Laboratory Flotation Column Research Articles

Numerical modelling of two-phase flow hydrodynamics of column flotation - validation against ERT data

ABSTRACT In mineral processing, column flotation is being used extensively for fine particle beneficiation. Although a few attempts were made to develop computational fluid dynamics (CFD) models for column flotation in the past, validation against comprehensive experimental data is yet to be demonstrated. This work employs an Eulerian-Eulerian model coupled with the standard k-ε turbulence model for modeling the hydrodynamics in the column flotation. The two-phase simulations were performed on both laboratory and industrial-scale column flotations. The two-fluid model was further modified with the appropriate interphase forces (Ishii-Zuber drag force and Tomiyama lift force) and population balance method (PBM) to accurately simulate and predict the bubble dynamics. The axial and radial gas holdup variations along with the axial liquid velocity are predicted at different air and liquid superficial velocities. The numerical predictions were then validated against the Electrical Resistance Tomography (ERT) data in a 4-inch laboratory column flotation and conductivity probe data in an industrial column flotation. The modified two-fluid model was able to predict accurate hydrodynamics close to ERT data. The gas holdup was found to increase with the superficial air velocity, liquid height, and liquid velocity. The gas holdup found uniformly dispersed radially at low superficial air velocities and heterogenous dispersed bubble flow at higher superficial velocities. A parabolic profile of liquid flow observed at higher superficial air velocities.

A Model of Froth Flotation with Drainage: Simulations and Comparison with Experiments

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.

Deinking of Recycled Paper by Coupling of the Enzyme Endo-Β- 1,4-D-Glucanasa and the Cellulose, and by using a Laboratory Flotation Column

AbstractDeinking of recycled office (MOW) paper was carried out by using a flotation column and adding separately sodium hydroxide, and the enzyme Cellulase Thricodema Sp., as defibrillators.The de-inked cellulose fibers were characterized according to the standards of the paper industry, to compare the efficiency of the deinking of each chemical reagent used to hydrolyze the fibers and defibrillate them.The computational simulation of the molecular coupling between the enzyme and cellulose was performed, to establish the enzyme-cellulose molecular complex and then to identify the principal amino-acids of endo-β-1,4-D-glucanase in this molecular link, which are responsible for the hydrolysis of the cellulose.Experimental results show the feasibility to replace sodium hydroxide with the enzyme Cellulase Thricodema Sp., by obtaining deinked cellulose with similar optical and physical properties.The use of the enzyme instead of sodium hydroxide avoids the contamination of the residual water; in addition to that, the column is operated more easily, taking into consideration that the pH of the system goes from alkaline to neutral.

Determination of the mass transfer rate constant in a laboratory column flotation using the bubble active surface coefficient

This study aimed to introduce a new approach to determine the mass transfer rate constant in a laboratory column flotation through an analogy to a mass transfer process. The RTD of the column at two separate bubble sizes were modeled using both conventional N-mixer and N-mixer with back-mixing flow models. The number of perfect mixers (N) and the corresponding backflow coefficients (λ) were optimized using MINLP solver in MATLAB environment where N = 5 with λ = 0.567 at the bubble size of 1.8 mm and N = 8 with λ = 0.643 at the bubble size of 0.8 mm were obtained. The bubble active surface coefficient was introduced as a new parameter to study the flotation kinetics. Thus, the mass transfer rate constant was determined for various operational conditions based on the bubble active surface coefficient and bubble loading measurements. Results showed that the mass transfer rate depends on the physical and surface properties such as particle size, particle surface hydrophobicity, and the carrying capacity. At the fine particle size, the mass transfer continues such that a long bubble retention time is required. However, for the coarse particles, bubbles reach their maximum loading capacity prior to the pulp-froth interface so that any additional retention time does not contribute to the flotation. Besides, at the coarse particle size, due to the poor stability of the aggregates, especially for particles with low surface hydrophobicity, the mass transfer rate and thereupon the bubble loading decreases remarkably. Moreover, the fine particles show lower flotation kinetics in the presence of the large bubbles.

