Flotation of coarse coal particles in the Reflux™ Flotation Cell

Flotation of coarse coal particles in the Reflux™ Flotation Cell

Minerals Engineering 149 (2020) 106224 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mine...

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Minerals Engineering 149 (2020) 106224

Contents lists available at ScienceDirect

Minerals Engineering journal homepage: www.elsevier.com/locate/mineng

Flotation of coarse coal particles in the Reflux™ Flotation Cell J.L. Sutherland , J.E. Dickinson, K.P. Galvin ⁎

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Centre for Advanced Particle Processing and Transport, Newcastle Institute for Energy and Resources, University of Newcastle, Callaghan, NSW 2308 Australia

ARTICLE INFO

ABSTRACT

Keywords: Coal Flotation Reflux Flotation Cell Coarse particle flotation

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.

1. Introduction In coal and mineral processing, froth flotation is usually applied to the so-called ultrafine particles, predominantly below 0.35 mm for coal and 0.1 mm for minerals. The performance of conventional froth flotation technology is known to vary significantly with particle size and mineralogy, with poorer hydrodynamic kinetics occurring at relatively fine sizes below 0.01 mm, and declining recovery at coarser sizes due to a combination of lower surface liberation and stronger bubble-particle detachment forces (Sutherland, 1948; Jowett, 1980; Sutherland, 1989; Jameson, 2012). At intermediate sizes, typically between 0.02 and 0.35 mm for coal, and 0.01 and 0.1 mm for minerals (Gaudin et al, 1931; Jowett, 1980), the technology is highly effective kinetically, with almost complete recovery possible at high grade. However, there is increasing interest to the recovery of coarser coal, and coarser, more poorly surface liberated, mineral particles (Jameson, 2010; Kohmuench et al., 2018; Jameson and Emer, 2019). In coal preparation, this application offers the potential for simpler and more robust circuits. In minerals processing, the application offers the ⁎

potential for rejecting unwanted gangue particles at coarser sizes, thus reducing the level of grinding required, and in turn the quantity of fine tailings and hence water consumption. The predominant factor resulting in an optimum size range in flotation is particle inertia. Coarser particles possess more inertia and thus when caught in turbulent eddies detach from bubbles predominantly due to centrifugal forces. This was quantified by Schulze (1982) in terms of the modified Bond number which describes the ratio of the detachment force to the force of attachment. Thus, to increase the maximum floatable particle size, it is essential to decrease the level of turbulence in the cell, potentially through minimising the energy dissipation rate (Jameson, 2010). Eriez has developed the Hydrofloat, a teetered bed separator, which uses a combination of air bubbles and upwards fluidization to assist the transport of coarse hydrophobic particles to the cell weir (Kohmuench et al., 2018). The system hydrodynamics are best described as quiescent, thus the tendency for bubble-particle detachment is minimized. Bubble-particle aggregates of low buoyancy are in turn recovered in the overflow. This cell is operated without a froth layer (Awatey et al,

Corresponding author. E-mail address: [email protected] (J.L. Sutherland).

https://doi.org/10.1016/j.mineng.2020.106224 Received 28 June 2019; Received in revised form 17 December 2019; Accepted 17 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.

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direction. Thus, the difference between the RFC and the other two systems could not be more significant. With downwards fluidization, the final product can be cleaned across the full-size range. However, it is necessary to achieve increased particle buoyancy to overcome the downwards convective flow that arises for a positive bias flux. It should be noted that the emphasis in the present study was on recovery as opposed to exploiting the full potential of cleaning, which has been covered in earlier studies involving coal particles with a top-size of 0.26 mm (Galvin and Dickinson, 2014; Galvin et al., 2014). Coarse coal flotation could simplify processing plant design. For example, the underflow of a 2 mm classifying screen on a typical coal processing plant will generally be split by a classifying cyclone into two separate streams (Luttrell et al., 2003). The coarser particles would be sent for gravity separation, with the finer particles, generally −0.35 mm, sent to the flotation circuit. With the ability to float particles up to 2 mm in diameter, the gravity separation and classifying cyclone could be replaced by a single flotation circuit. With this impetus, the objective of this study is to investigate the flotation of coal particles up to 2 mm in diameter, using the RFC. In this study, experiments involving the discrete addition of coal tracer particles provided a precise assessment for identifying the optimum hydrodynamic conditions, defined by the gas and the feed flux, for coarse particle recovery using the RFC. Considering that coal particles are naturally hydrophobic (Wills and Finch, 2016), and of high grade compared to typical mineral particles, (low ash) coals can provide an ideal model system to circumvent the effects of surface liberation and focus the interpretation of the results onto the effects of particle. The tracer particle investigation was then followed by more realistic experiments conducted on a continuous steady state basis at 5 wt% solids using industrial feed. The optimum conditions established from the tracer particle work were applied to this second part of the study. The continuous steady state experiments were benchmarked against the Tree Flotation Curve for particles −0.125 mm in size, and the Float/Sink method for particles +0.125 mm, based on separation at the relative density of 1.6.

Fig. 1. Schematic of experimental RFC set-up for tracer particle experiments.

