CFD validation for flyash particle classification in hydrocyclones

The investigation pertains to establishing a simulation methodology for understanding the flyash classification characteristics of a 76 and 50 mm diameter hydrocyclone where the work was carried out using commercially available CFD software. Comparative results on the simulated and experimental water throughput, split values are presented. Results indicted that there is a good match in water split between the experimental and simulated values with error values below 10% at different hydrocyclone designs. Further a discussion is made on the flow features at comparable ratio of cyclone diameter to spigot opening in the 76 and 50 mm designs. The vertical core region around the cyclone axis having static pressure equal to or below the atmospheric pressure is examined to be increasing in diameter from bottom of the spigot opening till the interface where vortex finder joins the main cylindrical cyclone body and remains more or less similar at the vortex finder outlet. The diameter of this zone at the spigot outlet is 0.6 and 5.4 mm at 3.2 and 9.4 mm spigot openings in case of 50 mm diameter hydrocyclone. The diameter of the core at spigot outlet is found to be around 9.2 and 11 mm at 15 and 20 mm for spigot openings in case of 76 mm diameter hydrocyclone.Classification of flyash particulates is simulated through discrete phase modeling using particles injection technique and the simulated results are further validated with suitably performed experiments. With 50 mm diameter hydrocyclone, reasonable predictions are observed at 9.4 mm spigot opening. Considerable deviation in particle distribution points with this hydrocyclone is observed at narrowest spigot diameter of 3.2 mm. The simulated values of d₅₀ in case of 50 mm diameter hydrocyclone are 8 and 10 μm at 9.4 and 3.2 mm diameter spigot openings. Better predictions are obtained with 76 mm diameter hydrocyclone at both 10 and 15 mm diameter spigot openings. Similarly, the simulated d₅₀ values are 14 and 20 μm at 15 and 10 mm diameter hydrocyclones. Possible reasons for deviations in the results relating the spigot opening, solids concentration at the underflow and in turn role of slurry viscosity on the air core diameter are proposed.     

An experimental study of size segregation in a batch jig

This paper presents the findings of an experimental investigation into size segregation in a 200 mm diameter batch jig that was conducted to provide background information for the development of a stratification model that accounts for the effects of both particle size and particle density on separation performance. The investigation focused on a simple system in which the only variable was particle size; i.e. binary systems involving 50% mixtures of two differently sized spherical glass beads from 14 mm to 4 mm diameter in 2 mm increments. The density of all beads was 2520 kg/m³. The study revealed four different types of size segregation patterns that may occur in a jig bed, and gave some indication of the factors that determine the transition from one type to another under the specific experimental conditions of the test work carried out. It also developed a conceptual picture of the dynamics affecting size segregation in batch jigs operated under equilibrium conditions and highlighted three mechanisms: the interplay between stratification and dispersive processes; interstitial tricking of smaller particles; and convective remixing of smaller particles in the bed. Interpretation of the findings suggests a compositional regime where one segregation mechanism dominates, i.e. the stratification/dispersion interplay, and that our modelling efforts should concentrate on this mechanism and this regime.     

Using a high pressure hydrocyclone for solids classification in the submicron range

A 10 mm hydrocyclone was operated using a barite suspension with a maximum particle size of d^max = 7 μm. The test rig was equipped with a piston diaphragm pump for pressures up to 60 bar. At 40 bar and 20 °C, cut sizes d₅₀ were obtained down to 0.7 μm; increasing the temperature to 50 °C resulted in d₅₀ values down to 0.5 μm for a throughput of 0.6 m³/h. Another experiment was conducted at 40 bar using a batch hydrocyclone technology. Only the overflow was recirculated to the feed box, whereas the underflow was discharged via a collection box. Increasing the number of recirculations increased the separation of fines in the submicron range. The results showed that after 20 min particles with d^max = 1 μm were obtained in the hydrocyclone overflow. After 120 min, the particles size distribution had a d^max = 0.5 μm and a mean size of d⁵⁰ = 0.2 μm. This procedure requires high energy consumption and is thus suitable only for fractionating small quantities of particles in the submicron range.     

