Dense Medium Cyclone Research Articles

Particle scale modelling of the multiphase flow in a dense medium cyclone: Effect of vortex finder outlet pressure

Dense medium cyclone (DMC) is widely used to upgrade run-of-mine coal in the coal industry. In practice, different designs of the outlet geometry of the vortex finder are used to achieve different purposes. However, the underlying mechanisms are not well understood. In this work, this phenomenon is studied numerically with reference to the effect of the pressure at the vortex finder. The simulation is carried out by use of a combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) (CFD–DEM). In the model, DEM is used to describe the motion of discrete coal particles, and CFD to describe the motion of medium slurry which is a mixture of gas, water and fine magnetite particles. It is shown that a relatively small change of the vortex finder pressure can cause significant variations of both the medium-coal flow and DMC performance. An important finding is that the flow direction of the axial velocity of the air phase in the “air-core” could reverse (changing from upward to downward) as the vortex finder pressure increases, which results in the downward viscous drag force on coal particles and consequently causes some low density coal to be misplaced to the reject/underflow. This work suggests that the control of the pressure at the outlet of the vortex finder is important for DMC performance.

Prediction of the performance of dense medium cyclones in coal preparation

The Dense medium cyclone (DMC) is a high-tonnage device that is widely used to upgrade run-of-mine coal in coal industry. Its complicated multiphase flow structure is difficult to investigate experimentally. In recent years, Computational Fluid Dynamics (CFD) and in particular, its combination with Discrete Element Method (DEM) have been shown to be effective in overcoming this difficulty. However, such a mathematical model, particularly the CFD-DEM one, is very time-consuming in computation and not suitable for engineering application. In this paper, based on the CFD and CFD-DEM simulated data, a PC-based mathematical model is formulated to predict the performance of DMCs under various conditions. It first discusses how such a model can be developed, with its validity examined against the collected plant data. Then, the effects of some key variables related to DMC geometry, operational conditions and materials properties are examined. It is shown that the proposed model can indeed offer a convenient way to quantify the effects of different variables, being useful in the design and control of DMCs under different conditions.

Medium Quality and DMC Performance: Effect of Clay and Fine Coal

A closed-loop medium circuit incorporating a 150 mm Dense Medium Cyclone (DMC) has been developed at the CSIRO QCAT laboratory to investigate the effect of changing the medium quality on medium stability. The dimensions of the cyclone were similar to the Dutch State Mines recommended values, except for the spigot. This was set at 35 mm since this provided the best operation and 2 mm colored tracers were available for determination of the partitioning performance of the DMC. For mediums with clay, magnetite, and/or fine coal, there is a considerable spread in the values obtained for overflow and underflow densities and the separation density of the tracers at each feed density. This is clear evidence that the composition of the medium (the medium quality) does influence the density differential and the effect becomes very significant for feed densities below 1.4. Empirically derived quantitative relationships were obtained for the underflow (RhoU) and overflow (RhoO) medium densities and separation density (RD50) using multilinear regression analysis. This improved quantitative understanding will allow the application of “smart sensors” to improve the level of process control that could be applied to a DMC circuit.

Medium Quality and DMC Performance: Influence of Small Coal

The issue of what is the size at which the particles in the Dense Medium Cyclone (DMC) separation system cease to be part of the medium and act as individual particles was considered. Previous work had assumed that the fine (less than about 0.150 mm) coal was part of the medium, but that the small (about 0.4 mm) coal particles in the medium behaved as individual entities. This hypothesis suggested that three effects should be observable from analysis of the data: 1. A significant reduction in the effective volume fraction of particles present in the medium; 2. The experimentally measured values of RD50 should be described by the effective volume fraction of magnetite and clay using Equation 4 after discounting the volume fraction of the small coal; 3. The increase in the RD50 values for systems with clay and fine coal are clearly related to their actual volume fractions. Obviously, these are also associated with Equation 4. Also, all three have been shown to be present. It can be concluded that even while the small coal is in the medium and would add to the apparent density of the medium, it does not contribute to the effective medium.

