Analysis of Electrical Phenomena Occurring in Thermally Assisted Mechanical Dewatering Processes (TAMD)-a Preliminary Study

Abstract

The design, modelling and simulation of solid-liquid separation processes remain a challenging area due to difficulties in quantifying distribution of the phases in the filter cake, and in acquiring reliable experimental data for model development. Usually, overall mechanical dewatering parameters, such as filtrate volume, average volume fraction of solids, or cake thicknesses are measured. A more fundamental approach is to determine the internal structure of filter cakes, i.e., the profile of the solid volume fraction, in order to investigate the role of the material behaviour during compression steps. In most cases, profiles of solid volume fraction have been investigated using electrical resistance meters (Shirato & Aragaki, 1972; Chase & Willis, 1991). During the measurements, iron pins are positioned at the wall of the filter cell, disturbing the liquid flow and cake formation. Some investigators studied profiles of solid volume fraction in filtration cakes with X-rays (Bierck et al., 1988; Tiller et al., 1990) or nuclear magnetic resonance (NMR) (Horsfield et al., 1989). In our knowledge, the investigation of the solid distribution in a filter cake by electrical tomography technique has not been reported before. This contribution describes preliminary studies for the application of electrical resistance tomography (ERT) for the determination of local volume fractions of solids in a filter cake during the thermally assisted mechanical dewatering process (TAMD) (Fernandez et al., 2005). ERT is a non intrusive and non-destructive technique, which does not influence cake. Moreover, electrical resistance tomography (ERT) belongs to real-time, low-cost, easy to implement and fast techniques which exploit differences in the electrical properties of solids and liquids. Electrical resistance tomography basically consists in reconstruction of the conductivity distribution from electrical data which characterize the electrical response of a medium. The electrical current flow in a saturated packed bed submitted to an electrical field is depending on the solid and liquid phases, namely the dispersed and the continuous phases, which have different properties. Several theories have been proposed for the transport of electrical current in mixtures (Helfferich, 1962). However, most of the theories are based on the assumptions of either a regular lattice-type arrangement or a completely random distribution of the components that are not well adapted for filtration cakes. Electrical conductivity of saturated packed bed varies with the mobility and affinity of ions with