煤泥浮选气泡比表面积的计算方法

为了从基于分水岭变换分割后的煤泥浮选泡沫图像上获得更加准确的气泡大小的分布情况,需要计算气泡的比表面积.由于气泡的不规则性,使用等效椭圆比等效圆来计算面积、体积更能反应真实的气泡情况.通过对比表面积进行不同的加权,比较表明,使用气泡投影的横截面积作为加权系数是描述气泡分布情况最适合的方法.同时还讨论了气泡特征与浮选操作条件之间的关系,为计算机在线控制浮选过程建立基础.     

Relationship between the bubble surface flux that overflows and the mass flow rate of solids in the concentrate of flotation processes

This paper presents the relationship between the bubble surface flux that overflows and the mass flow rate of solids in the concentrate. Even though this study was carried out in a flotation column, the knowledge derived from this paper may be applied to all froth flotation processes. The experimental set up was equipped with an image analysis system to estimate the froth bubble diameter and the air recovery. This study describes the difference between the bubble surface flux entering the froth zone (S_bI) and the flux that arrives to the top of the froth (S_bT) and then overflows to the concentrate (S_bO), the latter being most directly related to the mass flow rate of solids in the concentrate. It was observed that the superficial area of the overflow increased with increasing collector addition and air flow rate, but decreased with increasing froth depth and particle size distribution. Visual evidence and experimental results suggest that, it is common that the superficial area of air that overflows in the concentrate is covered by particles. Only when this condition is almost achieved does overflows occur; otherwise, a high level of coalescence and bubble bursting take place at the froth surface. This was concluded after finding compatible trends between the estimated and predicted mass flow rates of solids in the concentrate, when a tractable geometrical model was used (R² = 0.8).     

Effect of particle size on flotation performance of complex sulphide ores

Flotation processes occurring in the bulk and froth phases have a characteristic influence on the structural features and dynamics of the flotation froth. It is recognized that the structure and texture of a mineral froth is a good indicator of flotation separation performance. The surface froth feature and dynamics are presented by three features extracted from the digitized images of the froths, i.e. SNE, a rough indication of the average bubble size of the froth, froth stability and the average grey level of the froth, an indication of mineral loading. Particle size is an important parameter in flotation operation. Nowadays, particle size is often measured and controlled in flotation concentrators. In this study the dependence of the froth structures on the particle size variation was investigated on the batch flotation of a sulfide ore from the Merensky reef in South Africa, and the size by size recovery curves were studied as well. In general medium particles produced bubbles smaller than those observed in the presence of fine and coarse particles, and the recovery rates were larger. Entrainment was a contributory mechanism to the recovery of fine particles. The fluctuation of flotation indices on the particle size change can be diagnosed and predicted by the froth structures change with a high degree of accuracy.     

Evaluation of the selective detachment process in flotation froth

The improved selectivity between particles of varying degrees of hydrophobicity in flotation froths has been well documented in literature, especially in the deep froths utilized in flotation columns. The phenomenon is believed to be due to the selective detachment process whereby the least hydrophobic particles are released from the bubble surface upon bubble coalescence. To quantify the selective detachment process, column flotation experiments were performed under various operating conditions that provided varying amounts of reflux between the froth and collection zones. Entrainment was eliminated by the use of relatively coarse 250 × 75 micron material. The flotation column incorporated the ability to provide instantaneous stoppage of the process streams and separation between the collection and froth zones after ensuring steady-state operation of the column. The samples collected from the two zones and process streams were evaluated to quantify the flotation rate distribution of the particles comprising each sample. The flotation rate was used as an indicator of the degree of hydrophobicity and thus a relative measure of the binding force between the particle and bubble in the froth zone. The flotation rate data was used as input into well known flotation models to obtain the froth zone recovery rate and the quantity of material that refluxes between the collection and froth zones.     

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.     

高气泡表面积通量浮选柱浮选硫化铜矿参数的研究

通过浮选硫化铜矿的研究，检验高气泡表面积通量浮选柱的浮选性能，优化主要结构参数和操作参数。研究表明，高气泡表面积通量浮选柱采用多段相互独立的发泡器，可以在表观充气速率较高的情况下，不增大气泡直径，而增加气泡数量，从而有效地增大气泡表面积通量，并且减少了气泡在浮升过程中兼并的可能性，避免了疏水颗粒的脱落和因气泡携带能力不足导致的回收率损失，对分选效率的提高有着十分显著的效果。高气泡表面积通量浮选柱浮选硫化铜矿，在精矿品位下降不大的情况下，回收率可提高10个百分点以上。     

The relationship between the peak in air recovery and flotation bank performance

Air recovery, or the fraction of air entering a cell that overflows the cell lip as unburst bubbles, is an important measure of froth stability as it affects the flow of bubble surface to the concentrate. An experimental campaign was carried out over the first four cells of the rougher bank at a South African platinum mine in order to find the relationship between froth stability and flotation performance as a function of air flowrate.The results showed that a peak in air recovery was observed as the air rate increased. Furthermore, this corresponded to the air flowrate at which the highest overall recovery was obtained. This can be explained by understanding the resulting changes in the structural features of the froth such as bubble loading and the flow of bubble surface and suggests that improved flotation performance can be achieved by operating a bank under conditions that result in a maximum in froth stability.     

