CFD simulation of fine coal segregation and stratification in jigs

The literature review on discrete element (DEM) model analysis of jigging reveals that an idealized fluid behavior is assumed and the damping of the fluid motion across the mineral bed is generally ignored. A microscopic model based on the principles of Computational Fluid Dynamics (CFD) is used to simulate the liquid flow and stratification of coal particles with a wide size range and density distribution in jigging. Fluid motion is calculated by directly solving the Navier–Stokes equations. Coal particles are moved in a Lagrangian frame through the action of forces imposed by the fluid and gravity. 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 snapshots of particle configurations for the simulation of stratification in oscillating flow show that the model predicts the macroscopic behavior, such as segregation and stratification, of particles reasonably well.     

Experimental and simulation studies on the role of fluid velocity during particle separation in a liquid–solid fluidized bed

The fluidization technique has been in use for particulate material processing operations for many years. It has been widely believed that the ratio of size and density of the particulate components controls the separation efficiency. In this paper, it is demonstrated that fluid velocity during fluidization could assume an overriding significance when an improvement in separation efficiency is required. This was at first experimentally established by analyzing simpler particulate systems, and later a simulation scheme was adopted to study a wider range of particulate systems. The numerical scheme known as the discrete element method (DEM), that incorporates both the solid- and hydro-dynamic components of the interactive forces, served as an important tool in understanding the separation behavior of binary particulate systems in fluidized beds. It has been established that mere fluidization does not necessarily guarantee an optimal separation, especially when the particles differ widely in density and size.     

储气砂中浅层气溢出过程数值模拟研究

基于A.Nermoen的物理实验,文章进行了浅层气溢出过程数值模拟,区别于以往常用的欧拉-欧拉方法,文章采用了基于欧拉-拉格朗日方法的新型的离散粒子模拟方法MP-PIC（multiphase particle-in-cell）模型。揭示超压气体释放的机制,以及浅层气溢出过程中速度、压力的变化情况和变化规律。模拟得到的颗粒流化现象以及气泡的产生、运动和溢出现象与试验现象相一致,数模计算结果与实验资料两者相关度为0.94,相关性较好。临界流化速度和压力与土层的高度呈正相关,临界流化压力与实验结果较为接近,但由于马格努斯力（magnus）等作用力未考虑,导致临界流化速度相比于实验偏低。该模型方法能处理任意尺寸分布的颗粒,适用庞大的颗粒量,紧密耦合气固间相互作用,适用性好,准确性高,为数值模拟提供了便利。     

An experimental and theoretical study of the seepage migration of suspended particles with different sizes

This study experimentally investigates the effect of particle size, particle concentration and flow velocity on the migration of suspended particles of size 1.02–47 μm in porous media. The results show that at the same flow velocity, the peak values of the breakthrough curves decrease and corresponding pore volumes increase slightly with increasing particles size. The migration velocity of smaller suspended particles is even greater than water flow velocity, which is attributed to the size exclusion effect. With increase of the injected particle concentration, the deposition coefficients of small single particles increase at first and then tend to a steady state or even decrease slightly, explained by the maximum retention concentration. The dispersivity of small particles decreases with increasing velocity. However, at a high flow velocity, the hydrodynamic dispersivity becomes increasingly dominant with the increase of particle size. The deposition coefficients for large-sized particles are higher than those for small-sized particles, which is attributed to considerable mass removal due to straining. An analytical solution, considering the release effect of sorbed particles, is developed to account for the one-dimensional flow and dispersive effect using a source function method, and then three transport parameters—dispersivity, deposition coefficient and release coefficient—are fitted using the experimental results. Finally, suspended-particle migration is predicted by the proposed model for short-time constant-concentration injection and repeated three-pulse injection. Overall, particle size has a significant effect on the seepage migration parameters of suspended particles in porous media such as the particle velocity, dispersivity and deposition coefficient.     

