Optimization of Pulp Yield from Sorghum Stalk Using the Soda-Anthraquinone Pulping Process

The demand for pulp and paper products is increasing in Ethiopia, while wood-based raw materials are becoming scarcer and more expensive. There is a need to identify alternative, sustainable lignocellulosic resources for the production of pulp. Sorghum stalk is one of the major agricultural residues abundantly available in Ethiopia and has significant potential as a non-wood raw material for pulp and paper production. The purpose of this study was to investigate the suitability of sorghum stalk harvested from Raya area of southern Tigray for pulp production and to optimize the pulping yield and kappa number using the soda-AQ pulping process. The kappa number is an important parameter used to evaluate the residual lignin content of pulp and indicates the degree of delignification and bleaching requirement. The sorghum stalks were dried, chipped, and ground before pulping. Soda-AQ pulping was studied at different cooking temperatures (140–180 °C), NaOH concentrations (10–20%), and cooking times (80–120 min) using a Box-Behnken Design (BBD). Response surface methodology (RSM) was used to evaluate the effects of pulping factors on pulp yield and kappa number. The pulp yield ranged from 38.2% to 52.0%, as shown in the experimental results. The kappa number ranged from 7.13 to 28.2, depending on the pulping parameters. The kappa number decreased with increasing temperature, alkali concentration, and cooking time due to enhanced delignification. However, the pulp yield decreased under severe pulping conditions due to carbohydrate degradation. The quadratic regression models showed statistical significance and very good agreement with coefficients of determination (R2) of 0.9993 and 0.9997 for pulp yield and kappa number, respectively. Using desirability function analysis for numerical optimization, the optimum pulping conditions were identified as 161.95°C cooking temperature, 15.52% NaOH concentration and 99.38 minutes cooking time, with a predicted pulp yield of 49.14%, kappa number of 14.05, and desirability value of 0.730. The developed models were experimentally validated and found to be adequate with a minimal difference between the predicted and experimental results. The study affirmed that sorghum stalk is a promising and sustainable non-wood raw material for the production of pulp in Ethiopia. The optimized soda-AQ pulping conditions provided a good balance between pulp yield and lignin removal. Further studies on bleaching, paper properties, and pilot-scale production are recommended to assess its industrial applicability. Sorghum stalk, Soda–AQ pulping, Non-wood pulp, Delignification, Pulp yield, Kappa number, Response Surface Methodology.