Production of bleached softwood pulp by low pollution processes

Abstract

Some aspects of sulphur-free soda pulping with anthraquinone (AQ) additions and chlorine-free bleaching with oxygen-based treatments were investigated with the aim of producing bleached softwood pulp with kraft-like strength properties. The extent of soda-AQ pulping was studied by pulping to 32, 42 and 68 Kappa number. After oxygen delignification to 10, 15 and 20 Kappa number respectively, the extent of bleaching with ozone, alkali extraction, and hydrogen peroxide treatments (ZEP sequence) was studied. For comparison, oxygen delignified pulps were conventionally bleached with a sequential chlorine/chlorine dioxide, alkali extraction, and chlorine dioxide (D/CED) sequence. A reference kraft pulp at 45 Kappa number was similarly oxygen delignified and bleached. Kraft-like strength properties (as measured by tear/burst relationships) were attainable if soda-AQ pulping was terminated at high Kappa number and was followed by oxygen delignification and D/CED bleaching. This pulp had a 3% (on wood) higher total yield and consumed less active alkali (2.0% as Na2O) than the bleached reference kraft pulp. Soda-AQ pulping followed by oxygen delignification and ZEP bleaching produced pulp with strength properties that were at least 25% less than those of the reference kraft pulp after oxygen delignification and conventional bleaching with chlorine-based treatments. Although the combination soda-AQ pulping to low Kappa number and extended oxygen delignification adversely affected pulp strength properties, it was ZEP bleaching and ozone treatments in particular, which caused the majority of the strength loss.