Alteration of the equal settling ratio of fine particles by vertically-oscillating water

An equal settling ratio is an important factor in estimating particle separation accuracy. However, this factor is often calculated using the settling velocity in stationary water, there are no examples of calculation of the equal settling ratio in an actual separator. This is difficult because particle movement in a separator is very complicated, and even simple periodic motions, such as the oscillation field used with many separators, are ignored in many cases. The authors have previously reported on the relation between the equal settling ratio and the oscillation frequency by analysis of particle movement in vertically oscillating water, using spherical particles of glass (average size 435 μm) and zirconia (202 μm) which have the same settling velocity in stationary water. In this study, the influence of particle diameter on the change in the settling velocity in oscillating water was experimentally investigated for three pairs of glass and zirconia particles having different sizes under 0.5 mm, which have the same settling velocity in stationary water. The settling velocities of different-sized particles decreased at different rates in oscillating water, indicating that the equal settling ratio is reduced by water oscillation. We conclude that water oscillation improves the accuracy of size separation for glass particles over 300 μm and zirconia particles over 150 μm when glass and zirconia particles are separated from each other with the difference of these settling velocities.     

不同粒度颗粒在非恒定管流中跟随性试验

管道中固液两相流水击对管道和输送系统可能产生严重的破坏,而固液两相在这种非恒定流中的运动特性是计算最大水击压力变化的重要依据。采用粒子图像测速技术（Particle Image Velocimetry,PIV）,通过试验研究水击发生时,水平圆管中不同平均流速、颗粒粒径条件下,流体介质和粗颗粒在管道断面的速度分布以及粗颗粒跟随性的变化规律。研究结果表明:①在水击发生的不同时刻,圆管流中粗颗粒的流速在管道断面分布呈不规则的抛物线型分布,主要表现为靠近管道壁面底部的颗粒流速略小于靠近管道顶部流速,当颗粒粒径大于1.5 mm,平均流速小于2.5 m/s时,粗颗粒表现出明显的沉降特性;②粗颗粒的跟随性与颗粒受力有密切关系,其中颗粒速度与流体速度的变化量是影响颗粒受力的重要参数;③基于试验数据拟合得到了水击条件下粗颗粒跟随性系数k的经验公式,并分析了颗粒粒径、管道直径、两相流平均流速以及水击发生时间等不同参数对粗颗粒跟随性系数的影响,公式计算值与实测值之间的误差在5%以内。     

Analogue study of particle segregation in pyroclastic density currents, with implications for the emplacement mechanisms of large ignimbrites

Abstract Analogue flume experiments were conducted to investigate the transport and sedimentation behaviour of turbulent pyroclastic density currents. The experimental currents were scaled approximately to the natural environment in three ways: (1) they were fully turbulent; (2) they had a very wide range of particle sizes and associated Rouse numbers (the ratio of particle settling velocity to effective turbulent eddy velocity in the current); and (3) they contained particles of two different densities. Two sets of surge‐type experiments were conducted in a 5 m long, water‐filled lock‐exchange flume at five different volumetric particle concentrations from 0·6% to 23%. In one set (one‐component experiments), the currents contained just dense particles; in the other set (two‐component experiments), they contained both light and dense particles in equal volume proportions. In both sets of experiments, the population of each component had a log‐normal size distribution. In the two‐component experiments, the size range of the light particle population was selected in order to be in hydrodynamic equivalence with that of the dense particles. Dense particles were normally graded, both vertically and downstream, in the deposits from both sets of experiments. The mass loading (normalized to the initial mass of the suspension) and grain size of the dense component in the deposits decreased with distance from the reservoir and were insensitive to initial total particle concentration in the currents. On the other hand, in the two‐component experiments, the light particles were extremely sensitive to concentration. They were deposited in hydrodynamic equivalence with the dense particles from dilute currents, but were segregated efficiently at concentrations higher than a few per cent. With increasing particle concentration, the large, light particles were carried progressively further down the flume because of buoyancy effects. Deposits from the high‐concentration currents exhibited reverse vertical grading of the large, light particles. Efficient segregation of the light component was observed even if the bulk density of the current was less than that of the light particles. In both sets of experiments, marked inflexions in the rate of downstream decline in mass loading and maximum grain size of the dense component can be attributed to the presence of two different particle settling regimes in the flow: (1) particles with Rouse numbers >2·5, which did not respond to the turbulence and settled rapidly; and (2) particles with Rouse numbers <2·5, which followed the turbulent eddies and settled slowly. The results are applied to the transport and sedimentation dynamics of pyroclastic density currents that generate large, widespread ignimbrites. Field data fail to reveal significant departures from aerodynamic equivalence between pumice and lithic clasts in three such ignimbrites: the particulate loads of some large ignimbrites are transported principally in turbulent suspensions of low concentration. In some ignimbrites, the well‐developed inflexions in curves of maximum lithic (ML) size vs. distance can be attributed to the existence of distinct high and low Rouse number particle settling regimes that mark the transition from an overcharged state to one in which the residual particulate load is transported more effectively by turbulence.     

