Collision Efficiency Of Particles Research Articles

Fine and ultrafine flotation with the Concorde CellTM – A journey

The Concorde Cell is a high-intensity pneumatic flotation cell dedicated to fines and ultrafines. Its development by Professor Graeme Jameson and later on by Metso was driven by two considerations: a reduction in bubble size and an increase in the shear rate and energy dissipation rate increases the particle–bubble collision and attachment efficiencies of fine particles. The Concorde Cell is a technology which produces fine air bubbles with high carrying capacity, thereby allowing increased capacity per unit; smaller footprint and a greater recovery of valuable fine particles which can be lost in other flotation cells. In addition, with a smaller number of flotation stages or less recycle required to achieve the desired recoveries, operational expenditures like power, water and reagents can be reduced.In this article, the potential of the Concorde Cell is explored through six cases, from base metals to metallurgical coal. Its main operating parameters, Air-to-Pulp Ratio, froth washing, froth depth and tailings recycle ratio, are investigated and show that the metallurgical performance of the Concorde Cell follows expectations based on the literature. Amongst others, higher APR leads to higher mass pulls and higher recoveries but lower grades, deeper froth depth increases selectivity… But the data also demonstrate the typical complexity of flotation, whereby the negative impact of increased APR can be compensated with froth washing to maintain high grades at high recoveries for example.About ten industrial scale flotation projects worldwide are deploying the Concorde CellTM. The examples of a metallurgical coal washing plant in Australia and of a copper plant in Zambia prove the capacity of the Concorde Cell to better recover the fines and ultrafines while providing high selectivity. In the two case studies, the Concorde Cell outperformed the previous technology specifically on the finest fractions of the processed streams.

Experimental study on the effect of hydrodynamic conditions on flocculation and settling properties of fine-grain sediment

The flocculation of cohesive sediment particles is a function of the collision efficiency of sediment particles, which is mainly influenced by local flow hydrodynamics. A detailed study on local hydrodynamic characteristics in a novel stirred tank was done to measure the flow field of a turbulent flow. Instantaneous flow velocity fields were obtained and processed, and the average flow field was computed. Particle image velocimetry (PIV) results measuring the hydrodynamic characteristics (expressed as turbulent kinetic energy (TKE), local dissipation rate of TKE, velocity gradient or shear rate, turbulent shear stress, etc.) were analyzed and discussed. Particle tracking and automated image processing techniques were utilized to analyze the impact of the hydrodynamic conditions on the floc and settling properties. The differences in floc properties under the two turbulent shear states (i.e., steady-state (SS) and unsteady-state (US)) hydrodynamic conditions were examined. The results show that floc size and floc size distribution (FSD) of pure kaolin clay flocs are significantly influenced by the local dissipation rate of TKE and the turbulent intensity states. The floc size, df, 84, at local dissipation rate of TKE of 11 × 10−4, 29 × 10−4, 44 × 10−4, 83 × 10−4, and 142 × 10−4 m2/s3 is 97, 108, 107, 124, and 150 μm, respectively. It was found that by increasing the local dissipation rate of TKE, the FSD was skewed left. The US shear condition generated larger flocs (and in higher proportions) relative to the SS condition. The settling velocity and flux settling velocities of flocs formed under SS conditions are higher than those formed under US conditions. The fractal dimension of SS flocs also is higher than the fractal dimension of US flocs. These results highlight a significant impact of the states of hydrodynamics on floc properties such as floc size and FSD as well as the settling and morphological properties of flocs.

Determining collision efficiency in multi-bubble-particle systems in presence of turbulence

