煤颗粒与气泡黏附行为的试验研究

浮选微观模型认为,颗粒与气泡的黏附是实现浮选的关键步骤,对颗粒与气泡黏附规律的直接研究非常重要。采用自行设计搭建的颗粒与气泡碰撞、黏附行为测量装置,以内蒙古公乌素原煤为试验对象,直接观测了不同密度级的0.1~0.15 mm粒级煤样的黏附行为,并采用自行开发的多目标追踪软件进行分析。结果表明:煤颗粒在与气泡碰撞前会发生绕流,速度大小和方向均会改变,当煤颗粒与气泡碰撞时,煤颗粒的速度降为最低。煤颗粒在气泡表面的滑动速度先是逐渐增大,在气泡"赤道"位置处达到最大值,越过"赤道"后,煤颗粒的滑动速度逐渐减小,并最终黏附在气泡底部。煤颗粒与气泡的黏附效率随碰撞角的增大而降低,在碰撞角相同时,随煤样密度级的增大,黏附效率降低,临界黏附角减小。随煤颗粒沉降末速的增大,煤颗粒与气泡的黏附效率降低,临界黏附角减小。     

浮选中颗粒-气泡间相对运动研究进展

颗粒-气泡间相对运动的研究对浮选机理的认知至关重要,对新型浮选机的开发和提高浮选效率均具有指导意义,本文系统综述了颗粒-气泡间相对运动的研究进展。早期研究过程中,研究者忽略了颗粒和气泡性质的影响,将颗粒视为随流线运动的点,气泡视为刚性球体,利用流线方程对颗粒-气泡间的相对运动展开研究;随着认知过程的不断深入,颗粒和气泡物理化学性质的影响逐步得到了关注,研究者分别从颗粒惯性力、重力、形状和粗糙度以及气泡表面流动性等方面并展开了大量研究;颗粒-气泡间相对运动的试验研究多通过颗粒沉降法进行,研究对象由单个玻璃微珠发展为大量矿物颗粒,且出现了关于运动玻璃球与上升气泡之间相对运动的研究。研究表明,当颗粒粒度较细、密度较小时,利用流线方程对颗粒-气泡间相对运动的研究具有一定的适用性;当颗粒粒度较粗、密度较大时,需考虑正负惯性力、重力等因素对颗粒-气泡间相对运动的影响。此外,颗粒形状的不规则性会影响颗粒周围液体对颗粒的作用力,导致临界碰撞半径减小,且颗粒表面不规则的凸起会促进颗粒-气泡间水化膜的破裂,减少诱导时间,增大颗粒表面粗糙度有助于增强颗粒-气泡间的黏附强度。气泡表面的流动性可采用"滞留帽"模型进行分析,具有较好的适用性。对于颗粒-气泡间相对运动的试验研究主要采用颗粒沉降法,亲水玻璃微珠只能在气泡上半球滑行,到达气泡赤道位置附近后便离开气泡,疏水玻璃微珠会刺破颗粒-气泡间的水化膜,越过气泡赤道后会继续沿气泡表面滑行并最终黏附在气泡底部,煤颗粒与气泡的黏附效率随碰撞角和密度的增大而减小。然而目前的试验研究多集中于静水领域,对于浮选流场中颗粒-气泡间相对运动的试验研究尚需进一步探索。     

煤颗粒与气泡间相对运动过程的试验研究

针对当前颗粒与气泡间相对运动过程研究中理论推导居多、试验研究相对匮乏的情况,在阐述和分析两种颗粒与气泡间相对运动过程监测方法的基础上,提出了一套颗粒与气泡碰撞、黏附行为测量系统,并利用该系统对内蒙古公乌素原煤颗粒与气泡间的相对运动过程进行了观测。试验结果表明:在远场区,煤颗粒处于自由沉降状态,运动轨迹不受影响,进入近场区后,受气泡阻力的影响,煤颗粒逐渐偏离原来运动轨迹,趋于沿气泡切线方向运动;在远场区,煤颗粒以沉降末速向气泡逼近,随着二者间距离的不断减小,煤颗粒运动速度逐渐降低,且在二者碰撞时达到最低,之后煤颗粒沿气泡表面向下滑落,运动速度逐渐增大,当滑落至气泡"赤道"位置时达到最大,越过气泡"赤道"位置后又开始降低;当煤颗粒滑落至气泡中心以下0.34 mm位置时,彻底从气泡上脱落。     

