Stabilization of bubble size in a flotation cell

Conclusions  

Maximum size of floating particles in different flotation cells

Two flotation models, particle at the liquid-gas interface and particle-bubble aggregate, both based on balance of forces, were used for evaluation of experimental data relating the maximum size of floating particles d_max and their advancing contact angle. It was noticed, by comparing the experimental and model data, that for a given flotation device and material the maximum size of floating particle d_max increases with increasing particle hydrophobicity and at the same time the acceleration a, experienced by the d_max particle at the moment of rupture, decreases with particle hydrophobicity. The acceleration values change with cell dynamics and type of flotation device and are usually not available, therefore empirical apparent cell constants A, which characterize flotation dynamics and relate particle acceleration with advancing contact angle have been proposed instead. The values of A were determined by evaluation of experimental data relating d_max and advancing (detachment) contact angle for constant: particle density, medium density, surface tension, and flotation cell dynamics. Since A depends on particle density, a tentative formula was proposed to link A with density-independent flotation cell constant A_o. The values of A_o for selected flotation cells were calculated and presented.Using quartz as an example, it was shown in the paper that a positive advancing contact angle does not guarantee flotation because a prerequisite for flotation is non-zero receding contact angle.     

Micro- and nano-scale phenomena effect on bubble size in mechanical flotation cell

The micro/nano-scale phenomena affecting the bubble size observed in a mechanical flotation cell were investigated using an extensive dynamic surface property study with commercial laboratory instruments. The results highlighted two distinct dynamic surface properties: a rapid adsorption/desorption rate leading to undetectable dilatation surface elasticity, and a slow adsorption/desorption process taking place within minutes and leading to a significant increase in the dilatation elasticity of the interface. The latter is observed in highly surface-active reagent grade Polypropylene Glycol and in commercial Dowfroth 250 frothers while the rapid adsorption/desorption rate is a characteristic of weakly surface-active reagent grade 1-Pentanol and commercial Dowfroth 200.The bubble size distribution and therefore the Sauter mean diameter results also highlighted the unambiguous similarity in the effect of frother on bubble size between the I-Pentanol and the Dowfroth 200 and indeed between the Polypropylene Glycol and the Dowfroth 250. The present paper examines the similarity between the two groups in order to gain understanding on how the dynamic surface properties affect the bubble size in two-phase systems.     

Estimation of the bubble size and bubble loading in a flotation froth using electrical resistance tomography

Flotation process is widely used in mineral industry for the separation of valuable minerals from low-grade ore slurry. There are several parameters such as the bubble size and bubble loading that predict the efficiency of the flotation process. These parameters can be used for the control of the flotation process. There are already some techniques that can be used for online monitoring of these parameters, for example, the high-speed video imaging and a probe sensor based on electrical resistance tomography (ERT). These methods, however, suffer for some limitations. The high speed video imaging gives information only on the surface of the froth and in the previously proposed ERT based techniques the conductivity of the froth is typically modeled to be smoothly varying. However, in reality the froth is composed of different size of bubbles having highly conductive surface and very low conductive interior which configuration cannot be modeled with smoothly varying conductivity distribution. In this paper, we propose a computational approach in which the structure of the froth is modeled and both the bubble size and the conductivity of the boundary of the bubbles are estimated. The proposed approach utilizes data measured with the standard ERT probe. The estimated bubble size and conductivity of the boundary of the bubbles are compared to online measured camera based estimates of the bubble size and bubble loading. The proposed approach is evaluated with simulated measurements and real data from Pyhäsalmi Mine. The results show that there is a high correlation between the camera based and the ERT based estimates of the bubble size. Furthermore, some of the parameters obtained from the ERT based method correlate well with the camera based estimate of the bubble loading.     

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.     