十二胺反浮选胶磷矿的消泡机理研究

为了解决十二胺反浮选胶磷矿浮选泡沫量大、不易消散等问题, 通过在浮选过程中添加泡沫调整剂磷酸三丁酯(TBP)和CA-2035对浮选泡沫进行调整, 结果表明, 添加泡沫调整剂CA-2035对十二胺的浮选泡沫具有较好的消泡效果, 综合考虑泡沫量和精矿产率之间的关系, 确定了CA-2035的最佳用量为800 g/t。为了进一步研究泡沫调整剂CA-2035的消泡机理, 以石英纯矿物为研究对象, 采用表面张力、Zeta电位、红外光谱对浮选体系进行了分析, 发现表面张力的减小和Zeta电位的升高是浮选泡沫消散的主要原因; 红外光谱分析结果表明, CA-2035不会在石英表面发生吸附。     

A technique for measuring flotation bubble shell thickness and concentration

Froth structure is known to significantly affect flotation performance. However, little information is generally available on the loading of solids on bubbles in the froth. It has been noted in foam studies that the bubble shell is very viscous and distinctly separate from the inter-bubble lamellae. In froth flotation, these bubble shells contain hydrophobic solids selectively attached, while the inter-bubble lamellae contain both hydrophobic and hydrophilic solids, which are non-selectively entrained. In this technical note we introduce a measurement technique to measure directly the bubble shell thickness and solids concentration. This measurement allows the collection of particles due to attachment to bubbles and from transport in the inter-bubble lamellae to be estimated. It is found that an increase in the surfactant concentration decreases the solids concentration in the bubble shell. The average bubble shell thickness closely corresponds to the particle size, which is in agreement with previous studies.     

Froth transport characterization in a two-dimensional flotation cell

A study of the froth bubble transport in a two-dimensional (2D) flotation cell was performed. Experiments were developed as a 2 × 2 factorial design, in which the effect of superficial air rate (1.2–1.8 cm/s) and froth depth (2–4 cm) on the froth transport for a two phase (air–water) system was characterized.Using image analysis techniques, bubble residence times, air recovery, bubble path and bubble size increase through the froth were obtained. This information was complemented by froth surface velocity measurements using the Visiofroth system.It was found that bubbles transported from the pulp–froth interface up to the overflow, showed a minimum residence time for bubbles entering the froth near the lip wall. Also, the air-recovery significantly changes in a range of 7–20% at different operating conditions.Higher residence times promoted bubble size increase by coalescence for bubbles transported from the interface. Conversely, for lower residence times, a smaller increase in bubble size was observed.     

The implications of the froth recovery at the laboratory scale

This paper presents a critical review of the role of froth recovery in laboratory flotation kinetics tests. By conducting tests in the standard lab equipment (the Denver cell), it is demonstrated that, for typical scraping rates, the froth recovery is significantly lower than the 100% that is commonly assumed when interpreting lab kinetics data. Furthermore, it is shown that the curve of overall rate constant versus froth residence time, as defined by the scraping rate, is not linear, but increases quickly at faster scraping rates. These findings have important implications for scale-up. For one, differences in froth recoveries at the lab scale can lead to significant error in the modeled plant recoveries. For two, they undermine a key assumption used to derive and validate the linear relationship between collection rate constant and bubble surface area flux. This casts doubt on the assertion that it is only the collection rate—rather than the froth recovery, interface recovery, or some combination thereof—that is responsible for the observed collinearity between the bubble surface area flux and the overall rate constant.     

Simulating realistic froth surfaces

Image analysis as a tool for monitoring and controlling flotation froths has become quite widely used in the minerals industry. One of the challenges is to be able to use the image analysis data to predict the performance of a flotation cell. One of the key parameters to be predicted is the water recovery from the froth, since this is intimately related to the gangue recovery and thus the grade. Recent theoretical developments have allowed for the calculation of this water recovery based on the air rate, froth stability and the bubble size distribution. The problem is that all that can be seen of a froth is the bubble films at the top surface. This paper uses physics based simulations of the froth structure to show that the relationship between the size distribution of the films seen at the top surface and the underlying bubble size distribution is a complex one. The paper presents some preliminary results on the statistical relationship between these distributions.     

