Residence time distributions and mass transport in the froth phase of the flotation process (original) (raw)
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Modelling of froth transportation in industrial flotation cells
Minerals Engineering, 2004
Modelling of froth transportation, as part of modelling of froth recovery, provides a scale-up procedure for flotation cell design. It can also assist in improving control of flotation operation. Mathematical models of froth velocity on the surface and froth residence time distribution in a cylindrical tank flotation cell are proposed, based on mass balance principle of the air entering the froth. The models take into account factors such as cell size, concentrate launder configuration, use of a froth crowder, cell operating conditions including froth height and air rate, and bubble bursting on the surface.
Froth mean residence time measurement in industrial flotation cells
Minerals Engineering, 2008
Froth plays an important role in flotation processes preventing the pulp transport to the concentrate (short-circuit). Thus, it contributes to increasing the concentrate grade by gravity drainage of entrained particles, back into the pulp. Key parameters affecting the froth performance are the mean residence times of solids, liquid and gas in the froth. The froth mean residence time depends on the froth depth, gas flowrate, gas holdup , and flow regime. In this work, the froth mean residence times were evaluated from direct measurements of liquid and solid time responses in the froth of self-aerated copper flotation cells of 130 m 3. For this purpose the radioactive tracer technique was applied, using 82 Br as liquid tracer, and non-floatable mineral particles in three size classes (+150; À150 + 45;À45 lm) as solid tracers. All tracers were injected at the cell feed entrance, which allowed the tracer to circulate first through the rotor, and become well distributed over the whole cross-sectional area before entering the froth. Each tracer time response was measured on-line below the pulp/froth interface (input signal) and at the concentrate overflow discharge (output signal). The froth mean residence time was then obtained by difference between the average times of the froth input and output tracer signals, previously modelled. For the copper rougher flotation, the froth mean residence time (9-12 s) of non-floatable solids, derived from experimental measurement, was comparable with that obtained by measuring the gas flowrate and estimating the effective gas volume in the froth. While, the froth mean residence times of liquid and floatable solid were significantly larger, 21 and 24 s, respectively.
A compartment model for the mass transfer inside a conventional f lotation cell
International Journal of Mineral Processing, 2005
A model is developed by taking into account the simultaneous mechanisms of true flotation and entrainment in a conventional flotation cell. The total volume of the cell is divided into three compartments: pulp collection zone, pulp quiescent zone and froth region, with the mechanisms being modeled as occurring at the same time but originating at different places: true flotation from the collection zone and entrainment from the quiescent one. A particle is referred to as suspended in water or attached to an air bubble, depending upon its original state before crossing the pulp-froth interface (whether or not it remains in that state all the way to the concentrate launder). The model is obtained by solving a set of equations describing the mass conservation of solids and water between adjacent compartments. The principal mass transfer factors are identified as: the flotation rate constant, the mean residence time in the collection zone, the froth recovery of attached particles, the degree of entrainment through the froth and the water recovery from the feed to the concentrate. The development presented here allows the intricate nature of the mass transfer in a flotation cell to be reduced to one single equation, overcoming the need of numerical methods for simulation purposes. Moreover, it is shown that reliable prediction of grade and recovery can be obtained without detailed information on the pulp hydrodynamics or on any froth sub-process either than drainage, bubble bursting and bubble coalescence. D
Scale-up in froth flotation: A state-of-the-art review
Separation and Purification Technology
Froth flotation has been one of the most important and widely used methods to concentrate minerals since its introduction over a hundred years ago. Over the last few decades, in order to process more mineral while reducing capital costs, flotation equipment has become exponentially larger. The increase in tank volume, however, has brought new challenges in the operation and design of industrial flotation tanks. This review analyses the literature on flotation tank scale-up for the first time, contrasting several techniques and approaches used in both historical and state-ofthe-art research. The study of flotation scale-up is crucial for the optimisation of industrial plant performance and the maximisation of laboratory-scale research impact. While important advances in our understanding of flotation have been achieved, large flotation tank design and scale-up has, to a large extent, remained in-house know-how of manufacturing companies. This review of the literature relevant to flotation tank scale-up has resulted in a new classification, dividing the scale-up literature into two main areas of study, namely "Kinetic scale-up" and "Machine design scale-up". This review indicates that current scale-up rules governing the design of flotation tanks focus mainly on pulp zone kinetic parameters and neglect the effects on the froth zone, despite the importance of froth stability and mobility in determining flotation performance. Froth stability and mobility are closely linked to the distance the froth needs to travel, which increases with tank diameter. Although including internal elements, such as launders and crowders, has been the industrial solution for enhancing froth transport and recovery in larger tanks, the design and scale-up of these elements have not been thoroughly studied. Gaps in our knowledge of flotation are discussed in the context of addressing the scale-up problem, considering froth transport and froth stability. Addressing these gaps will pave the way for the design and operation of large flotation tanks of enhanced performance.
A Model of Froth Flotation with Drainage: Simulations and Comparison with Experiments
Minerals
The operation of a froth flotation column can be described by a nonlinear convection–diffusion partial differential equation that incorporates the solids–flux and drift–flux theories as well as a model of foam drainage. The resulting model predicts the bubble and (gangue) particle volume fractions as functions of height and time. The steady-state (time-independent) version of the model defines so-called operating charts that map conditions on the gas and pulp feed rates that allow for operation with a stationary froth layer. Operating charts for a suitably adapted version of the model are compared with experimental results obtained with a laboratory flotation column. Experiments were conducted with a two-phase liquid–bubble flow. The results indicate good agreement between the predicted and measured conditions for steady states. Numerical simulations for transient operation, in part for the addition of solid particles, are presented.
Dyna, 2011
Effective control of rougher fl otation is important because a small increase in recovery results in a signifi cant economic benefi t. Although many fl otation control strategies have been proposed and implemented over the years, none of them incorporate concentrate grade measurements at intermediate cells because these data are not usually available. On the other hand, there is much research on characterizing concentrate froth on the cell surface by image processing in order to extract information on froth color, bubble size, and speed that can then be used for developing expert control strategies, and some works have shown the possibility of estimating the concentrate grade. This work presents two multivariable model based predictive control (MPC) strategies for a rougher circuit. The fi rst strategy is based only on general tailings and concentrate grade measurements while the second one includes, beside these data, the intermediate cell grade estimates. Both strategies are compared with a fi xed control strategy. Simulation tests show that the recovery can increase by 1.7%, compared to the fi xed control strategy.
Froth recovery of industrial flotation cells
Minerals Engineering, 2008
The mass flowrate of particles (ton/h), entering the froth by true flotation, was evaluated from direct measurement of bubble load (ton/m 3 ) and gas flowrate (m 3 /h). This information, together with the concentrate mass flowrate, allowed the estimation of the froth recovery of floatable mineral in a 130 m 3 rougher flotation cell.
Powder Technology, 1998
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.