Statistical Modelling of Oil Removal from Surfactant/Polymer Flooding Produced Water by Using Flotation Column

The objective of this study was to investigate the effect of polymer (GLP-100) and surfactant (MFOMAX) towards the efficiency of oil removal in a flotation column by using the Response Surface Methodology (RSM). Various concentrations of surfactant (250, 372 and 500 ppm) and polymer (450, 670, and 900 ppm) produced water were prepared. Dulang crude oil was used in the experiments. Flotation operating parameters such as gas flow rate (1–3 L/min) and duration of flotation (2–10 min) were also investigated. The efficiency of oil removal was calculated based on the difference between the initial concentration of oil and the final concentration of oil after the flotation process. From the ANOVA analysis, it was found that the gas flow rate, surfactant concentration, and polymer concentration contributed significantly to the efficiency of oil removal. Extra experiments were conducted to verify the developed equation at a randomly selected point using 450 ppm of polymer concentration, 250 ppm of surfactant concentration, 3 L/min gas flowrate and duration of 10 min. From these extra experiments, a low standard deviation of 1.96 was discovered. From this value, it indicates that the equation can be used to predict the efficiency of oil removal in the presence of surfactant and polymer (SP) by using a laboratory flotation column.

Assessment of pH-responsive nanoparticles performance on laboratory column flotation cell applying a real ore feed

Nanoparticles (NPs) can promote the column flotation process in mining industry. Nanoparticles’ effects on column flotation process (copper recovery, grade and flotation rate constant) are assessed in Sarcheshmeh Copper Complex, Iran, through response surface methodology (RSM) optimization technique. The γ-Al2O3, α-Fe2O3, SiO2, and TiO2 nanoparticles are selected for these experiments. A flotation rate constant is chosen as a response to assess the effect of nanoparticles on flotation in its kinetic sense. The process pH and nanoparticle dosage are selected as the influential parameters. Results obtained from RSM indicated that the maximum percentage of Cu recovery and grade is obtained at pH of 12 and nanoparticle dosage of 6 kg/t, through α-Fe2O3 and γ-Al2O3 nanoparticles, respectively. Applying nanoparticles in particular γ-Al2O3 and α-Fe2O3 increases the Cu recovery by 8–10% together with the grade by 3–6% in a significant manner. It is revealed that nanoparticles could effectively be applied in enhancing the flotation performance.

Evaluation of Frother Types for Improved Flotation Recovery and Selectivity

A laboratory study was conducted to evaluate and compare the effectiveness of nine different frother types when used in a three-phase, continuously operating froth flotation system. The frothers included several that are commonly used in the industry (e.g., MIBC, 2EH, and F-1) as well as unique frother types (e.g., F-3). The tests were conducted in a 5-cm diameter laboratory flotation column that provided near plug-flow mixing conditions due to a length-to-diameter ratio of around 50:1. Test results indicate that F-1, MIBC, and MPC (in order of decreasing effectiveness) provided the weakest performance in terms of combustible recovery while F-2, MAC, and 2EH were the top three generating the highest separation efficiencies. When processing ultrafine coal, the ash content of the flotation concentrate ranged from 10% to 15% while recovering over 80% of the combustible material. F-3, F-4, and DIBC provided over 80% recovery of combustibles at the expense in the amount of hydraulic entrainment. The flotation performances were also closely examined in accordance with the fundamental properties of the nine tested frothers, and their correlations were addressed in detail.

