Reflux Flotation Cell Research Articles

Synergistic effects of Reflux Flotation Cell's downcomer and reverse fluidization bed in rejecting clay entrainment during particle separation

The Reflux Flotation Cell (RFC) technology is heralded as an innovative game-changer, promising significant advancements in particle separation, and offering generational benefits for the resources industry. This paper meticulously assessed the RFC's capabilities in tackling another critical industry challenge in resources processing— severe entrainment of waste clay particles during flotation separation and compared the performance with conventional mechanical flotation. Through flotation tests where kaolinite particles were separated from combustible materials, in-situ particle aggregation measurements and calculating air fractions in both flotation systems, this study identified the superior performance of the RFC in reducing kaolinite entrainment and producing a high product quality. This was attributed to a synergistic function between the RFC's downcomer for kaolinite dispersion and the RFC's fluidization bed for generating a low air fraction in reducing kaolinite entrainment. While kaolinite dispersion avoided the blocking of water drainage channels between air bubbles by aggregates, a low air fraction ensured a large gap between air bubbles to allow the displacement of entrained fine particles with fluidization water. Herein, the research findings will guide the resources industry to process low quality resources by successfully rejecting clay and other fine waste materials.

Understanding the Effects of Hydrophobic Particle Densities on Bubble-Liquid Motion and Gangue Entrainment in the RFC’s Reverse Fluidized Bed Through CFD Simulation

ABSTRACT The Reflux Flotation Cell (RFC) developed recently provides great potential in solving the emerging challenges, in terms of low plant capacity and low metal production, faced by the resources industry. However, the bubble-liquid motion and the mechanisms responsible for gangue entrainment in the RFC have not been well studied. In this study, chalcopyrite and coal were chosen as value materials and quartz was chosen as a gangue material, and the flotation of chalcopyrite or coal against quartz was conducted in the RFC. It was found that chalcopyrite flotation produced a much higher quartz recovery than coal flotation despite the same operating conditions employed, which indicated a great effect of hydrophobic particle density on gangue entrainment in the RFC. CFD simulations indicated that a higher density of hydrophobic particles increased the gas holdup in the RFC’s reverse fluidized bed and the boundary of the air phase, which promoted gangue entrainment. Meanwhile, the higher density of hydrophobic particles also inhibited the formation of a liquid recirculation loop in the reverse fluidized bed, weakening the displacement of gangue particles with wash water and resulting in higher gangue entrainment. The findings from this study will provide guidance on how to minimize gangue entrainment in chalcopyrite flotation in the RFC.

Effects of operating parameters on residence time distribution in a REFLUX flotation cell

Residence time distribution (RTD) is a critical parameter in design and optimization of flotation processes and machines. RTD measurements provide valuable indication into the distribution of residence times experienced by particulates (e.g., particles, oil droplets) within the system, enabling the identification of flow regimes and potential issues such as short-circuiting, dead zones or inadequate mixing. While previous studies in the literature have generally focused on RTD measurements in mechanical flotation cells, there are limited experimental studies available for pneumatic flotation cells. In this study, RTD measurements were performed to characterize the type of flow regime in the REFLUXTM Flotation Cell (RFC), and to investigate the impacts of operational parameters, including bias, feed flux, and gas flux, using a laboratory-scale RFC-100. An impulse-response methodology was employed with a KCl solution used as a tracer in a two-phase (liquid–gas) system. Large and Small Tanks in Series (LSTS), N-mixer in Series, and Perfect Mixer models were applied to a database created by experimental results. The results revealed that the fluid behavior in RFC-100 approached a plug-flow pattern rather than perfect mixing in the two-phase (liquid–gas) system. Furthermore, an increase in bias resulted in a decrease in mean residence time (MRT), and the tracer concentration inside the cell could be adjusted by varying the gas flux under the same feed flux.