Bubble-particle collision efficiency in the existing flotation literature is determined in a single bubble-particle system assuming an ideal flow field in absence of any turbulence which does not fully represent reality of the actual flotation systems. To address this knowledge gap, in this study, we numerically computed collision efficiency in a multi-bubble-particle system (particle diameter dP = 30 µm, bubble diameter dB = 1 mm) using a two-way coupled Eulerian-Lagrangian 3D computational fluid dynamics model with different solids concentrations (0.385 %–3.080 %) at turbulence intensities (Ti = 0 %–20 %). Collision efficiency of multiple particles with a single central bubble was determined in presence of different configurations of neighbouring bubbles. Two simplified multi-bubble configurations – monolayer and multilayer comprising different number of bubbles/layer, were investigated. It was noted that side-by-side collisions in monolayer configuration did not occur because particles did not have sufficient time and kinetic energy to migrate to a surrounding bubble. On contrary, in multilayer configurations, particles traversing around a leading bubble had a chance to get entrained in the rear wake and collide with a trailing bubble which resulted in higher collision efficiency compared to monolayer configurations. Modelling showed that bulk energy dissipation rate increased with number of bubbles per layer, number of bubble layers, turbulence intensity and solids concentration. For the lowest and highest solids concentration, the optimal collision efficiency occurred at Ti = 20 %, while for the intermediate solid concentration, it occurred at Ti between 4 % and 11 %. In all cases, collision efficiency increased with the energy dissipation rate.

Nanobubble-enhanced flotation of ultrafine molybdenite and the associated mechanism

It is well known that fine and ultrafine particles are difficult to recover through froth flotation because of low bubble–particle collision efficiencies. In this study, we first explored the influence of particle size on the flotation behavior of molybdenite. To address the problem of low recovery of ultrafine molybdenite, nanobubbles (NBs) were introduced to promote the flotation efficiency, and the enhanced effect was investigated by micro-flotation experiments. The results of the flotation experiments showed that as the particle size decreased, the recovery of molybdenite decreased dramatically, especially for particles smaller than 23 μm. The introduction of NBs can significantly facilitate ultrafine molybdenite recovery. Moreover, the strengthening mechanism was uncovered through zeta potential measurements, turbidity tests, microscopic imaging, and wettability analysis. The analyzed results illustrated that the attachment of NBs to ultrafine molybdenite reduced the repulsive force between particles, facilitating particle aggregation and enhancing the hydrophobicity of the mineral surface, which further enhanced the bubble–particle collision and attachment, thereby improving the flotation recovery.

Effect of hydrothermal pretreatment on surface physicochemical properties of lignite and its flotation response

Flotation is the common method for fine low rank coal upgrading while it is always inefficient due to the rough surface morphology and rich oxygen-containing functional groups. Hydrothermal pretreatment is widely used in dewatering for low rank coal. The physicochemical properties of coal surface considerably vary following the hydrothermal pretreatment. In this study, hydrothermal pretreatments at different temperatures for different times were applied to enhance lignite flotation. The hydrothermal pretreatment at 200 °C can significantly improve the flotation efficiency, while the temperature raised to 300 °C, the flotation recovery did not increase or even decrease. Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Zeta potential, Brunauer–Emmett–Teller, and particle size distribution analysis were conducted to explain the underlying mechanism. The contents of pore and crack on coal surface considerably decreased and more fresh and smooth surfaces exposed following hydrothermal pretreatment, according to SEM analysis. The FTIR spectroscopy and XPS showed that the contents of oxygen-containing functional groups were decreased, while the hydrophobicity of lignite was increased following pretreatment. The decrease of Zeta potential also indicated the decrease of oxygen-containing functional groups and increase of hydrophobicity on coal surface. However, excessive temperature of hydrothermal pretreatment could increase the pore diameter and fine particle fraction, leading to a higher collector consumption and lower bubble–particle collision efficiency. Consequently, the flotation recovery was not improved or even decreased at the hydrothermal pretreatment temperature of 300 °C.

Numerical investigations into the effect of turbulence on collision efficiency in flotation

At present, the theoretical understanding of the collisional interaction between particles and a single bubble under well-defined laminar flow conditions is relatively sound. However, there is a lack of experimental data and predictions on details of the interaction processes in stirred turbulent conditions, as exists in mineral flotation cells. This paper investigates the effect of turbulence on the efficiency of collision between particles and bubbles from a fundamental point of view. A computational fluid dynamics method is used to establish a bubble–particle collision efficiency model system for turbulent flow. The 3D model is used to systematically study the effect of fluid turbulence on bubble–particle collision efficiency. In order to isolate the phenomena caused by turbulent flow physics, the study is conducted in two parts: large-scale turbulence and small-scale turbulence, which are respectively larger and smaller than the bubble size. Large-scale turbulence is assumed to increase the speed of bubbles relative to the slurry, and in most cases, increase in the speed increases collision efficiency, though in some cases the gravitational effect can cause decrease in overall efficiency. Small-scale turbulence is treated through the effect of fluctuations on particle trajectories. Again, such turbulence increases collision efficiency when the turbulent particle Stokes number is less than or about unity, but has little effect for Stokes number much greater than unity. An equation for the increase in efficiency in the limit of small Stokes number is obtained from the simulations.

