颗粒气泡黏附科学——宏观尺度下颗粒气泡黏附研究进展及困境

颗粒气泡黏附指从颗粒与气泡相遇开始到液膜发生薄化破裂最后至三相润湿周边铺展形成稳定矿化气絮体的过程,是浮选中的核心作用单元。然而浮选颗粒气泡黏附机理至今仍不明确。黏附过程主要受颗粒气泡的表面物理化学性质及溶液化学条件影响,表面力及流体作用力协同支配微纳尺度下颗粒气泡间液膜薄化破裂行为。排液过程中气液界面的变形效应进一步增加了系统复杂性,上述因素使得颗粒气泡黏附的理论研究及试验探索步履维艰。早期关于颗粒气泡黏附的研究主要聚焦于黏附概率,其中宏观尺度下的诱导时间测试占据主导地位,通过诱导时间结果计算黏附概率。对国内外宏观尺度下颗粒气泡黏附概率模型及研究技术手段进展展开全面综述,并对现有技术瓶颈及局限进行分析。诱导时间测量仪及高速动态摄影技术大大促进了浮选工作者对颗粒气泡黏附的理解,“诱导时间与实际浮选回收率具有着良好的相关关系”也已经被广泛证明。然而因微纳尺度下的表面力及液膜薄化动力学信息的缺失导致宏观诱导时间并不能从基础层面揭示颗粒气泡的黏附机理,微纳尺度下颗粒气泡间相互作用力及液膜薄化动力学的定量测试表征是技术发展的必然趋势,其可为浮选微观矿化反应过程提供新的理论视角,同时也为难浮煤及难选矿浮选过程强化提供理论支撑。     

相互作用力及液膜排液动力学研究进展

颗粒气泡间相互作用力及液膜薄化破裂动力学是揭示浮选黏附机理的核心,也是近年来浮选胶体化学领域的研究热点。为深入明晰浮选黏附机理,对当前颗粒气泡间相互作用力及液膜排液动力学模型理论研究进展进行了系统综述。对于颗粒气泡间相互作用力,疏水引力可克服颗粒气泡间范德华力和静电斥力,诱发黏附。不同作用程范围内疏水力的来源机制不同:长程疏水力(20 nm)主要源于固液界面亚微米/纳米气泡桥接,而短程疏水力(20 nm)则主要源于固液界面水分子重排效应。由于疏水力强烈的吸引性和气液界面变形,颗粒气泡间疏水力的定量表征仍存在较大的挑战。对于颗粒气泡间液膜排液动力学模型,最具代表性的有Stefan-Reynolds平坦膜模型,Taylor模型和Stokes-Reynolds-Young-Laplace(SRYL)模型。Stefan-Reynolds及Taylor模型并未考虑排液过程中气泡表面曲率的变化,其应用存在着较大的局限性。SRYL模型则在描述液膜薄化速率的同时,兼顾了气泡表面在流体力和表面力等外力作用下的变形行为。在给定起始与边界条件下,SRYL模型通过数值迭代法与液膜排液试验测试结果对比,可以计算出颗粒气泡间的相互作用力信息;也可通过与相互作用力试验结果对比获得液膜排液数据。在今后的研究中,应重点将SRYL模型与试验测试相结合,对颗粒气泡间疏水力进行定量表征,揭示浮选黏附机理。     