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.     

Simple relationships for predicting the recovery of liquid from flowing foams and froths

Foams play an important role in a wide range of processes. In these processes the foams are often flowing and the amount of liquid carried within them is an important operating consideration. In froth flotation, for instance, the amount of water collect is intimately related to the amount of gangue collected, which in turn helps dictates the grade of the product obtained.This paper presents some theoretical relationships for the prediction of water recovery from a flowing foam. In the foams that have less than half the bubbles bursting at the top surface, the amount of water collected is independent of the amount of bursting and the rate is proportional to the gas rate squared and inversely proportional to the bubble diameter squared. When more than half the bubbles burst, there is a similar dependence on air rate and bubble size, but the fraction of the bubbles that bursts becomes a further factor.This relationship is demonstrated experimentally to provide a very accurate prediction for stable 2-phase foams. The relationship is theoretically applicable to the 3-phase froths found in flotation, but there are a few problems involved in obtaining the correct variable values to put into the relationships. The most problematic of these is the average size of the bubbles that are collected to the concentrate. It is also demonstrated how this equation can be used as an alternative method to image processing for estimating the bubble size in flotation froths.     

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.     

Effect of ultrasound on separation selectivity and efficiency of flotation

The effect of the use of ultrasound in the froth phase on the flotation performance has been investigated in relation to the flotation rate of a complex sulphide ore. A series of kinetic flotation tests with and without ultrasound were conducted in a flotation machine with a 2-L cell, in which an ultrasonic probe is located in the froth zone. The results indicate that there is a considerable effect of ultrasound on separation selectivity and efficiency in the flotation of a complex sulphide ore at intermediate and high level airflow rates whereas, no significant differences in the separation performance were obtained from the flotation with and without ultrasound at low airflow rates. In addition, the results of the size-by-size analysis show that a much better cleaning action in the froth was promoted for coarse particles rather than fine particles as a result of the use of ultrasound. As a result of increase in the bubble coalescence, it was found that the use of ultrasound in the froth is more effective at shallow froths. Therefore, either effective pulp volume can be increased with a negligible loss of flotation performance or the pulp density can be decreased to obtain better product quality with the use of ultrasound in shallow froths.     

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.     

Effect of entrainment in bubble load measurement on froth recovery estimation at industrial scale

Froth recovery was calculated in a 130 m³ mechanical cell of a rougher flotation circuit. This was done by bubble load determinations along with mass balance surveys. Valuable grade in the bubble load decreased in the −38 μm due to fine particles entrained to the chamber of the device. The effect of fine particle entrainment on froth recovery was evaluated. A comparison between results from the raw bubble load data (assuming all particles were transported by true flotation) with those from corrected bubble load information (subtracting fine particle entrainment) was carried out. Entrainment occurred due to hydraulic transport in the bubble rear, which corresponds to the worst case scenario for froth recovery estimation. Results showed that the relative error was less than 0.3%, which allowed validation of the bubble load measurement as an effective methodology for froth recovery estimation at industrial scale.     

Dynamic froth stability in froth flotation

A new measure for froth stability is introduced in this work, based on a dynamic stability test for non-overflowing froth columns. The dynamic stability factor represents the lifetime of a bubble in the froth, and is defined as the ratio of the total volume of froth at equilibrium to the volumetric gas rate introduced into the system. Experiments have been carried out at laboratory scale to measure the dynamic stability factor under different operating conditions. Air flowrate and frother concentration were the key operating variables. It was found that the equilibrium height and the dynamic stability factor depend significantly upon both the air flowrate and the frother concentration. Also, the dynamic stability factor and the fraction of air overflowing as unbroken bubbles in batch flotation tests were related and can be used to establish a stability criteria. These measurements will also allow a clearer quantitative link to be formed between froth stability with froth structure and flotation performance.     

浮选机矿浆相气泡图像采集系统的设计

在浮选优化控制领域，有关泡沫图像采集、处理的研究已被广泛应用。现有的研究绝大多数关注的是采集自浮选机泡沫相表面的图像数据，而对于矿浆相，由于液体本身环境恶劣，透光性极差，具有腐蚀性等因素，内部气泡图像难以获取，从而影响技术发展。针对此问题，文本设计了一种新的气泡图像采集系统，能够直接在矿浆溶液里原位采集图像数据，并提出利用深度学习语义分割模型实现矿浆相气泡图像的精确分割。实验结果表明，该系统在实验室环境下能够采集到清晰的矿浆相气泡图像，基于小样本利用预训练模型进行迁移学习，验证集的气泡分割精度达到0.943，具有重要的研究和应用价值。