Towards more predictive clarification models via experimental determination of flocculent settling coefficient value

Improved settleability has become an essential feature of new wastewater treatment innovations. To accelerate adoption of such new technologies, improved clarifier models are needed to help with designing and predicting improvement in settleability. In general, the level of mathematics of settling clarifier models has gone far beyond the level of existing experimental methods available to support these models. To date, even for simple one-dimensional (1D) clarifier models, no experimental method has been described for flocculent settling coefficient (rp). As a consequence, rp cannot be considered as a sludge characteristic and is used as a calibration parameter to achieve observed effluent quality. In this study, we focused on the development of an empirical function based on a simple and practical experimental approach for the calculation of the rp value from sludge characteristics. This approach provided a similar approach as currently taken for hindered settling coefficient calculations (Veslind equation) and allowed for the model to predict effluent quality, thus increasing the power of the 1D model. The threshold of flocculation (TOF), which describes the collision efficiency of particles, directly correlated with the effluent quality of the five tested activated sludge systems and was selected as experimental method. The proposed empirical function between TOF and rp was validated for four years of validating data with five different sludge types operated under different operational conditions and configurations. The good effluent quality prediction with this approach brings us one step closer in making the clarification models more predictive towards effluent quality and clarifier performance.

Agglomeration–Flotation of Finely Ground Chalcopyrite Using Emulsified Oil Stabilized by Emulsifiers: Implications for Porphyry Copper Ore Flotation

Flotation is the conventional method for processing porphyry copper deposits, one of the most economically important sources of copper (Cu) worldwide. The rapidly decreasing grade of this type of Cu ore in recent years, however, presents serious problems with fine particle recovery using conventional flotation circuits. This low recovery could be attributed to the low collision efficiency of fine particles and air bubbles during flotation. To improve collision efficiency and flotation recovery, agglomeration of finely ground chalcopyrite (CuFeS2) (D50 = 3.5 μm) using emulsified oil stabilized by emulsifiers was elucidated in this study. Specifically, the effects of various types of anionic (sodium dodecyl sulfate (SDS), potassium amyl xanthate (KAX)), cationic (dodecyl amine acetate (DAA)), and non-ionic (polysorbate 20 (Tween 20)) emulsifiers on emulsified oil stability and agglomeration–flotation efficiency were investigated. When emulsifiers were added, the average size of agglomerates increased, resulting in higher Cu recovery during flotation. This dramatic improvement in flotation efficiency could be attributed to the smaller oil droplet size in emulsified oil and their higher stability in the presence of emulsifiers. The utilization of emulsifiers during agglomeration–flotation not only lowered the required agitation strength for agglomeration but also shortened the agglomeration time, both of which made the process easier to incorporate in existing flotation circuits.

High air flotation efficiency of multiphase flow pump with the addition of dodecyl dimethyl benzyl ammonium chloride (DDBAC)

Abstract Recently, centrifugal multiphase pump–dissolved air flotation (CMP-DAF) has become an increasingly popular alternative to DAF that can achieve more stable performance and higher removal efficiency, and this method is widely used in sewage treatment. However, the nonuniformity of the bubble size and low adherence of the floc particles and bubbles, as well as the complicated raw water quality, pose great challenges to CMP-DAF, which does not meet the standards of water supply and drainage in practical use. In the present study, the surfactant dodecyl dimethyl benzyl ammonium chloride (DDBAC) was utilized as a flotation agent to further improve the flotation efficiency of the CMP-DAF process. DDBAC at a dosage of 0.2 mg/L was introduced to the air flotation of raw water to construct a flotation enhanced CMP air flotation system. The results showed that the average turbidity decreased to 0.433 ± 0.017 NTU, and effluent floc particles were present at 1,053 cnt/mL with an acceptable removal rate of 96.20%. In addition, 34.0% and 30.1% of UV254 and CODMn were removed, respectively. These results imply that DDBAC can increase the collision efficiency of bubble particles by reducing the diameter of the bubbles, which is conducive to forming larger flocs, and enhancing the shear resistance of the bubble–floc particles, thus improving the air flotation efficiency.

