Research Article Triboelectrostatic Separation of Fly Ash with Different Charging Materials A combination of mechanical sieving and triboelectrostatic separation were used to separate fly ash. The results indicate that a simple separation of unburned car- bon from fly ash is achievable at particle sizes of 74 and 44 microns. Subse- quently, triboelectrostatic separations were conducted via a louvered plate separa- tor. The results show that the final carbon content in the products, which can be as low as 1.5 % or as high as 60 % with different mineral components, can be further adjusted with the combination of sieving, louvered plate separator with a tribocharger made of different materials (copper and Teflon), and the location on the louvered plate where the fly ash particles were collected. Keywords: Combustion, Fly ash, Separation techniques Received: September 21, 2006; revised: November 21, 2006; accepted: November 29, 2006 DOI: 10.1002/ceat.200600289 1 Introduction Improved utilization schemes for fly ash can transform it from a waste material, with associated disposal costs, to a valuable product. Utilization of power plant derived fly ash has been impacted by recent shifts to low NO x burners, typically accom- panied by an increase in the unburned carbon concentration in the fly ash. The main application for fly ash continues to be Portland cement. However, increases in carbon content can make the fly ash unsuitable for such applications. The ASTM C618 specification limits loss-on-ignition (LOI) to 6 %, largely due to the fact that higher LOI levels often result in discolora- tion, poor air entrainment, and segregation of mix compo- nents [1]. The ability to efficiently extract high purity carbon or ash is important in the development of cost-effective benefi- ciation technologies. Post-combustion beneficiation can gener- ate valuable unburned carbon and inorganic fly ash products, and these two constituents can be collected and used as com- mercial products. The unburned carbon fraction can be re- cycled back to the burner as fuel or used as a catalyst, sorbent, activated carbon, or catalyst support [2–3]. The purified inor- ganic fraction can be utilized as a cement additive [4]. The constituents of fly ash can vary in size, density, electro- static, physical, and chemical properties, thus making the sepa- ration of the fly ash a very difficult task. However, a systematic combination of separation techniques based on the differences in size, density, electrostatic, and physical properties may help achieve the difficult separation task of extracting valuable products from fly ash. Sieving fly ash capitalizes on the tendency of carbon and minerals to have different particle size distributions within the fly ash . Carbon and mineral particle size distributions vary widely, and typically, there appears to be a clear trend for most of the unburned carbon to be contained in the larger size frac- tion of the fly ash as opposed to the minerals. For instance, fly ashes sieved through a 44 lm screen, exhibit a measurable re- duction in LOI [5]. Triboelectrostatic separation has been applied for the sepa- ration of coal and mineral particles [6–7]. Electrostatic benefi- ciation of fly ash to separate unburned carbon has been inves- tigated widely as an alternative to other post-combustion cleaning technologies [8–11]. During triboelectrification, or- ganic and mineral particles are charged with opposite polarity and separated by the use of an electrostatic separator. When two materials are in contact, electrons move until the energy level of the electrons in each material at the interface is equal- ized. The material with a higher affinity for electrons gains electrons and charges negatively, while the material with the lower affinity loses electrons and charges positively. On contact with a tribocharger, and depending on the material of which the tribocharger is made (i.e., copper), the organic particles (unburned carbon) become positively charged, and the inor- ganic mineral particles become negatively charged. The differ- ential charging of unburned carbon and its mineral impurities achieved in the triboelectrostatic method makes it possible to use a static high voltage separator to direct the unburned car- bon and mineral refuse into separate receivers, as shown in Fig. 1. Organic particles are attracted to the negative plate, and minerals are attracted to the positive plate. In laboratory ex- periments, unburned carbon samples deposited on the elec- © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com Yee Soong 1 Gino A. Irdi 1 Thomas R. McLendon 1 Sheila W. Hedges 1 Robert M. Dilmore 1 Craig Griffith 1 Vyacheslav Romanov 1 Igor Haljasmaa 1 1 U. S. Department of Energy, National Energy Technology Laboratory, Pittsburgh, United States of America. – Correspondence: Y. Soong (soong@netl.doe.gov), U. S. Department of Energy, National Energy Technology Laboratory, P.O. Box 10940, Pittsburgh, PA 15236, United States of America. 214 Chem. Eng. Technol. 2007, 30, No. 2, 214–219