Performance of used lubricating oil as flotation collector for the recovery of clean low-rank coal

Abstract

In this study, the possibility of used lubricating oil (ULO) as a collector to float low-rank coal was explored; a traditional coal collector, diesel oil (DO), was used for possible comparisons. Experimental measurements including Fourier-transform infrared spectroscopy, gas chromatography-mass spectrometry, and zeta potential were used to determine the chemical constituents of the collectors. Flotation tests were conducted to investigate the flotability, flotation rate, and flotation selectivity. The coal-water, coal-collector, and coal-bubble interactions were emphatically analyzed using X-ray photoelectron spectroscopy, water contact angle, coal-water-collector contact angle, and the designed particle-bubble wrap angle experiment. The results indicate that ULO contains more oxygenated functional groups, aromatic groups, and long-chain hydrocarbons than DO, leading to its strong interaction with low-rank coal. A significant increase in zeta potential was also observed in the ULO environment, which could decrease the repulsion between particles. Flotation results confirm that ULO can significantly enhance the clean coal yield and flotation rate, and present high flotation selectivity. Furthermore, the mechanism of ULO as a flotation collector was examined. The adsorption of ULO increased the carbon content and decreased the oxygen content on the low-rank coal surface. This is beneficial as it helps to decrease coal-water interaction. Meanwhile, ULO showed stronger interaction with the coal, and coal particles pretreated with ULO more easily adhere to the bubble surface. The application of ULO can promote low-rank coal flotation, and has an important role in saving coal resources and alleviating the impact of used oil on the environment.