Numerical simulation of behavior of fine coal in oscillating flows

A mathematical Euler/Lagrangian model based on computational fluid dynamics (CFD) approach is used to simulate the stratification of coal particles in the column jig. Interactions of wide size range and density distributing particles with the fluid have been simulated in the oscillating flow. Fluid motion is calculated by directly solving the Navier–Stokes equations by a SIMPLE approach. Particles are moved in a Lagrangian frame through the action of forces imposed by the fluid and gravity. The magnitude of forces acting on particles has been analyzed and compared. Particle effects on fluid motion are fed back at each time step through calculating the velocity disturbance caused by the particle. Particle–particle and particle–wall collisions are also considered. The simulation captures the essential features of jigging process. It was found that the pulsating amplitude has less effect on stratification on fine particles than frequency does in the jigging. The best stratification is also achieved at some intermediate values of amplitudes and frequencies.     

Numerical simulation of the in-line pressure jig unit in coal preparation

This paper presents a numerical study of the multiphase flow in an in-line pressure jig (IPJ), which is a high yield and high recovery gravity separation device widely used in ore processing but may have potential in coal preparation. The mathematical model is developed by use of the combined approach of computational fluid dynamics (CFD) for liquid flow and discrete element method (DEM) for particle flow. It is qualitatively verified by comparing the calculated and measured results under similar conditions. The effects of a few key variables, such as vibration frequency and amplitude, and the size and density of ragging particles, on the flow and separation performance of the IPJ are studied by conducting a series of simulations. The results are analyzed in terms of velocity field, porosity distribution and forces on particles. The findings would be helpful in the design, control and optimisation of an IPJ unit.     

Prediction of wear and its effect on the multiphase flow and separation performance of dense medium cyclone

Dense medium cyclone (DMC) is a high-tonnage device that is widely used to upgrade run-of-mine coal in modern coal preparation plants. It is known that wear is one of the problems in the operation of DMCs, but it is not well understood. In this work, the wear rate of DMC walls due to the impact of coal particles is predicted by a combined computational fluid dynamics and discrete element method (CFD-DEM) approach, using the Finnie wear model from the literature. In the CFD-DEM model, DEM is used to model the motion of discrete coal particles by applying Newton’s laws of motion and CFD is used to model the motion of the slurry medium by numerically solving the local-averaged Navier–Stokes equations together with the volume of fluid (VOF) and mixture multiphase flow models. According to the Finnie wear model, the wear rate is calculated according to the impact angle of particles on the wall, particle velocity during an impact and the yield stress of wall material; the relevant particle-scale information can be readily obtained from the CFD-DEM simulation. The numerical results show that the severe wear locations are generally the inside wall of the spigot and the outside wall of the vortex finder. The wear rate depends on both the operational conditions and solids properties. It increases generally with the decrease of medium-to-coal (M:C) ratio. For a given constant M:C ratio, the wear rate for thermal coal is higher than that for coking coal, especially at the spigot. Large particles may cause a non-symmetric wear rate due to the gravity effect. The effect of a worn spigot wall on the multiphase flow and separation performance is also studied. This work suggests that the proposed approach could be a useful tool to study the effect of wear in DMCs under different conditions.     

Numerical simulations of comminution slurries over complex topographies: Putting together CFD and pipeline integrity

The use of computational fluid dynamics gives new and interesting insights for risk analysis of cross-country ore hydraulic transport operations. In particular, they offer the possibility to predict, with reasonable accuracy, the progression and final condition of spills driven by pipeline leaks at selected locations, at a relatively modest computational cost. In this work, a depth-averaged, two-dimensional numerical model is used to simulate an ore concentrate pipeline rupture and subsequent spill, reproduced as a constant flow condition at the leak point. Although the model is well suited to solve the governing flow equations on arbitrary topographies by means of digital elevation models, two specific locations featuring relatively mild and steep slopes, are analysed with regard to their implications on the potential requirements for emergency team response. Results, obtained using different slurry rheologies, are compared with those obtained using a simpler, common flow resistance model derived for water flowing over rough surfaces.     