Effect of Bubble Surface Properties on Bubble–Particle Collision Efficiency in Froth Flotation

In research on the particle–bubble collision process, due to the adsorption of surfactants and impurities (such as mineral particles, slime, etc.), most studies consider the bubble surface environment to be immobile. However, in the real situation of froth flotation, the nature of the bubble surface (degree of slip) is unknown. Mobile surface bubbles increase the critical thickness of the hydration film rupture between particles and bubbles, and enhance the collision between particles and bubbles. Sam (1996) showed that when the diameter of the bubble is large enough, a part of the surface of the bubble can be transformed into a mobile state. When the bubble rises in a surfactant solution, the surface pollutants are swept to the end of the bubble, so when the bubble reaches terminal velocity, the upper surface of the bubble is changed into a mobile surface. This paper analyzes the collision efficiency and fluid flow pattern of bubbles with mobile and immobile surfaces, and expounds the influence of surface properties on collision efficiency.

Investigation of bubble-particle interactions in a mechanical flotation cell, part 1: Collision frequencies and efficiencies

In this study the influence of impeller speed and superficial gas velocity on hydrodynamic parameters, bubble-particle collision frequency and efficiency using pure quartz particles of different sizes in an aerated Denver flotation cell was investigated. Collision frequency in different parts of cell was calculated by local turbulent energy dissipation rate measurements. Collision efficiency due to gravitational, inertial, interceptional and turbulence mechanism was determined. The overall collision efficiency (Ec) was also investigated. The results showed that the bubble-particle frequencies for different particle sizes in the turbulent zone were 10–12 times higher than in the quiescent zone. Under various hydrodynamic conditions, minimum and maximum values of Ec were observed for −38 µm and −300 + 212 µm particles, respectively. The turbulence mechanism was the main collision mechanism for fine particles. The collision efficiency of coarse particles was mainly influenced by gravitational, inertial and interceptional mechanisms.

Analysis of dynamic interactions in a bubble-particle system in presence of an acoustic field

Use of an acoustic field in flotation is known to improve mineral recovery. However, studies in this area are rather limited and in general there is a lack of a mechanistic description of the collision and collection efficiency of particles in presence of an external acoustic field. This study aims to contribute to this knowledge gap by developing a simplified 3D numerical model of single bubble-particle interactions based on a discrete element method (DEM) based approach. Volume mode oscillatory behaviour of the bubble was modelled within the theoretical spherical shape limit (0.1 ≤ Bo ≤ 0.5) using 1D Rayleigh-Plesset equation in a quiescent liquid medium and one-way coupled to particle motion obtained through DEM. Interaction dynamics were simulated for various operating conditions involving three parameters, namely oscillation amplitude ratio (ε ≤ 0.1), excitation frequency (below and above resonance frequency) and bubble-particle surface-to-surface distance (∼1.0 to 10.6% of bubble radius). Regime maps were constructed to establish suitable combinations of these three operating parameters to represent the collision and attachment behaviour of a particle with the oscillating bubble. While conventional flotation models predict particle collision efficiency based on the nearest streamline adjacent to the bubble surface, application of an acoustic field on a bubble was shown to incur collision with a particle in the far field away from the interface due to oscillatory motion. It was noted that although such collisions occurred in the below-resonance-frequency regime (∼35 to 79 Hz), particle attachment did not occur due to weakening of the attractive capillary force. In the above-resonance-frequency regime (3.61–14.4 kHz), however, particle attachment was predicted and attachment probability increased in the vicinity of the bubble resonance frequency.

