颗粒气泡黏附科学——微纳尺度下颗粒气泡黏附试验研究进展

自20世纪30年代始,微纳尺度下的颗粒气泡黏附就引起了学者的广泛关注并逐渐涌现出一系列试验技术探索颗粒气泡黏附机理。在宏观尺度下颗粒气泡黏附研究进展的基础上,系统的对微纳尺度下颗粒气泡间相互作用力及液膜薄化破裂动力学试验技术研究进展进行综述。技术总体上可以分为两类:力测量法及排液法。排液法是通过光学显微干涉技术直接获得气液界面变形及排液动力学数据,通过耦合扩展DLVO理论及排液方程求解作用力信息,如单气泡撞板显微干涉技术、薄膜压力平衡技术及表面力分析仪等。力测量法则主要是借助胶体探针原子力显微镜(AFM)测试颗粒气泡间表面力及流体阻力,通过流体力学排液模型模拟液膜薄化破裂动力学行为。排液法和力测量法均发现疏水力是颗粒气泡间液膜快速薄化并破裂的根本原因,其中排液法所获得的疏水力倾向于一种长程作用力,而力测量法得到的疏水力为短程作用力,造成这种差异的原因仍不明确。随着AFM-反射干涉对比显微镜联用、变形体系力分析仪和薄液膜力分析仪等技术的问世,作用力和液膜排液的同步测试已经成为一种技术趋势,充分助力了浮选颗粒气泡黏附基础研究。基于现有研究进展应进一步开展颗粒气泡间疏水力的系统研究,通过借助不同检测技术的优势互补及分子动力学模拟等手段,有望从根本上阐明这一科学问题。     

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。     

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...     

低阶煤的油泡浮选研究进展

针对低阶煤表面亲水性强、可浮性差及浮选过程中捕收剂消耗量高等问题,国内外研究者研究了低阶煤的汕泡浮选。本文对低阶煤-油泡浮选试验、矿化理论及分选装置进行了归纳总结。低阶煤的油泡浮选试验表明,油泡表面的强疏水性可以提高低阶煤浮选回收率,降低捕收剂消耗量。诱导时间测试结果表明,低阶煤颗粒-油泡间的诱导时间要远短于低阶煤颗粒-气泡间的诱导时间。目前研究颗粒-气/油泡间水化膜薄化理论的模型主要有Stefan-Reynolds模型、Taylor方程、Stokes-Reynolds-Young-Laplace模型以及Stokes-Reynolds模型。油泡的制备方法主要有高温气化法和常温零调浆法。     

浮选过程中气-液界面特性研究进展

这是一篇矿业工程领域的论文。近年来，表面活性剂（起泡剂）在矿物浮选中的作用研究尤为重要，因其揭示了气泡-颗粒碰撞或粘附前的气-液界面特性信息，对浮选效果的优化具有重要理论指导意义。本文重点概述了气-液界面电位测试技术、气-液界面张力表征技术及气-液界面吸附特性测试技术，介绍了气-液界面吸附特性的图像轮廓-张力法（PAT）测试方法，阐述了不同气-液界面特性下Stefan-Reynolds模型、Taylor模型及Stokes-Reynolds-Young-Laplace及Stokes-Reynolds模型计算参数，并展望了气-液界面特性的研究工作。     

低阶煤颗粒-气/油泡间的疏水力常数研究

浮选实验表明油泡对低阶煤颗粒的捕收能力要远强于传统浮选过程的起泡。这主要是由于油泡表面被捕收剂覆盖,其表面疏水性要远高于气泡表面的疏水性。因此,在油泡浮选矿化过程中,低阶煤颗粒-油泡间水化膜的薄化速度要远快于煤颗粒-气泡间的薄化速度。诱导时间测试发现,随着DAH溶液浓度从10~(-7) mol/L增加到5×10~(-5) mol/L时,低阶煤颗粒-气泡间的诱导时间从93 ms下降到12 ms。随着DAH溶液浓度从5×10~(-5) mol/L增加到10~(-3) mol/L时,低阶煤颗粒-气泡间的诱导时间从12 ms增加到35 ms。当DAH浓度由10~(-7) mol/L(纯去离子水溶液)增加到5×10~(-5) mol/L,低阶煤颗粒-油泡间的诱导时间由35 ms降低到10 ms。随着DAH浓度的进一步增加到10~(-3) mol/L时,低阶煤颗粒-油泡间的诱导时间由10 ms增加到25 ms。为了从微观尺度下去表征油泡表面较气泡表面所具有的强疏水性,本文通过低阶煤颗粒-油/气泡间的诱导时间,利用non-DLVO理论及Stefan-Reynolds水化膜薄化模型,拟合出初始水化膜厚度h与疏水性常数K_(132)之间的关系,进而得到了低阶煤颗粒-油/气泡间的疏水力常数K_(132)与十二烷胺盐酸盐DAH溶液浓度的关系。疏水力常数K_(132)拟合结果表明,当DAH溶液的浓度为5×10~(-5) mol/L时,低阶煤颗粒-油泡间的疏水力常数K_(132)约为低阶煤颗粒-气泡间的疏水力常数K_(132)的3倍;当DAH溶液的浓度为10~(-6) mol/L时,前者是后者的15倍。因此,油泡表面较气泡具有更强的疏水性质。从而解释了低阶煤-油泡浮选矿化过程优于传统浮选过程的本质特征。     