Development of a laboratory-scale flotation column with inlet bubble size measurement

This paper presents the development of a small-scale (1 in.) laboratory flotation column with online measurement of bubble size. In contrast to the systems that use sampling-for-image techniques and have the viewing chamber located at the top of the flotation device, the system employed here measures the bubble size via image analysis using a bubble size analyzer located at the bottom of the column using a well-referenced sloped viewing chamber. The main advantage of this configuration is that it allowed the measurement and monitoring of the bubble size during flotation experiments (three-phase system). This was possible because the sizes of the bubbles were determined before they entered the column; moreover, the “cloudy effect” caused by fine particles in the bubble viewing chamber was minimized owing to the use of an expansion chamber (named the bubble injection chamber). The image capture, processing, and analysis were automated, which rendered it possible to determine the bubble size distribution and mean bubble size (D32) of consecutive images every 5 s using a simple image analysis procedure. Two-phase experiments conducted to evaluate the system showed that the mean bubble size (D32) and gas holdup were within the ranges found in the literature. As expected, the holdup was dependent on the bubble size, which was strongly dependent on the frother concentration. Finally, some exploratory experiments were conducted to test the apparatus in a three-phase system. Recovery-particle size curves for different bubble sizes showed that mass recovery increases as the mean bubble size decreases, and the coarser the particles, the greater is the effect of the bubble size on the recovery.

Hydrodynamic study of three-phase flow in column flotation using electrical resistance tomography coupled with pressure transducers

Bubble size distribution, bubble rise velocity, gas and solids hold-up are the extensively used hydrodynamic parameters apart from floatability data for column flotation design and performance evaluation. In the current work, the combined solids and gas hold-up distribution characteristics are studied using non-intrusive electrical resistance tomography (ERT) coupled with pressure transducers (PT) in a laboratory column flotation. The effect of superficial gas velocity, slurry feed flow rate and slurry height in the column on mean gas hold-up, combined solids-gas hold-up and its radial distribution is analysed for both two-phase and three phase slurry systems. Slurry experiments are conducted for three different solids concentrations. Using the modified sensitivity back projection (MSBP) algorithm the measured raw voltage data has been reconstructed into conductivity data and thereby estimating the equivalent phase concentration values by adopting Maxwell principle.. Mean gas and solids hold-ups extracted from ERT have been critically assessed for the column operating in various flow regimes. The ERT measured mean gas hold-up and solids hold-up values are in close agreement with the data estimated from pressure difference measurements. The results show an increment in the gas hold-up with an increase in the superficial gas velocity, whereas solids hold-up distribution is almost homogeneous for high gas velocities. The gas hold-up reduces with solids content at the fixed gas and slurry flowrates. The presence of solids render the bubble rise velocity thereby decreases the local gas hold-up with the solids percent. Further feed slurry flow rate and frother dosage effects on column hydrodynamics have been studied and the change in the phase hold-ups is quantified.

Concentration of rare earth minerals from coal by froth flotation

Rare earth elements (REEs) found in coal are in the form of minerals, ion-adsorbed onto clay surfaces or inner layers, or organically bound. Rare earth minerals such as monazite exist in coal and have grain sizes smaller than 5 µm. In this study, froth flotation was successful in concentrating rare earth minerals existing in a thickener underflow material derived from Fire Clay seam coal that contained around 431 ppm of total rare earth elements (TREE) on a dry ash basis. Conditioning with fatty acid followed by processing using multiple stages of conventional flotation produced a final concentrate containing 2,300 ppm TREE. Using a laboratory flotation column to limit hydraulic entrainment, the TREE content was further enriched to around 4,700 ppm, which equated to an enrichment ratio of 10:1..

Laboratory and CFD investigations of the two-phase flow behavior in flotation columns equipped with vertical baffle