The effect of hydrodynamic parameters on probability of bubble–particle collision and attachment

In this study the dependence of the impeller speed on the particle size variation was investigated on the quartz particles using laboratory mechanical flotation cell. Maximum recovery was obtained at 1100 rpm. For either more quiescent (impeller speed <900 rpm) or more turbulent (impeller speed >1300 rpm) conditions, flotation recovery decreased steadily. Furthermore, amount of collision probabilities is calculated using various equations. According to this study, maximum collision probability was obtained around 48.35% with impeller speed of 1100 rpm, air flow rate of 15 l/h and particle size of 545 μm and minimum collision probability was obtained around 2.43% with impeller speed of 700 rpm, air flow rate of 15 l/h and particle size of 256 μm. Maximum attachment probability was obtained around 44.16% with impeller speed of 1300 rpm, air flow rate of 75 l/h and particle size of 256 μm. With using some frothers such as poly propylene glycol, MIBC and pine oil, probability of collision increased, respectively. Maximum collision probability was obtained around 65.46% with poly propylene glycol dosage of 75 g/t and particle size of 545 μm.     

Fluid dynamics based modelling of the Falcon concentrator for ultrafine particle beneficiation

Enhanced gravity separators are widely used in mineral beneficiation, as their superior gravity field enables them to separate particles within narrow classes of density and size. This study aims to shed light on the Falcon concentrator’s ability to separate particles within size and density ranges lower than usual, say 5–60 μm and 1.2–3.0 s.g. respectively. As differential particle settling is expected to be the prevailing separation mechanism under such conditions, this study presents the workings of a predictive Falcon separation model that embeds phenomenological fluid and particle flow simulation inside the Falcon’s flowing film. Adding to the novelty of modelling the Falcon concentrator using a fluid mechanics approach, one point of practical significance within this work is the derivation of the Falcon’s partition function from fluid flow simulation results.     

Tangential velocity measurements in a conical hydrocyclone operated with a fibre suspension

The tangential velocity flow field in a conical hydrocyclone was measured using a self-cleaning pitometer. The influence of pulp fibre concentration on the tangential velocity profile was of particular interest.The measurements showed that the pulp fibres had a strong influence on the tangential velocity profile. When operating the hydrocyclone with pure water, the velocity profile showed the typical combination of free-vortex-like rotation close to the hydrocyclone wall and solid-body-like rotation in the proximity of the hydrocyclone axis. This profile changed significantly when pulp fibres were added. At a low fibre concentration (1.2 g/l), the fibres smoothened the transition between the free-vortex-like and the solid-body-like region of the velocity profile. The location of the maximum tangential velocity moved to a larger radius. At higher fibre concentration (7.5 and 11 g/l), the free-vortex-like behaviour in the outer area was virtually suppressed. Due to networking, the fibre suspension in the entire hydrocyclone behaved as a solid body.The radial acceleration profile and tangential rate-of-strain profile were determined. Based on these profiles, a hypothesis was proposed explaining the well-known observation that fractionation efficiency decreases significantly at higher fibre concentrations.     

Characterizing hydrocyclone performance for grit removal from wastewater treatment activated sludge plants

Typically, 15–45% of the mixed liquor (sludge) in biological wastewater treatment plants (WWTPs) consists of inorganic (fixed) suspended solids. A portion of these inorganic compounds is grit (sand) originating from the influent. Grit accumulation impacts WWTP design and operating costs as these unbiodegradable solids reduce the effective treatment capacity of the bioreactor and other unit operations that must be sized to carry this material.The goal of this study was to characterize the performance of a hydrocyclone to selectively separate grit from activated sludge. Laboratory experiments were conducted with a 13 mm diameter Krebs hydrocyclone treating sludge from eight WWTPs. Reduced efficiencies of 17 ± 7% on fixed suspended solids and 9 ± 6% on volatile suspended solids were obtained. Grade efficiency curves enabled the development of a modified definition for cut size useful for this application. The characterization of hydrocyclone performance for grit removal from activated sludge will enable modelling of the process for integration into wastewater treatment simulators used for process performance prediction and design.     

