Pneumatic transport, triboelectrostatic separation is a well-known technique for separating unburned carbon from coal combustion fly ashes to create ash products useful as pozzolanic additives in cement. Although considerable fundamental insight has been gained about particle interactions and charging during carbon-ash beneficiation, little work has been done to examine how particle concentration in the ash transport charging line, and how tribocharged particles colliding with electrodes and causing particle charge reversal, relate to carbon-ash separation performance. We show ash-carbon separation performance diminishes with increasing particle concentrations, especially for carbon-rich fly ashes, and exhibits a maximum related to the electric field strength. On the basis of this investigation, an improved separator geometry was established that helps to overcome the limits imposed by charge reversal. (C) 2008 Elsevier B.V. All rights reserved.