Delignification of high Kappa number soda-ODiMAQ pulps with a polyoxometalate

The objectives of this study were (a) to investigate the possibilities for increasing the yield of chemical pulp by using two stages of delignification and (b) to evaluate the possibility of replacing the kraft process with a more environmentally benign pulping system. High Kappa number kraft, soda-anthraquinone (AQ) and soda-octahydrodimethylanthraquinone (ODiMAQ) pulps were produced and then delignified to Kappa number 30 with aqueous solutions of the polyoxometalate Na_4.9+2 [SiV_0.9MoW _10.1O₄₀]. Pulp yields and strengths were determined. The properties of these pulps were compared with those of Kappa 30 pulps produced by single-stage kraft, soda-AQ, and soda-ODiMAQ pulping. The high Kappa pulps delignified with a polyoxometalate (POM) were significantly higher in yield than were the single-stage pulps. The highest yield two-stage pulp was the POM-delignified kraft pulp. However, all the resulting pulps were somewhat weaker than the single-stage kraft pulp. As the Kappa number of the initial soda-ODiMAQ pulps was increased from 65 to 83 and finally 96, the POM delignification of these pulps to Kappa number 30 resulted in a constant level of pulp yield and significantly decreased pulp strength. Strong pulps were nevertheless produced in all cases. Soda-ODiMAQ delignification to approximately Kappa number 65 followed by POM delignification could be a replacement for kraft pulping.