The feasibility of a slip velocity model for predicting the enrichment of chromite in a Floatex density separator

The Floatex density separator (FDS) is an industrial separator and works on the principle of hindered settling where the settling rate of a particle in suspension is affected by nearby particles. This phenomenon of hindered settling has been described by various authors using the concept of particle slip velocity. The feasibility of one such slip velocity model proposed by Galvin et al. to predict the separation of chromite in a plant scale FDS was examined. Previously this model was validated only at lab scale with synthetic mixture of various density particles. In order to use this model, the feed chromite ore was characterized and quantified into different density mineral classes and their percentages were estimated by using mineralogical grain count method. The performance of FDS was then predicted using slip velocity model in terms of weight recoveries and composition of different minerals in the FDS underflow product.     

Statistical representation of generalized distribution data for float-sink coal-cleaning devices: Baum jigs,Batac jigs,Dynawhirlpools

In an earlier paper (1978), Gottfried presented a method for combining distribution data for float-sink coal-cleaning devices into a single generalized distribution curve which, for a given device and feed size, is independent of specific gravity of separation. A non-linear, exponential-type equation was utilized to represent the generalized distribution curve, along with the corresponding generalized probable error. Distribution data for six common coal-cleaning devices have previously been treated by this method.This paper is an extension of two previous studies (Gottfried, 1978, 1980). The method described above is applied to three different float-sink coal-cleaning devices: Baum jig (replacing previously reported results), Batac jig and Dynawhirlpool separator. Results for the Baum jig and Batac jig reflect a two-stage separation process, with a set of generalized distribution curves obtained for each stage and another set for the overall separation. Several different feed size fractions are given for each vessel.     

滑源区粒序分布及颗粒粒径对碎屑流冲击作用的影响研究

滑体的运动速度、堆积形态、冲击力等因素决定了碎屑流的致灾程度。滑源区不同岩性特征和结构分布的差异导致了滑体粒序分布和颗粒粒径的差异。在运动过程中产生的碰撞、摩擦、跳跃,影响着滑坡碎屑流的致灾程度。在物理模型试验的基础上,运用三维离散元软件PFC3D,探究滑源区粒序分布及颗粒粒径对滑体运动速度、堆积形态、冲击力的影响。研究结果表明:碎屑流中各粒径颗粒的平均速度受颗粒粒径及滑源区初始粒序的共同影响,且初始粒序对各颗粒平均速度影响更大;在堆积形态方面,粒径大小对厚度方向上的粒序排布影响较大,而滑源区粒序分布对单种颗粒的堆积形态影响较大;在颗粒分选作用下,颗粒粒径成为控制峰值冲击力的主要因素,而滑源区粒序分布则通过决定滑体堆积形态控制了准静态堆积阶段碎屑流的冲击力。     

粗粒料级配缩尺后最大干密度试验研究

对某级配粗粒料,采用剔除法、等量替代法、相似级配法和混合法等4种不同缩尺方法,依据规范要求进行缩尺。缩尺后替代级配料的最大颗粒粒径分别为20、40、60 mm。对各替代级配料采用振动台法进行了最大干密度试验,基于试验成果,结合分形理论,提出了一种将最大干密度与级配及细粒含量之间关系归一化的方法,并拟合了最大干密度与试验前级配的分形维数、小于5 mm颗粒的含量及最大粒径之间的关系,据此可推求出原型级配料的最大干密度,探讨了缩尺方法对替代料试验前后粒径分布曲线变化幅度的影响,总结出颗粒破碎分形维数与试验前级配的分形维数、小于5 mm颗粒含量及最大粒径之间的关系式,据此可推求出填筑后原型级配料的颗粒破碎分形维数。     

