Effect And Possible Mechanism Of Sodium Borohydride Pretreatment On Hydrogen Peroxide Bleaching

With the continuous development of the concept of environmental protection, the entire paper-making and pulping industry pays more and more attention to clean pulping and green bleaching. Hydrogen peroxide bleaching is an important bleaching process in totally chlorine free bleaching(TCF). Hydrogen peroxide can be used in a variety of different bleachingprocesses.Resulting in bleached pulp with high yield, good whiteness stability, and low yellowing index. What’s more, the waste liquor does not contain chlorinated organic compounds, which can effectively reduce the toxicity of the bleaching effluents. However, because of the accumulation and introducionof transition metal ions, e.g., Fe3+, Mn2+, Cu2+, etc., in raw material and pulping process, the hydrogen peroxide can decompose to some extent due to the catalysis of metal ions. The decomposition will not only cause waste of the hydrogen peroxide, reducing the bleaching efficiency, but also cause the degradation of the cellulose. Therefore, in order to improve the efficiency and selectivity of the hydrogen peroxide, the appropriate pretreatment for the pulp will be important. Based on the fast-growing eucalyptus Kraft pulp.This paper mainly focused on the establishment of rapid method for determining the important parameters involved in the pulp. Moreover, the effects of sodium borohydride pretreatment on the hydrogen peroxide bleaching and the mechanism of the sodium borohydride pretreatment were reported. Specifically, a rapid method for determining carbonyl groups in pulps was established. The effect of the sodium borohydride pretreatment on the bleaching selectivity and the contents of the main components in bleaching effluent were investigated. Meanwhile, the mechanism of sodium borohydride pretreatment for peroxide bleaching was studied. The success of the present work can provide great guidance for the application of sodium borohydride pretreatment in peroxide bleaching and further exploring the protection mechanism in CMP.The content of carbonyl groups in pulps related to the sodium borohydride pretreatment. Firstly, method for determining carbonyl groups in pulps was established based on the phase reaction conversion technique of headspace gas chromatography. It is based on the reaction between sodium borohydride and carbonyl groups of pulps at an alkaline medium(0.1 mol/L Na OH), and the excess sodium borohydride is q uantitatively converted to hydrogen after the solution is acidified by sulfuric acid. The released hydrogen is then determined by GC equipped with a thermal conductivity detector. Thus, the carbonyl groups of pulps can be indirectly quantified. The results showed that the reaction was complete in 180 min at 90 oC, The relative standard deviations in the reproducibility test are within 2.0%, and a sensitivity whose limit of quantification(LOQ) is 1.26 μmol, and a demonstrated recovery ranging from 97 to 105%. The present method is simpler, more reliable, and more practical compared to the methods reported previously.And we investigated the effect of the sodium borohydride pretreatment on the hydrogen peroxide(H2O2) bleaching processes for the eucalyptus Kraft pulps. Then, the result was compared with the acid pretreatment. The results showed that the sodium borohydride pretreatment was proved more effective for the pulp bleaching. There were more hydrogen peroxide found in the bleaching effluents, more exce llent viscosity and kappa number than the acid pretreatment. Due to the reduction performance of sodium borohydride pretreatment, the carbonyl groups of pulps was reduced with the sodium borohydride pretreatment. It also noticed, compared with the non- pretreatment, there are no negative effect on the kappa number, brightness, and viscosity of the bleached pulp by the sodium borohydride pretreatment.Then, we reported the effect of different pretreatment on the main components, i.e., the dissolved lignin, methanol, carbonate in the bleaching effluents. The results indicate that the amount of residual hydrogen peroxide in the effluents decreased with the p H value in the acidic pretreatment; the maximum amount of methanol and dissolved lignin were found in the effluents when the pulp was pretreated at p H range from 3~ 4.5, respectively. However, more carbonate and oxalate were observed in the effluents that the pulp was pretreated at the lower p H value.In the final part of this paper, we reported the effects o f sodium borohydride pretreatment of peroxide bleaching process on the p H value, the amount of residual hydrogen peroxide, dissolved lignin and methanol in the effluents and the yellowing value of pulp. Then, we investigated the mechanism of the sodium borohydride pretreatment. Finally, with the glucose as a model compound, we studied the role of sodium silicate on the reaction between sodium borohydride and carbonyl. The experiment results showed that chemical pulp whiteness could effectively be improved at higher kappa number. So, we inference that sodium borohydride pretreatmen can significantly improve the whiteness of CMP. It also showed that the structure of dissolved lignin was evidently changed(due to the removal of carbonyl) and thus the activity of dissolved lignin was weakened by sodium borohydride pretreatment. This reduced both the peroxide consumption in the reaction on the dissolved lignin and the methanol formation in effluent. As a result, the efficiency of peroxide in pulp bleaching could be further improved. The reaction between sodium borohydride and carbonyl was promoted when added a little amount of sodium silicate to the reaction system. Temperature and reaction time ware the important factors which would influence the catalysis of sodium silicate.