Removal of Lignin-rich Surface Material from Unbleached Kraft Fibres

The composition and structure of the surface material of kraft fibres can affect properties of the pulp such as the reactivity in, for example, bleaching processes. In order to study the nature of the surface material, conditions for a gentle removal of fibre surface material from 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 PFI-mill, a high intensity mixer, and a disintegrator. The number of revolutions and the pulp concentration were varied and the removal of surface material was followed by chemical and microscopical analyses. These analyses revealed that there is lignin-rich material present on the fibre surface which can be removed by a mild mechanical treatment. The lignin content of the surface material abstracted was between 2.5 and 4.5 times higher than that of the fibres. The primary fines (obtained by fractionation of unbeaten pulp, 20μm), also originating from the fibre surface, had a lignin content almost 5 times higher than that recorded in the fibres. For analytical studies of the surface material, 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 efficient and suitable treatment for gradual removal of material from the fibre surface.