Experimental study of flow patterns in deoiling hydrocyclone

Using a two-component laser Doppler velocimeter, the axial velocity, tangential velocity and fluctuation velocity were measured in a 35-mm deoiling hydrocyclone. Air core occurs in the hydrocyclone when inlet flow rate is more than 2.00 m³/h. The fluctuation velocities with air core are greater than without air core. The velocity magnitudes are dependent on the inlet flow rate and the flow character is not change by changes in the inlet flow rate in hydrocyclone. The velocity fluctuations are greater near the core and near the wall. The results show that air core in the hydrocyclone will arose turbulence fluctuation. These studies are helpful to understand the separation mechanism of deoiling hydrocyclone.     

Gravity separation of coarse particles using the Reflux Classifier

A comprehensive study examining the potential of the Reflux Classifier to be applied to the beneficiation of coarser coal up to 8 mm in size was undertaken. It was demonstrated that efficient combustible recovery and control of the separation density to target low ash products could be achieved. The major finding from the study was the critical importance of providing sufficient fluidization water, though beyond the critical level the process was largely insensitive to the fluidization rate. It was concluded the required fluidization velocity is nominally 10 m/h per mm of top-size, hence for a nominal 4 mm top size the required velocity is 40 m/h. In an extended campaign the control of the process was investigated by varying the set point density from high to low levels and then returning the process to the original settings, and demonstrating a return to the original separation. Further analysis was conducted to determine the partition curves and the shift in the separation density with particle size. The variation in the D₅₀ with particle size approaches a level that is independent of the particle size. Previous data (Galvin et al., 2002, Galvin et al., 2004) covering particles up to 2 mm in size are consistent with the results from this study, involving feeds with top sizes of 4 mm and 8 mm. Beyond a particle size of 2 mm the Ep is typically less than 0.05 and approaches about 0.03 as the particle size increases to 8 mm.     

Determination of ferrosilicon medium rheology and stability

Ferrosilicon (FeSi) has a fast settling rate in dense suspension, attributed to its very high solids density, coarse particle size, more spherical particle shape and low medium viscosity. The fast-settling nature in dense suspension is a challenge to acquire reliable rheological data. A testing rig was set up to maintain a constant medium density during the rheology measurement by circulating the medium to keep FeSi particles well suspended. A rotational rheometer with a modified concentric bob-cup measuring system was incorporated in the testing rig. Taylor number (T_a) was calculated and a threshold T_a = 41.3 was used to identify the onset of unstable laminar flow owning to the Taylor vortices and turbulence formation in the measuring system. The data with T_a > 41.3 were excluded in the determination of rheological flow curves. Evaluation of the modified measuring system with Newtonian silicone oils of known viscosities confirmed that the system can produce true flow curves over the entire tested shear rate range for a stable laminar flow (T_a < 41.3). This data reduction procedure was applied to the FeSi medium rheology measurement. It demonstrates that FeSi medium exhibits a pseudoplastic trend with a yield stress. The apparent viscosities were calculated at two shear rates, 10 s⁻¹ that is assumed to be a typical shear rate in dense medium bath separators and 75 s⁻¹ for dense medium cyclones. A characteristic curve between apparent viscosity and medium density was established, which can be used in FeSi selection for dense medium separation. Medium stability was determined from the FeSi sedimentation rate measurement. It shows that medium stability was closely correlated with medium viscosity. A trade-off between stability and viscosity for optimal dense medium separation should be established.     