A Stokesian dynamics approach for simulation of magnetic particle suspensions

The dynamic behaviour of μm-scale ferromagnetic particles in suspension is of interest for various mineral beneficiation processes. It is, however, difficult to experimentally study such processes at the particle-level. In these instances it can be advantageous to resort to suitable particle simulation methods.Stokesian dynamics is a mesh-free numerical technique developed for suspensions of nm to mm size particles. The method inherently considers hydrodynamic interactions, but additional interaction models can be included depending on the system under investigation. We here present a Stokesian dynamics (SD) implementation, which allows for simulation of the motion of suspended magnetic particles in presence of an external magnetic field. The magnetic interaction model includes particle-field interactions as well as pairwise interactions between magnetised particles.Simulations are compared with experiments using a laboratory-scale flow cell. The method is shown to be realistic for studying ferromagnetic suspensions in mineral processing applications, and can be useful in understanding and predicting the efficiency of mineral separation processes.     

基于Euler-Lagrange方法的细粒煤脉动气流分选数值模拟

传统的欧拉模型和离散相模型难以准确模拟细粒煤气流分选过程,为此采用DDPM(Dense Discrete Phase Model)模型,引入相体积分数和软球碰撞模型,提出了一种基于离散相体积分数和颗粒间碰撞作用的新的细粒煤气流分选过程数值模拟方法。通过数值模拟与实验室连续分选实验,研究了分选机内流场分布和6~3 mm细粒煤脉动气流分选效果。结果表明:采用DDPM模型可有效模拟细粒煤气流分选过程中颗粒相对流场的扰动作用;分选柱内流场受被分选物料的影响,速度分布和压降值较加入颗粒前变化明显,且分选机处理量越高,压降变化越明显;实验条件范围内,DDPM模型模拟得到的各密度级重产物分配率均方根误差低于3%,且相同风量时,分选机处理量越大,分选密度越高。     

Velocity gradient and maximum floatable particle size in the Jameson cell

Due to efficient flotation of fine particle in the Jameson cell, it has been used in more than two hundred flotation plants since 1986. However, the performance of the Jameson cell decreases with the increase in the particle size in the feed. In some plants, coarse particles have to be fed to the Jameson cell because of operational and maintenance issues. Therefore, it is necessary to find the reasons of decrease of the cell performance caused by coarse particles as well as possible solutions of the problem.The present study has been carried out to delineate coarse particle flotation in the Jameson cell from a hydrodynamic point of view. In the study, flow characteristics of the fluid discharging from the downcomer to the separation tank have been investigated and a mathematical model, which can predict the velocity gradient, referred to as turbulence, has been developed in terms of fluid mechanics principles. Furthermore, experiments have been carried out to determine the maximum size of floating particle (d_max) using the developed model and experimental findings.     

The effect of distributor configuration on the hydrodynamics of the teetered bed separator

The teetered bed separator (TBS) is a gravity concentration device which operates on the principle of hindered settling. The hydrodynamics of the separator and fundamental particle interactions in the teeter bed were investigated for a two phase liquid–solid system with the coal particles of a wide size and density distribution considered as multiple solid phases. Seven distributor plates of varying aperture size and geometric arrangement were considered. The flow behaviour was predicted using the Multiphase Eulerian and Mixture Model in the commercial CFD package, Fluent 6.1. The simulations gave an indication of the dead zones, turbulent regions, wall effects and the specific flow pattern due to each plate. The model may be used to predict the most appropriate distributor configuration for the separation process based on the flow patterns and particle interactions in the unit. A laboratory scale TBS was constructed to investigate the separations achievable using the simulated designs. Coal particles sized between 2 and 0.038 mm with a specific gravity (SG) range of 1.2–2.0 were used as the material to separate. Partition curves were plotted for the separations carried out under modeled conditions and the Ecart probable (E_p) and cut point density (D₅₀) obtained. The simulations and experimental testwork indicated that Plate 3 may be the most appropriate distributor configuration for the investigated conditions due to the even velocity profile with minimum flow disturbances experienced in the coal separation process.     

Applicability of a coarse-grained CFD–DEM model on dense medium cyclone

The combined computational fluid dynamics and discrete element method (CFD–DEM) approach has been proved to be an effective tool to study the fundamentals of different particle–fluid flow systems, but suffers high computational cost problem. Recently, various treatments such as parcel–particle concept, coarse-grained model, similar particle assembly and representative particle model have been developed to reduce the computational cost of CFD–DEM approaches. These treatments are basically empirical and thus their applicability is likely system-dependent. Until now, there are still no general agreements on the formulation of those models and their accuracy and general applicability are largely unknown.In this work, a coarse-grained (CG) (CFD–DEM) model is developed to model the swirling multiphase flow in a dense medium cyclone (DMC) and the error caused by the CG concept is quantified by carrying out controlled numerical calculations to directly compare the simulated results between a standard CFD–DEM model and a CG CFD–DEM model. It demonstrates that when the flow is dilute, the results are independent on the size of the grain (also called as parcel or model particle in this work). Nonetheless, when the flow is dense, small discrepancies are observed between the two models. This work suggests that the CG CFD–DEM model is indeed a useful tool to quickly evaluate the flow and performance of large-scale DMCs and the simulation results should be useful at least qualitatively, if not quantitatively.     

