Full Bleaching of Eucalyptus Kraft-O Pulp with Peracids and Pressurized H2O2, 1996 Pulping Conference Proceedings

Jose Barna, Du Pont do Brazil S/A
Jorge L. Colodette, Federal University of Vicosa
Elias Salvador & Eleuice R. P. Maia, Federal University of Vicosa

The effects of chelation conditions, additives, temperature reaction time, pressurizing gas, reaction pressure and peroxide charge were evaluated in pressurized peroxide bleaching (PP) of a eucalyptus kraft-O pulp. The optimized Pp-stage was inserted into TCF and ECF bleaching sequences such as PxaQPp (Pxa=mixed peracids), QEopPxa=QPp, DPpPxaED and DEopPxaED and the results were compared with those of conventional TCF-peroxide (QP) and ECF-chlorine dioxide (DEopDED) bleaching ones. It was shown that adequate pulp pretreatment, the use of additives and optimum process conditions are critical for a successful Pp-stage. The Pp-stage is very effective in brightening the pulp while removing only a small fraction of the residual lignin. pulps bleached only with pressurized peroxide present high kappa numbers, good strength properties and poor brightness stability. The bleaching efficiency of the Pp-stage is greatly improved if the pulp is pretreated with Pxa. A bleached pulp of 90+ ISO brightness and of acceptable brightness stability and pulp strength was achieved after Pp bleaching of a pulp sample pretreated with 0.42% Pxa as H2 O2 . It was demonstrated that the PxaQ pretreated pulp responds better to Pp bleaching than the QPxa pretreated one. The use of the combination Pxa-Pp is very attractive to convert the standard bleaching sequence (DC)EopDED into ECF or TCF. The percent replacement ratio Pxa:ClO2 to convert this sequence into the DEopPxaED one is 1:2. Pulps bleached by both TCF and ECF sequences containing Pxa present good strength properties and brightness stability.