Iterative approach to weir drainage

Understanding liquid drainage in foam is an important step in determining the performance of a froth flotation system. The geometry of the flotation vessel has a major impact on drainage and thereby performance. In particular it is known that in a vessel geometry with sloping walls, a thin boundary layer of wet foam can appear near the wall, containing a high speed liquid jet that is sliding downwards. Although a zeroth order theory exists describing this liquid jet (Eur. Phys. J. E 8 (2002) 517), it has a number of unsatisfactory features which need to be rectified. The jet structure predicted does not match correctly onto the known state of the foam far from the wall. Also important physical mechanisms influencing the speed and liquid content of the jet are neglected. These problems can be corrected by iteratively improving the zeroth order solutions. The iterative approach indicates that bulk foam motion is an important effect influencing the jet boundary layer, and indeed that the foam is wetter at the wall than previously predicted.     

On the drift‐flux analysis of flotation and foam fractionation processes

Drift‐flux analyses are a common way of estimating the phase fractions and fluxes in vertical multiphase flows. However their use has had scant impact on the flotation and foam fractionation communities despite the publication of a number of such analyses over the past two decades. By recognizing that the physics that underpin the hydrodynamics of the froth are dissimilar to those that pertain to the bubbly liquid beneath, we present a drift‐flux analysis with two characteristic curves. A method for obtaining the characteristic curve for the froth from experimental data for the drainage of stationary foams is presented.     

Distillation tray efficiency and interfacial area

Three pure liquids, viz. water, ethanol and toluene were evaporated in air on a 10 cm dia. sieve plate to ascertain the effect of liquid properties on the mass transfer in the gas phase.

It was found that two extreme forms of dispersion exist on the tray, viz. a liquid dispersion (droplets) at high air velocity and low liquid hold-up, and an air dispersion (bubbles) at lower air velocity and higher liquid hold-up.

The mass transfer outside the jetting droplets is found to increase with increasing air velocity and with decreasing surface tension.

The mass transfer of the cellular foam is found to decrease as the air velocity increases, and is correlated with an internal circulation Re-number and a Fo-number for the contact time.

The difference in bubbling mechanism between bubbles rising from a single orifice and those rising from closely spaced orifices is discussed.     

Solids motion in flowing froths

Flotation is a widely used process within the minerals processing industry, as well as being used for water treatment and de-inking of recycled paper. The froth phase and its role in the separation achieved is as yet ill understood. A fundamentally based model of the behaviour of solids within a flowing froth allows for a fuller understanding of the froth phase of flotation vessels and process optimisation.This paper outlines a model for the motion of solids within a flowing froth. It builds on earlier work on the modelling of bubble and liquid motion within a flowing froth and includes all the effects of same phenomena that effect liquid motion, as well as including the effect of solids concentrations on liquid motion.The solids are divided into two classes for the purposes of modelling, namely the attached material, which follows the bubbles, and the unattached material, which mainly follows the liquid, but can move relative to the water by means of hindered settling and geometric and Plateau border dispersion. The attached material consists of hydrophobic particles, while the unattached material can consist of both hydrophobic and hydrophilic particles. Attached particles can become unattached due to coalescence or bursting.Results from simulations are shown to illustrate the movement and concentration of the solids from the pulp-froth interface to the upper, bursting surface and overflowing the weir.     

三相流化床液相径向和轴向扩散的研究

本文对二维扩散模型的示踪剂瞬态点源注入进行了解析及模拟计算.用微型电极检测不同径向位置的示踪剂浓度,由计算机对输出信号自动采集、实时分析,同时对三相流化床液相径向和轴向扩散系数等模型参数进行估值.     