计算流体力学在矿物加工行业创新中的价值

矿物加工流程主要包含矿物颗粒或矿浆流体相关的输运、破碎、混合、分离等过程。其中矿物颗粒之间的交互作用具有动态特性和流体相关特性。利用计算流体力学(CFD)技术可以从理论层面深入分析矿物加工流程中涉及的物料运动规律,进而帮助工程师更好地理解矿物加工过程,推动矿物加工创新。从矿物加工设备设计、选矿难题解决、选矿流程优化、选矿过程控制等方面阐述了CFD技术能够为矿物加工创新带来的价值。     

Computational fluid dynamics methods in research and analysis of mineral separation

Parameters of heterogeneous working fluid in magnetic-gravity, gravity and flotation machines are studied using methods of computational fluid dynamics. Numerical modeling of separation processes allowed graphical and numerical characterization of separation. The article bases suggestions on improvement of the designs of gravity and magnetic-gravity processing equipment and offers an approach to evaluating surface energy of mineral particles in flotation modeling.     

浅析聚磁介质在磁选机中的应用

高梯度磁选机是在其他强磁选机的基础上发展起来的一种新型强磁选机。通过整个高梯度磁选机工作体积的磁化场是均匀的,即工作体积中的任何一个颗粒受到的力相同,且磁化场中的磁介质被均匀磁化,同直径的磁介质在磁化空间的任何位置,其梯度的数量级是相同的,与一般磁选机相比较,磁场梯度大大提高,从而为磁性颗粒的选别提供强大的磁力克服流体阻力和重力,使微细粒弱磁性颗粒可以得到有效的回收。高梯度磁选是在能产生高梯度的聚磁介质上进行的,而高梯度的产生与聚磁介质的材质、形状、放置方式、相对尺寸及充填率等参数有密切关系,对磁选机的磁选指标有较大的影响。     

Particle collision modeling - A review

Over the past 100 years particle collision models for a range of particle inertias and carrier fluid flow conditions have been developed. Models for perikinetic and orthokinetic collisions for simple, laminar shear flows as well as collisions associated with differential sedimentation are well documented. Collision models developed for turbulent flow conditions are demarcated on the one side with the model of Saffman and Turner (1956) associated with particles exhibiting zero inertia and on the other side with the model of Abrahamson (1975) for particle velocities that are completely decorrelated from the carrier fluid velocities. Various attempts have been made to develop universal collision models that span the entire range of inertias in a turbulent flow field. It is a well-accepted fact that models based on a cylindrical as opposed to a spherical formulation are erroneous. Furthermore, the collision frequency of particles exhibiting identical inertias are not negligible. Particles exhibiting relaxation times close to the Kolmogorov time scale of the turbulent flow are subject to preferential concentration that could increase the collision frequency by up to two orders of magnitude. In recent years the direct numerical simulation (DNS) of colliding particles in a turbulent flow field have been preferred as a means to secure the collision data on which the collision models are based. The primary advantage of the numerical treatment is better control over flow and particle variables as well as more accurate collision statistics. However, a numerical treatment places a severe restriction on the magnitude of the turbulent flow Reynolds number. The future development of more comprehensive and accurate collision models will most likely keep pace with the growth in computational resources.     

高梯度磁选数学模型及计算机模拟的研究（Ⅱ）

利用有限差分法、龙格－库塔法等数值方法对高梯度磁选数学模型进行了数值求解，估计出了模型参数与影响因素之间的关系表达式，选择磁性产品的产率、品位及回收率等工艺指标为模型输出掼标，开发了高梯度磁选计算机模拟软件，在各种操作条件下对高梯度磁选非线性动态模型进行了验证和模拟研究，结果表明，模型的计算值和试验结果相，说明建模的方法是可行的，模型的结论是合理的，模型能比较全面地、正确地描述各种影响因素对高梯度磁选过程的影响。     

