Laureate Professor Kevin Galvin

This work assessed the potential of a single stage Reflux Classifier to upgrade itabirite iron ore to high grade at satisfactory recovery. Part I reported on the detailed findings of the experimental program and the physical transport of the particles through the system. A key purpose of the present paper was to deduce the underlying partition surface from relatively basic feed information on the Fe assays obtained as a function of the particle size. Conversion of the feed data into a simple binary description based on the density of hematite and density of silicates was used. This approach then provided a basis for applying the partition surface to a given feed to predict the separation performance. Data from the experiments were compared to values predicted from the partition surface. A least squares objective function was used to implicitly deduce the parameters governing the partition surface, notably the key exponent n and the Écarté Probable, Ep. Across 12 of the experiments, the exponent, n, governing the partition surface was found to be 0.26 ± 0.02. This result was in very good agreement with the value of 0.28 determined previously. The second key parameter, the Ep, was also determined for each of the experiments. The lowest Ep, found to be 365 kg/m3 for a low slimes viscosity, was also in good agreement with the result reported previously for a deslimed feed. This work provides confidence in the application of the partition surface to predict similar dense mineral separations, and stronger insights into the mechanisms responsible for the separation.

This study utilised a REFLUX¿ Classifier, with water as the fluidising medium, to classify feed suspensions based on particle size. This work built on an earlier approach, known as Split Fluidisation, to achieve sharp separations, focussing on the potential to control the separation size, and to deliver efficient separations at high throughputs. The partition curves were remarkably sharp, with the Imperfection, I = Ep/D50, typically less than 0.14 for separation sizes covering a range from 182 µm down to 44 µm, with virtually no ultrafine entrainment with the coarse underflow, and virtually no oversize particles misplaced in the overflow. The solids throughputs ranged from 39 t/m2/h down to 8 t/m2/h. Importantly, the study achieved a deeper understanding of how to control the separation size, with the fluidisation velocity identified as the key control variable. The underflow rate is then increased to a level sufficient to prevent the bed level from rising, while ensuring the solids throughput is below the limit applicable for a given separation size.

High grade iron ore resources are becoming depleted in Brazil, with relatively low-grade ores requiring more intensive concentration to achieve a premium product. Accordingly, a typical industrial itabirite concentration circuit includes desliming in hydrocyclones and concentration via reverse flotation, product thickening and filtration, with the slimes sent to tailings thickeners, and onto tailings storage facilities. This work examined the potential for applying a vastly simpler approach, a single stage of gravity separation using the Reflux Classifier. Here the classified feed, 90 % finer than 0.150 mm, is sent directly to the Reflux Classifier, leading immediately to a high-grade concentrate at high solids concentration. Part I describes the findings from a comprehensive series of experiments covering the effects of bed density set point, feed pulp density, throughput, fluidisation water rate and lamella channel spacing. The main program, based on an ore with 8 % goethite and 45 % hematite, achieved a feed upgrade from 37 % to 65.6+/-0.4 % iron and iron recovery of 72.9+/-0.4 % at 9 t/m2/h. A second feed with 1 % goethite and 57 % hematite was upgraded from 41 % to 66.3+/-0.4 % iron at an iron recovery of 84.7+/-0.5 % at 10 t/m2/h. (The grade of pure hematite is 69.9 % iron). It was essential to run the Reflux Classifier at a sufficient volumetric rate to achieve shear induced inertial lift of the coarse silica within the closely spaced inclined channels, to reject the gangue minerals from the high-grade product. The results demonstrate the technical feasibility of applying the Reflux Classifier to upgrade itabirite feeds.

The dynamics of a sub-millimeter air bubble rising at a bubble Reynolds number of about 100 in water in an inclined, laminar channel flow is investigated experimentally. In this configuration which is relevant in modern separation technologies for valuable particles, the bubble is undergoing a cross-stream motion, as the buoyancy force is not aligned with the undisturbed liquid flow. From measurements of bubble velocities and trajectories we estimate the drag and lift forces on the bubble at two different channel Reynolds numbers. The results are compared with their streamwise counterparts, i.e. in the configuration where the bubble rises largely along a streamline of the undisturbed liquid flow. For the lower channel Reynolds number, the cross-stream effects are only small. For the larger channel Reynolds number however, the drag coefficient is found to be notably larger than its streamwise counterpart. The lift coefficient may be either larger or smaller than its streamwise counterpart depending on the detailed local flow conditions. In particular, its value is non-zero when the bubble crosses the channel centerline where the shear rate is zero. These deviations are found to be closely connected with the bending of the bubble wake as well as the finite value of the angle formed between the bubble slip velocity and the velocity of the liquid flow.