颗粒气泡黏附科学:基于AFM和DWFA的颗粒气泡间疏水作用力研究

为探索颗粒气泡体系疏水力的长程及短程来源机制,分别采用原子力显微镜(AFM)和浮选动态润湿膜分析仪(DWFA)对气泡与同一疏水玻璃基板间的疏水力进行测试。AFM发现气泡与亲水性玻璃基板间的相互作用力为单调斥力作用,体系不存在诱发液膜失稳的引力作用项。疏水性颗粒气泡间的液膜是不稳定的,当AFM负载力到达19. 3 nN时,力曲线中观察到了明显的跳入黏附现象。疏水玻璃与气泡间的疏水力以3. 50 nm的衰减长度按单指数模型衰减,液膜在32. 96 nm临界破裂厚度处破裂。该疏水力倾向于一种短程力(50 nm),其源自界面的水分子重排熵效应。DWFA法同样发现亲水性玻璃基板与气泡间的液膜是稳定的,当总分离压力与气泡内部拉普拉斯压力相等时,液膜到达133 nm的平衡膜厚度。疏水性玻璃基板与气泡间的液膜是不稳定的,液膜发生快速薄化并分别在185 nm临界膜厚处破裂。对疏水力进行定量求解发现该力以47. 30 nm的衰减长度衰减,所获得的疏水力为一种长程作用力,该力源于固液界面纳米气泡空化效应。AFM和DWFA排液试验中所用的气泡尺寸分别为微米级及毫米级,疏水力受气泡本身的尺寸影响,与气泡表面的毛细波传播有关,在吸引力作用下大气泡表面会形成更强烈的毛细波震荡。由于疏水界面水分子的热力学不稳定性,这种界面波动会诱发疏水固液界面空化气泡的析出,增加了引力作用程。     

固-液界面纳米气泡稳定性及其强化浮选黏附机制研究进展

颗粒-气泡黏附是浮选核心作用单元,驱动其自发黏附的主要作用为疏水颗粒-气泡间疏水引力。作为长程疏水引力主要来源,界面纳米气泡对浮选界面调控有重要影响。从纳米气泡的基本性质、稳定性机理及浮选强化机制3个方面进行了系统讨论。纳米气泡异常稳定性和接触角一直是近20 a来的研究热点。经典物理学理论预测纳米气泡寿命在微秒尺度,而试验发现纳米气泡寿命通常可达数天以上。针对纳米气泡异常稳定性提出污染物层、动态平衡、三相线钉扎等假说,然而各假说均无法解释所有试验现象,其稳定性机理仍需要深入研究。纳米气泡接触角(气侧)远小于Young接触角,高密度气体导致的固-气界面能降低可能是接触角异常的主要原因。对纳米气泡强化浮选黏附机制进行了探讨,一方面界面纳米气泡可通过边界滑移促进颗粒-气泡碰撞过程中液膜排液,另一方面纳米气泡桥接使颗粒-气泡出现长程引力,同时颗粒-气泡间的DLVO力由排斥力转变为引力,从而促使颗粒-气泡黏附。目前已有试验表明纳米气泡在煤、磷酸盐、白钨矿及铁矿石等多种矿物的浮选中均有显著提升效果。在浮选日益精细化的背景下,纳米气泡强化技术可为浮选界面调控提供新的理论视角与技术手段,是未来浮选领域...     

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。     

浮选系统中颗粒与气泡的碰撞行为研究

浮选设备中气泡与颗粒的碰撞是一个复杂的力学过程,本文设计了一套电磁弹射装置,将颗粒以可控的速度和角度射向水中的静止气泡,用以简化模拟真实的浮选现象。高速相机被用来监测这一碰撞过程,通过对图像进行分析,提取了颗粒的运动参数以及气泡的变形信息,研究了颗粒与气泡的作用时间与液膜排液规律,并推导了一个数学模型来预测碰撞现象的时间尺度并提出了黏附判据。     

浮选系统中粗颗粒与气泡的碰撞行为研究

浮选设备中气泡与颗粒的碰撞是一个复杂的力学过程,本文设计了一套电磁发射装置,将颗粒以可控的速度和角度射向水中的静止气泡,用以简化模拟真实的浮选现象。高速相机被用来监测这一碰撞过程,然后对图像进行分析,提取了颗粒的运动参数以及气泡的变形信息,研究了颗粒与气泡的作用时间与液膜排液规律,并推导了一个数学模型来预测碰撞现象的时间尺度并提出了黏附判据。     

潮湿煤颗粒间液桥力的理论研究

通过颗粒间水分作用的分析建立了颗粒间液桥力与其亲水性、颗粒大小、水分含量之间的关系，得出：小颗粒间的液桥力大于大颗粒间的液桥力，亲水性颗粒间的液桥力大于疏水性颗粒间的液桥力，钳入角（水分）适中，颗粒间液桥力最大，当水分超过临界值时，颗粒间的吸引力变为排斥力。     

