Mild potassium hydroxide pulping of straw

Abstract

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).