The separation of lignin from the black liquor generated during alkaline pulping is reviewed in this article with an emphasis on chemistry. Based on published accounts, the precipitation of lignin from spent pulping liquor by addition of acids can be understood based on dissociation equilibria of weak acid groups, which affects the solubility behavior of lignin-related chemical species. Solubility issues also govern lignin separation technologies based on ultrafiltration membranes; reduction in membrane permeability is often affected by conditions leading to decreased solubility of lignin decomposition products and the presence of colloidal matter. Advances in understanding of such phenomena have potential to enable higher-value uses of black liquor components, including biorefinery options, alternative ways to recover the chemicals used to cook pulp, and debottlenecking of kraft recovery processes.

Lignin is a sustainable raw material with a high potential for use in the production of renewable products. While the market for lignin is slowly growing, lignin recovery via acid precipitation during the kraft pulping process requires the addition of chemicals that will impact the chemical balance of the pulp mill. This negatively affects both the environmental and business operations. Utilizing existing process streams as a source of chemicals will allow the mill to close the chemical loop and reduce emissions, which will have positive environmental impacts. This study investigated the internal production of sulphuric acid (H₂SO₄) and carbon dioxide (CO₂) for use in lignin separation (also called extraction) at a Swedish kraft pulp mill. The process simulation tool CHEMCAD was used to model and analyze the wet gas H₂SO₄ (WSA) process to produce H₂SO₄. The chemical absorption process using monoethanolamine (MEA) to capturing CO₂ was also analyzed. The utilization of the sulphur-containing gases to produce H₂SO₄ can generate an amount that corresponds to a significant lignin extraction rate. The CO₂ available in the flue gases from a mill well exceeds the amount required for lignin extraction.