Interfacial interaction of emulsion collector in enhancing low-rank coal flotation

The long-time accumulation of low-rank coal can induce many environmental problems and severe waste of carbon resources. Flotation technology based on gas/liquid/solid interfacial interaction remains an efficient way to recovery combustible matter and realize clean utilization of coal. The traditional collector, kerosene, has demonstrated its low flotation efficiency and environmental hazards for low-rank coal. In this work, the kerosene was highly dispersed through emulsification with the biocompatible surfactant alkyl polyglucosides (APG) and optimized based on HLB of emulsifiers and emulsion phase diagrams. The emulsion with the mixture of hydrophilic APG and hydrophobic Span65 as emulsifier exhibits the strongest dispersibility and stability due to the formation of a stable composite molecular adsorption film of the mixed emulsifiers at oil/water interface. Compared with kerosene, the prepared emulsion collectors exhibited much higher flotation efficiency with combustible matter recovery increasing by 31.4% and simultaneously consumption of kerosene decreasing by 57.8% under optimal operation conditions. The hydrophobicity of coal surface is obviously enhanced after treatment by emulsion with the contact angle increasing from 26.6° to 71.7°. High-speed camera observation showed that only 76.59 ms was taken for the impinging emulsion droplet to adhere to low-rank coal surface followed with a faster spreading, indicating stronger interfacial interaction between emulsion collector and low-rank coal surface. The proposed 'bridging action' of emulsifier molecule between oxygen-containing groups at coal surface and kerosene in emulsion could remarkably enhance the differences in surface hydrophilicity between combustible coal and other contents, resulting in high flotation efficiency. These results constitute a substantial step forward in technical development for resource recycling of low-rank coal.