The influence of ECF bleaching history on the performance of a final hydrogen peroxide stage

The performance of a final hydrogen peroxide stage, applied instead of the conventional chlorine dioxide stage in the ECF bleaching of eucalypt kraft pulps, has been studied. Two industrial bleached pulps, one from a DED sequence and one from a OQ(PO)D sequence, were further bleached in the laboratory with both D and P stages, separately, until 90 +/- 0.5% ISO brightness. The papermaking potential of the four fully bleached pulps - DEDD, DEDP, OQ(PO)DD and OQ(PO)DP was assessed. While chlorine dioxide charge was nearly the same for both sequences, a much higher peroxide charge was needed in DEDP due to bleaching history impact on transition metals, residual chromophores and on additives performance. The decomposition of peroxide in the final P stage of the DEDP and OQ(PO)DP sequences was estimated. Decomposition reactions contribute significantly to peroxide consumption, especially at higher temperatures. Depending on the bleaching history, autoxidation reactions could contribute to the brightness gain at the final P stage. The final P stage mainly benefits pulp brightness stability, beatability and tensile strength.