Studies on Hibiscus cannabinus and Hibiscus sabdariffa as an alternative pulp blend for softwood: An optimization of kraft delignification process

Abstract

Hibiscus cannabinus and Hibiscus sabdariffa, agro-based residues consist of bast and wood fibers which resemble to those of softwood and hardwood, respectively. The runkel ratio of core fibers H. cannabinus and H. sabdariffa is comparable to that of Picca abies, whereas, it is much less than those of hardwood like Eucalyptus tereticornis. The slenderness ratio of H. cannabinus is much closer to P. abies in comparison to H. sabdariffa whereas, it is 7.7 and 13% less than E. tereticornis. The flexibility coefficient of H. cannabinus and H. sabdariffa are slightly lower than that of P. abies but it is 59.6 and 57.0% are more than that of E. tereticornis. It indicates that morphological characteristics of core fibers of H. cannabinus and H. sabdariffa closely resemble to that of softwood except fiber length which can be compensated by long bast fibers. Due to identical pulping conditions, H. cannabinus and H. sabdariffa can be delignified together by kraft pulping process. The optimum cooking conditions for H. cannabinus and H. sabdariffa were found to be as, active alkali 16%, sulfidity 20%, temperature 160 °C, time (at temperature) 120 min and wood to liquor ratio of 1:4.5. An anthraquinone (AQ) dose of 0.05% at an active alkali dose of 13% (as Na 2O) produces the screening rejects and kappa number similar to that obtained by using 15% active alkali (as Na 2O). The reaction kinetics study indicates that delignification is of first order. Low sulfidity AQ additive kraft pulping at constant H-factor produces better strength properties compared to non-additive kraft cooks.