Hot water extraction for the purification of paper grade pulps

Abstract

Hot water treatments (HWT) have been industrially used in the pre-treatment stage of prehydrolysis Kraft (PHK) process, obtaining good results in the decrease of xylan content. Lately, studies have shown promising results when paper-grade pulp undergoes HWT to produce dissolving grade pulp in a flow-through (F-T) reactor, demanding a high water to solids ratio. This thesis aims to reduce xylan content in the original pulp using a batch reactor pulp in order to reduce the water consumption. The feedstock used is birch Kraft paper-grade pulp. Pulps produced under this process with acceptable viscosity values, >400 ml/g, possess a xylan content above the required for dissolving-grade pulps applications. On the other hand, pulps with acceptable xylan content, <7 %, have a viscosity too low to be applied as dissolving pulp. Furthermore, pseudo-lignin seems to appear in pulps that undergo extreme conditions (> 240OC).

In addition, diluted acid HWT were carried out to determine the efficiency of adding an acid catalyst in the reaction mixture. It was expected that the addition of an acid would reduce the need of extreme temperatures; and therefore, prevent from cellulose degradation while xylan content in pulp is removed up to the level of dissolving pulps, <7%. Two organic acids were used in the experiments. Results demonstrate that irrelevant improvement was obtained when 0.1 M acetic acid HWT was carried out, it allows small reduction of the temperature, but the selectivity lacks improvement. Addition of 0,1 M oxalic acid as a catalyst appreciably decrease the pH of the reaction; consequently, milder conditions were demanded to reduce the xylan content below 7%; however, the viscosity was significantly decreased.

The results demonstrate that HWT and diluted acetic acid HWT lead to hydrolyzates in which xylan and glucan were mainly found as furfural and hydroxymethylfurfural (HMF), respectively. Unlike HWT, xylan and glucan was recovered as monomeric xylose and glucose in hydrolyzates of diluted oxalic acid HWT.