New insight into the role of the emulsified diesel droplet size in low rank coal flotation

[Display omitted] •The optimal low rank coal flotation result is obtained with the emulsified diesel droplet size 7.68 μm.•The adsorption and spreading of ED on coal particle surfaces was evaluated by total organic carbon analyzer (TOC) and Fourier-transform infrared spectroscopy (FTIR).•The interaction between the oil droplets and coal particles was conducted using adhesion force measurement system.•The new function mechanism of the emulsified diesel droplet size on low-rank coal flotation was thorough proposed. Low-rank coal (LRC) is difficult to float using oily collectors due to poor floatability resulted from its abundant oxygen-containing groups. In recent years, numbers of emulsified collectors have been used to enhance LRC flotation. In this study, diesel-Span80 mixed collectors were added to high shear disperser producing emulsified diesel (ED) with a droplet size ranging from 12.51 to 4.58 µm to discuss the effects of droplets size on LRC flotation. The adsorption and spreading of ED on coal particle surfaces was evaluated by total organic carbon analyzer (TOC) and fourier-transform infrared spectroscopy (FTIR). The interaction between the oil droplets and coal particles was conducted using adhesion force measurement system. The flotation results indicated clean product obtained by using ED as the collector had the higher combustible recovery and lower ash content than conventional diesel (CD). It was interesting to find an optimal droplet size (7.68 μm) at which the ED flotation performance was maximized and beyond which the flotation performance deteriorated. The FTIR, adsorption amount and adhesion force results were all well consistent with the flotation results. The effect of ED droplet size on LRC flotation is controlled by ED dispersibility and concentration of adsorbed surfactant molecules on each ED droplet surface, playing a critical role in determining the optimal droplet size which delivers the largest hydrophobicity, maximum adsorption amount, highest maximum adhesion force and hence the best flotation performance.