Environmentally friendly pulping of rice straw to eliminate black liquor discharge

Rice straw pulping trials were carried out with aqueous ammonia mixed with caustic potash to eliminate problems associated with black liquor in nonwood pulping. This process likely can use the black liquor, which contains nitrogen, potassium, and ammoniacal lignin, 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. We studied the effects of flocculating conditions, such as dosage of 10% aluminum polychloride, dosage of 0.1% polyacrylamide, reaction temperature, and pH of black liquor to obtain suitable technological conditions. Analyses confirmed that major quantities of lignin and 23.7% nitrogen (dried basis), 6.2% potassium (dried basis) existed in the flocculating residues, so this is potentially 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.