Study On Lignin Removal Mechanism In Pre-hydrolysis Liquor Of The Dissolving Pulp From Bamboo

Dissolving pulp composed of high purity and quality cellulose is a kind of chemical pulp.In recent years,the demand of raw materials increases,and the cotton linter and wood can’t meet the quantity demanded of dissolving pulp,for the increasing production of viscose fiber.Prehydrolysis is an important stage for the production of high quality dissolving pulp.And,prehydrolysis liquor（PHL）contains large quantities of hemicellulose degradation products and a small amount of lignin.The hemicellulose has low fuel value and the hemicellulose degradation products extracted from the PHL can be converted into high value-added chemicals such as xylitol,ethanol and biodiesel.However,the presence of lignin inhibits the production of xylitol and ethanol.Therefore,it is important to effectively enhance the lignin removal and separate hemicellulose from the PHL.In this paper,the dissolving pulp was prepared from whangee through optimizing the cooking and bleaching production process.And the effects of boric acid addition on the prehydrolysis of whangee were studied,by adding different dosages of boric acid in the prehydrolysis stage.The structure of lignin in the PHL was also characterized,and the calcium carbonate and polyaluminium chloride（PAC）were used to treat the PHL,and the mechanisms of lignin removal were studied.The dissolving pulp of whangee was prepared with the prehydrolysis,kraft cooking and OP（oxygen delignification enhanced with peroxide）-Q（chelation）-P（peroxide bleaching）three stages bleaching process.The optimum process conditions of kraft cooking were obtained as followings:the alkali charge of 20%,a sulfidity of20%,the H-factor value of 1238.98.The prehydrolyzed kraft pulp was subjected to OP-Q-P three stages bleaching process.Optimization of the（OP）-stage was carried out using 4.0%H₂O₂ on pulp,the（Q）stage 0.30%ethylenediaminetetraacetic acid（EDTA）,and the（P）stage 5.0%H₂O₂.As a result,the bleached dissolving pulp had a brightness of 83.69%ISO,a DP of 498.27,the kappa number of 0.80,anα-cellulose content of95.61%,the ferric ion content of 45.76mg/kg,and the final yield of 27.46%.In order to improve the dissolution of hemicellulose in the prehydrolysis stage,the effects of boric acid addition on the prehydrolysis of whangee were studied,by adding different dosages of boric acid in the prehydrolysis stage.The final yields of the prehydrolysis stage obviously increased with the addition of BA.Additionally,the highestα-cellulose retention occurred at a BA dosage of 0.50%.Oligosaccharide was the main component of the PHL.With the addition of BA,acetic acid percentages in the total sugars and furfural percent in xylose obviously increased,which indicated that the PHL had more hemicellulose degradation products;mass balance analysis of the PHL and whangee indicated that the partly acid-soluble lignin in PHL likely come from acid-insoluble lignin in the whangee.By acid precipitation and rotary vacuum evaporation treated the PHL respectively,the extracted acid precipitation and scale were characterized by FTIR and ¹H-NMR spectroscopy and Py-GC/MS,and the structure of lignin in PHL was studied.The lignin in the PHL was mainly composed of syringyl lignin.The syringyl lignin hadβ-1,β-5,β-O-4 andβ-βbonds.At pyrolysis temperature 800℃,the phenolic aldehyde group,methoxyl and side chain hydroxyl of the syringyl and guaiacyl lignin were dropped,and a large number of substituted methyl,ethenyl and ethyl phenol were produced.For producing the high added value products using the PHL,the CaCO₃ and PAC were used to treat the PHL.One component of CaCO₃ treated the PHL,and the results showed that:the effect of CaCO₃ on the components of the PHL was by adsorption;and at the optimum dosage of 1.70%,the total sugar removal was 4.40%,the acid soluble lignin removal 6.09%,and the acid soluble lignin selective removal 58.06%.One component of PAC treated the PHL,and the results showed that at the optimum dosage of 200ppm,the total sugar removal was 5.63%,the acid soluble lignin removal 9.29%,and the acid soluble lignin selective removal 62.26%.The combination of CaCO₃ and PAC was better than that of one component,and enhanced the acid soluble lignin selective removal.The addition of calcium carbonate firstly was better than that of PAC.At CaCO₃/PAC dosage of 2.0%/200ppm,the acid soluble lignin selective removal（77.22%）was the highest,the total sugar removal（2.74%）was the lowest,and the acid soluble lignin removal was 9.29%.