2013); indeed significant levels of ultrafine particles are conveyed to the overflow. Therefore, it is necessary to classify the feed or the product to direct the finer particles to a more conventional cell. More recently, Jameson has also reported on a novel fluidized bed approach, called the Nova Cell (Jameson, 2010, 2017; Jameson and Emer, 2019). Again, the hydrodynamics are quiescent in nature. This device retains a froth zone so establishes cleaning over a wider size range. The air is introduced in the fluidization water upstream of the bed. In contrast to the Eriez Hydrofloat, the Nova Cell employs a froth layer at the top of the cell. Two product streams are generated; the finer product emerges from the froth layer and coarser product from within a collection zone below the froth layer. This second product is screened via an external recirculation circuit. In coal flotation, Jameson (2017) has reported recoveries approaching 95% for coal particles up to 2 mm. This paper is focussed on the recovery of coarse particles in yet another device, the Reflux Flotation Cell (RFC). This device, shown in Fig. 1, offers distinct advantages over conventional flotation devices in multiple ways due to its novel hydrodynamics. The RFC uses a system of inclined channels below the fluidization zone to greatly increase the segregation rate of the bubbles via the Boycott Effect (Boycott, 1920), preventing them from being entrained in the tailings, even at high gas, feed and fluidization wash water fluxes. Bubble volume fractions of the order 0.5 can be maintained through the entirety of the system (Dickinson and Galvin, 2014), in contrast to pulp zone volume fractions of only 0.1 to 0.2 in conventional flotation systems. Moreover, the top of the RFC is enclosed by a plenum chamber to deliver a uniform supply of downwards fluidization water for counter-current washing through a highly permeable, bed of rising bubbles that is less susceptible to the effects of bubble coalescence than pneumatic froth. Overall, the RFC exhibits quiescent hydrodynamics within the cell, enhanced by the downwards water fluidization, high bubble concentration, and the parallel inclined channels preventing large eddies. Given these benefits, it is appropriate to consider how coarse particles respond to the system’s hydrodynamics to determine whether the benefits can be extended in this way. Bubble-particle attachment arises within the high shear field in the downcomer. Thus, discharge from the downcomer remains a potential source of hydrodynamic disturbance. It is emphasized that in both the Eriez and Nova Cells the fluidization is introduced from below the device, while in the RFC, the fluidization is introduced from above the device, hence in the reverse

2. Experimental Two experimental methods were used to investigate the flotation of hydrophobic particles below 2 mm in diameter using the RFC under specific hydrodynamic conditions. The first series of experiments involved the flotation of coarse coal tracer particles of known density, added discretely or in number to the RFC in the absence of any other particles. A steady state was established in the system using fixed feed, tailings, fluidization water, and gas fluxes, with 40 ppm MIBC (methyl isobutyl carbinol) added to the feed and fluidization water to ensure a frother concentration well beyond the critical coalescence concentration, previously reported to be 11.2 ppm (Laskowski, 2003). Typically, around 70 tracer particles were individually added per experiment. The particles were collected via the overflow and underflow streams using fine sieves, as shown in Fig. 1, and the partition number reporting to the product overflow determined. To attribute the partitioning of the particles to the system’s hydrodynamics rather than to the specific particle properties, particles recovered via the overflow for one of the experiments using −0.710 +0.500 mm particles were reprocessed, and confirmed a similar recovery, or partition number, was obtained. The second form of experiment involved the continuous steady state flotation of coal slurry sourced directly from hydrocyclone feed, sampled from a coal preparation plant in the Hunter Valley, Australia. The slurry was screened to a top-size of 2 mm prior to conditioning. All coal samples were stored in water to minimize oxidation. The recovery of the intermediate to coarse, −2.0 +0.35 mm, coal particles was the primary goal of the experiments, however the flotation of the finer size fraction involving the −0.35 mm coal particles was also examined, motivated by the advantage of recovering the coal across the full-size 2

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range, in a single separation. Here, the analysis of the process separation focussed on the combustible recovery, which depended on the assays describing the so-called ash %.

downwards flow of liquid containing the feed particles. Here, the mean of the Sauter mean bubble diameters was calculated as 609 ± 49 µm from 3240 bubbles measured across different tracer particle experiments, and 693 ± 137 μm from 1080 bubbles for experiments involving the ultrafine particles. Fig. 2(b) shows the rectilinear downcomer (RD), which was used in the latter tracer particle experiments. A single rectangular channel, 0.034 m wide with a 0.0045 m perpendicular gap, had a porous surface 0.03 m wide and 0.15 m long located on one side of the channel. Air was supplied to the plenum behind the sparger via a small flexible hose, which was itself attached to a larger annular plenum around the outside of the upper section of the downcomer. The third downcomer, shown in Fig. 2(c), was located above the RFC, hence was more accessible. This tubular downcomer (TD), 0.254 m in internal diameter, was used for the industrial feed. It should be noted that each downcomer extended approximately 0.7 m into the vertical section of the RFC.

2.1. The Reflux Flotation Cell experimental apparatus A schematic of the RFC is given in Fig. 1. Feed entered the system via a downcomer, and exited into the vertical section of dimension 1.0 × 0.10 × 0.086 m3. Air bubbles were sheared from a porous sparger inside the downcomer. Below the vertical section were parallel inclined channels with an inclination of 70° to the horizontal. The channels were formed using 1.0 m long stainless steel plates, with a 0.009 m perpendicular channel spacing. The tailings were withdrawn from below the inclined channels. Fluidization wash water entered the system through a plenum chamber enclosing the top of the RFC. By controlling the tailings rate, it was possible to set the direction of the bias flux in the top portion of the RFC. A net flux of water in the downwards direction was defined as a positive bias, conversely a net flux of water in the upwards direction was defined as a negative bias. An annulus around the centred downcomer formed the product overflow outlet. The overall design of the RFC remained unchanged throughout the study, however three different downcomer designs were used during this study in an effort to generate flows containing more stable bubble formation exiting the downcomer over wider air to feed (pulp) ratios in the downcomer, using varying feed and air fluxes. Towards higher ratios, the sporadic formation of oversized bubbles occurred, and experimental conditions in which their formation dominated the flow were avoided. It is noted that several experiments, not reported here, demonstrated little difference in the separation performance between the downcomers. The annular downcomer (AD), seen in Fig. 2(a), was used for many of the tracer particle experiments. The feed liquid flowed through an annulus between an outer surface and 0.30 m long inner porous surface, with a 0.0045 m gap. Air was forced down through the enclosed inner tube. The air passed through the porous surface, sheared by the

