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An on-site evaluation of different flotation technologies for fine coal beneficiation
~
Vol. 7, Nos 5/6, pp. 699-714, 1994 Copyright(~)1994 Elsevier Science Ltd Printed in Great Britain 0892-6875/94 $7.00+0.00
Minerals Engineering,
) Pergamon 0892-6875(94)E0015-4
AN ON-SITE EVALUATION OF DIFFERENT FLOTATION TECHNOLOGIES FOR FINE COAL BENEFICIATION
M.C. HARRIS, J.-P. FRANZIDIS, A.W. BREED and D.A. DEGLON Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa
(Received 12 August 1993; accepted 18 October 1993)
ABSTRACT
This paper presents the results of an investigation of the performance of a number of different flotation cell technologies for the beneficiation of fine coal. The work was conducted on-site at the Grootegeluk Colliery in the northern Transvaal province of South Afi'ica, using a pilot-scale conventional colunm cell, a pilot-scale Jameson-type cell, and an air-sparged hydrocyclone (ASH). hz addition, characterisation and conventional batch flotation tests were conducted in the laboratory in the Department of Chemical Engineering at the University of Cape Town. All three units tested on-site demonstrated improved selectivity compared to conventional subaeration flotation, bt the colunm cell, optimum performance couM only be achieved at very low throughputs. Substantial losses of coal occurred in the coarser size fi'actions. The Jameson-type cell was able to operate effectively at about double the throughput of the column cell at shnilar recoveries. Coal recover y in the coarser size fractions was still poor, but better than that of the column cell. The ASH was characterised by a very high throughput, more than 150 times that of the column cell on the basis of solids capacity per unit cross-sectional area. However, the ASH required more than three times the reagent dosage of the other two units to achieve this. The ASH performed particularly well in the recovery of the coarser size fractions, but was less effective than the other cells on the finer size fi'actions. Overall, the best peu'brmancefor this application was that of the Jameson cell, owing to its higher capacity in comparison to the column cell. The high reagent requirement of the ASH makes this technology uneconomic in this application. Keywords
Coal flotation, column cell, Jameson cell, air-sparged hydrocyclone, particle size
INTRODUCTION
The increasing importance of fine coal beneficiation has received widespread recognition in South Africa. Discarding the fines onto dumps is expensive and environmentally hazardous, and represents a waste of a potentially valuable resource. The introduction of spiral plants on many South African collieries has to some extent addressed this problem. However, on most South African collieries coal that is too fine for spiral beneficiation is generally discarded, despite the fact that this material can represent as much as 5 % of run-of-mine production. 699
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M . C . HARRIS et al.
Froth flotation is used extensively worldwide for the beneficiation of fine coal, and it has been shown that most South African coals are also amenable to beneficiation by this method. Until recently, however, process options were limited to conventional subaeration flotation technology. This situation has now changed considerably, first with the advent of column flotation, followed by the introduction of a number of other novel flotation devices. With many South African collieries considering the introduction of flotation in the near future, there is a clear need to characterise the performance of these different technologies, in order to optimise equipment selection within the constraints of the requirements of each plant.
BACKGROUND This paper presents the results of an investigation of the performance of alternative flotation cell technologies for the beneficiation of coal fines, conducted on-site at the Grootegeluk Colliery in the northern Transvaal province of South Africa. Three flotation cell types with very different characteristics were investigated on a pilot scale, namely a conventional column cell, a Jameson-type cell, and an air-sparged hydrocyclone. Grootegeluk Colliery is situated in the Waterberg coalfield, which is South Africa's largest, containing approximately 46 % of the country's known coal reserves. The mine is owned and operated by the South African Iron and Steel Corporation Limited (ISCOR). Approximately 30 m tons of high volatile bituminous hard coal (consisting primarily of vitrinite, at 85 % by volume) are mined per annum by opencast methods. The run-of-mine coal has a high ash content, at around 40%. The washing plant produces a clean coal product, at about 10% ash, for use as a blend coking coal in ISCOR's steel works, and a middlings product, at around 33 % ash, as feed for the nearby Matimba power station. A fines treatment circuit is incorporated into the Grootegeluk washing plant, with spirals treating the -1000+300 I.tm coal and a flotation plant treating the -300 l.tm coal, the latter using conventional subaeration flotation cells. However, the flotation plant was not in operation at the time of this investigation (November/December 1992), owing to the fact that considerable difficulty was being experienced in achieving the grade required for addition of the concentrate to the metallurgical coal product (10% ash). At the time of the investigation, the -300 ~m coal fines were being discarded to the plant tailings. This investigation of alternative flotation cells was conducted on the material that would normally represent the Grootegeluk flotation plant feezl. Each unit was operated over a wide range of conditions over a period of 3 to 8 days in order to characterise the performance with respect to grade-recovery, throughput, reagent consumption, and particle size. In addition, a representative feed sample was collected over the period during which the testwork was conducted, and this was subjected to a detailed characterisation in the Department of Chemical Engineering at the University of Cape Town (UCT). As the conventional flotation plant at Grootegeluk was not in operation, a number of standard batch flotation experiments were also performed, for comparison with the results of the non-conventional flotation units tested on-site.
COAL CHARACTERISATION The representative sample of Grootegeluk coal fines that was collected during the onsite investigation was characterised in the laboratory at UCT using the following techniques: size, ash-by-size, float-and-sink and release flotation analyses. The float and sink analysis was performed using a centrifugal technique developed at UCT that enables rapid and accurate analysis of very fine coal samples [1].
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701
Flotation release analysis aims to establish the optimum performance that can be achieved by flotation [2]. The technique used in this investigation consisted of subjecting the sample to incremental flotation in a 3 litre Leeds batch flotation cell. Initially, the minimum reagent addition was used, and the most gentle operating conditions employed, so that only the most hydrophohic material reported to the concentrate. Gradually, by increasing the reagent addition and air rate, more and more material was recovered, until the cell was barren of floatable material. The results of the coal characterisation are summarised in Table 1. The results show that the sample was relatively coarse, with a high proportion (38.9 %) of + 150 ~m material. Material of this size is generally difficult to beneficiate by flotation. Only 34.8% of the material was in the optimum flotation range of - 1 5 0 + 2 5 I.tm, with 26.3% of the feed finer than 25 I.tm. T A B L E 1 Characterisation of Grootegeluk coal fines. SIZE ANALYSIS: Size Mass (%) (micron) +300 12.0 -300+150 26.9 -150+75 19.0 -75 + 2 5 15.8 -25 26.3 Overall 100.0
RELEASE ANALYSIS: Ash
(%) 24.5 37.0 35.9 36.3 51.7 39.0
FLOAT AND SINK ANALYSIS: Relative Cum. Cum. Density Floats Floats Mass (%) Ash (%) 1.35 29.1 2.9 1.40 37.8 4.3 1.45 43.9 5.4 1.50 47.5 7.0 1.55 51.0 8.5 1.60 54.3 9.1 1.70 58.2 11.6 1.80 60.8 12.9
Cum.
Cure.
Ash(%) Yield (%) 9.9 8.2 12.2 9.9 19.2 9.9 25.7 10.3 32.2 10.8 39.2 11.4 53.3 14.0 MACERAL COMPOSITION: 85 % vitrinite (by vol)
COAL RANK: High volatile bituminous "A" hard coal
The results of the ash determinations indicate that the -25 ~m size fraction had the highest ash content (in excess of 50%) and that the +300 I.tm material had the lowest ash content (about 24.5 %). The ash content of the intermediate size fractions was relatively constant, at between 35 and 37 %. The overall ash content was 39.0%. The high ash content of the -25 ~tm size fraction is typical of the fines from most collieries. The mineral component in most coals tends to be very friable and so the gangue concentrates into this "ultrafine" fraction during breakage. The float-and-sink analysis results indicate that the coal was fairly well liberated in terms of relative density properties. A yield of about 55% is indicated at 10% ash. The results of the release analysis indicate that the sample was not very floatable, and that the selectivity was poor. The lowest ash content obtained was greater than 9 %. A theoretical yield in the region of 25 % was indicated at an ash content in the region of 10%, compared to the value of about 55 % indicated by float-and-sink analysis.
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EQUIPMENT The Column Cell Column flotation has gained widespread recognition as an effective alternative to conventional flotation technology in many applications, including fine coal beneficiation. The technology has proved particularly effective in the selective separation of very fine material. The entrainment of gangue into the product can be significantly reduced, or eliminated, by the addition of wash water and the use of a deep troth. In addition, fine particle recovery can be enhanced by the quiescent collection environment. A problem with column cells, however, is their limited capacity in handling a high tonnage, low value material with a high product mass recovery, such as coal. The maximum throughput is constrained by the small area to volume ratio of this type of cell. Very large plants would be required to handle the tonnages produced by most collieries, so the capital investment required could be substantial. In addition, a number of flotation studies conducted on South African coals have shown that column cell performance can be adversely affected when the feed contains significant quantities of particles larger than 150 ~m [3, 4]. The "fine" coal which would represent the flotation feed at many South African collieries frequently contains a considerable proportion of + 150 ~m material. Grootegeluk Colliery is a good example of this. In this investigation, a pilot-scale column rig was used which was designed and built in the Department of Chemical Engineering at UCT. It consists of an agitated conditioning tank, variable speed peristaltic feed and tails pumps, a PID level controller, a compressor, an external "USBM" type sparger, a pressure and volumetric flow rate control panel and the column itself. A schematic diagram of the rig is shown in Figure 1. The colunm consists of 1.5 m lengths of PVC piping, with an internal diameter of 105 mm. The height of the column can be varied by the addition or removal of these column sections. The interface level is monitored using a conductivity probe, and maintained by PID control of the tailings pump rate. The topmost section, fitted with a launder box, is made from clear PVC, allowing the position of the pulp-froth interface to be visually monitored during column operation. A feed port situated at 1.0 m below the overflow lip was employed, and a column height of 6 m was used throughout the investigation. i,
LEVEL DETECTOR CONCENTRATE iFROTH ~!..: :, :. FEED SLURRY
PID CONTROLLER CONTROL PANEL FEED
i
PUMP
o0~goO
(2o0 i)
J oo oo
V~SH AND 8PARGERWATER
AGITATED FEED TANK
.,...-L2[_
COMPRE880R AIR ~I T A I : 8
PUMP (CONTROLLED)
Fig. 1 A Schematic diagram of the column cell pilot plant used on-site at Grootegeluk Colliery.
