Analysis of causes for high hydration and low drainage rate of wheat straw chemi-thermo-mechanical pulp

The wheat straw chemi-mechanical pulp has the characteristics of high yield, good mechanical performance and low production cost. However, there are significant problems in the papermaking process by such as high pulp hydration, poor drainage, and large energy consumption during the paper drying-process. It was found that during the chemical mechanical pulping of wheat straw, due to the residual alkali and temperature, the hemicellulose in the pulp was continuously dissolved, some of it entered the water phase, and some of it migrated to the fiber surface. Which reflected in the raise in cation demand for anionic substances in the aqueous phase, the increase in carboxyl content on the surface of pulp fibers, and the aggravation in hydration degree. Further research found that the high valence metal ions can combine the carboxylate radical on the hemicellulose molecule in the aqueous phase with the carboxylate radical on the pulp surface to form a covalent bond, so that the hemicellulose entering the aqueous phase can be re-adsorbed to the fiber surface, thus further increasing the degree of pulp hydration. Carboxymethyl starch was used to simulate hemicellulose, and the hemicellulose re-adsorption phenomenon verification experiments were reproduced in a calcium ion environment. The above findings indicate that during the production of wheat straw chemi-mechanical pulp process, the hemicellulose dissolved and re-adsorption not only leads to the increase of pulp hydration, but also results in poor water filtration during paper formation. Therefore, this study can provide a theoretical basis for optimizing the production process of chemi-mechanical pulp of wheat straw and controlling the technology and method of pulp hydration.