Flotation of coarse coal particles in a fluidized bed: The effect of clusters

Minerals Engineering 146 (2020) 106099

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Flotation of coarse coal particles in a fluidized bed: The effect of clusters Graeme J. Jameson , Lonn Cooper, Kitty K. Tang, Cagri Emer ⁎

T

Centre for Multiphase Processes, University of Newcastle, Callaghan, NSW 2308, Australia

ARTICLE INFO

ABSTRACT

Keywords: Coarse particle flotation Fluidized bed flotation Bubble clusters Cluster flotation Coarse coal flotation Froth dropback Froth recovery Flotation tailings Coarse tailings Dry stackable tails

This paper is concerned with the separation of coarse particles from siliceous gangue, in a fluidized bed froth flotation column, the NovaCell. Coarse particles are allowed to settle in the base of a column, and are fluidized by an upward-flowing stream of liquid. Air bubbles are introduced with the stream, which collide with hydrophobic particles in the bed, carrying them to the top of the vessel as flotation product. The paper describes the use of bubble clusters as a means of recovering particles in a flotation machine. Clusters are buoyant aggregates of bubbles and particles. Copious quantities were observed to form in the fluidized bed, rising to the top of the flotation column. They were present in such volumes that the froth was unable to absorb them all, and a thick cluster layer developed beneath the froth. This layer was drawn off and separated on a screen as a secondary flotation product, in addition to the conventional froth product. Here we describe the flotation of coal particles from different sources with a top size of 2 mm, in a NovaCell fluidized bed contactor. High combustibles recoveries were achieved, at acceptable ash contents, over the whole size range. The froth phase contributed approximately 65% of the total, the balance appearing on the screen. Particles of all sizes from 0 to 2 mm were represented in the froth discharge. The NovaCell gave two flotation products and two tailings streams. It was possible to control the tails from the fluidized bed so that the solids fraction was as high as 60–70%, in the size range 300 µm–2 mm. The results highlight the value of coarse particle froth flotation from a product de-watering perspective, when compared with flotation in conventional cells that are restricted to a top size of 500 µm. The ultrafine fraction less than 74 µm in diameter in the product was reduced by a third when the top size was increased to 2 mm from the normal top size of 500 µm for coal. The fact that low-buoyancy clusters can form beneath the froth, provides an alternative explanation for the phenomenon known as froth dropback.

1. Introduction Recently, a new form of froth flotation machine has been described (Jameson, 2010a, 2010b), in which a bed of coarse particles1 is maintained in a fluidized state by a circulating stream, which incorporates a high-shear bubble contactor. The fluidized bed is located in the base of a vertical column. Liquid that has passed through the fluidized bed rises to the top of the column, carrying elutriated fine particles. The system is arranged so that bubbles can be contacted with fine particles in the high-shear contactor. Coarse particles are collected in the fluidized bed. The bubbles with attached particles rise to the top of the column, where they can be collected as the flotation product. Thus the fluidized bed froth flotation cell, known as the NovaCell, is able to capture floatable particles that can be of the order of microns in size, up to sizes in the

millimetre range. The top size depends on the liberation characteristics of the ore or mineral to be treated. The research program described here began as an investigation into the flotation of coal particles with a top size of 2 mm. This is a convenient break point for coal preparation plants, because different separation devices are required for particles above and below this size. In the course of the inquiry, it was seen that a significant fraction of the flotation product was generated by clusters. Clusters, which are aggregates of bubbles held together by hydrophobic particles, have been observed before in flotation cells (Ata and Jameson, 2005; Ata et al., 2009) but their full significance has not previously been appreciated. In this investigation it was found that clusters formed very quickly in the fluidized bed, and were present in such quantities, that they could not be accommodated in the froth. Accordingly, they were removed from

Corresponding author. E-mail address: [email protected] (G.J. Jameson). 1 In base metal operations, fine particles are in the range 20–150 µm. Ultrafines are 0–20 µm and coarse particles are above 150 µm. In coal flotation, the size range sent to flotation is from 0–500 µm, depending on the floatability of the coal. ⁎

https://doi.org/10.1016/j.mineng.2019.106099 Received 3 April 2019; Received in revised form 27 September 2019; Accepted 17 October 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.

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the column in a continuous stream, and recovered on a screen, as a second flotation product. In this paper, we describe an investigation into the use of a fluidized bed froth flotation device, for the treatment of coarse coal particles, with a top size of 2 mm. The aim was to recover low-ash coal in a single step, and in so doing, to replace the two-stage process currently used. Further, we describe the formation and growth of bubble clusters in the fluidized bed; the characteristics of the clusters; the way that they could be used to produce a second flotation product; and the characteristics of the coarse tailings.

preparation plant, and in the plant itself. Grinding is not used. The fine coal fraction is beneficiated in a two-step process, see Fig. 1. The larger particles in the range, from 2 mm down to 250–500 µm, are treated by a gravity separation process, such as spiral classifiers, teeter bed separators or reflux classifiers. For the ultra-fines, flotation is used (MacKinnon and Swanson, 2013). The requirement for the two separate technologies arises from the limitations of current separation techniques, when dealing with very fine particles. It would be advantageous to be able to replace the two-stage systems with a single technology that could give high-grade products over the whole particle size range from 0 to 2 mm.

2. Coal flotation

3. The fluidized bed froth flotation column

Coal is a major commodity, which is used mainly for two purposes: thermal coal, which is burned to create electrical energy; and metallurgical (coking or steelmaking) coal, that is used in the production of crude steel. Our focus here is on metallurgical coal which is often naturally hydrophobic, and responds well to flotation. Metallurgical coal is used for steelmaking. In 2016, 12% of the coal produced in the world was used for for this purpose (BP Statistical Review, 2017; World Steel Association, 2018). Metallurgical coal is generally amenable to flotation, being regarded as ‘high quality’. However, thermal coals may also be treated by flotation. Some countries have rich reserves of coking coal. The largest producers are China and Australia. The largest exporters are Australia, the United States, and Canada; the largest importers are India and China (Australian Metallurgical Coal Report, 2018; Coal and Steel Report, 2009). In current coal preparation operations in Australia, particles less than 2 mm in diameter are referred to as ‘fine’ coal. This fraction originates from the mechanical processes used in the mining and handling of friable coal as the run-of-mine material is transported to the coal

The fluidized bed froth flotation device consists of a vertical column, with an upflow of water that is introduced into the base, sufficient to fluidize the coarser size fraction in the feed, see Fig. 2. The principles have previously been briefly described in Jameson (2010a, 2010b). The coarse particles settle in the bottom, and fine particles are elutriated out of the bed. For coal, the coarse dense particles consist mainly of silica and silicates. These particles accumulate in the bed, which becomes a hindered bed separator with respect to the coal. The particles are fluidized by a stream of water, consisting of part of the water from the overflow stream from the top of the cell, and water in the new feed. Fine air bubbles are generated in the external contactor, which provides a high-shear zone that is effective for the collection of very fine particles. The aerator was a simple laboratory vacuum jet pump, operating with air under pressure. The aerated mixture combines with new feed and enters into the base of the column. The feed is conditioned with conventional reagents such as diesel oil, to make the coal particles hydrophobic. In the system as a whole, water enters only in the feed stream. It can

Fig. 1. Typical Queensland fine coal circuit, after MacKinnon and Swanson, (2010, 2013). DMC: dense medium cyclone. D&R: drain and rinse screen. TBS: teeter bed separator. 2

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Fig. 2. Schematic of the NovaCell fluidised bed froth flotation cell.

