Lignin Separation and Purification from Black Liquor Precipitate

The spent liquid after the pulping process in pulp mill, generally known as black liquor contains the degraded lignin and carbohydrate. The P_H and the residual alkali of black liquor increased with the increase of total alkali used during the pulping process while the black liquor viscosity mainly depends on the dissolved lignin. The increase of alkali charge resulted higher dry solid yield due to the increase of both inorganic components and dissolved organic components. Analysis by Fourier transform infrared spectroscopy (FTIR) showed that the isolated lignin contain in each solvent, while gas chromatography(GC) analysis showed that black liquor composition. In this work, lignin precipitation was performed by the isolation of kraft black liquor at various process conditions, namely P_H, temperature and different solvent and influences exerted by these parameters on the equilibrium of lignin increases with decreasing P_H and temperature and with increasing ion strength of the black liquor used. The concentration of carbohydrates in lignin decreases with decreasing P_H and that an increasing amount of lower molecular weight lignin is precipitated at a higher precipitation yield.