Compare graphite flotation and coal flotation in Reflux Flotation Cell

The Reflux Flotation Cell (RFC) has been developed as a game-changing technology to improve fine particle flotation, coarse particle flotation and plant capacity. It has been proved successful for coal flotation. This study compared graphite flotation and coal flotation in the RFC under the operation conditions established for optimizing coal flotation with an objective of extending the RFC to mineral flotation. It was interesting to find that graphite flotation in the RFC produced much lower combustible recovery than coal flotation especially at a high gas flux despite the same surface property and feed size of graphite and coal particles tested. Through the modelling of fluid flow, it was found that the lower combustible recovery from graphite flotation was due to the lower sphericity of graphite particles. The flaky graphite particles promoted a greater dynamic drag force and a greater particle settling velocity compared to coal particles with a greater sphericity. This increased the relative velocity between particles and bubbles in the RFC downcomer, unfavourable for bubble-particle attachment in graphite flotation in the RFC. In addition, a higher gas flux environment in the RFC downcomer produced a higher bubble slip velocity, further increasing the relative velocity between particles and bubbles, exacerbating graphite flotation. Furthermore, the extra gas in the inclined channels pushed more graphite particles to the tailing stream given the higher particle settling velocity. This study indicates a direction of how to optimize graphite flotation in the RFC.

Coarse particle separation by fluidized-bed flotation: A comprehensive review

Since comminution within mineral beneficiation plants is the most energy consumer unit, the effective separation of coarse particles has generated considerable interest in the ore processing industry. Due to the inherent challenges faced with floating coarse particles using conventional flotation cells (CFCs), a number of attempts were made to develop new technologies that could efficiently and cost-effectively treat them. Fluidized-bed flotation cells (FBFCs), as novel developments such attempts, have proven a higher potential than CFCs for processing coarse particles. HydroFloat, NovaCell, and Reflux Flotation Cell are three main developed FBFCs for processing coarse particles. The quiescent nature and other characteristics of FBFCs allow for longer retention time, reduced particle buoyancy constraints, a near absence of coarse particle detachment from the bubble, and increased bubble-liquid segregation capacity, which leads to an excellent coarse particle recovery. These properties assist FBFCs in processing coarse particles environmentally friendly.

Flotation behaviour in reflux flotation cell – A critical review

• The roles of three major components of the Reflux Flotation Cell are introduced. • The flux mechanism formed in the boundary of major components is discussed. • Three theories adapted to characterise the Reflux Flotation Cell were explained. • The operating parameters of the Reflux Flotation Cell were evaluated. • Future research directions on the Reflux Flotation Cell are recommended. The development of new flotation equipment has to address the challenges confronting the minerals industry, the low flotation efficiency of fine and coarse particles and the need to process a higher volume of ores due to the decreased feed grades. By combining a downcomer, a fluidised bed and inclined channels within one equipment, the Reflux Flotation Cell (RFC) offers a general solution to improving fine and coarse particle flotation while maintaining a high throughput. Current research on the RFC has proven its success in coal flotation. Based on the published research work, this paper introduces the history of RFC development, illustrates the unique structure of the RFC and the role of each major component, discusses the flux mechanism formed in the boundary between the fluidised bed and inclined channel section, explains the main theories adapted to characterise the RFC and evaluates the effect of the operating parameters on its performance. This information provides a general picture of the RFC from both theoretical and practical perspectives. This paper also recommends future research on the RFC to expand its application from coal flotation to minerals flotation and also model the bubble motion in inclined channels and the flux mechanism in the boundary area, both of which dictate the performance of the RFC.

The effect of feed solids concentration on flotation performance using the Reflux Flotation Cell

