CLC-St-PAM flocculant purifying bagasse soda pulping black liquor and recycling lignin

Abstract: 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.