Upgrading Paper-grade Softwood Kraft Pulp to Dissolving Pulp by Cold Caustic Extraction

Cold caustic extraction has potential applications in the production of dissolving-grade pulps due to its ability to selectively remove hemicellulose from lignocellulosic materials. In this study, we demonstrate the conversion of paper-grade kraft pulp into dissolving pulp by a single-stage cold caustic extraction. Under the extraction conditions of 12 wt% NaOH lye, 11% pulp consistency, a temperature of 35℃, and 2 h, a paper-grade softwood kraft pulp was purified to high-grade dissolving pulp with 97.1% α-cellulose content, 1.2% pentosane content, and narrowed molecular weight distribution. The resulting extraction filtrate was concentrated by nano-filtration to obtain the hemicellulose content of 59.0 g/L, while the permeate was a clear Na OH solution with 10.9 wt% concentration. A process configuration was also proposed, integrating this cold caustic extraction process with existing pulp and paper production and multi-purpose utilization of the extraction filtrate.