Sustainable utilization and optimization of undersized wood chip residues in soda-anthraquinone pulping: insights from response surface methodology and handsheet properties

The wood chip pins and fines share the same characteristics as the accepted wood chips and are not typically used for pulping solely due to their dimension heterogeneities. These residues are pervasive in integrated pulp and paper mills globally, collectively representing an annual volume exceeding multimillion tonnes. By utilizing low chemical pulping dosage, it is practical to fabricate pulp through continuous cooking. We employed the response surface methodology (RSM) to evaluate and optimize the influence of three independent pulping variables: active alkali concentration, cooking time, and liquor-to-dry wood fines ratio. These variables were assessed for their impact on the physical-mechanical, optical, and structural properties of the handsheets. The analysis demonstrated that all investigated parameters exerted a statistically significant influence on handsheet properties; nevertheless, the impact of active alkali level was more pronounced than other variables. The optimal conditions for the pulp production were identified as 16.42% active alkali, a cooking duration of 63.93 min, and a liquor-to-wood ratio of 5.5. Under these conditions, the resulting paper demonstrated a CMT of 282 kN, a tensile index of 26.25 Nm/g, a tearing index of 7.58 mN m2/g, a bursting index of 6.47 kPa m2/g, a brightness level of 51.60%, and an opacity of 96.92%. Fourier transform spectroscopy and x-ray diffraction were also employed to track changes in the functional groups, crystallinity, and crystal size. The unbleached pulps exhibited a significant degree of crystallinity at 82.6%, with a defined crystal size of 2.53 nm, highlighting their well-ordered cellulose structure. This optimized process not only enhances the mechanical and optical qualities of the paper but also significantly contributes to sustainable waste management practices.