Dispersion behavior and attachment of high internal phase water-in-oil emulsion droplets during fine coal flotation

Abstract Flotation recovery of fine oxidized coal is frequently inefficient when using conventional oily collectors. A large amount of fine coal is discharged as tailings, leading to resource waste and environmental pollution. To solve the problem of high oil consumption and low efficiency associated with traditional flotation, research efforts have been focused on the use of oil-in-water emulsion to reduce oil consumption. Few studies concerning water-in-oil emulsion, especially the high internal phase water-in-oil (HIP W/O) emulsion, have been conducted. Our microflotation results showed a significant oil dosage reduction when a HIP W/O emulsion was used as a collector for coal flotation. The state of the HIP W/O emulsion dispersion in water and the emulsion droplets attachment at the coal surface are vital issues associated with achieving efficient flotation. In this study, optical microscopy and high speed video microscopy were employed to study the dispersion behavior and microstructure of the HIP W/O emulsion in water. This study found that the HIP emulsion tends to be dispersed as a W1/O/W2 double emulsion during mixing in water because of the migration of the surplus Span 80 in the oil phase to the O-W2 interface. The presence of NaCl in W1 is also essential for a stable W1/O/W2 double emulsion, which is formed when equilibrium of the Laplace and osmotic pressures between the W1 phase and W2 phase is obtained. In addition, the W1/O/W2 emulsion droplets are negatively charged, and the emulsion aggregation appears to be inhibited by electrostatic repulsion. More importantly, the Span 80 emulsifier at the O-W2 interface seems to facilitate hydrogen bond formation between the hydrophilic moieties of Span 80 and the oxygen containing functional groups at the coal surface, which is verified by the results from molecular dynamics simulation (MDS). The hydrogen bond formation plays a leading role in the attachment and/or spreading of the emulsion droplets at the oxidized coal surface. The HIP W/O emulsion could be a potential economical and efficient replacement for the traditional oily collectors as a promising collector for the flotation of fine oxidized coal.