A 2 m diameter REFLUX¿ Flotation Cell was fed at roughly 210 m3/h, equivalent to a flux of 1.9 cm/s, about twice the maximum rate used in conventional flotation cells. The coal feed slurry had 59¿64 wt% head ash and nominal size range -0.100 mm (Sauter mean size 0.004 mm). The air, wash water and underflow rates were 180 m3/h, 65 m3/h and 235 m3/h respectively, giving a positive downwards wash water bias flux of 0.2 cm/s. Product ashes of 11¿15 wt% were obtained at combustible recoveries of 58¿75 %, with results on or better than the tree curve. These initial results demonstrate that the beneficial hydrodynamics seen at laboratory scale are realised at full-scale. Also demonstrated is the use of a novel oil-agglomeration technique to obtain detailed performance versus size data, showing that high hydrophobic recoveries were being obtained at sizes down to 0.001 mm.

The kinetics of hydrophobic particle recovery by flotation declines with particle sizes below 20 µm. This work investigated a much-faster novel process, agglomeration, which utilises a high-internal-phase, water-in-oil emulsion as the binder. The effects of changes in the binder composition were studied by varying the emulsifier:oil ratio from 0.2:1 to 100:1, while maximizing the concentration of the dispersed aqueous phase within the binder. The oil consumption was assessed as an inferred film thickness surrounding the individual hydrophobic particles captured by the agglomeration. This film thickness decreased from 156 nm to as little as 4.5 nm by increasing the emulsifier concentration. This result is consistent with an increase in the emulsifier concentration causing an increase in the internal oil¿water interfacial area within the binder. Water permeation into the binder, which reduces the viscous resistance to particle capture, decreased to an asymptotic level beyond a 5 wt% emulsifier concentration.

A 2 m diameter REFLUX¿ Classifier was tested on coal feeds up to 6 mm top size. Solids feed rate was varied from 50 to 160 t/h at suspension concentrations up to 37 wt.% solids. Performance was consistent, with product ashes below 10 wt.% for +1 mm particles and reject ashes usually exceeding 70 wt.%. The density cut point could be freely varied and decreased with particle size to the power -0.114. Ep values less than 0.10 were obtained on the composite +0.5 mm material. Ep varied with particle size to the power -0.963. Hence, beneficiation performance for -4.0 + 0.50 mm material approached that of large dense medium cyclones (DMCs). Hence, REFLUX¿ Classifiers could be used to increase the capacity of existing plants with minimal capital expenditure. The primary screen aperture size could be raised to around 4 mm, which should significantly increase screen capacity. The +4.0 mm fraction could be sent to the existing DMC circuit, which with a lower fines loading would have increased capacity and medium recovery. The -4.0 mm fraction could be sent to a REFLUX¿ Classifier and if required its overflow could then be sent to a flotation circuit.

A cascading sequence of two full-scale REFLUX¿ Classifiers was tested using 1.0 mm top size coal. The gravity separation in the first stage gave a clean coal product down to about 0.03 mm. The second stage then deslimed this product with minimal coal loss. At volumetric feed rates up to roughly 100 m3/h (~ 50¿t/h solids), 30¿50¿wt.% ash feeds were reliably cleaned to give product ash values less than 10¿wt.% at overall combustible recoveries of 60¿80 %. At higher feed rates up to 240 m3/h (~ 100¿t/h solids), product ash values were in the range 8¿13¿wt.% at 43¿67% combustible recovery. One run processed 150 m3/h of a very problematic high-slimes feed with 35¿wt.% -0.038 mm slimes and 52¿wt.% head ash, giving a 14¿wt.% ash product at 56% combustible recovery. The deslime unit size partition Ep values increased exponentially with overflow rate, but remained less than 0.05 mm for overflow rates up to 100 m3/h. The cut point size increased with the deslime unit overflow flux. When the effects of cut point density and hindered settling were accounted for, there was good agreement with previous laboratory and pilot-scale test work.