Fig. 3. P&ID of the laboratory pilot plant.

leave in the two product streams: the underflow tailings and the overflow tailings. The level control on the overflow stream leaving the cell serves two purposes: to control the froth/pulp interface at a desired level, and to maintain the water balance. It can take the form of a gravity overflow as shown in Fig. 3, or a control valve actuated by a level sensing device such as a float or a pressure transducer. The fluidized bed flotation device is equipped with separate collection zones: the high-shear aerator for capturing the fine particles,

and the fluidized bed for removing the coarse. The feed particles have a wide distribution of sizes and densities, and hence of terminal velocities. Particles whose terminal settling velocity is lower than the chosen fluidization velocity will be elutriated out of the bed, leaving behind the coarser particles. Therefore, the composition of the bed is not the same as that of the feed. In current flotation practice, using Jameson Cells or other high-efficiency devices, flotation is limited to coal particles with a top size of 3

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500 µm (MacKinnon and Swanson, 2013), or possibly as low as 300 µm (Meenan, 1999) for mechanical cells or 150 µm for columns (Kohmuench et al., 2007). Based on our experience, the present system was designed to ensure that coal particles below 500 µm would be treated in the high-shear aerator, as well as in the fluidized bed. There is little point in putting coarser particles through the high-shear device, because they will most likely be unable to form stable attachments to bubbles. The vertical liquid velocity in the cell was decided by consideration of the terminal velocities Ut and the minimum fluidization velocities Umf of particles of coal and ash. The density of the coal was assumed to be 1600 kg/m3, and that of the ash was 2650 kg/m3, the same as silica or quartz. The superficial upflow velocity in the fluidized bed was such that the corresponding elutriation diameter of the coal and silica particles was 500 µm and 300 µm respectively. Thus particles smaller than these values would be elutriated out of the bed. At this velocity, it was observed that the bed expansion ratio, which is the ratio of the height of the fluidized bed to that of the static bed, was 1.8. The column was of sufficient height to ensure that the expanded bed height was below the height of the base of the offtake cone. New feed is injected into the recycle before it enters the cell. To increase the probability of capture, the recycle flowrate can be much greater than the new feed flowrate. The performance of the NovaCell is regulated by the recycle ratio, which is the ratio of the recycle flowrate to the flowrate of new feed. The flowrates to be compared could be the mass or the volumetric flowrates in the respective streams, depending on practical considerations. To maintain the fluidized bed, a minimum fluidization flowrate of new feed and recycled pulp is required, so in practice, the recycle flowrate rate has to be maintained at a prescribed value. The recycle ratio has the same function as residence time in a mechanical cell. To improve recovery, the recycle ratio can be increased, creating more than one opportunity for a given particle to be captured either in the high-shear contactor or in the bed itself. It should be noted that the principle of particle capture in a fluidized bed is utilised in another separation technology, known as the HydroFloat (Kohmuench et al., 2001, 2007, 2010; Kohmuench et al., 2013; Kohmuench and Thanasekaran, 2013;Mankosa et al., 2000, 2002). This is an air-assisted gravity separator, and is successful with the coarse particles (> 250 µm) in a mineral process stream. It operates without a froth layer, so it cannot offer any selectivity for fine mineral particles, which must be removed from the feed and treated separately. It uses water to fluidize the bed.

trial-and-error attempts with a range of designs. The key issues were the ability to ensure that the particle size distribution in the feed box remained constant, even when the level was dropping; and the composition of the sample drawn continuously from the feed box was the same as the initial composition in the feed box itself, after perhaps an hour of operation. Eventually, the system shown in Fig. 3 was devised. An upward-pumping marine impeller was placed in a cylindrical baffled feed box. A recirculation stream was drawn from the box, at the same level as the impeller, and returned to the base of the box. This arrangement gave very satisfactory results provided the liquid level in the feed box never fell below one impeller-diameter above the top of the impeller. Coarse particles that may have settled in the bottom of the box were entrained in the upwardly flowing recirculation stream and lifted into the box by the jet stream from the impeller. An isokinetic sampler was developed to ensure that the composition of the new feed sample was representative of the contents of the recirculation stream and the contents of the feed box. The effectiveness of the system was checked by measuring the percent solids in the box contents and the particle size distribution as a function of height within the liquid above the impeller at different liquid surface levels over time. The results were checked against samples withdrawn from the isokinetic sampler at different time intervals. For the continuous runs, the internal diameters of the transfer lines and the constant level device were chosen so that the liquid velocities were sufficient always to keep the coarse silica particles in suspension. To this end, the minimum superficial velocity in any of the transfer lines was 1 m/s. Although the equipment design was more demanding, runs from the continuous operations delivered a major advantage, in that adequate samples of solids for mass and ash determinations could be collected over the run time, for size-by-size analysis. In a batch test, the mass of the samples of solids in the product streams declined exponentially with time. After conditioning with diesel oil (1 kg/tonne) and MIBC (20 ppm), the new feed was pumped continuously into the NovaCell at the desired rate. The froth depth was 75–100 mm. The underflow tails stream was withdrawn at a constant rate, and the froth depth was controlled by a simple overflow arrangement, which had the additional function of maintaining the water balance over the cell, in conjunction with the overflow tailings pump. The level of the fluidized bed was controlled by the variable speed peristaltic pump operating on the underflow tails, Fig. 3. The pump speed is adjusted to give a high percent-solids in the underflow tailing stream leaving the pump, as shown in Fig. 7. Since the bed is ‘fluid’, the top of the bed is horizontal, and particles flow to the underflow outlet. In these experiments, the solids content was judged visually. Once the desired percent solids was attained, minor corrections were made to the pump speed to maintain the flow. When the speed was increased, more water was drawn from the column and the percent solids dropped; when the speed was too slow, the percent solids was too high so the speed was corrected. Samples collected during the flotation runs were dried and weighed, to determine the mass recovery. The dry particles were separated into size fractions, and the ash content of each fraction was determined in duplicate using the procedure of Australian Standard AS 1038-3-2000, Coal and Coke–Analysis and Testing, Part 3: Proximate analysis of higher rank coal, Section 3. (For reporting purposes, “combustibles” refers to the non-ash fraction in the dry solids fractions subjected to ash determinations.) Some samples were sized and subjected to ash determination at an external ISO-regulated laboratory, for checking purposes. Agreement was excellent.