Development of a Steady-State Partition Curve from a Dense Medium Cyclone Dynamic Model in Coal Beneficiation

AbstractA partition curve is used in dense medium separation to determine the efficiency of the separation of clean coal from discard. The method used to determine a partition curve in coal beneficiation is float and sink analysis. The float and sink dry masses are determined at each density fraction only after complete separation of the material has taken place. This means that the partition curve is a form of a steady-state model. A dynamic model for a dense medium separation circuit is available from first principles. This paper shows how a steady-state model is derived from the dynamic model to generate a partition curve. This partition curve is compared with plant measurements taken from a plant operation.

Modelling the Multiphase Flow in Dense Medium Cyclones

Dense medium cyclone (DMC) is widely used in mineral industry to separate solids by density. It is simple in design but the flow pattern within it is complex due to the size and density distributions of the feed and process medium solids, and the turbulent vortex formed. Recently, the so-called combined computational fluid dynamics (CFD) and discrete element method (DEM) (CFD-DEM) was extended from two-phase flow to model the flow in DMCs at the University of New South Wales (UNSW). In the CFD-DEM model, the flow of coal particles is modelled by DEM and that of medium flow by CFD, allowing consideration of medium-coal mutual interaction and particle-particle collisions. In the DEM model, Newton's laws of motion are applied to individual particles, and in the CFD model the local-averaged Navier-Stokes equations combined with the volume of fluid (VOF) and mixture multiphase flow models are solved. The application to the DMC studies requires intensive computational effort. Therefore, various simplified versions have been proposed, corresponding to the approaches such as Lagrangian particle tracking (LPT) method where dilute phase flow is assumed so that the interaction between particles can be ignored, one-way coupling where the effect of particle flow on fluid flow is ignored, and the use of the concept of parcel particles whose properties are empirically determined. In this paper, the previous works on the modelling of DMCs at UNSW are summarized and the features and applicability of the models used are discussed.

Processing of coal fines in a water-only cyclone

It is reported in the literature that a water-only cyclone (WOC), a centrifugal gravity concentrator, is an alternative to froth flotation to treat coal fines (below 0.5 mm). This unit overcomes the inherent limitations of froth flotation and the dense–medium cyclone techniques as it requires no chemicals or artificial medium. The literature dealing with WOC performance to treat coal fines is also limited and as a result it is not well established how the design variables affect the performance of a WOC while treating coal fines. Therefore, an attempt has been made to develop regression models based on factorial design of experiments to quantify the effects of major design variables of a WOC on the beneficiation characteristics of a typical coal fine sample. Further attempts have been made to provide possible explanations on the observed trends of the data based on simple hydrodynamic analyses.

Computational study of the multiphase flow and performance of dense medium cyclones: Effect of body dimensions

This paper presents a numerical study of the gas–liquid–solid flow in 1000 mm dense medium cyclones (DMCs) with different body dimensions, which includes the spigot diameter, cylinder length, cone length and inlet size by means of a computer model which we recently proposed. In this model, mixture multiphase model is used to describe the flow of the dense medium (comprising finely ground magnetite contaminated with non-magnetic material in water) and the air core, where the turbulence is described by the well-established Reynolds Stress Model. The stochastic Lagrangian Particle Tracking method is used to simulate the flow of coal particles. It is found that the spigot size is very sensitive to the performance. The operational head and medium split reporting to overflow, decrease dramatically as the spigot diameter increases. The density differential decreases rapidly, and then passes through a minimum and increases slowly. The long body including cylinder and cone is helpful to particle separation, particularly for fine and heavy particles. The inlet size plays a remarkable role on the performance on DMCs. The operational head, the density differential and the medium split increase dramatically as the inlet size decreases. Both the upward flow and the downward flow become very strong in the DMC with a small inlet when medium feed rate is constant, which results in a very low Ep.

