Alkali-extracted spruce bark residues for pulping and making of pulp sheets

Spruce bark residues obtained after alkaline extraction of polyphenols were studied as raw material for pulping and making of pulp sheets. Kraft pulping of mixtures containing wood chips and 5 % or 20 % wt. bark residues produced unbleached pulps with higher screened yields and lower kappa number than wood pulps. During bleaching, the evolution of brightness along with kappa reduction followed a similar trend in all pulps, and bleached mixed pulps with ISO brightness >89 % could be obtained with somewhat lower intrinsic viscosity than wood pulps. The high content of calcium and other metal elements in spruce bark, which was partially reduced by the alkaline extraction, did not seem to impair the pulping and bleaching of the pulps. Sheets made from mixed pulps had rather similar optical and mechanical properties than sheets from wood pulps, and interestingly, sheets produced from mixed pulps with 20 % bark residues showed higher bulk as a function of internal bond strength, which may be exploited in packaging boards. Pulping of bark residues to produce mixed pulps would thus maximize the utilization of spruce bark streams after extracting the polyphenols, while increasing the pulp output in the mills.