Electro-assisted filtration of microfibrillated cellulose: the impact of the degree of fibrillation

Abstract

Efficient dewatering is necessary to achieve an economically sustainable large-scale production of microfibrillated cellulose (MFC) because the low solids content of the final product (< 3 wt.%) results in high costs related to transportation and storage, and problems for products with water incompatibility. Mechanical dewatering is preferred to thermal drying due to its lower energy demand, but MFC has a very high filtration resistance, which implies that an excessive filter area is necessary. Thus, to improve the dewatering, electro-assisted filtration may be used. In this study a bench-scale dead-end filter press was modified and the electro-assisted filtration of MFC, with two degrees of fibrillation, was investigated. The impact of the degree of fibrillation was clear when either pressure or electric field were applied separately. It was more challenging to dewater MFC with a higher degree of fibrillation using conventional filtration due to a greater surface area being subjected to the liquid flow. The opposite was found when using an electric field alone: the more fibrillated material has a higher surface charge and thereby is impacted more by the electric field. A combination of pressure and electric field resulted in a greatly improved dewatering rate, but no significant difference could be observed between the two qualities. After dewatering, the water retention value was slightly decreased, but the material still showed a gel-like behaviour, although the network strength was slightly reduced, as seen by a reduction in yield stress, storage and loss moduli. This was plausibly due to a decrease in the surface area and/or deformed network.