Nanobubble column flotation of fine coal particles and associated fundamentals A. Sobhy a, b , D. Tao a, c, ⁎ a Department of Mining Engineering, University of Kentucky, Lexington, KY 40506, USA b Central Metallurgical Research and Development Institute, Helwan, Cairo 11421, Egypt c College of Chemical Engineering, China University of Mining and Technology, Xuzhou, China article info abstract Article history: Froth flotation is a widely used, cost effective particle separation process. However, its high performance Received 29 January 2013 is limited to a narrow particle size range between approximately 50 to 600 μm for coal and 10 to 100 μm Received in revised form 5 April 2013 for minerals. Outside this range, the efficiency of froth flotation decreases significantly, especially for Accepted 20 April 2013 difficult-to-float particles of weak hydrophobicity (e.g., oxidized coal). Available online 14 May 2013 This study was aimed at enhancing recovery of an Illinois fine coal sample using a specially designed flotation column featuring a hydrodynamic cavitation nanobubble generator. Nanobubbles that are mostly smaller than Keywords: Cavitation 1 μm can be formed selectively on hydrophobic coal particles from dissolved air in coal slurry. Results indicate Coal that the combustible recovery of a -150 μm coal increased by 5–50% in the presence of nanobubbles, depending Froth flotation on process operating conditions. Nanobubbles also significantly improved process separation efficiency. Nanobubble Other major advantages of the nanobubble flotation process include lower frother dosage and air consumption since nanobubbles are produced from air naturally dissolved in water, thereby resulting in considerably lower operating costs. © 2013 Elsevier B.V. All rights reserved. 1. Introduction A 77% of the total global coal production is used by China, USA, India, Russia and Japan (U.S. Energy Information Administration, 2009). United States is one of the largest coal producing countries with an annual production of more than one billion short tons of clean coal. There are two main types of coal, low rank coal with 47% of world reserve and high rank coal with 53% of world reserve. Coal ranking is determined by degree of transformation of the original plant materials to carbon. Therefore, low rank coal which can be subdivided into lignite and subbituminous is low in carbon and high in hydrogen and oxygen contents. On the other hand, high rank coal which can be subdivided into bituminous and anthracite is high in carbon and therefore energy value but low in hydrogen and oxygen contents. These different types of coal have different uses. For example, lignite is mainly used in power generation. Bituminous and subbitumi- nous are used in power generation, cement manufacture and other industrial applications. Anthracite is mainly used as smokeless fuel. A 40% of worldwide electricity is generated from coal and 70% of steel produced today uses coal (World Coal Association, 2012). Froth flotation is commonly used in the coal industry to clean -100 mesh or -150 μm coal particles from gangue minerals. This process separates solid particles based on their differences in physical and surface chemistry properties. It is most efficient and cost effective for particles within a narrow size range, nominally from 50 μm to 600 μm for coal and from 10 μm to 100 μm for minerals (Feng and Aldrich, 1999; King, 1982; Trahar and Warren, 1976). The lower and upper particle size limits are due to the low probability of collision and the high probability of detachment, respectively (Ralston and Dukhin, 1999; Tao, 2004; Yoon, 2000). The previous studies have shown that flotation recovery of coal particles outside the optimum size range and/or of poor floatability can be enhanced by use of nanobubbles (Tao et al., 2008). Nanobubbles can be produced using ultrasonic or hydrodynamic cavitation principle (Farmer et al., 2000; Johnson and Cooke, 1981; Zhou et al., 1997). Nanobubbles preferentially nucleate at the surface of hydrophobic particles (Zhou et al., 1997) because work of adhesion between a solid particle and water is always smaller than work of cohesion of water. Furthermore, work of adhesion decreases with increasing solid surface hydrophobicity measured by the contact angle. Nanobubbles can nucleate on ultrafine particles without the need for collision, which is often the rate-determining step in froth flotation for ultrafine particles (Weber and Paddock, 1983; Yoon and Luttrell, 1989). Nanobubbles generated on a particle surface also serve as a secondary collector, improving the probability of adhesion and minimizing the need for the hydrophobizing chemical reagents (Luttrell and Yoon, 1992; Zhou et al., 1997). In addition, particles are less likely to detach from tiny bubbles due to their lower ascending velocity and centrifugal force associated with the detachment step, reducing the probability of detachment. The objective of this study was to develop an innovative cavitation nanobubble flotation process based on understanding of nanobubble International Journal of Mineral Processing 124 (2013) 109–116 ⁎ Corresponding author at: Department of Mining Engineering, University of Kentucky, Lexington, KY 40506, USA. Tel.: +1 859 257 2953; fax: +1 859 323 1962. E-mail address: daniel.tao@uky.edu (D. Tao). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.04.016 Contents lists available at ScienceDirect International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro