Studies On The Technology And Mechanism Of Clean Chlorine Dioxide Bleaching On Wheat Straw Pulp

As one of the most experienced countries, China is the world's major producer of non-wood pulps, whose paper production of non-wood fiber material is the largest all along. The proportion of grass weed pulp in non-wood pulps is the largest at about 60%~65%, which mainly refers to wheat straw pulp. Under the new environmental requirements, the discharge amount of absorbable organic halides (AOX), one vital crucial toxicant in paper industry, has become one forced implementation indicator. With the stern prohibition of elemental chlorine bleaching, the bleaching modes of non-wood pulp, mainly refers to wheat straw pulp currently, are being accelerated and transformed to elemental chlorine free (ECF) and totally chlorine free (TCF) bleaching methods. ECF bleaching technology based on chlorine dioxide is one of the most important technologies being studied and expanded presently. Clean chlorine dioxide bleaching technologies on wheat straw pulp and its mechanism are investigated detailedly in this paper, for the purpose of affording certain technical supports and theory bases for realizing the transform of bleaching modes from traditional CEH to ECF bleaching of wheat straw pulp, which will facilitate the industrial process of ECF clean bleaching of wheat straw pulp and even the whole non-wood pulp.The efficiency of chlorine dioxide delignification and oxygen delignification and corresponding bleaching sequences were investigated comparatively. The results indicated that effects of ClO2 delignification (D0) on the modification of residual lignin were better than that of O2 delignification (O). Under the conditions of lower than and near to the removal rate of O2 delignification, the bleachability of D0 stage pulp was superior to that of O stage pulp. With the same removal percentage of delignification, the final brightness of bleached pulp by D0A/QP sequence and D0EP sequence were 11.2 and 10.3 points higher than the ones bleached by OA/QP sequence respectively. The CODCr loads of the D0 stage filtrate and corresponding mixed effluents by three stages of D0A/QP sequence were lower than those of O stage filtrate and corresponding mixed effluents by three stages of OA/QP sequence.Enhanced chlorine dioxide bleaching of wheat straw pulp by synergetic effect of alkaline extract and xylanase treatment were investigated. Results demonstrated that the improvement of chlorine dioxide delignification with xylanase treatment was not significant. The effectiveness of chlorine dioxide delignification was not increased distinctly whether by xylanase pretreatment prior-delignification (XD) or by xylanase treatment post-delignificatiion (DX). Using XDP and DXP bleaching sequences, the aided bleaching effect of xylanase was not obvious. The final brightness was increased only as much as 1.5 points. Meanwhile, pulp treated with xylanase reduced the consumption of hydrogen peroxide by 25%-33.3%. With peroxide-reinforced extraction (EP) and using XDEPP and DXEPP bleaching sequences, the aided bleaching effectiveness of xylanase was increased significantly, with the final brightness enhanced by 4.5 points. With the same sequences, xylanase treated pulp saved the charge of chlorine dioxide up to 17%-20%, while still maintaining the final brightness at the control level.The modification effects of four kinds of alkaline extraction modes on lignin of chlorine dioxide delignified wheat straw pulps were explored, and various corresponding ECF bleaching sequences were investigated comparatively. Results indicated that in D0ED1 bleaching sequences the modification effects of alkali extraction modes on the residue lignin of D0 stage were different. As far as the subsequent D1 stage was concerned, the bleachability order of pulps, from easiest to hardest, was D0EOP>D0EP>D0EO>D0E. It was demonstrated that in D0ED1 bleaching sequences the influence of pH value on the brightness of D1 stage was significant, the optimal final pH range was 6.0~6.5% and the optimized charge of NaOH was 0.6%~0.8% under the conditions of present experimentation.The activation effectiveness of aldehydes on chlorine dioxide bleaching of non-wood pulps was investigated. The rate and efficiency of chlorine dioxide delignification were enhanced and facilitated effectively through adding 1.0%-1.5% formaldehyde to D stage. Compared to the control, the kappa numbers both in the D stage and in DE or DEP stage were decreased somewhat, simultaneously, the actual consumption of ClO2 was increased, which demonstrated that the improvement of formaldehyde on chlorine dioxide bleaching was certain. Except for formaldehyde, a certain extent of activation effect can be obtained by adding other aldehydes or aldoses, such as