Reaction kinetics and mechanisms of filler clay-induced discolouration of bleached mechanical pulp

Earlier studies have shown that accessible iron in filler clay may increase the discolouration of mechanical pulp under papermaking conditions. Moreover, this discolouration could have a considerable effect on the bleaching chemical consumption. In order to avert process conditions which increase the discolouration, it is of interest to identify the kinetics and mechanisms involved in the clay-induced discolouration. In this study, the kinetics of the clay-induced discolouration of a hydrogen peroxide bleached thermomechanical pulp (TMP) were assessed. It was found that the initial release rate of iron during co-storage of TMP and a specific commercial kaolin clay was high, and that the resorption on the TMP was a quite fast process. The results indicated that during the first 30 min of co-storage at 23-65 degrees C, more than 50% of the iron that would be released after co-storage for 24 h at 65 degrees C was already released. However, the iron catalyzed discolouration was slower and highly temperature dependent. There was a considerable increase in the clayinduced discolouration rate with increasing temperatures in the range from 50 degrees C to 65 degrees C upon storage for 5 h. Thus, it should be possible to reduce the potential impact of clay on the discolouration of TMP by avoiding prolonged contact between clay and TMP at high temperatures. It was shown that the main part of the chromophores formed at temperatures above 50 degrees C was irreversible in terms of a subsequent chelating stage.