Application of reactive oily bubbles to bastnaesite flotation

Reactive oily bubble, defined as air bubbles covered with a thin layer of kerosene containing collectors, was used to float a major rare earth mineral, bastnaesite from rare earth ores. Both fatty acid and hydroxamic acid were used to generate reactive oily bubbles. The flotation of bastnaesite with reactive oily bubble was investigated by zeta potential, zeta potential distribution and induction time measurement and micro-flotation tests. The results showed a quicker attachment to bastnaesite and a stronger collecting power of reactive oily bubbles containing 100 ppm fatty acid than conventional air bubbles, resulting in an enhanced bastnaesite recovery. The flotation recovery of bastnaesite by reactive oily bubbles containing hydroxamic acid is lower than that by conventional air bubble flotation where the bastnaesite was pre-conditioned by hydroxamic acid in aqueous phase. During induction time measurement, no attachment is observed between bastnaesite particles and reactive oily bubbles containing hydroxamic acid, illustrating the importance of collector type in reactive oily bubble flotation technology. These findings suggest the superior performance of reactive oily bubble technique than conventional bastnaesite flotation method only when proper collector is used to generate the reactive oily bubbles.     

Fundamental study of reactive oily-bubble flotation

A novel concept of reactive oily bubbles (i.e., bubbles covered by a thin layer of oil containing oil-soluble collectors) as a carrier in flotation is proposed. In addition to the role of fine particle agglomeration by oily films, the surface properties of air bubbles coated with a thin oil film can be better controlled for the desired selectivity by adding certain types and concentrations of water insoluble collectors into the oil phase. Oily bubbles attain a much higher contact angle than air bubbles, ensuring a strong collecting power, favorably for floating both coarse and fine particles. The reactive oily bubble flotation can eliminate the addition of collector to the aqueous phase, avoid undesired synergetic interactions among collectors, activators, depressants and dispersants present in slurry, minimize undesired activation of gangue particles and significantly reduce the amount of collectors needed.The electrokinetics of kerosene droplets in aqueous collector solutions was measured as a function of solution pH. The results clearly showed that the surface charge and hence the surface properties of oil droplets can be finely tuned by controlling the type of the collectors to suit the desired flotation needs. The attachment of collector-containing oily bubbles on silica, sphalerite and galena surfaces was investigated with contact angle measurement. The concept of using reactive oily bubble to achieve selective flotation was demonstrated in microflotation tests.     

低阶煤颗粒-气/油泡间的诱导时间研究

为表征低阶煤颗粒-气/油泡间矿化过程的差异,通过Sutherland理论下固体颗粒进入泡沫产品的总概率(E)和浮选速率常数(k)之间关系,并结合低阶煤颗粒-气/油泡的浮选速率试验,求得了低阶煤颗粒-气/油泡间的诱导时间。浮选实验研究表明,在相同的捕收剂消耗量下低阶煤-油泡浮选产率均高于低阶煤-气泡浮选产率。诱导时间测试表明,低阶煤颗粒-油泡间的诱导时间(35 ms)要明显低于低阶煤颗粒-气泡间的诱导时间(93 ms)。上述实验结果表明,油泡表面的疏水性要强于传统浮选气泡表面的疏水性。然而,进一步利用Sutherland理论中固体颗粒进入泡沫产品的总概率和浮选速率常数之间的数学关系,并结合低阶煤颗粒-气/油泡的浮选速率试验求得的低阶煤颗粒-气/油泡间的诱导时间分别为9.67和8.46 ms,其与诱导时间测试仪分别测量的诱导时间差异很大。这主要是由于在实际浮选过程中气/油泡的上升速度分别为23.26和22.68 cm/s,其远高于2015EZ型诱导时间仪测试过程中气/油泡碰撞速度(2.0 cm/s)。因此,诱导时间理论计算表明气泡-颗粒间的碰撞速度对颗粒-气泡间的诱导时间影响很大。上述研究结果表明油泡浮选效果优于传统浮选的内在原因在于低阶煤颗粒-油泡间的诱导时间小于低阶煤颗粒-气泡间的诱导时间。     