Oil recovery from oil in water emulsions using a flotation column

A flotation column was used to recover oil from oil in water emulsions. The feed oil concentrations investigated were relatively high, in the range of 0.25 to 8 percent by volume. Previous studies using conventional flotation cells dealt with very dilute systems where the oil concentration was less than 0.1 percent by volume. The oil recovery was found to decrease with an increase in the feed oil concentration. The addition of wash water to the froth zone of the column had little effect on the oil recovery. The oil recovery decreased with increasing feed flow rate and surfactant concentration. The effect of increasing the gas flow rate was to initially increase the oil recovery. The oil recovery data were analyzed using a kinetic model. The order of the flotation kinetics was found to be 0.6. The gas hold-up behaviour of the recovery and the froth zones of the flotation column is found to correlate well with the drift-flux model.     

Diesel Oil Removal by Froth Flotation Under Low Interfacial Tension Conditions II: Continuous Mode of Operation

Abstract

The objective of this study was to investigate the relationship between interfacial tension (IFT) and foam characteristics and the efficiency of diesel oil removal from water in a continuous froth flotation column. The effects of operational parameters, including surfactant concentration, salinity, oil-to-water ratio, foam height, air flow rate, and hydraulic retention time (HRT) on the oil removal were investigated in the continuous mode of a froth flotation operation and compared to batch operation results. Unlike the batch system, for the continuous system used in the present study, having only branched alcohol propoxylate sulfate sodium salt surfactant (C_14–15(PO)₅SO₄Na) and NaCl present in the solution yielded such poor foam characteristics that a stable froth which overflowed the flotation column could not be produced, so the addition of sodium dodecyl sulfate (SDS) as a froth promoter was used to improve the foam stability. Unlike the batch froth flotation system with only C_14–15(PO)₅SO₄Na, the continuous froth flotation with the mixture of C_14–15(PO)₅SO₄Na and SDS, it was not possible to find a SDS and a NaCl concentration at which both ultralow IFT and good foaming were both achieved. Foam formation, stability, and production rate were found to be crucial parameters to the froth flotation efficiency. The continuous froth flotation system offers a high diesel oil removal of 96% in the single stage unit. Demonstration of efficient operation in the continuous mode in this work is important to the practical application of froth flotation in large scale processing.     

On Fiber Rejection Loss in Flotation Deinking

Reducing fiber rejection loss in flotation deinking is very important to conserve natural resources and reduce the cost of secondary fibers in paper recycling. This study examined two aspects of the problem, fiber consistency in the rejection stream and rate of froth (or wet stream) rejection. Flotation experiments were conducted using both nylon and wood fibers in column and commercial bench‐scale flotation deinking cells. It was found that increased froth stability resulted in a lower fiber consistency in the wet reject stream. However, it also increased rejection rate of the wet stream. As a result, the total fiber rejection loss was increased with the increase of froth stability. The results obtained suggest that controlling froth stability through reducing frother application and froth rejection are effective ways to reduce fiber yield loss in flotation deinking. This study also experimentally measured water and fiber drainage in fiber suspended froth to explain the effect of froth stability on fiber consistency in the reject stream using froth drainage dynamics.     

Dimensional analysis of foam drainage

Dimensional analysis of the process of steady drainage of liquid from foam has shown that, if surface and inertial effects are neglected, the drainage rate non-dimensionalised as a Stokes-type number may be described by a function of the liquid volume fraction only. In addition it is demonstrated that a simple power law relationship between these two quantities is a good approximation to channel-dominated and node dominated foam drainage models and that the approach enables the prediction of the dependency of drainage upon bubble size. The power law approach requires two adjustable constants which is the minimum required since a priori estimates of both surface shear viscosity and viscous losses in the vertices of the foam are problematical. In addition, a simple dimensionless parameter has been introduced that can modify the power law model to take into account capillary suction so that transients in one-dimensional drainage problems can be modelled.     