In this research, a numerical approach and a series of laboratory tests have been used to investigate the effect of vertical baffling and height to diameter ratio on the axial mixing in flotation columns. The baffle is a plate located perpendicular to the cross section of the column with a length of 2.8m and thickness of 0.4cm. The computational domain is a column with a circular cross section having a height of 3.2m and a diameter of 10cm. Three-dimensional simulations were executed using Eulerian two-phase computational fluid dynamics (CFD) models for both non-baffled and baffled columns. In order to reduce computing demand and simplify the problem, it was assumed that the column is already filled with water, and air enters from the lateral and upper surfaces of a cylindrical sparger with a length of 15cm and a diameter of 1cm located vertically at the bottom of the column. To validate the simulation results, a series of laboratory flotation column experiments have been performed under the above-mentioned conditions. Three-dimensional simulations were executed using an Eulerian two-phase model for both non-baffled and baffled columns. The simulated pressure values on the wall at 0.2m and 2.8m height of the non-baffled column were in good agreement with experimentally measured values with the highest relative difference of <3.07%. Comparison of the computational results for the non-baffled and baffled columns showed that baffling can reduce water axial velocity up to 16.96%, which consequently reduces the axial mixing in the column and increases flotation recovery. Study of the effects of height to diameter ratio showed that effect of baffling in columns with lower aspect ratios is more prevalent for reducing the axial mixing.

Hydrodynamic study of two phase flow of column flotation using electrical resistance tomography and pressure probe techniques

Gas dispersion characteristics are very important to design and selection of flotation process for the fine particle separation. Although the separation of flotation process heavily depends on hydrophobicity of given solid particles, but their rate of attachment and detachment depends on local gas dispersion characteristics. In this work, gas dispersion characteristics in a 100 mm laboratory column flotation have been investigated by exploring the flow behavior in terms of the local and mean gas hold-up, bubble rise velocity and bubble size distribution across the column. This experimental data has been used to demonstrate the validation of the two-fluid CFD model predictions in the column flotation. The column hydrodynamics are estimated by using high speed dual planar Electrical Resistance Tomography (ERT) system and pressure transducers. In all the experiments, water is used as the liquid phase and air (bubbles) as the disperse phase. Using the ERT system, measurement of two phase distributions are examined for a wide range of design and operating conditions of the column including different spargers and frother dosage, where the flow changes from homogenous to transition bubbly flow. It is confirmed by ERT that the gas-holdup increases with an increase in the sparger porosity, air superficial velocity, liquid height and liquid feed flow rate. To test the reliability of ERT measurements, simultaneously a set of pressure transducers is used to obtain the sectional average gas holdup and validated. It is observed that bubble density occupancy is more at center region of the column and gas holdup value increases with the increase of gas superficial velocity. Dynamic gas disengagement technique is utilized to measure bubble rise velocity and sauter mean bubble diameter. At end this ERT data has been used to validate two-fluid CFD model (modified with suitable drag and lift forces via user defined functions) predictions in the column and found a close agreement.

Multiphase CFD Modelling of Mineral Separators Performance: Validation Against Tomography Data

This paper aims on application of computational fluid dynamic (CFD) modelling on two-phase and multiphase flows in 75 mm conventional hydrocyclone, 350 mm dense medium cyclone (DMC) and 100 mm diameter laboratory column flotation. Along with classification experiments, a dual-planar high speed electrical resistance tomography (ERT) is used to measure the internal flow dynamics in situ in 75 mm hydrocyclone and column flotation at different operating conditions. Experiments are carried out in 75 mm hydrocyclone for different operational parameters and feed solids content varying in the range of 0–30 wt%. Results are presented in terms of the air core diameter and solids volume fraction contours using Maxwell’s conductivity equation. It is confirmed by ERT that the air core occupied area and wall solids conductivity levels decreases with increasing the feed solids concentration. Similarly, ERT is used to estimate the mean gas hold-up and radial distribution in order to understand the hydrodynamics of the column flotation. The effect of air superficial velocity and liquid flow rate on gas hold-up is discussed. Algebraic slip mixture model modified with lift, drag and rheology is used to predict the air core size, the solid concentration distribution and classification performance for the hydrocyclone. Validation of air core size and mean solid volume fractions by ERT measurements with the CFD simulations is attempted. The CFD predicted air core radius and medium segregation in the DMC is validated against literature based gamma ray tomography data. Further discrete phase model (DPM) is used to track and measure the residence times of different size and density coal particles. CFD predicted gas hold-up in the column flotation is also validated against in-house measured ERT and imaging data.

