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IIT Hyderabad

Multi-phase simulations of turbulent driven flow in a dense medium cyclone with magnetite medium have been conducted in Fluent, using the Algebraic Slip Mixture model to model the dispersed phases and the air-core, and both the Large Eddy Simulation turbulence model (LES) and Reynolds Stress Models (RSM) for turbulence closure. The predicted air-core shape and diameter were found to be close to the experimental results measured by gamma ray tomography. It is possible to use the LES turbulence model with ASM multi-phase model to predict the air/slurry interface accurately. Multi-phase simulations (air/water/medium) show appropriate medium segregation effects but over-predict the level of segregation compared to that measured by gamma ray tomography near the wall. This is believed to be because of unaccounted back-mixing of the dispersed phase due to turbulence in the basic Algebraic Slip Mixture model. The predictions of accurate axial segregation of magnetite medium are investigated using the LES turbulence model in conjunction with the multi-phase mixture model and viscosity corrections according to the feed particle loading factor. At higher feed densities the agreement between the Dunglison [Dunglison, M.E., 1999, A general model of the dense medium cylone, PhD thesis, JKMRC, University of Queensland] correlations and experimental measurements and the CFD is reasonably good, but the overflow density is lower than the model predictions. It is believed that the excessive underflow volumetric flow rates are responsible for under prediction of the overflow density. The effect of size distribution of the magnetite has been fully studied. As expected, the ultra-fine magnetite sizes (i.e. 2 and 7 microns) are distributed uniformly throughout the cyclone. As the size of magnetite increases, more segregation of magnetite occurs close to the wall. © 2006 Elsevier B.V. All rights reserved.

Department of Chemical Engineering

World University root node

International Journal of Mineral Processing

Prediction of magnetite segregation in dense medium cyclone using computational fluid dynamics technique

Numerical simulations of changes in feed medium solids on dense medium cyclone performance were performed using a multi-phase mixture CFD (Computational Fluid Dynamics) model for medium and air-core coupled with Lagrangian model for coal particles for a 350mm DSM cyclone. The turbulence was resolved using Large Eddy Simulation (LES). The mixture model considered the interactions between water and solid phases in terms of hindered settling, lift and Bagnold forces at high feed medium solid loadings. The medium properties were modified by changing the particle size distribution and concentration. Three different medium sizes (ultrafine, superfine and fine) were used in the simulations. The effect of medium stability and rheology on DMC performance is related to feed medium size in terms of density differential and medium segregation. The simulations predicted low Ep (Ecort probability) values with finer medium which gives high separation efficiency on density. A reduction in cyclone efficiency observed for a given feed medium solids distribution at higher feed medium concentrations due to an increase in slurry viscosity.

Proceedings of the 16th Australasian Fluid Mechanics Conference, 16AFMC

Numerical analysis of the changes in dense medium feed solids on dense medium cyclone performance

A computational fluid dynamics (CFD) model of the dense medium cyclone (DMC) has been developed, using Fluent, by coupling component models for the air-core, the magnetite medium and coal particles. Simulations of turbulent driven flow in a dense medium cyclone with magnetite medium showed that the predicted air-core shape and diameter were close to experimental results measured by gamma ray tomography. Multiphase simulations (air/water/medium) using the large Eddy simulation (LES) turbulence model, together with viscosity corrections according to the feed particle loading factor, gave accurate predictions of axial magnetite segregation, with results close to gamma ray tomography data. Addition of lift forces and viscosity correction improved the radial magnetite segregation predictions especially near the wall. Predicted density profiles are very close to gamma ray tomography data, showing a clear density drop near the wall. At higher feed densities the agreement between the empirical correlations of [Dungilson, M.E., 1998. A model to predict the performance of the dense medium cyclone for low and high density applications, In: Seventh JKMRC Conference, Brisbane, Australia, 67-84; Wood, J.C., 1990. A performance model for coal-washing dense medium cyclones, Ph.D. Thesis, JKMRC, University of Queensland] and the CFD are reasonably good, but the overflow density from CFD is lower than the empirical model predictions and experimental values. It is believed that excessive underflow volumetric flow rates are responsible for under prediction of the overflow density. The partition characteristics of the DMC for particles between 0.5 and 8 mm in diameter were modeled using Lagrangian particle tracking. For the first time, the pivot phenomenon, in which partition curves for different sizes of coal pass through a common pivot point, has been successfully modeled using CFD. The values of Ep predicted by the Lagrangian particle tracking are very close to the experimental values although cut-point predictions deviate slightly. This comprehensive CFD model provides a tool for new DMC design with clear advantages over approaches based on constructing and trialling new designs experimentally. © 2006 Elsevier Ltd. All rights reserved.

