Abstract
The pulping of birch wood using dilute aqueous nitric acid solution under atmospheric pressure was studied. The pulping conditions, delignification reaction course, and waste-liquid recovery were studied in detail. The optimum cooking conditions were 9.2% nitric acid for 4.3 h at 85 °C. The pulp yield at these conditions was 51.1%, the lignin content was 5.1%, and the brightness was 50%. The delignification course during the pulping involved two phases, namely, bulk and residual phases. The bulk phase was 0 to 1.5 h long, and the delignification level was 92.6% of the total dissolved lignin. The residual phase was 1.5 h to 4.5 h long, and the delignification level reached about 7.4% of the total dissolved lignin. The waste liquid could no longer be used after six rounds of recycling. The contents of nitrogen and organic matter in the organic fertilizer prepared using the final-round of recycled waste liquid were in line with the indicators for the preparation of organic fertilizer.
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