The mineral processing industry relies heavily on froth flotation to beneficiate complex minerals. The performance of the process depends on a multitude of chemical reagents affecting the solid mineral particles as well as the air bubbles used to collect the valuable mineral particles. In flotation, bubbles and froths are transiently stable by the use of a frothing agent or the presence of inorganic electrolytes in the process water. This review presents the primary stages characterizing bubble coalescence. The effect of flotation reagents and inorganic electrolytes on the stability of bubbles and the mechanisms which delay the coalescence of bubbles are discussed. Recently it had been proposed that solid particles external to the flotation system may be intentionally added to stabilize the froth phase, which has attracted attention from the flotation community. This gave rise to additional studies on the topic. The final section of the paper was therefore designed to capture the progress made on this particular subject. The use of solid flotation aids offers a promising future to increase the recovery of valuable particles as these external particles may be customized. The advantage of solid particles over soluble reagents is that they may be recovered and re-used making them environmentally and economically attractive.

The depletion of high-grade and coarse-grain ores has led to an increasing demand for the development of efficient separation technologies for low-grade and fine-grain ores. However, conventional froth flotation techniques are not adequate to efficiently recover fine and ultrafine particles (typically <10-15 μm) due to the low collision probability between these particles and the relatively large bubbles used in the process. The introduction of microbubbles has shown promise in enhancing particle recovery, making it a subject of significant interest. Thus, this review focuses on microbubble generation methods that have the potential to be scaled up for industrial applications, with a specific emphasis on their suitability for froth flotation. The methods are categorized based on their scalability: high-hydrodynamic cavitation, porous media/medium-dissolved air, electrolysis/low-microfluidics, and acoustic methods. The bubble generation mechanisms, characteristics, advantages and limitations of each method and its applications in froth flotation are discussed to provide suggestions for improvement. There is still no appropriate technology that can optimize bubble size distribution, production rate and cost together for industrial froth flotation application. Therefore, novel approaches of combining multiple methods are also explored to achieve the potential synergic effects. By addressing the limitations of current microbubble generation methods and proposing potential enhancements, this review aims to contribute to the development of efficient and cost-effective microbubble generation technologies for fine and ultrafine particles in the froth flotation industry.

The stability of bubbles laden with particles of different densities has been investigated. Capillary-held bubbles were produced and coated with particles across the density range 1.2 – 3.6 gcm-3. The materials used were poly(methyl methacrylate) (PMMA), glass, and anatase. The interaction of the bubbles, once brought into contact, was monitored using high-speed video recording. Visual inspection indicated that denser particles were more easily displaced during the contact of the bubbles, and therefore that the PMMA particles provided a particle barrier more resistant to coalescence. The coalescence events yielded information on the surface properties of the bubble and the detachment of particles. Attached particles commonly dampen the oscillation of the coalesced bubbles through viscous drag and change in surface properties (e.g. area exclusion principle). The dampening of the oscillation generally leads to a reduced mass of particle detaching from the bubble surface. It was found that the different materials investigated did not offer clear evidence on the effect of particle detachment on the bubble surface properties in the present systems. On the other hand, the detachment of different particle materials seemed to be consistent with one another when comparing the attachment and detachment forces exerted on the particles based on their density, size, and hydrophobicity. It was concluded that particles of lower density are more effective at stabilising interfaces and thus particle density is an important parameter in the selection of materials for the handling of dispersions.

Coal preparation plants are increasingly turning to alternative sources of water to reduce their consumption of fresh water. These alternative sources can vary significantly in ionic content. This review discusses the effect of dissolved inorganic electrolytes on coal flotation. It is argued that increasing process monitoring will enable a better understanding of the effects of salts on the flotation process. A pragmatic approach in monitoring is essential to differentiate significant factors from insignificant ones controlling the performance of coal flotation units in salt water. The effects of inorganic electrolytes on the air–water interface and the solid–liquid interface are reviewed to assist in the interpretation of industrial results.