Fibrous materials for viscose filtration

Abstract

its degree of anisodiametry, i.e., the ratio of the length Z to the diameter d of the particles, is small so that these particles break through and clog the spinneret holes. Short-cut fibres with an I/d ratio of 170-250 are preferred as BFM. In filtration of filter presses use is usually made of woven fabrics of cellulosic fibres although woven or nonwoven materials of a synthetic fibre (Khlorin, polypropylene, Capron, etc.) give faster filtration and last longer. The advantages of cellulosic fibres are obviously that they give a purer filtrate and that they are more efficient in terms of dirt entrapment over a given filtration area. The difference in the behavior of filtration materials of cellulosic and synthetic fibres can be assumed to arise from differences in the surface and adsorption properties of the fibres. Cellulosic fibres are more hydrophilic and filter out the impurities by an adsorption-clogging mechanism whereasfiltratio n through a material of synthetic fibres is based on the principle of mechanical clogging. These types of filtration are illustrated in Fig. i. In adsorption-clogging filtration the impurity particles are adsorbed on the channel walls along their whole length so that with complete clogging (Fig. la) the filter material adsorbs a large number of particles. In partial, clogging too, the number of particles adsorbed to the channel walls may be considerable (Fig. ib). In mechanical clogging, on the other hand, only relatively large particles are entrapped and complete clogging occurs when a small number of particles are entrapped merely at the channel inlet (Fig. Ic). Partial clogging also takes place only at the channel inlet (Fig. id). Since the volume of filtrate depends mainly on the diameter of the channel and very much less on its length the productivity and capacity of a filter are significantly lower with mechanical than with adsorDtion clo~~in~. In this article an attempt is made to describe the mechanism of filtration and, more particularly, to determine the part played by the physicomechanical properties of the filter material and thereby to provide a basis for selecting the optimal type of fibrous filter material for the built-up layer. The experiments were carried out with chemically modified viscose rayon staple fibre (MVR) and Khlorin staple fibre. Being of the same nature as the bulk of the impurity particles in the viscose, rayon staple can be assumed to possess considerable adsorption capacity for these particles so that in this case filtration should proceed mainly on the principle of adsorption. Khlorin fibre, on the other hand should filter by a proces6 of mechanical clogging. The filtration t~'sts were conducted on experimental apparatus with a total filtration area of 0.i m 2. The filter was of the vertical type with a horizontal filter frame fitted with gauze of 500 ~ mesh. The viscose was of the grade used for rayon staple and contained 8.8% cellulose and 6.4% alkali and had a viscosity of 55-65 sec. The swelling of the fibre material is a 10% alkali solution in terms of the change in the fibre diameter [i, p. 331] was determined toge~her with the