摘要：
Lignin is mainly obtained as a byproduct from pulp industry, which is the only renewable aromatic carbon source generated in nature, containing 3 major structures, 4-hydroxyphenyl, guaiacyl, and syringyl structures. Lignin can be efficiently transformed into highly value-added fine chemicals such as phenolic monomers and other high-grade biofuels such as alkanes through catalytic depolymerization methods. With the growing concern over the excessive emission of greenhouse gases and the depleting stocks of fossil fuels, the lignin conversion has become an important part of the renewable energy strategy for the governments. In chemical pulping, black liquor is a process stream containing both organic and inorganic components. One of those organic components is lignin which can be separated effectively by using acidification-flocculation. This present study describes the precipitation of lignin by acidification-flocculation of black liquor produced by soda pulping of bagasse. Firstly, the cross-linked cationic starch (CLC-St) was obtained by the cross linking reaction and etherification reaction with cassava starch, 5-hydroxymethylfurfural and 3-Chloro-2-hydroxypropyltrimethylammonium as raw material. Then cross-linked cationic starch grafted by polyacrylamide (CLC-St-PAM) flocculant was prepared using the CLC-St as backbone through the graft copolymerization reaction with acrylamide. The structures of CLC-St-PAM were characterized with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electronic microscope (SEM). The results showed that PAM was grafted into the base of CLC-St successfully, the grafting copolymerization reaction occurred on the surface of the starch particle, which was in agreement with the SEM micrographs, and the aggregation phase of CLC-St-PAM was changed to amorphous aggregation state. The CLC-St-PAM flocculant was added to the black liquor of bagasse soda pulping to form lignin-rich precipitate under different pH value conditions (varying from 7 to 2). The effects of selective important factors, such as dosage of CLC-St-PAM flocculant, pH value of black liquor, on the obtained lignin mass precipitated, the concentration of acid soluble lignin, and chemical oxygen demand (COD) in filtrate were studied systematically. It showed that organic matter and COD in black liquor had been efficiently removed under suitable flocculating conditions. When the pH value was 2 and the dosage of CLC-St-PAM was 0.015 g/L, the lignin recovery rate reached 89.02%±0.54% and the COD removal rate reached 69.06%±1.42%; meanwhile, the concentration of acid soluble lignin was reduced from (0.561±0.021) to (0.187±0.008) g/100 mL, and the color of the liquor was strongly modified from dark brown to pale yellow. The recovered lignin was characterized by nuclear magnetic resonance (1HNMR) and gel permeation chromatography (GPC). The results indicated that the recovered lignin consisted of syringyl and guaiacyl units and was connected together by C-O and C-C linkages. There was large variation among the molecular weight of the recovered lignin molecules, the GPC spectrum of recovered lignin contained 2 continuous peaks, and the number-average molecular weights of these 2 peaks were 105 691 and 11 270 g/mol, respectively. In the acid-flocculation process, pure lignin molecules were precipitated in high pH value, and with the further decrease of the pH value, more lignin-hemicellulose complexes were formed and precipitated.%该文通过制备耐酸型絮凝剂,以酸析絮凝法处理蔗渣制浆黑液高效回收木质素.首先以木薯淀粉、5-羟甲基糠醛以及3-氯-2-羟丙基三甲基氯化铵为主要原料制备交联阳离子淀粉(cross-linked cationic starch, CLC-St),然后以CLC-St 为骨架与丙烯酰胺在引发剂作用下进行接枝共聚反应,合成了交联阳离子淀粉接枝聚丙烯酰胺(cross-linked cationic starch grafted polyacrylamide, CLC-St-PAM)絮凝剂,并通过红外光谱、X射线衍射和扫描电子显微镜等手段对 CLC-St-PAM 进行结构和形貌分析.结果显示,交联阳离子淀粉分子与丙烯酰胺成功发生了接枝共聚反应,反应主要发生在淀粉颗粒表面,接枝产物呈现为无定形的聚集态.以CLC-St-PAM为絮凝剂处理pH值为2~7间的蔗渣碱法制浆黑液,系统考察了pH值、CLC-St-PAM用量等因素对木质素的回收率、滤液中酸溶木质素含量和化学耗氧量(COD)的影响,结果表明,酸析-絮凝处理可以有效的去除制浆黑液中的酸不溶木质素、酸溶木质素以及COD,在pH值为2时投入0.015 g/L的CLC-St-PAM,木质素回收率高达89.02%±0.54%,COD去除率也达69.06%±1.42%;同时,酸溶木质素质量浓度由原来的(0.561±0.021) g/100 mL下降至(0.187±0.008) g/100 mL,黑液由原来的棕黑色变为清澈的淡黄色.不同pH值回收得到的木质素结构通过核磁共振氢谱和凝胶渗透色谱表征,结果表明酸析-絮凝过程中首先析出来的是相对纯净的木质素分子,随着体系pH值的进一步下降,更多与半纤维素通过稳定的共价键形成的木质素-半纤维素复合体也逐渐沉淀析出.
