Froth Zone Research Articles

Effect of aliphatic alcohol-based and polyglycol polymer-based foaming agents on the water-liquid-vapor interface by means of molecular dynamics

The effect of foaming agents, or frothers, on the water-liquid-vapor interface is studied through molecular dynamics at different frother concentrations. The frothers are aliphatic alcohol-based, such as hexanol, methyl isobutyl carbinol (MIBC), and octanol, and polyglycol polymer-based, such as DF-200 and DF-250. The aim is to delve into the stability they confer to the interfaces they participate in and elucidate some of the differences between these two types of widely used frothers. The results indicate that frothers at the interface tend to be located outside the liquid phase towards the high-density vapor phase and that this behavior is more pronounced in polyglycol-based frothers. The structure of water is most affected by polyglycol-type frothers whose polar groups induce the water to adopt orientations that maximize hydrogen bonding. This effect increases with concentration. Unlike alcohols, which produce thin and dry interfaces with stability determined by the repulsion of their polar heads, polyglycol-based frothers produce thicker interfaces with structured-water whose stability is determined by an extended network of hydrogen bonds that produces water lamellae and plateau borders between bubbles, preventing coalescence. The results suggest that hydrogen bonding in polyglycols prevails over the other existing molecular forces in stabilizing the froth zone.

Study on inhibition mechanisms of detachment of coal particles from oily bubbles in flotation column

The cyclonic microbubble flotation column is an efficient separation equipment in the beneficiation process of metallic and non-metallic minerals. However, the higher detachment rate of coarse particles in the pulp, pulp-froth interfacial zone, and froth zone is an issue that needs to be addressed urgently in column flotation. Compared with traditional air-water bubbles, oily bubbles have higher hydrophobicity, which is beneficial for strengthening the bubble-particle attachment and inhibiting the bubble-particle detachment. Therefore, oily bubbles were introduced into the cyclonic microbubble flotation column to reduce the particle detachment rate. Firstly, an improved cyclonic microbubble flotation column was set up, and traditional air-water bubbles were placed by oily bubbles through the combined use of an atomizer and a bubble generator during experiments. Subsequently, we investigated the feasibility of using oily bubbles as a flotation carrier to regulate the particle detachment rate, improve the particle recovery, and explore the mechanism behind this. Column flotation results showed that recovery rates generated from experiments with oily bubbles are higher than those with air-water bubbles under different operation conditions, and the particle detachment rate in the interfacial region was significantly reduced with the use of oily bubbles. Recoveries and flotation rate constants generated from collection zones for each particle size range indicated that the use of oily bubbles can increase the upper limit of flotation particle size compared with the use of air-water bubbles. Calculated detachment rates showed that the use of oily bubbles can effectively reduce the detachment probability of particles with different sizes in the collection zone. Combined with the observation of ultrasonic-assisted oscillation, it is proved that the oil coating film on the oily bubble significantly improves the resistance of oily bubbles to turbulent disturbance. In addition, the increase in hydrophobic force between oily bubbles and particles and the growth of the three-phase contact periphery are beneficial to reducing the particle detachment probability due to turbulent disturbance in the collection zone. Results from bubble coalescence experiments showed that the existence of oily bubbles significantly reduced bubble coalescence time and increased the damping coefficient. Compared with air-water bubbles, oily bubbles have a stronger damping effect on the oscillation caused by bubble coalescence, inhibiting the violent bubble surface deformation and the induced particle detachment during bubble coalescence. Therefore, the introduction of oily bubbles is a promising idea to reduce the detachment probability of coarse particles and increase the upper limit of particle size in column flotation.

The Impact of Restricting Air Intake in Self-Aspirated Flotation Cells at Los Pelambres Concentrator

