Response of Mixed Pacific Northwest Softwood RDH-Kraft Pulps from High to Low Kappa Levels Using a Conventional Bleaching Sequence, 1991 Pulping Conference Proceedings

In the conventional-kraft pulping process, extended delignification of softwood to low kappa numbers (<20) has="" not="" generally="" been="" practiced="" because="" of="" lower="" selectivity="" and="" bleachability.="" however,="" the="" rdh-kraft="" pulping="" process,="" with="" its="" improved="" selectivity,="" has="" demonstrated="" the="" ability="" to="" cook="" softwood="" pulps="" to="" less="" than="" 16="" kappa="" number="" in="" both="" commercial="" and="" laboratory="" observations.="" the="" focus="" of="" this="" study="" is="" to="" evaluate="" the="" bleaching="" response="" of="" mixed="" pacific="" northwest="" softwood="" rdh-kraft="" pulps="" (cooked="" to="" 16,="" 24="" and="" 31="" kappa="" numbers)="" to="" a="" five-stage="" (dc)(eo)(d)(e)(d)="" bleaching="" sequence="" with="" and="" without="" brownstock="" oxygen="" delignification.="" the="" environmental="" impact="" of="" extended="" delignification="" is="" also="" discussed.="">

An RDH-Kraft pulp cooked to 16 kappa number, bleached by a (DC)(EO)(D)(E)(D) sequence at 0.21 kappa factor and 70% D-substitution in the (C)-stage, demonstrated that lignin in pulp reacts equally well with conventional bleaching chemicals compared to an RDH-kraft pulp (31 kappa) delignified with oxygen prebleaching to 18 kappa number followed by the same bleaching sequence. As a result, these two RDH pulps produced the same 89% GE final brightness. A combination of extended RDH cooking and oxygen prebleaching reduces softwood kappa number to below 10 leading to an AOX value of 1.0 kg/ODT pulp, an effluent color value of 53 kg/ODT pulp and a BOD5 value of 4.0 kg/ODT pulp. The fully bleached RDH-Kraft low kappa number pulp had the same or better pulp strength versus its high kappa counterpart.