4. Experimental Flotation was carried out in a NovaCell laboratory cell of diameter 100 mm and overall height 1680 mm, with the circuit depicted in Fig. 3. The apparatus was constructed so that it could be operated in batch or continuous mode. In continuous mode, it is necessary to prepare sufficient sample to last over the length of a run, which could have been 1–1½ h. The feed solids flowrate was typically 0.5 kg/min, so one run required 45 kg of prepared solids. Runs 4 and 5 were conducted in one day (see below), so 90 kg of feed was prepared, and Runs 6, 7 and 8 which were conducted on one day requiring 120 kg. The number of runs that could be conducted therefore depended on the mass of solids in the sample available. In batch mode, a charge of 5 kg of solids was placed directly into the base of the column, and overflow slurry was pumped through the bed. Timed samples of product were taken from the froth launder, and the wedge-wire screen. Flotation was continued until no coal particles could be seen in the overflow. The residual particles in the cell constituted the tailings. In continuous mode, the feed solids were placed in the feed box with the appropriate volume of water. The development of a system to deliver a stream of constant composition from the feed box, with fastsettling siliceous particles up to 2 mm in diameter, required numerous

5. Feed samples Feed samples were kindly provided by a coal operation in the Hunter Valley, New South Wales, Australia. They were drawn from various points around the fine coal circuit. As shown in Fig. 4, samples 4

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observations.

• The average d • •

• •

80 of the froth product was 804 µm, and that of the screen product was 1728 µm. The froth is capable of capturing coal particles that are much larger than is possible with existing technologies. The froth contained less entrained ash than expected, possibly due to the presence of coarse particles that propped open the gaps between bubbles, assisting drainage. The NovaCell delivers two tailings streams: an overflow stream that is diverted from the recycle stream, and an underflow stream from the fluidized bed. The underflow is deslimed by the action of the liquid rising through the bed, and contains particles that are mainly in the size range 300–2 mm. The overflow tailings is less than 500 µm. The underflow tailings represent 60% on average of the overall tails flow, and can be discharged at a high solids fractions, of the order of 60–70% solids. The mean d80 of the combined product for the continuous runs was 1390 µm, and that for the combined tailings was 1222 µm. These streams will be much easier to de-water than the streams from current flotation technologies.

It was not possible to optimise the operation of the cell due to constraints on the mass of feed sample available. It is likely that the ash values could be reduced by varying standard operating parameters such as froth depth, air rate and washwater addition. In the continuous runs, the fraction of product reporting to the froth was generally higher than that to the screen. Removal of +500 µm coal by cluster formation and transfer to the screen, seems to act as a backstop process. It appears that particles preferentially enter the froth layer. However, if the froth is overloaded, the excess particles in the form of clusters gather beneath the froth, from where they can be captured and removed in a side stream, to appear in the screen product. It will be seen in the following tables that the froth is well able to retain coarse coal particles. See for example the combustible recoveries in Tables A4, A5, Appendix A. Examination of the distribution of masses between the various output streams shows that the 500 µm wedge-wire screen was very efficient. In Runs 4–7, no wash water was applied to the particles on the screen, but it is seen that the mass of fine particles less than 500 µm in the screen product was essentially zero. The data for the overflow tails, which is the liquid that has passed through the screen, show that the coal is concentrated in the < 75 µm size band, and these particles drained easily from the coarse particles retained on the screen.

Fig. 4. Fine coal circuit at a Hunter Valley coal preparation plant, showing sampling points for flotation testing.

were taken from (a) the product from a desliming screen, with a nominal top size of 16 mm; (b) feed to the desliming cyclone; and (c) the feed to spirals. The samples were prepared with a top size of 2 mm, by screening. In some tests, the coarse particle fraction was augmented by taking some of the particles in the product of the desliming screen and crushing them to a top size of 2 mm. Diesel oil was used as collector, and MIBC as frother. The coal particles of all sizes floated very well. 6. Results and discussion 6.1. Test results Table 1 summarises the test results for batch and continuous flotation tests. It is important that these results are considered in conjunction with the detailed reports on individual runs shown in the Supplementary Files where the results are given on a size-by-size basis. The results for the continuous tests were outstanding. Very high combustibles recoveries were achieved for all runs, with short residence times. (The residence time is calculated by dividing the volume of the cell below the level of the underflow tails exit pipe by the volumetric flowrate of the new feed.) Key results for continuous runs:

6.2. Comments on continuous test runs In the Supplementary Files, Appendix A shows the flotation results on a size-by-size basis, for all batch and continuous runs. Appendix B gives the particle size distributions of the various streams in the continuous runs: feed, froth product, sieve bend (screen) product, combined product, underflow tailings, overflow tailings and the combined tailings. For each of the runs shown in Appendix A, three data sets are tabulated on a size-by-size basis. These are the distributions of mass, the % ash in sample and the combustibles distribution, for each stream: the feed, the froth product, the sieve bend product, the underflow tailings and the overflow tailings, together with the combined product and the combined tailings. When examining the data for a particular run, it is important to consider the three data sets as a whole, as otherwise incorrect deductions may be made. For example, Table A4 shows the data for Run 4. If we look at the sieve bend product ashes alone, it may appear that the product is richer in ash than the feed, which should be impossible. However, further inspection shows that the combined product ash is less than the feed ash for all size fractions. Take for example the size band between 500 and 700 µm. The sieve bend product

• Combustibles recovery ranged from 92.8 to 96.7%, average 94.2%. • Yield ranged from 83.5 to 88.7%, average 86.3% • Product ash was in the range 10.5–14.8%, average 12.4%. • Flotation feed ash ranged from 15.1 to 25.5%, average 20.6%. The feeds were drawn from different sources. • Tailings ash was in the range 49.8–79.7%, average 65.0%. • The residence time of 1.7 mins is relatively short, compared with other flotation devices. • Generally, more of the product came from the froth layer than from

the overflow screen. The device appears to be self-correcting to some extent. If the froth becomes loaded with particles, the excess can be carried out as clusters to the overflow stream. Cluster formation is discussed in greater detail in Section 8 Experimental 5

6

54.6

95.3

37.5 30.7 68.2

49.1

40.5

89.5

Yield, froth product (%) Yield, screen product (%) Overall yield, %

Combustibles recovered to froth product, % Combustibles recovered to screen product, % Combustibles recovery, overall, %

– – 78.9

Ash in underflow tails, % Ash in overflow tails Ash in combined tails (%) Mass fraction of −75 µm particles in product stream (%)

– – 77.7

18.5 13.7 15.8

25.6

35.9

16.0 15.6 15.8

Ash in feed, %

42.7

37.1 47.1 84.2

0.8 – – – –

54.8

Ash in froth product, % Ash in screen product, % Ash in combined product (%)

Fraction of combustibles recovery reporting to froth, %

40.7

0.8 – – – –

Superficial air velocity Jg, (cm/s) New feed flowrate (L/min) Feed percent solids (%) Feed solids flowrate (kg/min) Residence time (min)

Batch

Batch

Test type

Primary desliming cyclone feed

Run 2

Spiral feed

Run 1

Feed source

Table 1 Summary of test data and results.