Characterization of large size flotation cells

Design and operating conditions of large size mechanical flotation cells were evaluated by comparing it with the actual operating conditions in a plant. The objective was to determine the time scale-up factor, typically based on empirical rules. Experiments were conducted on the rougher flotation circuit at Minera Escondida Ltd. The circuit consisted of self-aerated mechanical cells of 160 m³, arranged in six parallel banks with nine cells each.The rougher circuit flotation kinetics was evaluated from direct sampling and local mass balances around each cell of the bank. Adjusted overall mass balances were also developed. This information was used to fit different kinetic flotation models, and it was found that the rectangular distribution function was the most appropriate to describe the distributed rate constant for industrial operation. Then, a rougher flotation simulator was developed to describe the actual operation in terms of the operating variables (mass flow rate, solid percentage, feed grade) and the actual volumetric flow rate entering to each cell. In this study feed pulp samples were taken in parallel from the rougher circuit and were simultaneously floated in laboratory. The kinetic behavior was then modeled at a laboratory batch scale in order to determine the time scale-up factor between laboratory batch flotation data and industrial size flotation. The time scale-up factor observed for large sized cells, 160 m³, was found reasonably similar to those previously determined for self-aerated mechanical cells, but of lower size, operating at similar recoveries. In addition, the relative effect of mixing, between laboratory batch and an industrial flotation bank was quantified by the ϕ parameter, separating the impact that kinetic and mixing changes have on the time scale-up factor.In general, the rougher flotation operation was found to reach the predicted metallurgical target, and that the optimal separability criterion was also respected.The diagnostic generates information about the internal state of the process and helps to identify potential improvements for design, operation and control of the circuit.     

Effect of impeller rotational speed on the size dependent flotation rate of galena in full scale plant cells

The first three rougher cells in the lead circuit of the Elura concentrator (formerly Pasminco Australia Limited) were selected as the plant cells for investigation. Metallurgical surveys were performed and various hydrodynamic measurements taken, allowing the galena flotation rate constant and the bubble surface area flux (S_b) in these cells to be calculated over a wide range of gas flow rates, and at two impeller rotational speeds. It was determined that altering the impeller rotational speed did not significantly change the rate constant dependency on S_b when flotation was considered on an unsized basis.The analysis was further extended to examine the same cells parameters on a size-by-size basis. The results obtained have been used to identify differences in the flotation behaviour of the various particle size fractions, independently of surface hydrophobicity. It is shown that the physical conditions for effective flotation of fine (<9 μm) and coarse (>53 μm) particle size fractions differ substantially, suggesting that a specific hydrodynamic environment will favour a high flotation rate for fine galena, which may be detrimental to the recovery of coarse galena, and vice versa. These observations are in accord with metallurgical practice that suggest that it is difficult to improve fine particle flotation without also compromising coarse particle stability efficiency simply by modifying the cell hydrodynamics alone. A fundamental flotation model was applied to quantify differences in the flotation rate of the various particle size fractions with impeller rotational speed.     

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 effect of mothers on bubble size distributions in flotation pulp phases and surface froths

Froth dynamics, i.e. the stability and mobility of the froth, are crucial indicators of various important features of flotation systems. For example, it is desirable that the froth collapses as soon as possible after it is skimmed off the cell in order to curtail losses in throughput. On the other hand, if the froth is too prone to collapse, it will not be sufficiently stable to support its load prior to skimming. Likewise, the mobility of the froth gives similar information on the performance of the flotation cell. For example, a sharp contrast can be observed between dry viscous (immobile) froths and watery runny froths with high mobility. Several authors have recently shown that analysis of the structure of the froth in a flotation cell can be used to assess the performance of the cell. This implies that there is a close relationship between the bubble size distribution in the pulp and froth phases.Until very recently, it was not possible to verify this hypothesis direct, since reliable measurements of bubble size distributions in especially the froth phase could not be obtained. With recent improvements in the machine vision technology originally developed at the University of Stellenbosch, it is now possible to measure bubble size distributions and stability in froth structure with a high degree of accuracy. Unlike previous methods, these improved algorithms can provide a detailed map of flow patterns in the froths, which can give a significantly better idea of operating conditions in the flotation cell. Consequently, in this experimental study the bubble size distribution in the pulp phase of a laboratory flotation cell was measurement with a capillary tube system (UCT bubble size analyser), while the bubble size distribution in the froth phase was measured by use of digital image analysis. The relationship between these bubble size distributions in the pulp and froth phases is discussed.     

微泡浮选中气泡尺寸影响分析与参数优化

微泡对于微细颗粒的浮选具有极大优势已得到广泛认可.为此,通过对浮选机理分析并结合相关经验知识,从捕集概率、浮选速率及回收率3个方面分析了气泡尺寸对矿物浮选效果的影响.对浮选车间旋流-静态微泡浮选柱采用正交实验法,采集现场生产数据,对泡沫层厚度、矿浆浓度、药剂添加量3个工艺参数对浮选指标的影响进行了分析,得出上述3个工艺参数对于浮选效果的影响程度依次降低,并提出了现场最优参数优化条件,便于现场生产人员操作应用.     