Evaluation
of different flotation technologies
703
The Jameson Cell The Jameson cell was developed jointly in the 1980's by Mount Isa Mines and Professor GJ Jameson of the University of Newcastle, in Australia [5]. It represents a significant modification of the original conventional column design, by the incorporation of a slurry-air contacting system based on the ejector principle. Slurry and gas come into intimate contact in a downcomer prior to entering the cell, promoting high particle collection kinetics. On entering the cell, the mineralised bubbles disengage from the tailings and enter the froth zone, which is handled in a fashion similar to that of the conventional column cell. Thus the major advantage ascribed to the Jameson cell is its increased capacity and reduced size (height) in comparison with the conventional column cell, while maintaining comparable selectivity by also employing a deep, washed froth zone. An additional advantage, however, is that the air required for flotation is induced by the cell, so that compressed air is not required. The Jameson-type flotation cell used in this work was designed and built in the Department of Chemical Engineering at UCT. A schematic diagram of the test rig is shown in Figure 2. The cell is operated using the same ancillary equipment as that used for the operation of the column cell. The cell consists primarily of the topmost (clear) piece of the column cell described above, with a downcomer suspended down the middle of the cell. The downcomer is 1.2 m in length, 19 mm in diameter, and manufactured from perspex. Aeration is induced in the downcomer by pumping the feed through a stainless steel orifice plate located at the top of the downcomer at a pressure of between 3 and 4 bar. The operation of the cell has been adapted to incorporate a recycle stream from the tailings to combine with the fresh feed stream to the aerator (see Figure 2). In this way, a constant flow rate can be maintained to the aerator, independent of any variation in the feed rate. This allows optimum aerator performance to be maintained, even at low volumetric feed rates. The feed to the system, the recycle stream and the tails from the cell were all pumped using variable speed peristaltic pumps. AERATOR[~
J
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9
SLURRY FEED TO AERATOR
INTERFACE
"Fi~
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i RECIRCULATION STREAM
CONTROL
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Fig.2 A Schematic diagram of the Jameson-type cell pilot plant used on-site at Grootegeluk Colliery.
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The Air-Sparged Hydrocyclone (ASH) The air-sparged hydrocyclone, or ASH, represents a radical departure from other flotation cell designs, both in appearance and mode of operation. It was designed in the early 1980's by Professor JD Miller of the University of Utah. Flotation takes place in a centrifugal field, which promotes the very rapid collection of fine particles [6]. It is similar in operation to a conventional cyclone, except that it contains an inner porous cylinder surrounded by a jacket that is pressurised with air. As slurry, fed tangentially, moves axially down the central cylinder, hydrophobic particles collide with (and attach to) air bubbles sparging through the cylinder walls. The particle-bubble aggregates move to the cyclone axis, and tbrm a froth phase, which moves axially up the ASH to be discharged in the overflow via a vortex finder. The tailings slurry is discharged at the base. The main advantage ascribed to the ASH is an extremely high throughput per unit volume, for a flotation performance comparable to conventional flotation cells. In previous studies conducted on a coal from the Witbank coalfield of South Africa, it was shown that it was possible to beneficiate the coal fines at a rate of up to 300 times greater than that possible using conventional flotation, and in the region of 1500 times greater than that which could be achieved in a column cell. However, it was also found that the unit required more than three times the optimum column cell collector dosage for effective operation [7, 8]. The ASH rig used in this work is shown schematically in Figure 3. It consists of a 900 1 rubber-lined agitated conditioning tank; a feed system comprising a centrifugal pump, bypass stream, valve and magnetic flow meter; a compressed air supply and the ASH unit itself. The ASH is 46 mm in diameter and either 300 mm or 500 mm in length. The inner porous cylinder is made from sintered bronze with an average pore size of 2 p.m. The outer non-porous jacket is made of PVC. An inlet measuring 15 mm x 12 ram, and a 46 mm long vortex-finder was used for this investigation.
CONCENTRATE
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VORTEX
POROUS CYLINDER
~ 1
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Fig.3 A Schematic diagram of the air-sparged hydrocyclone pilot plant used on-site at Grootegeluk Colliery.
FEED SLURRY
AGITATED FEED TANK (900 I)
Evaluation of different flotation technologies
705
EXPERIMENTAL Batch Flotation Experiments As the conventional flotation plant at Grootegeluk was not in operation, a number of standard batch flotation experiments were performed in the laboratory at UCT, on the sample collected during the on-site testing, for comparison with the results of the non-conventional flotation units. A total of 9 tests were performed using a modified 3 litre Leeds batch flotation cell. The range of operating conditions investigated in these tests is summarised in Table 2. T A B L E 2 Operating conditions employed in the investigation. CONDITIONS COMMON TO ALL THE TEST UNITS COllector Paraffin II Frother
2-Et Haxanol
i
LABORATORY BATCH CELL Collector dosage (I/t) Frother dosage (ml/t)
Teats: 9 0.3 - 12.3 II Pulp density (%) 150 - 2 0 0 ~ Air rate (l/mln)
7 6
COLUMN CELL Height (m) Slurry feed rate (I/min) Collector dosage (I/t) Wash water rate (I/min)
Tests: 20 6 Diameter (cm) 0.7 - 2.0 Air rate (I/rain) 2 - 10 Frother dosage (ml/t) 0.4 - 1.1 Froth height (cm)
10.5 6-11 50-320 20 - 40
JAMESON CELL Height (m) Slurry feed rate (I/mln) Collector dosage (I/t) Wash water rate (ltmin)
1.6 1.0 2 0.7
Tests: 14 Diameter (cm) - 4.0 Air rate (l/rain) 10 Frother dosage (ml/t) - 1.9 Froth height (cm)
AIR-SPARGED HYDROCYCLONE Height (m) Slurry feed rite (I/min) C o l i a ~ r dosage (I/t) Vortex finder diam. (mm)
Testa: 55 0.3; 0.5 Diameter (cm) 25 - 70 Air rate (l/rain) 3 - 40 Frother dosage (ml/t) 16.0; 21.5 Underflow config.
10.5 2 - 12 5 0 - 320 20; 25
4.6 120 - 3 6 0 60 - 1200 orifice; pedestal
The On-Site Investigation The on-site investigation was carried out in the fines beneficiation plant at Grootegeluk Colliery. The feed used throughout the test programme was obtained from a sump which receives the underflow from a sieve-bend cutting at 300 I.tm. This material would normally represent the feed to the Grootegeluk flotation plant. The sieve-bend overflow (-1000+300 I.tm) material forms the feed to the spiral plant. Slurry feed was obtained via a pipe fitted to the sump especially for test purposes, and was controlled by means of a valve. All the units were operated in a semi-batch mode. At the start-up of a particular unit, the feed tank was filled, and the required dose of collector and frother was added. Tests were then conducted on the unit until the level in the feed tank was too low for further operation, at which point the unit would be shut down, and the whole process would be repeated as often as required. Each unit was operated over a wide range of conditions so as to thoroughly characterise its performance in this application. The range of operating conditions employed on each of the cells is sunmaarised in Table 2. All the samples generated in the investigation were analysed for ash content in the laboratory at UCT, using the standard method. In addition, the concentrate and tailings samples generated in a number of the tests were subjected to size and ash-by-size determination.
706
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RESULTS Feed Variability During On-Site Operation It was not possible to operate the flotation units in parallel owing to equipment constraints, so each unit was operated in turn for a period ranging from a few days to more than a week. Reconstitution of the size distributions of some of the concentrate and tails samples from the on-site and laboratory tests indicated that the feed to the different flotation units on the mine varied from day-to-day and test-to-test. The extent of this variation is indicated in Table 3, which shows the variation of, and the average of, the amount of + 150 and -25 pm material in the feed to the different units during the time that they were tested. The composite sample, on which the batch cell was operated, was collected incrementally over the entire period of the on-site tests, and as such should represent a distribution close to the average tbr the test period. TABLE 3 Variability of the feed during the on-site operation.