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 kinetic 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.

The selectivity of a novel mineral beneficiation technology utilizing a water in oil emulsion as the binder for mineral agglomeration has been investigated. This novel technology can agglomerate fine and ultrafine hydrophobic mineral particles in less than 7 s thus providing an option for the beneficiation of low-grade ores or rare earth metal oxides, metal sulphides or platinum group metals (PGMs). However, the selectivity of the process is compromised by the interaction of emulsifier-sorbitan mono oleate (SMO), commonly known as Span® 80, with the gangue. SMO is the emulsifier used to make the emulsion binder. In this work, we show that the SMO interacts with the transition metal ions adsorbed on the gangue particles possibly via the sorbitan sugar group thus hydrophobizing and leading to the agglomeration of the gangue. This gangue-SMO interaction reduces the selectivity of the agglomeration process. Treating the ore slurry with ethylene diamine tetraacetic acid (EDTA), prior to agglomeration, prevents the agglomeration of gangue as the EDTA removes the adsorbed metal ions and brings them into the solution. The EDTA treatment is shown to improve the selectivity of the agglomeration process significantly. We also show that SMO can act as a generic collector as it can hydrophobize particles of a variety of transition and rare earth metal oxides.

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.

A high internal phase water in oil emulsion has been used as the binder in a novel agglomeration process for achieving selective and ultrafast recovery of hydrophobic particles. For the first time, the novel agglomeration was applied to the selective recovery of chalcopyrite particles from a model feed formed from equal portions of high grade chalcopyrite (27.5 wt% Cu grade) and pure silica, with Sauter mean diameter of the chalcopyrite of 11.0 µm. While very high copper recovery was achievable when sufficient binder was added, the selectivity was found to depend on the solution chemistry. The separation performance achieved in a period of 7 s of agglomeration was found to be very strong for a pH of 8.5, 9.0, and 9.5, but very poor at a pH of 10.5. Interestingly, this pH dependence was not observed in froth flotation experiments. Further, the aging of the model feed had an impact on the agglomeration but not on the flotation. These effects were attributed to the emulsifier, SMO, being released from the binder used in the agglomeration experiments.

Agglomeration of ultrafine hydrophobic particles can be performed using high-internal-phase (HIP), water-in-oil emulsions as the binder. The ultrafast particle recovery achieved using these emulsions can be attributed to the presence of thin, permeable oil films as its organic phase. The internal aqueous phase of these emulsions contains salt, which drives water absorption through these permeable oil films during agglomeration, greatly reducing the effects of the lubrication resistance to particle collision, adhesion and hence agglomeration. In this study, the water permeation was quantified by studying the growth of cylindrical rivulets placed in fresh water. The rivulet diameter increased approximately with the square root of time, suggesting a diffusion-limited process. Water transport rates increased about three fold with increasing internal salt concentration of the emulsion from 0.5 wt% to 10 wt%.

A novel hydrophobic binder consisting of tightly packed drops of aqueous salt solution, stabilized by thin films of oil, in the form of a high-internal-phase water-in-oil emulsion was used to agglomerate ultrafine hydrophobic particles in seconds to a size sufficient for their capture on a 150-µm screen. Almost complete recovery of the particles, extending from sizes of more than 100 µm to less than 500 nm, was achieved. Examination of the process revealed that the agglomeration appears to be governed primarily by the length scale of the thin oil films, on the order of 30 nm, and their ability to quickly and efficiently deliver organic liquid to the particles. Moreover, it appears that the hydrodynamic resistance that develops when a particle is driven toward an interface is reduced because of the permeability of the films. Water permeation driven by osmosis also appears to assist the transport of the particles toward the interface. (Figure Presented).

When a system of identical spheres settles under conditions of negligible surface and inertial forces an idealised form of sediment consolidation unfolds amenable to a universal description. We have described this complex process using a simple constitutive model expressed as an elementary scaling law in time, t, applied at the local particle level. The free-volume surrounding a particle consists of two volume contributions occupied by fluid, one portion fixed and the other portion variable, the latter of which declines with t¿2 . A comprehensive system of analytical equations was derived using this one idea, and associated boundary conditions, to describe all aspects of the batch settling process. An experimental system exhibiting negligible surface and inertial forces was used to validate the model and hence assess the merits of the scaling law. Excellent agreement was achieved. The precise physics responsible for this scaling law, and the applicable boundary conditions, remain unclear at this stage. Hence this work is likely to motivate further work in this area, concerned with the dynamics of random consolidation of settling spheres.

