Froth recovery measurements in an industrial flotation cell

The froth phase serves an important role in upgrading the final concentrate in flotation. At present, the techniques that are used in the mineral industry to determine the effect of froth phase on the metallurgical performance of plant scale flotation cells have limitations.The aim of this paper is to investigate the performance of the froth in an industrial flotation cell. A unique device has been developed which is able to decouple the froth zone from the pulp zone. The device consists of two concentric tubes. The inner tube acts as a dropback collection chamber or catcher. The particles that return from the froth phase fall directly into the catcher and are collected as froth dropback. This technique is capable of measuring plant scale flotation cell froth recovery as well as providing valuable information on froth dropback particles.The froth recovery measurements were carried out in a rougher bank of a copper concentrator treating sulphide minerals. The dropback device is designed so that it can be immersed into an industrial size flotation cell and plant froth recovery measurements can be taken at any given location. During the experiments, the bubbles laden with valuable mineral particles entered the device from the flotation cell, subsequently rising to form a froth layer at the top of the device. The particles that detached or drained from the froth zone were collected in the dropback collection chamber whereas the concentrate sample was collected through a launder. By sizing and chemical analysis of the concentrate and dropback samples, the froth recovery was estimated on the basis of the valuable component. The effect of air rate on the froth recovery was also investigated. Metallurgical grades of the froth dropback device samples for different particle size ranges were compared to those of the concentrator to better understand the froth dropback mechanism.     

Flotation frother mixtures: Decoupling the sub-processes of froth stability,froth recovery and entrainment

The flotation process consists of two distinct phases: the pulp and froth phase. One of the main roles of the froth phase is to create a suitable environment for the separation of floatable, valuable minerals from non-selectively recovered, entrained gangue minerals. As a result the froth phase plays a significant role in the metallurgical performance of industrial flotation cells. Froth stability is important for the recovery of valuable minerals. However, a stable froth may contribute to increased entrainment and, consequently, a lower grade.This study compares the effect of frother mixtures with that of their single component frothers on the froth stability, froth recovery and entrainment of a platinum-bearing UG2 ore using polyglycol and alcohol frothers. The study showed that frother mixtures resulted in a greater froth stability than either of their component frothers. The increased froth stability was reflected in increased froth recoveries and greater overall recoveries. However, the important aspect in the use of frother blends was that they altered the froth structure and resulted in a lower degree of entrainment. This, together with the increased recovery, resulted in higher grades of valuable mineral recovered to the concentrate when using the frother mixtures.     

Modified-three-product column (3PC) flotation of copper–gold particles in a rougher feed and tailings

Results of copper–gold rougher-flash flotation in a three-product column (3PC) are presented, and separation parameters are compared with results of an existing industrial small mill (rougher–scavenger–cleaner treating 100–120 t/day). This 3PC cell yielded 2%–5% richer Cu and Au concentrate grades at higher recoveries (5% R_Cu and 15% R_Au, respectively) while consuming only 5% of the circuit energy and occupying 10% of the actual footprint. The flotation of the fresh tailings from the same industrial circuit was also successful in recovering more than 40% of the copper losses and 25% of the gold losses with concentrate grades that were high enough to permit recycling of the recovered copper and gold. The cleaning action of the cell is due to the selective separation of the third product (froth drop-back), which corresponded to a low-grade material amenable for discard. This third product is primarily composed of misplaced (entrained) gangue particles, and the mass recoveries were found to be a function of froth height with an optimal value varying from 40 to 60 cm. Results are discussed in terms of the high potential of this 3PC cell and the positive impact on the design of a more simplified and efficient flotation flowsheet.     