湍流条件下气泡-颗粒间碰撞效率的数值计算

为了研究浮选过程中的气泡与微细粒煤泥之间的碰撞效率,提出了一种基于数值模拟软件FLUENT的碰撞效率数值计算方法,并采用此方法模拟了微观尺度下不同颗粒粒径、颗粒密度、气泡直径、流场湍流强度下单气泡与颗粒碰撞行为,计算出气泡-颗粒间的碰撞效率,得到各因素对单气泡与颗粒碰撞效率的影响规律。结果如下:颗粒粒度和气泡尺寸是影响气泡颗粒间的碰撞效率的主要因素。随着颗粒粒度、密度以及湍流强度的增加,碰撞效率增大。在静水中,碰撞效率随气泡尺寸的增大而增大;在湍流中,随着气泡尺寸变大,碰撞效率呈减小趋势。     

基于高速动态的颗粒气泡脱附过程动力学研究

通过颗粒气泡脱附高速动态测试系统,研究了颗粒气泡脱附过程动力学。运用Image-Pro Plus图像处理软件测量颗粒气泡间接触角、三相润湿周边,计算颗粒气泡间毛细黏附力随颗粒运动时间的变化。结果表明：颗粒从气泡表面脱附主要分为气泡拉伸变形接触角增大和气泡滑动三相润湿周边减小两个阶段。气泡拉伸阶段,三相润湿周边固定在颗粒表面,接触角由平衡接触角增大到前进接触角;气泡滑动阶段,接触角保持不变,三相润湿周边滑动减小。毛细黏附力在气泡脱附过程中随接触角增大而增大,随三相润湿周边滑动而减小,当外力超过颗粒气泡间临界黏附力时,颗粒从气泡表面脱附。     

浮选中起泡剂浓度对气泡尾涡区特征及颗粒卷吸的影响

为探明浮选中起泡剂浓度对气泡及其尾涡区特征的影响，预测颗粒在浮选气泡尾涡区的卷吸概率，通过实验室自制的拟上升气泡装置及粒子图像测速系统，研究了不同浓度仲辛醇条件下的气泡尾涡区流场特征，分析了气泡表面流体分离行为及尾涡特征；通过高速摄像系统的同步观测，研究了气泡的形态特征以及颗粒在气泡尾涡区的轨迹和分布概率。研究结果表明：随着起泡剂浓度的增加，气泡的尺寸略有减小而长径比逐渐增大；气泡边界层分离角随起泡剂浓度的增加逐渐增加，存在临界分离角196.70°；气泡引起的尾涡主要集中在流场流速小于0.09 m/s的区域，气泡尾涡区高度随起泡剂浓度的增加逐渐减小，存在最小尾涡区临界值为气泡直径的1.06倍；颗粒在气泡尾涡区存在3种运动轨迹，颗粒卷吸的运动轨迹可以分为3个显著阶段，颗粒受力是导致其卷吸类型区别的关键因素；随着起泡剂浓度的增加，颗粒被卷吸的范围与概率逐渐减小。影响气泡尾涡及颗粒运动的临界浓度均为1.6×10^-4 mol/L。研究结果明晰了起泡剂浓度对矿物颗粒在气泡尾涡区卷吸的机理，为微细矿物浮选技术的发展提供了有价值的指导。     

加载方式与冲击速度对煤裂纹扩展规律的影响

为研究加载方式与冲击速度对煤裂纹扩展规律的影响,采用非对称加载半圆弯曲试件开展了冲击载荷下Ⅰ型、Ⅱ型及Ⅰ-Ⅱ复合型3种加载方式下多种冲击速度的断裂试验与数值模拟,研究了加载方式和冲击速度对煤样动态断裂裂纹扩展过程、裂纹起裂时间和煤样破坏形式的影响规律。结果表明:煤样在Ⅰ型动态加载下,裂纹呈直线形式;在Ⅰ-Ⅱ复合型和Ⅱ型加载下,裂纹呈向左上方扩展的曲线形式;3种不同加载方式下,煤样裂纹起裂时间均随冲击速度增大而减小,其影响逐渐减弱,煤样在Ⅱ型加载下起裂时间明显大于其他2种加载方式;煤样破坏程度均随冲击速度的增大而加大,Ⅰ型加载影响较小,Ⅰ-Ⅱ复合型和Ⅱ型加载影响较大。     