The REFLUX¿ Classifier is a gravity separation device that consists of a system of inclined channels located above a fluidized bed. Here we report for the first time the results obtained processing a minerals sands feed using narrow 6 mm channels that promote a laminar-shear separation mechanism that enhances the separation based on density. The feed had a head grade of approximately 5 wt% heavy minerals of density greater than 2800 kg/m3, with the majority in the size range from 50 to 150 µm. The overall recovery of the heavy minerals component was approximately constant at 85% for solids throughputs of 10-18.5 t/(m2 h). The unrecovered heavy mineral had an average density close to that of the gangue, hence was deemed unrecoverable by gravity separation. The recovery of the denser and more liberated zirconium mineral exceeded 95%. So this work demonstrated the potential for the heavy mineral to be upgraded by a factor of 16 or more in a single stage operation.

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¿±¿0.5¿cm/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 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 0.35. wt% 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 25. s. Very good combustible recoveries were obtained, with the +38. µm portion increasing from 92.3% to 98.5% with increasing gas flux. The partitioning of particles below 38. µm decreased with decreasing particle size until separation became governed by hydraulic entrainment, clearly evident at a particle diameter of ~1.65. µm.

This paper is concerned with the separation of cenosphere particles from fly ash. Cenospheres are hollow alumina silicate micro-shells found in fly ash. They are positively buoyant in water, thus allowing gravity-separation to be used to achieve separation from negatively buoyant fly ash particles. In this study an Inverted Reflux Classifier, a combination of parallel inclined channels and a vertical fluidized bed, was used for the first time to recover and concentrate cenospheres from a real fly ash feed obtained from a coal fired power station. The effects of different operating parameters such as the feed rate, product rate, and fluidization rate were investigated. The device was fed at a solids flux of about 2600 kg/(m2 h). A product grade of 76% was achieved from a feed with a grade of only 0.51%, corresponding to an upgrade of 149. Here, the recovery of the cenospheres was 42%. By increasing the overflow product rate, a significantly higher recovery of 64% was achieved, but at a reduced upgrade of 33. In both cases most of the losses were attributed to the relatively fine cenosphere particles being entrained to the underflow.

Abstract The enhanced separation of valuable positively buoyant cenosphere particles from negatively buoyant fly ash particles using an Inverted Reflux Classifier (IRC) was examined. The effect of the suspension density on the recovery and concentration was examined in the IRC by operating at different feed pulp densities ranging from 10 wt% to 46 wt%. Using a sufficiently high fly ash concentration, it was hypothesised that a powerful bulk streaming phenomenon develops (Batchelor and Van Rensburg, 1986) within the inclined channels, driving the segregation between the positively and negatively buoyant species. With the feed flow rate, fluidization rate, and flow split to overflow and underflow fixed, the recovery of the cenospheres increased from 61.7% (at 10.1% solids) through to an optimum recovery of 89.9% (at 38.1% solids), before declining rapidly to a recovery of 60.2% (at 46.4% solids). The performance at the optimum of 38.1% pulp density was remarkable, with 3.1 t/(m² h) solids throughput, a single-stage cenosphere recovery of 89.9% and upgrade of 58.6, and throughput advantage over a conventional fluidized bed of 54. Detailed analysis indicated that the inclined channels produced an underlying throughput advantage of 18, with a further factor of 3 attributed to the bulk streaming phenomenon. The separations were also assessed in terms of the partitioning of the cenospheres between the overflow and underflow exit streams, with the sharpest size classification evident at the optimum feed pulp density, with the d<inf>25</inf> = 31.5 µm, d<inf>50</inf> = 36.5 µm, and d<inf>75</inf> = 50.0 µm. The separation was then investigated using different feed flow rates, providing the basis needed for ensuring optimum performance in future pilot scale investigation of this novel technology.