2.2. Tracer particle experiments Fig. 1 shows the arrangement for adding individual tracer particles, with the frother-water solution recycled via a 0.5 m3 feed tank using peristaltic pumps. Approximately 70 particles were added to the system individually through an interlock consisting of two 0.025 m ball valves in series. For experiments involving the sequential addition of grouped tracer particles, the feed tank was replaced with a 0.15 m3 baffled mixing tank. Instead of introducing the particles through the feed interlock, all tracer particles were introduced into a 0.15 m3 baffled mixing tank, as per Fig. 1. Ten minutes was allowed for the particles to enter the RFC and partition to the product or tailings. In some experiments, multiple size fractions were processed at once. Hence, each run generally involved ~200 particles for a single size fraction, to ~1000 tracer particles for multiple size fractions. The higher number of particles involved in this experiment is captured by narrower confidence intervals. A third system configuration involved the circulation of ultrafine hydrophobic coal particles, with the addition of tracer particles. Very fine hydrophobic particles tend to improve bubble stability by increasing the interfacial elasticity, making the film more resilient when subjected to non-uniform transient thinning (Horozov, 2008; Hunter et al., 2008; Ireland, 2009). This should in turn lead to higher coarse particle yield, as fewer particles are lost due to bubble coalescence. Moreover, the fine particles might prove effective in enhancing the adhesion of the coarser particles at the bubble surface, effectively locking the particles in place (Rahman et al., 2012). Therefore, the effect of ultrafine hydrophobic particles on coarse tracer particle yield was also examined, with the tracer particles and ultrafine particles simultaneously added to the feed tank, as illustrated in Fig. 1. In these experiments, the ultrafine coal particles, wet screened at 63 μm, were circulated with the feed at a solids concentration of 0.44 wt%. A Malvern Mastersizer 3000 was used to measure the size distribution of the dilute ultrafine slurry, giving a P80 of 45.3 µm. The fine coal particles were sourced from RFC rougher-cleaner product and had a low ash content of approximately 7.7%, as reported by Jiang et al., (2016). This coal was further conditioned with diesel in these experiments, and hence, was regarded as being highly hydrophobic. While high recovery of the ultrafine particles was not the aim of this experiment in itself, partitioning to the product of the ultrafine particles was favoured due to their hydrophobic nature. The tracer particle experiments involved hydrophobic coal particles of density 1250–1300 kg/m3, as determined by the Float/Sink method. Coal in this density range is generally well liberated with very low mineral matter content (Lin et al., 2000; O’Brien et al., 2011), hence high combustible content. As a result, it is reasonable to assume that the particles in this density fraction are also homogeneous in their hydrophobicity, and the most hydrophobic of the coal sampled (Holuszko and Mastalerz, 2015). The tracer particles were wet-sieved into size

Fig. 2. Illustration of the (a) annular (AD), (b) rectilinear (RD) and (c) tubular (TD) downcomers from the side, with projections (not to scale) of each downcomer exit respectively in (d), (e) and (f). 3

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fractions using sieve sizes of 2.00, 1.40, 1.00, 0.710, 0.500 and 0.355 mm. All coal tracer particles were stored in water to prevent oxidation. The most reliable method of conditioning individual tracer particles was by fully submerging the particles in the collector, diesel oil. After 5 min of immersion, the particles were then submerged in water prior to their individual addition to the system as a tracer particle. This approach ensured consistent surface properties and overcame the difficulty of trying to apply a fixed “kg/t” addition to each particle. A major innovation of the RFC is the de-coupling of the overflow liquid flux and the gas flux, allowing the tailings rate to be set as required. This condition arises due to the presence of the inclined channels, and the freedom of the system to self-establish a final interface within this inclined channel zone. The airflow to the sparger was set to the required level, and the feed and wash water pumps switched on to fill the system. As soon as the bubbly liquid began to exit the overflow, the tailings pump was switched on. The desired overflow rate was thus obtained by adjusting the underflow tailings rate. The system was then allowed to reach steady state. Tracer particles, saturated with diesel collector, were added individually to the system via the pair of interlocking valves. An accurate account of every particle added to the interlock was recorded for number balancing.

Fig. 3. Tracer particle partition to product as a function of gas flux for three particle size fractions, using the annular downcomer (feed flux = 3.3 cm/s, bias flux = −0.45 cm/s).

3.1. Tracer particle experiments 3.1.1. Discrete addition of individual tracer particles Fig. 3 shows the effect of the gas flux on the partition number to overflow for the individually added tracer particles at a feed flux, jf, of 3.3 cm/s, and a fluidization flux of 0.14 cm/s. At this feed flux, which is high relative to fluxes in conventional flotation systems, high gas fluxes over a wide-ranging air to feed (pulp) ratio in the downcomer of 0.15–0.91 could readily be examined without concern of bubble instability and the formation of oversized bubbles in the liquid-bubbly mixture. The data show that the yield declined as the gas flux increased. Yield also decreased with the doubling in particle size from −0.710 +0.500 mm to −1.40 +1.00 mm, although over the size fractions −0.710 +0.500 mm and −0.500 +0.355 mm comparable yields are evident. In the limit of a low gas flux, the partition to overflow is relatively high over a broad particle size range. The partition to overflow represents a binary partitioning or the probability, p, of a tracer particle reporting to the overflow. By assuming a Bernoulli distribution, the statistical variance was given by p (1-p), and a 95% confidence interval determined for each data point, based on the total population of tracer particles added. Around 70 particles were individually added per experiment, thus resulting in the moderately wide error bars in Fig. 3. Nevertheless, the trend of decreasing partition number with increasing gas flux was clearly evident for the two particle size fractions above 0.500 mm.

