Atmospheric Acetic Acid Pulping of Rice Straw II: Behavior of Ash and Silica in Rice Straw during Atmospheric Acetic Acid Pulping and Bleaching

Summary A detailed examination was made of the behavior and distribution of ash and silica during atmospheric acetic acid pulping and subsequent bleaching of rice straw. Ash-rich pulps (in unbleached pulp, about 18%; and in bleached pulp, 16%) with matchable strength properties for conventional alkaline pulps were obtained from rice straw by acetic acid pulping. More than 50% of the ash and about 75% of the silica in rice straw were retained in the pulp after pulping. Because only those acid-soluble mineral components were dissolved during acetic acid pulping, the ash remained in the pulp consisted mainly of silica (92.9% of the ash in unbleached pulp and 97.3% of the ash in bleached pulp). Although part of the ash dissolved during bleaching, the ash content of bleached pulp was still 15.5%, a value much higher than that in other conventional pulps. This type of ash might be good as a filler for paper. Fractionation analysis and the profile and mapping of silica by scanning electron microscope combined with an energy dispersive X-ray analysis (SEM-EDXA) indicated that the silica in the bleached pulp was located mainly in epidermal cells and not in other elements, such as fibers and parenchyma, and that the silica-rich epidermal cells were scattered throughout the pulp as single cells or in bundles.