A novel (DPmo) combination stage for more efficient delignification and bleaching of hardwood and softwood kraft pulp

摘要

The objective of this study was to evaluate different variations of the combination of a hot chlorine dioxide (D) stage and an acidic, molybdate catalyzed hydrogen peroxide stage (Pmo) and to compare to other state-of-the-art options for a first bleaching stage after oxygen delignification and prior to an alkaline extraction stage. Basic studies were conducted with oxygen delignified eucalyptus Kraft pulp provided by South American pulp mills. Surprisingly and in contrast to previous scientific and patent literature, the (DPmo) process variant (a short hot chlorine dioxide stage followed without intermediate washing by an extended acidic, molybdate catalyzed hydrogen peroxide stage) turned out to be the most effective with regard to kappa number reduction and brightness gain. There was a moderate viscosity loss, which was even smaller than observed in a single D stage. The novel (DPmo) stage outperforms single stages such as a hot acid stage (A), an ozone stage (Z), a Pmo or a D stage. It is also superior to the other possible variations of the combination, i.e. (PmoD) with reversed order or the simultaneous application of chlorine dioxide and hydrogen peroxide in a Dpmo stage. It was found that a temperature lower than 90 °C can be compensated by a higher catalyst concentration and/or an extended retention time. Full bleaching sequences containing the (DPmo) stage were run and compared to corresponding sequences with a single D stage. Significantly less total chlorine dioxide is needed in the (DPmo) combination sequences compared to the conventional bleaching sequence. The savings in chlorine dioxide should easily overcompensate the higher total demand of hydrogen peroxide which is due to the additional use in the combination stage. Measurements of mechanical properties of fully bleached hardwood pulp have shown that the (DPmo) stage does not have a negative impact. First results of bleaching trials with softwood pulp from a New Zealand Kraft pulp mill show a savings potential in chlorine dioxide consumption as well. In contrast to hardwood pulp, conditions for softwood pulp bleaching have to be chosen more carefully in order not to harm the pulp quality. Thanks to a recently developed molybdate recovery and recycle process, the (DPmo) stage has become a viable candidate for existing or future bleaching lines.