Historical perspective
The roles of particles in multiphase processes: Particles on bubble surfaces

https://doi.org/10.1016/j.cis.2015.08.008Get rights and content

Highlights

  • Review of the effect of particles on air–water interfaces

  • Particles within and outside the foam film may contribute to bubble stability.

  • A bubble surface bearing particles is a composite surface with elastic properties.

  • Local surface properties of bubbles during coalescence are poorly characterized.

  • Local surface properties are needed to understand particle detachment.

Abstract

Particle-stabilised foams (or froths) form the fundamental framework of industrial processes like froth flotation. This review provides an overview of the effects of particles on bubble surfaces. The characteristics of the particles have a profound effect on the stability of the bubbles although the stabilisation mechanisms may differ. It is well known that layers of particles may provide a steric barrier between two interfaces, which prevents the coalescence of bubbles. Although perhaps considered of lesser importance, it is interesting to note that particles may affect the bubble surface and momentarily suppress coalescence despite being absent from the film separating two bubbles.

Foams are at best metastable and coalescence occurs to achieve a state of minimum energy. Despite this, particles have been reported to stabilise bubbles for significant periods of time. Bubble coalescence is accompanied by a release of energy triggered by the sudden change in surface area. This produces a distinctive oscillation of the bubble surface, which may be influenced by the presence of incompressible particles yielding unique surface properties. A survey of the literature shows that the properties of these composite materials are greatly affected by the physicochemical characteristics of the particles such as hydrophobicity and size.

The intense energy released during the coalescence of bubbles may be sufficient to expel particles from the bubble surface. It is noted that the detachment of particles may preferentially occur from specific locations on the bubble surface. Examination of the research accounts again reveals that the properties of the particles may affect their detachment upon the oscillation of the bubble surface. However, it is believed that most parameters affecting the detachment of particles are in fact modifying the dynamics of the three-phase line of contact. Both the oscillation of a coalescing bubble and the resulting detachment of particles are highly dynamic processes. They would greatly benefit from computer simulation studies.

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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