Recycling pretreatment black liquor in wheat straw chemi-thermo-mechanical pulping (CTMP) for sustainable packaging paper production

Abstract This research examines recycling potassium hydroxide-based pretreatment black liquor in wheat straw chemi-thermo-mechanical pulping (CTMP) to improve process sustainability and cut operational expenses. By reusing the black liquor across five cycles, the solid content increased from 6.2 % to 14.8 %, concentrating both organic and inorganic elements without adding new chemicals. About 80 % of the solids were degradation products of cellulose, hemicellulose, and lignin, with potassium and silicon making up the remaining 20 %. The recycling did not harm fiber structure, pulp strength, or papermaking ability, as shown by FTIR, SEM, and Micro-CT analyses. No new functional groups appeared, though slight increases in hydroxyl and carbonyl bands were noticed. These findings reveal that black liquor recycling in wheat straw CTMP is a viable method to boost liquor concentration, reduce freshwater use, and lower energy and costs associated with evaporation and transportation. This study supports cleaner production in non-wood pulping by enabling effluent reuse and valorization within a circular process.