This study examines the separation performance achieved using the Reflux Flotation Cell (RFC) when treating low economic value coal waste streams over a range of feed solids concentrations. Feed solids concentrations of 1.3, 2.4 and 5.7 wt% were examined using a fine coal suspension of nominally 50 wt% head ash, covering the typical range emerging from the overflow of a desliming hydrocyclone. This work was also extended to include much higher levels of ultrafine gangue through the addition of significant levels of silica, resulting in an extreme feed ash of 97 wt% head ash, the aim being to assess whether these levels of gangue minerals affect hydrophobic particle capture. For the actual plant feed, as the feed solids concentration was increased the flux of hydrophobic particles fed to the RFC increased in a fixed ratio to the flux of hydrophilic particles. These results showed a greater gas flux was required to maintain combustible recovery as the feed flux of hydrophobic particles increased. The relationship between bubble surface coverage and hydrophobic particle recovery was also investigated. Estimates of the average bubble surface coverages, ranging 6–18%, showed a decreasing trend between recovery and surface coverage. By establishing a strong positive liquid bias flux, ∼0.7 cm/s in magnitude, the product ash was maintained within a narrow band of 7.0–11.9 wt% ash across the series of experiments, demonstrating consistent and strong product desliming over the up to 16-fold increase in feed solids concentration.

Investigating the System Flotation Kinetics of Fine Chalcopyrite in a REFLUX™ Flotation Cell using a Standardised Flotation Cell Reference Method

• Performance ratio was proposed to compare froth flotation technologies. • Performance was based on the relative cell product flux at a given recovery. • Product cleaning was quantified using a selectivity parameter. • Reflux Flotation cell was compared to a batch mechanical cell. • Performance ratio ranged from 3 to 20 fold. There is increasing recognition of the need to develop and optimise the individual sub-processes of flotation. The REFLUX™ Flotation Cell offers the opportunity to achieve this goal within a single system, fundamentally by decoupling the water recovery and hence entrainment from the imposed gas flux. In this study, the flotation kinetics achieved using a batch mechanical cell were determined for a pure chalcopyrite feed less than 0.1 mm. These results were then used to assess the performance of the REFLUX™ Flotation Cell operated under continuous steady state conditions. A standardised flotation cell reference method was established based on the ratio of the product flux of the REFLUX™ Flotation Cell relative to the plug-flow equivalent result for a batch mechanical cell. This analysis, conducted as a function of the particle size, indicated a performance ratio of ∼3–20 fold. Moreover, the analysis provided insights into the effects of the operating conditions on the kinetics of the REFLUX™ Flotation Cell. This work was then extended to establish an assessment of the counter current washing achieved as a function of the particle size for when the pure chalcopyrite ore was combined with ultrafine silica.

Maximizing recovery, grade and throughput in a single stage Reflux Flotation Cell

The Reflux Flotation Cell (RFC) utilises the Boycott Effect to decouple the overflow water flux from the gas flux, permitting in principle high product grade and recovery at a vastly higher volumetric feed flux. This study investigated this relationship between concentrate grade, recovery, and volumetric feed throughput using a single flotation stage and feed fluxes spanning 1–9 cm/s, well beyond that used in conventional flotation. Coal flotation tailings and hydrocyclone overflow provided convenient representations of “binary” feeds for the experiments, constituting liberated hydrophobic and hydrophilic particles. The results demonstrated robust recoveries through the preservation of the gas to feed flux ratio with increasing feed flux, while minimising the gas flux strengthened the capacity to maintain high product grade using inverted fluidization water as the wash water. Remarkably, a high product grade (low product ash%) was maintained over the extreme feed flux range by ensuring a net downwards flux of wash water delivered through the upper fluidized bed of bubbles. Coal Grain Analysis (CGA), an optical imaging technique, identified the maceral composition of the feed particles and validated, with close agreement, the RFC steady state separation performance. Indeed, under continuous operation the RFC data demonstrated an overall positive shift in performance relative to that of the standard tree flotation curve. The findings showed strong preservation of product grade and recovery using a single RFC stage, over a seven-fold increase in the feed flux relative to conventional flotation systems.

