Laureate Professor Kevin Galvin

© 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.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.

© 2015 Elsevier Ltd.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 d25 = 31.5 µm, d50 = 36.5 µm, and d75 = 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.

© 2015 Elsevier Ltd. All rights reserved.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.

© 2015 Elsevier Ltd. All rights reserved.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.

© 2016 The Institution of Chemical Engineers.Fine coal may be separated from an aqueous suspension of coal and mineral particles through the application of a pure oil. The pure oil preferentially wets and agglomerates only the coal, forming a high quality, granular product. However, the use of the pure oil also comes at a relatively high cost and this cost prohibits commercial implementation of this process. In this work a new, economic binder, was introduced. This binder consisted of a high internal phase water-in-oil emulsion which was 95. vol% water and 5. vol% organic. This type of binder was selected as it possessed the hydrophobic surface functionality of oil while the space filling functionality of the binder was primarily satisfied by the dispersed water droplets within the emulsion. The application of this emulsion in the agglomeration process led to a 10-fold reduction in the organic liquid dosage required to achieve agglomeration as compared a pure oil binder. It was also observed that the agglomeration time required when using the emulsion binder was one order of magnitude less than required when using a pure oil binder. This variation was considered to result from the five orders of magnitude difference in the viscosity of the two binders.

© 2014 American Chemical Society.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.

© 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.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.