The high efficiency of a froth flotation process is limited to a narrow particle size range. Nanobubbles integrated into a specially designed flotation column expanded the particle size range for effective froth flotation as a result of increased probabilities of particle-bubble collision and attachment and reduced probability of detachment. Results show that combustible recovery of a −150 μm coal sample increased by 5%–50% in the presence of nanobubbles. Nanobubble also increased the kinetic flotation rate constant by more than 40%. Other major advantages of the nanobubble flotation process include lower collector dosage since nanobubbles are produced selectively on hydrophobic particle surface.

Coal cleaning is often conducted using wet physical separation processes such as heavy medium vessels or spirals at coal preparation plants to remove impurities such as ash, sulfur, and mercury. However, the resultant clean coal product still contains a significant amount of impurity due to the fact that impurities are not well liberated from coal particles ranging from several millimeters to inches in size at which wet cleaning processes take place. A cleaner coal product can be obtained if a dry process is avaialble to further clean pulverized and thus better liberated fine coal at the power plant prior to its combustion. In this study, a novel rotary triboelectrostatic separator (RTS) was investigated for its application to dry cleaning of fine coal samples acquired from the power plants in the state of Illinois. The proprietary RTS is characterized by an innovative high-efficiency rotary charger, charger electrification, laminar air flow, etc. Compared to existing triboelectrostatic separators, the RTS offers significant advantages in particle charging efficiency, solids throughput, separation efficiency, applicable particle size range, etc. Important process parameters such as charger rotation speed, injection and co-flow rate, and feed rate were investigated for their effects on separation performance.

Froth stability plays a crucial role in a froth flotation. This study was conducted to investigate the impact of nanobubbles on froth stability in flotation column. Hydrophobized particles of different degrees of hydrophobicity were employed in the flotation experiments to investigate their interactions with nanobubbles. The results indicated that nanobubbles and hydrophobic particles of a contact angle up to about 70 degrees significantly enhanced the froth stability. More importantly, it was demonstrated, for the first time, that froth stability was well maintained by nanobubbles in the presence of strongly hydrophobic particles with a contact angle of more than 85 degree.