Enhancing Ion Flotation through Decoupling the Overflow Gas and Liquid Fluxes

Conventional ion flotation is hydrodynamically constrained by coupling of the gas flux and liquid flux that report to the concentrate. This constraint has greatly limited the industrial application of ion flotation, despite its remarkable effectiveness in extracting ionic species down to very low concentrations, of order 1 ppm. Previous work demonstrated that these hydrodynamic constraints could be significantly relaxed using the reflux flotation cell (RFC), a system incorporating parallel inclined channels to improve bubble-liquid segregation. However, it was found that bubble coalescence placed an additional limit on performance. In this study the impact of coalescence was minimized by reducing the volume reduction from 20 to 5, ensuring sufficient liquid reported to the concentrate with the bubbles. Under these conditions, an equivalent adsorptive recovery was achieved using the RFC at feed fluxes up to four times those in the conventional system. The maximum adsorptive extraction rate achieved with the RFC was three times that for the conventional system. A refined experimental methodology was used to quantify much more accurately the relative hydrodynamic limits of conventional and RFC operation. The previously neglected issue of split-zone segregation, resulting in smaller bubbles in the lower part of the cell, was also investigated.

Study on the effects of some parameters on the flotation performance of Vang Danh coal fines -0.3 mm in the reflux flotation cell

In the world, the coal fines usually are upgraded in flotation columns (Jameson, Pneufloat, Microcell,...). The reflux flotation cell is a novel type of flotation column that uses a system of inclined channels to enhance bubble - liquid segregation in flotation. Additionally, in this type of device, it is used a special froth washing mechanism with pressurized water to ameliorate the quality of cleaned coals. This type of device has a simple structure, high capacity and allows to receive cleaned coals of high quality. This paper presents the test results of coal fines samples -0,3mm from Vang Danh Mine by flotation in a laboratory Reflux flotation cell. In the test works, some process parameters are investigated and optimized: flotation time, pulp density, feed rate, froth height, wash water, and flotation reagents regime.

Flotation of coarse coal particles in the Reflux™ Flotation Cell

Froth flotation is a separation process that has widespread application throughout the coal and minerals industries. Conventional flotation technologies are effective in recovering coal up to about 0.35 mm and dense minerals up to about 0.10 mm. There are significant benefits to both the coal and minerals industries in increasing the upper particle size. This paper reports on an investigation of coarse coal flotation up to a nominal 2 mm size using the Reflux Flotation Cell (RFC). The first part of the investigation involved the flotation of coal tracer particles added to the system individually, with the RFC operating at a specific hydrodynamic condition defined by the feed and gas fluxes. The particles were strongly hydrophobic, having relative densities (compared to water) within the range 1.25 – 1.30. The particles were saturated by the collector, diesel oil, prior to each experiment to ensure consistent hydrophobicity. The tracer particle experiments revealed a general trend of decreasing coarse particle yield with increasing gas flux. The yields were highest at gas fluxes below 0.5 cm/s, and largely independent of the volumetric feed flux over the range 0.9 to 6.0 cm/s. The data revealed a dependence on the gas volume fraction in the overflow, with the highest recoveries observed when held at nominally 0.80 or less. The second part of the investigation involved the flotation of industrial coal slurry at 5 wt% solids concentration. The performance of the RFC was judged with reference to Tree Flotation for particles below 0.125 mm, and Float/Sink separation at a relative density of 1.6 for particles −2.0 +0.125 mm. Again, high recoveries of 82% to 97% for fractional sizes spanning −2.0 +0.125 mm were achieved provided the gas flux was below 0.5 cm/s. Product grade surpassed the tree curve using inverted fluidization water, without any recovery losses over the full particle size range.

Recovery and cleaning of fine hydrophobic particles using the Reflux™ Flotation Cell