Test Batch cell
Column cell
Unit Jameson cell
ASH
35 - 38
41 - 5 6
25 - 38
20 - 36
material in feed (%)
AVG: 36
AVG: 48
AVG: 29
AVG: 32
Mass of -25 micron
26 - 30
18 - 23
31 - 41
20 - 40
AVG: 27
AVG: 22
AVG: 37
AVG: 29
Mass of + 1 5 0 micron
material in feed (%) I
It is apparent that the feed size distributions to the different flotation cells varied considerably, This phenomenon is most apparent in the case of the ASH tests, as a result of these runs having been carried out over a longer period of time. Despite the feed size fluctuations, however, all the test units could be operated to produce acceptable results. Flotation Results As stated previously, all the units were operated over a very wide range of conditions in order to characterise their performance in this application as thoroughly as possible. The limits of each unit were explored, particularly with respect to throughput and reagent addition. As a result, many tests were conducted at conditions which resulted in very poor performance. This was particularly true of the ASH, on which a large number of tests (55) was performed. This unit proved to be extremely sensitive to the collector dosage; below about 20 l/t, no selective separation of concentrate was possible at any combination of levels of the other operating parameters. In order to simplify the analysis of the results and the comparison between the units, only those tests which were close to, or on, an optimum grade-recovery pertbrmance line for each unit are considered in this paper. On this basis, a global illustration of the "good" results obtained in the flotation tests on each of the units is presented in Figure 4. This shows the relationship between coal recovery (on a dry, ash free basis) and the proportion of ash, or noncombustible gangue, recovered to the concentrate. The 10% ash line shown in Figure 4 defines the amount of ash that can be recovered in the concentrate at a given coal recovery, while still maintaining the required product grade. Points above the line are indicative of unacceptable performance. It can be seen that with the exception of the conventional laboratory batch cell, the pertbrmance "envelope" of all the units was remarkably similar. Each of the three alternative technologies demonstrated the ability to achieve the required grade at coal recoveries of up to 40 %, which corresponds to a yield of about 28 %. The batch cell, however, was not able to produce an acceptable product grade in any of the tests.
Evaluation of different flotation technologies 14-
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+ JAMESON
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A-S N []
..............................................................................................................................................~,.._......_:::~.~:...... ~ ............................... D "'"'" 10 % ASHUNE J"" ~EQUIRED ~...=,-.gi ......................................I c o N e t m ~ r E J .............. ..... + I ORADEI /
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o
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. C O A L R E C O V E R Y (%)
Fig.4 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery for the different flotation units.
Recovery-by-Size Results The concentrate and tailings samples generated in a number of the flotation tests were subjected to size and ash-by-size determination, in order to determine the performance characteristics of each unit with respect to recovery and selectivity as a function of particle size. Tests were selected to cover a range of overall coal recoveries from about 20% to the maximum possible (typically 50 to 60%), at the best product grade. The optimum performance of each unit at the target product grade of 10% ash was approximately in the middle of this range. The relationship between coal recovery (dry, ash free) and concentrate ash recovery tbr the + 300 I.tm size fraction is presented in Figure 5 for each of the cell types. It can be seen that only the conventional batch cell and the ASH were capable of being operated so as to recover appreciable quantities of coal in this, the coarsest size fraction. Both units were able to achieve a recovery of up to 45 %. Next best was the Jameson cell at 16%, while the coal recovery from this fraction in the column cell was extremely poor, at less than 5 %. At low coal recoveries, the selectivity of the four cells was very similar. At higher coal recoveries, the selectivity achieved by the ASH was somewhat better than that obtained in the batch cell. Figures 6, 7, 8 and 9 show the analogous results for the 150 to 300 I.tm size fraction, the 75 to 150 I.tm size fraction, the 25 to 75 Izm size traction and the -25 Izm size fraction respectively. In the 150 to 300 Izm size fraction, coal recovery followed the same trend as observed for the + 300 l.tm fraction, with both the ASH and the batch cell achieving the highest coal recoveries (63 % and 57 % resp,), with the column cell again recovering the least material, at about 28 %. At higher coal recoveries, the ASH demonstrated markedly superior selectivity to the other cells; the selectivity achieved by the Jameson cell was particularly poor. In the 75 to 150 I.tm size fraction, differences in the maximum coal recovery achieved by each unit were less marked, ranging from 52 % for the Jameson cell to 68 % for the ASH. For this fraction, the column cell demonstrated the best selectivity, followed by the ASH, while the Jameson cell selectivity was superior to the batch cell at lower coal recoveries, and poorer at higher recoveries.
708
HARRIS et al.
M.C.
In the 25 to 75 I.tm size fraction, which is generally regarded as the optimum size material for beneficiation by flotation, the column cell achieved by far the best performance with respect to both coal recovery and selectivity. The selectivity of the ASH was similar, but the maximum coal recovery was substantially lower (58 % vs 80%). Once again, the Jameson cell was more selective than the batch cell at lower recoveries, but the maximum recovery achieved in the batch cell was somewhat higher (71% vs 58%). 14--mCOLUMN
12-
+300 MICRON SIZE FRACTION
JAMESON /.t
A-S.H
10-
'-~LAB BATCH
.....................................................................................................
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-
0
0
1'0
20
:30 40 COAL RECOVERY (%)
50
60
70
Fig.5 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the + 300 I.tm size fraction for the different flotation units.
14-
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-
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150 BY 300 MICRON SIZE FRACTION /
,
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0
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iO
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COAL RECOVERY (%)
Fig. 6 The recovery of ash, or noncombustible gangue, as a function of" clean coal recovery in the -300+ 150 ~m size fraction for the different flotation units.
Evaluation of different flotation technologies
709
In the -25 I.tm size fraction, the superiority of the column cell is again apparent. Once again, the Jameson
cell was superior to the batch cell at lower coal recoveries, and poorer at higher recoveries. Notable, however, is the very poor performance with respect to both recovery and selectivity achieved by the ASH on the "ultrafine" fraction.
i
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75 BY 150 MICRON SIZE FRACTION
t ...........................................................................................................................................................................................
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Fig.7 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the -150 +75 ~m size fraction for the different flotation units.
25 BY 75 MICRON SIZE FRACTION
-mCOLUMN JAMESON --)K-A-SLH
-eLAB BATCH
ui
.,o, nZ W
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~ ~:) COAL RECOVERY (%)
110
70
80
I
go
The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the -75 +25 ~m size fraction for the different flotation units.
710
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HARRIS et al.
25
-~-
LC°'u" 20-
-25 MICRON SIZE FRACTION
L
t/ ~"'" -tI . . . . . . . . . . . . . . ...................................................
/ :.-
...........
/
/
./.~ ....................
,12f/
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~" 10-
4"
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eoA, . s c o v s . Y (~)
Fig.9 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the -25 I~m size fraction for the different flotation units.
DISCUSSION The results in Figure 4 indicate that each of the flotation devices tested during the on-site pilot plant investigation was capable of beneficiating the Grootegeluk coal fines more efficiently than the conventional batch cell. In addition all the devices tested on-site were able to achieve coal recoveries in the region of 40% at ash contents below the required product grade of 10% ash. The flotation plant at Grootegeluk Colliery is at present not in use, as the conventional cells are unable to meet the required product ash specification, which suggests that the batch cell results obtained in this investigation can be taken to represent a reasonable approximation of the conventional flotation plant performance. The performance characteristics of the different flotation units at optimum conditions are summarised in Table 4. This presents the optimum result achieved on each of the cells at, or below, the required grade (10% ash), along with a summary of the performance with respect to particle size, the solids and volumetric capacities of the units on the basis of both unit cross-sectional area and unit volume, the reagent requirement of each unit for optimum performance, and the concentration of solids in the product slurry. It should be noted that although none of the batch experiments met the required grade criteria, the results of a test in which a yield was obtained that was similar to the optimum yield obtained on the other units has been included ltbr comparative purposes. In Table 4, the similar overall pertbrmance of the three units tested on-site is clearly apparent. The optimum yield differed by only about 1%, although the product grade produced by the ASH was somewhat better than either the Jameson or the colunm cells. On the basis of recovery by particle size, however, it can be seen in Table 4 that the performance of the different units on the fine (-25 I.tm), intermediate (-150+25 I~m) and coarse (+150 lam) fractions of the feed was very different. These differences were considerably larger than could be accounted for by variation in the particle size distribution of the feed to each unit. The column cell performed by far the best of all the units on both the fine and intermediate sizes, while performing very poorly on the coarse material (particularly the + 300 l.tm material, as shown in Figure 5). The poor coarse particle performance occurred despite the high proportion of this fraction in the teed
Evaluation of different flotation technologies
711
during the column tests, as indicated in Table 3. These results are in agreement with a number of other investigations of column flotation of South African coals [3, 4]. It is clear that if this technology is to be introduced for fine coal beneficiation in South Africa, the feed should be accurately sized to below 150 Izm. The + 150 l.tm material would in any case be more suited to spiral beneficiation. T A B L E 4 A comparison of the performance characteristics of the different flotation units at optimum conditions.
Optimum performance: Yield (%): Clean coal recovery (%): Concentrate ash (%): (required eonc grade: 10 % ash) Clean coal recovery/grade by Ilze: Coarse (+ 150 micron): Intermedlats (150x25 micron): Fine (-25 micron): Solids capacity of unit: XSectlonal area (t/hr/m2): Unit volume (Vhr/m3): Volumetric capacity of unit: XSectional area (m3/hr/m2): Unit volume (m3/br/m3): Reagent requirements: Collector (paraffin) (I/t): Frother (2-Et Hex) Iml/t): So,de In product slurry (%):
Laboratory Batch Cell
Column Cell
25.8 36.1 11.6#
28.4 38.4 9.9
Rec (%) 22.8 42.3 51.0
AIr-Sparged Hydrocyclone
27.7 38.7 9.8
27.1 39.5 7.2
Ash (%) Rec (%) 4.4 31.1 6.9 53.7 30.6 15.2
Ash (%) Rec (%) 3.3 24.5 43.4 6.9 16.6 48.1
Ash (%) Rec (%) 11.6 6.0 63.4 10.1 76.2 17.7
4 150
Teat Unit Jamelon-Type Cell
Ash (% 3.9 6.4 15.6
0.9 0.2
2.1 1.3
137 274
4.9 0.8
13.9 8.7
2090 4180
8 150 11
8 150
28 580 25
[# Unableto meetrequiredproductgrade]
The performance of the Jameson cell with respect to coal recovery as a function of particle size was remarkably similar to the batch cell in all the size ranges. Both cells followed the same trend as the column cell, with recovery increasing with decreasing particle size. However, both the fine and intermediate fractional recoveries were lower, with the difference made up by an increased recovery in the coarse fraction. The batch cell was characterised by a less selective separation, however, particularly in the finer sizes, and was therefore unable to achieve the required grade. The significant difference in the performance of the Jameson cell and the column cell with respect to particle size was a little surprising, as it has been suggested in the literature that the Jameson cell is most efficient for the collection of very fine particles [5, 9]. The main difference between the operation of the two cells was the fact that the Jameson cell was operated at a feed rate of more than double that of the colunm in nearly all the tests. This certainly accounts for the poorer selectivity of the Jameson cell relative to the column (see Figures 5 to 9), as more than double the concentrate was recovered from the same cell cross sectional area. The other important difference, however, was the use of a recycle stream in the operation of the Jameson cell used in this investigation. This probably accounts for the improvement in coarse particle recovery. The probability of particle-bubble detachment prior to recovery is high for coarse particles, so the recycle steam gives such particles more than one chance to be recovered.