Using SPH one-way coupled to DEM to model wet industrial banana screens

Large banana screens with multiple decks are used extensively in the process separation of many valuable export commodities. They are high capacity vibrating screens with a curved profile. Discrete Element Method (DEM) modelling using non-spherical particles has previously provided significant insight into the operation of these dry industrial screens. Here we introduce the use of Smoothed Particle Hydrodynamics (SPH) to model the flow of slurry (water and fine material) through a double deck banana screen. This paper firstly reports on the underlying DEM model of the coarse particulate flow on a full-scale banana screen. We then use Smoothed Particle Hydrodynamics (SPH) to model the transport of fine particle slurry over and through the double deck banana screen. Finally, we combine the DEM with SPH models using a one-way coupling to simulate the effects of adding a slurry flow to coarse particulates on the banana screen. The key outcomes from this study are that; SPH is ideally suited for the high speeds and the high fragmented and filamentary nature of the fluid flow through the screen deck openings; the fluid only (SPH) model of slurry behaves similarly to the DEM approach in that more fluid is screened as the velocity slows, except near the earlier panels on the top deck; and, use of a porous media derived from DEM in one-way coupled approach with SPH produces clear and reasonable changes in fluid structure, separation and wetting of the screens consistent with slurry behaviour. Specifically, the fluid layer was much thicker in the coupled case, with slurry being trapped inside a coarse particle bed and which is sensitive to the fluid viscosity.     

磁流体静力选矿中非磁性矿粒的磁浮力分析

从流体力学方程和Maxwell电磁场方程出发,对磁流体静力选矿系统中非磁性矿粒所受磁浮力进行了重点分析和计算,并在此基础上,对磁场强度梯度方向垂直向下时非磁性矿粒在磁流体中所受合力进行了推导。     

重介质旋流器流场湍流数值计算模型的选择

简述了重介质旋流器理论研究的现状,给出了计算流体力学基本方程及各种湍流数值计算模型.在相同边界条件下采用k-ε,RNG,RSM三个模型对重介质旋流器流场进行试探性数值模拟,并与相同边界条件的试验测量结果进行比较,最后选定RSM模型用于重介质旋流器流场数值模拟的湍流模型.     

Magnetic separation of hematite and limonite fines as hydrophobic flocs from iron ores

Magnetic separation of weakly magnetic iron mineral fines in the form of flocs, which is termed Floc Magnetic Separation (FMS) process, has been studied in the present work, in order to find a substitution for high-intensity or high-gradient magnetic separators to treat the ores with weakly magnetic iron minerals in the fine size range. This study was performed on a hematite ore and a limonite ore that were finely ground to be micron particles, through the hydrophobic flocculation induced by sodium oleate and kerosene to make flocs. The experimental results have shown that the FMS process is effective to recover hematite and limonite fines at a middle magnetic field intensity, greatly increased the separation efficiency, compared with the conventional magnetic separation at the same conditions. By applying the process to the fine hematite ore containing 30.5% Fe, a concentrate assaying 64% Fe with 82% recovery has been produced. It has been found that the separation efficiency of the FMS process closely correlates with the main parameters of hydrophobic flocculation such as sodium oleate addition, conditioning time and kerosene addition. This finding suggests that the high efficiency achieved by the FMS process might be attributed to the considerable increase of the magnetic force on the iron mineral fines in the form of hydrophobic flocs in a magnetic field, thus the fines can be held by the separation plates in a magnetic separator and then be collected as magnetic concentrates.     

CFD modeling and analysis of the multiphase flow and performance of dense medium cyclones

A computational fluid dynamics (CFD) model is proposed to describe the multiphase flow in a dense-medium cyclone (DMC). In this model, the volume of fluid (VOF) multiphase model is first used to determine the initial shape and position of the air core, and then the so called mixture model is employed to describe the flows of the medium, coal particles and air, where the turbulence is described by the Reynolds stress model. The validity of the proposed approach is verified by the reasonably good agreement between the measured and calculated results in terms of separation efficiency. On this base, this model is used to quantify the effects of the ratios of spigot to vortex finder diameters (U:O) and medium to coal (M:C) on performance. The results are shown to be generally comparable to those reported in the literature. It reveals that when vortex finder or spigot diameter is varied at the same U:O ratio, the offset and medium split nearly remain the same, however, the coal feed rate and E_p are different under the conditions considered. It is also shown that the fish-hook phenomenon is observed when spigot diameter is equal to or slightly larger than vortex finder diameter, and a normal operation becomes less stable with decreasing U:O ratio. The key phenomena predicted are explained by the calculated inner flows.