Abstract
There is a growing demand for minerals, particularly due to the increased commitment to green technologies. The global grade is constantly decreasing, and the few “newly” discovered deposits are not sufficient to counteract this trend, leading to the need of exploiting low-grade ores with progressively complex mineralogy. To liberate valuable minerals in low-grade and finely disseminated ores, fine grinding is often required. This produces fine particles which, in froth flotation, are slowly and poorly recovered due to their low probability of collision with conventional size air bubbles. In froth flotation circuits, small bubbles can be generated by various methods such as hydrodynamic cavitation and can be classified by size as fine bubbles (≤0.01 to 100 μm), microbubbles (1 to 100 μm), and ultrafine bubbles/nanobubbles (≤0.01 to 1 μm). It is known that tiny suspending solid particles or a substrate cavity in a liquid may promote bubble nucleation which could benefit the floatability of fine particles. In fact, surface nucleated fine bubbles have been referred to as promoters for fine particle aggregation, increasing their apparent size and, consequently, mitigating the low efficiency of collision. Other benefits of fine mineral particles frosted with fine bubbles include the reduction of collector consumption and a faster particle-bubble attachment. Several size measurement methods have been recently applied, including focused beam reflectance measurement (FBRM), atomic force microscopy (AFM), laser diffraction, nanoparticle track analysis (NTA), and dynamic light scattering (DLS), often combined with zeta potential measurement features. However, some of these techniques are limited regarding their accuracy and precision for measurements below 1 μm. Those which are considered to be high-resolution techniques are not adapted for in-situ measurements either. Although nanobubble stability is reported to be high, lasting hours after their generation, for systems that include components other than water and gas, sampling might disturb the behavior of the system. Moreover, flotation processes may be considered “contaminated” due to the presence of solid particles and flotation reagents that might be used as nucleation sites. Therefore, in a complex system, are we measuring and observing nanobubbles? Would those nano entities be organic surfactant assemblages or colloidal particles, for example? Is my data a real measurement or noise? Although we cannot neglect the existence of nanobubbles and their positive impacts on the recovery of fine mineral particles, we have to rely on the available (but limited) technology. This is posing a difficult challenge in scaling-up measurements and analyzing pilot or industrial scale systems to validate the advantages of the introduction of fine and ultrafine bubbles in flotation processes. The differences between different measurement techniques will be presented as well as future research perspectives regarding the role of fine bubbles in froth flotation.