CFD Modeling of Dense Medium Cyclone

A number of empirical models are in existence for the dense medium cyclone (DMC) and in recent years this subject has been broached with computational fluid dynamics (CFD). The dense medium presents a centrifugal field within the cyclone body. The coal particles separate in this field due to various forces acting on them. Hence, CFD is ideally suited for the modeling of the DMC. The Large Eddy Simulation (LES) method for resolving the turbulence was used in the CFD simulation of a 76 mm dense medium cyclone. In particular, the magnetite was modeled as three granular fluids. In the simulation the diameter of the vortex finder and spigot are varied to compare with the experimental data of P. A. Verghese and T. C. Rao. The results obtained using LES turbulence model is found to be accurate in terms of the cut density and the slope of the distribution curves. Thus, the three granular fluid modeling of the magnetite stream is a computationally simpler method for the analysis of DMC.

Development and Evaluation of the CAVEX Dense Medium Cyclone

The CAVEX dense medium cyclone (DMC) was developed in the later part of the 1990s as a result of the expertise developed by Weir engineers in slurry pumping. The inlet area of the cyclone is designed to minimize turbulence and to reduce wear at the feed entry point, which provides more energy for particle separation at a given feed pressure. A parametric study was performed on a 150 mm diameter unit to quantify separation efficiency as a function of feed pressure, apex diameter, medium density, and cone angle. The added energy in the cyclone was confirmed by comparing the stability of the medium in the CAVEX unit with that provided by a common commercial unit having the same dimensions. A 500 mm unit was installed in parallel with an identically sized industrial unit in an operating preparation plant treating 12 × 1 mm coal. The separation efficiency values achieved by the CAVEX DMC were found to be higher than those obtained by the standard industrial unit and the amount of improvement increased with a decrease in particle size. The data from the pilot-scale and in-plant tests are presented and discussed in this article.

Predictive Control of Screen Process Efficiency

The durability of critical wear components, the ability to predict their remaining life, and their improved operating efficiency was identified by the Australian coal industry to be a long-term strategic objective. Polymer screen panels used in high-capacity multisloped screens (banana screens) are a critical wear component that can affect both availability of the plant and plant performance. As a screen panel wears more oversize material is presented to downstream processing unit operations such as spirals and flotation plants, this oversize material has detrimental affects on the operating efficiency of these unit operations and can result in considerable yield losses. In addition, the maintenance of the screen panels is often done on a reactive basis or carried out at times when it is not needed resulting in unnecessary stoppages to the plant. If the plant operators can predict when the screen apertures/misplaced material reaches a point where yield loss is excessive and screen maintenance is required then reactive screen maintenance can be eliminated. This article describes the plant work undertaken during the course of the project, the results of the plant work, and the next steps towards developing a model describing the wear and the effect this wear has on downstream processing. Two plants were chosen for the test work; the first a plant with desliming banana screens cutting at nominally 1.8 mm with the oversize (30 mm × 1.8 mm) reporting to a primary 1 m DMC and the undersize to classifying cyclones and spirals. The second plant was desliming the 50 mm top-size feed again using banana screens at a cut-point of nominally 0.4 mm; the 50 mm × 0.4 mm was cleaned using a primary 1 m dense medium cyclone and the desliming screen undersize nominally 0.4 mm × 0 mm was treated using column flotation. Both these circuits incorporated secondary processing of the primary DMC rejects using secondary DMCs to produce a thermal coal. Screen apertures were measured over a period of 10 to 12 weeks using an optical-image analysis technique and samples taken of the screen products either just prior to the aperture measurement or immediately after.

