Journal of Colloid and Interface Science 338 (2009) 558–565

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis The detachment of particles from coalescing bubble pairs
Seher Ata *
Centre for Multiphase Processes, University of Newcastle, NSW 2308, Australia

a r t i c l e

i n f o

Article history:
Received 29 April 2009
Accepted 1 July 2009
Available online 4 July 2009
Keywords:
Froth flotation
Froth
Bubble coalescence
Particle detachment
Bubble oscillation

a b s t r a c t
This paper is concerned with the detachment of particles from coalescing bubble pairs. Two bubbles were generated at adjacent capillaries and coated with hydrophobic glass particles of mean diameter 66 lm.
The bubbles were then positioned next to each other until the thin liquid film between them ruptured.
The particles that dropped from the bubble surface during the coalescence process were collected and measured. The coalescence process was very vigorous and observations showed that particles detached from the bubble surfaces as a result of the oscillations caused by coalescence. The attached particles themselves and, to some extent the presence of the surfactant had a damping affect on the bubble oscillation, which played a decisive role on the particle detachment phenomena. The behaviour of particles on the surfaces of the bubbles during coalescence was described, and implications of results for the flotation process were discussed.
Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction
Froth flotation is widely used in the separation of mineral particles [1,2]. The process has also been applied successfully in several other areas such as coals cleaning, de-inking of recycling paper fibres, waste water treatment and removal of oils from water
[3].
The initial stage in mineral flotation is the grinding of ore in water to release the valuable mineral particles from the waste or gangue. In a suitable cell or column, reagents are added to the slurry of ground ore which render the surfaces of the valuable particles hydrophobic, while leaving the gangue mineral hydrophilic. Frothing agents are also added into the suspension of particles to facilitate the formation of a stable bubble and also froth. The suspension is then aerated and the hydrophobic mineral particles preferentially attach to the rising bubbles, bringing them into the froth phase at the top of the cell where they are collected as concentrate or product.
Not all the particles that attached to the bubbles in the cell are transferred to the concentrate. Some of them detach either in the collection zone [4,5] where the bubble-particle interaction takes place or in the froth layer [6–9]. A particle that has become attached to the surface of a bubble can become detached when the forces that hold it at the surface are exceeded by the detachment forces [10,11]. In the collection zone, the main cause for particle detachment is the presence of turbulence [4,12,13]. Turbulence is necessary to bring bubbles and particles together for collision
* Fax: +61 2 4960 1445.
E-mail address: Seher.Ata@newcastle.edu.au
0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.07.003 and attachment and therefore is inherent to most flotation devices.
Flows created by turbulence are characterised by the formation of eddies of different length scales. The bubble-particle couplet caught in the centre of eddies experience centrifugal forces which tend to dislodge the particle from the bubble [14–16]. Turbulence within a flotation cell is also known to lead to oscillation and deformation of bubbles which can result in detachment of particles
[17,18].
Detachment of particles also occurs in the froth layer although the mechanism may differ from that in the collection zone. It has been conjectured [6–8] that detachment largely occurs at the pulp–froth interface. As the bubble-particle aggregates move towards the pulp–froth