Production of high-value Dissolving Wood Pulp (DWP) from sawdust waste.

Abstract

This study adopts a circular economy framework, aiming to divert waste from landfills and channel it towards more lucrative value chains. A novel approach for producing dissolving wood pulp from wood sawdust waste, focusing on three sawdust species: hardwood, softwood, and a mixture of hardwood species was undertaken. The two-step low-temperature chemical process (60-80℃) involved delignification using ammonium persulphate and subsequent bleaching to remove residual lignin, and hemicellulose, and enhance pulp brightness. This approach provides a more environmentally friendly alternative with reduced chemical, water, and energy intensity compared to complex traditional processes involving several stages. The delignification process was optimised on a benchtop level for reaction time, chemical concentrations, liquid-to-wood ratios, and temperature following respective benchtop bleaching treatments. The resulting bleached sawdust pulp, with yields ranging from 38-46% and an ISO brightness of 86-88%, proved unsuitable for viscose production due to the low intrinsic viscosity (180-310 ml/g). However, microcrystalline cellulose production remained viable requiring a final acid hydrolysis step to modify the degree of polymerization. Scaling up to a pilot-scale operation, the study included characterisation of the final microcrystalline cellulose using various analytical techniques such as wet chemistry, thermogravimetric analysis, microscopic and spectroscopic techniques, chromatographic and mass spectrometry methods, particle size analysis and colourimetric methods of analysis. Characterization using these methods showed some close similarities between the prepared microcrystalline cellulose samples and commercially available microcrystalline cellulose. Microcrystalline cellulose yields varied between 30-42%, the degree of polymerisation ranged between 257-398, brightness ranged between 79-86%, pH between 5-7 and the crystallinity index between 76-84%, with physical attributes like particle size requiring further modification for commercial applications. By-product recovery explored the potential of ammonium sulphate, benefiting both agricultural and non-agricultural applications. A techno-economic assessment, incorporating recovery methods, indicated a return on revenue of 78-84%, an internal rate of return of 67-70%, and a payback period of 1-2 years, with marginal effects from acid (hydrochloric) recovery. The study highlighted the highest potential in microcrystalline cellulose derived from hardwood, with bleached softwood sawdust showing promise for lyocell production due to its high degree of polymerization. Overall, the methodology not only demonstrated environmental benefits but also economic viability, making it a promising avenue for sustainable wood waste utilization.