Grinding Pretreatment Of Industrial-Grade Magnesium Oxide And Its Effect On The Bleaching Of High-Yield Pulps

Poplar is a fast-growing hardwood species and a good raw material for the chemi-thermomechanical pulp, because of its characteristics such as shallow wood color and good liquid absorbency. CTMP has been widely applied in specialty paper, newsprint and the intermediate layer of the coated paperboard as one of the main high-yield pulps (HYPs). Sodium hydroxide is a traditional alkali source in the alkaline peroxide bleaching process. But many drawbacks have been discovered, including dissolved the wood chemical compositions, decreased the bleaching yield, and increased the chemical oxygen demand (COD) of the effluent, and so on. 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 recent years, some researchers haved used MgO or/and Mg(OH)2 partially substituting NaOH as the alkali source in peroxide bleaching of poplor CTMP to decrease the dissolving of the wood chemical compositions, improve the bleaching yield, reduce the bleaching cost and the cost of the treatment of the effluent.This thesis had studied the grinding pretreatment of the industrial-grade magnesium oxide and its effect on the bleaching of high-yield pulps (poplar CTMP). During the course of the study, an industrial-grade MgO was ground by a basket mill in order to get the MgO with different particle sizes at the first. And then, the response surface methodology was used to optimize the condition of the alkaline hydrogen peroxide bleaching process of poplar CTMP, including:alkali charge, H2O2 charge, bleaching time and bleaching tempreture. Secondly, at the optimal bleaching condition, an industrial-grade MgO partially substituting NaOH was used as the bleaching alkali source in the alkaline peroxide bleaching process of poplar CTMP to get an optimal substitution percentage. Subsequently, under the condition of the optimal substitution percentage of MgO, the MgO with different particle sizes was used in the alkaline peroxide bleaching process of poplar CTMP. The strength and optical properties of the bleached pulps were tested. The aim is to investigate the effect of industrial-grade MgO with different particle sizes on the bleaching of poplar CTMP.The reaults showed that, during the grinding pretrement process of the industrial-grade MgO by using a basket mill, the particle size of MgO would reduce from 17.4 to 2.9μm when the grinding time increased from 0 to 60 min. The notable factors that influence the brightness of the bleached pulps were H2O2 charge and the interaction between alkali charge and H2O2 charge; the alkali charge has a big influence on the bulk of the bleached pulps; and alkali charge, bleaching tempreture and bleaching time have a big influence on the bulk of the bleached pulp. The optimal bleaching condition obtained by using the response surface method as follows:3% alkali charge,6% H2O2,90℃, and 150 min. Under this condition, the properties of the resultant bleached pulps were as follows:ISO brightness of 75.0%, bulk of 2.41 cm3/g, and tensile index of 32.6N-m/g. In this thesis, an industrial-grade MgO was partially used as the alkali source in the peroxide bleaching of poplar CTMP. Considering the properties of the bleached CTMPs, the resultant bleaching effluent load, and the bleaching cost, the optimal substitution percentage of MgO for NaOH was chosen 50%(molar ratio). The decrease of the MgO particle size had a negative influence on the bulk, light-scattering coefficient, and opacity of the bleached pulps, but was beneficial to improving the tensile and tear index of the pulp. When the MgO particle size reduced from 17.4 to 2.9μm, the tensile and tear index would increase by 10.2% and 5.6%, respectively. Moreover, the smaller the MgO particle size, the greater the effect on the strength properties of the BCTMP. The reduction of the particle size of the industrial-grade MgO was not always helpful to improve the brightness of the poplar BCTMP. The reason responsible for this may be the change of the transition metal content during the grinding treatment for the industrial-grade MgO applied in the bleaching process. The particle size of MgO has to be controlled in order to get an optimal brightness. The maximum ISO brightness (73.2%) was obtained while the particle size of MgO was 4.3μm (3% alkali charge,6% H2O2,90℃,150 min,0.2% DTPA,2% NaSiO3, and 20% of pulp consistency). Under this condition, the other properties of the bleached pulps were as follows:bulk of 2.52 cm3/g, light-scattering coefficient of 49.5m2/kg, tensile index of 25.3 N-m/g, tear index of 2.72 mN-m2/g, and opacity of 86.9%.