Fractional Isolation and Purification of Lignin from Pulping Black Liquor

Fractionation and purification of lignin form pulping black liquor by the organic acid precipitation method was studied in this paper to meet the demands of resource utilization of pulping black liquor. The narrow molecular weigh distribution fraction of lignin could be obtained by citric acid, as shown by gel permeation chromatography (GPC). The effects of reaction temperature, pH of acid evolution endpoint and speed of centrifuge were evaluated for the process of classification. The results of three factors quadratic orthogonal combination experiment demonstrated that the optimal condition of fractionation and purification was as follows: citric acid saturated solution, 40°C, pH 5.0 and centrifuge speed is 1000rpm. Under these conditions, the Mn, Mw and Mz of lignin fraction was 1147, 1901 and 2854, the dispersing coefficient D was measured to be 1.65 which smaller than the unfractionated lignin. The FT-IR and UV spectroscopic studies demonstrated that the classification and purification have no distinct effects on the structure and the integrity of lignin molecular was keeped.