Minerals Engineering

A comprehensive CFD model of dense medium cyclone performance

A major operating cost in dense-medium separation is in replacement of lost medium solids. The loss of medium solids, being costly, plays a crucial role in determining the economics of any preparation operation. Coal washeries that employ dense-medium cyclones often attempt optimization of the processes by varying the vortex or the spigot diameter and the feed relative density. While these changes help in closer control of the separation process, they also result in medium losses due to changes in the medium split ratio (ratio of the medium flow rate in overflow to underflow). Since medium solids are lost by adhesion to products and as magnetic separator effluent, the effect of the change in medium split ratio on the drain-and-rinse screens and, hence, the magnetic separator circuit needs to be studied. In Tata Steel's coal washeries, at Jharkhand India, which employs primary and secondary dense-medium cyclones in series to produce clean coal, middlings and rejects, reducing the relative density of feed medium, had an insignificant effect on the medium split ratio. On the other hand, changing the cone ratio (ratio of the overflow diameter to the underflow diameter) changed the relative density and the flow rates through the cyclone outlets, thus affecting the performance of the magnetite recovery circuit. A systematic study through laboratory tests and a detailed plant sampling campaign helped in identifying the causes of magnetite loss. Upon implementation of the recommendations, the magnetite losses decreased, resulting in a saving of approximately US$27,500 per annum. The study also helped in evolving some checkpoints for plant operators for identifying magnetite losses. © 2006 Elsevier B.V. All rights reserved.

An analysis of medium losses in coal washing plants

Numerical simulations of turbulent driven flow in a dense medium cyclone with magnetite medium have been conducted using Fluent. The predicted air core shape and diameter were found to be close to the experimental results measured by gamma ray tomography. It is possible that the Large eddy simulation (LES) turbulence model with Mixture multi-phase model can be used to predict the air/slurry interface accurately although the LES may need a finer grid. Multi-phase simulations (air/water/medium) are showing appropriate medium segregation effects but are over-predicting the level of segregation compared to that measured by gamma-ray tomography in particular with over prediction of medium concentrations near the wall. Further, investigated the accurate prediction of axial segregation of magnetite using the LES turbulence model together with the multi-phase mixture model and viscosity corrections according to the feed particle loading factor. Addition of lift forces and viscosity correction improved the predictions especially near the wall. Predicted density profiles are very close to gamma ray tomography data showing a clear density drop near the wall. The effect of size distribution of the magnetite has been fully studied. It is interesting to note that the ultra-fine magnetite sizes (i.e. 2 and 7 μm) are distributed uniformly throughout the cyclone. As the size of magnetite increases, more segregation of magnetite occurs close to the wall. The cut-density (d50) of the magnetite segregation is 32 μm, which is expected with superfine magnetite feed size distribution. At higher feed densities the agreement between the [Dungilson, 1999; Wood, J.C., 1990. A performance model for coal-washing dense medium cyclones, Ph.D. Thesis, JKMRC, University of Queensland] correlations and the CFD are reasonably good, but the overflow density is lower than the model predictions. It is believed that the excessive underflow volumetric flow rates are responsible for under prediction of the overflow density. © 2006 Elsevier Ltd. All rights reserved.

Numerical simulation of magnetite segregation in a dense medium cyclone

A critical assessment is presented for the existing fluid flow models used for dense medium cyclones (DMCs) and hydrocyclones. As the present discussion indicates, the understanding of dense medium cyclone flow is still far from the complete. However, its similarity to the hydrocyclone provides a basis for improved understanding of fluid flow in DMCs. The complexity of fluid flow in DMCs is basically due to the existence of medium as well as the dominance of turbulent particle size and density effects on separation. Both the theoretical and experimental analysis is done with respect to two-phase motions and solid phase flow in hydrocyclones or DMCs. A detailed discussion is presented on the empirical, semiempirical, and the numerical models based upon both the vorticity-stream function approach and Navier-Stokes equations in their primitive variables and in cylindrical coordinates available in literature. The existing equations describing turbulence and multiphase flows in cyclone are also critically reviewed.

Coal Preparation

A review of flow modeling for dense medium cyclones

Hydrocyclones are widely used in the mining and chemical industries. An attempt has been made in this study, to develop a CFD (computational fluid dynamics) model, which is capable of predicting the flow patterns inside the hydrocyclone, including accurate prediction of flow split as well as the size of the air-core. The flow velocities and air-core diameters are predicted by DRSM (differential Reynolds stress model) and LES (large eddy simulations) models were compared to experimental results. The predicted water splits and air-core diameter with LES and RSM turbulence models along with VOF (volume of fluid) model for the air phase, through the outlets for various inlet pressures were also analyzed. The LES turbulence model led to an improved turbulence field prediction and thereby to more accurate prediction of pressure and velocity fields. This improvement was distinctive for the axial profile of pressure, indicating that air-core development is principally a transport effect rather than a pressure effect. An increase in feed flow rate will increase the air-core diameter by increasing the centrifugal force on fluid elements, which in turn increases the tangential velocity component, thereby lowering the pressure at the hydrocyclone axis near the apex. It has been observed that when the spigot diameter increased, the air-core diameter increased, since the tangential velocity increases when underflow diameter increases. As expected, a rise in the viscosity of liquid reduces the air-core diameter at constant feed velocity, by lowering the pressure drop over the cyclone. This LES turbulent multi-phase CFD model, once extended to particulate phase flow in hydrocyclone, would be a useful tool for studying design dimensions. More importantly, alternative geometries may be examined swiftly. © 2006 Elsevier B.V. All rights reserved.