2.3. Continuous steady state flotation experiments The findings of the tracer particle experiments were applied to the continuous steady state flotation experiments, using coal slurry at 5 wt % solids. In contrast to the tracer particle experiments, no recirculation stream was used in the continuous experiments. Independent feed and wash water tanks were employed, and the overflow and underflow streams sampled separately. Before a run commenced, the feed was subdivided into multiple 20 L buckets ensuring consistent composition, and then conditioned with diesel collector to 2 kg/t using an agitator. A baffled mixing tank, functioning as the feed tank, was filled with the coal slurry. While the tank was mixing, 40 ppm of MIBC was added. As additional buckets of feed were added to the tank throughout a run, MIBC was also added to each of the buckets. The feed buckets were added at regular intervals to preserve the hydrostatic head and the mixing in the tank. Once steady state was reached, the overflow product, underflow tailings and feed streams were sampled and weighed. These samples were subsequently wet split over a 0.125 mm screen, with the passing fraction wet split a second time at 0.038 mm, generating two fractions: −0.125 +0.038 mm and −0.038 mm. The +0.125 mm particles were dried and then dry sieved. Additional feed samples were collected to construct “ideal” separation curves. The Tree Flotation method was performed on the −0.125 mm fraction, to achieve in principle the best possible series of separations for the finer sized particles. The RFC combustible recovery and product ash were later compared to the Tree Flotation curve over the same particle size range. The +0.125 mm fractions underwent a Float/Sink separation at a relative density of 1.6, with the float and sink portions then analysed for ash content, providing an ideal combustible recovery and product ash for each particle size fraction.

3.1.2. Continuous addition of tracer particles in groups Clearly, the discrete addition of individual particles in isolation is not very representative of continuous flotation. Therefore, experiments were undertaken using the continuous addition of coarse particles. For the sake of comparability, the operating conditions applied were deliberately similar to the earlier tracer particle experiments. Hence, the feed flux, fluidization flux and overflow flux were all fixed at approximately 3.2 cm/s, 0.14 cm/s and 0.55 cm/s, respectively, giving a liquid split ratio of the overflow to feed flux of 0.17. The gas flux was 3.0 cm/s. Fig. 4a shows the partition to product of tracer particles added as distinct size fractions (open circles) and added as a mixture of four size fractions (closed circles). The open circles each represent individual runs conducted over ten minutes. The closed circles all represent a single run, involving ~1000 particles, covering four size fractions, added to the system over ten minutes. It is clear that adding all tracer particle size fractions simultaneously to the system was detrimental to the partition numbers. It is unclear why the coarse particle recovery should be poorer when different size particles are present at the bubble surface or between neighbouring bubbles. It is noted that the bubble

3. Results This section divides into two parts involving particles up to 2 mm in size. The first is concerned with the flotation of the tracer particles, and the second with the continuous flotation of coal slurry. Combustible recovery was primarily reported for the continuous flotation experiments, while the partition number, which is analogous to the yield, was reported for the tracer particle experiments. 4

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Fig. 4c. Grouped tracer particle partitioning to overflow with ultrafine particles circulated in the feed, for a gas fluxes of 1 and 3 cm/s, feed flux = 3.2 cm/s, and bias flux = −0.4 cm/s, using the annular downcomer. Curves are a guide for the eye only.

Fig. 4a. Tracer particle partitioning to overflow when introduced as individual size fractions across five experiments (open circles), or grouped together with all four sizes added in a single run (closed circles). The gas flux = 3 cm/s using the annular downcomer, feed flux = 3.2 cm/s, and bias flux = −0.4 cm/s. Curves are a guide for the eye only.

in Fig. 3 for a gas flux of 1 cm/s. It should be noted that experiments involving higher gas fluxes were susceptible to the occasional generation of large air voids within the annular downcomer. This periodic disturbance had the potential to be damaging to the recovery of the coarse particles. For most of the experiments involving the annular downcomer, however, no air voids were observed in the system for gas fluxes, jg, of 0.5 and 1.0 cm/s. Small air voids appeared infrequently at jg = 2.0 cm/s, while slightly larger voids appeared at a similar frequency at jg = 3.0 cm/s. In general, the air voids became larger and more frequent as the gas flux was increased. An alternative rectilinear downcomer (RD) was therefore introduced to eliminate the formation of the air voids.

size was nominally 0.5 mm in diameter so the particles were generally large by comparison. Both experiments involved an exit gas volume fraction of 0.86, so coalescence at the exit point may have contributed to losses. This effect, which is examined later, may be more significant in the presence of coarse particles of vastly different size. A comparison of the tracer particle yield with and without ultrafine particles is shown in Fig. 4b, both at a high gas flux of 3 cm/s. It is clear that the presence of ultrafine particles greatly increased the yield of the smallest tracer particles from 26.7% to 93.7%, however, the increase in yield diminished with increasing particle size. These results are in agreement with literature, suggesting that in the presence of the ultrafine particles, coarse particle recovery is improved (Rahman et al., 2012). All liquid fluxes in and out of the cell, including the liquid flux to the overflow, were equal in the runs shown in Figs. 4a and 4b. Fig. 4c shows the significant effect of lowering the gas flux from 3 cm/s to 1 cm/s, in the presence of the ultrafines. The partitioning to product increased significantly, even beyond that achieved using narrow particle size ranges as shown in Fig. 4a. For the 1.18 mm particles (−1.40 +1.00 mm size fraction), the partition was very similar to that shown