– – 72.9

18.7 17.1 18.1

33.4

56.5

88.6

38.5

50.1

41.1 31.0 72.0

0.8 – – – –

Batch

Primary desliming cyclone feed

Run 3

72.8 84.9 76.6 10.7

12.8 18.1 13.5

25.0

87.3

96.7

12.3

84.4

76.8 11.9 88.7

0.9 3.8 11.6 0.2 1.3

Cont.

Mixture of cyclone and spiral feeds

Run 4

75.0 85.7 79.7 13.3

13.2 19.2 14.8

25.5

75.5

95.5

23.4

72.1

61.9 21.6 83.5

1.1 3.0 15.1 0.5 1.7

Cont.

Mixture of cyclone and spiral feeds

Run 5

55.9 62.5 58.7 13.5

9.7 13.4 11.2

18.0

60.6

92.8

36.6

56.3

51.1 34.6 85.8

0.9 3.0 18.2 0.5 1.7

Cont.

Crushed screen oversize + spiral feed + cyclone feed

Run 6

55.0 67.1 60.3 14.3

11.5 13.3 12.2

19.2

57.2

92.9

39.7

53.1

48.5 37.0 85.5

1.1 3.0 14.0 0.5 1.7

Cont.

Crushed screen oversize + spiral feed + cyclone feed

Run 7

45.5 57.3 49.8 9.5

9.6 11.2 10.5

15.1

46.5

93.0

49.7

43.3

40.6 47.5 88.1

0.9 3.0 17.5 0.6 1.7

Cont.

Crushed screen oversize + spiral feed + cyclone feed + wash water

Run 8

G.J. Jameson, et al.

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had a very high ash content, 57.2% from a feed of 19.5%, but the froth product had a low ash of 8.8%. The ash in the combined product was 9.9%, which is very acceptable. The mass distributions between froth and sieve bend products in this size fraction were 9.4% and 0.2% respectively. The froth product was totally dominant. It seems that the low-ash coal mostly passed into the froth, while high-ash composites were unable to enter the froth, and and accordingly were captured in the sieve bend product stream. This explanation is consistent with the observations in Section 8. Experimental observations. Runs 4 & 5. Mix of spiral feed and primary cyclone feed. In Runs 4 and 5, material was drawn from several sources, to modify the ash and the particle size distribution in the feed, and the balance between the coarser and finer fractions. Run 5 was a repeat of Run 4 with slightly increased residence time (1.3 mins vs. 1.7 mins), percent solids in the feed (11.6% vs. 15.1%) and air rate. Key results: The particle size distribution of the feed was relatively uniform, on average 59% greater than 500 µm and 41% in the sub500 µm fraction. The overall results for Run 4 and Run 5 are similar. The combustibles recovery is slightly lower (96.7% vs. 95.5%), the yield is slightly lower (88.7% vs. 83.5%), and the product ash is slightly higher (13.5% vs. 14.8%) in Run 5, all due to differences in the operating conditions. The sieve bend in the pilot plant had a nominal cut-point of 500 µm, which is confirmed by the data. Tables A4-A and A5-A show that the sieve bend product had very few particles less than 500 µm, which was expected. However, Tables A4-B and A5-B show that the ash in these undersize particles was very high, suggesting that they were mainly composites, that for some reason were entrained in the discharge from the sieve bend. Because of the very low concentrations of these particles in the feed, they played little role in determining the combustibles recovery or the product ash content. Points to note, from Tables A4-C and A5-C:

below 500 µm. Thus from Table A7-A, the masses in the sieve bend fractions above 500 µm are generally much higher than those in the froth product, and the same can be seen for the combustibles recoveries for the froth and screen streams, at the same dividing size, Table A7-C. The wedge-wire screen appears to give a fairly precise split at 500 µm, as there are very few particles in the sub-500 µm fraction in the screen product. Above 500 µm, the results confirm that the screen product streams are greater in mass and in combustibles, than the froth product streams. Much of the screen product could have come from the crushed deslime screen oversize. The ash values in the sieve bend fractions below 500 µm were quite high, in the range 21.6% to 50.8%. If the sieve bend were operating perfectly, these particles should not have been present in the product, which is the overflow from the wedge-wire screen. However, the mass contributions of these particles were negligibly small, indicating that the screen was highly efficient. The fact that the particles that were retained in the product in the < 500 µm fraction contained high ash (and consequently were of high density) may be related to the way the sieve bend operates. High-ash particles would have a higher inertia than particles of coal alone, and would tend to overshoot the gaps between the wires, and would have a reduced tendency to drop between the screen wires and into the underflow. In practical terms, the contribution of these particles to the ash in the product is very small. Comparison of the results in Tables A4 and A7 shows that the overall recovery of combustibles in Run 4 (96.7%) was markedly better than that in Run 7 (92.9%). It should be noted that the residence time was lower in Run 4. Further investigations on this point are required. The overall product ash in Run 8 compared to Run 6 and 7 was lower with wash water applied to the screen. The reduction occurred because of improved ash rejection in the froth product, rather than the screen product. When wash water was applied, the majority flowed back into the flotation cell in the recycle flow. It is likely that the improved recovery resulted from dilution of the coal slurry in the flotation cell, which would reduce the concentration of ultrafine ash particles entrained into the froth.

• The combustibles recoveries for all size fractions were very high, ranging from 90.8% to 98.0%. • In these tests, the mass fraction of the recovered coal reporting to the froth product was much higher than that from the sieve bend. • The combustibles recovery in the froth was relatively uniform across

6.3. Distribution of combustible recoveries between froth and screen

all size fractions, so the NovaCell was just as effective at recovering the coarse particles as the finer ones, in the froth phase.

In the continuous runs, it was found that the floatable particles were distributed between the froth and the screen. Table 2 shows the combustibles for each of the runs, and the average values. It is seen that on average, 65.4% of the combustible particles were delivered from the froth. The screen was very efficient in separating the coarse particles above 500 µm into the screen product. Coarse particles above 500 µm were also recovered in the froth.

Runs 6, 7 and 8. Mixture of crushed deslime screen oversize, with spiral feed and cyclone feed. As shown in Fig. 4, the product from the desliming screen had particles with a top size of 16 mm. Some of this material was crushed and mixed with samples of spiral and primary cyclone feeds, to boost the proportions of the coarser fractions in the feed to the NovaCell. The particle size distribution of the feed was relatively uniform, with 53.8% greater than 500 µm and 46.2% in the sub-500 µm fraction. These runs were conducted on the same day with the same feed and with similar conditions, to check reliability and repeatability of the test procedure. Comparison of the results for Runs 6, 7 and 8 shows that repeatability was very good. In Run 6, The overall combustibles recovery was 92.8%, the yield was 85.8% and the product ash was 11.2%. In Run 7, the combustibles recoveries range from 88.2% for the coarsest particles, of which there was little in the feed, to 95.8% for the fraction between 150 and 300 µm. The overall recovery was 92.9%. The overall yield was 85.5% and the product ash was 12.2%. In Run 8, wash water was applied to the wedge-wire screen. The combustibles recoveries for Run 8 range from 90.1% for the finest particles, of which there was little in the feed, to 95.5% for the fraction between 150 and 75 µm. The overall recovery was 93.0%. The overall yield was 88.1% and the product ash was 10.5%. When the distributions of mass and combustibles in the froth and the sieve bend products in Run 7 are compared, an interesting trend can be seen. There is a cross-over in behaviour for the particles above and

6.4. Recovery of coarse particles in the froth phase It is sometimes asserted that coarse particles do not float, or that Table 2 Mass distribution of recovered combustible particles between the froth and the screen.