柴油对浮选泡沫稳定性影响的试验研究

泡沫稳定性是影响浮选过程效率的重要参数之一。为了探究柴油对浮选泡沫稳定性的影响,借助泡沫扫描分析仪(FOAMSCAN)研究了气液两相体系下不同浓度的柴油与体积分数20×10-6的甲基异丁基甲醇(MIBC)混合溶液的起泡能力与泡沫稳定性,采用动态液膜分析装置分析了泡沫间液膜的最终状态,进一步明晰了柴油对泡沫稳定性的影响机制,并通过细粒煤浮选及气液固三相泡沫稳定性试验探讨了柴油对实际浮选体系泡沫性质及浮选效果的影响。气液两相体系泡沫稳定性试验表明,随着柴油浓度的增加,溶液起泡能力和泡沫稳定性逐渐降低。泡沫间液膜测试结果说明,柴油浓度加大使得泡沫间液膜由最终的平衡状态转为破裂状态,液膜稳定性变差,气泡更容易兼并甚至破裂,该结论与气液两相泡沫稳定性试验结果保持一致。浮选结果表明,柴油用量较低时,随着柴油浓度增加,最大泡沫层高度和半衰期逐渐增大,浮选精煤产率也随之增大,这主要是由于柴油改善煤样表面疏水性以及细粒煤的稳泡作用所致;但当柴油用量增加到一定浓度后,最大泡沫层高度和泡沫半衰期减小,浮选精煤产率减小,一方面,柴油油滴进入泡沫间液膜中,在范德华力等力的驱使下,泡沫间的液膜逐渐薄化直至形成经典的油滴架桥现象,最终导致气泡兼并甚至破裂,另一方面,柴油油滴竞争吸附起泡剂分子,使得气液界面的起泡剂浓度降低,从而导致泡沫稳定性降低,柴油具有一定的消泡作用。     

A study of bubble–particle interaction using atomic force microscopy

Interaction between solid particles and air bubbles is central to froth flotation. Measurement of such interaction forces has only recently been possible with the invention of the atomic force microscopy (AFM). In this paper, the AFM colloidal probe technique was used to measure hydrodynamic interaction forces between a solid sphere attached to an AFM cantilever and an air bubble placed on an AFM piezoelectric stage at different approach speeds. Strong repulsive forces due to the hydrodynamic interaction were established and quantified for both hydrophobic and hydrophilic particles, and bubbles in deionised water and 1 mM KCl aqueous solutions. No surfactants were used. In the case of hydrophobic spheres, strong attraction between the surfaces, in addition to the repulsive hydrodynamic force, was observed, leading to the rupture of the intervening water film due to submicroscopic bubbles and the attachment of the particle to the air bubble at relatively large separation distances, which were of the order of 500–2000 nm. In the case of hydrophilic spheres, the rupture of the intervening water film and the attachment of the particle to the air bubble did not take place. An analysis of the AFM data was carried out to obtain the interaction force and relative separation distance. Theoretical hydrodynamic force calculation shows agreement with experimental data for larger separation distances. Deviations at shorter distances are related to the deformation of air–water interface due to the particle approach and surface forces.     

Dynamics of dewetting and bubble attachment to rough hydrophobic surfaces – Measurements and modelling

The influence of solid surface roughness (hydrophobic Teflon®) on the timescale of the ascending air bubble (R_b = 0.74 mm) attachment and the kinetics of the spreading of the three-phase contact (TPC – gas/liquid/solid) line was studied. The moment of the rising bubble collision with a horizontal Teflon® plate immersed in ultrapure water was monitored using fast video recordings (4000 fps). It was shown that, depending on the solid surface roughness, the time of the TPC formation was significantly different. Similarly to our previous studies, it was shorter for higher roughnesses. Using high-frequency video acquisition, an additional factor, kinetics of the spreading of the TPC line associated with various bubble shape changes during TPC formation, could be determined. The registered attachment kinetics and bubble shape variations were very reproducible for smooth and very rough Teflon® surfaces, whereas for Teflon® of intermediate roughness, up to five different attachment scenarios were observed, with a relatively large standard deviation of time of TPC formation. Numerical calculations used for simulation of the bubble collisions with a horizontal solid wall with precisely controlled hydrodynamic boundary conditions revealed that the experimentally observed timescales of the bubble attachment and spectacular bubble shape variations can be accurately (qualitatively) reproduced for each roughness of the Teflon® plate studied. Good agreement between experimental and numerical data is, in our opinion, rather strong evidence for air-induced rupture of the liquid film formed between the colliding bubble and the hydrophobic solid plate. This supports the hypothesis that depending on the solid surface roughness, different amounts of air entrapped in solid surface irregularities could drastically change the solid surface hydrodynamic boundary conditions and, consequently, the kinetics of spreading and formation of the TPC.     

