Bio-lubricant production from black liquor as a sustainable biorefinery strategy

Abstract The paper and pulp industry annually generates approximately 170 million tons of black liquor, yet research directly targeting its utilization remains scarce. Addressing this gap, our study tackles the challenge of revalorizing black liquor through innovative techniques. Concurrently, the lubricant industry’s pursuit of eco-friendly alternatives necessitates effective thickeners for semi-solid lubricant formulations derived from vegetable oils. Our research focuses on revalorizing weak black liquor via supercritical water hydrolysis, presenting a novel avenue for semi-solid bio-lubricant production. We obtained and treated four lignin waste fractions, including original black liquor and depolymerized variants through supercritical water hydrolysis, followed by additional processes such as spray-drying, oven-drying, and ethyl acetate extraction. These lignin fractions were then incorporated into epoxy-modified vegetable oils to formulate sustainable lubricants. Evaluation of these formulations revealed promising rheological and tribological properties. The resulting castor oil-based bio-lubricants not only exhibited excellent stability and appearance but also demonstrated notable performance improvements in rheological and tribological tests, noticing a maximum wear scar size reduction of around 50%, while maintaining friction factor values. Our findings highlight the potential of depolymerization hydrolysis in enhancing the lubricating properties of castor oil-based formulations, thus advancing the revalorization of weak black liquor. Graphical Abstract