Diesel Oil Removal by Froth Flotation Under Low Interfacial Tension Conditions I: Foam Characteristics,and Equilibration Time

Abstract

Froth flotation is a surfactant‐based separation process which is suitable for treating dilute wastewaters. To achieve high performance for the froth flotation operation, the combination of an ultra‐low interfacial tension (IFT) between excess oil and excess water phases, high foam production rates, and high stability of the foam produced, must be attained. To obtain the ultra‐low interfacial tensions, a Winsor Type III or middle phase microemulsion has to be formed. In this study, branched alcohol propoxylate sulfate sodium salt with 14–15 carbon number and 4 PO groups (Alfoterra 145–4PO) was used to form microemulsions with diesel oil. From the results of this work, an increase in surfactant concentration decreased the IFT, and increased foam stability. To obtain the minimum IFT in the region of a Winsor Type III microemulsion, the addition of 5 wt.% NaCl was needed. However, this optimum salinity does not result in effective froth flotation due to poor foam characteristics. The results indicate that both the IFT and the foam characteristics should be optimized to achieve high efficiency of oil removal in a froth flotation operation. Unlike the previously‐studied ethylbenzene system, agitation of the solution before introduction into the flotation column yielded the lowest diesel oil removal efficiency because of the poor foam characteristics compared to either unagitated systems or systems allowed to equilibrate for one month.     

Direct measurement of entrainment in large flotation cells

A key response variable to describe the metallurgical performance in a flotation cell is the concentrate grade. The mineral feed characteristics and conditioning determine the grade of the particles attached directly to the surfaces of the bubbles by true flotation, while the operating conditions, such as gas rate, bubble size, froth depth and others, determine the amount of gangue recovered by entrainment, which finally decreases the concentrate grade.In this paper, the recovery of liquid and solids by entrainment was evaluated by direct measurement of the fraction of liquid and solids reported to the concentrate in a 130 m³ mechanical flotation cell.The liquid and solids entrainment, per size classes (+ 150; − 150 + 45; − 45 µm), was measured by the radioactive tracer technique. The procedure consisted of introducing a tracer impulse at the cell feed entrance. The tracer time response was monitored on-line at the concentrate overflow and at the tailings discharge. Also, in order to obtain the quantitative distribution of the feed, samples were taken periodically from the concentrate and tailings streams, for a period of 4 residence times, during the tracer tests. This allowed the quantification of the mass of tracer reporting to both streams.Experimental results confirmed that solids entrainment with the froth was strongly dependent on particle size, and that the entrainment factor (EF = solid/water recovery ratio) was similar to that reported for smaller size cells under similar operations conditions.     

Convective-dispersive gangue transport in flotation froth

The transport of gangue through flotation froth has been described by solving the convection-diffusion equation. Gangue recovery is predicted to be proportional to liquid recovery, which is consistent with experimental observation. In addition, it is seen that the dependency of gangue recovery upon particle size is due to processes within the pulp phase rather than the froth, insofar as the transport of particles in a given froth is approximately independent of size. The importance of maintenance of positive bias in column flotation, previously stressed by other workers, is reinforced. This model utilises a simplified representation of the froth and, as a consequence, it does not necessarily give accurate gangue recovery estimates for practical flotation processes. However, the convective-diffusive model does illuminate the physical processes behind gangue recovery in the concentrate which will aid the development of automatic control strategies.     

Performance characteristics of a tower fermenter with mechanical foam control

For a bubble column (BC) treating foaming liquids, the characteristics of a rotating-disk mechanical foam-breaker (MFRD) fitted to the BC were examined. The foaming behaviour of the BC and the foam-breaking behaviour of the MFRD under various operating conditions were related to the changes in liquid hold-up in the foam. The gas hold-up in a mechanical foam-control system (MFS) with the MFRD was confirmed to be greater than the gas hold-up in a non-foaming system (NS) including antifoam agent (AF). Comparison of the volumetric mass transfer coefficient between the MFS and the non-foaming system, in terms of the specific power input, also demonstrated the superiority of mechanical foam control for oxygen transfer performance.     

Studies of downcomer behaviour for foaming and non-foaming systems

Froth behaviour in downcomers has been studied experimentally. The foaming systems studied are n-butanol/water and n-pentanol/water. The non-foaming system used is air/water. Results obtained show that downcomer limitation is a function of downcomer size, type of system and liquid/gas loadings. To minimize downcomer limitations, the downcomer should be sized to minimize the height of froth/foam. A theoretical model of froth height in downcomers has been derived. Foam heights obtained from this model approximate closely the height of froth in the downcomer for both the foaming and non-foaming systems.     

