Cellulose from Softwood via Prehydrolysis and Soda/Anthraquinone Pulping

Abstract

A two stage process based on a hydrolytic pretreatment and a soda/anthraquinone pulping stage has been studied to produce bleachable-grade cellulose from residual softwood sawdust, concretely a mixture of ground spruce (Abies alba) and pine (Pinus insignis). The hydrolytic pretreatment, which is carried out in an isothermal plug-flow reactor, solubilizes the hemicelluloses and substances of low molecular weight, and partially depolymerizes the lignin which can then be more easily extracted during the pulping stage. Soda/AQ pulping was performed on the pretreated fiber in a stirred batch reactor. The resulting cellulose pulps have low contents of residual hemicelluloses and ash, and a low enough amount of residual lignin which means that a chlorine-free bleaching procedure may be used. The effect of the process variables on the chemical characteristics and yields of the unbleached cellulose are presented. The combination of an autohydrolytic pretreatment at 231 °C for 5.5 min and a pulping stage at 170 °C for 150 min using 0.1% of anthraquinone, results in a cellulose yield of 64% of the potential. Because of the pretreatment the cellulose is partially depolymerized. This cellulose has a kappa number of 16.6, a non detectable content of hemicelluloses by HPLC analysis, an ash content of 0.60%(w/w) and an average degree of polymerization of 300.