Precipitation of Kraft Lignin: Yield and Equilibrium

Abstract: Kraft pulping is the dominant pulping process used in the world today. The material efficiency in modern kraft pulp mills is, however, only 40−55% and the final product consists mainly of cellulose. Recently, a novel process called “LignoBoost” has been introduced to separate lignin from kraft black liquor. The separated lignin can be utilized either as a solid fuel or as a raw material for the production of carbon fibres or chemicals. It makes it possible for a traditional pulp mill to become a combined biorefinery. There are four major steps in the LignoBoost process: precipitation of lignin, filtration of lignin, re-dispersion of the lignin suspension and, finally, washing of lignin. The filtration and washing steps have been investigated extensively already, so it is of interest to gain more knowledge regarding the precipitation of lignin. Two kraft black liquors were used in this work: a mixed hardwood/softwood black liquor and a softwood black liquor. Cross-flow filtration was used to fractionate lignin with a different molecular weight from the softwood black liquor. The precipitation of lignin was performed by acidifying the black liquor at various process conditions, namely pH, temperature and ionic strength of the black liquor. The influences exerted by these parameters on the precipitation yield of lignin were investigated. The molecular properties (average molecular weight and functional groups) of precipitated lignin, together with a carbohydrate analysis of the black liquor and precipitated lignin, were determined. Finally, the dissociation degree of the phenolic groups on the lignin molecules was estimated using the Poisson-Boltzmann cell model. The results show that the precipitation yield of lignin increases with decreasing pH and temperature and/or with increasing ionic strength of the black liquor. There is an increasing amount of lignin with a lower molecular weight that is precipitated at a higher yield. Within the same precipitation conditions, the lignin fraction with the highest molecular weight tends to have the highest yield. According to NMR analysis of lignin, the content of methoxyl groups decreases for softwood lignin but increases for mixed hardwood/softwood lignin at a higher yield, whereas the content of phenolic groups increases at a higher yield for both types of lignin. The content of carbohydrates decreases with increasing yield. In a highly electrolyte solution (such as black liquor), the dissociation degree of the phenolic groups on the lignin molecules is related to the alkalinity and temperature of the precipitation conditions, but less so to an increase in the ionic strength or the molecular weight of the lignin.