3.1.3. Examining the effect of feed throughput using tracer particles The effect of the feed flux on tracer particle yield was investigated over four fractional sizes, ranging from 0.355 mm to 2.0 mm, using the rectilinear (RD) and annular (AD) downcomers. A total of 36 experiments was conducted, using different volumetric feed, fluidization and gas fluxes, involving individual tracer particles of a defined size fraction, in the absence of ultrafine particles. Fig. 5 shows the particle partitioning as a function of the gas flux. Here it is evident that the yield decreased as the gas flux increased, and that the rate of decline was higher for the coarser particles. Fig. 6(a) and (b) shows the particle partitioning as a function of the feed flux. Here, for a given gas flux and particle size, similar yields were achieved using very different feed fluxes. For instance, for particle size fraction −0.710 +0.500 mm, and gas flux of 1.0 cm/s, yields of between 89% and 99% were obtained for feeds of 1.1 cm/s to a very high 6.0 cm/s. For −1.40 +1.00 mm particles, the yields are lower, but are largely independent of the feed flux over the range of 0.9–3.3 cm/s. Thus, the tracer particle yield for each particle size was dependent on the gas flux, while relatively insensitive to the volumetric feed flux. In conventional flotation, there is evidence of an increased probability for particle detachment when bubbles effectively collide with the froth zone, the collisions producing an inertial deceleration force that dislodges the particles, meaning coarser particles are more susceptible to detachment (Falutsu, 1994; Rahman et al., 2012). It is emphasized that the conditions in the RFC are very different. Here a flooded state arises due to the use of either a high gas flux, feed flux, fluidization flux, or combination, resulting in a uniform gas hold-up of order 0.5 (Dickinson and Galvin, 2014). Hence, no traditional froth zone exists, hence there is no inertial effect associated with the bubbles joining a froth zone. However, the gas volume fraction at the point of

Fig. 4b. Grouped tracer particle partitioning to overflow with and without ultrafine particles circulated in the feed, with a gas flux of 3 cm/s, feed flux = 3.2 cm/s, and bias flux = −0.4 cm/s, using the annular downcomer. Curves are a guide for the eye only. 5

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Fig. 5. Tracer particle partition to overflow as a function of gas flux for various feed fluxes and particle sizes.

overflow, Ø, increases to higher levels of perhaps 0.6 to 0.9, depending on the gas and overflow liquid flux. The overflow emerges as a pneumatic froth, satisfying the relation (Dickinson and Galvin, 2014), jOF + jg = jg/Ø. If this final volume fraction, Ø, is too high, then coalescence becomes significant. The tracer partitioning data of Figs. 5 and 6 are also shown in Fig. 7 as a function of the gas volume fraction in the overflow, given by the gas flux divided by the total flux of gas and liquid to overflow. The data show a trend, with the partition number decreasing as the exit volume fraction of the gas exceeds 0.8. This result is thought to be due to the reduced interstitial liquid between the bubbles (Ireland, 2009). For monodisperse bubbles, one can consider the critical gas fraction represented by randomly packed spheres (Bouvard and Lange, 1992) given by Ø = 0.64. Below this critical value, the bubbles are spherical and typically flow independently (Langevin, 2017). For ordered packing of monodisperse spheres, Ø = 0.74, while for wet foams and dry foams, Ø exceeds 0.90 and 0.99, respectively. Accordingly, we conclude that at a high gas fraction in the overflow, perhaps nearing 0.90, there is insufficient interstitial liquid available to dissipate mechanical energy, causing particle detachment, coalescence, and reduction in the tracer particle yield. There is a further discontinuity in the gas hold-up in the vicinity of the downcomer exit. Examination of the yield versus the gas fraction exiting the downcomer revealed no clear correlation. However, the coarse particle recovery was dependent on the gas flux contribution, to

first order, perhaps in part due to a rise in the gas volume fraction of the overflow. However, as discussed later, there may be other factors responsible for the decrease in yield. Certainly, at a low gas flux of 0.5 cm/s the tracer particle yields were generally very high across the full particle size range. This finding was therefore adopted in the continuous steady state separations. 3.2. Continuous steady state separation using industry feed The rate of flotation of different coal maceral types, and their floatability has been shown to strikingly vary with particle size using an imaging technique referred to as Coal Grain Analysis. For instance, Ofori, O’Brien and Firth (2007) obtained approximately 90% recovery for low-density particles of up to 0.75 mm in size having > 95% vitrinite. However, the recovery of 0.75 mm particles composed of inertite, liptite, minerite, and their composites, reduced to below 10%. Considering that the tracer particle study detailed in the previous section involved exclusively low-density coal tracer particles, it was thought prudent that a more realistic feed be examined. Hence, a series of experiments was performed using an industrial feed of 5 wt% solids. The base-line conditions used in the continuous steady state experiments involved a gas flux of 0.5 cm/s and a volumetric feed flux of 1.1 cm/s. The effect of the gas flux was then explored in later experiments. In addition to using a negative and neutral bias flux, two further experiments were completed using positive bias fluxes of 0.18 cm/s and

Fig. 6. (a) Tracer particle partition to overflow as a function of feed fluxes for −0.71 +0.50 mm particles, and (b) −1.40 +1.00 mm particles. 6

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Fig. 7. Tracer particle partition as a function of gas volume fraction in the overflow stream for various feed and gas fluxes, and particle sizes. Positive (net downwards) and negative (net upwards) liquid bias fluxes were utilised.

