Historical perspectiveThe roles of particles in multiphase processes: Particles on bubble surfaces
Graphical abstract
Introduction
Foams and thin liquid films are interesting experimental systems for fundamental studies on surface forces [1]. What makes the results of such studies even more meaningful is the broad range of industrial applications in which foams are found such as the food industry, personal care products, and flotation [2]. For the sake of generality, it should be noted that research in emulsion systems can be meaningful to foam systems although foams may be more sensitive due to increased perturbations of the bubbles during the segregation of the bubbles on the surface of the continuous liquid phase, when compared to emulsions (i.e., lower density difference between the oil and water phases) [3].
Among the various systems in which foams are present, froth flotation is probably one of the most challenging systems. Adamson and Gast [4] noted that “[froth flotation] is particular in that empirical practice has been in the lead, with theory struggling to explain”. As such, froth flotation offers an excellent working example for many phenomena, including bubble–particle interactions. In flotation, the addition of frothers or electrolytes in a sufficient concentration can be used to delay or prevent the coalescence of bubbles [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Like surfactants, particles may also be used to prevent the coalescence of bubbles [25], although this is not their specific role in flotation. Nevertheless, it is important to consider the impact of both the target mineral particles and the bubble stabilising additives on bubble stability.
Froth flotation is used for the selective separation of valuable minerals from gangue (i.e., valueless minerals) [26]. In this process, the mineral first undergoes comminution to liberate the valuable minerals. The particles are placed in an aqueous medium to form a slurry in which a collector is added. The collector is a surfactant, which tends to adsorb at the solid–water interface and it is used to produce a hydrophobic layer on the surface of the target mineral particles. Air is introduced into the vessel in the form of fine bubbles which ultimately contact the hydrophobic particles. During the collision of a particle with an air bubble, a thin film is formed. Attachment of a particle occurs when the thin liquid film drains to a critical thickness and ruptures, which is followed by the propagation of a three-phase line of contact (TPLC). The lower density of the bubble–particle aggregate, imparted by the air bubble, causes the segregation of the particle-loaded bubbles from the liquid (pulp) zone to the surface of the flotation vessel, leading to formation of a froth phase. Illustrations of the froth structure and phenomena are shown in Fig. 1. The liquid drainage from the froth entrains some of the unattached particles back to the pulp phase. The decrease in water content in the froth zone causes the bubbles to deform and produce liquid lamellae. The stability of the bubbles to film rupture (i.e., coalescence) and coarsening (which is negligible in flotation), is determined by the properties of the interface, which are controlled by the addition of inorganic electrolytes, frothers, particles or a combination thereof.
In a process like froth flotation, the collapse of the froth phase is desirable once the froth has been collected. The froth is thus transient and a partial collapse, in the form of bubble coalescence, is observed in the flotation vessel. As suggested in Fig. 1, a number of processes occur in the froth zone. These processes are largely regulated by the properties of the froth zone. The latter are incidentally regulated by the processes taking place in the froth zone. The term froth is used to describe the bubble-rich mixture that forms on top of the liquid in a flotation cell. In the wider literature, such mixtures are referred to as foams. Here, foam is sometimes used, although it is understood that froth and foam are interchangeable. As well, it is worth noting that in foam applications, the contact angle of the particles is kept well below 90° since hydrophobic particles (θ > 90°) would act as froth breakers (i.e., anti-foaming agent). Thus, although the contact angle of foam-stabilising particles is found to be below 90°, such particles are considered ‘hydrophobic’ by industrial standards. The terminology used throughout this manuscript reflects this standard.
It has been noted that bubbles are more stable in the froth phase whilst loaded with particles [28]. This review explores the various roles of particles on bubble stability. The formation of a steric barrier preventing the contact of the bubble surfaces is probably one of the most effective ways to stabilise bubbles. However particles are found to partake in the stability of bubbles through other interfacial effects, which are worth reviewing.
From an energy consideration, the mixing of two immiscible phases to form a dispersion leads to an unstable system. Coalescence of the bubbles is one means to reduce the surface energy. The coalescence process releases energy, which is dissipated in the form of the oscillation of the bubble surface. Particles, which are present at the interface, are found to affect the surface properties of the bubbles under dynamic conditions. The surface properties of the bubble have a considerable impact on the detachment of particles. Although the processes of film thinning and rupture have been extensively investigated, the post-coalescence phenomena have not received the same attention. Thus this review also expands on the topic of coalescence dynamics, which includes the detachment of particles.
Section snippets
Stabilisation of bubbles by particles at the interface
Aqueous foams were traditionally stabilised by molecular species dissolved in the liquid phase [29]. However, there is a growing body of works on particle-stabilised foams in recent years [30], [31], [32], [33], [34]. Surfactants with the dual function of foaming agent and particle hydrophobising agent may also be added. Foaming of particulate systems, with or without surfactant, is generally improved when the conditions promote the coagulation of the particles [25]. This has been shown to be
The effect of particles on dynamic surface properties
The coalescence of bubbles is driven by a decrease in total energy produced by a reduction in surface area and a decrease in curvature of the resulting bubble (i.e., increase in the radius of curvature) [112], [113], [114]. The excess energy is released to the surroundings. Probably the most interesting measurable features, in connection to the release of energy associated with the coalescence of bubble events, are the generation of sound and the observation of an oscillation of the interface.
Observation from flotation experiments
One of the roles of the froth phase is to control the quality of the concentrate. As such, the froth phase is used as an enrichment zone to remove entrained particles and, if necessary, attached lower grade particles. There are numerous accounts of the effect of operational variables on the behaviour of the froth phase, however, the exact processes/mechanisms taking place within the froth phase are for the most part speculative. Nevertheless, extensive research efforts are devoted to
Conclusions
This review examines the various effects of particles at air–liquid interfaces. The role of particles in stabilising bubble surfaces is quite well characterized. Particles attached to bubble surfaces act as a steric barrier although their effectiveness is dependent upon many factors such as concentration, particle hydrophobicity and size among others. Interestingly, bubbles can be (transiently) stabilised when apparently no particles are within the thin liquid film (i.e., contact region). It is
Acknowledgements
The Australian Research Council is acknowledged for financial support through DP120102305. José Hamilton Tavares and Ludmila de Oliveira e Souza (UNSW) are thanked for their contribution to the original experimental work presented in this manuscript (Fig. 24, Fig. 25).
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2022, Advances in Colloid and Interface ScienceCitation Excerpt :In froth flotation, particle-armoured stability determines the possibility of particles from bubbles in bubble coalescence and fluid turbulence, consequently impacting the recovery of valuable minerals. The studies on particle-armoured bubbles stability are focused on the following aspects: effect of particles on bubble coalescence, detachment of particles from forced vibrating bubbles, detachment of particles when bubbles coalescence, TPCL on the particles [88]. The role of particles in the stability of foams has been reviewed by Hunter et al. [89].