BLEACHING PROCESSES FOR THE PRODUCTION OF MECHANICAL AND CHEMIMECHANICAL PULPS OF HIGH-BRIGHTNESS

Several processes used for bleaching mechanical and chemi-mechanical PulPs at high brightness levels are compared: (I) single-stage peroxide bleaching (P) at high consistency (HC) and at medium consistency (MC) with and without recovery of the residual peroxide; (2) double-stage bleaching sequences such as peroxide followed by hydrosulphite (PY) and double-stage peroxide bleaching (PP) at medium consistency in each bleaching stage (MC-MC) and at medium consistency in the first stage and at high consistency in the second stage (MC-HC). Laboratory bleaching results obtained for a commercial CTMP pulp of a spruce/balsam mixture were used as the input data for mass balance calculations which were done using the computer simulation package GEMS. The process flow diagrams with the white water recirculation strategy are described. The processes are compared in terms of peroxide charge required to reach a given ISO-brightness level, the estimated bleaching costs, the maximum ISO-brightness values obtained as well as yellow shade, luminosity and brightness reversion of the bleached pulps after thermal treatment. It is shown that the double-stage bleaching sequence using peroxide at high consistency in the first stage and hydrosulphite at medium consistency in the second (PY) is the most cost-effective process in addition to giving maximum achievable brightness (84.2% ISO), highest luminosity (L. = 96.1), lowest yellow shade (b. = 5.51) and brightness reversion (DELTA1.5%) after thermal treatment.