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Chlorination effluent recycle in kraft pulp bleaching

University of British Columbia

When performed at an industrial scale, kraft pulp bleaching commonly utilizes recycled chlorination effluent for brown stock dilution. Much of the information regarding this practice pertains to pulp quality. Some of the previous research done in this field failed to separate its effect from the effect of countercurrent effluent recycle for pulp washing. In addition, there are some shortcomings of the batch techniques that have been used to simulate effluent recycle in the laboratory. Thus little is known about the effect of this practice on effluent quality. Accordingly, the objective of this research project was to determine the effect of chlorination effluent recycle for brown stock dilution on the quality of kraft pulp bleaching effluent. Chlorination effluent recycle was investigated in conjunction with the variables of multiple, mode, and chlorine dioxide substitution. Effluent quality was characterized in terms of toxicity, total chlorinated organic compounds, and chlorate. Pulp quality was also measured. A full 2⁴ factorial design was carried out with 24 bleaching runs. The experimentation was performed in the laboratory using a two-stage (CE) bleaching sequence. A specially designed continuous laboratory-scale pulp chlorination apparatus, which was operated with an unbleached pulp throughput of 6 odg/min, was used to execute the first stage. By producing sufficient quantities of effluent at steady state in a short period of time, this apparatus enabled the efficacious use of trout bioassays. The second stage was performed on a batch basis. Results from the laboratory experiments were compared to those from an industrial-scale bleach plant. The chlorination stage toxic emission factor for rainbow trout decreased by 314 m³/adt as the level of recycle was increased from 4.5 to 22.5 m³/adt. The effects of multiple, mode, and substitution were also significant. Effluents from the extraction stage were much less toxic than those from the chlorination stage. The first stage toxic emission factors for trout and luminescent marine bacteria were not correlated. In addition, the concentration of organochlorine compounds and the number of toxic units were not correlated. A novel method for determining the toxic emission factor from median lethal time data was also developed. Recycle had no significant effect on the total production of chlorinated organic compounds, which were quantified together in terms of AOX. A regression equation, which includes the effects of multiple and mode as well as the interaction between multiple and substitution, was formulated for the AOX results. Recycle had no significant effect on the production of chlorate. A regression equation, which includes the variables of substitution and mode, was formulated for the chlorate results. A hypothesis, which involves the reactions of chlorine and chlorine dioxide with the phenolic hydroxyl groups of lignin, was developed to explain several characteristics of this equation. Recycle also had no effect on pulp quality, which was measured after the extraction stage with the kappa number and the viscosity. Mode and multiple were the only factors that significantly affected the kappa number. A hypothesis, which involves the reactions of chlorine and chlorine dioxide with dissolved organic material in the chlorination stage, was developed to explain the effect of mode on delignification. None of the variables had a significant effect on the viscosity.

Text

2010-11-12

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http://hdl.handle.net/2429/29938

Chemical and Biological Engineering

University of British Columbia

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