Effect of particles on bubble size distribution and gas hold-up in column flotation

In flotation systems, it is generally assumed that the bubble size presents an inverse correlation with the gas hold-up. However, this hypothesis is not systematically true. The focus of this work is to study the effect of solid particles on the bubble size distribution and gas hold-up, as well as the correlation between the bubble size distribution and gas hold-up in column flotation. Experiments were conducted in two and three-phase systems using a laboratory flotation column (5.6cm internal diameter, total height of 650cm), and mixtures of quartz (hydrophilic gangue) and talc (naturally hydrophobic mineral), classified in four different size fractions. For the studied flotation system, experimental results are compared with literature correlations and models to reveal that hydrophobic particles affect the gas hold-up through three different mechanisms modifying the Sauter mean diameter and rise velocity, namely (1) surface interactions, and the joint antagonistic effect of (2) bubble loading and (3) coalescence.

Predictive control of the bubble size distribution in a two-phase pilot flotation column

Among the gas dispersion properties affecting flotation column behavior, the bubble size distribution (BSD) is one of the most important. Its control could positively contribute to optimize the metallurgical performance. Experimental tests are carried out in a laboratory flotation column working with air and water. The objective is to regulate the BSD to a desired distribution set-point. BSD is measured in real-time using an image analysis method and a dynamic non-linear model (Wiener) for BSD, based on a log-normal distribution, is identified. Finally, a constrained model predictive controller is designed to control the BSD. The proposed approach leads to good control results, thus confirming the possibility to use this strategy to optimize the valuable mineral recovery by adequately selecting the BSD for a given particle size distribution.

Identification of a non-linear dynamic model of the bubble size distribution in a pilot flotation column

Gas dispersion properties play an important role in flotation as they partially determine the metallurgical performance. The objective of this work is to obtain a dynamic model of the bubble size distribution in a two-phase (air and water) pilot flotation column. The steps are: 1) to measure and count the bubbles from digital images taken by a camera, 2) to estimate a log–normal distribution of the bubble sizes, 3) to estimate a Wiener model whose outputs are the mean and standard deviation of the distribution while the inputs are the superficial gas velocity set-point and the superficial shearing water velocity set-point. The size and number of bubbles in each image are evaluated by a bubble detection technique based on circular Hough transform (CHT). This technique allows overcoming issues related to the detection of large single bubbles as well as clusters. Tests are carried out in the laboratory flotation column using different concentrations of frother and air flow rates. Results are compared with visual counting, as well as with a commonly used bubble detection method based on circular particle detection (CPD). The estimated number of bubbles is very similar to what is obtained with a visual count. Compared to CPD algorithm, CHT significantly improves D32 estimation (error of 3% instead of 18% with the former) with a comparable processing time. Then, the mean and standard deviation of a log–normal distribution are estimated by maximizing a likelihood function thereby leading to very good fits for the distributions. Finally, a series of model identification tests are conducted by manipulating the superficial gas velocity set-point and the superficial shearing water velocity set-point (i.e. air and shearing water flow rate set-points through the sparger). A Wiener model is then estimated to predict the corresponding mean and standard deviation of the log–normal distribution. Validation tests confirm the quality of the non-linear model.

