Understanding the Effect of Precipitation Process Variables on Hardwood Lignin Characteristics and Recovery from Black Liquor

Lignin has an enormous valorization potential as a sustainable raw material. The most common source of industrial lignin is black liquor, a byproduct from the kraft process. However, the variability in color and chemical characteristics of precipitated lignin is a constituent, hindering its use as a raw material. The structural complexity and heterogeneity of the lignin molecule make it challenging to determine the underlying mechanisms behind the chemical and physical changes that occur during precipitation. This work used a statistical design of experimental approach in Minitab to study how fractionation process variables influence the lignin mass yield and characteristics. The specific phenomena governing hardwood lignin solubility and precipitation in black liquor were investigated, and the relationships between chemical and physical properties and lignin color were analyzed. We found a positive correlation between temperature, mass yield, molecular weight, and dark coloration. For pH < 4, condensation of molecular fragments and sorption onto larger chains seem to drive the precipitation process.