Study On The Aspen CTMP Hydrogen Peroxide Bleaching By Substituting Magnesium Oxide For Sodium Hydroxide

CTMP (Chemi-Thermo Mechanical Pulping) is one of the widely applied high-yield pulping methods. At present, the common method of high-yield pulps bleaching is based on alkaline H2O2using NaOH as the alkali source. But the strong alkalinity of NaOH can cause some degradation of hemicellulose, and therefore the pulp yield decreases and the effluent load increases. Meanwhile, the strong alkalinity of NaOH may also lead to the formation of new chromophore structure in lignin, resulting in the alkali-induced darkening and lower pulp brightness after bleaching. In this study, MgO was used to instead of partial NaOH as the alkaline bleaching alkali source, hoping to solve the above problems.Aspen CTMP was used as the raw material in the present study. The alkali was at3.0%and3.5%alkali charge with different substitution percentages of MgO for NaOH, respectively (Other bleaching parameters were fixed). The characteristics of the effluent and the physical properties and chemical composition of the bleached pulp were studied to investigate the influence of substitution percentage of MgO for NaOH on the effluent characteristics, physical properties of pulp, and chemical composition of the resultant pulp. When the brightness of the bleached pulp was the maximum, we investigated the influence on the effluent characteristics and physical properties of pulp handsheets by extending the bleaching time.The results showed that the effluent load decreased greatly with the increase of substitution percentage of MgO for NaOH from0to100%. For example, at the3.0%alkali charge, the CODCr decreased by62.3%, the cationic demand decreased by34.2%, and the residual hydrogen peroxide of the effluent increased by about164%. Meanwhile, the bulk of the resultant pulp would increase by13.8%, but the pulp strength decreased. The tensile index decreased by35%, and the tear index decreased by43%. However, the brightness increased firstly and then decreased, and reached the maximum value of76.6%ISO at the substitution percentage of25%. The fiber length of the pulp decreased by12%, while the fines content increased by31.3%. The benzene alcohol extractives, lignin, holocellulose, and pentosan content of the pulp increased. It showed that in the bleaching process, with the increase of substitution percentage of MgO for NaOH, the dissolution of the chemical compositions of the pulp decreased, thus the effluent load reduced. At the same substitution percentage of MgO for NaOH, the cationic demand and CODCr from3.5%alkali charge process were higher than those from3.0%alkali charge process, respectively, but the residual hydrogen peroxide was lower; as well as the brightness and bulk of the bleached pulp. Meanwhile, the tensile index, tear index and fiber length were higher; the fines content of the pulp and the dissolution of the chemical composition of the pulp were much lower. When the brightness of the bleached pulp was maximum (3.0%alkaline,25% substitution MgO for NaOH), we extended the bleaching time from120minutes to150minutes. And the residual H2O2content of the effluent decreased from22.70%to18.10%, the cationic demand increased from10.20meq/L to11.20meq/L, the CODCr increased from5300mg/L to5700mg/L; the brightness of the bleached pulp handsheet increased from76.60%ISO to77.11%ISO, the bulk decreased from2.85cm3/g to2.75cm3/g, both the tense index and tear index increased a little bit.