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。     

煤泥盐水浮选技术

盐水对浮选体系下颗粒与气泡行为的影响规律进行了综述。浮选矿浆中的无机盐电解质在提高精煤可燃体回收率的同时也增加了脉石矿物的回收。反电荷阳离子在煤粒表面吸附改善颗粒疏水性,颗粒-气泡间的液膜排液速度因双电层被压缩而加快;同时电解质兼有起泡剂的效果,能有效阻止气泡兼并聚合。另一方面,盐水浮选体系下细粒脉石颗粒的水流夹带和聚合截留现象严重,可燃体回收与脉石颗粒非选择性上浮之间的矛盾激增。     

高质量浓度曲拉通X-100对煤泥浮选的抑制效应及机理

表面活性剂在煤泥浮选领域中应用广泛,且表面活性剂常被用于煤泥浮选促进剂,本文从另一个方面探究了表面活性剂在过量的情况下对煤泥浮选的抑制效应及其机理。以低灰煤粒和表面活性剂曲拉通X-100为研究对象,采用X射线光电子能谱(XPS)表征曲拉通X-100在煤表面的吸附状态,静态接触角测定仪测定曲拉通X-100对煤表面疏水性的影响,并探究了不同质量浓度曲拉通X-100液滴在煤粒表面的润湿与铺展情况,采用诱导时间测量仪分析不同质量浓度曲拉通X-100水溶液中的气泡与煤粒的黏附情况,最后通过紫外分光光度计定量表征浮选槽中残留曲拉通X-100质量浓度与不同浮选时间下浮选结果的对应关系。结果表明:高质量浓度曲拉通X-100会抑制煤粒上浮,随着浮选试验的进行,曲拉通X-100质量浓度逐渐降低,低质量浓度曲拉通X-100会促进煤粒被上浮气泡黏附而浮出;高质量浓度曲拉通X-100能够在煤粒表面发生有效吸附,该吸附属于物理吸附,且在一定程度上提高了煤表面的疏水性;高质量浓度曲拉通X-100水溶液更容易润湿煤表面,从而减缓了气泡-煤粒黏附过程中的液膜薄化与破裂速率;在高质量浓度曲拉通X-100水溶液中,气泡表面因罩盖有曲拉通X-100分子,导致气泡表面发生改性,难以与煤粒发生有效黏附。高质量浓度曲拉通X-100主要通过对气泡改性,以及减缓气泡与煤粒碰撞-黏附过程中的液膜薄化-破裂速率来抑制煤粒的浮选。     

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.     

On the role of cavitation in particle collection in flotation – A critical review. II

Research in applying hydrodynamic cavitation to recovery of natural resources during the last decade is reviewed. The existence and formation of tiny bubbles or gas nuclei (with diameter from microns down to nano sizes) in natural water were verified from both direct and in-direct measurements, thus providing a foundation for applying hydrodynamic cavitation to flotation systems. The interactions between tiny bubbles and fine particles in aqueous slurry were analysed based on particle surface properties and types of gas nuclei present in water. Tiny bubbles generated by hydrodynamic cavitation were found to increase contact angle of solids and hence attachment force, bridge fine particles to form aggregates, minimize slime coating, remove oxidation layers on particle surfaces, and in consequence reduce reagents consumption. Experiments on generating tiny bubbles by hydrodynamic cavitation revealed that the energy dissipation levels for cavity formation in a flowing liquid could be much lower than predicted, depending on the content of dissolved gases, presence of free gas nuclei and design of cavitation tubes. Application of hydrodynamic cavitation to fine and coarse particle flotation, high intensity conditioning, oil agglomeration of fine coal, and oil sands processing has confirmed the role of tiny bubbles formed by cavitation in improving recovery efficiency. Based on the characteristics of vapor cavity bubbles, increased flotation kinetics by hydrodynamic cavitation could be attributed to a dual role: some collapsing cavity bubbles serving to break interfacial layers on particle surfaces, while other cavity bubbles attaching to those freshly exposed mineral surfaces. The role of water vapor and other gases within cavity bubbles in particle–bubble attachment remains to be explored, as does the action of frothers. Incorporating hydrodynamic cavitation into flotation systems to take advantage of its unique features is expected to develop the next generation of flotation machines.     

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.     

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.     

Fundamental aspects of bubble–particle attachment mechanism in flotation separation

Analysis of bubble–particle mechanism is important for improving our understanding of flotation process. The research presented integrates microflotation experiments, bubble–particle attachment time measurements, and colloid and surface characterization and analysis. The bubble–particle attachment time was inversely related to the flotation recovery and the minimum attachment time matched the maximum flotation recovery, which occurred around mutual isoelectric point for the glass particles and air bubbles. Bubble–particle force measurements, performed with an Atomic Force Microscope (AFM), showed a similar trend. Additionally, the adsorption isotherm of the glass–dodecyl amine hydrochloride (DAH) system indicated that there are the three adsorption regions, and the flotation recovery reached its maximum value in the second region of DAH adsorption on the glass surface. All results obtained in this study showed the important role of colloidal forces affected by surfactant adsorption in bubble–particle attachment.