Upgrading of paper-grade pulps to dissolving pulps by nitren extraction: Optimisation of extraction parameters and application to different pulps

Abstract

Abstract Xylans were selectively removed from paper-grade pulps by nitren extraction to produce dissolving pulps. Extraction parameters were optimised for a birch kraft pulp regarding time, temperature, liquor/pulp ratio, and total nitren charge. Furthermore, the applicability of the method was investigated for two other kraft pulps obtained from eucalyptus and mixed softwood, and for one beech sulfite pulp. Extracted pulps were characterised regarding their carbohydrate content and Cuen viscosity. The nitren charge was a decisive factor for xylan removal and pulp purity. The combination of a high nitren concentration and low liquor/pulp ratio was most effective for xylan removal. However, a high liquor/pulp ratio with a lower nitren concentration proved to be more selective and minimised cellulose degradation as well. Glucomannans were almost insoluble under the extraction conditions investigated. Therefore, softwood pulps were not suitable for the upgrading of chemical pulps to dissolving pulps by nitren extraction. On the other hand, hardwood pulps obtained by kraft and sulfite processes contained 96–97% cellulose after nitren extraction.