[Oral Session 3] DISSOLVED AND COLLOIDAL SUBSTANCES RELEASED INTO WATER FROM BLEACHED CHEMICAL PULPS

Dissolved and colloidal substances released from pulps into process waters can accumulate and give detrimental effects on papermaking, especially in paper machine systems with a high degree or water system closure. It has been reported in the literature that beating of chemical pulps in the stock preparation can result in considerable dissolution and dispersion of fibre substances. Three ECF- and four TCF-bleached pulps delivered in dry form to the Sappi Austria AG fine paper mill in Gratkorn, including both softwood and hardwood pulps, were beaten in a Valley laboratory beater. Refiner beating in the mill was investigated by analysis of samples taken before and after the refiners. The dissolved and colloidal substances in the waters were separated from the fibres by centrifugation and were ana lysed by standard determinations and by GC analysis of carbohydrates and lipophilic extractives (resin). Considerable amounts of organic material was dissolved and dispersed already during slushing of the pulps. During beating in the Valley beater the concentrations increased further by 6-32%. Beating in the paper mill released little organic material from the pulps. The released dissolved and colloidal substances were mainly composed of hemicelluloses. with xylans as the dominant group. both for hardwood and softwood pulps. More lipophilic extractives were released from birch pulps than from eucalypt and softwood pulps. In mill samples. starch was the dominating dissolved material. Starch is used in the mill both in the wet end and as a coating color component. It was concluded that the amounts of dissolved and colloidal substances coming in with chemical pulps to the wet end of papermaking are dependent mainly on the cfficiency of pulp washing in the pulp mill. Only some additional fibre material is dissolved or dispersed during beating.