Studies On High Brightness Bleaching Of Wheat Straw Chemi-mechanical Pulp And Its Mechanism

The pulping properties of wheat straw materials by chemi-mechanical pulping process were investigated and compared in this thesis. Investigations on influences of impregnation using three kinds of pretreatments process(alkaline peroxide pretreatment, pectinase /alkaline peroxide pretreatment, neutral sodium sulfite pretreatment ) on unbleached chemimechanical pulps including optical and physical properties, were carried, such as coarse pulp brightness, coarse pulp strength, results of fiber screen classified parts , and specific energy consumption of refing process and COD load of effluent generated by pulping process were also contrasted. Results shows that higher pulp brightness, lower pollution load of effluent generated by refining process, better pulp strength and bleaching performance can be gathered through chemi-mechanical pulping process.Therefore, alkaline hydrogen peroxide pretreatment is a feasible method for wheat straw chemical mechanical pulping process.High brightness of wheat straw chemi-mechanical pulp bleached by peracetic acid and sodium borohydride combined with alkaline peroxide was gained. The final pulp brightness could be 72.38% ISO under the Pa optimal condition(peracetic acid dosage 1.0%, bleaching time 120min, pH 5, pulp consistency 20%), associated with two stage 5.0% H₂O₂ for multi-stage bleaching (PaP₁P₂ bleaching process)of wheat straw chemical mechanical pulp; The final brightness of wheat straw chemi-mechanical pulp bleached by NaBH₄ (NaBH₄ dosage 1.0%, bleaching time 60min, pH 10-11, pulp consistency 10%)associated with two stage 5.0% H₂O₂ could be 73.98%ISO. After storage for period of time, the appearance color of wheat straw becomes darker and deeper due to the changes of lignin from wheat straw materials, which adding the bleaching difficuty. The brightness of pulp made by wheat straw material storaged for 12 months could only be achieved at 67.09%ISO and 70.83%ISO respectively, using the same bleaching process(PaP₁P₂, P₁P₂B1.0 ), Results shows that the short storage-time material is important to wheat straw chemi-mechanical bleaching.The research on effluent from bleached pulp indicated that both COD value and BOD value of bleaching effluent from wheat straw chemi-mechanical pulp P₁P₂B bleaching stages are lower than that of PaP₁P₂, which are 149.06 kg/t pulp and 384.24 kg/t pulp respectively. The molecular structures of pollutant in these two kinds of effluent are similar, both contain aromatic compounds and aliphatic compounds, which can be determined by chromatography and mass spectrometry. The content of esters organic compounds and 4,4`-(1-methylethylidene) bis- organicas from Pa bleaching effluent is obviously higher than the content of organicas from NaBH₄ bleaching effuent. This could be one of the reasons which result in the pollution load of Pa bleaching effluent is higher than that of NaBH₄ bleaching effluent.There is an obvious difference for wheat straw chemi-mechanical brightness of pulp bleached by H₂O(24%,6%,8%,10% dosage)after acidfication, the pulp least brightness gain can be reached by 9.85 % ISO, the maximal brightness gain can be reached by 16.12 % ISO, and b* value decreased after acidficationPulp yellowing experiment results show that wheat straw chemi-mechanical pulp brightness stabilization is significantly improved using 1.0% peracetic acid or 1.0% NaBH₄ compared with H₂O₂ bleaching results; in contrast, the treatment efficiency for pulp brightness stabilization treated by 1.0% NaBH₄ is more obvious than the pulp stabilization treated by 1.0% peracetic acid. Improvement of brightness stabilization can be judged by quantity of carboxyl, because of the quantity of carboxyl of lignin from pulp treated by NaBH₄ decreasing, the pulp brightness stabilization is improved.Determination of nitrobenzene oxidation products of lignin by GC and spectra analysis for ¹H-NMR show that the lignin of wheat straw materials, chemi-mechanical pulps ,pulp bleached by alkaline H₂O₂ and pulps treated by H₂O₂/NaBH₄ is a G-S-H type lignin (the proportion of guaiacylpropane units 40%～48%, syringylpropane units is about 41%～53.10%, and p-hydroxyphenylpropane units is about 6%～9%).Amount of auxochrome phenolic hydroxyl(0.29 unit/C9)in lignin from H₂O₂ bleaching chemi-mechanical pulp is slightly higher than one of material(0.28 unit/C9), so amount of auxochrome phenolic hydroxyl can not be effectively decreased by alkaline peroxide bleaching process. Conjugated carbonyl content in lignin from H₂O₂ bleaching chemi-mechanical pulp decreases by 26.43% from 0.1063 unit/C9 to 0.0782 unit/C9, the content of conjugated carbonyl in lignin from pulp reduced by NaBH₄ decreases further by 69.95% from 0.0782 unit/C9 to 0.0235 unit/C9. Conjugated carbonyl in lignin from wheat straw chemi-mechanical pulp was reduced from Cα=O structure to Cα-OH structure by NaBH₄ reducing process after H₂O₂ bleaching process, this is the reason for pulp brightness improvement and pulp brightness stablilization increase.