THE LEACHING OF LIGNIN AND CARBOHYDRATE FROM HIGH YIELD PULP FIBRES SUSPENDED IN WATER

When high yield chemimechanical sulphite pulp fibres were suspended in water, both lignin and carbohydrate were leached out of the pulp. The rate of leaching fitted a theory developed previously for the diffusion of macromolecules through the water-swollen fibre wall. The average intrafibre diffusion coefficients for lignin and carbohydrate were several orders of magnitude lower than the free diffusion of these macromolecules in solution. Leaching over a long period indicated a polydispersity of diffusion coefficients. This was related to an increase with time in the molecular weights of macromolecules removed from the fibre. Although the rate of leaching increased markedly with temperature, the diffusion coefficient of lignin decreased, while that of carbohydrate increased by a factor higher than that predicted by the Stokes-Einstein equation. These contrasting results were interpreted in terms of the molecular weights of the diffusing species and hydrogen bonding of carbohydrate to the fibre wall. The rate of leaching decreased with an increase in pulp yield, increasing cationic strength and pH of the wash water, decreasing refining energy, pretreatment of pulp with formaldehyde, and the use of polyethyleneoxide/alum retention aid in the wash water. Decreasing pH of the cooking liquor and the use of anthraquinone in the pulping increased the leaching of lignin but did not affect the leaching of carbohydrate. The effects observed could be related either to swelling or degradation of the fibre wall.