Analysis and optimization of mineral processing and coal-cleaning circuits — Circuit analysis

In mineral processing and coal cleaning operations, there are two classes of unit operations used in the separation processes. The first type — crushing, grinding, pelletizing and settling — modifies the size distribution of the feed. The second type — heavy media, sizing, flotation, magnetic, electrostatic and jigging — separates the particles. In these latter unit operations, the size distribution is not changed; rather, the feed stream is split into a product and waste stream based on subtle physical and physicochemical differences between the different types of feed particles. In separation unit operations, the mathematical functions, representing the behavior of the unit operations, are easy to manipulate. Thus, a number of theorems, useful to both the plant designer, and the plant engineer, can be proven.The work presented in this paper makes the fundamental assumption of linearity as do all current plant design analysis systems. The linearity assumption states that in a separation operation, there is no particle-particle interaction effecting the probability that a particle will be selected for the product or waste stream. In other words, if one doubles or triples the feed rate to a unit operation, the fraction of particles of a given characteristic selected to appear in the product stream remains unchanged. In actuality, this assumption is not true because higher feed rates affect the behavior of separation operations. However, when designing a plant, one can calculate the feed rate to a given unit operation and then select a piece of equipment large enough to handle the computed tonnage. Thus, the designed circuit behaves linearly.     

钙质砂压缩波速与物理性质参数关系研究

开展钙质砂纵波波速与物理性质参数关系的试验研究,对珊瑚礁地基无损检测及工程物探具有重要的理论指导意义和工程应用价值。控制试样的物性参数状态,利用制样装置进行波速测量,揭示钙质砂纵波波速与物性参数的相关关系。试验结果表明：含水率是影响纵波波速的主要因素,与纵波波速呈二次函数关系。CT扫描结果表明,试样粒径越大,颗粒内部孔隙越丰富,这导致试样孔隙比越大,波速故而越小,与粒径、孔隙比呈负相关关系。在密实度相同、不均匀系数不同时,级配良好的砂样纵波波速差异不大。经过侧限压缩后,含水率一定时,粉土纵波波速与孔隙比呈二次关系,且与密度呈良好的线性关系。利用粉土易压缩且与纵波波速紧密变化的特点,可积极探索声波在粉土地基密实度检测的工程应用。     

Hindered settling of sand grains

The sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects. The behaviour of various particulate systems involving sand has been compared with the widely used Richardson–Zaki expression. The general form of the expression is found to hold, in as much as remaining as a suitable means to describe the hindered settling of irregular particles. The sedimentation exponent n in the Richardson–Zaki expression is found to be significantly larger for natural sand grains than for regular particles. The hindered settling effect is therefore greater, leading to lower concentration gradients than expected. The effect becomes more pronounced with increasing particle irregularity. At concentrations around 0·4, the hindered settling velocity of fine and medium natural sands reduces to about 70% of the value predicted using existing empirical expressions for n. Using appropriate expressions for the fluidization velocity and the clear water settling velocity, a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes.     

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

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.     

Experimental study of particle migration under cyclic loading: effects of load frequency and load magnitude

The study of particle migration in porous media under cyclic loading is the key to understand the mechanism of mud pumping hazard in railway embankments. This paper presents a series of particle migration tests, in which soil particles migrate into an overlying gravel layer under cyclic loading. The results show that the increase in loading frequency and load magnitude leads to more particle migration upwards at a greater rate, implying that the train speed and axle loads affect the extent of mud pumping. The slurry turbidity in the gravel layer increases to a steady state value with time. Soil particles smaller than 5 μm have the potential to diffuse into the entire gravel layer, and larger particles tend to aggregate in the bottom layer of the gravel. The backward erosion gradually develops deeper into the soil layer, and there is a maximum erosion depth associated with each load frequency and load magnitude. As for the mechanism, the pore water pressure oscillates because of liquid sloshing. Its amplitude is much larger in the gravel layer than that in the soil layer due to their difference in permeability. The axial hydraulic gradient acts as a pumping effect to stimulate the migration of soil particles. Increasing load frequency is conducive to the generation of a stronger pumping effect at the gravel–soil interface. Increasing load magnitude does impact not only the extent of pumping effect, but also the development of an interlayer which plays an important role in promoting particle migration.

