ELSEVIER Powder Technology 98 ( ~ o98 ) 6 i-73 POWDER "IEi NOWGY Interpretation of the effect of froth structure on the performance of froth flotation using image analysis A.W. Banford a.,, Z. Aktas h, E.T. Woodburn ': "BNFL Enghleering Group. Consultam T Services, Risley. Warrington WA3 6AS. UK "Ankara Unirersity. Chemical Engineering Department, 06 !00 Tandogmz. Ankara, Turkey ' Chemical Engineering D~TJartment, UMIST. PO Brtt 88, Manchester M60/QD, UK Received 12 January 1997. received in revised form 15 January t998: accepted 17 February 1998 Abstract The recovery and grades of a coal flotation concentrate are significantly dependant on the water content of tile overitowing frotll. The drainage of water from the froth is associated with the coalescence and bursting of bubbles in the froth. This work was undertaken to investigate the possibility of regulating the performance of a flotation cell by using Image Analysis to define a desired bubble size in the concentrate, and subsequently to use measured deviations to control bubble coalescence by the compensating addition of suffactants. The image analysis was done on a transputer-enhanced Micro Vax computer off-line, using video images of experimental semi-batch runs. Individual runs were done using a single surfactant which had to act both as a frother and collector, two surfactants 2-eth) ! hexanol and Triton X-405 being used. Data fro,n the image analysis was interpreted together with experimental measurements of particle and water recoveries in terms of a previously formulated froth kinetic model. Correlation of the point rates of overflow of dry-mineral-matter-free (dmmf) coal in terms of the model produced physically plausible parameters. © 1998 Elsevier Science S.A. All rights reserv,::d. Keywords: hnage analysis: Froth structure: Froth flolation: Model I. Introduction Froth flotation is a process for separating a mixture of two species of finely ground solids, depending upon differences in the species surface properties. The particles are suspended in water, :rod air bubbles are passed through the pulp. The pulp separation is envisaged as bubble-particle interception followed by selective attachment of the hydrophobic species, the resulting bubble-particle aggregates rising through the pulp. The selectivity of the separation can sometimes be improved by the addition of a reagent which is preferentially adsorbed by one component, and which by so doing enhances the particle hydrophobicity. When the loaded bubbles cross the upper surface of the pulp they entrain water and a froth will form. As the entrained water contains solids which have not been selectively attached to the bubble surface, the inter- bubble water content will increase nfineral fraction in the froth. As the bubbles rise through the froth, the water content is reduced by drainage which is related to bubble coalescence and rupture. These visual effects can be quantified by Image Analysis. Refs. [I.21 and M.R. Noor Mohamed (personal * Corresponding author. 0032-5910/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PIIS0032-5910(98)00032.1 communications, Chemical Engineering Department, UMIST, Manchester, UK, 1993) have shown that the rates of rupture and coalescence in the froth can be regulated on- line, by the addition of reagents. The drainage of water from the froth back to the pulp has two identifiable stages. As the bubbles cross the upper surface of the pulp into the froth they are spherical and are well- dispersed in water, this is the kugeischaum. Drainage in this stage is under gravity and is determined by the velocity profile of the water between adjacent rising bubble surfaces. The water near to the bubble surfaces will rise with the bubbles but water centrally located will drain downwards. As drainage proceeds the bubbles will crowd closer together and drainage will fall to zero as the bubbles reach a close packed structure. This limiting kugelschaum will how- ever still have a significant water content. For the water con- tent to fall, further capillary drainage has to occur. This occurs when the bubbles deform from their initial spherical shape to ellipsoids, when the radii of curvature of the bubble shells arc no longer uniform, this will lead to a polygonal structure. a polyederschaum. The froth structure at this stage comprises inter-bubble lamellae with parallel walls, and the formation of plateau borders at the junction of three bubbles. The lamel-