The fate of chlorine species during high temperature chlorine dioxide bleaching

Losses of chlorine dioxide do occur during ECF bleaching by side reactions that produce chlorite, chlorate and other inactive chlorine species. The fate of chlorine species under normal chlorine dioxide bleaching is quite well known but not so under high temerature (> 90 degrees C). This study evaluated the fate of chlorine species and bleaching/delignification performances of normal and high temperature chlorine dioxide stages for eucalyptus kraft-O-2, pulps. The normal D (30min/60 degrees C) and D-HT (120min/95 degrees C) bleaching stages were compared under similar conditions, after a mild pulp alkali extraction (E). At end pH 3, the DE treated pulp showed 2.5% ISO higher brightness and 46% higher kappa number (1.9 units) as compared to DHTE treated pulp. Highest DE and DHTE bleaching performances are achieved at end pH 4.0 and 3.0, respectively. For a similar ClO2 dose (kappa factor 0.20 at end pH 3.0) DHTE bleaching produced 46.3% and 9% less AOX and OX in the filtrate and pulp, respectively, in relation to the DE. The high temperature chlorine dioxide treatment generates less chlorinated organics and chlorate and more chloride ions. No chlorite is formed at end pH 3.0 both in DE and DHTE treatments. Increasing D-stage pH from 3 to 5 reduces formation of chlorinated organics (OX and AOX) and chlorate but increases formation of chlorite. Alkaline extraction (E) reduces pulp organically bound chlorine (OX) by about 59% after the D stage and by about 44% after the D-HT stage. Formation of AOX in extraction stage filtrates is about 17 and 22% of that in the D and D-HT stage filtrates, respectively. Chloride formation in the extraction stage is about 17% of that in the D stage and about 10% of that in the D-HT stage. Formation of chlorate in the extraction filtrates was only 8% of that formed in the D or D-HT stages. Addition of 1% formaldehyde to chorine dioxide bleaching improves both DE and DHTE performances, but more so in the former case, particularly at end pH 4.