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The reason for the drop in fine particle recovery in the Jameson cell is not clear fi'om the data. However, it can be postulated that the improved collection of clean coarse particles at optimum operating conditions resulted in a higher degree of rejection of the fines, which were of much less amenable to selective separation (see Figure 9, and Table 4). The effect of a recycle stream on Jameson cell performance certainly warrants further investigation. The performance as a function of particle size of the ASH was very different to that of the other cells. The performance in the coarse fraction was by far the best of all the cells tested, while the performance in the fine fraction was substantially poorer. This agrees with previously reported results that the ASH is less efficient at fine sizes [7]. The good coarse particle recovery probably can be ascribed to the very high collector addition to this unit relative to the other cells. Coarse particle hydrophobicity would be higher at higher collector additions, rendering these particles more floatable. The high proportion of coarse particles in the ASH concentrate probably accounts i-or the improved product grade of this cell relative to the others. Figures 4 and 5 indicate that the coarser fractions were the most amenable to selective separation. In Table 4 it can be seen that the ASH had by far the greatest capacity of all the units tested in the on-site investigation, irrespective of the basis used for the comparison. In terms of solids capacity per unit cross-sectional area, the ASH was able to handle 137 t/hr/m 2, more 150 times the column cell capacity on an equivalent basis. On the basis of unit volume, the difference was even greater at 1400 times the column capacity. As discussed previously, the solids capacity of the Jameson cell, while nowhere near that of the ASH, also proved to be substantially better than that of the column. The unit was found to perform optimally at about 2.1 t/hr/m 2, more than double the column capacity on the basis of unit cross-sectional area, and more than six times the column capacity on the basis of unit volume. The results obtained in the investigation indicated that the frother requirements of the batch, column, Jameson-type and ASH cells were similar*. In addition the column and Jameson-type cell collector dosage requirements were tbund to be similar. However, the ASH collector dosage was found to be considerably greater than that of either the column or Jameson-type cells. In Table 4, it can be seen that the ASH required the addition of 28 litres of reagent per ton of solids at optimum conditions, more than three and a half times the collector dosage required by the column and Jameson cells, and seven times the dosage required by the batch cell. A large number of tests were conducted on this unit in which every attempt was made to reduce the amount of collector required. However, below about 20 l/t, no upgrading of any kind occurred in the unit. In a previous investigation carried out using the same ASH pilot plant on coal from a colliery in the Witbank coalfield of South Africa, the collector dosage required was also in the region of three times the dosage required in a column cell [7]. This increased collector requirement, in comparison to that required in other cell types, seems to be a problem inherent in coal flotation using an ASH. However, recent investigations conducted on coal on larger scale units in the USA have indicated that the collector requirement reduces substantially with scale-up, and approaches that of other cells for units of around 16 inch diameter [10]. This clearly requires wider confirmation, which is difficult on such large capacity units. At this stage, it has to be concluded that unless the processed coal is used in an application in which oily collectors are of value (e.g. liquefaction), using the ASH for the beneficiation of South African coal does not appear to be economically feasible. In Table 4 it can be seen that the ASH produced the best result with respect to the highest solids concentration in the product slurry, at 25 %. The column was marginally better than the Jameson cell (11% vs 8 %), probably because of its lower throughput. The difference is probably not significant. Once again, the advantage of the ASH cannot offset the very high collector requirement. * The h i g h e r ffother addition to the A S H reported in Table 4 c a n be ascribed to the high collector d o s a g e , as the frother w a s a d d e d as a p r o p o r t i o n o f this. H o w e v e r , the results o f other tests suggest that the unit could h a v e been operated with a fi'other addition similar to thal e m p l o y e d on the other units without a n y p r o b l e m .
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CONCLUSIONS The results of this investigation indicate that each of the novel flotation technologies that was tested on-site at the Grootegeluk Colliery was capable of achieving a satisfactory performance at the required product grade of 10 % ash. In tests conducted using a conventional laboratory batch cell, it did not prove possible to achieve the required product grade at any of the conditions that were tested. While the three units tested on-site achieved a very similar overall performance at optimum conditions, the performance with respect to particle size was very different. The column cell performed by far the best of all the units on both the fine and intermediate sizes, while performing very poorly on the coarse material. The Jameson cell performed better than the column on the coarse material, probably due to the use of a recycle stream, but was less effective on the finer sizes, probably because it was operated at a considerably higher throughput. The ASH was particularly effective on the coarse size fractions, but was significantly poorer than either the column or the Jameson cells on the finer sizes. The ASH had by far the greatest capacity of all the units tested in the on-site investigation, and the column cell the lowest. In terms of solids throughput, the ASH was able to handle more 150 times that of the column on the basis of unit cross-sectional area, and 1370 times on the basis of unit volume. The Jameson cell was able to handle more than double the column throughput on the basis of unit cross-sectional area, and six times the throughput on the basis of unit volume. At optimum conditions, the collector requirements for the column and Jameson cells were found to be very similar, at around 8 l/t. However, the collector requirements of the ASH were considerably higher. This unit could not be operated below a collector addition of about 20 l/t, while about 28 1/t was required at optimum conditions. The collector requirement was therefore too high for this technology to be economically feasible in this application. It remains to be established whether this problem only occurs on small scale pilot units, such as the 2 inch cell used in this investigation. The results of this investigation support a postulate that certain cell technologies are more appropriate than others, depending on the size and grade-by-size distribution of the feed. Overall, the Jameson cell demonstrated the best performance for this particular application, owing to its significantly higher throughput in comparison to the column cell, and better coarse particle performance. The use of a recycle stream, while substantially improving the flexibility of the operation of the cell, also appears to improve coarse particle recovery. This certainly warrants further investigation.
ACKNOWLEDGEMENTS This project was sponsored by the South African Department of Mineral and Energy Affairs, and their support in gratefully acknowledged. In addition, the authors would like to thank the following persons: The personnel at the Grootegeluk Colliery for their assistance and co-operation during the perfbrmance of the on-site testwork. Helen Divey, Lorna Wall and Steve Amos for performing the coal characterisation, and batch flotation tests in the UCT laboratory.
REFERENCES 1.
ME 7/5-~-M
Franzidis, J.-P. & Harris, M.C., A new method for the rapid float-sink analysis of coal fines. J. S. Aft'. Inst. Mitt. Metall., 86, No. 10, 409 (1986).
714 2, 3.
.
.
.
7o 8. 9.
10.
M.C. HARRISet al. Dell, C.C., Release analysis, a new tool for ore dressing research, htst. Mitt. Metall. Symposium on Recent Developmentx in Mineral Dressing, London (1952). Franzidis J,-P., Harris M.C. & O'Connor, C.T., Review of colunm flotation practice on South African mines. Cohmm Flotation '91, Proceeding of an h~ternational Cot~rence on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 479, (1991). Harris, M.C., Franzidis, J.-P., O'Connor, C.T. & Stonestreet, P., An evaluation of the role of particle size in the flotation of coal using different cell technologies., Minerals Engineering, 5, nos 10-12, 1225-1238 (1992). Jameson, G.J. & Manlapig, E.V., Applications of the Jameson Flotation Cell. Column Flotation '91, Proceeding of an htternational Conference on Column Flotation (G. E. Agar, 13. J. Huls, D. B. Hyma, eds)Sudbury, Ontario, 673-687 (1991). Ye, Y., Gopalakrishnan, S., Pacquet, E. & Miller, J., Development of the air-sparged hydrocyclone - a swirl-flow flotation column. Cohmm Flotation '88 (K. V. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 305-313 (1988). Breed, A.W., Beneficiation of South African Coal Fines using the Air-Sparged Hydrocyclone. M.Sc. Thesis, University of Cape Town, (1992). Von Holt, S., An Investigation into Column Flotation of South African Coals. M. Sc. Thesis, University of Cape Town, (1992). Atkinson, B.W., Griffin, P.T., Jameson, G.J. & Espinosa-Gomez, R., Jameson cell test work on copper streams in the copper concentrator of Mount Isa Mines Limited. Proceedings ~fthe XVIlI International Minerals Processing Congress, the Australasian Institute of Mining and Metallurgy, Sydney, Australia, 826 (1993). Miller, J.D., personal conamunication, (1993).