Investigating froth stability: A comparative study of ionic strength and frother dosage

Valuable mineral recoveries and grades are strongly dependent on the stability of the froth phase within the flotation system; a stable froth zone allows for the efficient transportation of material for downstream processing, while entrainment of gangue is directly proportional to the amount of water recovered from the froth phase (Engelbrecht and Woodburn, 1975, Zheng et al., 2006a, Zheng et al., 2006b, Neethling and Cilliers, 2002). In an attempt to reduce the naturally floatable gangue (NFG) present in platinum bearing ores and to improve the grades of the valuable minerals, the use of high depressant dosages have been investigated. These high depressant dosages have significantly destabilised the froth phase in a number of studies (Bradshaw et al., 2005, Martinovic et al., 2005, Wiese, 2009). Wiese et al. (2010) has shown that an increase in frother dosage impacted the stability of the froth resulting in enhanced recovery of valuable minerals. In a separate study by Corin et al. (2011), the effect of an increase in the ionic strength of plant water on the stability of the froth was considered. As water restrictions become more stringent, many operations are recycling and reusing their process water causing an increase in the amount of dissolved ions present in the water, the effects of which are little understood. The same study also considered depressant addition at high dosages. It was noted that an increase in the ionic strength of the plant water increased the froth stability. This paper therefore compares the outcomes of increased frother dosage with increased ionic strength, and attempts to better understand the factors which influence the froth stability. The data presented in this paper forms part of much larger studies and complementary data has been published elsewhere (Wiese and Harris, 2012, Manono et al., 2012, Manono et al., 2013).     

Froth flotation of a Merensky Reef platinum bearing ore using mixtures of SIBX with a dithiophosphate and a dithiocarbamate

The Bushveld Igneous Complex (BIC) in northern South Africa has the largest deposit of platinum group elements (PGEs) in the world. In trace amounts, these are closely associated with base metal sulphides (BMS). Froth flotation is used as a bulk sulphide recovery to beneficiate these PGE ores. To maximise the recovery of the PGEs it is required to improve the recovery of the BMS. The chemical additives used largely determines the performance of the froth flotation process. Consequently, mixtures of collectors were used in batch froth flotation tests in an attempt to improve concentrate grades and recoveries of BMS from a Merensky Reef platinum ore. The mixtures consisted of a xanthate (SIBX) with a dithiophosphate (DTP) or a dithiocarbamate (DTC). Each mixture was tested at mole ratios of 80:20 and 60:40, with the xanthate the major component. An increase in nickel recovery was observed with all mixtures relative to pure SIBX at the expense of concentrate grade. The mixtures of DTC with SIBX increased the cumulative nickel recovery by 11%, while the mixtures with DTP increased it by 10%. Copper recovery increased by 6% with the DTP mixtures. No significant improvements in the copper recoveries and grades were observed with the mixtures of SIBX with DTC compared to pure SIBX.     

A new method for flotation rate characterization using top-of-froth grades and the froth discharge velocity

A new methodology is proposed for flotation characterization in industrial operations. The approach considers the mineral recovery to be proportional to both the top-of-froth (TOF) grade and the froth discharge velocity down a bank of cells. The procedure allows for the identification of the fractional recovery profile from the discharge velocities and the TOF grades. In addition, if the total recovery of the bank is available, the cell recoveries can be estimated by scaling the fractional recoveries. For this purpose, a single parameter was used to scale the recoveries for each sampling survey in order to obtain the kinetic response along the flotation banks. Industrial tests were performed in two rougher banks; one bank consisted of six 250 m³ self-aerated cells in a 1-1-1-1-1-1 arrangement, and the other bank consisted of nine 130 m³ self-aerated cells in a 1-2-2-2-2 arrangement. The results showed good agreement with the recovery profiles obtained from the cell-by-cell mass balances along two industrial flotation banks.     

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.     