The Reflux Flotation Cell (RFC) consists of a vertical vessel located above a series of parallel inclined channels. This novel system, which is the inverse of an existing gravity separation system known as the Reflux Classifier, provides a powerful mechanism for enhancing bubble-liquid segregation. The usual link between the imposed gas flux and water recovery in the froth product is decoupled through this mechanism, resulting in the establishment of a concentrated bubbly zone throughout the upper section of the cell, and no froth zone. A downwards fluidization arrangement promotes strong washing of the flotation product, to remove the hydrophilic slimes from the hydrophobic concentrate. The mechanism also provides for strong control of the bias flux, allowing a significant positive bias flux to be established. A feed suspension of fine coal tailings containing hydrophobic coal and hydrophilic mineral matter was subjected to the novel flotation, providing an ideal basis for studying the interplay between the hydrophobic particle recovery, inferred by the combustible recovery, and the product grade, inferred by the mineral matter content of the product, expressed by the ash %. The feed ash % was about 43 wt%. The product cleaning was compared to firstly, the results from the tree flotation method, and secondly, a newer method known as Coal Grain Analysis (CGA). The RFC results were found to lie to the left of the tree curve, converging to the limit described by the CGA analysis. This work suggests the RFC and CGA provide a form of mutual validation on the limits of cleaning, though more work will be needed to confirm this finding. The RFC was operated with a volumetric feed flux of the order 1 cm/s, a rate comparable to that of conventional flotation systems, with the bias flux ranging from 0.0 cm/s to 1.9 cm/s. For a fixed gas flux of 1.1 cm/s, increases in the liquid bias flux from 0.0 cm/s to 1.0 cm/s resulted in a decrease in product ash from 18.2% to 7.8%, and a reduction in combustible recovery from the order 87% to 75.6%. Operating at the lowest gas flux of 0.6 cm/s, and the strongest bias flux of 1.9 cm/s yielded a low product ash of 6.7% at the reduced combustible recovery of 64%.

Enhancing extraction in ion flotation using the boycott effect

While Ion Flotation is highly effective in recovering ionic species at very low concentrations, the approach has struggled to find economically viable application due to hydrodynamic constraints. Recovery and upgrade decline very rapidly with increases in the volumetric feed flux (volumetric rate per unit of vessel area), largely due to the direct coupling of the liquid product flux with the input gas flux. We demonstrate a way to overcome the hydrodynamic limitations using the Reflux Flotation Cell (RFC), a system incorporating parallel inclined channels. These produce a powerful increase in the bubble-liquid segregation velocity, allowing decoupling of the liquid product flux and input gas flux. The system achieved an increase in the feed flux by up to four-fold for the same recovery and upgrade, with the adsorptive extraction rate up to three times greater than achieved by conventional ion flotation. Ultimately bubble coalescence limited the final performance in part due to the significant volume reduction (ratio of feed to product flux) that was targeted.

The kinetics of Fast Flotation using the Reflux Flotation Cell

This paper examines the kinetics of Fast Flotation of very fine, hydrophobic particles using a rectangular-sectioned downcomer in the Reflux Flotation Cell (RFC). Three separate downcomers of equal height and breadth, but different channel widths of 2, 4.5, and 9 mm, were used to investigate the recovery of particles ranging from 1 to 100 µm in diameter. The RFC utilises parallel inclined channels to enhance the segregation rate of rising bubbles in liquid, a phenomenon known as the Boycott Effect. This work was focused on the contribution of the downcomer kinetics to the Fast Flotation. Feed fluxes over the range of 0.9–8.6 cm/s were examined, with residence times in the downcomer of less than 1 s. Recovery due to entrainment was made consistent by fixing the portion of liquid reporting to the overflow to 10% of the feed volumetric rate, independent of the gas flux employed. Kinetic rate constants for the particle recoveries were analysed by varying flotation parameters such as particle size, gas flux and feed flux, obtaining downcomer kinetic constants of up to 419 min−1. Distinct kinetic behaviour was found for the 1–10 µm and 10–100 µm coarser particles, with the kinetics of finer particles being more dependent on the particle size. The kinetics using the narrower downcomer widths (4.5 mm and 2 mm) relied more on the gas flux than the feed flux, while the kinetics using the wider downcomer width showed more dependence on the feed flux. Scaling laws for the kinetic rate constant were derived empirically based on the downcomer channel width, particle size, gas flux and feed flux. These scaling laws provided insight into the underlying mechanisms and a basis for exploring the potential flotation performance over a wide range of system operational conditions. The narrower downcomers exhibited better performance at the feed fluxes <10 cm/s. Above this feed flux, the wider downcomer of 9 mm performed better, especially for coarser particles above 10 µm. A basis for downcomer system design is provided based on the kinetic scaling laws.

