Study On The Process And Properties Of Triploid Poplar P-RC APMP Pulps By Substituting Magnesium Oxide For Sodium Hydroxide

P-RC APMP (Preconditioning followed by Refiner Chemical treatment, Alkaline Peroxide Mechanical Pulp) process is a highly efficient, clean and high-yield pulping process. However, many wood compositions can be dissolved during the pulping process due to the strong alkalinity of sodium hydroxide (NaOH) as the alkali source in the peroxide bleaching process, resulting in a high content of anionic trash in pulp and COD in the effluent, and causing the loss of pulp yield. Magnesium oxide (MgO) is a weak alkali, that can be used as a substitution of sodium hydroxide in the peroxide bleaching process to solve those problems. Triploid poplar P-RC APMP was used in this study. NaOH was substituted with MgO as the alkali source in the high-consistency retention stage of the P-RC APMP process.First, the effects of bleaching duration time, NaOH and H2O2dosages in the high-consistency retention stage of triploid poplar P-RC APMP on the pulp physical and optical properties were investigated. The three parametes were optimized mainly based on the brightness and the bulk of the handsheets. The results showed that the optimized parameters were90min bleaching time,3.5%NaOH and3.5%H2O2.Secondly, the influences of various substitution percentages of MgO for NaOH on physical and optical properties of the pulp were studied under the optimized process conditions. The results showed that, under the optimized process conditions, with the increase of the substitution percentage of MgO for NaOH in the high-consistency retention stage (from0%to60%), the optical properties of the handsheets were improved, but the physical strength properties were decreased. As the MgO substitution percentage was40%, the resultant P-RC APMP pulp could have a2.35ISO units brightness increase, a higher bulk and light-scattering coefficient, while the tensile index and the tear index were decreased by23.9%and22.4%, respectively, compared to those without any MgO substitution.Then, the reaction courses and the effluent characteristics of the bleaching filtrate by substituting MgO for NaOH in the high-consistency retention stage of P-RC APMP process were investigated to explore the potential advantages of the process. The results showed that MgO substitution for50%NaOH, resulted in a lower pH of residual bleaching liquor by1.2-1.5units than those of the NaOH-based process. In addition, the dissolution of chemical components (carbohydrates, pentosans and lignin) was all decreased, resulting in a higher pulp yield. In the high-consistency retention stage, benzene alcohol extractives were not completely removed, instead, the dissolved benzene alcohol extractives could be adsorbed onto the fibers at the later stage of bleaching. The dissolution of the carbohydrates and lignin could be divided into two stages:the former45minutes was the first stage in whichompositions were largely and quickly removed; the period beyond45minutes was the second stage in which less lignin and carbohydrates were dissolved slowly.In the high-consistency retention stage, when MgO substituting for50%NaOH, the effluent had much lower conductivity, lower turbidity, less anionic trash and lower CODcr load. These indicators were decreased by25.6%,11.0%,38.4%and18.6%, respectively, compared to those without any MgO substitution.In the high-consistency retention stage, when MgO substituting for50%NaOH, the major changes were that peroxide bleaching decreased the dissolution of xylose and avoided the dissolution of arabinose.