泡沫浮选分离法的最新发展动态

泡沫分离是根据吸附的原理,向含表面活性物质的液体中鼓泡,使液体内的表面活性物质聚集在气液界面上,在液体主体上方形成泡沫层,将泡沫层和液相主体分开,就可以达到浓缩表面活性物质和净化液相主体的目的。泡沫分离是以气泡作分离介质来浓集表面活性物质的一种新型分离技术。本文注重介绍了泡沫浮选分离法的发展历史及其发展状况,另外也论述了其最新的发展动态,以及其优缺点。     

Coalescence in a steady-state rising foam

Steady-state rising foam is an ideal medium for studying foam coalescence phenomena. The coalescence process in this type of foam is characterized analytically. The spatial distribution of bubble size depends on the effect of individual coalescence events on the bubble size distribution, the rate at which they occur, and the rise rate of the bubbles. The film rupture rate is assumed to depend on the film area and the film drainage time, and is calculated using several simple models. This analysis agrees well with measurements conducted in a laboratory foam cell. However, the time-scale of the experiments indicated that film drainage was occurring predominantly in the hydrodynamic regime, and that non-equilibrium rupture mechanisms were primarily responsible for the observed coalescence.     

The effect of different flotation operating parameters on the froth properties and their relation to clean coal ash content

Froth properties and their relation to the concentrate grade play an important role in monitoring flotation running conditions and predicting flotation concentrate quality. In this paper, the correlation between the froth properties and clean coal ash content was investigated under complicated conditions where the frother dosage, gas velocity, and froth height were changed together. For the froth properties under study, their degree of correlation with clean coal ash content decreased in the order of homogeneity, water recovery, gray value, and froth velocity. The coefficient of determination (R²) of the fitting relationship between homogeneity and clean coal ash content was as high as 0.9028, because homogeneity has a close correlation with the foam structure and foam destabilization behaviors.     

Bubble growth in secondary foams

When mechanical foambreakers are used in chemical engineering processes to control an undesired primary foam, a secondary foam is often produced. Secondary foams consist of small bubbles, have a high liquid hold-up, and cannot be condensed further by mechanical foambreakers. Secondary foams can be converted into liquid and gas in a coalescence column by drainage, diffusion, and breakage of the lamellas. This paper presents a new model to determine the velocity of bubble growth and the time of coalescence of secondary foams. The experimental investigations have been carried out with surfactants in aqueous solutions.     

Fluid Flow and Kinetic Modelling in Flotation Related Processes: Columns and Mechanically Agitated Cells—A Review

In this paper, fluid flow and kinetic models related to minerals flotation process are presented and the advantages and limitations of using this type of models are discussed. The modelling of such processes was firstly developed assuming perfect mixing for the whole system as a black box. Then, a more realistic approach was developed recognizing the interaction between two zones, the particle–bubble collection zone and the froth transport zone.

From a hydrodynamic point of view, experimental data showed that single large mechanical flotation cells can deviate significantly from perfect mixing, while the mixing conditions in a flotation bank of mechanical cells (three to nine cells in series) can be well described as a series of continuous perfectly mixed reactors. From plant experience, it was observed that performance of large industrial pneumatic flotation columns, originally regarded as a counter-current operation, also operate closer to a single perfectly mixed reactor.

Advances in the field of modelling and design of flotation cells and columns, have been achieved because the fluid flow regime, the mass transport conditions at the pulp/froth interface and the froth transport mechanisms are better known and understood. Key parameters such as the bubble surface area flux, related to the bubble generation and the rate of particle collection, bubble loading related to the mass transport across the pulp-froth interface and froth recovery, which is mainly related to the gas residence time in the froth, are relevant for a deeper understanding of this type of equipment.