Two-stage fast flotation of coal tailings using reflux flotation

Low pulp density and low grade slurries in the coal and minerals industries are discharged as waste to tailings dams, incurring significant losses of valuable particles. This paper investigates the rapid processing and cleaning of hydrocyclone overflow coal slurry using two laboratory scale Reflux Flotation Cells in series as a means to economically beneficiate low quality tailings streams. The Reflux Flotation Cell incorporates a novel arrangement of inclined channels to enhance bubble-liquid segregation, enabling extremely high gas rates and liquid rates per unit of vessel area. Hence, in the first stage, fast flotation is employed to rapidly recover fine coal particles using a feed flux of 11.4±0.5cm/s, up to an order of magnitude increase in the throughput rate over conventional flotation systems. First stage product was then sent to a second stage for counter-current washing using fluidisation wash water to produce a fully deslimed product, having ash percent in agreement with the minimum ash attainable using flotation as determined through tree flotation analysis. The results demonstrate the potential for two-stage Reflux Flotation to deliver high throughput at a high separation efficiency from low quality slurry, with a fivefold reduction in the required vessel footprint, thus overcoming the principal economic deterrent of having to install banks of large-scale flotation cells.

Fast flotation of coal at low pulp density using the Reflux Flotation Cell

Fast particle flotation is accomplished by maximising three fundamental aspects: the kinetics of particle–bubble attachment, the bubble interfacial flux for particle extraction, and the rate of bubble–liquid segregation. In practice, it has been impossible to extend all three aspects simultaneously using conventional flotation devices. Hence, significantly higher processing rates using a single flotation cell has not been possible. Here, the Reflux Flotation Cell has been used in this work to address all three aspects in unison in a single stage of separation. This novel system permits throughput rates well beyond conventional flotation standards. Stable operation using extreme gas and feed fluxes is accomplished using a system of parallel inclined channels located below the vertical portion of the cell. In this paper a highly diluted coal feed comprised of well-liberated coal particles at 0.35wt% solids, was prepared from hydrocyclone overflow. The volumetric feed flux was increased to nearly 10 times the typical conventional level, achieving an extremely low cell residence time, in the order of 25s. Very good combustible recoveries were obtained, with the +38μm portion increasing from 92.3% to 98.5% with increasing gas flux. The partitioning of particles below 38μm decreased with decreasing particle size until separation became governed by hydraulic entrainment, clearly evident at a particle diameter of ∼1.65μm.

Maximizing bubble segregation at high liquid fluxes

This study is concerned with a common class of problem involving two phase separation of a dispersed gas flow from a continuous liquid flow under extreme processing conditions. Relatively fine spherical bubbles of order 500μm were generated in the presence of a surfactant under a high shear rate within a rectangular, multi-channeled, cuboidal downcomer. Liquid fluxes, as high as 176cm/s through each channel of the downcomer, sheared bubbles from a sintered surface mounted flush to the channel wall before disengaging the downcomer flow into a vertical vessel. Both high feed fluxes, up to 15cm/s, and high gas fluxes, up to 5.5cm/s, ensured a high gas holdup beneath the downcomer and the hindered rising of the bubbles. Enhanced bubble–liquid segregation was achieved using an arrangement of parallel inclined channels incorporated below the main vertical chamber. This novel device, referred to as the Reflux Flotation Cell, prevented the entrainment of bubbles to the underflow, and significantly reduced the liquid flux to overflow, even in the absence of a conventional froth zone. Extreme upward bubble surface fluxes of up to 600s−1 were achieved, while counter-current downward liquid fluxes reached 14.4cm/s, arguably four times the bubble terminal rise velocity. Hence successful phase separation was achieved while operating well beyond the so-called flooding condition arising from extreme levels of gas and feed fluxes. This hydrodynamic arrangement should find application in increasing surfactant extraction rates in foam fractionation and ion flotation, gas absorption, and even particulate flotation.