Performance characteristics of pilot plant dense media hydrocyclone for beneficiation of coal and 3-D CFD simulation

Dense-medium separators have proven to be the most efficient processes for removing the undesirable material from run-of-mine coal. The application of high-pressure feed injection into dense-medium cyclones to provide an elevated centrifugal force has recently been found to allow efficient separation performances for the treatment of fine coal (i.e., <1000 μm). However, high-pressure injection requires specialized pumps and results in relatively high maintenance requirements. the Current study involves experimental investigation of separation performance characteristics of the dense media hydrocyclone (DMC). A pilot plant DMC has been designed and fabricated for performance characterization. Experiments have been conducted on 300 mm dense medium cyclone treating coal in the size range of −6 to +2 mm using magnetite as the medium under operating conditions. The operating variable was the specific gravity of the medium, feed inlet pressure and feed inlet flow rate. The ash contents of the feed coal reporting to the overflow and underflow have been analyzed qualitatively. The result indicates that the use of magnetite as dense medium in DMC resulted in the yield of clean coal, which is 5% more when the air core is suppressed as compared to the same conditions when the air core remains. A 3-D geometry is created in Gambit to support the experimental findings by using CFD simulation. It is interesting to observe that experimental findings agree well with the simulation results.

Considerations on the Kinetics of Froth Flotation of Ultrafine Coal Contained in Tailings

This article presents a kinetic evaluation of froth flotation of ultrafine coal contained in the tailings from a Colombian coal preparation plant. The plant utilizes a dense-medium cyclones and spirals circuit. The tailings contained material that was 63% finer than 14 µm. Flotation tests were performed with and without coal “promoters” (diesel oil or kerosene) to evaluate the kinetics of flotation of coal. It was found that flotation rates were higher when no promoter was added. Different kinetic models were evaluated for the flotation of the coal from the tailings, and it was found that the best fitted model was the classical first-order model.

Flotação do carvão contido em um rejeito carbonoso

Os processos envolvidos no beneficiamento do carvão produzem efeitos nocivos para o meio ambiente, principalmente pela quantidade e natureza dos rejeitos que são gerados. Esses rejeitos, comumente denominados “piritosos”, mesmo não apresentando altos conteúdos de pirita, constituem-se em um material ácido que causa efeitos nocivos ao meio ambiente, principalmente aos corpos de água. Esses rejeitos, geralmente, são depositados em barragens de rejeitos. O rejeito carbonoso estudado nessa pesquisa provem de um processo de beneficiamento que envolve ciclones de meio denso e espirais. A caracterização revelou que ele possui teor de cinzas de 56% e poder calorífico de 5.800 BTU/lb, sendo que o conteúdo de enxofre é 1,2%. Em termos de granulometria, o material é considerado ultrafino já que 63% é menor que 0,014 mm. O conteúdo de matéria carbonosa deste rejeito é facilmente recuperado por flotação, como se demonstra nesse trabalho. É possível recuperar 74% da matéria carbonosa e obter um produto com 7,3% de cinzas e poder calorífico de 14.225 BTU/lb em base seca.

Dense Medium Cyclone Control – A Reconsideration

The current process management and control of dense medium cyclones is based on the measurement of feed medium density and pressure. This paper considers the relationships between the controlling factors and the relative density of separation and Ep. These relationships have been developed from the analysis of over 20 data sets on modern large cyclones in which important factors such as overflow and underflow densities, feed solids flow rate, medium to coal ratio, and loading to the spigot were recorded.

CFD–DEM study of the effect of particle density distribution on the multiphase flow and performance of dense medium cyclone

A mathematical model is developed to study the coal-medium flow in a dense medium cyclone (DMC) of 1000 mm body diameter. In the model, the motion of coal particles is obtained using the Discrete Element Method (DEM) facilitated with the concept of “parcel–particle” while the flow of medium as a liquid-magnetite mixture Computational Fluid Dynamics (CFD) based on the local averaged Navier–Stokes equations. In addition the Reynolds Stress Model (RSM) is adopted to describe the anisotropic turbulence, the Volume of Fluid (VOF) model is used to describe the air-core position and multiphase mixture model used to estimate the flow of fine magnetite particles. The simulated medium and coal flows allow estimates to be made of pressure drop, efflux stream medium densities and partition curves for coal particles of different sizes and densities. These estimates are compared favourably with industrial scale measurements of a 1000 mm DMC operating under similar conditions. On this base, the effect of particle density distribution that represents the major difference between two major coal type, i.e., coking coal and thermal coal, is studied. The results are analysed in terms of medium flow pattern, particle flow pattern, partition performance and particle–fluid, particle–wall and particle–particle interaction forces.