气泡、油泡和活性油质气泡在浮选中的应用和机理探讨

浮选是高效回收矿产资源应用最广泛的技术方法。气泡作为浮选载体在浮选过程中有着举足轻重的作用。以气泡-油泡-活性油质气泡为线索,对比了传统气泡与改性后油泡(气泡表层包裹一薄层油性捕收剂)、活性油质气泡(气泡表层包裹一薄层含有捕收剂的中性油)的浮选特性。通过浮选动力学分析了气泡与油泡、活性油质气泡浮选的区别,传统气泡浮选与改性后的油泡浮选均为2步反应,而活性油质气泡实现了1步浮选,大大降低了气泡与矿物颗粒间的黏附功,提高了浮选效率。从油-水界面表面活性剂解离度这个角度分析了活性油质气泡的表面性质,指出活性油质气泡的表面电性由表层中性油中添加的捕收剂和p H决定。通过DLVO理论计算了不同气泡与矿物颗粒间的相互作用能,从理论上解释了活性油质气泡浮选指标更好的原因。活性油质气泡在选矿中的成功应用表明,活性油质气泡与矿物表面的作用均强于传统气泡与矿物表面的作用,即活性油质气泡对矿物具有更强的捕收能力,相较于气泡和油泡的浮选,活性油质气泡浮选有利于提高浮选效率,降低捕收剂用量。活性油质气泡作为浮选载体从气泡这一特殊视觉为浮选行业开辟了一个崭新的研究领域。     

Influence of the propagation of three phase contact line on flotation recovery

Froth flotation is considered the most effective process of beneficiating low grade ores and is widely used in the base metals industry. For effective flotation, the attachment of mineral particles to air bubbles is important and has been studied by many researchers by measuring quantities such as attachment time, film-thinning time and induction time. This paper identifies an important step in the bubble–particle attachment process, namely, the expansion mechanism of the three phase contact (TPC) line between liquid, solid and air. It has been shown that the TPC expansion time is determined by the drainage of the surrounding fluid. It is influenced by factors such as pulp chemistry surrounding the particle, variations in surface forces and pressure inside the bubble. It has been observed experimentally that the TPC expansion time bears square root relationship to attachment efficiency. In this work, it has been argued that the attachment efficiency is related to the TPC circle radius propagation.     

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。     

电解质对低阶煤油泡浮选矿化过程的影响

浮选矿浆中离子的种类与浓度直接影响着矿物颗粒和气泡的表面电位,进而支配着浮选矿化过程。从热力学和动力学两个方面入手,通过DLVO理论探究了不同电解质对煤粒和油泡间的相互作用势能的影响,结合其在不同电解质条件下诱导时间的差异,最终通过相应条件下的油泡浮选试验来证实电解质对低阶煤-油泡浮选矿化黏附过程的影响。结果表明:对于NaCl和CaCl_2两种电解质,随其浓度的增大,煤粒和油泡表面电位的负值均不断减小,煤粒和油泡间的能垒也不断降低,当CaCl_2浓度为100 mmol/L时,煤粒和油泡间的相互作用不存在能垒;并且随这两种电解质浓度的增大,黏附过程的诱导时间逐渐减小,相应地可燃体回收率不断提高,且相同的电解质浓度下,CaCl_2电解质对其相互作用能垒和诱导时间的降低程度更大,可燃体回收率更高。而对于AlCl_3电解质,当其浓度大于20 mmol/L时且随浓度的增大,煤粒和油泡间的相互作用能垒和诱导时间不断增大,相应地可燃体回收率则不断降低。     

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.     

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.     

油泡-低阶煤颗粒间的黏附特性

在浮选矿浆中矿物颗粒能够有效地黏附在气泡上完成矿化过程是浮选技术的关键。用油泡代替气泡,研究难浮低阶煤颗粒与油泡间的相互作用及其黏附特性对于弄清其矿化机制至关重要。从矿物表面电性入手,探究了溶液p H值和表面活性剂对煤样与油泡间作用力的影响,运用DLVO理论计算了相互作用力的大小,并通过诱导时间这一重要参数评价了黏附效率。实验结果表明,DLT煤样和油泡的等电点分别出现在p H值2~3和p H值3~5之间;当p H=3时溶液中煤样与油泡之间的相互作用不存在能垒;p H3时开始出现能垒,并随p H值的增大而增大。2-乙基己醇和双十二烷基二甲基溴化铵的加入能够有效地减小煤样与油泡之间的能垒并降低诱导时间,最大降幅分别为74.35%和86.45%。     

Poly (N-isopropylacrylamide) (PNIPAM) as a flotation collector: Effect of temperature and molecular weight