淀粉分子结构对钛铁矿与浮选气泡间相互作用的影响研究

淀粉是由糖苷键结合的多糖聚合物,可作为调整剂运用于钛铁矿反浮选脱硅。矿物颗粒与浮选气泡 的相互作用是决定矿物浮选效率的关键因素,而不同的淀粉由于分子结构的差异对其有重要影响。研究了在不同 玉米淀粉作用下的钛铁矿颗粒与气泡的碰撞及黏附过程。采用自行搭建的颗粒-气泡相互作用观测装置,发现不 同淀粉组分对钛铁矿颗粒与气泡的碰撞概率及黏附概率有重要影响。结合接触角检测和激光粒度测试试验推测 支链淀粉含量越高的淀粉分子更容易团聚矿物颗粒从而增大其与气泡的碰撞概率,且其更容易减小钛铁矿疏水性 从而降低其与气泡的黏附力。最后基于试验结果和前人理论,建立了颗粒与气泡碰撞及黏附过程的数学模型,并 讨论了淀粉分子结构在模型中的作用。     

含水煤样动态拉伸能量演化与破坏特征试验研究

为研究不同含水煤样动态拉伸变形破坏过程的能量耗散规律,利用分离式霍普金森压杆(SHPB)试验系统,对不同含水煤样进行冲击加载下的动态劈裂试验,并结合超高速数字图像相关(DIC)试验系统对煤样动态拉伸破坏过程进行观测。基于试验结果分析,获得了煤样破坏过程能量耗散特性随含水率的变化规律,分析了含水率对破碎煤样分形维数的影响。研究结果表明,冲击载荷下应力波是煤样内部大量微损伤结构及原生孔隙、空隙损伤演化的主控因素,煤岩体破碎是一个能量吸收与耗散的过程,随着冲击载荷的增加煤样耗散能密度呈线性增大,但当入射能较小时煤样耗散能密度值相差不大;试样分形维数随加载气压的增加而增加,且增加速率有减小趋势,同种加载气压下,饱和煤样的分形维数最大,干燥煤样的最小;煤样破坏主要以拉伸劈裂为主,破坏裂纹沿加载方向发育,率先在圆盘中部起裂,随后萌生多条次生裂纹,次生裂纹随加载气压的增大而增多,低加载气压下,劈裂裂纹在煤样中的扩展时间较长,扩展速度较慢;基于数字图像技术发现冲击载荷下饱和煤样中部出现多个主应变集中域,且范围逐渐扩大最终沿径向发育贯通。     

淀粉分子结构对钛铁矿与浮选气泡间相互作用的影响研究

淀粉是由糖苷键结合的多糖聚合物,可作为调整剂运用于钛铁矿反浮选脱硅。矿物颗粒与浮选气泡 的相互作用是决定矿物浮选效率的关键因素,而不同的淀粉由于分子结构的差异对其有重要影响。研究了在不同 玉米淀粉作用下的钛铁矿颗粒与气泡的碰撞及黏附过程。采用自行搭建的颗粒-气泡相互作用观测装置,发现不 同淀粉组分对钛铁矿颗粒与气泡的碰撞概率及黏附概率有重要影响。结合接触角检测和激光粒度测试试验推测 支链淀粉含量越高的淀粉分子更容易团聚矿物颗粒从而增大其与气泡的碰撞概率,且其更容易减小钛铁矿疏水性 从而降低其与气泡的黏附力。最后基于试验结果和前人理论,建立了颗粒与气泡碰撞及黏附过程的数学模型,并 讨论了淀粉分子结构在模型中的作用。     

The effect of microhydrodynamics on bubble-particle collision interaction

Interactions between particles and bubbles are influenced by hydrodynamic forces of the aqueous medium in which the flotation process takes place. This paper investigates the effect of liquid hydrodynamic forces working at short inter-surface separation distance, referred to as microhydrodynamic forces, on the bubble-particle collision (encounter) interaction. The full equation of particle motion around an air bubble with either a mobile surface (e.g., the potential flow) or an immobile surface (e.g., the Stokes flow) has been solved and analyzed numerically. The effect of particle density, size and film thickness (i.e., inter-surface separation distance) on the bubble-particle collision angle and efficiency has been examined. The new results were compared against the results obtained under the condition that microhydrodynamic effect has been ignored (the conventional theory). The effect of microhydrodynamics on the collision angle and efficiency has been found significant. Generally, the microhydrodynamic effect decreases the collision efficiency due to retarding the particle approach to the rising bubble surface. There also exists a critical set of particle size and density, where the collision angle is minimal, for both the mobile and immobile bubble surfaces. Away from the critical particle size and density the collision angle increases to 90°.     

无烟煤颗粒对浮选体系中气泡运动及兼并行为的影响

为探究颗粒浓度及粒度对浮选体系中气泡运动及兼并行为的影响,以无烟煤颗粒为研究对象,采用单气泡兼并行为观测系统,对不同条件下的气泡兼并行为进行了研究.试验结果表明:随着无烟煤颗粒浓度的增加,溶液的表面张力及气泡挣脱直径逐渐下降,但下降趋势逐渐变缓;随着颗粒粒度的增加,二者变化趋势不明显;气泡的运动速度随着颗粒粒度和浓度的...     

