This paper presents experimental results on the effect of adding high doses of sodium sulphite (Na2SO3) to replace other sources of alkali, such as Na2CO3 or NaOH, in high-yield Neutral Semichemical Pulping (NSSC) process. Sodium sulfite (Na2SO3) was used as delignification agent in the chemical stage of eucalyptus globulus pulping. A 12" Bauer refiner was employed in the mechanical stage. Pulp yields ranged within 70-90% and, results show that physical and mechanical properties increased significantly as yields decreased. Moreover, the defibration energy consumption and beatings energy also showed a significant decrease.
Those pulps featuring yields near 80%, obtained with high sodium sulfite loads, presented similar physical and mechanical properties as chemical pulps. At the largest sulfide dose used here, no significant increase in chemical consumption was observed, especially at high yields. This study shows that physical and chemical properties are enhanced by high hemicelluloses content, and process selectivity to remove lignin from the middle lamella, which leads to a fiber integrity and reduced mechanical stress.