Poly (N-isopropylacrylamide) (PNIPAM), a temperature responsive polymer, was tested for its potential use as a collector in a quartz flotation system. The effect of PNIPAM on the surface characteristics of quartz particles were studied using induction time, contact angle and zeta potential measurement and analysed in terms of the probability of bubble/particle attachment and the probability of formation of stable bubble/particle aggregates. It was found that probability of bubble/particle attachment of quartz significantly increases in the presence of PNIPAM, particularly at temperatures above the lower critical solution temperature (LCST) of 32 °C. Furthermore, the probability of bubble/particle attachment increases with increasing PNIPAM molecular weight. This was attributed to the increased hydrophobicity of the quartz surface as well as the decrease in the double layer repulsion between bubbles and particles. This leads to the conclusion that PNIPAM could act as an effective collector in a flotation system.     

Effect of mechanical and chemical clay removals by hydrocyclone and dispersants on coal flotation

Fine minerals, mostly clays, are known to have a detrimental effect on coal flotation. This paper focuses on the effect of mechanical and chemical removals of fine minerals by hydrocyclone and dispersants on coal flotation. The experimental results showed that the flotation recovery slightly increased from medium acidic to medium alkaline ranges. The flotation experiments carried out with dispersants at different dosages showed that the dispersants did not enhance the flotation recovery significantly. However, the removal of the fine fraction from the feed using a hydrocyclone significantly increased the flotation recovery. The bubble–particle attachment tests also indicated that the attachment time between an air bubble and the coal particles increased in the presence of clay particles. These attachment time results clearly showed that the clay particles adversely affected the flotation of coal particles by covering the coal surfaces which reduced the efficiency of bubble–coal attachment. An analysis based on the colloid stability theory showed that the clay coating was governed by the van der Waals attraction and that the double-layer interaction played a secondary role. It was also concluded that the best way to increase the flotation recovery in the presence of clays was to remove these fine minerals by mechanical means such as hydrocylones.     

A conceptual model for gold flotation

This paper is aimed at producing a conceptual model for gold flotation based on the discussion of a number of experimental results where the behaviour of free and refractory gold has been studied under different chemical and physical conditions. A review of the literature suggests that there have been numerous studies on the flotation of free gold particles and refractory sulphides, but these investigations have typically focused on the individual flotation behaviour of each gold bearing species in synthetic mixtures and not when they are present together in “real” ores in the same pulp. The model discussed here shows that the flotation of refractory gold follows a similar trend to the recovery of refractory pyrite and pyrrhotite and is mainly affected by chemical conditions in the pulp such as redox potential, aeration conditions, copper activation, reagent synergism and galvanic interaction. Refractory gold is usually recovered by true flotation that is hydrophobic particle-bubble attachment, unless under certain conditions the physical transport of water and gangue provides a washing effect and detaches some of the sulphide material from the air bubbles. The flotation recovery of free gold is largely affected by physical constraints like the shape and size of the particles, the degree of water and gangue transport to the froth, the stability of the froth, and the extent of bubble loading of sulphide particles which can provide a barrier towards the hydrophobic bubble attachment of free gold. In each individual study the results suggest that the recovery of free gold follows a proportional trend with regard to water and gangue recovery. However, there is an inverse relationship between the true flotation of free and refractory gold due to the fact that free gold particles cannot attach to air bubbles properly in the presence of physical barriers.     

Detachment of coarse composite sphalerite particles from bubbles in flotation: Influence of xanthate collector type and concentration

The force required to detach sphalerite ore particles from air bubbles has been measured in flotation concentrates, for particles in the size range of 150–300 μm and 300–600 μm with different degrees of liberation. An electro-acoustic vibrating apparatus, that produces typical force conditions experienced in a flotation cell, was used to measure particle–bubble detachment as a function of the vibrational acceleration. Sodium isopropyl xanthate (SIPX) and potassium amyl xanthate (PAX) collectors were used in flotation, at different concentrations. At a fixed frequency of 50 Hz, the maximum vibrational amplitude at which a particle detaches from bubble was used to calculate the particle detachment force. It was shown that changes in surface hydrophobicity (contact angle), due to variations in reagent conditions have significant impact on particles detaching from bubbles. On average, detachment of particles from oscillating bubble correlated well with xanthate concentration and hydrocarbon chain length of xanthate ions. Particles (300–600 μm) with high contact angle obviously required higher force to detach from bubbles than similar particles with lower contact angle. This correlated well with the flotation response at the different reagent conditions. SEM analysis of particles after detachment showed that fully liberated particles attached to bubbles more readily and also gave higher detachment force than composite particles. Moreover larger detachment forces were observed, on average, for particles with irregular shape compared to particles with rounded shape of the same size range.     