Transported probability density function based modelling of soot particle size distributions in non-premixed turbulent jet flames

The need to establish actual particle size distributions (PSDs) of soot emissions from the nanoscale upwards, along with the current global indicators based on soot mass, stems from increasingly strict regulatory demands. In the current work, a mass and number density preserving sectional model is coupled with a transported probability density function (PDF) method to study the evolution of soot PSDs in two non-premixed turbulent jet flames at Reynolds numbers of 10,000 and 20,000. The transported PDF approach is closed at joint-scalar level and includes mass fractions of gas phase species, soot sections, as well as enthalpy, leading to a fully coupled 78-dimensional joint-scalar space, treating interactions between turbulence and gas phase/soot chemistry as well as radiation without further approximation. The gas phase chemistry features 144 reactions, 15 solved and 14 steady-state species and an acetylene-based soot inception model is calibrated using comprehensive detailed chemistry up to pyrene and applied to a well-stirred/plug flow reactor configuration. The derived nucleation rate is subsequently applied in the turbulent flame calculations. Soot surface growth is treated via a PAH analogy and oxidation via O, OH and O2 is accounted for. The sectional model features 62 sections covering particle sizes in the range 0.38 nm ≤ dp ≤ 4.4 µm and includes a model for the collision efficiency of small particles ( ≤ 10 nm) based on the Lennard–Jones potential. The computed results reproduce the evolution of the PSDs with encouraging accuracy. It is also shown that the distribution of soot in mixture fraction space is affected by local extinction events.

Estimation of flotation rate constant and collision efficiency using regression and artificial neural networks

ABSTRACTThe effects of particle characteristics and hydrodynamic conditions on the flotation rate constant (k) and bubble–particle collision efficiency (Ec) of pyrite and chalcopyrite particles were investigated. Experimental results showed that k increases with increase of bubble surface area flux (Sb) and Ec. Artificial neural network (ANN) and multivariable linear regression procedures were used to predict both k and Ec based on the particle characteristics and hydrodynamic conditions. Multivariable linear regression resulted in R2 of 0.6 and 0.93 for k and Ec, respectively. Using an ANN model, R2 as high as 0.98 was achieved in modeling the Ec with regard to the available parameters. The proposed ANN model can be reliably used to determine both k and Ec parameters in froth flotation.

Innovations in the flotation of fine and coarse particles

Research on the mechanisms of particle-bubble interaction has provided valuable information on how to improve the flotation of fine (<20 µm) and coarse particles (>100 µm) with novel flotation machines which provide higher collision and attachment efficiencies of fine particles with bubbles and lower detachment of the coarse particles. Also, new grinding methods and technologies have reduced energy consumption in mining and produced better mineral liberation and therefore flotation performance.

Agglomeration Kernel of Bipolar Charged Particles in the Presence of External Acoustic and Electric Fields

ABSTRACTA theoretical study on the agglomeration kernel of bipolar charged particles in external fields is presented in order to investigate particle agglomeration under the combined effects of a direct-current (DC) electric field and an acoustic field, while taking into consideration particle collision efficiency. Three agglomeration kernels are considered in our model, namely Coulomb agglomeration, electric agglomeration, and acoustic agglomeration. The model performance is validated by comparing its predictions with the available exact solutions of peculiar cases. Our results indicate that the collision efficiency increases when an electric field is applied simultaneously with an acoustic field. The size of the total agglomeration kernel formed by the three processes listed above increases with the external electric field intensity and the sound pressure level (SPL). Based on this model which includes collision efficiency, the electric and acoustic agglomeration kernels both increase rapidly as the agglomeration nucleus particle size increases, eventually approaching saturation for large diameters. On the other hand, the total agglomeration kernel decreases as the sound frequency increases within the investigated range. The agglomeration kernels of particles with different relative dielectric constants, such as fly ash particles, solid carbon particles and water droplets, are investigated as well.

