Studies of Chlorine Dioxide Substitution in Mills
A consistent picture emerges of chlorine dioxide substitution both in the laboratory and at mill scale. This picture is consistent with our understanding of the fundamental chemistry of lignin oxidation discussed earlier. These trends are summarized in Figure 4 (from Axegård, 1986b). When elemental chlorine is used as the bleaching agent, there is a proportional formation of AOX (including EOX). This EOX contains the spectrum of organochlorines ranging from chlorophenolic compounds to dioxins. As chlorine dioxide is substituted for elemental chlorine there is a concomitant reduction in AOX (Figure 4 A) and EOX (Figure 4 B). The reduction in polychlorinated phenols and dioxins is more rapid, as shown in Figure 4 C and D. It is thus possible to reduce dioxin levels to close to or below analytical detection limits1 and to reduce the level of chlorination (e.g. tri- to di- to mono-chloro compounds; Figure 4 D). With conventionally delignified softwood pulp (Kappa no.30) at 100% substitution with chlorine dioxide, levels of chlorinated organic compounds measured as AOX, are less than 0.5 kg/tonne of pulp (depending on wood species and process) are achievable after secondary treatment.
The overwhelming evidence is that substitution of chlorine dioxide for elemental chlorine substantially reduces environmental loadings of organochlorines. Especially significant is the fact that it reduces the chlorine to carbon ratio and the formation of the more hydrophobic organochlorines.
Several studies have been conducted at mills where chlorine dioxide has been substituted for elemental chlorine. A study of the bleachery effluent at the Crofton mill (Morgan et al., 1991) showed low levels of organochlorines at 60% substitution and reductions of most chlorinated phenols to levels below the detection limit at 100% substitution. Further reductions in the concentrations of chlorophenols occur after secondary treatment. Recent surveys of mills in the U.S. and Canada (some sampled over a significant period of time) using 70 and 100% chlorine dioxide substitution have shown that levels of chlorophenols are very low or nondetectable (Table 1). The data from these mills are extensive, with, in some cases, a total of 60 analyses available. For these it was possible to calculate mean concentrations (assuming non-detects as zero) and, on the assumption of a normal distribution of the data, the upper 95th percentile concentrations (Table 1). The data clearly show that, concentrations of chlorophenols in final effluent from mills with 100% chlorine dioxide substitution are substantially lower (Table 1).