Filler clay induced discolouration of bleached mechanical pulp

Today's pulp and paper industry is highly exposed to cost reduction pressures. The bleaching chemical cost may constitute 50-70% of the total chemical costs in the production of magazine paper grades. Thus, it is important to avert process conditions which enhance discolouration and consecutive increase in the bleaching chemical consumption. In this study, the effect of different commercial kaolins (china clays) on discolouration of thermomechanical pulp was assessed. The purpose of the work was to acquire new knowledge about clay-induced discolouration of mechanical pulp in order to minimize discolouration of pulp in the production process of supercalendered magazine paper. Results showed that commercial kaolins can reduce the ISO-brightness of mechanical pulp by up to seven points under papermaking conditions. Mill experiences demonstrated that this discolouration had a considerable effect on the bleaching chemical consumption in a supercalendered magazine paper mill. The total bleaching chemical cost increased by at least 50% due to clay-induced discolouration. The clay-induced discolouration of mechanical pulp was ascribed to the accessible iron in kaolin. This study revealed that the degree of clay-induced discolouration varies considerably between different commercial kaolins. Results indicated that the bleaching sequence and the metal ion profile of the mechanical pulp influences the degree of iron-and clay-induced discolouration upon pulp storage. It is recommended that producers of paper consisting of mechanical pulp and kaolin make demands on the potential clay-induced discolouration properties of the clay. The clay-induced discolouration test method developed in this work can be used for an acceptance check.