Controlling particle trajectory in free-fall electrostatic separators

The purity and recovery of concentrates obtained in industrial free-fall electrostatic, separators can be increased by preventing the impacts between the particles and the electrodes. The aim of this paper is to analyze the possibility to control particle trajectories in such separators by modifying the conditions of particle admission in the interelectrode space. The parametric equations of a charged particle trajectory in the electric field between the electrodes of a free-fall separator served for writing a numerical modeling program in MATLAB. The ten control factors of the. free-fall electrostatic separation process were employed as input variables of this program: particle charge and dimension, trajectory start point coordinates, feed input angle and initial velocity, electrode length and inclination, interelectrode spacing, and applied high voltage. A custom-designed laboratory free-fall electrostatic separator (length of the electrodes: 1000 mm; standard interelectrode spacing: 300, mm; and nominal high voltage: 90 kV), provided with a fluidized-bed tribocharger, was employed for validating the conclusions of the numerical modeling. The geometrical data of this separator were taken into account for computing the initial velocity nu(o) of the particles entering the electric field zone. Another important parameter of the numerical model, which is the granule charge q, was attributed the value measured at the exit of the fluidized-bed tribocharger. The numerical simulations were performed for the two values of the feed input angle: alpha = 4 degrees and alpha = 8 degrees, considering polyethylene therephtalate (PET) and polyvinyl chloride (PVC) particles of different sizes. A good agreement was found between the theoretical predictions and the results of the experiments carried out with two binary mixtures: 50% PET/50% PVC and 10% PET/90% PVC. Both the numerical modeling and the experimental study demonstrated that the feed input angle alpha influences the outcome of the electrostatic separation to a great extent.