Cenospheres are hollow spherical particles formed as part of the fly ash waste of coal-fired power stations. In a previous paper Kiani et al. (2015) investigated the recovery and the concentration of these particles using an Inverted Reflux Classifier (IRC) at a laboratory scale, of cross-section 0.100 m × 0.086 m, achieving a throughput advantage over a conventional fluidized bed by a factor of 54. The present paper investigated the potential to achieve scale-up, utilizing a pilot scale device with cross-section 0.3 m × 0.3 m. The product grade and recovery were examined as a function of the solids yield by varying the product volumetric rate relative to the feed volumetric rate. The performance data were compared directly with those obtained at the smaller laboratory scale. Agreement was excellent. The performance was also examined as a function of the feed slurry flux, with good agreement again evident at the laboratory and pilot scales. Overall, the separation performance was excellent, with a cenosphere recovery of about 80% achievable at a high upgrade of 19 while a recovery of 75% was achieved at an upgrade of 38. Here the feed solids flux was 4.2 t/(m² h). It is noted that much higher upgrade was achieved at a recovery of about 80% in the former study by operating at a lower solids feed flux. This paper provides the necessary basis for proceeding with a full scale implementation of this technology.

Traditional sink-float methods for measuring the density distribution of particulate samples rely on expensive and toxic heavy liquids. An alternative method has been developed which uses aqueous glycerol solutions in a laboratory-scale Reflux Classifier run in semi-batch mode. The high viscosity of these solutions promotes laminar high-shear flow in the channels which suppresses the effect of particle size on separation performance. Thus this technique was able to accurately measure the yield-ash curve of coal samples, and from this their density distribution could be inferred. Applying this approach to feed, product and reject samples enabled calculation of the density partition separation performance. Samples were collected from two case studies: a laboratory-scale continuous Reflux Classifier and a single spiral start from a full-scale coal handling and preparation plant. In both cases the partition curve measured by the new method was within experimental uncertainty of the partition curve measured by the standard sink-float method.

A high internal phase (HIP) water-in-oil emulsion was used as the binder in the selective agglomeration of fine coal from an aqueous suspension of coal and mineral particles. Traditionally, this agglomeration is achieved by a pure oil, hydrophobic, binder. However, the high cost associated with using pure oil makes the process economically unfeasible. Therefore, the emulsion binder introduced in this work was motivated by the economic need to reduce the amount of organic liquid required in the process. The effect of the agitation time during the agglomeration process and the composition of the emulsion on its performance as a binder were investigated. The best result obtained was for a HIP emulsion made from 3 wt % aqueous NaCl and diesel oil with sorbitan monooleate as the emulsifier. This emulsion had a dispersed phase volume fraction of 0.94 and achieved a 7.5-fold reduction in the amount of organic liquid required to achieve agglomeration.

A novel flotation system was used to process fine coal feeds supplied from coal preparation plants. The system consisted of an inverted fluidized bed arranged above a system of inclined channels. High fluidization (wash water) fluxes were imposed through a distributor enclosing the free-surface, producing strong positive bias of up to 2.4 cm/s, ideal for desliming. High gas fluxes of up to 2.6 cm/s, in excess of the flooding condition, were also imposed. The presence of the inclined channels prevented the entrainment of gas bubbles into the tailings stream. This paper, which is the third in a series, examines, for the first time, the hydrodynamic performance of this system on two actual plant feeds, each known to be difficult to wash. The first feed was a poorly liberated coal with particle size <260 µm and 69% feed ash. The second was a well liberated coal with nominal size <125 µm and 83% less than 38 µm. The product ash was shown to decrease significantly with an increasing fluidization flux to gas flux ratio. The single stage flotation system demonstrated a performance capable of matching the Tree Flotation Curve with some cases in fact surpassing this result. © 2014 Elsevier Ltd. All rights reserved.

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%. © 2013 Elsevier Ltd.

It is common practice in the coal industry to use heavy organic liquids to fractionate coal samples on the basis of density. However, concerns over worker health and the influence of these liquids on coal carbonisation properties are prompting the search for alternative water-based methods. Previous work has already shown that 0.038-0.25 mm samples can be very effectively separated using pure water in a Reflux Classifier with narrow 1.7 mm channels. Narrow channels give laminar flow with high shear rates which promotes density-based separation. Processing coarser particles requires wider channels and the laminar flow condition is lost, reducing performance. This work tested whether using viscous glycerol solutions to restore the laminar flow condition could improve the separation performance of the laboratory Reflux Classifier for larger particles. For 0.25-2.0 mm coal particles, using 50 wt.% glycerol solution in 6 mm channels, the Reflux Classifier was able to match the float-sink yield-ash curve across the entire yield range. For 2.0-16 mm coal, using 70 wt.% glycerol solution in 24 mm channels, the Reflux Classifier gave results which were at worst only 1.0 wt.% ash units off the float-sink curve. Hence the Reflux Classifier can potentially replace the float-sink method for measuring the washability of small bore core samples and producing clean coal composites. © 2014 Elsevier Ltd. All rights reserved.

