Environmentally friendly pulping process for rice straw to eliminate black liquor discharge

Rice straw pulping trials were carried out with aqueous ammonia mixed with caustic potash to eliminate the black liquor problem in nonwood pulping. The black liquor, which contains nitrogen, potassium, and ammoniacal lignin, potentially can be used as a fertilizer for agricultural production. Excess ammonia in the black liquor was recovered by batch distillation. The black liquor was further treated for reuse by coagulation under alkaline conditions. Effects of flocculating condition, such as dosage of 10% aluminum polychloride, dosage of 0.1% polyacrylamide, and reaction temperature and pH of black liquor, were studied systematically by experiment to obtain suitable technological conditions. Infrared and other analytical techniques confirmed that major quantities of lignin, 23.73% nitrogen (dried basis), and 6.24% potassium (dried basis) existed in the flocculating residues, which has the potential to be a good solid fertilizer. The amount of delignification and the pulp screen yield for the process remained steady at 83%–85 % and 38-40 %, respectively, when reusing the supernatant four times.