Acid precipitation and purification of wheat straw lignin

The acid precipitation and purification of lignin from wheat straw soda pulping effluent were studied. Prior to lignin precipitation, the suspended solids and dissolved silica were removed from the effluent to avoid lignin contamination. Suspended solids and dissolved silica were removed by centrifugation and selective precipitation at pH 5, respectively. After these treatments, the pH of the effluent was reduced to an optimum pH value of 3.5 with sulfuric acid in order to precipitate lignin. About 80% of the total lignin was recovered. Effluent color and COD were simultaneously reduced by 82 and 71%, respectively. Lignin precipitate purification was carried out by washing with aqueous acidic solutions. Only the time and the agitation rate influenced the washing process. The purity of the lignin was very high when the time and rate of agitation during washing were low. Under optimum conditions, lignin with a purity of 99.5% can be obtained.