Froth flotation is a widely used method in various industries such as mineral processing, food processing, waste water treatment, paper deinking, ion separation, and oil sand processing [[39], [40], [41], [42]]. In the mineral industry, froth flotation is commonly used to separate particles within the size range of 20 to 100 μm for base metal ore [17]. The flotation efficiency decreases outside this range. This is attributed to several factors affecting coarse and fine particles flotation such as poor mineral liberation for coarser particles and the low momentum of particle for finer particles [17,[43], [44], [45]]. As high-quality ores continue to deplete, low-grade ores have emerged as new sources of minerals. These deposits often consist of complex and finely disseminated minerals, requiring fine grinding, typically below 20 μm, and sometimes ultrafine grinding below 10 μm, to achieve sufficient liberation for froth flotation separation [17].

Separation of fine and ultrafine particles through froth flotation is challenging and often results in significant losses [46,47]. For example, Jameson et al. demonstrated that the recovery for the ultrafine particle is around 30% in the Jameson cell, which is still considered a better option for fine particle flotation than a mechanical cell [17,48]. As illustrated in Fig. 1(a), the conventional flotation bubbles (CFBs) are characterized with rapid rising bubbles with sizes between 600 and 2500 μm and fast liquid flow around the bubbles. This leads to low collision efficiency between fine particles and large bubbles due to high drag forces acting on low inertia particles, resulting in the fine particles following the flow around the bubbles rather than colliding with them [47,49,50]. In addition, the attachment probability is relatively low and detachment probability is relatively high for the large bubbles relative to interaction with small bubbles [47,51]. That is to say, small bubbles can achieve a higher collision rate through relatively low liquid flow around the bubble due to its low rising velocity, which can be overcome by low momentum of fine particles, as illustrated in Fig. 1(b). In addition, once attached, the particles are more likely to remain attached.

A microbubble is defined as a bubble with a mean size under 100 μm in froth flotation with the benefits of more effective bubble-particle collision [52,53]. However, microbubbles are associated with a number of disadvantages such as having low lifting force and velocity from the dominant effects of drag force over buoyancy, resulting in high residence time, water recovery, and particle entrainment [49,54,55]. Despite these disadvantages microbubbles remain a viable option for the recovery of fine particles.

Microbubbles cannot be formed in an energy efficient way by conventional methods such as mechanical rotor-stator methods which generate shear force onto the injected air. As a result, this paper explores alternative mechanisms for industrial scale microbubble generation. Similar to particle processing, where a specific size range is achieved through particle breakage and crystallization using top-down and bottom-up methods, microbubble generation can also be achieved through bubble breakage and nucleation [56]. Bubble generation methods can be categorized based on the underlying mechanisms, such as mechanical stirring and porous media, which employ bubble breakage, and water electrolysis and dissolved air, which use bubble nucleation for bubble formation [[57], [58], [59], [60], [61], [62]]. However, hydrodynamic cavitation and acoustic method utilize both mechanisms, so they cannot be categorized in a simple way [37,63,64]. As bubble size decreases, nucleation may become more energetically favorable, similar to grinding methods [56]. However, the exact boundary for the energy efficiency is not clear and depends on the bubble generation method.

The characteristics of bubbles generated from various methods are summarized in Fig. 2. The bubble diameter and polydispersity index (PI) range of various bubble generation methods. PI is an indicator that shows whether the bubble size is uniform. Reproduced from ref. [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. The polydispersity index (PI) is calculated using the span equation:PI=d90−d102∗d50which takes into account the sizes at 10, 50, and 90 vol% of the cumulative particle size distribution (d10, d50, and d90). Hydrodynamic cavitation produces bubbles with a wide range of sizes, ranging from nanometers to micrometers, but its polydispersity index is relatively high compared to other methods [1]. On the other hand, porous media generates relatively monodisperse bubbles than other industrialized methods such as hydrodynamic cavitation and impeller methods, but it produces larger bubbles than most of the other techniques [2]. Dissolved gas and electrolysis methods generate medium-sized bubbles, with the PI for electrolysis showing the lowest value among the industrialized methods [5,7]. Meanwhile, the microfluidic method produces monodisperse bubbles, with the lowest PI value observed among all the methods discussed [8]. Overall, there is no bubble generation method that covers a wide range of bubble size in a controlled way by generating monodisperse bubbles at high production rate to be applied to industrial applications.

The scale-up factor for bubble generation is identified as a critical consideration. For industrial-scale flotation cells, both energy efficiency and mass production capabilities are important factors to consider. Bubble generation methods are categorized into three groups based on their scalability: scalable, semi-scalable, and potentially scalable. The degree of scalability (scalable vs semi-scalable) is defined based on whether the technique is commercialized in industrial mineral froth flotation or other large-scale processing industries such as wastewater treatment [11,65,66]. Other methods which are not used industrially but can be potentially used in froth flotation are categorized as potentially scalable.

This review covers both conventional and prospective microbubble generation methods that are and can potentially be applied in mineral flotation. These methods are categorized by scalability: scalable of hydrodynamic cavitation and porous media, semi-scalable of dissolved air and water electrolysis, and potentially scalable of microfluidic and acoustic methods [67]. In each section, bubble generation mechanisms are briefly introduced, followed by size control, and its use in other industries before discussing its application in froth flotation or potential scale-up methods. The review is expected to contribute for the better control of bubble generation to enhance and optimize bubble utilizing scaled-up processes based on specific industrial needs.