AOX formation in ECF bleaching at different kappa numbers--influence of oxygen delignification and hexenuronic acid content.

Application: This study gives additional insight on the AOX formation in ECF bleaching. The information may be used to take appropriate measures when addressing environmental issues.

The aim of this study was to determine how the adsorbable organic halogen (AOX) formation in the elemental chlorine tree (ECF) bleaching effluent was influenced by oxygen delignification before bleaching and by the hexenuronic acid (HexA) content of the pulps. Softwood industrial chips cooked to different kappa numbers were oxygen delignified, treated in a hot acid stage ([A.sup.*]), or not pretreated at all before ECF bleaching. We then compared the AOX levels in the ECF effluents from bleaching of these pulps.

The results showed a higher formation of AOX for oxygen delignified pulps than for non-oxygen delignified pulps compared at the same kappa number before ECF bleaching. The hexenuronic acid (HexA) content of the pulps had a major impact on the AOX levels in tire DO effluent. The oxygen delignified pulps had a higher HexA content than the non-oxygen delignified pulps compared at the same kappa number. This explains the higher AOX formation from oxygen delignified pulps. Tire AOX formed due to HexA probably consists of simple chlorinated dicarboxylic acids. It was reduced in the filtrates upon storage. The results indicate that, with the appropriate treatment of the effluent in a pulp mill, the major part of this AOX originating from HexA can be removed before it reaches the recipient.

Bjorklund is a PhD student and Germgard is professor at Karlstad University, SE-651 88 Karlstad, Sweden; Jour is senior R&D engineer and Forsstrom is senior R&D engineer with Eka Chemicals AB, SE-445 80 Bohus, Sweden. Email Bjorklund at [e-mail].