In the conventional preconditioning followed by refiner chemical treatment, alkaline peroxide mechanical pulping (P-RC APMP) process, sodium hydroxide (NaOH) is used as the alkali source. In this study, magnesium oxide (MgO) was used to partially substitute for NaOH as the alkali source (substitution ratio of up to 50%) in the second-stage impregnation of the triploid poplar P-RC APMP process. The changes in pH values of the residual bleaching liquor, yields, and chemical components of the treated wood chips in the second-stage impregnation and also the physical properties of the resultant P-RC APMP pulps with and without MgO substitution were investigated to explore the potential advantages of the process. The results showed that the partial MgO substitution for NaOH resulted in a lower pH of residual bleaching liquor and a higher treated wood chips yield than those of the NaOH-based process. In addition, the dissolution of benzene alcohol extractives, lignin, pentosans, and carbohydrates were all decreased. The resultantly yielded P-RC APMP pulp has a higher bulk, opacity, and light-scattering coefficient, but lower strength properties and a slightly lower brightness.

Magnesium-based alkali is another attractive alkali source in the peroxide bleaching process of high-yield pulps. This work investigated the effluent characteristics in the high-consistency retention stage and pulp properties of triploid poplar P-RC APMP (that is, so-called preconditioning followed by refining chemical treatment, alkaline peroxide mechanical pulp) by partially substituting MgO for NaOH. The results showed that the pH, dissolved and colloidal substances (DCS), chemical oxygen demand (CODCr), cationic demand, and conductivity of the effluent in the high-consistency retention stage decreased with the increase of the substitution percentage of MgO for NaOH. Meanwhile, both the light-scattering coefficient and the bulk of the pulps increased, but the tensile index and internal bond decreased. The tensile index and internal bond had a good linear relationship with bulk.

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