Dithionite bleaching of thermomechanical pulp — chemistry and optimal conditions

The study described bleaching of thermomechanical pulp (TMP) mainly by a reductive bleaching agent, sodium dithionite. The theoretical part concentrated on the elucidation of lignin-preserving bleaching of mechanical pulps by introducing general concepts and reaction mechanisms. It also discussed novel analytical possibilities to determine sulphur-containing anions originated from dithionite. The primary objective of the experimental part was to find out relevant factors affecting achievable brightness gain in the lignin-preserving bleaching of pressure groundwood (PGW). The bleachability and brightness stability were characterised by employing brightness measurements after the sodium dithionite treatment. Observations were further confirmed by filtrate analyses and total sulphur measurements of TMP. Reaction pathways of dithionite bleaching were studied by analysing var-ious sulphur anions (bleach liquor and filtrates) and also the sulphur chemical-ly bound to fibres. The effects of bleaching factors were studied mainly by de-signing the experiments with the Taguchi method. The new information on re-actions of dithionite during the bleaching process further developed the postu-lated mechanism of lignin-preserving bleaching. The quantities of different sulphur constituents in filtrates and pulps after bleaching with respect to the added dithionite under laboratory-scale conditions could be presented. The efficiency of bleaching improved with an increase in temperature. Hence, tem-perature was the most important factor concerning the efficiency of dithionite bleaching, but it could not be extracted from other variables. The dosage and composition of dithionite grade were also significant factors and they should be optimised carefully for each mill-scale application with PGW.