ALLEVIATION OF THE ENVIRONMENTAL IMPACT OF BLACK LIQUOR FROM PAPER MILLS THROUGH NOVEL LIGNIN PRECIPITATION AND SEPARATION TECHNIQUES

摘要

Black liquor disposed from paper mills using rice straw as a feedstock has extremely high COD and BOD values thus causing serious environmental problems. The high silica content of the liquor does not allow its conventional processing to recover the alkali and using it in straw digestion. A novel process was developed to simply and efficiently separate lignin by decreasing the pH value using anionic surfactant, thus decreasing pronouncingly the COD and BOD values. A continuous system for lignin precipitation was developed and the optimum feeding rates of flocculant/black liquor mixture and sulfuric acid were determined. Lignin flocs form immediately and are large enough to be retained on 24-mesh filter cloth. The retained fluffy lignin mass was squeezed using a compression unit simulating industrial filter belt press. The spent liquor remains after lignin separation was aerobically treated using sludge process to further decrease its COD and BOD values. Lignin precipitation efficiency of 80% was achieved at pH value of 2.5, the COD and BOD values of black liquor were decreased by 95% and 92%, respectively. The squeezed lignin mass is easy to handle for subsequent utilization. The spent liquor complies with the standards for discharging in public sewer.