Physically emulsifying oily collectors to produce optimal droplet sizes for flotation

•Oil droplets ranging from 12.2 to 0.3 μm were produced by physical emulsification.•The best flotation performance was achieved at an optimal oil droplet size.•Flotation deteriorated at a droplet size smaller than the optimal one.•A maximum oil adsorption rate was achieved at the optimal droplet size.•A smaller optimal droplet size was required for more hydrophilic coal particles. Chemical emulsifiers have been used in laboratories to improve the efficiency of oily collectors by reducing their droplet sizes, but their application in flotation plants is limited due to their strong frothing abilities. In this study, recently developed physical emulsification techniques were used to emulsify diesel for coal flotation and diesel emulsions with a droplet size ranging from 12.2 to 0.3 μm were successfully produced. It was interesting to find an optimal droplet size at which the flotation performance was maximized and beyond which the flotation selectivity deteriorated. A smaller optimal droplet size was identified for the coal sample with a higher degree of surface oxidation. Theoretical modelling showed that the reduction of oil droplet size decreased the collision rate of oil with coal particles, but increased the oil adhesion efficiency on particle surfaces, the opposite effects leading to an optimal droplet size to provide a maximum oil adsorption rate. The decrease of surface hydrophobicity significantly reduced the oil adhesion efficiency and the adsorption rate, and therefore a smaller optimal droplet size was required for coal particles with lower surface hydrophobicity. The results from this study can guide the flotation plants to identify optimal oil droplet sizes for their coal samples and a suitable emulsification technology to produce the optimal oil droplet sizes to maximise the flotation performance.