Vol. 7, Nos 5/6, pp. 699-714, 1994 Copyright(~)1994 Elsevier Science Ltd Printed in Great Britain 0892-6875/94 $7.00+0.00
Minerals Engineering,
) Pergamon 0892-6875(94)E0015-4
AN ON-SITE EVALUATION OF DIFFERENT FLOTATION TECHNOLOGIES FOR FINE COAL BENEFICIATION
M.C. HARRIS, J.-P. FRANZIDIS, A.W. BREED and D.A. DEGLON Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa
(Received 12 August 1993; accepted 18 October 1993)
ABSTRACT
This paper presents the results of an investigation of the performance of a number of different flotation cell technologies for the beneficiation of fine coal. The work was conducted on-site at the Grootegeluk Colliery in the northern Transvaal province of South Afi'ica, using a pilot-scale conventional colunm cell, a pilot-scale Jameson-type cell, and an air-sparged hydrocyclone (ASH). hz addition, characterisation and conventional batch flotation tests were conducted in the laboratory in the Department of Chemical Engineering at the University of Cape Town. All three units tested on-site demonstrated improved selectivity compared to conventional subaeration flotation, bt the colunm cell, optimum performance couM only be achieved at very low throughputs. Substantial losses of coal occurred in the coarser size fi'actions. The Jameson-type cell was able to operate effectively at about double the throughput of the column cell at shnilar recoveries. Coal recover y in the coarser size fractions was still poor, but better than that of the column cell. The ASH was characterised by a very high throughput, more than 150 times that of the column cell on the basis of solids capacity per unit cross-sectional area. However, the ASH required more than three times the reagent dosage of the other two units to achieve this. The ASH performed particularly well in the recovery of the coarser size fractions, but was less effective than the other cells on the finer size fi'actions. Overall, the best peu'brmancefor this application was that of the Jameson cell, owing to its higher capacity in comparison to the column cell. The high reagent requirement of the ASH makes this technology uneconomic in this application. Keywords
Coal flotation, column cell, Jameson cell, air-sparged hydrocyclone, particle size
INTRODUCTION
The increasing importance of fine coal beneficiation has received widespread recognition in South Africa. Discarding the fines onto dumps is expensive and environmentally hazardous, and represents a waste of a potentially valuable resource. The introduction of spiral plants on many South African collieries has to some extent addressed this problem. However, on most South African collieries coal that is too fine for spiral beneficiation is generally discarded, despite the fact that this material can represent as much as 5 % of run-of-mine production. 699
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Froth flotation is used extensively worldwide for the beneficiation of fine coal, and it has been shown that most South African coals are also amenable to beneficiation by this method. Until recently, however, process options were limited to conventional subaeration flotation technology. This situation has now changed considerably, first with the advent of column flotation, followed by the introduction of a number of other novel flotation devices. With many South African collieries considering the introduction of flotation in the near future, there is a clear need to characterise the performance of these different technologies, in order to optimise equipment selection within the constraints of the requirements of each plant.
BACKGROUND This paper presents the results of an investigation of the performance of alternative flotation cell technologies for the beneficiation of coal fines, conducted on-site at the Grootegeluk Colliery in the northern Transvaal province of South Africa. Three flotation cell types with very different characteristics were investigated on a pilot scale, namely a conventional column cell, a Jameson-type cell, and an air-sparged hydrocyclone. Grootegeluk Colliery is situated in the Waterberg coalfield, which is South Africa's largest, containing approximately 46 % of the country's known coal reserves. The mine is owned and operated by the South African Iron and Steel Corporation Limited (ISCOR). Approximately 30 m tons of high volatile bituminous hard coal (consisting primarily of vitrinite, at 85 % by volume) are mined per annum by opencast methods. The run-of-mine coal has a high ash content, at around 40%. The washing plant produces a clean coal product, at about 10% ash, for use as a blend coking coal in ISCOR's steel works, and a middlings product, at around 33 % ash, as feed for the nearby Matimba power station. A fines treatment circuit is incorporated into the Grootegeluk washing plant, with spirals treating the -1000+300 I.tm coal and a flotation plant treating the -300 l.tm coal, the latter using conventional subaeration flotation cells. However, the flotation plant was not in operation at the time of this investigation (November/December 1992), owing to the fact that considerable difficulty was being experienced in achieving the grade required for addition of the concentrate to the metallurgical coal product (10% ash). At the time of the investigation, the -300 ~m coal fines were being discarded to the plant tailings. This investigation of alternative flotation cells was conducted on the material that would normally represent the Grootegeluk flotation plant feezl. Each unit was operated over a wide range of conditions over a period of 3 to 8 days in order to characterise the performance with respect to grade-recovery, throughput, reagent consumption, and particle size. In addition, a representative feed sample was collected over the period during which the testwork was conducted, and this was subjected to a detailed characterisation in the Department of Chemical Engineering at the University of Cape Town (UCT). As the conventional flotation plant at Grootegeluk was not in operation, a number of standard batch flotation experiments were also performed, for comparison with the results of the non-conventional flotation units tested on-site.
COAL CHARACTERISATION The representative sample of Grootegeluk coal fines that was collected during the onsite investigation was characterised in the laboratory at UCT using the following techniques: size, ash-by-size, float-and-sink and release flotation analyses. The float and sink analysis was performed using a centrifugal technique developed at UCT that enables rapid and accurate analysis of very fine coal samples [1].
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Flotation release analysis aims to establish the optimum performance that can be achieved by flotation [2]. The technique used in this investigation consisted of subjecting the sample to incremental flotation in a 3 litre Leeds batch flotation cell. Initially, the minimum reagent addition was used, and the most gentle operating conditions employed, so that only the most hydrophohic material reported to the concentrate. Gradually, by increasing the reagent addition and air rate, more and more material was recovered, until the cell was barren of floatable material. The results of the coal characterisation are summarised in Table 1. The results show that the sample was relatively coarse, with a high proportion (38.9 %) of + 150 ~m material. Material of this size is generally difficult to beneficiate by flotation. Only 34.8% of the material was in the optimum flotation range of - 1 5 0 + 2 5 I.tm, with 26.3% of the feed finer than 25 I.tm. T A B L E 1 Characterisation of Grootegeluk coal fines. SIZE ANALYSIS: Size Mass (%) (micron) +300 12.0 -300+150 26.9 -150+75 19.0 -75 + 2 5 15.8 -25 26.3 Overall 100.0
RELEASE ANALYSIS: Ash
(%) 24.5 37.0 35.9 36.3 51.7 39.0
FLOAT AND SINK ANALYSIS: Relative Cum. Cum. Density Floats Floats Mass (%) Ash (%) 1.35 29.1 2.9 1.40 37.8 4.3 1.45 43.9 5.4 1.50 47.5 7.0 1.55 51.0 8.5 1.60 54.3 9.1 1.70 58.2 11.6 1.80 60.8 12.9
Cum.
Cure.
Ash(%) Yield (%) 9.9 8.2 12.2 9.9 19.2 9.9 25.7 10.3 32.2 10.8 39.2 11.4 53.3 14.0 MACERAL COMPOSITION: 85 % vitrinite (by vol)
COAL RANK: High volatile bituminous "A" hard coal
The results of the ash determinations indicate that the -25 ~m size fraction had the highest ash content (in excess of 50%) and that the +300 I.tm material had the lowest ash content (about 24.5 %). The ash content of the intermediate size fractions was relatively constant, at between 35 and 37 %. The overall ash content was 39.0%. The high ash content of the -25 ~tm size fraction is typical of the fines from most collieries. The mineral component in most coals tends to be very friable and so the gangue concentrates into this "ultrafine" fraction during breakage. The float-and-sink analysis results indicate that the coal was fairly well liberated in terms of relative density properties. A yield of about 55% is indicated at 10% ash. The results of the release analysis indicate that the sample was not very floatable, and that the selectivity was poor. The lowest ash content obtained was greater than 9 %. A theoretical yield in the region of 25 % was indicated at an ash content in the region of 10%, compared to the value of about 55 % indicated by float-and-sink analysis.
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EQUIPMENT The Column Cell Column flotation has gained widespread recognition as an effective alternative to conventional flotation technology in many applications, including fine coal beneficiation. The technology has proved particularly effective in the selective separation of very fine material. The entrainment of gangue into the product can be significantly reduced, or eliminated, by the addition of wash water and the use of a deep troth. In addition, fine particle recovery can be enhanced by the quiescent collection environment. A problem with column cells, however, is their limited capacity in handling a high tonnage, low value material with a high product mass recovery, such as coal. The maximum throughput is constrained by the small area to volume ratio of this type of cell. Very large plants would be required to handle the tonnages produced by most collieries, so the capital investment required could be substantial. In addition, a number of flotation studies conducted on South African coals have shown that column cell performance can be adversely affected when the feed contains significant quantities of particles larger than 150 ~m [3, 4]. The "fine" coal which would represent the flotation feed at many South African collieries frequently contains a considerable proportion of + 150 ~m material. Grootegeluk Colliery is a good example of this. In this investigation, a pilot-scale column rig was used which was designed and built in the Department of Chemical Engineering at UCT. It consists of an agitated conditioning tank, variable speed peristaltic feed and tails pumps, a PID level controller, a compressor, an external "USBM" type sparger, a pressure and volumetric flow rate control panel and the column itself. A schematic diagram of the rig is shown in Figure 1. The colunm consists of 1.5 m lengths of PVC piping, with an internal diameter of 105 mm. The height of the column can be varied by the addition or removal of these column sections. The interface level is monitored using a conductivity probe, and maintained by PID control of the tailings pump rate. The topmost section, fitted with a launder box, is made from clear PVC, allowing the position of the pulp-froth interface to be visually monitored during column operation. A feed port situated at 1.0 m below the overflow lip was employed, and a column height of 6 m was used throughout the investigation. i,
LEVEL DETECTOR CONCENTRATE iFROTH ~!..: :, :. FEED SLURRY
PID CONTROLLER CONTROL PANEL FEED
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Fig. 1 A Schematic diagram of the column cell pilot plant used on-site at Grootegeluk Colliery.
