Comparison of the effect of particle size on the flotation kinetics of a low-rank coal using air bubbles and oily bubbles

separation method that is widely used in coal beneficiation for the separation of fine coal particles from associated minerals in water slurries. Since the cumulative recovery of a component in the concentrate is proportional to flotation time, the flotation process can be considered as a time-rate recovery process (Emad, Mohammad, and Bahram, 2010). Therefore, the general mathematical flotation models that incorporate both a recovery and a rate function can describe flotation timerecovery profiles. The kinetics of coal flotation, as a measure of coal recovery as a function of time, have been studied in detail by many researchers. Different kinetic models have been proposed and published in the literature. The order and the rate constant are strongly dependent on the flotation conditions, such as the particle size and size distribution, the bubble concentration, the reagent concentrations, and the operating parameters (Zhang et al., 2013). The particle size has been reported to affect the flotation kinetics to a great extent (Yoon, Luttrell, and Asmatulu, 2002; Ntengwe and Witika, 2011). Numerous researchers have studied the aspects of flotation kinetics while paying special attention to particle size (Meftuni and Ibrahim, 1997; Rubinstein and Samygin, 1998; Graeme, 2012). However, the effect of particle size on the flotation kinetics of a low-rank coal has been relatively little discussed. It is difficult to achieve a high yield or combustible matter recovery by flotation of fine low-rank coal, due to the poor hydrophobicity. The presence of polar hydroxyl, carbonyl, and phenol groups and some peroxide-type oxygenated moieties on the surface of low-rank coal decreases the coal hydrophobicity (Xia, Yang, and Liang, 2013). Researchers have investigated many ways to improve the floatability of low-rank coal by using pretreatment prior to flotation and surfactant additions during flotation. Recently, Xia and Yang (2013) designed an oily bubble flotation process using oily bubbles instead of the conventional air bubbles to increase the flotation recovery of low-rank/oxidized coal. The oily bubble flotation (OBF) process yielded a higher combustible matter recovery and lower ash content than the conventional air bubble flotation (ABF) process. An oily bubble is an air bubble covered with a thin layer of kerosene containing collectors (Liu et al., 2002). The OBF process has been successfully applied in bitumen flotation. A superior performance was noted when using OBF, compared to the use of conventional ABF, when applied to Comparison of the effect of particle size on the flotation kinetics of a low-rank coal using air bubbles and oily bubbles