0.43 cm/s. The four experimental run conditions are summarised in Table 1. Note that the tubular downcomer was used in these experiments. The fractional combustible recoveries are shown in Fig. 8 as a function of the particle size. It is evident the results from all of the experiments virtually overlap one another, irrespective of the level of wash water applied, and the imposed direction of cleaning. The mean cumulative combustible recovery from the four runs was 86.8% ± 1.9% (95% confidence interval). These results are in agreement with experiments, not reported here, conducted at 1 wt% feed concentration, suggesting recovery by particle size was not diminished by the feed

solids concentration. A summary of mass balance data for the run involving a 0.18 cm/s bias flux is given in Appendix A. The data show that the product pulp density was 26.4 wt%, significantly higher than the feed of 4.7 wt%. Despite the relatively high gas fraction to overflow of 0.85, high recovery of the coarsest fractions remained, supposedly due to the stabilising effect of the ultrafine particles in the feed that contained 34 wt % below 38 µm. Nevertheless, it is reasonable to assume operation at higher gas fractions would result in a loss in recovery, for example, when insufficient liquid is carried to the overflow. Fig. 9 clearly demonstrates the effect of increasing positive bias flux, that is a downwards water flux, on the product ash content, particularly over the finer size fractions. A positive increase in the bias flux from −0.12 cm/s to 0.43 cm/s produced a reduction in the −0.038 mm product ash from 34.7% to 14.6%, well below the feed fractional ash of 63.7% ± 2.0%. It is noted however, that across the runs the cumulative feed head ash % shifted moderately from 39% to 34%, hence not all this reduction is due to the washing. Yet, the data in Fig. 8 show high recoveries over the full particle size range, without the typical “elephant’s trunk” decline in flotation recovery for coal particles above 0.2 mm (Firth, 2015; Jameson, 2017; Kohmuench et al., 2018). This is a significant achievement given that fine hydrophilic particle rejection via

Fig. 8. Fractional combustible recovery as a function of particle size at 5 wt% feed solids loading for bias fluxes of −0.12, 0.02, 0.18, 0.43 cm/s, and gas volume fraction in the overflows, Ø.

Fig. 9. Fractional ash of the RFC product, with relative errors included, from negative to increasingly more positive bias fluxes of −0.11, 0.02, 0.18, 0.43 cm/s.

Table 1 Run conditions for processing 5 wt% feed. Flux (cm/s)

Liquid Split (%)

Feed

Wash Water

Overflow

Bias

1.1 1.1 1.1 1.1

0.05 0.10 0.27 0.54

0.17 0.08 0.09 0.11

−0.12 0.02 0.18 0.43

16 7 8 10

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Fig. 10. Recovery-ash tree curve for the −0.125 mm particle size fraction, and the RFC recoveries achieved using increasing bias fluxes.

Fig. 12. Fractional ashes and relative errors of the RFC product (bias flux = 0.18 cm/s) and the Float/Sink method for the +0.125 mm particles.

wash water and coarse particle recovery have generally not been achievable in a single flotation stage. The RFC products were benchmarked against the Tree Flotation method for particles in the −0.125 mm size fraction and via the Float/ Sink separation for the +0.125 mm fraction, as shown in Figs. 10 and 11. Fig. 10 shows the RFC product ash shifting from right to left with the bias flux shifting from negative to positive, at a constant combustible recovery of 75%. The closest correspondence between the RFC and the Tree Curve occurred using the near neutral bias flux of 0.02 cm/s, while a negative bias flux resulted in the RFC separation lying to the right of the curve, and a positive bias flux well left to the knee of the curve. The fractional recoveries for a positive bias flux of 0.18 cm/s are plotted in Fig. 11 with the Float/Sink separation performed on the feed at a relative density of 1.6. The combustible recovery of the Float/Sink separations increased only slightly with increasing particle diameter, achieving a cumulative recovery of 88.8% at 11.0 ash%. The fractional recoveries obtained by the RFC were higher than for the Float/Sink benchmark up to nominally 1 mm particles, while the recovery of the −2.00 +1.00 mm particles were only slightly lower. The RFC cumulative recovery over −2.00 +0.125 mm was 93.6% at 12.4 ash%. The opposing trend directions in Fig. 11 are explained by considering the product ash values. Fig. 12 shows that the ash content of

