Transition from spray to roping in hydrocyclones

Abstract

Although roping in hydrocyclones is a problem that has been studied by many researchers, we do not yet have a theory that relates all the variables involved. Several experimental works with different approaches such as mechanical energy balance, work on hydrocyclones with water only and hydrocyclone air core measurements with different instruments in the laboratory and plant have attempted to explain the roping phenomenon. They have addressed design variables such as apex to vortex diameters and different cone angles as well as operating variables such as inlet solid concentration, particle size, pressure and overflow and underflow flow rates and concentrations in order to understand their effect on the roping condition. In this paper we intend to verify some of the conclusions of these studies and establish inlet pressure and particle diameter as the variables that, in combination, lead from spray to roping. We present a computational model using Computational Fluid Dynamics (CFD) to study a 75-mm laboratory hydrocyclone operating at a variable flow rate and with three different particle diameters. The Reynolds Stress Model and Eulerian multiphase model were used to model the turbulence and interaction of phases, respectively. The solid particles are described with the kinetic theory of granular flows (KTGF). Physical coherence and accuracy were compared with experimental data, where errors are within the expected range for an engineering prediction. The results indicate that transition from spray to roping generates an increase in the inlet flow pressure and/or particle diameter at a constant solid concentration in the feed flow. For 20-µm-diameter particles an increase results in a decrease in the discharge angle, although it always has a spray shape, for 34-µm diameters increasing the inlet pressure generates a semi-rope discharge and for 70-µm-diameter particles a small increase in the inlet pressure generates a roping condition. Transition to roping is characterized by a decrease in the air core diameter and discharge angle due to slow rotational velocity when particle diameter increases or higher accumulation of the solid fraction in the apex when inlet pressure increases. The passage from spray to roping occurs with a change in the frequency spectrum of the pressure oscillations in the walls of the hydrocyclone, with high amplitudes at low frequencies for spray discharge and noisy signals in all spectrums and damped low frequencies for roping.