Fluidized bed desliming in fine particle flotation – Part II: Flotation of a model feed

This is second in a series of papers concerned with the performance of a novel technology, the Reflux Flotation Cell. Part I examined the system hydrodynamics, commencing with a gas–liquid system and examination of the fluidization boundary condition. The desliming, or potential to reject entrained fine gangue particles from the product overflow, was investigated by introducing hydrophilic particles. In Part II, a model feed consisting of hydrophobic coal particles and hydrophilic silica was introduced. The separation of these two components was investigated across an extreme range in the applied gas and wash water fluxes, well beyond the usual limits of conventional flotation.The Reflux Flotation Cell challenges conventional flotation cell design and operation in three ways. Firstly, the upper free-surface of the flotation cell is enclosed by a fluidized bed distributor in order to fluidize the system in a downwards configuration, counter-current to the direction of the rising air bubbles. Secondly, a system of inclined channels is located below the vertical section of the cell, providing a foundation for increasing bubble–liquid segregation rates. Thirdly, the system is operated with a bubbly zone, hence in the absence of a froth zone. This combination of conditions provides for the establishment of a high volume fraction of bubbles in the bubbly zone, of high permeability, ideal for promoting enhanced counter-current washing of the rising bubbles, and hence high quality desliming. The arrangement permitted operation at extreme levels in the value of the fluidization (wash water) flux and the gas flux, with the fluidization flux set at up to 2.1cm/s and the gas flux set at up to 4.7cm/s for a mean bubble size, db, of 1.5mm. These gas and wash water fluxes corresponded to a bubble surface flux of 188m2/m2s and a positive bias flux of 1.7cm/s. Thus the operating regime was shown to be far broader than that achieved by conventional flotation, thereby confirming the robust nature of the system. The model flotation feed provided a basis for establishing the flotation performance across this vast regime of operation. Full combustible recovery of fine coal and full rejection of mineral matter were achieved, with good agreement with the Tree Flotation curve. At extreme levels of wash water addition it was possible to selectively strip poorer floating coal particles from the bubble surface, and in turn achieve beneficiation results significantly better than those defined by the Tree Flotation method.

Fluidized bed desliming in fine particle flotation – Part I

This is the first of a series of publications concerned with a novel system that transforms the hydrodynamics of flotation. This system, referred to as a Reflux Flotation Cell, consists of a vertical flotation zone, with a system of parallel inclined channels below. The system is enclosed at the top by a fluidization distributor, while a central port is used to discharge the overflow product. The inclined channels located below the vertical section enhance the segregation of the bubbles from the tailings flow, permitting separations to be conducted at bubble surface fluxes well beyond the normal flooding condition, while also permitting extreme wash water fluxes. The system hydrodynamics produces spherical bubbly-foam, with a bubble volume fraction of order 0.5, ideal for counter-current washing, and hence desliming.This paper addresses two objectives. The first concerns the fluidization boundary condition at the top of the device. We identify for the first time a conundrum that arises when Drift Flux theory and fluidization theory are used to describe the effect of wash water addition in flotation. A subtle but nevertheless significant change in the predicted bias flux arises when the system is formally fluidized, resulting in the wash water reporting with the overflow, and hence failing to provide the desired desliming. Our experimental work, however, demonstrated that the applied fluidization leads to strong positive bias, with a downwards liquid flux and in turn powerful desliming of hydrophilic particles. Indeed the system behaved as though the wash water was introduced below rather than at the upper boundary.The second, and most important objective was to assess the system hydrodynamics with respect to extreme gas and wash water fluxes using firstly a particle-free system, and secondly assess the desliming achievable using a system containing hydrophilic particles. Thus in Part I the system was free of hydrophobic particles. The enhanced bubble–liquid segregation arising from the system of inclined channels permitted very high gas fluxes, sufficient to achieve a bubble surface flux of 144m2/m2s, well beyond the theoretical flooding limit of ~100m2/m2s (Wace et al., 1968). This high bubble surface flux was especially significant given this occurred during the application of extreme bias fluxes, as high as 2.5cm/s passing downwards. Experiments involving a silica feed were used to quantify the performance of the desliming, covering extreme gas and fluidization (wash) water fluxes. Silica rejection from the product exceeded 99%.