VOLUME 85, OCTOBER 2007 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 739 INTRODUCTION F roth flotation is a process designed to separate hydropho- bic particles selectively in an aqueous medium, in which gas bubbles are dispersed. Hydrophobic particles selectively attach to the gas bubbles, forming aggregates. If the aggregate density is lower than the medium density the aggregates float to the top of the separation cell, where they overflow into a launder (Schulze, 1993; Shergold, 1984). The flotation of mineral sulphides and oxides, for example, operates most efficiently when the particle diameter is between 10 and 150 µm (Shergold, 1984). Coarse and fine particles are often not recovered during the flotation process, or are recovered poorly. Flotation is achieved in part by increasing the hydrophobicity of the particles (Lucassen-Reynders and Lucassen, 1984). The degree of hydrophobicity can be expressed by the contact angle, the angle at the three-phase line of contact between the mineral, The Limits of Fine and Coarse Particle Flotation Carlos de F. Gontijo, Daniel Fornasiero and John Ralston * Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, Adelaide, SA 5095, Australia the aqueous phase and the air bubble (Gaudin, 1957). It is accepted that the higher the contact angle of a mineral surface, the more readily it is wetted by air, and is thus more hydropho- bic (Lucassen-Reynders and Lucassen, 1984; Gaudin, 1957). Particle hydrophobicity or contact angle is dependent on the type and distribution of species present on the mineral surface (Crawford et al., 1987). Generally, the mineral particle surface may be covered with hydrophobic (e.g. collector, polysulphide) and hydrophilic species (oxide, hydroxide, and sulphate) as well as with different mineral phases, as found in composite particles (Prestidge and Ralston, 1995). Recovery decreases with increas- ing particle size because of detachment and decreases at small particle sizes due to inefficient collision (Dai et al., 2000). The flotation behaviour of quartz particles was studied over the particle size range from 0.5 µm to 1000 µm and for advancing water contact angles between 0º and 83º. Flotation was performed in a column and in a Rushton turbine cell. Particle contact angle threshold values, below which the particles could not be floated, were identified for the particle size range 0.5–1000 µm, under different hydrodynamic conditions. The flotation response of the particles, either in a column or in a mechanically agitated cell with a similar bubble size, was comparable. Turbulence plays a role, as does bubble-particle aggregate velocity and bubble size. The stability of the bubble-particle aggregate controls the maximum floatable particle size of coarse particles. For fine particles, the flotation limit is dictated by the energy required to rupture the intervening liquid film between the particle and bubble. Flotation of very fine and large particles is facilitated with small bubbles and high contact angles. These results greatly extend our earlier observations and theoretical predictions. On a étudié le comportement de flottation de particules de quartz pour des tailles de particules comprises entre 0,5 µm et 1000 µm et des angles de contact de l’eau de 0º et 83º. La flottation a été réalisée dans une colonne et dans une cellule munie d’une turbine Rushton. Les valeurs de seuils des angles de contact, en dessous desquels les particules ne pouvaient pas flotter, ont été identifiées pour une gamme de particules de 0,5-1000 µm, dans différentes conditions hydrodynamiques. La réponse de flottation, dans une colonne ou dans une cellule agitée mécaniquement avec une taille de bulles similaire, est comparable. La turbulence exerce une influence, tout comme la vitesse des agrégats de bulles et de particules et la taille des bulles. La flottation des particules très fines et des particules larges est facilitée avec des bulles petites et des angles de contact élevés. Ces résultats élargissent de façon importante nos observations et prédictions théoriques antérieures. Keywords: coarse particle flotation, detachment, stability, critical contact angle, kinetic theory of flotation * Author to whom correspondence may be addressed. E-mail address: john.ralston@unisa.edu.au