Acid Sulfite Pulping of Hemp: Effects of Processing Conditions on Lyocell Pulp Quality

Abstract

Industrial hemp is a renewable lignocellulosic feedstock with strong potential for regenerated cellulose fibers such as lyocell manmade cellulosic fibers (L-MMCF). Production requires pulps with high α-cellulose content, low residual lignin, and sufficient intrinsic viscosity (IV) to ensure dissolution in N-methylmorpholine N-oxide and stable fiber formation. While acid sulfite (AS) pulps historically supported viscose manufacturing due to high cellulose accessibility, alkaline pulps now dominate industrial production because of scalability and operational efficiency. This study systematically evaluates applicability and limitations of AS pulping for hemp-derived feedstocks by examining effects of pH, liquor-to-solid ratio, temperature, and pulping time on lignin removal, α-cellulose retention, and IV. Feedstock influences were assessed by comparing bast and hurd fractions as well as decorticated and mechanically disrupted bast fibers, and AS pulps were benchmarked against soda pulps. AS pulping increased apparent α-cellulose through preferential removal of non-cellulosic components but consistently retained higher lignin levels than soda pulps. Further delignification reduced IV, constraining dissolving pulp quality. Soda pulps achieved lower lignin while maintaining α-cellulose and IV within ranges suitable for L-MMCF spinning. Overall, although AS pulping is historically associated with enhanced cellulose accessibility, the AS conditions examined here did not meet the combined lignin and IV requirements.