acetaldehyde, glyoxal, glucose, et al. The effectiveness of formaldehyde was relevant to the initial kappa number of unbleached pulps. The higher of the initial kappa numbers of unbleached pulps, the better of activating effect of formaldehyde on D stage. By adding 1.5% formaldehyde to D stage and with DEPP bleaching sequences, the unbleached Soda-AQ wheat straw pulps with a initial kappa number of 21.7 reached a final brightness of 81.7%, 2.5 points higher than the control. Correspondingly, some physical strength properties decreased to a certain extent.Various ECF bleaching effluents were qualitative analyzed and quantitative determined using GC-MS analyzer. Results showed that the pollutant load of D0 stage was remarkably lower than that of O stage and EP stage. The analysis of bleaching filtrates by GC-MS analyzer indicated that except for including the main pollutants such as the benzoquinones with the content of 41.9% in D0 filtrate, D0 stage filtrate also contained a small portion of organic chlorine compounds, mainly in the form of chloro-acetone and chlorinated aromatic compounds. Additionally, as for O stage comprising of the main aliphatic hydrocarbons with the content of 51.2%, followed by aromatic aldehydes, there is no any content of ketone in O stage filtrate, which is a significant characteristic different to the D0 stage filtrate. The main pollutant ingredients of EP filtrate were aliphatic hydrocarbons and fatty acids, while the main pollutant types of D1 stage were fatty acids, with lower content of aliphatic hydrocarbons relatively. As with the EP stage, the main pollutant compositions of P filtrate were referred to aliphatic hydrocarbons and fatty acids with the total content of 85.9%.Using two-step methods of enzyme hydrolysis and weak acid hydrolysis, various lignin were isolated from unbleached and bleached wheat straw pulp during the process of ECF bleaching, and then, lignin structure were analyzed through a serious of methods such as FT-IR, UV-VIS, GPC, TG, 31P-NMR, 1H-NMR and so on. FT-IR spectroscopy analysis showed that the carboxyl content of chlorine dioxide delignified pulp was increased, with the subsequent bleaching the carboxyl absorption gradually decreased and then disappeared in the end. UV spectrum analysis showed that with the proceeding beach the characteristic absorption peak of aromatic ring at 280nm gradually disappeared in both D1 pulp and P pulp. Thermal gravimetric (TG) analysis indicated that before 200℃the thermal weight loss of lignin was not obvious and after 200℃the loss percentage began to decline rapidly until 500℃, after 500℃the loss trends became changing flatly and mildly. GPC analysis showed that after ECF beaching sequences as D0EPD1 and D0EPP the number average molecular weight, the weight average molecular weight and polydispersity had declined to some extent.Lignin 1H-NMR analysis showed that the proton concentrations of methoxyl were relatively high, second only to that of aliphatic methyl and methylene. Lignin units of unbleached wheat straw pulp mainly included guaiacyl and syringyl units and a few amounts of p-hydroxylphenyl units. The content of three kinds of units as G, S and H types were all decreased gradually. Especially, the content ofα-proton in the structure ofβ–O-4 andβ-1 also reduced to some extent, which demonstrated that the rapture ofβ–O-4 and demethylation reaction were the main approaches by which large molecules were degraded into small ones. Moreover, the rapture of ether bonds not only degraded large lignin molecules into small ones, but also produced new phenolic hydroxyl groups, which can enhance the reactivity of lignin.31P-NMR analysis indicated that the content of aliphatic hydroxyl was higher than the ones of phenolic hydroxyl of benzene ring. After chlorine dioxide delignification and oxygen delignification the content of aliphatic hydroxyl and total phenolic hydroxyl increased somewhat, mainly due to the contribution of the rise of the content of condensed phenolic hydroxyl and p-hydroxylphenol phenolic hydroxyl. After chlorine dioxide delignification and oxygen delignification the phenolic hydroxyl content of both syringyl and guaiacyl declined to some extent, moreover, the drop percentage of the former was higher than the later, indicating that the degradation of syringyl was the principal degradation form during chlorine dioxide delignification and oxygen delignification. As for the change of carboxyl content, almost to the zero content in unbleached wheat straw pulp, the carboxyl content after ClO2 and O2 delignification were raised significantly, which was one prominent characteristic of ClO2 and O2 delignification. In terms of the comparison of hydroxyl content between D0 lignin and O lignin, the distinction of the contents of various hydroxyl and carboxyl were not significant.