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。     

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。     

CFD modelling of bubble–particle attachments in flotation cells

In recent years, computational fluid dynamic (CFD) modelling of mechanically stirred flotation cells has been used to study the complexity of the flow within the cells. In CFD modelling, the flotation cell is discretized into individual finite volumes where local values of flow properties are calculated. The flotation effect is studied as three sub-processes including collision, attachment and detachment. In the present work, these sub-processes are modelled in a laboratory flotation cell. The flotation kinetics involving a population balance for particles in a semi-batch process has been developed.From turbulent collision models, the local rates of bubble–particle encounters have been estimated from the local turbulent velocities. The probabilities of collision, adhesion and stabilization have been calculated at each location in the flotation cell. The net rate of attachment, after accounting for detachments, has been used in the kinetic model involving transient CFD simulations with removal of bubble–particle aggregates to the froth layer.Comparison of the predicted fraction of particles remaining in the cell and the fraction of free particles to the total number of particles remaining in the cell indicates that the particle recovery rate to the pulp–froth interface is much slower than the net attachment rates. For the case studied, the results indicate that the bubbles are loaded with particles quite quickly, and that the bubble surface area flux is the limiting factor in the recovery rate at the froth interface. This explains why the relationship between flotation rate and bubble surface area flux is generally used as a criterion for designing flotation cells. The predicted flotation rate constants also indicate that fine and large particles do not float as well as intermediate sized particles of 120–240 μm range. This is consistent with the flotation recovery generally observed in flotation practice. The magnitude of the flotation rate constants obtained by CFD modelling indicates that transport rates of the bubble–particle aggregates to the froth layer contribute quite significantly to the overall flotation rate and this is likely to be the case especially in plant-scale equipment.     

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.     

The role of colloidal precipitates in the interfacial behavior of alkyl amines at gas–liquid and gas–liquid–solid interfaces

Alkyl amines are widely used as cationic collectors in froth flotation. They are generally referred to as weak electrolyte collectors and frequently form solid insoluble precipitates. Much work has been done to measure the interfacial tension and contact angle regarding alkyl amine collectors. However, the role of these colloidal precipitates in adsorption phenomena and the kinetics involved in the adsorption have not been systematically studied and decoupled from those of soluble species in flotation. Using dodecyl amine (DDA) as the model surfactant, four sets of intimately linked measurements were made on the dynamic surface tension of amine true solutions and solutions containing precipitates to demonstrate the difference in the surface activity kinetics of colloidal precipitates and soluble species. The effects of the interfacial kinetics on the bubble size in a two-phase system and on the contact angle in a three-phase system were also studied to understand the migration of precipitates between different interfaces. The experimental data showed that kinetic effects existed whenever precipitates were involved, but were absent from true amine solutions. A three-stage mechanism (migration–adsorption–aggregation) was proposed and tested to explain the kinetic behavior associated with precipitates. The size of air bubbles generated in the true solutions was not affected by the surface age of bubbles, while bubble size was affected by the surface age in the presence of precipitates. The observed kinetics of contact angle evolution suggested that the migration of DDA species between air–water–interfaces occurred.     

An exponential decay relationship between micro-flotation rate and back-calculated induction time for potential flow and mobile bubble surface

Flotation researchers have long hypothesised that particles have inherently different flotation rates under the same operating conditions because they have different induction times in the flotation cell. The relationship between flotation rate constant and induction time, however, has yet to be explored. Here we analysed the relationship between micro-flotation rate and back-calculated induction time for galena and sphalerite particles. The floatability of the particles was controlled by depression with potassium chromate (galena) and activation with copper sulphate (sphalerite). The bubble rise velocity vs. size in the micro-flotation experiments was determined by high speed video microscopy and followed the prediction for bubbles with the fully mobile air–water interface. Therefore, the theoretical analysis of the micro-flotation results was carried out, based on the potential flow model for water flow around a mobile bubble surface. The relationship between micro-flotation rate constant and back-calculated induction time was found to rapidly decay exponentially. In this model, flotation rate constant is highly sensitive to induction time. For example, a doubling or tripling of induction time results in an order-of-magnitude decrease in flotation rate constant.