搅拌叶轮尺寸对双叶轮浮选机性能的影响

通过对常规机械搅拌式浮选机进行改造,将其搅拌系统设计为2个转子(离心叶轮和搅拌叶轮)配1个定子的结构模式,开发了双叶轮浮选机.为了考察搅拌叶轮直径对双叶轮浮选机性能的影响,试验设计了直径为32、34和36 m m的搅拌叶轮,考察了直径对充气性能、悬浮性能和分选性能的影响.试验表明:浮选机的充气、悬浮和分选性能,与搅拌叶轮的结构特征和浮选机的转速有关.随着转速的增加,充气量越来越大,固体颗粒悬浮质量分数越来越小,而分选效率先增大后减小;在实验室现有搅拌叶轮尺寸条件下,叶轮直径为36 mm时,充气、悬浮和分选效果都达到最佳状态.     

A model to relate the flotation rate constant and the bubble surface area flux in mechanical flotation cells

Two recent flotation models developed by the authors are discussed, viz. the bubble population balance model and the attachment-detachment model. The bubble population balance model describes the history of a bubble population in a flotation cell in terms of the sub-processes of bubble breakage and coalescence. The attachment-detachment model allows for the presence of a gas phase in the flotation cell, both in terms of a gas residence time and the attachment and detachment of mineral particles to/from bubbles. When combined the two models predict a near-linear region about a point of inflexion on the (simulated) response between the flotation rate constant (k) and the flux of bubble surface area through the flotation cell (S_b). It is proposed by the authors that this region corresponds to the linear k−S_b relationship observed in a recent research project on flotation kinetics in mechanical flotation cells by Gorain et al. (1997).     

Recognition of the operational statuses of reagent addition using dynamic bubble size distribution in copper flotation process

The output PDF (probability density function) shape of surface bubble size in froth flotation is believed to be closely related to the operational statuses of reagent additions. An online operational status recognition method for quality evaluation of reagent addition is presented based on adaptive learning of the dynamic distribution features of the surface bubble size. We avoid the bubble over-segmentation problem by exploring an improved image segmentation algorithm to get the accurate bubble size statistics taking account of the local regional distribution of the image brightness value. By utilizing the kernel density estimation, we obtain the PDF and CDF (cumulative distribution function) of the bubble size statistics effectively. The distribution features of the bubble size statistics under the PDROS (pre-defined reagent operation statuses) are learned by FNC (furthest neighbor clustering), successively, the current health status of the reagent addition in the test time period is inferred by Bayesian inference according to the dynamic change of the bubble size PDFs; what is more, the statistical distribution features under PDROS are updated online according to the disturbance of the process operation conditions. This status recognition method is tested and practically applied in a copper ore beneficiation plant. The experimental results on the real production data demonstrate that this method outperforms other quasi machine vision based production condition recognition methods with much lower error recognition rate. It paves the way for the realization of an optimal control and fault diagnosis for reagent addition in the flotation process operation.     

粗细粒级差异化给矿对浮选柱选别性能的影响

粗细粒级矿物具有不同的浮选特性,浮选柱主要应用于精选作业的细粒级矿物分选,对粗颗粒矿物回收率较低,限制了浮选柱的应用。在泡沫层分选理论的基础上,以纯石英矿物(纯度大于99%)为代表矿样,将其分成150~280μm和-15μm粗细粒级两个组分,采用Ф100 mm×2 000 mm浮选柱开展试验考察粗细粒级差异化给矿对选别性能的影响。在一个试验中将粗细粒级矿物混合给入浮选柱泡沫层之下进行常规浮选,在另一个试验中将粗细粒级矿物差异化给入浮选柱泡沫层之上和泡沫层以下分别进行泡沫层分选和常规浮选。试验对比结果表明,粗细粒级差异化给矿提高了浮选柱精矿回收率,对粗颗粒矿物回收效果提升更为显著。     

射流浮选柱内纵向气泡均匀性的试验研究

在实验室射流浮选柱上进行了纵向气泡分布规律影响因素的试验.利用自制充气量测试仪测定浮选槽内不同深度的充气量,以样本均值和样本变异系数为指标,研究了射流器给料压力、喉管直径、喷嘴直径对浮选槽内充气量及纵向气泡分布均匀性的影响.结果表明：在试验范围内,压力越高,喉管越细,浮选槽内充气量越大,纵向气泡均匀性越好;当压力一定时,大喷嘴气泡均匀性明显好于小喷嘴.通过流体力学初步分析,解释了基本试验现象.     