Direct numerical simulations of collision efficiency of cohesive sediments

A clear understanding of the collision efficiency of cohesive sediment particles is critical for more accurate simulation of the flocculation processes. It is difficult, if not impossible, to carry out laboratory experiments to determine the collision efficiency for small particles. Direct Numerical Simulation (DNS) is a relatively feasible approach to describe the motion of spherical particles under gravity in calm water, and thus, to study the collision efficiency of these particles. In this study, the Lattice Boltzmann (LB) method is used to calculate the relative trajectories of two approaching particles with different ratios of sizes and densities. Results show that the inter-molecular forces (i.e., van der Waals attractive force, electrostatic repulsive/attractive force, and displacement force), which are usually neglected in previous studies, would affect the trajectories, and thus, lead to an overestimation of the collision efficiency. It is found that to increase the particle size ratio from 0.1 to 0.8 only slightly increases the collision efficiency, since the force caused by fluid-solid interaction between these two particles is reduced. To increase the submerged particle density ratio from 1 to 22, however, would significantly decrease the collision efficiency. Earlier analytical formulations of collision efficiency, which only consider the effects of particle size ratio, have significantly overestimated the collision efficiency (change from 0.01 to 0.6) when the particle size ratio is around 0.5.

Effect of bubble size and velocity on collision efficiency in chalcopyrite flotation

In flotation processes, bubble diameter (db), bubble velocity (vb), and turbulence are the key factors involved in particle-bubble interactions. The collision efficiency (EC) is used as an indicator to assess the extent of these interactions. In this work, the bubble surface is assumed mobile with potential flow conditions dominating the particle-bubble collision efficiency. The collision probability has been determined by Schulze and Generalized Sutherland Equation (GSE) models in the particle size range of 1–100 μm. Bubble diameters of 0.08, 0.12, and 0.15 cm and bubble velocities of 10, 20 and 30 cm/s were selected to study the flotation of chalcopyrite. The results reveal that the collision efficiency of ultra-fine particles (1–10 μm) is generally improved with bubbles of finer sizes, e.g. db = 0.08 cm compared to those of larger sizes, i.e. db = 0.12 and db = 0.15 cm. Also, in the same particle size range, EC decreases with increasing the bubble velocity. The best agreement between Schulze and GSE models for ultra-fine particles at all bubble sizes is achieved at the bubble velocity of 30 cm/s. The maximum EC of chalcopyrite (0.12) using the GSE model is found to occur for coarser particles of 70–100 μm in size at bubble conditions of vb = 30 cm/s and db = 0.12 cm. Results reveal that for a given bubble diameter increasing the bubble velocity from 10 to 30 cm/s makes the inertial force more effective on finer particles. A detailed interpretation of the effect of bubble diameter and its velocity on particle-bubble interaction of chalcopyrite is discussed from a theoretical point of view.

Effect of surface slip on the relative motion and collision efficiency of slippery spherical particles

The motion of a pair of spherical particles suspended in a viscous fluid is considered under conditions of Stokes flow. The particle surfaces allow the fluid to slip according to the Navier–Maxwell–Basset law. Batchelor and Green׳s mobility functions determining the relative particle motion during interception are computed with high accuracy using a boundary-integral method, and their dependence on the slip coefficient is discussed. The numerical results confirm that particle collision in the presence of surface slip occurs with a finite impact velocity. As the slip coefficient decreases, and thereby the particle surfaces become increasingly slippery, the collision efficiency in uniaxial elongational and simple shear flow increases monotonically from zero for no-slip surfaces to a finite limit for perfectly slippery surfaces.

An Early Stage of Aerosol Particle Transport in Flows Past Periodic Arrays of Clear Staggered Obstructions: A Computational Study

Transport of aerosol particles is a fundamental phenomenon in many environmental and industrial applications. Among the several computational fluid dynamical schemes used to study this problem, the lattice Boltzmann methods (LBMs) have shown great promise. Using a 2-D LBM model coupled with a Lagrangian formalism, this study investigates an early stage of particle–surface collisions in a free-stream flow over a semi-infinite array of staggered obstructions at operating conditions of woven-wire screens. After an initial validation of the model, the particle–surface collision efficiency with different diameters is then examined as functions of the number of staggered obstructions, obstruction morphology, and separation distance between two center points of obstructions. Particle motion mechanisms include drag, lift, and Brownian forces. Enhanced collision efficiency of particles to obstructions due to the presence of multiple staggered cylindrical obstructions is identified and highlighted. Based on these i...