Improving dissolving wood pulp quality using Brown-stock fractionation

Dissolving wood pulp (DWP) contains high levels of cellulose and has various applications. Production of dissolving wood pulp is accompanied by various challenges such as equipment operational problems and high bleaching costs. These are mainly due to lignin and other impurities contained in wood. Further, these impurities impose threats to the dissolving pulps applications such as the viscose process and the manufacture of finished rayon products. Removal of these contaminants at the early production stages proved effective in meeting by the challenges. Hydrocyclones achieve the separation of heavy and light pulp components in a process known as fractionation. In the case of dissolving wood pulp, hydrocyclones fractionate the pulp fibres into coarse fibres and fine fibres known as fines. Fines are the reject materials and are associated with impurities such as wood resins. Wood pulp was fractionated at 0.8% consistency using a Kadant laboratory hydrocyclone at mass reject rates of 5% and 11%. Pulp properties and application properties were determined and compared to unfractionated pulp. The fractionated pulps showed higher cellulose contents and lower levels of fines and associated resins. Bleaching of the fractionated pulps under a standard ODEDH bleaching sequence showed higher levels of delignification, as measured by brightness, and more favourable application properties. These findings were used to optimise the bleaching sequence by reducing the amount of chlorine dioxide applied in the second chlorine dioxide stage. The optimised bleaching sequence produced pulps with satisfactory fibre and application properties.