REMOVAL OF LIGNIN-RICH SURFACE MATERIAL FROM UNBLEACHED KRAFT FIBERS

The composition and structure of the surface material of kraft fibres can affect properties of the pulp such as the reactivity in, for examp le, bleaching processes. In order to study the nature of the surface m aterial, conditions for a gentle removal of fibre surface material fro m unbleached kraft fibres have been evaluated. An unbleached softwood kraft pulp with a lignin content of 6.2% (kappa number 32) was treated mechanically using three different types of laboratory equipment: a P FI-mill, a high intensity mixer, and a disintegrator. The number of re volutions and the pulp concentration were varied and the removal of su rface material was followed by chemical and microscopical analyses. Th ese analyses revealed that there is lignin-rich material present on th e fibre surface which can be removed by a mild mechanical treatment. T he lignin content of the surface material abstracted was between 2.5 a nd 4.5 times higher than that of the fibres. The primary fines (obtain ed by fractionation of unbeaten pulp, 20 mu m), also originating from the fibre surface, had a lignin content almost 5 times higher than tha t recorded in the fibres. For analytical studies of the surface materi al, a mechanical treatment of the kraft fibres in a disintegrator at a pulp concentration of 4.5% and at 20.000-200.000 revolutions is an ef ficient and suitable treatment for gradual removal of material from th e fibre surface.