Ash thermochemical behaviors of bamboo lignin from kraft pulping: Influence of washing process

Abstract

To improve the efficiency and safety of the thermochemical conversion process of lignin residues from the bamboo paper factory, the residual lignin of black liquor was precipitated by concentrated hydrochloric acid. It was further washed using a hydrochloric acid solution with a pH of 2 and deionized water to obtain lignin samples. The ash of lignin samples with different washing processes was prepared using a muffle furnace according to standard method (GB/T 36057-2018). Their surface morphology, chemical compositions and thermochemical behaviors were investigated by cold field emission scanning electron microscope, X-ray fluorescence analyzer, X-ray diffractometer, and synchronous thermal analysis. The results showed that the washing process changed the type and content of chemical compositions in lignin ash and affected its thermochemical behaviors. The ash content of L1 sample (13.76%) decreased to 6.13% (L2 sample) and 4.35% (L3 sample) when the pH value of washing filter was 4 and 7. The efficiency of ash removal was 68.39%. The LA1 sample was composed of Si, S, O, Na, Cl, K, Ni, Zr, and Pd, especially for Cl (46.49%) and K (41.85%). The washing process decreased Cl and K. LA2 and LA3 samples had the high content of Si, S and O. This increased the Ra and F, and deceased Rb, Rb/a, Rb/(a+p), Fu, and S/A. Three weight loss peaks of the DTG curve and three prominent endothermic peaks of the DSC curve corresponded to Cl release, Na and K release, and S release for LA1 sample. Compared with LA1, LA2, and LA3 had a slight weight loss. The washing process increased the efficiency and safety of the thermochemical conversion of lignin residues in the bamboo papermaking factory.