Application of closely spaced inclined channels in gravity separation of fine particles

This paper is concerned with the gravity separation of fine particles in a Reflux Classifier, a fluidized bed device with a system of parallel inclined channels located above. A significant advance is reported here over what was previously possible, through the application of a recent discovery described by Galvin et al. (2009). By using closely spaced inclined channels it is possible to achieve significant suppression of the effects of particle size, and hence produce a powerful separation on the basis of density. Experimental work was undertaken on the continuous steady state separation of coal and mineral matter, with a very narrow channel spacing of 1.77 mm used to process a feed finer than 0.5 mm in diameter, and a channel spacing of 4.2 mm used to process coarser feeds finer than 2.0 mm. These results are compared with previous findings reported in the literature for wider channels. The gravity separation performance is shown to be remarkably high, with a significant reduction in the variation of the separation density with particle size, and a significant reduction in the Ecart probable error, E_p. For example, over the particle size range 0.25–2.0 mm the composite E_p for the size range decreased from 0.14 for the wide channels used in previous studies to a typical level of 0.06 for the closely spaced inclined channels used in the present study. The separation performance was also shown to be insensitive to feed pulp density and feed solids throughput over a very broad range.     

Speed analysis of quartz and hematite particles in a spiral concentrator by PEPT

Activated quartz and hematite tracers between 1180 and 1700 μm in diameter were tracked in two sections of the first two turns of a spiral concentrator to ascertain information on valuable and gangue particle motion. The tracking took place while the tracer was flowing in a 20% solids by mass iron ore slurry. The direct activation of mineral particles, combined with the use of an adjustable height circular assembly of modular positron emission particle tracking detectors, has made this tracking possible. The tracer particle trajectories and speeds are presented in this paper. An early separation into two streams (concentrate or loss to tailings) can be seen for certain runs of the hematite tracers. Tracers speeds and radial position from the centre of the spiral give more details about the presence of the slurry film’s secondary flow in the middle zone of the spiral trough. Particle inward and outward migration speed in this secondary circulation is presented. The speed of this migration is approximately 0.12 m/s inward and 0.15 m/s outward for the set-up used.     

Density based separations in the Reflux Classifier with an air–sand dense–medium and vibration

The Reflux Classifier is a device consisting of parallel inclined channels above a fluidized bed. Water-based versions of the system have been successfully employed in industry for gravity separation of ?2 + 0.25 mm coal and mineral matter. In this study an air-fluidized system was investigated using a single 2 m long inclined channel with 100 mm wide channels and 20 mm perpendicular spacing. Sand (?355 + 125 micron) was used as a dense–medium and vibration at two distinct levels was used to improve fluidization stability. Tracer particles of ?6.35 + 1.00 mm nominal diameter and 1300 to 2400 kg/m³ density were used to study the effects of the vibration energy and vibration direction on the separation efficiency. The device was able to separate coal particles from a maximum of 8 mm down to 1 mm on the basis of density; hence this method has potential for industrial application. Results were analysed using a simple 2-parameter dispersion–convection model. Dense particles had negative slip velocities, low-density particles had positive slip velocities and the slip velocities were proportional to particle diameter.     

Sand production with VSI crushing and air classification: Optimising fines grading for concrete production with micro-proportioning

Crushed sand fines have a pronounced effect on fresh concrete rheological properties, which can be controlled through a concrete micro-proportioning approach, i.e. optimisation of the particle size distribution (PSD) in the very fine range of the grading (⩽250 μm). The paper describes a study where possibilities of producing crushed sand by combining high-speed (70 m/s) vertical shaft impact (VSI) crushing and static air classification are explored to enable the micro-proportioning approach in full-scale aggregate and concrete production. In addition, the effect of rock resistance to fragmentation (crushability) on the shape improvement and fines generation during high-speed VSI crushing is experimentally studied.The VSI crushing experiment results show that an acceptable level of crushed sand particle (1.25–8 mm) equi-dimensionality (flakiness index lower than 5–8%) can be achieved for all processed feeds, regardless of the parent rock crushability or initial particle shape. The amount of fines smaller than 63 μm and 125 μm generated during high-speed VSI crushing is very strongly governed by the resistance to fragmentation of the processed rock materials. Analysis of the air classification results show possibilities of modelling a variety of different crushed sand fine particle (⩽250 μm) grading curves independently of the rock type, amount of fines or grading of the crushed sand product after the VSI crushing.     