Evaluation
of different flotation technologies
703
The Jameson Cell The Jameson cell was developed jointly in the 1980's by Mount Isa Mines and Professor GJ Jameson of the University of Newcastle, in Australia [5]. It represents a significant modification of the original conventional column design, by the incorporation of a slurry-air contacting system based on the ejector principle. Slurry and gas come into intimate contact in a downcomer prior to entering the cell, promoting high particle collection kinetics. On entering the cell, the mineralised bubbles disengage from the tailings and enter the froth zone, which is handled in a fashion similar to that of the conventional column cell. Thus the major advantage ascribed to the Jameson cell is its increased capacity and reduced size (height) in comparison with the conventional column cell, while maintaining comparable selectivity by also employing a deep, washed froth zone. An additional advantage, however, is that the air required for flotation is induced by the cell, so that compressed air is not required. The Jameson-type flotation cell used in this work was designed and built in the Department of Chemical Engineering at UCT. A schematic diagram of the test rig is shown in Figure 2. The cell is operated using the same ancillary equipment as that used for the operation of the column cell. The cell consists primarily of the topmost (clear) piece of the column cell described above, with a downcomer suspended down the middle of the cell. The downcomer is 1.2 m in length, 19 mm in diameter, and manufactured from perspex. Aeration is induced in the downcomer by pumping the feed through a stainless steel orifice plate located at the top of the downcomer at a pressure of between 3 and 4 bar. The operation of the cell has been adapted to incorporate a recycle stream from the tailings to combine with the fresh feed stream to the aerator (see Figure 2). In this way, a constant flow rate can be maintained to the aerator, independent of any variation in the feed rate. This allows optimum aerator performance to be maintained, even at low volumetric feed rates. The feed to the system, the recycle stream and the tails from the cell were all pumped using variable speed peristaltic pumps. AERATOR[~
J
CONCENTRATE
9
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Fig.2 A Schematic diagram of the Jameson-type cell pilot plant used on-site at Grootegeluk Colliery.
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The Air-Sparged Hydrocyclone (ASH) The air-sparged hydrocyclone, or ASH, represents a radical departure from other flotation cell designs, both in appearance and mode of operation. It was designed in the early 1980's by Professor JD Miller of the University of Utah. Flotation takes place in a centrifugal field, which promotes the very rapid collection of fine particles [6]. It is similar in operation to a conventional cyclone, except that it contains an inner porous cylinder surrounded by a jacket that is pressurised with air. As slurry, fed tangentially, moves axially down the central cylinder, hydrophobic particles collide with (and attach to) air bubbles sparging through the cylinder walls. The particle-bubble aggregates move to the cyclone axis, and tbrm a froth phase, which moves axially up the ASH to be discharged in the overflow via a vortex finder. The tailings slurry is discharged at the base. The main advantage ascribed to the ASH is an extremely high throughput per unit volume, for a flotation performance comparable to conventional flotation cells. In previous studies conducted on a coal from the Witbank coalfield of South Africa, it was shown that it was possible to beneficiate the coal fines at a rate of up to 300 times greater than that possible using conventional flotation, and in the region of 1500 times greater than that which could be achieved in a column cell. However, it was also found that the unit required more than three times the optimum column cell collector dosage for effective operation [7, 8]. The ASH rig used in this work is shown schematically in Figure 3. It consists of a 900 1 rubber-lined agitated conditioning tank; a feed system comprising a centrifugal pump, bypass stream, valve and magnetic flow meter; a compressed air supply and the ASH unit itself. The ASH is 46 mm in diameter and either 300 mm or 500 mm in length. The inner porous cylinder is made from sintered bronze with an average pore size of 2 p.m. The outer non-porous jacket is made of PVC. An inlet measuring 15 mm x 12 ram, and a 46 mm long vortex-finder was used for this investigation.
CONCENTRATE
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Fig.3 A Schematic diagram of the air-sparged hydrocyclone pilot plant used on-site at Grootegeluk Colliery.
FEED SLURRY
AGITATED FEED TANK (900 I)
Evaluation of different flotation technologies
705
EXPERIMENTAL Batch Flotation Experiments As the conventional flotation plant at Grootegeluk was not in operation, a number of standard batch flotation experiments were performed in the laboratory at UCT, on the sample collected during the on-site testing, for comparison with the results of the non-conventional flotation units. A total of 9 tests were performed using a modified 3 litre Leeds batch flotation cell. The range of operating conditions investigated in these tests is summarised in Table 2. T A B L E 2 Operating conditions employed in the investigation. CONDITIONS COMMON TO ALL THE TEST UNITS COllector Paraffin II Frother
2-Et Haxanol
i
LABORATORY BATCH CELL Collector dosage (I/t) Frother dosage (ml/t)
Teats: 9 0.3 - 12.3 II Pulp density (%) 150 - 2 0 0 ~ Air rate (l/mln)
7 6
COLUMN CELL Height (m) Slurry feed rate (I/min) Collector dosage (I/t) Wash water rate (I/min)
Tests: 20 6 Diameter (cm) 0.7 - 2.0 Air rate (I/rain) 2 - 10 Frother dosage (ml/t) 0.4 - 1.1 Froth height (cm)
10.5 6-11 50-320 20 - 40
JAMESON CELL Height (m) Slurry feed rate (I/mln) Collector dosage (I/t) Wash water rate (ltmin)
1.6 1.0 2 0.7
Tests: 14 Diameter (cm) - 4.0 Air rate (l/rain) 10 Frother dosage (ml/t) - 1.9 Froth height (cm)
AIR-SPARGED HYDROCYCLONE Height (m) Slurry feed rite (I/min) C o l i a ~ r dosage (I/t) Vortex finder diam. (mm)
Testa: 55 0.3; 0.5 Diameter (cm) 25 - 70 Air rate (l/rain) 3 - 40 Frother dosage (ml/t) 16.0; 21.5 Underflow config.
10.5 2 - 12 5 0 - 320 20; 25
4.6 120 - 3 6 0 60 - 1200 orifice; pedestal
The On-Site Investigation The on-site investigation was carried out in the fines beneficiation plant at Grootegeluk Colliery. The feed used throughout the test programme was obtained from a sump which receives the underflow from a sieve-bend cutting at 300 I.tm. This material would normally represent the feed to the Grootegeluk flotation plant. The sieve-bend overflow (-1000+300 I.tm) material forms the feed to the spiral plant. Slurry feed was obtained via a pipe fitted to the sump especially for test purposes, and was controlled by means of a valve. All the units were operated in a semi-batch mode. At the start-up of a particular unit, the feed tank was filled, and the required dose of collector and frother was added. Tests were then conducted on the unit until the level in the feed tank was too low for further operation, at which point the unit would be shut down, and the whole process would be repeated as often as required. Each unit was operated over a wide range of conditions so as to thoroughly characterise its performance in this application. The range of operating conditions employed on each of the cells is sunmaarised in Table 2. All the samples generated in the investigation were analysed for ash content in the laboratory at UCT, using the standard method. In addition, the concentrate and tailings samples generated in a number of the tests were subjected to size and ash-by-size determination.
706
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RESULTS Feed Variability During On-Site Operation It was not possible to operate the flotation units in parallel owing to equipment constraints, so each unit was operated in turn for a period ranging from a few days to more than a week. Reconstitution of the size distributions of some of the concentrate and tails samples from the on-site and laboratory tests indicated that the feed to the different flotation units on the mine varied from day-to-day and test-to-test. The extent of this variation is indicated in Table 3, which shows the variation of, and the average of, the amount of + 150 and -25 pm material in the feed to the different units during the time that they were tested. The composite sample, on which the batch cell was operated, was collected incrementally over the entire period of the on-site tests, and as such should represent a distribution close to the average tbr the test period. TABLE 3 Variability of the feed during the on-site operation.
Test Batch cell
Column cell
Unit Jameson cell
ASH
35 - 38
41 - 5 6
25 - 38
20 - 36
material in feed (%)
AVG: 36
AVG: 48
AVG: 29
AVG: 32
Mass of -25 micron
26 - 30
18 - 23
31 - 41
20 - 40
AVG: 27
AVG: 22
AVG: 37
AVG: 29
Mass of + 1 5 0 micron
material in feed (%) I
It is apparent that the feed size distributions to the different flotation cells varied considerably, This phenomenon is most apparent in the case of the ASH tests, as a result of these runs having been carried out over a longer period of time. Despite the feed size fluctuations, however, all the test units could be operated to produce acceptable results. Flotation Results As stated previously, all the units were operated over a very wide range of conditions in order to characterise their performance in this application as thoroughly as possible. The limits of each unit were explored, particularly with respect to throughput and reagent addition. As a result, many tests were conducted at conditions which resulted in very poor performance. This was particularly true of the ASH, on which a large number of tests (55) was performed. This unit proved to be extremely sensitive to the collector dosage; below about 20 l/t, no selective separation of concentrate was possible at any combination of levels of the other operating parameters. In order to simplify the analysis of the results and the comparison between the units, only those tests which were close to, or on, an optimum grade-recovery pertbrmance line for each unit are considered in this paper. On this basis, a global illustration of the "good" results obtained in the flotation tests on each of the units is presented in Figure 4. This shows the relationship between coal recovery (on a dry, ash free basis) and the proportion of ash, or noncombustible gangue, recovered to the concentrate. The 10% ash line shown in Figure 4 defines the amount of ash that can be recovered in the concentrate at a given coal recovery, while still maintaining the required product grade. Points above the line are indicative of unacceptable performance. It can be seen that with the exception of the conventional laboratory batch cell, the pertbrmance "envelope" of all the units was remarkably similar. Each of the three alternative technologies demonstrated the ability to achieve the required grade at coal recoveries of up to 40 %, which corresponds to a yield of about 28 %. The batch cell, however, was not able to produce an acceptable product grade in any of the tests.