Mass distribution of combustibles Froth product, % Screen product, % Overall combustibles recovery, % Fraction of recovery attributable to the froth, %

Run 4

Run 5

between 84.4 12.3 96.7 87.3

Run 6

Run 8

Average

froth and screen products 72.1 56.3 53.1 23.4 36.6 39.7 95.5 92.8 92.9

43.3 49.7 93

61.8 32.3 94.2

75.5

46.6

65.4

32.1

40.4

60.7

Run 7

57.2

Mass fraction of coarse particles above 500 µm in the froth product Fraction of coarse particles in 63.1 51.4 30.8 24.6 the froth, %

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1344 804 1728 1390 1436 211 1222

Run 7 Crushed screen oversize + spiral feed + cyclone feed

1300 540 1710 1350 1650 180 1340

Run 6 Crushed screen oversize + spiral feed + cyclone feed

1300 640 1740 1350 1600 280 1250

1520 640 1710 1550 1700 350 1520

The cumulative particle size distribution curves for Runs 4–8 are shown in Appendix B. For convenience, the passing size for 80% of the mass of particles, has been abstracted for each of the streams and runs, as shown in Table 3. The average d80 of the combined product is 1390 µm, which is far coarser than the size of the product from conventional flotation cells. Similarly, the size of the combined tailings is very high, at 1222 µm. The large sizes of these particles are of considerable practical importance, in terms of the water lost to tailings, the sizes of thickeners and other de-watering devices, and the stability of tailings dams. 7. Product and tails de-watering One of the problems with the flotation of ultrafines, is the de-watering of the product coal. This is because of the size of the particles, which usually have a top size of up to 500 µm (Mackinnon and Swanson, 2012, 2013). Accordingly, they require mechanical devices such as vacuum, hyperbaric, and centrifugal filters. With the NovaCell, the product and tailings streams are much coarser, and de-watering should not be an issue. Table 3 shows the d80’s of the streams arising from the five continuous runs in this program. All the streams are relatively coarse except for the overflow tailings, which contains ash material that has been elutriated out of the fluidized bed, and has passed through the wedgewire sieve bend. The tailings streams from the overflow and underflow streams could be thickened before disposal, in which case only the overflow tails would require treatment. However, if the product and tailings streams were separately combined, it is seen that the average d80 for the product is 1390 µm and that for the tailings is 1222 µm. Particles in both streams will be much easier to de-water than the particles in the streams from fine particle flotation using current technologies. 7.1. Flotation product dewatering To illustrate this point, the fraction of the froth flotation product that is less than 75 µm has been calculated for two cases, using the data shown in Appendix A, Supplementary Files. Here, the mass fractions in the froth product are presented on a size-by-size basis. The top size of the feed in our experiments, and hence that of the product, was 2 mm. The masses in each size band are shown in the Appendix A, from which it is possible to deduce the fraction of the overall froth product for each run, that was smaller than 75 µm. The results are shown in Table 4. The particle size distribution of the feed to flotation will be dependent on the coal type and the history of the particles leading up to the flotation step. It is known that even when coarse material is to be filtered, the presence of ultrafine particles can have a major impact on the filtration time and the residual moisture. The results in Table 4 serve to demonstrate the difference in the fraction of ultrafines in the flotation product obtained from coarse particle flotation and conventional flotation. The Supplementary Files give the mass distributions in size fractions from 0–2 mm, from which it is possible to calculate the

1300 1200 1740 1350 1080 150 1000 Feed Froth product Sieve bend product Combined product Underflow tails Overflow tails Combined tails

1300 1000 1740 1350 1150 95 1000

Run 4 Mixture of cyclone and spiral feeds

Run 5 Mixture of cyclone and spiral feeds

coarse particles suffer from dropback out of the froth. While we cannot discount the dropback hypothesis, the data show that a substantial fraction of the froth recovery is due to coarse particles. Table 2 presents the recoveries of coarse combustible particles in the range 500 µm–2 mm, as a fraction of the overall combustibles recovery for each run. The wedge-wire screen has a theoretical cut size of 500 µm, which is a convenient point at which to separate coarse from fine particles. It is seen that the average fraction of froth product recovery for particles above 500 µm, was 40.4% for the five continuous runs. It is clear that coarse particles can be recovered in the froth, in substantial quantities. 6.5. Particle size distributions for the continuous runs

Stream

Table 3 Summary of the 80% passing size d80 for the product and tailings streams, continuous runs.

Run 8 Crushed screen oversize + spiral feed + cyclone feed + wash water

Average

G.J. Jameson, et al.

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Table 4 Fraction of ultrafine particles in the froth product for coarse vs. conventional flotation. Run number:

Run 4

Run 5

Run 6

Run 7

Run 8

Average

Coarse particle flotation, top size 2 mm Mass of combined product, 0 to 2 mm Mass of < 75 µm particles in combined product Fraction of combined product in the < 75 µm size band

88.7 9.5 10.8

83.5 11.1 13.3

85.8 11.6 13.5

85.5 12.3 14.3

88.1 8.3 9.5

86.3 10.6 12.3

Conventional flotation, top size 500 µm, (no screen product) Mass of froth product, 0 to 500 µm Mass of < 75 µm particles in froth product Fraction of froth product in the < 75 µm size band

32.0 9.4 29.6

33.0 10.3 31.1

36.1 10.8 29.9

37.3 11.1 29.9

28.2 7.7 27.4

33.3 9.9 29.6

Table 5 Distribution of the tailings solids mass fractions in various streams. Run Run Run Run Run Run

4 5 6 7 8

Underflow

Overflow

Total

Fraction in underflow, %

7.7 9.2 8.2 8.1 7.7

3.6 7.3 6.0 6.4 4.2

11.3 16.5 14.2 14.5 11.9

68.3 55.9 57.9 55.9 64.4

Average

60.5

fraction of the combined products (froth plus screen) from Runs 4 to 5 that were in the size fraction less than 75 µm. From Table 4, the average value was 12.3%. In conventional flotation, using a column such as the Jameson Cell, the feed would not contain any particles above 500 µm, so if we consider only the particles less than this diameter, we can find the fraction of ultrafines less than 75 µm in the flotation product. The data shown in Table 4 show that on average, the fraction of ultrafines if the same coal were floated in a conventional cell would be 29.6%, compared with the value of 12.3% for the full feed range 0–2 mm. Thus in a conventional cell, which cannot treat coarse particles, the fraction of ultrafines less than 75 µm would be higher by a factor of 2.4, than in coarse particle flotation in the NovaCell device. The reduction in the finest fraction is a very substantial improvement that would greatly reduce the difficulty of dewatering the flotation product.