This article describes the impact of restricting the air intake in industrial 250 m3 WEMCO flotation cells at Los Pelambres concentrator. The influence of air restriction on the hydrodynamic and metallurgical performance of this type of machine was evaluated. The experiments were conducted in single flotation cells and entire rougher banks. In all cases, the gas holdup was measured to estimate the effectiveness of the obstruction system to decrease the air concentration. In single cells, axial profiles for solid percentage and particle size were evaluated. In addition, mass balances were conducted to assess the copper recoveries and concentrate features. In individual cells, air restriction led to a decrease in the gas holdup. However, this slight change was enough to obtain a more stable froth zone and a better solid suspension. The latter was observed as: (i) a higher P80 below the pulp–froth interface, (ii) a less diluted pulp at this level, (iii) a slightly higher Cu recovery, and (iv) a coarser concentrate product. A mineralogical analysis of the concentrate sample also showed the presence of coarser liberated Cu-sulfide particles. The results in single cells suggested an improvement in the recovery of coarse particles via more intense solid suspension. The air intake was also restricted in three rougher banks to assess the impact of air obstruction on the overall performance of the respective circuit. Eleven out of fourteen cells were operated with air restriction, which led to a significant improvement in recovery of 0.9%–1.6% (absolute), at a 95% confidence level. Size-by-size mass balances were also conducted for the rougher circuits, which proved that the recovery improvements were justified by the simultaneous increase in the recovery of coarse and fine particles. Thus, a restriction in the air intake showed that a decrease in the gas holdup (and in the bubble surface area flux) was compensated by better solid suspension and a higher collision efficiency in the draft tube. The former promoted the recovery of coarse particles in the quiescent zone, whereas the latter improved the interaction between bubbles and fine particles. Further developments are being made to implement a regulatory control strategy for the air intake in self-aspirated flotation cells and to use this approach for optimizing industrial flotation banks.

Decoupling the pulp and froth effect of ultrafine particles on Itabirite iron ore flotation

The reverse flotation of Itabirite iron ore is typically performed after a desliming step in which most of the sub-10 µm particles are removed. This is done to limit processing problems such as poor flotation recovery of SiO2 due to slime coatings, increased slurry viscosities, unmanageable froths and increased losses of hematite particles due to entrainment. However, the rejection of ultrafine particles contributes to large losses in iron recovery and may contribute to the instability of tailings dams. The aim of this study is to decouple the effects of ultrafine particles in the pulp and froth phases to better understand the processing capabilities of the fines fraction. The flotation test work was performed in a special-purpose continuously operated laboratory flotation cell that has a deep froth section to simulate plant-scale froth conditions. Five conditions of fines addition were investigated at 3 different froth heights and a solids concentration of 50%. The behaviour of the pulp and the froth were decoupled using froth stability and froth recovery tests. This showed that, even though there is an exponential increase in froth stability at increasing fines quantities, the performance of the pulp has the overriding effect on the SiO2 recovery. Pulp zone, froth zone and overall SiO2 recoveries showed that recoveries increased by about 3.6% for every 1% decrease in fines content. This meant a large increase of 20% in SiO2 recovery when going from undeslimed to deslimed feed. This was driven largely by increases in the pulp zone recoveries, which were about 4% for every 1% decrease in fines content. The froth recoveries, on the other hand, decreased by between 0.75% and 2.5% for every 1% decrease in fines content, depending on froth height. This paper explores further insights into iron ore flotation processing at different slimes concentrations.

Ultrafine Particle Flotation in a Concept Flotation Cell Combining Turbulent Mixing Zone and Deep Froth Fractionation with a Special Focus on the Property Vector of Particles

Froth flotation faces increasing challenges in separating particles as those become finer and more complex, thus reducing the efficiency of the separation process. A lab flotation apparatus has been designed combining the advantages of agitator-type froth flotation for high turbulences and column flotation with a deep froth zone for a fractionating effect, also enabling a study on the effect of different particle property vectors. A model system of ultrafine (<10 µm) particles was used for flotation to study how the separation process is influenced by the ultrafine property vectors of shape and wettability. To evaluate the new apparatus, flotation tests were carried out in a benchmark mechanical flotation cell under comparable conditions. Higher wettabilities result in higher recoveries, but the results show that optimum levels of hydrophobicity vary for different particle shapes. Different behaviours are observed for differently shaped particles, depending on their wettability state. The entrainment of unwanted gangue is reduced with increasing froth depth. While higher recoveries are obtained for the benchmark cell, the newly developed apparatus produces concentrates with higher grades. Our findings contribute to ultrafine flotation techniques and especially our understanding of the complex effect of particle shape in combination with the other property vectors.

