Polar–nonpolar collector hybrids boosted nanoemulsion for improving low-rank coal flotation

Low-rank coal (LRC), with abundant oxygen-containing functional groups and strong hydrophilic polarity, is challenging for the effective surface adsorption of traditional nonpolar collectors and high flotation efficiency. In this study, a boosted nanoemulsion with average droplet size of 130.3 nm was prepared for improving the LRC flotation, using the low-energy emulsification method with hybrids of polar (methyl oleate) and nonpolar (diesel) collectors. The difficult-to-float mechanisms of LRC were first investigated through surface morphology and elemental composition. The dynamic stability, macroscopic appearance, and droplets size distribution of nanoemulsions boosted by different collectors were systematically explored by multiple-light and dynamic-light scattering methods, respectively. The results indicated that the nanoemulsion with hybrids of polar–nonpolar collectors had stronger stability and relatively low droplet size, compared to those of nanoemulsion with individual polar collector. In addition, the LRC flotation experiments identified that the hybrids boosted nanoemulsion had better combustible matter recovery rate (89.44%) than those collectors without emulsification process (74.90%). Moreover, the wrap angles and adhesion/desorption forces of particles-bubbles, as well as adhesion behaviors and wetting heat variations of particles-nanoemulsions, were measured to identify the improving mechanisms of nanoemulsions for LRC flotation. The polar groups of emulsifier and collector could synergistically adsorb on the oxygen-containing functional groups of LRC surface to reduce the hydrophilic polarity, thereby accelerating the adsorption of nonpolar groups and improving the flotation efficiency. This research can provide new insight into the role of nanoemulsion with different collectors in LRC flotation.