Comparison of Oxalate Formation in Peroxide Bleaching of Softwood and Hardwood Mechanical Pulps

Four mechanical pulps, namely, spruce TMP, aspen CTMP, maple CTMP and encalyptus CTMP, were subjected to alkaline peroxide bleaching under various conditions, and their oxalate formation was compared in terms of bleaching conditions, pulp brightness, brightness gain and peroxide consumption. Results show that under similar bleaching conditions, the softwood TMP always produced less oxalate than the three hardwood CTMP pulps. However, the oxalate formation at a given brightness gain depends on the unbleached pulp brightness, and pulps with higher original brightness produced less oxalate for a given brightness target. The difference in the amount of oxalate formed per unit of brightness gain was small between the spruce TMP, maple CTMP and encalyptus CTMP. For the same pulp the Mg(OH)(2)-based peroxide process produced similar or slightly more oxalate than the NaOH-based process for a given brightness target. Kinetic models were also developed to compare the oxalate formation process and the brightening process.