Optimisation of air rate and froth depth in flotation using a CCRD factorial design – PGM case study

Air rate and froth depth are the most commonly adjusted levers in PGM flotation plants. The optimisation of these levers on each flotation cell has traditionally been done by varying either air rate at a fixed froth depth or vice versa. This approach does not consider the interaction relationship between air rate and froth depth and this effect on flotation performance.Factorial type experimental designs are best suited for investigating interaction effects between variables. This paper presents the use of a factorial type of experimental design being the (CCRD) Central Composite Rotatable Design for plant scale flotation optimisation of air and froth depth. The results obtained include three dimensional response surfaces and models of flotation response variables such as 4E PGM recovery and grade as a function of air rate and cell level. This paper illustrates the experimental methodology and discusses the results for normalised 4E PGM grade and recovery for a rougher cell treating a Platreef ore.These results indicate that interaction effects of air and froth depth are significant and are more pronounced at conditions of higher air and shallower froth depth. In addition, indices which are based on an optimisation objective such as grade multiplied by recovery and/or grade multiplied by recovery squared allows application of this technique as an optimisation tool. These indices can be used to determine an optimum operating range for air and level with the consideration of interaction effects.     

Beneficiation of the Nechalacho rare earth deposit: Flotation response using benzohydroxamic acid

Rare earth minerals (REM) may be beneficiated using a variety of separation techniques with froth flotation being the most commonly applied in operating REM separation plants. Many different collectors have been tested with traditional REM such as bastnäsite. The most successful are the hydroxamic acids. The Nechalacho deposit contains many REM for which there is minimal processing information in literature. Thus, experimental investigation into their flotation behaviour is required.This work focuses on understanding the effect of a benzohydroxamic acid collector on the flotation of a REM enriched pre-concentrate from the Nechalacho deposit (after lab-scale gravity and magnetic separation steps). The lab-scale flotation response of the REM is quantified using mass recoveries, water recoveries, QEMSCAN and chemical assay data and TOF-SIMS in order to study the recovery, entrainment and kinetics of different REM. Three different flotation schemes are investigated to determine the effect of a secondary addition of collector as well as the addition of lead ions, a known activator in hydroxamic acid flotation. It is important to note that the goal of this work is to determine fundamental flotation properties of the different REM in the Nechalacho deposit. As such, the process investigated here is not indicative in any way of the current process design for this deposit.     

A novel scale-up approach for mechanical flotation cells

Planning industrial flotation operation and earlier flotation equipment sizing are commonly based on batch flotation testing, where ideal operating conditions can be provided. Each plant has its own batch flotation standards and typically uses a time scale-up factor in order to compare laboratory and plant flotation performance. However, flotation scale-up is more complex, and it is not yet completely understood.In this work, a novel scale-up approach was developed, where the effects of the hydrodynamic regime (mixing), solid segregation (effective residence time) and froth recovery on the plant flotation rate were identified and evaluated. Each effect was then described by means of correction factors applied on the batch flotation rate, which was considered the optimal condition. These factors can be determined from laboratory and plant experimental data. This methodology was successfully applied at the rougher copper flotation plant of Codelco Norte Division, Codelco-Chile, for cells of 160 and 300 m³.     

The froth stability column: Measuring froth stability at an industrial scale

Froth structure and stability are known to play important roles in determining mineral flotation recovery and selectivity. However, measuring froth stability in a consistent manner remains a significant challenge, especially at an industrial scale. Following preliminary tests on a copper concentration plant, a quantitative dynamic stability measure is investigated in this study in order to extend the results over a wider range of conditions and on a different ore body. The technique is based on the Bikerman foam test and uses a non-overflowing froth column to quantify froth stability.Experiments were carried out using an automated version of the froth stability column under different operating conditions. Air flowrate was the key operating variable. Tests were reproduced on a single flotation cell of a Platinum Group Metals concentrator. The froth stability factor, β, was measured for each operating condition, and compared with the air recovery in the cell, α, which was measured using image analysis. The froth stability column results gave the same trends as image analysis. In particular the froth stability factor was found to be linearly related to the actual fraction of air overflowing the cell.The metallurgical results clearly indicated that changes in air flowrate result in variations in flotation performance that can be attributed to changes in froth stability. The results showed that high froth stability conditions occur at intermediate air flowrates, and result in improved flotation performance. It is found that the froth stability column is a simple, cost-effective and reliable method for quantifying froth stability, and for indicating changes in flotation performance.     