Effect of platy gangue minerals in sulfide flotation: Part 2: Mechanisms

The presence of the usually unwanted, hydrophilic ore particles (gangue) in a mineral flotation froth plays a critical role. While they reduce the grade of the concentrate, they are also an important influence on the dynamical structure of the froth and its ability to transport value particles into the launder for ultimate recovery. This paper discusses how the shape and size of these gangue particles affect their transport through the froth zone under conditions typical of commercial sulfide flotation, but without hydrophobic ore particles present.Advective transport of gangue particles via entrainment depends on slurry drag due to the upward motion of air bubbles and the drainage effects of gravity on the liquid and solid phases confined in the network of fluid channels in the froth. In a companion paper (Bhambhani et al., 2023a) a significant increase in gangue transport rates through the froth zone was observed when a copper sulfide ore was mixed with platy mica (higher aspect ratio) compared to when the same ore was mixed with globular (lower aspect ratio) silica. It was hypothesized that this increased upwards transport rate was related to the greater upward drag experienced by the platy mica particles as compared to the lower aspect-ratio silica particles of comparable size. This hypothesis was tested by conducting froth sampling experiments, reported here, under increased upward drag conditions for the suspension and thus increased upward advective forces on the particles. It was observed that the entrainment vs particle size curves shifted upwards for the coarser sizes for both mica and silica. This was unexpected. It was additionally hypothesized that as the particle size approached the size of the channels (Plateau borders), through which settling particles must pass if they are to move downward, the particles become more confined and on average experience less downward motion. Platy mica particles of a certain sieve size are more susceptible to confinement in the channels than their lower aspect-ratio counterparts due to their reduced mass per particle. This was suggested from froth sampling experiments where drag and drainage were balanced in one case, and more drag dominated in the other.

Double‐sensor conductivity probe applied in a flotation system

AbstractThis study proposed a new approach for measuring bubble size distribution, bubble mean diameter, Sauter mean bubble diameter, and gas holdup using a double‐sensor conductivity probe in an air/water two‐phase system bubble column. The results for the two‐phase system were compared and calibrated using analyses from bubble images taken by a digital camera from the side of the column wall. Good agreement was observed between the two techniques. The same double‐sensor conductivity was used in an air/water/solids three‐phase system. The conductivity probe captured the change in bubble dynamic behaviour inside the pulp phase; however, the presence of the solids made it more challenging to measure. As a result, the VisioFroth commercial package, using images taken from the top of the froth layer, could be used in conjunction with the double‐sensor conductivity probe to show the dynamic evolution of mineralized bubbles from the pulp zone to the froth zone in a flotation process.

Effects of a cyclonic microbubble flotation column operating parameters on coal process responses

ABSTRACT Processing fine and ultrafine coal particles from primary resources is essential for sustainable development. As one of the most recently developed enrichment equipment, cyclonic microbubble flotation columns (FCMC) showed high efficiency in upgrading various fine minerals. However, understanding the effect of FCMC operating variables on coal upgrading process responses remains a black box and needs fundamental assessments. To fill the gap, this study examined the influence of circulating pump pressure and froth height on the recovery of different coal particle size ranges to assess and explore fundamental FCMC performance. The experimental results established that increased froth height would simultaneously reduce the collection and froth recovery. Coarse particles in the high froth height were detached due to bubble coalescence or rupture, while the liquid drainage reduced fine particle pollution of froth products. Increasing circulating pump pressure would increase the collection zone’s turbulence, lead to coarse particle detachment, and reduce collection recovery. Meanwhile, increasing circulating pump pressure could promote froth stability and decrease froth detachment in the froth zone. The calculated first-order rate constant (k c) confirmed that the flotation rate constants of coarse (−0.5 + 0.25 mm) and fine (−0.074 mm) particles were low, while the intermediate particles (−0.125 + 0.074 mm) had faster flotation kinetics.

INVESTIGATION OF THE ENTRAINMENT OF FINE SIZED CALCITE AND CHROMITE PARTICLES BY A FLOTATION COLUMN WITH NEGATIVE BIAS REGIME

Negatively Biased Flotation Column (NBFC) is a tool, which is developed for the flotation of coarse-sized ores. Unlike the conventional flotation column, in this column, there is no froth zone and wash-water is usually not used. Therefore, hydrodynamically entrainment of fine-sized particles into the concentrate is possible, and this may result in a decrease in the grade or yield of the concentrate.
 In this study, operating parameters that can affect the entrainment of fine-sized particles were investigated. Calcite with a grade of 99.4% (CaCO3) and chromite concentrates with a grade of 54.8% (Cr2O3) were used in the experiments. Parameters such as superficial water flowrate, particle size, air flow rate, frother dosage and particle density were used in the experiments. As a result of the experiments, it was determined that the amount of entrainment increased as the particle size and density decreased, and decreased with increasing superficial water flowrate, air flow rate and frother dosage.

