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

Several processes used for bleaching mechanical and chemi-mechanical P ulPs 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) an d double-stage peroxide bleaching (PP) at medium consistency in each b leaching stage (MC-MC) and at medium consistency in the first stage an d at high consistency in the second stage (MC-HC). Laboratory bleachin g results obtained for a commercial CTMP pulp of a spruce/balsam mixtu re were used as the input data for mass balance calculations which wer e done using the computer simulation package GEMS. The process flow di agrams 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 maximu m ISO-brightness values obtained as well as yellow shade, luminosity a nd brightness reversion of the bleached pulps after thermal treatment. It is shown that the double-stage bleaching sequence using peroxide a t high consistency in the first stage and hydrosulphite at medium cons istency in the second (PY) is the most cost-effective process in addit ion to giving maximum achievable brightness (84.2% ISO), highest lumin osity (L. = 96.1), lowest yellow shade (b. = 5.51) and brightness reve rsion (DELTA1.5%) after thermal treatment.