the RFC product, and by inference the mean density of the coal particles recovered, was highest at the finer sizes, and then declined with increasing particle size. Of course, recovery due to entrainment was regarded as negligible, a reasonable assumption given the particle sizes involved. Therefore, the coarser particle recovery was favoured by the more hydrophobic, lower density particles. The results in Figs. 10 and 11 were consistent with separate experiments involving a dilute feed of 1 wt% solids, demonstrating that the 5 wt% solids concentration made little difference to the separation performance. It was observed in the tracer particle study that high gas fluxes were detrimental to the recovery of the coarse particles. To determine if this observation held true for the continuous steady state separations, two further runs using gas fluxes of 1.0 and 2.0 cm/s were undertaken. The operating conditions for these runs are summarised in Table 2, and the combustible recoveries compared in Fig. 13 to those obtained at the 0.5 cm/s gas flux using a 0.18 cm/s bias flux. Fig. 13 shows that for particles above 0.125 mm, the fractional recoveries all decreased with a doubling in gas flux from 0.5 to 1.0 cm/s. The −2.0 +1.4 mm fraction was the worst affected, with the recovery dropping from 84.9% to 62.2%. Doubling to 2.0 cm/s gas flux resulted in recoveries further decreasing, with the −2.00 +1.40 mm fractional recovery reaching 7.5%. Negligible change in the fractional recoveries below −0.355 mm is apparent. It is clear higher gas fluxes hindered the recovery of the coarser particles. Although the volume fraction of the exit overflow varied across the three runs, this variation was negligible given the uncertainty in the values. Therefore, the reduction in recovery is attributed here to the increase in the gas flux. 4. Discussion It has been shown that lower gas fluxes delivered via a downcomer are more effective in floating coarser hydrophobic particles. It was concluded that the optimum gas flux was 0.5 cm/s for a feed containing relatively well-liberated, hydrophobic coarse particles. While tracer particle recovery was higher at lower gas flux values, in a flotation Table 2 Run conditions for processing 5 wt% feed with increasing gas flux. Flux (cm/s)

Fig. 11. Fractional recoveries with 95% confidence intervals using the RFC (bias flux = 0.18 cm/s) and Float/Sink method for the −2.00 +0.125 mm particles. 8

Liquid Split (%)

Gas

Feed

Wash Water

Overflow

Bias

1.0 2.0

1.0 1.0

0.52 0.50

0.40 0.37

0.12 0.13

38.5 36.3

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associated with the gas-liquid discharge from the downcomer. Bubble coalescence at the overflow exit due to high bubble volume fractions can also lead to lower recoveries, as is suggested in Fig. 7. It is worth considering potential reasons for the observed reduction in coarse particle recovery at gas flux values nominally beyond 0.5 cm/ s. At the point of discharge, the bubbles transition from the high shear environment within the downcomer to the relatively quiescent environments above and below the point of exit in the RFC. It is hypothesized that particle detachment predominately occurs during this transition. Given the coarse particle flotation data show a strong change in performance at gas fluxes of order 0.5 cm/s, the nature of the hydrodynamics of the bubble/liquid transition was studied by observing the discharge flow pattern at different gas fluxes lesser and greater than 0.5 cm/s. A two-phase, gas-liquid experimental setup was adopted, though no tracer particles were added to the system. A digital camera was used to record video footage of the bubble plume upon exiting the downcomer. Still frames of the video footage can be seen in Fig. 14 for gas fluxes increasing from 0.1 cm/s up to 0.7 cm/s, using air to feed ratios of 0.3, 1.3 and 1.8. The flows in Fig. 14 compare appropriately to the Jameson Cell tank void fractions reported by Harbort et al. (2003) at air to feed (pulp) ratios of 0.3 to 0.54. At an air to feed ratio of 0.94 however, void fraction through the Jameson cell demonstrated signs of flooding, with bubbles reported to exit the tailings, and only 10% of the cell volume devoid of bubbles. In the experiments of Fig. 14, a low feed flux of 0.38 cm/s (and low gas fluxes) was used to avoid flooding the system to observe the flow exiting the downcomer. With increasing gas flux, a bubble plume was observed to rise rapidly en-masse due to its low density, causing the bubbles to transport at a rate presumably greater than the terminal rise velocity of the bubbles in isolation. This action represents a sudden reversal in the direction of the gas bubbles, as seen in Fig. 14 (c), resulting in an increased inertial force acting on the bubbles. Particles attached to bubbles undergoing this sudden upwards acceleration would likely experience similar inertial forces, and possible detachment. The pulsation from the peristatic feed pump, evident in Fig. 14 (c), may also have contributed to this affect. However, a higher feed rate should mitigate the inertial effect to some degree, dispersing the gas bubbles in the plume, with the bulk of the liquid flow continuing downwards towards the tailings. The results in Fig. 6 support this hypothesis, showing recoveries independent of changes in the feed flux. Therefore, in general, gas fluxes beyond a critical value lead to significant increases in coarse particle detachment, while increases in the feed flux demonstrated relatively minor effects on coarse particle recovery.

Fig. 13. Fractional recovery of the RFC for gas fluxes 0.5, 1.0 and 2.0 cm/s.

system those lower gas fluxes would ultimately limit the so-called carrying capacity of the system, potentially resulting in a reduced recovery for certain feeds. However, this was not evident from the 5 wt% solids experiments reported here that achieved 87% combustible recovery at a 65% mass pull. Of course, coarse particle recovery should lean towards elevated mass rates due to the large particle mass per unit of bubble-particle contact area. In recent years, there have been significant developments in flotation systems in achieving coarse particle flotation, brought about by promotion of quiescent hydrodynamic conditions by fluidizing particles through the base of the system (Jameson, 2017; Fosu et al., 2015). The RFC is very different, with fluidization introduced from above the system. The system is relatively quiescent, given the absence of mechanical agitation and, under normal operation, the absence of an interface between the bubbly pulp zone and froth zone. The inclined channels also prevent the development of large scale eddies in the lower section of the system, while the downwards fluidization promotes more stable conditions in the upper section. Coarse particle detachment from air bubbles has been attributed to centrifugal forces that arise under turbulent flow conditions in mechanical cells, when the centrifugal force exceeds the adhesion attachment force (Ralston et al., 1999). Scaling laws relating the detachment to the Bond number have been reported (Schulze, 1982; Goel and Jameson, 2012). Intense conditions also exist within a downcomer, and strong forces develop at the discharge point of the downcomer. Therefore, coarse particle detachment in the RFC is most likely

Gas Flux = 0.1 cm/s

Gas Flux = 0.5 cm/s

Gas Flux = 0.7 cm/s

(a)

(b)

(c)

Fig. 14. Side view of the bubble plume formation at the downcomer exit as the gas flux was increased from 0.1 to 0.7 cm/s (jf = 0.38 cm/s, jb = −0.10 cm/s).