Fig. 5a. Clusters in a NovaCell. Particle top size, 2.8 mm. Average bubble diameter 0.5 mm.

the data in the Appendices shows that this is indeed the case. Essentially all the overflow particles are less than 300 µm, and are predominantly in the size band less than 75 µm. The ash content of these particles is very high, so they are presumably mainly silt or clay. The application of wash water to the wedge-wire screen had the desired effect, in that the mass fraction of particles less than 500 µm in the underflow tails is reduced from 2.4%, Table A6, to 1.7%, when wash water was applied as shown in Table A8. The wash water applied to the screen had more effect on the ash content of the froth product, than on the screen product, due to dilution of the slurry in the cell.

7.2. Tailings dewatering

8. Experimental observations

The mass fraction of feed that reports to the tailings is dependent on two parameters: the fraction of non-coal matter in the feed, and the recovery of combustibles for a given run, all on a size-by-size basis. The smaller particles will report to the overflow stream, while the coarser particles that have settled in the fluidized bed will be discharged in the underflow tails. Table 5 shows the distribution of the solids in the tailings streams, for the continuous runs – Runs 4–8. The data are shown in the Supplementary Files. The values represent the percent of the feed in each run, that ends up in the various tailings streams. Thus in Run 4, 11.3% of the total feed is found in the tailings, and the fraction in the underflow is 7.7/11.3 = 68.3%. The tailings stream in total is not a high fraction of the feed solids, because a high fraction of the feed was recovered as combustibles product. For the tailings, we see that 60.5% of the reject on average, reports to the underflow. The top size of the underflow particles is 2 mm. The lower limit is determined in principle, by the liquid rising velocity in the column, and the theoretical elutriation cutoff was 300 µm for silica particles. Inspection of the data in Appendices A and B in the Supplementary Files confirms that this was the case. The overflow tails should consist mainly of siliceous ash particles that had been elutriated out of the fluidized bed, with a top size of 300 µm, a value determined by the fluidization velocity. Inspection of

8.1. Cluster formation A highly significant discovery made in the course of this project was the creation of two product streams from a single flotation cell: the froth product as is usual in froth flotation devices; and a second product that is withdrawn from beneath the froth layer, and separated on the wedge-wire screen. The second product depended crucially on the formation of particle clusters. The fluidized bed froth flotation cell shown in Fig. 2 was constructed with transparent walls, and it was possible to see what was happening in the cell whilst in operation. It was seen that clusters of bubbles and coal particles were formed, as shown in Figs. 5a and 5b. The clusters consist of macro-flocs of bubbles held together by particle bridges. The particles must be hydrophobic.2 Clusters have been observed before (Gaudin, 1932; Glembotskii et al., 1963; Klassen and Makrousov, 1963; Jameson and Allum, 1984; Hernandez et al., 2004; Ata and Jameson, 2005; Perez-Garibay et al., 2014) but their 2 It is not clear whether the bubbles are flocculated by the particles, or vice versa. Clusters are not aerated flocs. There is no attractive force between particles, as would exist between flocculated colloidal particles.

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the froth, relates to the density of the particle-laden bubbles and clusters. In a suspension with a high concentration of floatable particles, most bubbles will carry many more than a single particle, and accordingly, there will be a spectrum of bubble-particle densities. Most bubble-particle aggregates will be of low density relative to the fluidized bed, and will rise out of the bed and into the pulp phase. Lightly loaded bubbles or clusters will continue to rise up the column and into the froth phase. In some circumstances, the density difference between a cluster and the pulp will be so low, that it can rise slowly to the frothpulp interface, but have insufficient buoyancy to continue into the froth. It should be realised that the froth consists of bubbles with a finite load of attached particles, with liquid entrained between the bubbles. The entrained liquid will have the same solids content as that of the slurry in the pulp from which it has arisen. Some loaded bubble clusters may have sufficient buoyancy to rise in the pulp, but be too dense to rise into the froth so they remain at the interface. It is likely that such clusters are formed in mechanical cells, and they will probably remain beneath the froth until they coalesce with lowermost froth bubbles, or the particles will simply detach under the influence of the turbulent agitation in such devices. In the NovaCell, the level of the disruptive forces is relatively low, and the clusters can be removed in a sidestream. Cluster collection. In the operation of the NovaCell device shown in Fig. 2, it is necessary to withdraw an overflow stream from the top of the column. The stream passes through a simple gravity leg to maintain the froth depth at the desired level. Part of the stream is removed as an overflow tailings, to maintain a water balance over the cell, and the balance is returned with new feed, as a means of fluidizing the bed of particles in the base of the cell. In the initial experiments, a cluster layer as shown in Fig. 2 was clearly seen, and a collection cone was constructed which had a triple purpose: it would enable liquid to be captured from the top of the column for recycling back to the base of the fluidized bed; it could remove clusters from the layer beneath the froth; and it could catch particles that had fallen out of the froth, if any. It was evident that the cluster layer would be a productive source of clean coal matter, so the stream withdrawn through the cone was directed to a wedge-wire screen or sieve bend whose effective cut point was 500 µm. It was found that the fraction of particles less than 500 µm that had passed through the screen, was negligible, indicating that the recycle liquid, which carried ash particles less than 300 µm in diameter, drained easily from the coarser particles. The experiments showed that it is possible to take a feed that has a wide range of particle sizes, from zero upwards. Coarse feed particles pack closely in the fluidized bed, and the gentle environment reduces their tendency to detach from the bubbles, improving recovery. The coarsest coal that has been floated with the NovaCell fluidized bed technology is 5.4 mm. The absolute upper limit has not yet been determined. Particles at the fine end of the spectrum are collected by small bubbles generated in the high-shear aeration device. While much of the coal product is removed in the froth layer, the removal of clusters in the overflow stream leads to a valuable additional separation mechanism. A limitation of conventional flotation cells is imposed by the mass of coal particles that can be carried on the surfaces of the bubbles in the froth, as determined by the so-called carrying capacity, tonnes/hr/m2 of column area. Normally, excess coal will be rejected into the tailings, if the feed rate is too high. In the NovaCell, the collector-cone cluster stream is a backstop, which allows the new feed rate to be doubled at least, thereby doubling the production rate per unit cell area.