Cenospheres are hollow, low-density particles found in power station fly ash. They have many commercially-useful properties which make them a valuable by-product. However, recovering cenospheres from fly ash is difficult due to their low concentration and fine size. Experiments were performed to test the novel approach of using an Inverted Reflux Classifier. In this configuration, the particles are fluidised by adding wash water from above which helps to wash any entrained dense material from the overhead product. Inclined channels are mounted at the base to minimise the loss of buoyant cenospheres in the waste underflow stream. Experiments were performed at both laboratory scale (80 mm × 100 mm cross-section) and pilot scale (300 mm × 300 mm cross section) using mixtures of cenospheres and silica, all nominally less than 100 µm in size. In batch tests, the bed expansion behaviour of the positively-buoyant cenospheres in the Inverted Reflux Classifier was found to be analogous to the behaviour of negatively-buoyant particles in the standard configuration. Continuous steady-state experiments were performed using feeds with suspension solids concentration varying from 0.3 to 9.5 wt.% solids and a buoyant cenosphere grade of 0.5 to 65 wt.%, with a range of fluidisation wash water rates, and degree of volume reduction (ratio of volumetric feed to product rate). Both units delivered high recoveries and product grades. An increase in volume reduction (decreasing overflow rate for a given feed rate), caused a drop in recovery and an improvement in grade. The throughput advantage compared to a conventional teetered (fluidised) bed separator was over 30 in some cases. Both laboratory and pilot-scale units displayed similar behaviour and the results were also consistent with existing correlations for negatively-buoyant particles in the standard Reflux Classifier. Hence this technology has clear potential for recovering and concentrating cenospheres from fly ash. © 2014 Elsevier B.V.

This study was concerned with the interaction between a gaseous dispersion of fine particles travelling in the horizontal direction and discrete drops of water falling vertically through the dispersion. A simple analytical model of the particle-drop collision was developed to describe the particle recovery by the drops as a function of the water flux, covering two extremes of relative velocity between the particles and drops. The Discrete Element Method was used to validate the analytical model. Further validation of the model and insights were obtained through experimental studies. The physical process of wetting was observed to be important in influencing the tendency of particles to become engulfed by the drops of water, or to either adhere to the drops or by-pass the drops altogether. Hydrophilic particles were readily engulfed while hydrophobic particles, at best, adhered to the surface of the drop, or failed to attach. Moreover, the recovery of the hydrophilic silica particles was significantly higher than the recovery of hydrophobic coal particles, with the selectivity ratio approximately 1.5. Spherical ballotini particles were the most sensitive, with a notable increase in recovery when cleaned, and evidence of increased recovery with increasing particle size. The recovery of irregular shaped silica flour particles, however, was largely independent of the particle size. A similar result was observed for irregular coal particles, though the recoveries were all lower than relatively more hydrophilic ballotini or silica flour. Crown Copyright © 2014.

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, d, of 1.5mm. These gas and wash water fluxes corresponded to a bubble surface flux of 188m/ms 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. © 2013 Elsevier Ltd.

Two pilot-scale Reflux Classifiers (600 mm × 600 mm cross-section) arranged in a cascading sequence were used to beneficiate fine -2 mm coal. The first Reflux Classifier performed a density separation that produced a coal product contaminated with fine high-ash slimes. This was then washed in the second Reflux Classifier to remove the fine clays and mineral matter. This combination reliably produced a clean coal product and allowed gravity separation performance to be extended from the usual eight-fold limit of upper to lower size to a much broader size range. Performance was similar to previous laboratory-scale results units with cross-sectional areas of only 100 mm × 80 mm each. Hence, full-scale desliming units can be confidently designed based on laboratory trials. The cut size varied linearly from 0.04 to 0.24 mm with increases in the overflow channel velocity from 25 to 55 m3/(m 2 h). The Ep values increased from 0.02 to 0.07 mm (Whitten factor a from 2 to 6) over the same range. The linear dependence of the cut size on velocity in the Reflux Classifier was consistent with the theory and with the significant throughput advantage of the technology. © 2014 Copyright Taylor & Francis Group, LLC.