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Minerals Engineering 149 (2020) 106224

5. Conclusions

at 5 wt% feed concentration was high. Under a positive bias flux, the recoveries by ash bettered the Tree Curve for the fines, and the Float/ Sink benchmark for particles up to nominally 1 mm in size. Gas fluxes above 0.5 cm/s were again demonstrated to reduce coarse particle recovery drastically. Video footage of bubble plumes at the exit of the downcomer, over a range of gas fluxes, revealed that there was a strong tendency for the bubbles to immediately accelerate upwards upon exiting the downcomer. This produces an inertial force acting on the particles, which is higher for increasingly coarse particles. Hence, increasing the gas flux was concluded to similarly increase the inertial forces experienced by particles, and therefore the magnitude of the forces responsible for detachment.

Coarse particle flotation was examined as a function of different hydrodynamic conditions in the Reflux Flotation Cell (RFC). Two approaches were used. The first involved the addition of hydrophobic coal tracer particles up to 2 mm in diameter, one at a time. The overflow product and underflow tailings streams were passed over separate screens, allowing the partitioning of the particles to the product to be measured. A broad range of feed and gas fluxes were used, to define a window of optimum operating conditions for coarse particle recovery. Experimental results showed that the coarse particle recovery was very high at low gas fluxes, while beyond a gas flux of 0.5 cm/s the recovery started to decline, decreasing markedly at gas fluxes beyond 1.0 cm/s. The second experimental approach undertaken involved the continuous steady state flotation of industrial feed at the operating conditions identified in the tracer particle study as ideal for coarse particle recovery. Particles were processed at 5 wt% solids to examine the effect of particle–particle interactions on coarse particle recovery. These results were then compared to two benchmarks: −0.125 mm particles to the Tree Flotation Curve, and +0.125 mm particles to a Float/Sink separation at a relative density of 1.6. The recovery of coarse particles

Declaration of Competing Interest The authors acknowledge the financial support of ACARP (Australian Coal Association Research Program) for this research on the Reflux Flotation Cell technology. The University of Newcastle, Australia has an R&D Agreement with FLSmidth, Denmark and an IP policy that extends benefits to inventors.

Appendix A The tables below summarises the mass balanced data for the experiment involving 5 wt% feed concentration and a bias flux of 0.18 cm/s, as presented in Figs. 8, 9, 11, 12 and 13. For the sake of data reconciliation, mass balancing was performed on the solids rate, head ash, fractional mass, and fractional ash distribution in the feed, overflow and underflow streams. A comparison of the raw data to the balanced data revealed only slight changes (see Tables A1 and A2). The liquid bias rate is merely the difference between the wash water rate and the liquid overflow rate. In the table above, the bias is said to be positive because the downwards fluidization provided by the wash water is greater than the overflow rate. This generates a net-downward movement of liquid through the system, which is beneficial for the rejection of ultrafine gangue particles. Conversely, a negative liquid bias occurs when the wash water rate is less than the overflow rate.

Table A1 Liquid and solids rates for the continuous run performed with a positive bias flux of 0.18 cm/s and 5 wt% feed solids concentration. Stream

Feed Overflow Underflow Wash Water Bias Air

Water

Solids (raw)

Solids (mass balanced)

Rate (mL/ min)

Flux (cm/s)

Rate (g/ min)

% solids

Rate (g/ min)

% solids

5168 435 6026 1294 858 2385

1.08 0.09 1.26 0.17 0.18 0.50

252.7 156.2 90.4

4.7 26.4 1.5

252.6 164.1 88.6

4.7 27.4 1.4

Table A2 Fractional ash by size mass balanced data corresponding to Table A.1. Top Size Fraction (mm)

2.00 1.40 1.00 0.710 0.500 0.355 0.250 0.180 0.125 0.038

Feed

Product

Reject

Separation

Mass %

Ash %

Cum. Ash %

Mass %

Ash %

Cum. Ash %

Mass %

Ash %

Cum. Ash %

Yield %

Rec. %

Cum. Rec. %

2.5 7.3 8.2 9.0 7.3 6.8 5.5 4.8 14.2 34.4

13.0 14.1 15.4 16.8 18.5 19.7 21.9 24.3 31.4 63.5

13.0 13.8 14.6 15.3 16.0 16.6 17.2 17.9 20.8 35.5

3.0 9.3 10.9 12.3 10.2 9.6 7.7 6.4 16.3 14.2

8.0 8.6 9.7 11.3 13.2 14.8 16.7 16.9 16.7 15.8

8.0 8.5 9.0 9.8 10.6 11.3 11.9 12.4 13.2 13.6

1.4 3.6 3.4 2.9 2.0 1.7 1.4 1.7 10.3 71.7

33.4 41.0 49.7 59.9 68.7 73.3 75.0 76.1 74.3 81.0

33.4 38.8 43.2 47.6 50.8 53.3 55.2 57.1 63.4 76.0

80.2 83.0 85.7 88.6 90.3 91.6 91.0 87.5 74.4 26.9

84.9 88.3 91.5 94.5 96.3 97.2 97.1 96.0 90.4 61.9

84.9 87.4 89.3 91.0 92.1 92.9 93.4 93.6 93.0 86.9

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