Fly ash from a coal fired plants is an accepted additives for cement mortar or concrete which increases compressive strength and sulfate resistance, reduces cost as long as levels of unburned carbon below 5% are maintained. Tests to investigate technical feasibility of the triboelectrostatic process for separating unburned carbon from coal fly ash were conducted. Using a batch type separator composed of two parallel plates and an ejector-tribocharger, the charge density and the separation efficiency were evaluated under various operating conditions. It was found that the relative humidity of the surrounding air was the most influencing factor on the charge density as well as the separation efficiency. The maximum charge density was obtained at the air flow rate of 286 L/min, and the air temperature of 74degreesC when operated at solids loadings less than 290 g/m(3) and at lower than 20% relative humidity. At the optimum conditions, a fly ash of 11% LOI could be processed into a 4.5% LOI product with a recovery over 80%.