Low chlorine dioxide bleaching of kraft pulp using nitrous acid treatment

Non-chlorine bleaching of hardwood kraft pulp (LUKP) with a small addition of chlorine dioxide was investigated. Nitrous acid (Ak) treatment conducted at pH 1.3 and 70-90 degreesC for 30-180 min was proposed as the pre-treatment of oxygen (O) beaching. The A(k) treatment distinctly degraded the syringyl lignin model compounds veratrylglycerol-beta -syringyl ether (VS) by the C alpha -C beta cleavage yielding veratraldehyde, and kraft lignin was also degraded. The kappa number (Kn) of pulp decreased during the A(k) treatment. O bleaching was improved by the A(k) pre-treatment. A(k)-O bleaching gave a higher viscosity level than Ak-peroxide bleaching. Dioxins and coplanar polychlorinated biphenyls (PCBs) formed by chlorine dioxide (D) bleaching with a very large excess of chlorine dioxide following A(k)-O were determined. Formation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was a little above the minimum level of detection, but 2,3,7,8-tetrachlorodibenzofran (TCDF) was clearly determined. It was shown that an excess of chlorine dioxide may possibly cause formation of TCDFs. Therefore the sequence using the A(k)-O bleaching and a small addition of chlorine dioxide, A(k)-O-D, was proposed for LUKP.