Sodium hydroxide is the primary alkali source in the peroxide bleaching of HYPs. While the strong alkalinity of sodium hydroxide can increase the non-effective decomposition of peroxide, and cause the decomposition of partial carbohydrates in pulp. Meanwhile, the strong alkalinity of NaOH can still lead to the formation of new chromogenic groups in lignin structure units that is the alkali-induced darkening, so the brightness of the bleached HYP is decreased. In this work, we investigated the effluent characteristics and pulp properties from the softwood TMP bleaching process by substituting MgO for NaOH. However, the changes in pH value of the residual bleaching liquor and also the resultant TMP pulps with and without MgO substitution were investigated to explore the potential advantages of the process. The results showed that the addition of MgO could make the alkalinity of whole bleaching system weak. The pH, CODCr, cationic demand and conductivity of the effluent decreased with the substitution percentage of MgO for NaOH increasing. Meanwhile, both the opacity and bulk of the pulps increased, but the tensile index decreased.