Cationic flotation of nonsulfide minerals

From the analysis of information on flotation of quartz, barite, hematite and diamond spar using cationic reagents (amines), the authors show deficiency of thermodynamic approach to explain flotation results by one type of adsorption due to ion–electrostatic mechanism. The discussion offers hypothesis that says that at low pH collecting ability of a reagent is connected with hydrophobic attachment of the reagent ions in adsorption layer. In alkaline range of pH, the collecting ability is conditioned by formation and precipitation of ionomolecular associates in the adsorption layer of a mineral. These types of adsorption attach particle surface which is preliminarily made hydrophobic by ion–electrostatic mechanism. These adsorption types are active at bubble–liquid interface and can go to this interface upon rupture of water film between a particle and a bubble. According to the suggested hypothesis, liquid tension in the film becomes nonuniform and a surface force arises and expels kinematic constraint for particle–bubble attachment. The analytical review of the collected test data on cationic reagents proves the suggested hypothesis. The causes of breakdown of correlation between surface pressure and collecting ability for initial conditions of flotation are explained.     

A comparison of methods for measuring the induction time for bubble–particle attachment

Froth flotation is an exceedingly complex physicochemical process. The convenience of distilling much of the complexity of the particle–bubble interactions into a single parameter has led to the continuing popularity of the classical ‘induction time’ to quantify the threshold for particle–bubble attachment to occur. Despite this popularity and the simplicity of the concept, there is no single universal method of evaluating the induction period.In this paper, we begin with a critical review of the available techniques for estimating the induction period. These are: back-calculation from experimental (micro)flotation tests; pushing a particle toward a stationary bubble (or vice versa) using an atomic force microscope (AFM); pushing a bubble toward a stationary bed of particles in the ‘Induction Timer’; pushing a bubble toward a stationary solid surface using the ‘integrated thin film drainage apparatus’ (ITFDA); and dropping particles onto a submerged stationary bubble using the ‘Milli-Timer’ device. Each one of these methods has advantages and disadvantages, and the best choice depends on the application.In the experimental section, we present quantitative comparison of the induction periods estimated using two different techniques, namely the Induction Timer and the Milli-Timer. The same particles were tested in each device, under the same conditions. It was found that by tuning the operation of the particle pick-up device, similar estimates of induction period could be obtained to the estimates made by direct observation with the Milli-Timer. In the former device a bubble is driven toward a particle bed at a controlled rate, whereas in the latter a particle’s motion is governed by the hydrodynamics. The potential to match these presents an intriguing prospect for better understanding the bubble–particle interaction, and the possibility to ‘calibrate’ the simpler Induction Timer against direct observations.     

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°.     

Determination of activity/selectivity ratio in physical and chemical adsorption of a reagent

Under discussion is the particle and bubble interaction in froth flotation. Water flow from an interlayer between a particle and a gas bubble under effect of hydrophobic component of wedging pressure is studied. It is assumed for a mineral to be extracted, that electrostatic interaction slightly influences the particle and bubble contingence and the liquid interlayer thinning. For this reason, particular attention is given to the effect exerted by mineral particle surface hydrophobicity on water flow rate from the interlayer. It is found that water flow rate under influence of hydrophobic component of wedging pressure is less than water flow rate under physical adsorption of a reagent. The authors hypothesize that hydrophobization creates areas on the mineral particle surface, where the reagent species active relative to gas–water interface attach in accordance with the polarity equalizing rule. Physically adsorbed reagent species pull out water from the interlayer after the interlayer rupture and, thus, remove the kinetic constraint of the particle–bubble attachment.