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 500 µm 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 176 cm/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 15 cm/s, and high gas fluxes, up to 5.5 cm/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 600 s-1 were achieved, while counter-current downward liquid fluxes reached 14.4 cm/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.

An empirical equation for calculating the slip velocity of a species in any homogeneous suspension is proposed. The Richardson and Zaki (1954, Trans. Inst. Chem. Engng, 32, 35-53) and Lockett and Al-Habbooby (1973, Trans. Inst. Chem. Engng 51, 281-292; 1974, Powder Technol., 10, 67-71) equations are special cases of the proposed equation, and arise when all species are of the same density. Therefore, the main value of the proposed equation is in describing the slip velocities of particles in suspensions containing species of different density. In this short note results from one experimental system are presented, and shown to be consistent with the model. The model is also consistent with the explanation used by Moritomi et al. (1982) to account for phase inversion in fluidized beds. The model is appealing in its simplicity, and should find favour in the design and control of process equipment. The new model predicts the strong segregation effects observed in suspensions containing particles of different density, and hence represents an immediate improvement on the Lockett and Al-Habbooby equation. Its application is expected to cover all homogeneous suspensions, in which the particles are all more dense than the suspension. At this stage its validation has been limited to low concentrations of dense particles settling through a fluidized bed of low density particles as occurs in a Teetered Bed Separator, and to phase inversion conditions in fluidized beds. It is hoped that this note might lead to a much more extensive validation of the model by workers using vastly different particle species.

Transport of dry solid particles to a liquid is relevant to a number of emerging applications, including 'liquid marbles'. We report experiments where the transport of dry particles to a pendent water droplet is driven by an external electric field. Both hydrophilic and hydrophobic materials (silica, PMMA) were studied. For silica particles (hydrophilic, poorly conductive), a critical applied voltage initiated transfer, in the form of a rapid 'avalanche' of a large number of particles. The particle-loaded drop then detached, producing a metastable spherical agglomerate. Pure PMMA particles did not display this 'avalanche' behaviour, and when added to silica particles, appeared to cause aggregation and change the nature of the transfer mechanism. This paper is largely devoted to the avalanche process, in which deformation of the drop and radial compaction of the particle bed due to the electric field are thought to have played a central role. Since no direct contact is required between the bed and the drop, we hope to produce liquid marble-type aggregates with layered structures incorporating hydrophilic particles, which has not previously been possible.

Density-based separators have been used as a preferred method of concentrating minerals and upgrading coal for several centuries. However, significant technological advances continue to be developed and introduced into commercial practice. The main focus of development has been the improvement in separation efficiency over a larger particle size range and the ability to achieve effective density-based separations for particles as small as 10 microns. The success of enhanced gravity separators in the recovery of ultrafine particles has resulted in their implementation into grinding circuits in an effort to prevent overgrinding and reduce circulating loads while also reducing the consumption of the chemicals used in conventional recovery processes and circuits. A review of recent innovations in density-based separations is provided in this publication along with the impact on the strategies and circuits used to process a wide range of minerals and coal.

Chemical engineering at the University of Newcastle has introduced a "Systems Thinking" approach in response to the changing needs of today's young engineers, particularly in relation to sustainable development and interaction with the wider community. The basic concepts are reinforced to the students in the form of case studies. The activities cover a broad range of traditional chemical engineering principles, including fluid mechanics, heat and mass transfer, process flowsheeting, and design. The case studies have the additional dimensions of life cycle modelling, environmental impact assessment, and direct interaction with the broader community. In this paper, two examples, involving Building Design and On-Site Water Management, are presented, including a brief description, desired learning outcomes, results and general observations. Generally, it was found that the case studies provided an excellent framework for establishing a systems approach to arriving at solutions, and acted as a focus for quantitative analysis using the various tools taught during the course. Most importantly, the material presented assisted students to understand the practices which contribute to the transition to a sustainable society.