Optimizing lignin extraction from Kraft black liquor using protic Ionic liquids

Abstract

A variety of protic ionic liquids (PILs) were utilized to extract lignin from black liquor (BL). The PILs and a control IL, [Emim][Ac], were evaluated to extract lignin under different conditions. The impact of temperature, time and the IL,PIL:BL were the factors evaluated for extracting lignin. [Eth][Ac] was successful in extracting 75.3 ± 3.4% of the lignin (dry-basis) from BL at 95 °C for a PIL:BL(20:1)(w:w) and a mixing time of 4.5 h. The lignin extracted (13.4 ± 0.5%) was lowest for [Pyrr][La] under the same conditions. The characterization of the extracted lignin by FTIR confirmed that the observed peaks for the extracted lignin were in agreement with the peaks for a standard lignin. The conditions for the lignin extracted from BL using [Eth][Ac] were optimized in a three-factor, three-level Box-Behnken design (BBD). The factors (levels) of optimization parameters included time (0.5 h, 4.0 h, 7.5 h), temperature (60 °C, 95 °C, 130 °C), and the PIL:BL (10:1, 15:1, 20:1)(w:w). The BBD data was employed to develop a quadratic prediction model for lignin extraction as a function of the parameters affecting the process. A comparison of the model prediction with the experimental data confirmed the model predicted values were correlated with the experimental results. The optimum lignin extraction of 70.0% predicted by the model was 5.0% less than the experimental value, 75.0 ± 2.9%. The experimental quantity of lignin extracted with [Emim][Ac] under the same conditions was 77.6 ± 2.1%. The study established that the lignin extraction using PIL is comparable with the acidification-based commercial extraction, such as LignoBoost® and LignoForce® for lignin extraction from BL.