Evaluation of different flotation technologies 14-
707 ÷
COLUMN
g¢ tU
12-
+ JAMESON
10-
A-S N []
..............................................................................................................................................~,.._......_:::~.~:...... ~ ............................... D "'"'" 10 % ASHUNE J"" ~EQUIRED ~...=,-.gi ......................................I c o N e t m ~ r E J .............. ..... + I ORADEI /
8 ......................................................................................................................................................
•
0
g
,,J
w
/"'"
•
t
|
÷
...................................................................................................... i..~._.:~:':........................ I t .................................................................................................. d2m...-'" 7"'"
+
..-'" ~
+
)l(
-t*
~.-'~ Od
"
o
;o
. C O A L R E C O V E R Y (%)
Fig.4 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery for the different flotation units.
Recovery-by-Size Results The concentrate and tailings samples generated in a number of the flotation tests were subjected to size and ash-by-size determination, in order to determine the performance characteristics of each unit with respect to recovery and selectivity as a function of particle size. Tests were selected to cover a range of overall coal recoveries from about 20% to the maximum possible (typically 50 to 60%), at the best product grade. The optimum performance of each unit at the target product grade of 10% ash was approximately in the middle of this range. The relationship between coal recovery (dry, ash free) and concentrate ash recovery tbr the + 300 I.tm size fraction is presented in Figure 5 for each of the cell types. It can be seen that only the conventional batch cell and the ASH were capable of being operated so as to recover appreciable quantities of coal in this, the coarsest size fraction. Both units were able to achieve a recovery of up to 45 %. Next best was the Jameson cell at 16%, while the coal recovery from this fraction in the column cell was extremely poor, at less than 5 %. At low coal recoveries, the selectivity of the four cells was very similar. At higher coal recoveries, the selectivity achieved by the ASH was somewhat better than that obtained in the batch cell. Figures 6, 7, 8 and 9 show the analogous results for the 150 to 300 I.tm size fraction, the 75 to 150 I.tm size fraction, the 25 to 75 Izm size traction and the -25 Izm size fraction respectively. In the 150 to 300 Izm size fraction, coal recovery followed the same trend as observed for the + 300 l.tm fraction, with both the ASH and the batch cell achieving the highest coal recoveries (63 % and 57 % resp,), with the column cell again recovering the least material, at about 28 %. At higher coal recoveries, the ASH demonstrated markedly superior selectivity to the other cells; the selectivity achieved by the Jameson cell was particularly poor. In the 75 to 150 I.tm size fraction, differences in the maximum coal recovery achieved by each unit were less marked, ranging from 52 % for the Jameson cell to 68 % for the ASH. For this fraction, the column cell demonstrated the best selectivity, followed by the ASH, while the Jameson cell selectivity was superior to the batch cell at lower coal recoveries, and poorer at higher recoveries.
708
HARRIS et al.
M.C.
In the 25 to 75 I.tm size fraction, which is generally regarded as the optimum size material for beneficiation by flotation, the column cell achieved by far the best performance with respect to both coal recovery and selectivity. The selectivity of the ASH was similar, but the maximum coal recovery was substantially lower (58 % vs 80%). Once again, the Jameson cell was more selective than the batch cell at lower recoveries, but the maximum recovery achieved in the batch cell was somewhat higher (71% vs 58%). 14--mCOLUMN
12-
+300 MICRON SIZE FRACTION
JAMESON /.t
A-S.H
10-
'-~LAB BATCH
.....................................................................................................
/f ,! ! /
S .....................................................................
> 0 0 w n3: u) <
:~:~ ............................................................................................
/
J
.J
.............................................................................................................
1"
6 ..................................................................
J~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-
0
0
1'0
20
:30 40 COAL RECOVERY (%)
50
60
70
Fig.5 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the + 300 I.tm size fraction for the different flotation units.
14-
-m-
-
10-
g:
i11
150 BY 300 MICRON SIZE FRACTION /
,
....................................................................
.........
...............................
Z...../
...................................................................... i?..I......................... y,:~ ................. .~
8
..............
...................................................................
,o w "I"
6"
4
iii
.
o i
0
1'0
20
:30
iO
80
60
70
COAL RECOVERY (%)
Fig. 6 The recovery of ash, or noncombustible gangue, as a function of" clean coal recovery in the -300+ 150 ~m size fraction for the different flotation units.
Evaluation of different flotation technologies
709
In the -25 I.tm size fraction, the superiority of the column cell is again apparent. Once again, the Jameson
cell was superior to the batch cell at lower coal recoveries, and poorer at higher recoveries. Notable, however, is the very poor performance with respect to both recovery and selectivity achieved by the ASH on the "ultrafine" fraction.
i
C+~LUMN
1
A.Sk.
I
75 BY 150 MICRON SIZE FRACTION
t ...........................................................................................................................................................................................
15uI
-oo Z
+i/-J ..................................................................................................... 10- ......................................................................................................
0
o
1'o
~o
~o
~o
so
~o
io
8o
go
COAL RECOVERY (%)
Fig.7 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the -150 +75 ~m size fraction for the different flotation units.
25 BY 75 MICRON SIZE FRACTION
-mCOLUMN JAMESON --)K-A-SLH
-eLAB BATCH
ui
.,o, nZ W
y..'JY"
Ol
0
Fig.8
1'0
20
:30
~ ~:) COAL RECOVERY (%)
110
70
80
I
go
The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the -75 +25 ~m size fraction for the different flotation units.
710
M.C.
HARRIS et al.
25
-~-
LC°'u" 20-
-25 MICRON SIZE FRACTION
L
t/ ~"'" -tI . . . . . . . . . . . . . . ...................................................
/ :.-
...........
/
/
./.~ ....................
,12f/
ul 0
~" 10-
4"
0
eoA, . s c o v s . Y (~)
Fig.9 The recovery of ash, or noncombustible gangue, as a function of clean coal recovery in the -25 I~m size fraction for the different flotation units.
DISCUSSION The results in Figure 4 indicate that each of the flotation devices tested during the on-site pilot plant investigation was capable of beneficiating the Grootegeluk coal fines more efficiently than the conventional batch cell. In addition all the devices tested on-site were able to achieve coal recoveries in the region of 40% at ash contents below the required product grade of 10% ash. The flotation plant at Grootegeluk Colliery is at present not in use, as the conventional cells are unable to meet the required product ash specification, which suggests that the batch cell results obtained in this investigation can be taken to represent a reasonable approximation of the conventional flotation plant performance. The performance characteristics of the different flotation units at optimum conditions are summarised in Table 4. This presents the optimum result achieved on each of the cells at, or below, the required grade (10% ash), along with a summary of the performance with respect to particle size, the solids and volumetric capacities of the units on the basis of both unit cross-sectional area and unit volume, the reagent requirement of each unit for optimum performance, and the concentration of solids in the product slurry. It should be noted that although none of the batch experiments met the required grade criteria, the results of a test in which a yield was obtained that was similar to the optimum yield obtained on the other units has been included ltbr comparative purposes. In Table 4, the similar overall pertbrmance of the three units tested on-site is clearly apparent. The optimum yield differed by only about 1%, although the product grade produced by the ASH was somewhat better than either the Jameson or the colunm cells. On the basis of recovery by particle size, however, it can be seen in Table 4 that the performance of the different units on the fine (-25 I.tm), intermediate (-150+25 I~m) and coarse (+150 lam) fractions of the feed was very different. These differences were considerably larger than could be accounted for by variation in the particle size distribution of the feed to each unit. The column cell performed by far the best of all the units on both the fine and intermediate sizes, while performing very poorly on the coarse material (particularly the + 300 l.tm material, as shown in Figure 5). The poor coarse particle performance occurred despite the high proportion of this fraction in the teed
Evaluation of different flotation technologies
711
during the column tests, as indicated in Table 3. These results are in agreement with a number of other investigations of column flotation of South African coals [3, 4]. It is clear that if this technology is to be introduced for fine coal beneficiation in South Africa, the feed should be accurately sized to below 150 Izm. The + 150 l.tm material would in any case be more suited to spiral beneficiation. T A B L E 4 A comparison of the performance characteristics of the different flotation units at optimum conditions.
Optimum performance: Yield (%): Clean coal recovery (%): Concentrate ash (%): (required eonc grade: 10 % ash) Clean coal recovery/grade by Ilze: Coarse (+ 150 micron): Intermedlats (150x25 micron): Fine (-25 micron): Solids capacity of unit: XSectlonal area (t/hr/m2): Unit volume (Vhr/m3): Volumetric capacity of unit: XSectional area (m3/hr/m2): Unit volume (m3/br/m3): Reagent requirements: Collector (paraffin) (I/t): Frother (2-Et Hex) Iml/t): So,de In product slurry (%):
Laboratory Batch Cell
Column Cell
25.8 36.1 11.6#
28.4 38.4 9.9
Rec (%) 22.8 42.3 51.0
AIr-Sparged Hydrocyclone
27.7 38.7 9.8
27.1 39.5 7.2
Ash (%) Rec (%) 4.4 31.1 6.9 53.7 30.6 15.2
Ash (%) Rec (%) 3.3 24.5 43.4 6.9 16.6 48.1
Ash (%) Rec (%) 11.6 6.0 63.4 10.1 76.2 17.7
4 150
Teat Unit Jamelon-Type Cell
Ash (% 3.9 6.4 15.6
0.9 0.2
2.1 1.3
137 274
4.9 0.8
13.9 8.7
2090 4180
8 150 11
8 150
28 580 25
[# Unableto meetrequiredproductgrade]
The performance of the Jameson cell with respect to coal recovery as a function of particle size was remarkably similar to the batch cell in all the size ranges. Both cells followed the same trend as the column cell, with recovery increasing with decreasing particle size. However, both the fine and intermediate fractional recoveries were lower, with the difference made up by an increased recovery in the coarse fraction. The batch cell was characterised by a less selective separation, however, particularly in the finer sizes, and was therefore unable to achieve the required grade. The significant difference in the performance of the Jameson cell and the column cell with respect to particle size was a little surprising, as it has been suggested in the literature that the Jameson cell is most efficient for the collection of very fine particles [5, 9]. The main difference between the operation of the two cells was the fact that the Jameson cell was operated at a feed rate of more than double that of the colunm in nearly all the tests. This certainly accounts for the poorer selectivity of the Jameson cell relative to the column (see Figures 5 to 9), as more than double the concentrate was recovered from the same cell cross sectional area. The other important difference, however, was the use of a recycle stream in the operation of the Jameson cell used in this investigation. This probably accounts for the improvement in coarse particle recovery. The probability of particle-bubble detachment prior to recovery is high for coarse particles, so the recycle steam gives such particles more than one chance to be recovered.
