Leaching of chlorine compounds and metallic elements after the first chlorine dioxide bleaching

During chlorine dioxide bleaching, various reactions take place and both inorganic and organic chlorine compounds (chloride Cl-, chlorite ClO2-, chlorate ClO3-, adsorbable organic chlorine AOX) are present. It is important to understand the behaviour of various chlorine species in order to achieve better operating conditions in washing in modern bleaching processes. The aim of this research was to clarify the dynamic behaviour of some specific chlorine compounds, COD and metallic elements during leaching after the first chlorine dioxide bleaching of softwood kraft pulp. The main observations were: the leaching of compounds took place in two steps, the time scale depended on the compound itself, and the properties of the leaching liquor had a substantial effect on the leaching result. Almost 80% of removal occurred during the fast phase, in approximately ten minutes for COD and two minutes for various chlorine compounds and metals. The organic chlorine in pulp was removed effectively both during alkaline and neutral leaching and it seems that an adequate washing result could be obtained with slightly milder conditions than at recent mill practice.