Boundary conditions for gas rate and bubble size at the pulp–froth interface in flotation equipment

This paper describes the effective boundary conditions for the gas dispersion parameters of bubble size, superficial gas velocity and bubble surface area flux, in mechanical and column flotation cells. Using a number of previously derived correlations, with appropriate simplifying assumptions, and experimental data reported from plant practices, the boundary conditions were identified. Thus, it was shown that these constraints typically allow for a mean bubble diameter range of d_b = 1–1.5 mm and superficial gas rate of J_g = 1–2 cm/s, in order to maximize the bubble surface area flux, S_b = 50–100 s⁻¹. Under these conditions there is no carrying capacity limitation, while keeping a distinctive pulp–froth interface.     

浮选体系中气泡运动特性研究进展

在浮选体系中,气泡的运动特性直接影响着矿物浮选的选别效果,因此,研究气泡的运动特性,对于优化浮选工艺与改善浮选效果有着重要意义。本文首先简述了浮选中常见的气泡生成方式,主要有机械搅拌发泡、气泡发生器发泡、电解发泡及超声波发泡四种方式;气泡参数中介绍了气泡形态特征参数、运动特征参数和分布特征参数的含义和计算公式;然后重点阐述了浮选中起泡剂、捕收剂、抑制剂、浮选机叶轮参数、浮选柱气泡发生器及充气量对气泡参数的影响;归纳了纳米气泡在微细粒矿物浮选中的应用;最后指出,完善气泡测量方法、侧重于气泡在实际浮选环境中的运动特性研究以及研发新型纳米气泡生成设备将是未来的发展方向。     

A review of turbulence measurement techniques for flotation

Flotation is one of the most important primary separation processes in the minerals industry. As far as the mechanism of flotation is concerned, turbulence is one of the key parameters determining flotation performance because it affects three main processes: suspension of particles, air dispersion and particle-bubble collision, attachment and detachment. To study turbulence in industrial flotation cells, both numerical simulation and experimental measurement can be performed. Development of turbulence models and validation of Computational Fluid Dynamic (CFD) numerical simulation need experimental data obtained from turbulence measurement techniques that can be used in the three phase abrasive opaque environment present in a flotation cell. In this paper, the different techniques which have been used to characterise turbulence in the literature are reviewed in terms of their basic principles, system structure, range of application and limitations. Laser Doppler Anemometry (LDA), Particle Image Velocimetry (PIV), Constant Temperature Anemometer (CTA) and the Aeroprobe are all techniques that have been widely used to characterise the turbulence created in flotation machines operating with only fluid (or fluid and air). They cannot however be used when the concentration of solids is high as commonly occurs in a flotation machine. Techniques that have been identified that have the potential to be used to produce accurate measurements in three phase flows include Positron Emission Particle Tracking (PEPT), Piezoelectric Vibration Sensor (PVS) and Electrical Resistance Tomography (ERT). It is envisaged that applications of PEPT in three phase flotation cells will mostly be confined to studies at the laboratory scale. ERT has been tested in flotation cells filled with water and air but needs more development before it can be applied confidently in industrial scale flotation units. PVS, on the other hand, has been validated at laboratory scale and has been applied successfully for measuring turbulence in large scale operating flotation machines.     

浮选气泡群与矿浆作用模拟可行性研究

浮选柱常被用于极细粒矿物(譬如铜、银、铅等贵金属或是重金属)的精选,现阶段也用于粗选和扫选。浮选柱做为无机械搅拌的充气式浮选设备,其浮选时间一般较快,进行关于浮选柱的相关研究时,肉眼不易于观察,一般需要借助高速摄像机进行浮选观察;同时浮选柱在浮选时,需要非常平稳的分选环境,而且对矿物的解离程度要求高。本文应用大型通用有限元分析软件ANSYS,使用模型,以VOF法进行气液俩相界面捕捉,同时以控制变量法建立不同变量影响因素下的浮选气泡群上升模型。模拟气泡群在浮选柱中各影响因素作用下,气泡群与沉降矿浆作用过程。结合模拟与实验分析,在一定速度范围内,气泡群与颗粒沉降群之间的作用具有单个颗粒与气泡之间相互作用的特征,同时根据浮选柱最大输送能力模型和模拟结果分析,浮选中的气泡群与矿浆作用是可行的。