Numerical studies of the effects of medium properties in dense medium cyclone operations

A mathematical approach is proposed to describe the multiphase flow in a 1000 mm industrial dense medium cyclone. A mixture multiphase model is employed to describe the flow of the dense medium (comprising finely ground magnetite contaminated with non-magnetic material in water) and the air core, where the turbulence is described by the well established Reynolds stress model. The stochastic Lagrangian particle tracking method is used to simulate the flow of coal particles. The proposed approach was qualitatively validated using literature and industrial data and then used to study the effects of medium properties including medium density, magnetite type and non-magnetic content. It is found that as the medium density increases, the pressure drop increases, resulting in a high pressure gradient force on coal particles and reduced separating efficiencies. The segregation of magnetite particles becomes serious as magnetite particle size increases, which leads to a high density differential and a high off-set. The viscosity of medium decreases and the segregation of magnetite particles become significant with the decrease of non-magnetic content, resulting in a high density differential and off-set.

An Analysis of Mass Balance and Fractional Particle Size Distributions of Coal and Magnetite in a Dense-Medium Cyclone Circuit

In this study, a complete mass balance of magnetite and coal in the various parts of a dense-medium cyclone (DMC) circuit was determined and fractional size distributions of magnetite and coal were analyzed for the circuit. The DMC overflow product contained 71.34% of the feed coal, whereas 88.35% of the feed magnetite reported to the DMC underflow. The majority of the magnetite (about 86%) was removed by the sieve bends in both the DMC underflow and overflow streams. Sixty-one percent of the raw coal within a size range of 0.50–20.00 mm was recovered as clean coal with an average ash content of 15.30%. About 77% of the feed magnetite having a size range of 75–600 µm was obtained from the underflow of the drain-and-rinse screen belonging to DMC underflow.

CFD-DEM modelling of multiphase flow in dense medium cyclones

Dense medium cyclone (DMC) is a high-tonnage device that is widely used to upgrade run-of-mine coal in coal preparation. It is simple in design but the flow pattern within it is complex due to the size and density distributions of the feed and process medium solids, and the turbulent vortex formed. This paper presents a mathematical model to describe this flow system by means of combining Discrete Element Method (DEM) with Computational Fluid Dynamics (CFD). The DEM is used to model the motion of discrete particles by applying Newton's laws of motion. The CFD is used to model the motion of slurry medium by numerically solving the local-averaged Navier–Stokes equations facilitated with the Volume of Fluid (VOF) and Mixture multiphase flow models. The approach is shown to be able to reproduce typical flow phenomena in DMCs. The effect of medium-to-coal ratio and the so-called “surging” phenomenon are analyzed. An attempt is made to explain the observed phenomena in terms of particle–particle, particle–fluid and particle–wall interaction forces. The approach offers a convenient way to examine the flow and performance of DMC in relation to geometric, material and operational conditions.

Applicability of a Dense-Medium Cyclone and Vorsyl Separator for Upgrading Non-Coking Coal Fines for Use as a Blast Furnace Injection Fuel

Replacement of metallurgical coke by high injection rates of thermal coal into the blast furnace is an important technology as it reduces the cost of hot metals significantly. However, one of the main problems that prevents the use of thermal coals is their high mineral-matter contents. Although, the ash content of coals to be injected in a blast furnace should be as low as possible, a maximum of 16% ash is acceptable. A non-coking coal sample from Chhattisgarh area, India, having a feed ash content of around 27% was collected for beneficiation studies to a grade acceptable for the injection purposes. A series of experiments were conducted in a 76-mm diameter dense-medium cyclone (DMC) and a Vorsyl separator (VS). It is observed that a clean coal having around 16% ash can be produced using both the cyclones if the variables are properly optimized. Further, it is observed that at the same ash level the yield of clean coal was 5%–6% more in VS than in DMC. It has also been demonstrated that at the same ash level, the magnetite medium stability in a VS was better than a DMC.