Mild potassium hydroxide pulping of straw

This research focused on the pre-extraction of rice and wheat straws by potassium hydroxide (KOH) at 90 ​°C prior to alkaline pulping to remove silica and some of the lignin and hemicelluloses. Pre-extraction removed 76 and 73% lignin, 50.5 and 26.8% pentosans and 81 and 87% silica from the rice and wheat straws, respectively. Pre-extracted rice and wheat straws were subjected to KOH pulping at varying KOH charges (8.4, 11.2, 14, and 16.8% as KOH on straw), cooking times (60, 90, and 120 ​min), and cooking temperatures (70 and 90 ​°C). Pulp yields of 43.6 and 51.0% with kappa numbers16.2 and 20.2, for rice straw and wheat straw, respectively, were obtained at the conditions of 12% alkali for 120 ​min of cooking at 90 ​°C. KOH rice straw and wheat straw pulps were bleached. The ISO brightness was 76.8 and 80.1%, respectively, by the D0(EP)D1bleaching sequence on using a fixed total ClO2 consumption of 25 ​kg ClO2/tonne pulp. The strength properties of the pulps at the lower cooking temperature were inferior to that of corresponding pulps produced at a higher cooking temperature. Dissolved silica, lignin, and hemicelluloses were separated from the spent black liquor. The potassium-rich spent liquor was used as a soil amendment (fertilizer). •KOH pulping of straws was carried out at atmospheric pressure in two stages.•Most of the lignin and silica were removed in the first stage.•Second stage reduced kappa number to get a bleachable grade pulp.•Pulp yield in low temp pulping was higher than high temp pulping.•Papermaking properties of low temp pulping was inferior to high temp pulping.