A 20 mm-diameter electrical hydrocyclone was developed and tested. The presence of the underflow results in a smaller d(50) than the absence of the underflow. Additionally, the higher the feed flow rate, the smaller d(50), Effect of central rod diameter and initial pH of suspension on the separation cut size was examined with and without underflows. The d(50) decreases with the increase of central rod diameter and initial pH values in both with and without the underflow. The zeta potential of particles increases proportionally with the initial pH of suspension. The electrical potential exhibits a stronger effect when the suspension has a higher pH. The d(50) becomes smallest under largest central rod diameter, highest pH and longest slurry collecting box condition operates with the positive electrical potential at side wall in the presence of the underflow. Based on the experimental data, general semi-empirical correlations were proposed and the effect of each parameter on d(50) was examined. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.