Pulsating Drop Module Used to Study Surfactant Behaviour in Flotation
The choice of surfactant influences the recovery yield of minerals during flotation separation processes. Scientific instruments such as the Theta force tensiometer with Pulsating Drop Module are now being used to measure the rheological properties of bubble surfaces.
The principle of flotation is simple. In the flotation tank, fine bubbles are dispersed in the pulp containing finely ground ore particles which, depending on their hydrophobicity, attach to the air bubbles. Thus, valuable ore is lifted to the top of the tank, where it is skimmed off, while gangue minerals stay in the pulp.
The dispersed air bubbles play a key role in the process. Their properties are strongly influenced by the composition of the fluid medium, which consists of water and flotation reagents, such as frothers, collectors and depressants.
Frothers are surface-active substances that have an effect on the formation and behavior of the air bubbles. The frother molecules adsorb on the bubble surface creating an adsorption layer. During the adsorption, the surface tension decreases until it reaches an equilibrium value.
Under dynamic conditions, the kinetics of adsorption and desorption of surfactants to and from a solution have a major effect on the behavior of bubbles. Understanding the rheological properties of the adsorbed layers is important for the characterisation of commercial surfactants.
The capillary wave and oscillating bubble techniques were the first two methods developed to measure surface dilatational elasticity. The bubble oscillation method, developed by Lunkenheimer and Kretzschmar, uses harmonic interfacial disturbance to measure the surface dilatational elasticity.
A small bubble is formed at the end of a capillary tip and a membrane is used to create harmonic oscillation. This technique allows measurement of the surface dilatational elasticity at relatively high frequencies.
The modified pendant drop technique works on the same principle as the oscillation bubble method. The high resolution and excellent accuracy provides a useful tool to study the dynamics of adsorption layers. The method can be applied to liquid-gas as well as liquid-liquid interfaces.
The Pulsating Drop Module was provided by Attension (part of the Biolin Scientific Group, a global provider of analytical instruments). It is a compact, optional module to the Theta optical tensiometer for measuring surface dilatational elasticity based on the modified pendant drop technique.
In this study, the Attension Pulsation Drop Module was used to investigate the effect of two well-known commercial frothers; Nasfroth 240 (NF240) and Dowfroth 250 (DF250), on dilatational elasticity and viscosity of the bubbles surface.
The aim was to investigate how the surface properties changed when the concentration of the frothers was increased from below the CCC point (critical coalescence concentration) to above the CCC point.
The increasing concentration of surfactant caused a different effect on the surface elasticity in the presence of the two different types of frothers. In the NF240 solution the increasing concentration caused higher elasticity due to the decrease of surface tension. However, in the presence of DF250 with increasing frother concentration, the elasticity of the interface decreased.
This phenomenon could be explained by the faster molecule exchange. The higher the concentration, the faster the exchange and the lower the elasticity.
This study shows that the pulsation drop technique is an accurate way to investigate the rheological properties of interfaces with relatively high frequencies and is a valuable tool for characterising commercial surfactants used in the flotation process.