Using Top-of-Froth Conductivity to Infer Water Overflow Rate in a Two-Phase Lab-Scale Flotation Column

The metallurgical performance of a flotation machine is largely defined by phenomena occurring in the froth zone. The water content in the froth affects recovery by influencing froth stability and mobility and, at the same time, reduces grade by mechanical entrainment of gangue particles in the overflow water. Efficient operation requires a compromise between the water carried by bubbles from the collection zone and that which overflows. It is believed that the most suitable operating strategy could be based on the measurement of froth water content, as a strong correlation with water overflow is anticipated. This work reports the testing results of an in situ electrical conductivity sensor continuously measuring the froth zone water content in a laboratory-scale flotation column. The test program included simultaneous measurement of froth conductivity and water overflow rates for changes in gas flow rate and frother concentration. The results show a stronger dependence of the measured top-of-froth water content on frother concentration than on the gas flow rate. A relatively linear trend was shown between top-of-froth water content and water overflow rate for a given air rate and frother.

A Method to Detect Abnormal Gas Dispersion Conditions in Flotation Machines

This short communication presents a methodology to detect abnormal gas dispersion conditions in flotation machines. These abnormal conditions are characterized by the significant presence of cap-shaped bubbles. The approach considers the use of a bubble size analyzer to measure gas dispersion at industrial scale. The detection of abnormal conditions is critical when estimating bubble size by automated software. Otherwise, the estimates are significantly biased, since irregular bubbles are typically removed from the analysis. From the recorded images and the respective black and white representation, the variability of the shadow percentage caused by the bubbles in the vision field, abruptly increases in the presence of cap-shaped bubbles. Experimental conditions with coefficients of variation lower than 60–70% in the shadow percentage represent spherical and spherical-ellipsoidal regimes. In the former, automated bubble sizing software has proved to be sufficiently effective in obtaining reliable results. In the latter, different segmentation techniques have been proposed to obtain satisfactory results. Abnormal conditions are detected under coefficients of variation greater than 80% in the shadow percentage. The presence of cap-shaped bubbles causes inefficiencies in the collection of hydrophobic minerals in the pulp zone as well as disturbances in the separation stage (froth zone). Therefore, the detection of these irregular bubbles is suitable to provide feedback to flotation processes, allowing gas dispersion to be driven towards normal operating conditions.

State Estimation of a Flotation Column using Fundamental Dynamic Models

Two on-line model-based state estimators are used in a semi-batch flotation column system based on a two-phase fundamental dynamic model. The column is modeled as two interconnected plug-flow reactors, representing the pulp and the froth zones. The model accounts for the appearance and breakage of three bubble size classes. The unknown states representing the gas holdup through the column have been estimated, assuming that the gas holdup of the bubble size classes at the exit on top of the column can be measured. It is confirmed that the proposed estimator for a two-phase case well predicts the gas holdup propagation through a lab-scale two-phase semi-batch column flotation based on experimental data. The performance of the model-based ensemble Kalman filter is comparable to that of the Luenberger observer with the same operating conditions. Gas holdup propagation was better captured by the Luenberger observer for state estimation in this simplified version of a flotation system. However, the ensemble Kalman filter has an acceptable performance while being a better option than the linear Luenberger observer for state estimation of more complex cases, such as the continuous nonlinear three-phase model of a flotation column with parameter uncertainty. Thereby, the ensemble Kalman filter algorithm is used to estimate the gas holdup through the column and the concentration of attached and free minerals in the upward and downward flows in the case of a three-phase continuous flotation column.

