Abstract:In the pulp and paper industry different types of pulp or fiber fines are generated during the pulping (primary fines, mechanical fines), and/or the refining process (secondary fines). Besides fibers, these cellulosic microparticles are a further component of the paper network. Fines, which are defined as the fraction of pulp that is able to pass through a mesh screen or a perforated plate having a hole diameter of 76 µm, are known to influence the properties of the final paper product. To better understand the effect and properties of this material, fines have to be separated from the pulp and investigated as an independent material. In the present study, fines are isolated from the pulp fraction by means of a laboratory pressure screen. To allow for further processing, the solids content of the produced fines suspension was increased using dissolved air flotation. Morphological properties of different types of fines and other cellulosic microparticles, such as microfibrillated celluloses (MFC) are determined and compared to each other. Furthermore, handsheets are prepared from these materials and properties, such as apparent density, contact angle, modulus of elasticity, and strain are measured giving similar results for the analyzed types of fines in comparison to the tested MFC grades. The analysis of the properties of fiber fines contributes on the one hand to a better understanding of how these materials influences the final paper products, and on the other hand, helps in identifying other potential applications of this material.
Understanding separation of poly-disperse particle suspensions according to the particles size is of great importance to product quality. Previous experimental studies of suspension flow through coiled tubes report different results for spherical and elongated particles, e.g., larger and thus heavier elongated particles are faster than smaller ones. We use Euler-Lagrange simulations, as well as experiments, to measure the residence time distribution of fibres with different size in coiled tubes with different curvatures. Fluid flow through the coiled tubes was simulated as toroidal flow, i.e., the pitch of the tube was neglected. Fibres are one-way coupled to the fluid, and their movement in the cross section, as well as their orientation is predicted based on the assumption of an infinitely dilute suspension. We find that in coiled, dilute suspension flow of fibres the ratio of particle settling velocity to the secondary flow speed determines the fibre motion in the tube cross section. For low Reynolds number and thus larger effect of gravitation, fibres are found to concentrate in distinct orbits. Long fibres form flocs propagating through the torus whilst small fibres are well mixed and thus retained in the tube. We found that fibre-fibre interaction and the formation of flocs and not fibre-fluid interaction is key to the size based separation.
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