Upgrading softwood bleached kraft pulp to dissolving pulp by cold caustic treatment and acid-hot caustic treatment

Two chemical processes for hemicellulose extraction have been applied on a softwood kraft pulp in an attempt to convert paper pulp into dissolving grade pulp. The cold caustic extraction process (CCE) was performed under conventional and unconventional conditions. Alkali concentration ranged from 3% to 12% and temperature from 25 to 110 °C. As expected, the results showed that NaOH concentration had a predominant effect on xylan and glucomannan removal. The effect of temperature was more complex since the best removal was observed below 40 °C or above 80 °C. The unexpected good results obtained at high temperature would be due to the contribution of some alkaline degradation. Under the best conditions, 80% of the xylan and 60% of the glucomannan were removed. The cellulose II content was measured by solid state NMR spectroscopy. Formation of cellulose II increased with the concentration of caustic soda and was correlated to the extraction of hemicellulose. A second process consisting in an acid stage at high temperature (up to 150 °C) followed by a hot caustic extraction (A-HCE) was also tested. Promising results in terms of hemicellulose removal were obtained when temperature in A was above 140 °C. This might be due to a combination of acidolysis during A and peeling reactions during HCE. However, cellulose was partly depolymerized because of the severity of the A step. The two processes were compared in the context of getting to a viscose grade dissolving pulp.