     

基于B超成像的泥沙起动流速测量方法

床面上泥沙颗粒的起动测量一直是泥沙运动规律研究的难题。为此,提出了一种基于B超成像技术的泥沙起动流速测量方法。在模型试验水槽中,该方法利用B超仪获取水下地形及其附近粒子的运行图像,通过图像处理技术分析统计床面附近运动粒子的成像光斑个数,并分析其与流速之间的关系。结果表明,B超成像光斑个数在泥沙起动过程中存在一个突变过程,且该突变过程与泥沙起动运动相对应,可用来判定泥沙起动及其对应的起动流速,并利用该流速下的床面地形变化验证了该方法的正确性。该方法具有无扰动无干扰、适合于清水和浑水、易于实现自动化测量的特点。     

基于离散元模拟的二元颗粒柱坍塌特性研究

常见的地质灾害如滑坡、泥石流、岩崩等通常都涉及不同形状的颗粒物质运动,这些形状不同的颗粒又多具有不同的尺寸和含量。基于典型的颗粒柱坍塌试验,首先根据试验方法确定了离散元模拟所需的各项参数,然后采用随机多面体方法生成了可控制长细比的大颗粒,利用离散元法就不同大颗粒含量下形态变化对二元颗粒柱坍塌特性的影响开展研究,研究结果表明:(1)利用离散元法可以较好地重现室内试验中小球和多面体组成的二元颗粒系统的颗粒柱坍塌过程;(2)在不同长细比的不规则大颗粒和小球组成的二元颗粒柱系统中,当大颗粒含量高于临界含量值20%时,二元颗粒柱坍塌持续的时间随非球形大颗粒长细比的增加而增加;(3)在不同长细比的不规则大颗粒和小球组成的二元颗粒柱中,当大颗粒含量高于临界含量值20%时,在相同百分比的大颗粒含量下,大颗粒长细比的增加会提高大颗粒平均配位数以及降低颗粒的运动能力,大颗粒间形成更强的互锁作用,降低了颗粒柱的整体流动性,使其最终堆积高度更高、最大跑出距离更短以及更小的归一化动能峰值。(4)在不同长细比的不规则大颗粒和小球组成的二元颗粒柱中,小颗粒可以较为明显降低大颗粒间摩擦及互锁作用,增加流动性,降低大骨料形态对坍塌过程的影响。     

New method and equations for determining attachment tenacity and particle size limit in flotation

A new method and simple, yet accurate, equations for determining the tenacity of particle attachment and the particle size limit in flotation were developed by applying the force analysis of the gravity–capillarity coupling phenomena controlling the bubble–particle stability and detachment. Approximate solutions to the Young–Laplace equation were used to develop simple equations for the tenacity of attachment of particles with diameter up to 20 mm. Simple equations for the maximum size of floatable particles were derived as explicit functions of the particle contact angle, the surface tension, the particle density and the mean centrifugal acceleration of turbulent eddies. For the typical particle size and contact angle encountered in flotation, the analysis showed that the bubble size has little effect on the tenacity of particle attachment. The prediction for the largest size of floatable particles is compared with the experimental data and signifies influence of turbulence on the particle detachment.     

粗粒料级配缩尺后压实密度试验研究

对某级配粗粒料采用混合法、剔除法、等量替代法和相似级配法等4种不同缩尺方法进行缩尺。缩尺后替代级配料的最大粒径分别为10、20、40和60 mm。对各替代级配料进行了相对密度试验。同时,为完善试验成果,又人工随意配置了3种级配料用于试验。最大、最小干密度试验分别采用振动台法和松填法。研究同一原型级配料不同方法缩尺后各级配土石料在同一压实功能情况下的最大、最小干密度与土料级配参数、最大粒径之间的关系。提出了一种将干密度与级配之间关系归一化的方法,并拟合了干密度与Cu、Cc及最大粒径之间的关系,据此可推求原型级配料的最大干密度。