Modeling breakage rates in mills with impact energy spectra and ultra fast load cell data

A new era in modeling particle size distribution in grinding mills started at the beginning of 2000s. A direct estimation of breakage parameters became possible via computation of collision energy by discrete element method (DEM) and material breakage data.The material breakage data can be obtained for primary modes of breakage. In this study, impact and abrasion are assumed to be the primary modes of particle breakage, which are readily studied in the laboratory. The impact breakage mode is studied in a drop-weight apparatus and in a specialized device known as the ultra fast load cell. The abrasion mode of breakage is studied in a laboratory scale ball mill. Next, the particle breakage versus energy data is converted into breakage rates via impact energy spectra of the grinding mill computed by a DEM code. The fundamental material breakage information is converted into energy based breakage distribution function.The verification of the modeling concepts is shown for a 90 cm laboratory scale ball mill. In the batch mill, approximately a 10 kg mass of limestone in the 30 mm size is ground with around 100 kg of 50 mm steel ball charge. The breakage rate and the breakage distribution functions constitute the parameters of the energy based batch population balance model. It is shown that accurate particle size distribution predictions are possible with this modeling approach for different grinding regimes.     

Combining Positron Emission Particle Tracking and image analysis to interpret particle motion in froths

Previous research into particle motion in the froth zone has focussed on constructing detailed CFD models that describe the behaviour of particle classes with different properties; density, size and hydrophobicity. These models have been reasonably successful in predicting trends in the separation behaviour and how it can be manipulated. Models of separation sub-processes cannot readily be verified experimentally due to the opacity and fragility of froth systems.Positron Emission Particle Tracking (PEPT) can be applied to particles in froth flotation systems to observe the behaviour of individual particles in a mixed particle–liquid–gas system. However, measuring the particle position alone is not adequate as its behaviour is also affected by instantaneous froth events such as bubble coalescence. To link the observed particle behaviour to the froth behaviour requires multi-modal measurements. Video footage of a rising foam column was recorded simultaneously with PEPT data, so that the PEPT tracer trajectory could be explained in terms of foam structure and events. A time weighting function of cubic splines with kernel width 200 ms was used to remove the effects of signal noise. An ascending 70 μm hydrophilic tracer accelerated within vertical Plateau borders and decelerated in Plateau borders angled away from vertical. The tracer trajectory showed velocity peaks and troughs when it was contained in nodes in a rising foam. When the tracer descended within a foam showing convective roll, coalescence events and subsequent foam deformation directly influenced the tracer trajectory.     

Exploratory modelling of grinding pressure within a compressed particle bed

With increasing industry interest in high pressure roll grinding (HPGR) technology, there is a strong incentive for improved understanding of the nature of grinding pressure that exists in the interior of a compressed particle bed. This corresponds to the crushing region of the HPGR. The relationship between applied pressure (stress) to the particle bed and induced pressure (stress) within particles and at contact points between particles is of particular interest. A detailed parametric investigation is beyond the scope of this exploratory paper. However, this exploratory investigation does suggest some interesting behaviour.The compressed particle bed within an 80 mm diameter piston has been modelled using Particle Flow Code for three dimensions. PFC3D is a discrete element code. The total number of simulated particles was 1225 and 2450 for two beds of different thickness. Particle diameters were uniformly distributed between 4 and 4.5 mm. The results of the simulations show that stress intensity within the simulated particle beds and within the observed particles increased with increase of the applied stress. The intensity of the average vertical stress in the selected particles tended to be comparable with the intensity of the pressure applied to the surface of particle bed and was only occasionally higher. However, the stress at contact points between particles could be several times higher. In a real crusher, such high stress amplification at contacts will quickly decrease due to local crushing and a resultant increase the size of the contact area. Therefore, its significance is likely to be relatively small in an industrial context.The modelling results also suggest that failure within the particle bed will progress from the crushing surface towards the depth of the bed.     

