Alkaline pre-extraction of birch wood prior to alkaline pulping

The existing chemical pulp mills offer a unique starting point for the implementation of future multi-product biorefineries. For the pulp industry, the biorefining process may be envisaged as the removal and use of material that does not contribute to paper and cellulose products, particularly the material which is normally dissolved in the black liquor and combusted in the recovery boiler. In that sense, the recovery of dissolved hemicelluloses in a separate stage prior to pulping for the production of added-value products may potentially improve the overall economy of a chemical pulp mill. The alkaline extraction of hemicelluloses prior to alkaline pulping provides an additional possibility to isolate carbohydrates from wood in reasonably high yields, besides the currently widely investigated acid- and auto-catalyzed prehydrolysis. Furthermore, the use of aqueous sodium hydroxide solution in a pre-extraction process is compatible with kraft or soda-anthraquinone pulping processes, and particularly for the production of paper grade pulp, this process is more likely to preserve fibre strength properties than prehydrolysis followed by alkaline pulping. The objective of this work was the recovery of high molecular weight hemicelluloses, mainly xylan, through alkaline extraction. At the same time, the amount of co-extracted lignin was to be minimized. Birch wood chips were used for the optimization of the pre-extraction conditions. In addition to temperature and sodium hydroxide concentration, the effect of chip thickness was studied. The optimized extraction conditions were found to be NaOH concentration of 2.5 M at 95 °C. Lignin and carbohydrate mass balances were calculated for pre-extraction trials performed at these conditions. Approximately 6% of the original dry wood material was measured in the extracted liquor as polymeric xylan and 1% as co-extracted lignin. The selectively precipitated xylan revealed a weight average molecular weight (Mw) of 20 kDa as determined by size exclusion chromatography. For further purifying the extracted xylan, the diluted solution was concentrated by a factor of 8 by means of pressure-driven membrane filtration. This separation process allows the recovery of a large part of the sodium hydroxide in the permeate stream and makes a subsequent xylan precipitation via acidification more economical. The permeate stream containing the recovered sodium hydroxide may be recycled to the cooking liquor preparation. Preliminary tests confirmed that xylan can be isolated in higher purity when the pH is maintained at a higher level during the precipitation step since lignin remains more soluble under these conditions