Reduction of adsorbable organically bound halogens (AOX) formation at near-neutral pH chlorine dioxide bleaching of softwood kraft pulp
2020
Starrsjö, Sara | Sevastyanova, Olena | Sandström, Peter | Fiskari, Juha | Boman, Maria | Lindström, Mikael E.
Recently, a new type of bleaching sequence, Elemental Chlorine Free (ECF) light with one D stage, has been developed. It combines the efficiency and high selectivity of chlorine dioxide (ClO₂) bleaching with more environmental friendly oxygen based bleaching chemicals. This work examines the effect of pH on the formation of adsorbable organically bound halogens (AOX) in an intermediate D stage – a single ClO₂ stage at the middle of an ECF light bleaching sequence. Carbon dioxide (CO₂) is used to generate a bicarbonate buffer in situ, stabilizing the pH during the bleaching. Near-neutral pH is hypothesized to decrease the formation of strongly chlorinating species, so that the AOX formation is reduced. The results indicate that a near-neutral pH D stage can reduce the AOX content in the effluents with up to 30%. The ISO brightness was unchanged to a lower ClO₂ consumption. The pulp viscosity was slightly higher after near-neutral pH D stage, but to its disadvantage a lesser delignification and removal of HexA was obtained. The degradation of HexA correlated well with the AOX, affirming earlier theories that HexA has a major impact on the AOX formation. The higher amounts of residual HexA and lignin resulted in more thermal yellowing of the pulps bleached with a near-neutral pH D stage.
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