The Use of Mg(OH)2 in the Final Peroxide Bleaching Stage of Wheat Straw Pulp

Abstract

Magnesium-based alkali is an attractive alkaline source for the peroxide bleaching of high-yield pulp. However, little information is available on Mg(OH)2 application in the final peroxide bleaching stage of wheat straw pulp. The use of Mg(OH)2 was demonstrated as a partial replacement for NaOH in the peroxide bleaching of a chelated oxygen-delignified wheat straw pulp. The yield, viscosity, and strength properties of bleached pulp significantly increased with increasing replacement ratio of Mg(OH)2, while the chemical oxygen demand load (COD) of filtrate was decreased. For similar brightness of bleached pulp at a 24% replacement ratio of Mg(OH)2, the tensile and tear indices were higher, by 2.1 Nm.g-1 and 1.75 mN*m2.g-1, respectively, than that of control pulp bleached with NaOH as the sole alkaline source. When the MgSO4 was eliminated and the dosage of Na2SiO3 was decreased in the bleaching process, the tear and burst indices of the bleached pulp were also enhanced, with the brightness maintained. Scanning electron microscopy (SEM) showed that more swelling occurred in the fibers of bleached pulp from the Mg(OH)2-based bleaching process. Fiber analysis indicated that peroxide bleaching with Mg(OH)2 increased the proportion of fiber lengths between 0.20 to 1.20 mm and 1.20 to 7.60 mm.