Analysis of the Dynamics of Rougher Cells on the Basis of Phenomenological Models and Discrete Event Simulation Framework

Due to the increase in the amount of copper sulphide minerals processed through concentration processes and the need to improve the efficiency of these production processes, the development of theoretical models is making an important contribution to generating a better understanding of their dynamics, making it possible to identify the optimal conditions for the recovery of minerals, the impact of the independent variables in the responses, and the sensitivity of the recovery to variations in both the input variables and the operational parameters. This paper proposes a method for modeling, sensitizing, and optimizing the mineral recovery in rougher cells using a discrete event simulation (DES) framework and the fitting of analytical models on the basis of operational data from a concentration pilot plant. A sensitivity analysis was performed for low, medium, and high levels of the operative variables and/or parameters. The outcomes of the modeling indicate that the optimum mineral recovery is reached at medium levels of the flow rate of gas, bubble size, turbulence dissipation rate, surface tension, Reynolds number of bubble, bubble–particle contact angle, superficial gas velocity and gas hold-up in the froth zone. Additionally, the optimal response is reached at maximum levels of particle size and density and at minimum levels of bubble speed, fluid kinematic viscosity and fluid density in the sampled range. Finally, the recovery has an asymptotic behavior over time; however, the optimum recovery depends on an economic analysis, examining the marginalization of the response over time in an operational context.

A dynamic framework for a three phase hybrid flotation column

A comprehensive dynamic fundamental model with Danckwerts’ boundary conditions at the pulp/froth interface has been developed for a continuous hybrid flotation column that includes a mechanically agitated section at the bottom. The model accounts for three phases, namely the water, gas, and solid particles (hydrophobic and hydrophilic), in one space dimension. The modeling framework is based on three subsystems, including a well-mixed reactor and two plug-flow reactors representing pulp (collection) and froth (cleaning) zones that are interconnected through the boundaries. The model considers the micro-scale processes taking place in the column, such as bubble-particle attachment and bubble coalescence. The resulting mathematical model is a coupled set of nonlinear heterodirectional hyperbolic partial differential equations for the pulp and the froth and a set of ordinary differential equations for the well-mixed zone. The movement between phases are given by liquid upflow, liquid downflow, and gas upflow. An important parameter in this simulation is the bubble size, which directly affects the gas holdup, and consequently the distributions of all states, including the concentration of attached minerals through the column. Qualitative validation of the model is provided by testing its predictions against experimental data from the literature for two-phase systems. Finally, the effect of some important parameters such as gas flow rate, agitation, particle size on gas holdup, distributions of value minerals, grade, recovery, and attachment/detachment rates have been studied.

Effects of Operating Parameters on the Froth and Collection Zone Recovery in Flotation: An Industrial Case Study in a 10 m3 Cell

The effects of flotation operation parameters, including froth depth, air flowrate, and frother dosage, on the froth and collection zone recovery and the flowrate of particles into the froth phase were investigated in a 10 m3 industrial cell. The results showed that froth recovery increases upon increasing air flowrate and frother dosage, as well as reducing froth depth. While all tested parameters affected the particles that entered into the froth phase, air flowrate and frother dosage showed the most and least significance, respectively. When the air flowrate, frother dosage, and froth depth were 146 m3/h, 150 mL/min, and 5 cm, respectively, froth recovery was found to be above 84%. Also, the effect of the parameters studied on collection zone recovery was different from their effect on the froth zone, with air flowrate having the greatest impact on the former.

Modelling and boundary optimal control design of hybrid column flotation

AbstractA three‐phase continuous hybrid flotation column that seeks to obtain the benefits of both mechanical cells and flotation columns is modelled as the interconnection of a CSTR representing the well‐mixed zone and two plug‐flow reactors (PFR) representing pulp and froth zones. The plant model accounts for the micro‐scale processes such as bubble‐particle collision and attachment and the appearance and breakage of bubbles. This complex distributed parameter system (DPS) is described by sets of nonlinear coupled conservation counter‐current hyperbolic partial differential equations (PDEs) and one set of ordinary differential equations (ODEs). The dynamic conservation law‐based model for the continuous hybrid flotation column including well‐stirred, pulp (bubbly), and froth zones, is utilized in an optimal model‐based controller design. This linear quadratic regulator (LQR)‐based controller accounts for optimality, stability, and performance. The controller design utilizes a linear model obtained by linearization at operating steady states of interest. A full‐state optimal feedback control law is designed and controller performance has been demonstrated through a numerical simulation of physically meaningful and relevant plant operating conditions. The LQR‐based optimal controller outperforms proportional‐integral (PI)‐based control by more than an order of magnitude in terms of a return to steady state after a perturbation in the initial condition.