The effect of reagent addition strategy on the performance of coal flotation

The recovery and grade of concentrates obtained by flotation is partly dependent upon froth structure. The flotation froth structure can be modified, and hence the flotation performance modified, by the addition of reagents.A direct control system was developed which permitted pre-determined addition profiles of reagent and water to be added to a cell. A series of experiments were performed using different strategies of reagent (Triton x-405) addition for the flotation of Bickershaw coal.A transputer enhanced Micro-Vax Computer was employed to grab images of flotation froths and to process them quantitatively in a parallel procedure. Using image processing techniques characterization of froth structures was achieved. The image processing procedures that were intended to run on-line, were examined and developed using video tapes of experimental runs, off-line.An essential stage in the development of such a system is the understanding of the adsorption behaviour of the reagent in terms of the kinetics, the distribution of reagent between the solid and solution phases, and the resulting effect on the froth structure. A kinetic model was developed to describe the adsorption of the reagent on the particles. The experimental recoveries and froth structure were investigated in terms of reagent distribution and the froth structure.     

Investigation of the interactive effects of the reagent suite in froth flotation of a Merensky ore

Interpretation of the behaviour of reagents in flotation is made complex by the existence of secondary effects, which can override the desired effects that enable effective separation, in addition to a predominant role of a flotation reagent, and the incomplete liberation of the mineral particles. Moreover, accumulation of dissolved ions due to the water recycle and reuse practice in processing circuits, which alters the chemical environment in the pulp and impacts the overall performance of the process, adds to this complexity. Therefore, it is often challenging to precisely assess individual contributions of the reagents to the overall performance, and hence the need for a holistic approach in evaluating the behaviour of reagents in flotation. Factorial design of experiments offers such an approach. This study used a factorial design approach in an effort to determine the interactive effects between chosen flotation reagents; collector, depressant, frother and water quality on the metallurgical performance of a Merensky ore. Increasing the ionic strength of the plant water increased recoveries, and the effect on decreasing concentrate grade was minimal, therefore the practice of water recycle and reuse should not impose any adverse impact during beneficiation of the PGM-bearing ore from the Merensky reef. It was also observed that higher recoveries were obtained with high ionic strength plant water than high frother dosage, indicating that the use of saline water in froth flotation of certain ores can replace the use of frothers, thus reducing costs. The factorial design approach can be useful in evaluating the simultaneous effects of such variables in the flotation process, thereby giving better understanding of their overall effect on the whole process.     

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.     

Visual information model based predictor for froth speed control in flotation process

Image processing sensors are emerging as an important measurement option in mineral processing, mainly due to their non-intrusive characteristics. Their principal application areas have been the determination of ore size distributions in grinding and froth features in flotation. The incorporation of visual information in control loops is the logical step. However, the excessive processing required brings a new problem that must be solve: to count with a strategy able to provide a measured value for each visual sensor in the plant. A first approach is to assign one computer to each sensor yielding a distributed architecture, but this means the implementation of a huge computer network. A more efficient alternative is alternated sampling, but the succeed of this option is limited to the existence of virtual sensors capable of give accurate values that must be use during the unsampled period. In this paper we begin by reviewing classical image processing algorithms used in flotation froth feature extraction. Then a new method is introduced for the characterization and recognition of visual information using dynamic texture techniques. Finally we developed a dynamic texture based virtual sensor for the prediction of froth speed in the unsampled period, tested with industrial data.     

