A chemo-enzymatic approach to converting a conventional Kraft pulp to a higher value, dissolving pulp grade

Unlike Kraft pulps which are predominantly used to make paper, dissolving pulp grades are a highly uniform, highly purified cellulose feedstock that can be derivatized to produce higher value products such as textiles (rayon) and thermoplastics. Several companies are currently exploring ways to modify a Kraft mill to now produce dissolving grade pulps. However, Kraft pulping tends to maximise pulp yield and therefore hemicellulose retention. As this is detrimental for dissolving pulp applications, aggressive chemical steps are typically used to remove the residual hemicellulose, compromising pulp yield and the quality of the dissolving pulp. However, enzymes offer a more directed approach to hemicellulose removal. When xylanases and oxalic acid were compared for their ability to solubilize the hemicellulose present in a Kraft pulp, it was anticipated that the oxalic acid would be more effective. Unexpectedly, both treatments resulted in limited hemicellulose removal (~20%), while the lack of specificity of the oxalic acid compromised the overall cellulose/pulp yield. When a cold caustic extraction (CCE) was assessed for its ability to remove the hemicellulose present in a Kraft pulp, although most of the hemicellulose could be removed, the cellulose reactivity for subsequent derivatization was substantially reduced, likely due to lowered cellulose accessibility to chemicals. However, subsequent mechanical refining could increase pulp reactivity from 32% to 55% and when endoglucanase was combined with refining pulp reactivity was increased to 75%. A combination of CCE, endoglucanase treatment and subsequent refining was able to convert a Kraft pulp into an acceptable dissolving pulp grade.