Extended oxygen delignification via the double-stage process, reinforced with peroxide in the second stage

The effect of hydrogen peroxide addition to the second stage of a double-stage oxygen delignification system was evaluated. The effects of simple washing, chelation and acidification between stages was studied. The main process variables of the peroxide reinforced second stage oxygen delignification were optimized, including alkali charge, reaction consistency and the use of additives. The peroxide reinforced double-stage oxygen delignification results were compared with those of conventional double-stage oxygen delignification. Oxygen delignified pulps obtained by the reference system, (OO)process, and by the peroxide aided system at high consistency in the second stage, (Oq)Ophc-process, were bleached by ECF sequences. The results indicated that peroxide addition in the second oxygen stage is more efficiently effected at high consistency. Inter-stage metals removal is preferably done by acidification which seems to activate lignin and/or remove hexenuronic acids. The use of DTPA as part of the bleaching liquor in the peroxide aided second oxygen stage is not recommended. Adequate alkali adjustment in the second stage is imperative for successful performance of this stage. Comparison of different oxygen delignification systems must be done at similar final pH values. The chemical cost for bleaching the (Oq)Ophc-treated pulp with the DEopDED ECF sequence was 39.6% lower than that of the (OO)-treated one. However, the total cost for delignification and bleaching was about 9% higher for the (Oq)Ophc-delignified pulp. Despite the slightly higher cost, the (Oq)Ophc-process is advantageous on a mill closure approach because it decreases the kappa number entering the bleach plant by 38% and the chlorine dioxide requirement by 44%, in relation to the (OO)-process.