Effects of Energy Input on the Laboratory Column Flotation of Fine Coal

This study explored the effects of energy input and different energy increase patterns on the separation performance of a laboratory cyclonic–static microbubble flotation column in fine coal flotation. The energy input was changed by adjusting the circulating pump power and pulp residence time. Continuous flotation tests were designed by using five feed rates (i.e., different pulp residence time), which were 53.33, 80.00, 106.40, 133.60, and 160.00 g/min, and various useful power of the circulating pump (22.44, 30.14, 38.50, 46.86, 56.10, and 64.46 W). Results show that concentrate combustible recovery initially increased before reaching saturation distribution with increasing energy input absolute value. An ash content of 10.90% and combustible matter recovery of 92.84% were obtained at the energy input of 10098.00 J which is called the flotation saturation energy Es. Es is the essential condition and guarantee for a complete flotation process. A low ash content of clean coal was obtained with low energy input. With the increase of energy consumption, the additional coals recovered were coarse particles with low ash content and fine particles with high ash content. However, the ash content did not exhibit a significant change with the increase of energy input in this investigation. During the flotation process, a minimum critical flotation time T and critical circulation pump useful power P are required. If the P and T of a variable are less than the critical value, high combustible matter recovery could not be obtained by adding another variable to increase energy input. A reasonable mode of energy input was proposed that the absolute value of the energy input reaches Es; meanwhile, it ensures that each value of the P and T is greater than the critical values.

Automatic control of bubble size in a laboratory flotation column

Gas dispersion properties play an important role on determining the metallurgical performance of flotation processes. Indeed, bubble surface area flux (Sb) has been found to correlate with the collection zone flotation rate constant and is therefore a potential variable to regulate in order to achieve a target performance. Sb can be calculated as a combination of two other gas dispersion properties, namely, gas superficial velocity and the Sauter bubble mean diameter. This communication describes the design and implementation of a feedback control system for the Sauter bubble mean diameter in an attempt towards controlling Sb. In order to track changes on bubble size, the Sauter bubble mean diameter was derived from an adaptive bubble size probability density function (PDF). A Gaussian mixture model was used to represent the bubble size PDF and its parameters were updated on-line and in real-time by a dedicated computer from sequentially incoming images. To improve bubble size controllability, a frit-and-sleeve sparging device was installed. This device allows modifying bubble size by manipulating the water flow rate circulating through a sleeve surrounding a porous ring (shear water). A Wiener model is used to represent the dynamic relationship between the shear water flow rate and the Sauter bubble mean diameter. A nonlinear control, based on the internal model control (IMC) structure, was designed and implemented. Tracking and regulation performance against gas velocity and unmeasured frother concentration variations were then successfully evaluated in a two-phase system.

Flotation studies on low grade graphite ore from eastern India

A low grade graphite ore from eastern India was beneficiated by flotation to improve its quality. The ore was composed of 87.80% ash and 8.59% fixed carbon. Primary coarse wet grinding (d80: 186μm) followed by rougher flotation in Denver flotation cell using diesel as collector and pine oil as frother yielded a rougher concentrate. Regrinding (d80: 144μm) of this rougher concentrate was opted for further liberation of graphite. It was followed by cleaning in laboratory flotation column. This combined process of relatively coarse primary grinding followed by regrinding and cleaning in flotation column resulted in final concentrate of 7.44% yield with 89.65% fixed carbon and 6.00% ash. This approach of two-stage grinding to recover the flake graphite at the coarsest possible grind can help to minimize grinding energy costs. A conceptual flow sheet which is cost effective was developed based on this methodology.

Prediction of Gas Holdup in a Flotation Column by Artificial Neural Network

Flotation column is one of the separation technologies investigated for removal of gangue mineral from coal fines. The effect of process variables, such as gas flow rate, feed flow rate, and frother dosage on gas holdup, were investigated. In this study, the measurement of gas holdup in presence of coal fines was carried out in a laboratory flotation column by a phase separation method. Detailed parametric study was performed to observe the effect of slurry concentration, gas flow rate, slurry flow rate, and frother concentration on gas holdup. An artificial neural network approach with three layers was systematically employed to predict gas holdup using some of the experimental results. The rest of the experimental results were successfully compared with the model predictions with less than 5% average error. In this work, emphasis was made on random selection of training data and small network. The developed trained network was also able to capture the non-linear prediction of gas holdup with new operating conditions that were not used in the training process and enhanced the physical understanding of the process.