On the collection of valuable minerals along rougher flotation banks

The collection of valuable minerals along rougher flotation banks was evaluated using top-of-froth measurements, which consist of sampling the froth surface. Considering non-selective froths observed in rougher flotation circuits, it was found the top-of-froth grades were similar to the bubble load grades, which allowed for a direct estimation of the collected mineral characteristics (size distribution, mineralogy, and grade). Thus, the flotation process evolution can be characterized by analyzing the collection of valuable minerals by true flotation instead of the classical mass balances approach, which strongly depends on the reliability of the tailings grade data. In addition, comparison among different valuable minerals was observed without significant sampling disturbances caused by solid entrainment.It is expected the top-of-froth measurement will be a useful tool for evaluating the effect of operating variables such as pulp conditioning, air flowrate and solids percentage on the true collection process along a flotation bank.     

Case studies on the performance and characterisation of the froth phase in industrial flotation circuits

This paper deals with two separate case studies investigating the froth phase performance and characterisation of two industrial rougher/scavenger flotation circuits. Froth phase performance was quantified using a mass balance approach to estimate froth zone recovery. Measured characteristics of the froth phase included frother solution concentration determined by gas chromatography, and the time taken for an equilibrium froth sample to decay to one-half of its original froth height. The latter measurement is referred to as the ‘froth half-life’ and is strongly linked to froth stability. Special methods and techniques developed to preserve frother in solution and to measure froth half-life are briefly described. The frother type in the first case study was a mixture of straight and branched alcohols, whilst the frother type in the second case study was a mixture of alcohols, aldehydes and triethoxybutane. The first case study focussed on a flotation circuit treating a low grade ore containing only a small fraction of floatable copper sulphide minerals, while the second case study focussed on a flotation circuit treating a higher grade complex sulphide ore containing significant quantities of chalcopyrite, galena, sphalerite and pyrite.It was found that froth zone recovery of valuable mineral generally decreased down-the-bank of the two industrial rougher/scavenger circuits. Moreover, decreases in froth zone recovery significantly limit the overall cell recovery of valuable mineral achievable from the plant scavenger cells. However, the decrease in froth zone recovery could not be linked to the removal of frother from the pulp solution to the concentrate product in the preceding rougher flotation stages. Measurements of residual frother in solution suggested that, approximately, only 5–10% of the added frother was removed into the rougher/scavenger concentrate, with the remainder appearing in the scavenger tailings. This finding suggested there was apparently adequate frother in solution in the scavenger stages.There was, however, a correlation to the froth half-life, with the froth half-life also generally decreasing down-the-bank. A simple, empirical model, based on the froth half-life and froth residence time of gas, is proposed here to predict froth zone recovery. Further, it is proposed that the froth stability, as measured by the froth half-life, is strongly linked to the presence of particles in the froth, with poorly mineralised scavenger froth characterised by a short half-life and, potentially, a low froth zone recovery. The importance of particles on froth stability was confirmed in separately conducted laboratory experiments. These experiments also demonstrated the wide variation in froth stability behaviour between different frother types.     

基于长程时空特征与多尺度外观特征的锌精选工况识别北大核心

锌精选作为锌浮选的最后一道流程,其工况直接决定锌浮选最终产品质量。现有基于卷积网络的浮选工况识别方法具备挖掘隐藏特征的能力,取得了良好效果,但仍存在表征能力有限、模型参数大等问题。为此,提出了基于长程时空特征与外观特征的锌精选工况识别模型。首先,提出基于分离三维卷积网络(Separable 3D Convolutional Neural Network,S3D CNN)与注意力机制的泡沫视频相邻帧间短程时空特征提取方法,获得特征聚焦的泡沫视频相邻帧间短程时序信息。然后,在短程时空特征的基础上采用双向卷积长短时记忆网络(Bi-directional Convolutional Long Short-Term Memory,BiConvLSTM)提取泡沫视频帧间的长程时空特征,获取泡沫视频帧间的长程动态时序信息。最后,采用基于残差网络和迁移学习的二维卷积网络提取泡沫图像的多尺度外观特征,并融合长程时空特征,对锌精选工况进行识别。实验结果表明,与现有卷积网络方法相比,所提模型在工况识别精度和模型参数上性能更佳。