Synchronous silicon removal and viscosity reduction in the soda-oxygen pulping of wheat straw

The black liquor (BL) obtained by straw pulping can hardly be applied to conventional alkali recovery systems because of its high concentration of silicon and viscosity. Soda-oxygen pulping can synchronously deposit silica on the surface of the cellulose to reduce the silicon content and viscosity of BL remarkably. In this paper, the BLs of wheat straw soda-oxygen pulping obtained at different end points (pH < 10, 11.5 < pH < 12) and conventional soda-anthraquinone (soda-AQ) were obtained. The extent of silicon removal and viscosity reduction before and after centrifugation or membrane filtration as well as the thermodynamic properties of the BLs were investigated. Compared with that achieved by soda-AQ, over 45% silicon was removed from BL after soda-oxygen cooking at a similar delignification level. The total solid (TS) concentration of the soda-oxygen BL was easily concentrated by up to approximately 50%. SiO₂ can be further removed by simple centrifugation and membrane filtration, and its TS could be increased to 60% at 300 mp s. With cooking end point further decreased pH < 10, the centrifugated BL had the lowest silica content, the highest volumetric isothermal expansivity (VIE) value, and the lowest pyrolysis temperature.