Bubble coalescence and its effect on dynamic foam stability

Abstract

A ir-sparged bubble columns have broad applications as both reactor and separator units. Mass transfer rates in such systems .are determined by the interfacial area, which is controlled by the size of bubbles.Bubbles are commonly produced by sparging, that is, pumping gas through a capillary or frit into the bulk liquid. Fine bubbles can also be produced by cavitation at the trailing edge of the impeller blade (e.g. mechanical flotation cells), or by breaking the air supplied under pressure by shearing forces. Since a bubble size is the primary factor that determines efficiency of gas-liquid contactors, the efficiency of spargers is frequently evaluated using this parameter. This usually involves measurement of the size distributions of the bubbles produced by different spargers (Yoon et al., 1989) and is drown up on the assumption that the size of the generated bubbles is determined by the sparging mechanism and not by secondary processes (such as bubble coalescence). This is a critical point in evaluating the spargers and the objective of this paper is, first, to test this assumption, and, second, to define the conditions under which the bubble size measurement can characterize the efficiency of the sparging system. The size of bubbles generated using a single-hole sparger, a multi-hole sparger and a flotation cell were measured. While the size of bubbles generated from the single-hole sparger does not depend on frother concentration, the bubble size strongly depends on frother concentration when the multi-hole spargers, or flotation cell, are utilized. The bubble size ceases to depend on frother at the concentrations exceeding the value characteristic for each frother and referred to as the critical coalescence concentration (CCC). All the bubble size vs. frother concentration curves converge on a single curve. Aqueous solutions of the frothers that are characterized by low CCC values form foams quite stable under dynamic conditions. Since bubble coalescence can be inhibited by frother concentrations exceeding the CCC, sparging systems can be studied and compared only at frother concentrations that exceed CCC.On a mesure la taille des bulles produites par un aerateur a orifice unique, un aerateur a orifices multiples et une cellule de flottation. Alors que la taille des bulles produites avec I'aerateur a orifice unique n'est pas liee a la concentration d'agent moussant, la taille des bulles depend fortement de la concentration d'agent moussant dans le cas des aerateurs a orifices multiples ou de la cellule de flottation. La taille des bulles cesse d'hre reIi6e a I'agent moussant lonque la concentration excede la valeur caracteristique de chaque agent moussant (concentration de coalescence critique ou CCC). Toutes les courbes de taille des bulles par rapport a la concentration d'agent moussant se superposent. Les solutions aqueuses des agents moussants qui sont caracterises par de faibles valeurs de CCC forment des mousses plut6t stables dans des conditions dynamiques. Considerant que la coalescence des bulles...

A quantitative review of the transition salt concentration for inhibiting bubble coalescence

“…Fine bubbles are produced by cavitation at the trailing edge of the impeller blade in mechanical flotation cells [30]. Castro et al [31] and…”

“…Cho and Laskowski [30] to characterize the effect of different salts on froth stability. The CCC is a concentration above which bubbles do not coalesce and reach an almost constant bubble size.…”

Effect of ionic strength on bubble coalescence in inorganic salt and seawater solutions

“…Certain inorganic electrolytes have been shown to mimic the role of frothers by inhibiting bubble coalescence leading to a decreased bubble size in the pulp phase. 1,[3][4][5][6][7][8][9][10] Salts are not intentionally added to achieve bubble size reduction due to the high concentrations (0.02-0.8 M) required, 7,11 when compared to frothers (5-20 ppm). However, in an effort to minimize fresh water use, many industrial flotation circuits have introduced the use of underground water or seawater as conventional practice, thus inorganic salts may be present naturally.…”

“…Air is either drawn in from the vacuum created by the impeller or is forced in with the use of an external blower Typically organic chemicals, known as frothers, are employed in flotation circuits to facilitate air dispersion into fine bubbles and to stabilize froth formation. Cho and Laskowski 1 introduced the concept of a critical coalescence concentration (CCC), which is defined as the frother concentration at which bubble coalescence inhibition is at a maximum. The CCC is now an accepted parameter in the characterization of frothers.…”

Break-up in formation of small bubbles: Comparison between low and high frother concentrations

“…Most literature on the role of frother is based on coalescence inhibition (Harris, 1976;Laskowski, 2003). Cho and Laskowski (2002) introduced the term ''critical coalescence concentration" (CCC) to describe the concentration when minimum bubble size in a swarm is reached. Table 1 lists the CCC95 (i.e., the concentration giving 95% reduction in bubble size relative to water alone) of some frothers under typical flotation conditions, confirming the low concentration required.…”