Equilibrium of Lignin Precipitation

Lignin is possibly one new product from the paper pulp mill. Lignin of relatively high purity can be separated from black liquor by using the so called “LignoBoost” process. The first step of the LignoBoost process is the precipitation of lignin by lowering black liquor pH. The efficiency of this step depends on the properties of black liquor and the process conditions in the precipitation step. In this paper, a mixed softwood/hardwood black liquor was studied. The influence of pH, temperature and ionic strength on the solubility of lignin was investigated. Two different measurement methods were used to determine the lignin concentration in the filtrate: the Klason method and the standard UV method at a wavelength of 280 nm. Results showed that lignin solubility decreased when the pH and temperature of black liquor were lowered and the ionic strength in the black liquor was increased. When compared with earlier studies, based on softwood black liquor, it was found that the yield in the precipitation step was lower when the mixed softwood/hardwood black liquor was introduced. Another conclusion was that the Klason lignin method and the standard UV method gave different results of lignin concentration of the filtrate. However, the Klason lignin method is the more accurate method.