Pretreatment of Chlorine Dioxide Solution for Pulp Bleaching

Adsorbable organic halogen (AOX) is mainly produced by the reaction of residual lignin and chlorinated bleaching agents during bleaching processes. The formation of AOX can be reduced via optimization, addition of chemical additives, and/or pretreatment processes. Only a few studies exist on the effective purification of chlorine dioxide solutions and its effect on AOX reduction. In this paper, the effects of the extraction pressure, temperature, pH, and extraction time on the chlorine gas removal rate (CR) were evaluated. In addition, a response surface model was established to optimize the chlorine gas removal. The temperature of chlorine gas separation was optimized at 39.85 °C, with a separation pressure of –51.20 kPa and solution pH of 1.94, providing an estimated CR of 70.93% under the optimum separation conditions. Chlorine dioxide solutions with and without chlorine gas removal were used for pulp bleaching. AOX formation was reduced by 69.57% at the same 2.0% chlorine dioxide dosage, while the pulp brightness decreased by 3.0%ISO. In addition, AOX formation was reduced by 27.90% at the same bleached pulp brightness (80.00%ISO). The results provide theoretical support for the production of high-purity chlorine dioxide solutions and the development of clean pulping and papermaking processes.