How to Improve Brightness Stability of ECF bleached Softwood and Hardwood Kraft Pulp

摘要

A comparison of different ECF bleaching sequences regarding the resulting brightness stability was made. The results show an improved stability for brighter pulps and no direct influence of ozone, hot acid hydrolysis or hot peroxide stages. The brighter the pulp, the more stable brightness becomes. Provided removal of double bonds is effective (final Kappa number is below 1), this is generally valid. However, the final bleaching stage and the conditions used in this stage does affect brightness stability. A pulp bleached in its final stage with chlorine dioxide (D_2) more likely loses brightness in aging than pulp bleached with a final P stage. Bleaching with chlorine dioxide leaves quinoid structures in the pulp, with a pronounced tendency for more reversion. Because the oxidation of quinones is easily accomplished with hydrogen peroxide brightness stability is improved with a final P stage. D stage brightness reversion decreases with the application of very high temperature and/or long retention time. Reversion is lowest following a hot D_1 and a final P stage.