Nanobubbles (NBs) are gas-filled cavities in liquid, typically <1 µm in diameter. Unlike conventional bubbles, NBs exhibit extraordinary stability (lasting days or even months) due to their very small size and charged interfaces. They exist as surface NBs (on solid–liquid interfaces) or bulk NBs (spherical in liquid). NBs normally carry a negative zeta potential (−10 to −100 mV), generating electrostatic repulsion that slows gas dissolution. As a result, NBs can remain suspended for extended periods. In practice, NBs diffuse randomly (Brownian motion) and carry reactive gas species (e.g. OH radicals) that
can promote colloidal interactions. NBs also present a dramatically high surface-area-to-volume ratio, enabling vigorous gas–liquid mass transfer and interfacial reactions (e.g. oxidation).
Mechanisms in Mineral Processing
Nanobubbles have been shown to enhance several unit operations in mineral processing, especially flotation and dewatering. Key mechanisms include:
Enhanced Collision and Attachment: NBs act as secondary collectors, bridging fine particles to larger air bubbles. They increase effective bubble surface area and carry gas to particle surfaces. For very fine particles (e.g. < 20 µm) that normally have low collision probability, NBs dramatically increase “apparent collision radius”. For example, AFM studies on graphite showed surface NBs grow the effective contact angle to ~164° (versus 71° hydrophobic flat), raising attachment likelihood. In practical tests (coal, ilmenite, muscovite), adding NBs increased both flotation rate and recovery, as particles readily collided with bubble–NB aggregates.
Fine Particle Agglomeration: NBs can adsorb onto hydrophobic particle surfaces, making particles stick together (“nano-agglomeration”). Two fine particles brought close can trap a NB between them, effectively welding them into a flocculated cluster. This increases apparent particle size, easing flotation. In coal and phosphate systems researchers observed NB-promoted agglomerates sedimenting or floating more readily, improving recovery of slimes.
Hydrophobicity Enhancement (Surfactant Amplification): NBs often carry adsorbed surfactant/collector molecules which they can deliver to particle surfaces. This sometimes reduces the total collector dose needed. One study found NB injection allowed the same ilmenite recovery with significantly less OHA, indicating NBs improved collector utilization. In addition, NB presence by itself can increase particle hydrophobicity.
Reduction of Reagent Use: As noted, NBs can reduce required frother/collector consumption. This occurs both by improving particle–bubble attachment efficiency and by generating reactive oxygen species (e.g. OH radicals) that break down refractory coatings. In practice, operators report lower reagent addition when using NB generators.
Improved Flotation Kinetics: The flotation rate constants often increase in the presence of NBs. The muscovite kinetics study showed faster flotation rates with NB pretreatment, fitting well to standard kinetic models. Faster kinetics arises because more bubbles/nanobubbles per volume and better collision efficiency shorten the induction time of flotation.
Dewatering and Tailings Treatment: NBs also improve dewatering of fine tailings. In bench-scale filtration tests, NB conditioning increased filtration rates by ~20–30% for fine quartz and iron ore slimes.
The mechanism is twofold:
(a) NBs raise particle hydrophobicity, reducing water retention and making a more permeable filter cake (b) NBs reduce capillary forces in the cake, allowing water to drain more freely.
The observed result was up to 20% faster flux for ultrafines and ~30% reduced dewatering time for iron ore tailings.
Tailings Valorization: In addition to flotation, NBs have been applied to tailings processing. Studies showed that using NBs in flotation could upgrade low-grade tailings without regrinding. By effectively floating fine apatite from the coarse iron-bearing waste, NB-assisted flotation recovered ~10–35% more P₂O₅ (phosphate) than conventional bubble flotation. This demonstrates how NBs can unlock value from mine waste streams.
In all cases, the role of NBs is largely physicochemical, they act as mobile nanoscopic collectors. They adsorb to particle surfaces (via hydrophobic or electrostatic attraction) and to carrier bubbles, forming a “two-stage” attachment (particle–NB, then NB–bubble) that dramatically boosts collision and adhesion. By bridging fine particles and diminishing the effect of tailings water films, NBs improve recovery and kinetics and allow reagent savings.
Conclusion
Nanobubbles offer a promising approach for improving the recovery and handling of fine and ultrafine mineral particles. Their ability to enhance particle–bubble interactions, promote fine-particle agglomeration, improve flotation kinetics and increase the filtration rate of fine tailings has been demonstrated across several mineral systems. The technology can also reduce collector and frother requirements under suitable operating conditions. Overall, nanobubbles provide a potential route for improving mineral recovery and tailings dewatering, particularly where conventional flotation and filtration are limited by the behaviour of fine particles.
References
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