Fig. 5b. Clusters in a mechanical cell at Buchanan Borehole Colliery, Hunter Valley, Australia. Bubble diameter 0.5 mm.

importance in the flotation process has not been fully recognised. Cluster formation is favoured when the concentration of floatable particles is high, as occurs in the fluidized bed zone of the cell. Clusters are quite stable to mild agitation (Chen et al., 2015a, 2015b). When air bubbles were introduced in the aerator and passed into the fluidized bed, they immediately began to form clusters with the coal. Being less dense than the silica suspension, the clusters floated quickly to the top of the bed and rose in the column. The fluidized bed of silica has a density higher than that of the coal matter, so all fully-liberated coal particles floated out of the fluidized bed and into the zone above. Elutriation also occurs, and fine coal particles whose diameter is less than 500 µm approximately, are carried upwards by the flowing liquid. These particles can be captured by bubbles above the bed. Clusters in the fluidized bed. A range of behaviours was observed. Some coal particles larger than 500 µm formed clusters in the bed and rose swiftly to the top of the column. Others, although attached to bubbles, could not be lifted by a combination of buoyancy and the liquid drag force, but were too light to fall back into the silica-rich fluidization zone. Thus they gathered immediately above the fluidized bed as a concentrated layer. (The cutoff point of 500 µm was determined by the superficial rise velocity of the water, which equalled the theoretical terminal velocity of 500 µm coal particles. Similarly, at the same water velocity, the corresponding particle size for silica is 300 µm.) Since there is a constant flow of air, new bubbles rising through the bed enter this layer, and rapidly form buoyant clusters that rise in the column to the froth-pulp interface. A proposed mechanism of cluster formation is shown in Ata and Jameson (2005). Observation shows that when particles attach to bubbles, they tend to be swept to the rear, leaving the front of the bubble free of particles, as shown in Figs. 5. Furthermore, because of the lack of attached particles, an un-loaded bubble will rise faster and overtake a loaded bubble or cluster ahead in its rise path, colliding with particles on its lowermost face. When this happens, the bubble that has been overtaking the cluster joins it, thereby increasing its buoyancy. It suggests that even if a rising bubble already has particles attached, the front of the bubble will be clear, and will be in a position to accept further collisions with bubbles ahead in its rise path. In the current circumstance, the collision process is further assisted because the cluster layer above the bed is essentially static. Clusters beneath the froth layer. When bubbles, particles and clusters arrived at the top of the column, it was observed that some formed a distinct layer there, beneath the froth. Normally, when bubbles arrive at the top of the liquid, they rise through the froth-liquid interface and enter the froth. In the present experiments, it appears that the capacity of the froth to absorb discrete bubbles and particles is limited. The most likely reason for the creation of the bubble layer beneath

8.2. Coarse particles removed from the overflow stream The overflow stream carries clusters of floatable coal out of the cell. The coarse particles are separated on a 500 µm wedge-wire sieve bend as shown in Fig. 6. To maintain a water balance, part of the recycle stream was 10

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Fig. 6. Screen product discharging from wedge-wire screen. No screen water was used in these tests except in Run 8. Product was collected manually using a rubber squeegee.

Fig. 7. Underflow tailings discharging from a continuously-operating NovaCell. From Jameson and Emer, 2019.

removed as overflow tailings. The high-shear aerator has been designed to produce bubbles of diameter 500 µm, which collide immediately after formation with fine and ultrafine particles in the recycle stream. These particles, once attached to a bubble, remain with it. When the stream enters the fluidized bed, the bubbles can rise into the froth layer, or they can combine into clusters and pass out in the overflow.

particles may be collected by bubbles in the liquid phase, which do not appear in the froth product. Accordingly, the concept of “froth recovery” has been introduced, to account for the mass of floatable particles that appear in the froth product, as a fraction of the particles that have been calculated to have been collected in the liquid phase (Vera et al., 1999; Alexander et al., 2003; Ata, 2012; Gharai and Venugopal, 2015). It has been assumed that the cause of the discrepancy between the recoveries in the collection zone and the froth zone is the “drop back” of particles from the froth. Thus some particles that have risen into the froth phase are thought to have become disengaged from the bubbles and have dropped back into the liquid. Drop back has been an elusive quantity to measure, and there have been a number of attempts (Falutsu and Dobby, 1989; Seaman et al., 2004; Ata and Jameson, 2013; Rahman, Ata and Jameson, 2012, 2013, 2015a, 2015b). The prevalence of bubble-particle clusters in the region beneath the froth may lead to a new interpretation of the phenomenon of ‘froth dropback’. It is possible that particles that had been collected in the slurry did not appear in the froth product, for the simple reason that they never entered the froth at all, but gathered in a layer beneath the froth until they detached from supporting bubbles and fell back into the slurry. It is certainly possible that particles will detach from the froth and drop back, especially coarse particles. However, the contribution of cluster formation to low froth recoveries should be taken into account.

8.3. Underflow tails discharge When a fine coal stream is processed at a top size of 2 mm, the gangue mineral associated with the coal matter will obviously be present in the same size range, from 0–2 mm. The gangue is usually a mixture of silica (quartz), silicates, and various aluminosilicates. For present purposes, it will be regarded as quartz or silica with a density of 2650 kg/m3. In continuous operation, the silica particles, being higher in density than the coal matter, will settle in the fluidization zone of the NovaCell, except for the finer silica slimes, which will be elutriated out of the bed by the fluidization water. The design of the bed is such that the elutriated silica particles are less than 300 µm in diameter, so the underflow tailings will consist mainly of siliceous particles in the size range 300 µm–2 mm. These particles are removed through a duct in the side wall of the column, level with the top of the fluidized bed. The position of the entry to the duct effectively fixes the upper boundary of the fluidized bed. The removed material forms an underflow tailings. In the P&ID diagram shown in Fig. 3, the peristaltic pump has a variable speed drive, so the underflow tailings flowrate can be controlled to give a high solids concentration as shown in Fig. 7. In a full-scale operation, the flowrate could be determined by a non-nuclear pulp density sensor acting on a control valve. The solids content can be as high as 70%, so this tailings stream can be ‘spadeable’, requiring little or no further dewatering. Fig. 7 shows an underflow tailings stream discharging from the NovaCell. The particles in this stream are in the size range 250–710 µm, from a chalcopyrite separation. The underflow from the flotation application described here would be similar, although the size range would be 300 µm–2 mm.

10. Summary and conclusions In this paper, we have described a new flotation device, that can recover particles over a broad particle size range. The device, known as the NovaCell, has been applied to the flotation of coarse coal samples, with a top size of 2 mm. In the NovaCell, coarse particles are allowed to settle in the base of a flotation column, where they can be fluidized by an upwardly flowing stream of liquid, which contains small air bubbles. As in conventional flotation cells, bubbles carrying hydrophobic particles rise into a froth layer at the top of the column, and pass out as a froth product. However, an important outcome from this work was a demonstration of the value of clusters as a means of improving the recovery of hydrophobic particles, not only in the froth phase but also in a recycle stream withdrawn from the column. Clusters have previously been observed, but their effectiveness in creating an entirely new product stream from a flotation cell has not previously been

9. Clusters and drop back In the modelling of flotation cells, the recoveries in the froth and liquid phases are often treated separately. It has been postulated that 11