Large eddy simulation of hydrocyclone-prediction of air-core diameter and shape

The biased subdivision of slurry feed to a number of unit operations in parallel results in each unit operating under different density volumetric flow rate, feed concentration. Consequently some of the units may operate under sub-optimal conditions leading to reduced operating efficiencies of sizing and cleaning units. The objective of this work was to identify the reasons for biased slurry subdivision in the feed distributor of Washery-II, West Bokaro, Tata Steel, Jamshedpur, India. A lab model of the feed distributor was set up at R&D using the principles of geometric similarity and dynamic similarity. A number of experimental investigations were carried out based on Design Of Experiments and the results analysed using ANOVA. The variation of feed rate of coal and its size distribution through different outlet ports were quantified through two terms, the coal split ratio and the variation index. The design of the insert, the number of water inlets, and the feeding position of coal in the distributor were the most important factors affecting the variation index. In case of coal split ratio, apart from the first two variables, none of the others had a significant effect. Four water inlets with symmetrically arranged outlets, equal water flow rate in the inlets and the material being fed in an annular ring into the distributor was found to reduce both the coal split ratio and the variation index. Upon implementation of the recommendations the bias in distribution was reduced and the efficiency of the cyclones improved and hence the coarse clean coal yield increased. © 2006.

Modelling of feed distributor at coal washeries

Simplicity in design and minimal floor space requirements render the hydrocyclone the preferred classifier in mineral processing plants. Empirical models have been developed for design and process optimisation but due to the complexity of the flow behaviour in the hydrocyclone these do not provide information on the internal separation mechanisms. To study the interaction of design variables, the flow behaviour needs to be considered, especially when modelling the new three-product cyclone. Computational fluid dynamics (CFD) was used to model the three-product cyclone, in particular the influence of the dual vortex finder arrangement on flow behaviour. From experimental work performed on the UG2 platinum ore, significant differences in the classification performance of the three-product cyclone were noticed with variations in the inner vortex finder length. Because of this simulations were performed for a range of inner vortex finder lengths. Simulations were also conducted on a conventional hydrocyclone of the same size to enable a direct comparison of the flow behaviour between the two cyclone designs. Significantly, high velocities were observed for the three-product cyclone with an inner vortex finder extended deep into the conical section of the cyclone. CFD studies revealed that in the three-product cyclone, a cylindrical shaped air-core is observed similar to conventional hydrocyclones. A constant diameter air-core was observed throughout the inner vortex finder length, while no air-core was present in the annulus. © 2006 Elsevier Ltd. All rights reserved.

Study of flow behaviour in a three-product cyclone using computational fluid dynamics

The flow behavior in hydrocyclone is quite complex. This complexity of flow processes has led designers to rely on empirical equations for predicting the equipment performance. The publications on empirical models of the hydrocyclone far out-number few fluid-flow-modeling attempts. Empirical models correlate a classification parameter, such as the cut-size, with device dimensions and slurry properties. However, these can only be used within the extremes of the experimental data from which the model parameters were determined. On the other hand, models based on Computational Fluid Dynamics (CFD) techniques have proven to be useful in simulating fluid flow in hydrocyclones, and in predicting the separation efficiency of solid particles in the separator for a wide range of operating and design conditions. The shape and size of a hydrocyclone separator has a direct influence on the internal flow structure of the continuous phase and, thereby, the separation of the particulate phase. Hydrocylcones usually have a single inlet that distributes the feed stream near the end wall between the vortex finder and the sidewall. Effect of spigot diameter, i.e., 10 and 20 mm and inlet water velocities (5.91-12.35 m/s) on the water splits and particle classification in the hydrocyclone have been studied. The cut size of the hydrocyclone, operated at very low pulp density, has been predicted using discrete phase modeling technique. The studies revealed mat with an increase in feed flow rate and decrease in spigot diameter the cyclone sharpness of separation improves. These predictions were found similar in line with the experimental observations. © 2004 Elsevier B.V. All rights reserved.

CFD modelling of hydrocyclone-prediction of cut size

The forced convective heat transfer characteristics for incompressible power-law fluids past a bundle of circular cylinders have been investigated numerically. The cylinder-to-cylinder hydrodynamic interactions have been approximated via a simple cell model. The momentum and energy equations have been solved using a finite difference based numerical method for a range of physical and kinematic conditions. The role of the two commonly used thermal boundary conditions, namely, constant temperature or constant heat flux, on heat transfer characteristics has also been studied. Extensive numerical results elucidating the effect of shear-thinning viscosity on the values of Nusselt number have been obtained for Peclet numbers ranging from 1 to 5000, Reynolds number in the range 1-500, flow behaviour index 1 ≥ n ≥ 0.5 and three values of voidages, namely, 0.4, 0.5 and 0.6, typical of tubular heat exchangers and tube banks. Under all conditions, varying levels of enhancement in Nusselt number are observed due to shear-thinning behaviour. The surface averaged Nusselt number shows strong dependence on the values of voidage, power-law index, Reynolds and Peclet numbers. The paper is concluded by presenting comparisons with the scant experimental results available in the literature. © 2004 Elsevier Ltd. All rights reserved.