Influence of final bleaching stage on ECF brightness development, refineability and pulp properties

In this paper, the chemistry of chlorine dioxide (D) and hydrogen peroxide (P), two predominant chemicals used in final bleaching stages, is reviewed. Two distinct final bleaching stages, DD and DP are compared with respect to their ability to produce not only very bright and optically balanced pulps but pulps with low brightness reversion. Comparison is also made between DD and DP stages in terms of refineability and final pulp properties. Hot chlorine dioxide delignification (DHT) resulted in higher brightness pulps over conventionally delignified (D0) pulps. A final D bleaching resulted in higher brightness pulp than that of a P bleached pulp, but the pulp did not keep this brightness advantage upon aging. Incorporating a peroxide polishing step to DD bleaching provided not only higher brightness pulp than that of DP bleaching, but also more stable pulps; a low post color (PC) number and color index (CI) values. All these properties can be achieved at lower chemical consumption, an advantage the only DD bleached pulps can provide when compared to DP bleached pulp. Refining pulps bleached at the higher pH end is easier than bleaching at low pH. Between DD and DP bleached pulps, the former provided a better Tensile and Tear Index than the latter.