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reported or explored. Here, it has been shown that cluster formation is an effect of first order significance, when applied to the recovery of hydrophobic particles in high concentrations. They formed within and above the fluidized bed, and rose to the top of the column to form a layer beneath the froth. Some clusters, together with heavily-laden single bubbles, were sufficiently buoyant to rise to the top of the liquid in the column, but did not have sufficient buoyancy to force their way into the froth. Instead, they formed an integral layer beneath the froth, which could be withdrawn as a separate flotation product. For convenience, the cluster stream was placed on a 500 µm screen, which separated the oversize particles as a high-quality flotation product. The slurry that passed through the screen was split into a tailings stream containing fine particles, and a stream that was aerated with fine bubbles in a highshear contactor. This stream was combined with new feed, and returned to the base of the cell to fluidize the bed of coarse particles. In another innovation, a second tailings stream was withdrawn, from the fluidized bed section in the column. This had the effect of maintaining the height of the fluidized bed at the desired position. Further, it was possible to design the discharge system so that it acted as a gravity separator, from which the coarse solids were withdrawn at a high concentration, and free water was allowed to return to the column. In summary, the NovaCell was configured to operate with two flotation product streams, from the froth and the wedge-wire sieve bend respectively; and two tailings streams, one from the overflow containing only fine particles, and the other from the top of the fluidized bed with coarse solids that had settled there. The results showed that the NovaCell flotation machine can successfully recover high-grade coal particles from a feed with a top size of 2 mm, in batch or continuous mode. The continuous tests are more revealing, because they represent a steady-state operation. The continuous tests showed:

The ash values in the various feeds were highest in the fractions less than 75 µm. High recoveries of combustibles were achieved for this fraction, although the product ashes were higher than the average, indicating that some of the ash material was entrained into the flotation product, or possibly that there is a high proportion of composites in the finest fractions. These could be in the form of fine silica particles with small inclusions of coal matter. It was not possible to determine the operating conditions that would minimise the product ash, due to limitations of time and feed availability. The conventional parameters that could be varied would be the froth depth, the air rate and the use of wash water. The optimum operating conditions could best be found, using a cell operating in continuous mode with feed drawn from an operating plant. Phenomena to do with cluster formation have been described. Clusters were easily formed and were sufficiently strong to be withdrawn from the flotation cell in a continuous suspension. Clusters have been observed in large quantities in the flotation of coarse chalcopyrite particles as well as coal. It is suggested that cluster formation could lead to sub-optimal froth recoveries attributed previously to drop back. Detachment of particles from the froth has been proposed as a cause of low froth recoveries. Another contributing factor could be cluster formation, in that some hydrophobic clusters could be of neutral or only slightly positive buoyancy, and could never enter the froth at all. Acknowledgements This project was funded by the Australian Coal Research Program, Project C25021, “Coarse particle flotation for the plant of the future”. This support is gratefully acknowledged. The NovaCell is the subject of patents, vested in Newcastle Innovation Limited on behalf of the University of Newcastle, NSW, Australia. Granted patents include US 9,085,000 Method and apparatus for flotation in a fluidized bed, 21 July 2015 and similar patents in Australia and elsewhere.

• Combustibles recovery ranged from 92.8 to 96.7%, average 94.2%. • Mass recovery or yield ranged from 83.5 to 88.7%, average 86.3%. • Flotation feed ash ranged from 15.1 to 25.5%, average 20.6%. • Product ash was in the range 10.5 to 14.8%, average 12.4%. • Tailings ash was in the range 49.8 to 79.7%, average 65.0%.

Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mineng.2019.106099.

Generally, more of the product came from the froth layer than from the overflow screen. Using the data shown in Table 1, the average proportion of total recovery that was attributable to the froth product was 65.4%. The device appears to be self-correcting to some extent. If the froth becomes loaded with particles, the excess can be carried out as clusters in the overflow stream. The froth was well able to deal with coarse particles. The froth product contained particles over the whole size range in the feed, from 0 to 2 mm effectively. The average fraction of particles in the froth product above 500 µm, represented 40.4% of the froth recovery. The residence time in the continuous runs was only 1.7 mins, indicating that the flotation rate constant is high, much higher than is found with mechanical cells, which in any case are unable to capture large particles. The froth product contained less entrained siliceous ash than expected, possibly due to the presence of coarse particles that propped open the gaps between bubbles, assisting drainage. The average d80 of the froth product in the continuous runs was 804 µm, and that of the screen product was 1728 µm. The froth is capable of capturing coal particles that are much larger than is possible with existing technologies. Each of these streams will be much easier to de-water than the flotation products from conventional machines, which are much finer. The underflow tailings represented approximately 60% of the overall tails flow, and could be discharged at a high solids fraction, of the order of 60–70% solids, a high value that would have valuable consequences for tailings dewatering and disposal, and tailings dam stability.

References Alexander, D.J., Franzidis, J.P., Manlapig, E.V., 2003. Froth recovery measurement in plant scale flotation cells. Miner. Eng. 16, 1197–1203. Ata, S., Jameson, G.J., 2005. The formation of bubble clusters in flotation cells. Int. J. Miner. Process. 76, 123–139. Ata, S., Chen, Z., Jameson, G.J., 2009. Improving the capture of coarse particles using bubble clusters. SME Annual Meeting, Denver, Colorado, 22–25 Feb. 2009. Ata, S., 2012. Phenomena in the froth phase of flotation—a review. Int. J. Miner. Process. 102, 1–12. Ata, S., Jameson, G.J., 2013. Recovery of coarse particles in the froth phase – a case study. Miner. Eng. 45, 121–127. Australian Metallurgical Coal Report, 2018. Minerals Council of Australia. www.minerals. org.au. BP Statistical Review of World Energy, 2017. https://www.bp.com/content/dam/bpcountry/de_ch/PDF/bp-statistical-review-of-world-energy-2017-full-report.pdf (accessed 14 March 2019). Chen, Z., Ata, S., Jameson, G.J., 2015a. Behaviour of bubble clusters in a turbulent flotation cell. Powder Technol. 269, 337–344. Chen, Z., Ata, S., Jameson, G.J., 2015b. Breakup and re-formation of bubble clusters in a flotation cell. Miner. Eng. 71, 16–20. Coal and Steel Report, 2009. World Coal Association, www.worldcoal.org. Falutsu, M., Dobby, G.S., 1989. Direct measurement of froth drop back and collection zone recovery in a laboratory flotation column. Miner. Eng. 2 (3), 377–386. Gaudin, A.M., 1932. Flotation. McGraw-Hill, New York. Gharai, M., Venugopal, R., 2015. Modelling of flotation process – an overview of different approaches. Miner. Process. Extract. Met. Review 37 (2), 120–133. Glembotskii, V.A., Klassen, V.A., Plaksin, I.N., 1963. Flotation. Primary Sources, New York. Hernandez-Aguilar, J.R., Coleman, R.G., Gomez, C.O., Finch, J.A., 2004. A comparison between capillary and imaging techniques for sizing bubbles in flotation systems. Miner. Eng. 17, 53–61. Jameson, G.J., Allum, P., 1984. A Survey of Bubble Sizes in Industrial Flotation Cells.

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