Effects of flotation operational parameters on froth stability and froth recovery

SYNOPSIS The effect of flotation operational parameters on froth stability and froth recovery was studied. Froth stability was measured using a special column. To determine the froth recovery, the froth height change model and froth height exponential model were used. It was found that since the interactions between the pulp and froth zones affect the time of froth formation, the exponential model is more suitable than the froth height change method for determining the froth recovery. The results showed that superficial air velocity and collector dosage have, respectively, the highest and lowest effect on the froth recovery, while froth recovery decreases sharply with increasing froth height. Keywords: froth stability, froth recovery, superficial air velocity, collector dosage, frother dosage.

Effects of emulsified kerosene nanodroplets on the entrainment of gangue materials and selectivity index in aphanitic graphite flotation

In this study, kerosene emulsions with different average droplet sizes (700 nm, 854 nm, and 996 μm) were prepared to investigate their effects on the entrainment of gangue materials and the selectivity index in the flotation of aphanitic graphite. The calculation results based on Neethling and Cillers’s model indicate that a decrease in the average droplet size of emulsified kerosene would result in a decrease in the entrainment of gangue materials, which corresponds to the observations of froth layer height, maximum water recovery rate, and bubble size in the top section of the froth zone. This may be ascribed to the enhanced collision probability between graphite particles and oil droplets, as well as the increased modified flotation rate constant compared to the micro-scale droplets. It is also noted that the selectivity index in graphite flotation increases as the average droplet size of kerosene emulsions decreases. This is largely due to the synthetic effect of the mitigated gangue entrainment and the intensified collision between graphite particles and oil droplets in the presence of nano-scale oil droplets.

A Method to Predict Water Recovery Rate in the Collection and Froth Zone of Flotation Systems

This paper describes a method to predict water recovery rate into and through the foam in a bubble column operating under different gas rates, froth depths, and frother types and concentrations. Three frothers were considered: Metil Isobutil Carbinol (MIBC), a proprietary blend of alcohols, aldehydes, and esters commercialized under the name PINNACLE® 9891, and a PGE-based Dow Froth 1012 (DF1012). The water rate entering into the froth (foam) layer from the bubbly (collection) zone was estimated as the water rate overflowing the column when operating at a thin stable foam layer, i.e., 0.5 cm. It was observed that the rate at which water entered into the froth phase could be modelled as a unique linear function of the gas holdup below the froth, regardless of the frother chemistry. This is a fundamental result not previously found in the literature that also facilitates the calculation of the froth zone water recovery for deeper froths. The water recovery in the froth was found to be an inverse logarithmic function of the average liquid residence time in the froth. Although the same trend was observed for the three frothers tested, they did not converge into a single function, which suggests that frother chemistry plays a role in determining froth structure and then needs to be incorporated when modeling water transport in the froth. Finally, the water overflow rate calculated as the product of the water rate into the froth and froth water recovery predicted the actual measured values fairly well. The water transport model here proposed provides a simple representation of the interactions between collection and froth zone and its relation to easily measure operating variables.

The effect of impeller-stator design on bubble size: Implications for froth stability and flotation performance

The impeller in a mechanical flotation tank plays a key role in keeping particles in suspension, dispersing and breaking-up air bubbles, and promoting particle-bubble collision. However, the turbulent regime generated by the impeller can also affect the pulp-froth interface, destabilising the lower regions of the froth and affecting the overall flotation performance. The effects that pulp zone design modifications have on the froth are, however, poorly understood and have not been well-studied.In this work, we study the impact of impeller-stator design on the performance of a large laboratory-scale flotation tank. Two different impeller designs, with and without a stator, were assessed under a range of air flow rates to determine changes in pulp bubble size, froth stability, and metallurgical recovery. The results allow us to quantify, for the first time, the reduction in bubble size in a three-phase flotation system and the improvement in froth stability due to the use of a stator, and thus the enhancement in flotation performance. An inverse relationship is found between the pulp bubble size and froth stability. It is shown that the impeller designs that exhibited smaller bubble sizes resulted in higher froth stability values and also higher flotation recoveries. These findings provide insights into the links between pulp and froth zone phenomena, paving the way for improvements in flotation tank design that lead towards flotation optimisation.