Comparative study on hydrogen peroxide bleaching of soda-organosolv and Kraft rice straw pulps

Rice straw was cooked with three levels of alkali content (12, 14 and 16%) in cooking liquor by soda-organosolv (ethanol, diethylene glycol, dimethyl formamide) process. The effect of alkali addition on yield, kappa number, brightness and viscosity of organosolv unbleached pulps was studied. The TCF bleachability of these organosolv pulps and kraft pulp was carried out using a simple three stage peroxide bleaching sequence without oxygen pre-bleaching. All tested pulps were bleached under identical conditions during each stage. After full peroxide bleaching sequences of three alkali-organosolv pulps, some similarity in behaviour of brightness and peroxide consumption for each alkali percentage was observed. The final brightness of 63-70% ISO was attained for all tested pulps. The chemical charge required to reach this level of brightness varied for different pulps (despite the equal initial content of the residual lignin), which was directly related to starting brightness values. The high bleached yield for organosolv pulps (approximately, in range of 90–91% o.d.p) indicates to the limited organosolv carbohydrate degradation during peroxide bleaching. The strength properties of bleached organosolv pulps were higher than kraft pulp. No relation was found between improvement in brightness and lignin removal during hydrogen peroxide bleaching.