712
M.C.
HARRIS et al.
The reason for the drop in fine particle recovery in the Jameson cell is not clear fi'om the data. However, it can be postulated that the improved collection of clean coarse particles at optimum operating conditions resulted in a higher degree of rejection of the fines, which were of much less amenable to selective separation (see Figure 9, and Table 4). The effect of a recycle stream on Jameson cell performance certainly warrants further investigation. The performance as a function of particle size of the ASH was very different to that of the other cells. The performance in the coarse fraction was by far the best of all the cells tested, while the performance in the fine fraction was substantially poorer. This agrees with previously reported results that the ASH is less efficient at fine sizes [7]. The good coarse particle recovery probably can be ascribed to the very high collector addition to this unit relative to the other cells. Coarse particle hydrophobicity would be higher at higher collector additions, rendering these particles more floatable. The high proportion of coarse particles in the ASH concentrate probably accounts i-or the improved product grade of this cell relative to the others. Figures 4 and 5 indicate that the coarser fractions were the most amenable to selective separation. In Table 4 it can be seen that the ASH had by far the greatest capacity of all the units tested in the on-site investigation, irrespective of the basis used for the comparison. In terms of solids capacity per unit cross-sectional area, the ASH was able to handle 137 t/hr/m 2, more 150 times the column cell capacity on an equivalent basis. On the basis of unit volume, the difference was even greater at 1400 times the column capacity. As discussed previously, the solids capacity of the Jameson cell, while nowhere near that of the ASH, also proved to be substantially better than that of the column. The unit was found to perform optimally at about 2.1 t/hr/m 2, more than double the column capacity on the basis of unit cross-sectional area, and more than six times the column capacity on the basis of unit volume. The results obtained in the investigation indicated that the frother requirements of the batch, column, Jameson-type and ASH cells were similar*. In addition the column and Jameson-type cell collector dosage requirements were tbund to be similar. However, the ASH collector dosage was found to be considerably greater than that of either the column or Jameson-type cells. In Table 4, it can be seen that the ASH required the addition of 28 litres of reagent per ton of solids at optimum conditions, more than three and a half times the collector dosage required by the column and Jameson cells, and seven times the dosage required by the batch cell. A large number of tests were conducted on this unit in which every attempt was made to reduce the amount of collector required. However, below about 20 l/t, no upgrading of any kind occurred in the unit. In a previous investigation carried out using the same ASH pilot plant on coal from a colliery in the Witbank coalfield of South Africa, the collector dosage required was also in the region of three times the dosage required in a column cell [7]. This increased collector requirement, in comparison to that required in other cell types, seems to be a problem inherent in coal flotation using an ASH. However, recent investigations conducted on coal on larger scale units in the USA have indicated that the collector requirement reduces substantially with scale-up, and approaches that of other cells for units of around 16 inch diameter [10]. This clearly requires wider confirmation, which is difficult on such large capacity units. At this stage, it has to be concluded that unless the processed coal is used in an application in which oily collectors are of value (e.g. liquefaction), using the ASH for the beneficiation of South African coal does not appear to be economically feasible. In Table 4 it can be seen that the ASH produced the best result with respect to the highest solids concentration in the product slurry, at 25 %. The column was marginally better than the Jameson cell (11% vs 8 %), probably because of its lower throughput. The difference is probably not significant. Once again, the advantage of the ASH cannot offset the very high collector requirement. * The h i g h e r ffother addition to the A S H reported in Table 4 c a n be ascribed to the high collector d o s a g e , as the frother w a s a d d e d as a p r o p o r t i o n o f this. H o w e v e r , the results o f other tests suggest that the unit could h a v e been operated with a fi'other addition similar to thal e m p l o y e d on the other units without a n y p r o b l e m .
Evaluation of different flotation technologies
713
CONCLUSIONS The results of this investigation indicate that each of the novel flotation technologies that was tested on-site at the Grootegeluk Colliery was capable of achieving a satisfactory performance at the required product grade of 10 % ash. In tests conducted using a conventional laboratory batch cell, it did not prove possible to achieve the required product grade at any of the conditions that were tested. While the three units tested on-site achieved a very similar overall performance at optimum conditions, the performance with respect to particle size was very different. The column cell performed by far the best of all the units on both the fine and intermediate sizes, while performing very poorly on the coarse material. The Jameson cell performed better than the column on the coarse material, probably due to the use of a recycle stream, but was less effective on the finer sizes, probably because it was operated at a considerably higher throughput. The ASH was particularly effective on the coarse size fractions, but was significantly poorer than either the column or the Jameson cells on the finer sizes. The ASH had by far the greatest capacity of all the units tested in the on-site investigation, and the column cell the lowest. In terms of solids throughput, the ASH was able to handle more 150 times that of the column on the basis of unit cross-sectional area, and 1370 times on the basis of unit volume. The Jameson cell was able to handle more than double the column throughput on the basis of unit cross-sectional area, and six times the throughput on the basis of unit volume. At optimum conditions, the collector requirements for the column and Jameson cells were found to be very similar, at around 8 l/t. However, the collector requirements of the ASH were considerably higher. This unit could not be operated below a collector addition of about 20 l/t, while about 28 1/t was required at optimum conditions. The collector requirement was therefore too high for this technology to be economically feasible in this application. It remains to be established whether this problem only occurs on small scale pilot units, such as the 2 inch cell used in this investigation. The results of this investigation support a postulate that certain cell technologies are more appropriate than others, depending on the size and grade-by-size distribution of the feed. Overall, the Jameson cell demonstrated the best performance for this particular application, owing to its significantly higher throughput in comparison to the column cell, and better coarse particle performance. The use of a recycle stream, while substantially improving the flexibility of the operation of the cell, also appears to improve coarse particle recovery. This certainly warrants further investigation.
ACKNOWLEDGEMENTS This project was sponsored by the South African Department of Mineral and Energy Affairs, and their support in gratefully acknowledged. In addition, the authors would like to thank the following persons: The personnel at the Grootegeluk Colliery for their assistance and co-operation during the perfbrmance of the on-site testwork. Helen Divey, Lorna Wall and Steve Amos for performing the coal characterisation, and batch flotation tests in the UCT laboratory.
REFERENCES 1.
ME 7/5-~-M
Franzidis, J.-P. & Harris, M.C., A new method for the rapid float-sink analysis of coal fines. J. S. Aft'. Inst. Mitt. Metall., 86, No. 10, 409 (1986).
714 2, 3.
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.
.
7o 8. 9.
10.
M.C. HARRISet al. Dell, C.C., Release analysis, a new tool for ore dressing research, htst. Mitt. Metall. Symposium on Recent Developmentx in Mineral Dressing, London (1952). Franzidis J,-P., Harris M.C. & O'Connor, C.T., Review of colunm flotation practice on South African mines. Cohmm Flotation '91, Proceeding of an h~ternational Cot~rence on Column Flotation (G. E. Agar, B. J. Huls, D. B. Hyma, eds) Sudbury, Ontario, 479, (1991). Harris, M.C., Franzidis, J.-P., O'Connor, C.T. & Stonestreet, P., An evaluation of the role of particle size in the flotation of coal using different cell technologies., Minerals Engineering, 5, nos 10-12, 1225-1238 (1992). Jameson, G.J. & Manlapig, E.V., Applications of the Jameson Flotation Cell. Column Flotation '91, Proceeding of an htternational Conference on Column Flotation (G. E. Agar, 13. J. Huls, D. B. Hyma, eds)Sudbury, Ontario, 673-687 (1991). Ye, Y., Gopalakrishnan, S., Pacquet, E. & Miller, J., Development of the air-sparged hydrocyclone - a swirl-flow flotation column. Cohmm Flotation '88 (K. V. Sastry, ed) SME Annual Meeting, Phoenix, Arizona, 305-313 (1988). Breed, A.W., Beneficiation of South African Coal Fines using the Air-Sparged Hydrocyclone. M.Sc. Thesis, University of Cape Town, (1992). Von Holt, S., An Investigation into Column Flotation of South African Coals. M. Sc. Thesis, University of Cape Town, (1992). Atkinson, B.W., Griffin, P.T., Jameson, G.J. & Espinosa-Gomez, R., Jameson cell test work on copper streams in the copper concentrator of Mount Isa Mines Limited. Proceedings ~fthe XVIlI International Minerals Processing Congress, the Australasian Institute of Mining and Metallurgy, Sydney, Australia, 826 (1993). Miller, J.D., personal conamunication, (1993).