The limits of fine particle flotation

Understanding the limits of fine particle flotation is the key to the selective separation of fine mineral particles. Fine particles have low collision efficiencies with gas bubbles and float slowly. There has been a great deal of work aimed at overcoming the inefficient collision of small particles with rising air bubbles. This review deals with the influence of bubble size, particle aggregation, different flow conditions, particle induction time, as well as the action of surface and capillary forces on fine particle–bubble capture. Recommendations for practice are given.     

宽粒级煤炭颗粒在三相流化床中的分选实验研究

拓宽浮选粒度上限是浮选领域挑战的难题之一。应用三相流化床技术,在上升水流中引入气泡流,对比有无气泡对煤炭颗粒床层膨胀度的影响,得出气泡流的引入能够加强低密度颗粒的流化。并在单独上升水流、上升水流+气泡流和上升水流+气泡流+捕收剂(煤油)3种水流条件下,进行煤炭分选实验。结果表明,气泡流的引入,能够大幅减少低密度级颗粒悬浮所需的上升水流速度,气泡流对低密度级颗粒的影响强于高密度级,气泡的存在强化了颗粒密度、弱化了粒度对分选的影响;在上升气泡流中添加捕收剂后,低密度级颗粒与气泡形成颗粒-气泡结合体,低密度级颗粒便能够在较低的上升水流速度下进行分选,低速上升水流稳流度较低,保证了大颗粒与气泡的稳定性。添加捕收剂后,各粒级颗粒分选所需上升水流速度较为接近,各粒级精煤灰分在10%左右。     

CFD modelling of bubble–particle collision rates and efficiencies in a flotation cell

Computational fluid dynamic (CFD) modelling of a Denver-type flotation cell has been performed. Bubble–particle collision rates in different parts of the cell have been calculated from the local turbulent velocities, and the size and number concentrations of bubbles and particles obtained from CFD modelling. The probability of collision due to streamline effect of fine particles moving around the bubble has also been estimated. The local attachment rate based on the collision rate and collision probability is then calculated and found to decrease as particle size decreases. This is consistent with the decrease in flotation recovery of fine particles as observed in flotation practice. The magnitudes of the collection rate constants obtained from CFD modelling indicate that transport rates of the bubble–particle aggregates to the froth layer may contribute quite significantly to the overall flotation rate in plant-scale equipment.     

Mineralization of Bubbles during Flotation

The study is conducted for the dynamics of thinning of liquid film separating mineral particle and gas bubble at the moment of their collision in foam flotation. It is shown that for floatable and depressed minerals, the values of potentials of particles and the advancing contact angle as well as the values of potentials of bubbles and the surface tension of gas - liquid interface can be obtained by solution of problems on thinning of liquid film and determination of meniscus shape.     

Bubble–particle detachment in a turbulent vortex I: Experimental

The mechanics of the detachment of particles from bubbles in the flotation process in a turbulent environment are unclear. The traditional hypothesis assumes a bubble–particle aggregate is trapped inside an eddy of equivalent size, and the attached particles rotate at the same speed as the eddy. The rotational movement subjects the attached particles to a centrifugal force. It is theorised that particles detach when the centrifugal force is greater than the capillary force, but this hypothesis has not yet been experimentally proven.This work is an experimental study of bubble–particle detachment in a rotating eddy. A special experiment was designed to obtain a strong confined vortex, and bubble–particle aggregates were introduced into the cavity without destroying the vortex structure. This newly developed method, which provides a realistic analogue of the turbulent conditions in a flotation cell, is well suited to the study of an important sub-process of flotation in a turbulent field, namely, the stability of single bubble–particle aggregates.Particles can detach from bubbles by a number of ways, including inertial actions induced by rapid changes in direction, and disruption due to coalescence of colliding bubbles. In this paper, we focus on a particular mechanism, in which bubbles are observed to rotate in a turbulent vortex. Particles can be held on the surface of the bubble by surface tension, and the radial centripetal force induced by the rotation is sufficiently high, particles may detach. Experiments are described in which the process of particle detachment due to centrifugal movement, was captured by a high-speed video camera, and the necessary physical parameters, especially the rotational velocity of the particles, were extracted. For the first time, centrifugal movement of the particle on the bubble surface inside a vortex was observed, and the theory of detachment due to centrifugal forces in the turbulent field was experimentally proven.