Acid Precipitation of Lignin from Alkali Black Liquor: A Study on the Optimization of Lignin Recovery Using H₂SO₄ and HCl

In this study, black liquor derived from an alkali-catalyzed organosolv process was used as the source for lignin extraction, with the aim of maximizing lignin yield and exploring its utilization in sustainable applications. To reduce the pH of the alkali black liquor from 13 to below 2, sulfuric acid (H<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>SO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf>) and hydrochloric acid (HCl) were employed for acidification. The results showed that H<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>SO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> produced the highest solid precipitate yield, with a lignin yield of 86.09% and a corresponding solid yield of 26.19% at a reaction time of 12 hours. In contrast, HCl required larger volumes but achieved higher lignin purity, reaching 92.57% compared to 86.63 % for H<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>SO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf>. The process efficiency was examined in terms of acid volume consumption and lignin precipitation yield. While H<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>SO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> offered better efficiency in terms of lower acid volumes, HCl demonstrated superior lignin and solid yields, making it more effective in maximizing product recovery. Moreover, the high initial ash content (72.1 %) in the black liquor, which was linked to the solid yield, was significantly reduced after acid precipitation, improving the purity of the extracted lignin. The Klason lignin content was enhanced through the process, demonstrating the effectiveness of the acid precipitation in isolating lignin with reduced inorganic contaminants. These findings contribute to the development of more efficient lignin valorization processes, supporting the broader goal of sustainable biomass utilization by converting black liquor into high-purity lignin suitable for further applications in bioplastics, adhesives, and biofuels.