CFD simulation of a centrifugal air classifier used in the aggregate industry

The aggregate industry in Sweden is investigating methods to improve the quality of manufactured sand (aggregate smaller than 2 mm produced by crushing rock) for concrete production. A common way to improve the shape of the particles is to use a vertical shaft impact (VSI) crusher. However, the crushing process creates a large amount of fines (particles smaller than 63 μm) that are not desirable in concrete mixes. The aggregate industry in Sweden is therefore investigating methods to reduce the amount of fines produced by the manufacturing of sand. One method being investigated is air classification.A centrifugal air classifier used in the aggregate industry was investigated using computational fluid dynamics (CFD) to improve the understanding of the influence of the geometric design of the classifier on the cut size and the resulting particle size distribution. Simulations were performed with a CFD model using an Euler–Lagrange approach. The simulation results show that the classification results are affected by air flow velocity, particle shape, particle size, the geometry of the air classifier and turbulence in the air flow.     

Acid leaching and rheological behaviour of a siliceous goethitic nickel laterite ore: Influence of particle size and temperature

This study investigates the isothermal, batch, H₂SO₄ acid leaching behaviour of siliceous goethitic (SG) nickel (Ni) laterite ore and its links to pulp rheology. Specifically, the effect of feed ore particle size (−0.2 vs −2.0 mm), leaching temperature (70 vs 95 °C) and pulp rheology on Ni and pay metal, cobalt (Co) extraction kinetics and yield was studied for 4 h on 40 wt.% solid dispersions at pH 1. The leaching behaviour was distinctly incongruent, reflecting the disproportionate proliferation of major gangue mineral’s constituent elements (e.g., Fe, Al, Mg, Na, Si) alongside Ni and Co in the pregnant leach solution. At 70 °C, Ni/Co extraction rates were notably lower (<20%) in contrast with 95 °C where a significant increase in Ni/Co extraction to 78/77% and 74/77%, respectively, for the −0.2 and −2.0 mm feeds occurred. The slurries displayed a non-Newtonian, shear thinning Bingham plastic rheological behaviour of which the viscosity and shear yield stress increased markedly in the course of 4 h leaching. The pulp viscosity and shear yield stress were greater at lower temperature than at higher temperature and they were also greater in slurries with finer than coarser feed particles. The dynamic pulp rheology, however, had no marked effect on the overall Ni/Co extraction rates. Whilst the feed ore particle size had no remarkable impact on overall Ni/Co extraction, it led to noticeably higher acid consumption and enhanced slurry rheology in the finer sized ore. The mechanism of leaching the SG ore followed a two-stage, first order chemical reaction-controlled shrinking core model, the kinetics of which gave higher rate constants and lower activation energies for the release of Ni, Co, Fe and Mg in the first stage. A faster leaching process involving more reactive minerals during the first 30 min is envisaged to be followed by leaching of the more refractory minerals.     

Single and binary component sorption of the fission products Sr2+, Cs+ and Co2+ from aqueous solutions onto sulphate reducing bacteria

This study investigates the removal of the fission products Sr²⁺, Cs⁺ and Co²⁺ in single and binary metal solutions by a sulphate reducing bacteria (SRB) biomass. The effect of initial concentration and pH on the sorption kinetics of each metal was evaluated in single metal solutions. Binary component equilibrium sorption studies were performed to investigate the competitive binding behaviour of each metal in the presence of a secondary metal ion. Results obtained from single metal equilibrium sorption studies indicated that SRB have a higher binding capacity for Sr²⁺ (q_max = 416.7 mg g^?1), followed by Cs⁺ (q_max = 238.1 mg g^?1), and lastly Co²⁺ (q_max = 204.1 mg g^?1). Among the binary systems investigated, Co²⁺ uptake was the most sensitive, resulting in a 76% reduction of the sorption capacity (q_max) in the presence of Cs⁺. These findings are significant for future development of effective biological processes for radioactive waste management under realistic conditions.