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Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell part 2: Effect on gas holdup
Minerals Engineering,Vol. 8, No. 12, pp. 1557-1570, 1995 Elsevier Science Lid Printed in Great Britain
Pergamon 0897,-~7S(95)00118--2
0892-6875/95 $9.5o+o.oo
STUDIES ON IMPELLER TYPE, IMPELLER SPEED AND AIR FLOW RATE IN AN INDUSTRIAL SCALE FLOTATION CELL PART 2: EFFECT ON GAS HOLDUP
B. K . G O R A I N § , J . - P . F R A N Z I D I S ' ~ a n d E . V. M A N L A P I G §
§ Julius Kruttschnitt Mineral Research Centre, Isles Road, Indooroopilly, Australia 4068 1" I~.partment of Chemical Engineering, University of Cape Town, South Africa (Received 8 December 1994; accepted 24 July 1995)
ABSTRACT Gas holdup was measured at different locations in a 2.8 m3 portable industrial scale subaeration flotation cell, treating zinc cleaner feed at Hellyer Concentrator in Tasmania, Australia. The cell was fitted in turn with four different impeller-stator systems, and operated over a range of airflow rates and impeller speeds. The gas holdup was found to increase with increase in impeller speed as well as with increase in airflow rate, the manner in which it increased depended on the impeller type. Values ranged from 2% to 33%, with the greatest values produced by the Outokumpu impeller. Keywords Industrial flotation cell, Outokumpu impeller, Dorr--Oiiver impeller, Agitair Chile-X impelle:r, Agitair Pipsa impeller, Gas holdup, Aeration rate, Impeller speed
INTRODUCTION An investigation into the effect of impeller type, impeller speed and air flow rate on the bubble size (db) at different localions in an industrial scale flotation cell was recently reported by the authors [1]. The work was carried out in the zinc cleaner feed circuit at Hellyer Mine operated by Aberfoyle Resources Limited in Tasmania, Australia. The 2.8 m 3 test cell was fitted in turn with four different impellers commonly used in flotation cells, and operated at various combinations of air flow rates and impeller speeds, at values around those recommended by the manufacturers. This paper presents the results of a parallel investigation into the effect of the same variables (impeller type, speed and air flow rate) on gas holdup (eg) at the same six locations at which d b was measured. The investigations were carried out in April and May 1994. It should be emphasised that these experiments were not conducted to compare the performance or characteristics c,f the different impellers used. The impellers were not necessarily operated under optimum (recommended) conditions, or in a cell of optimum design, and were utilised solely to provide a range of hydrodynamic conditions for the study.
1557
1558
B.K. Gorain et al.
EXPERIMENTAL Equipment and operating conditions The design of the test cell, a schematic diagram of the rig, and details of the impellers used in the testwork, are given in Gorain e t al. [1]. Feed to the cell was taken from the feed to the head of the zinc cleaner circuit; concentrates and tailings were returned to the same circuit. Details of the operating conditions prevailing during the testwork may also be found in the previous paper. For each of the four impellers tested, the air holdup was measured at four different flow rates at each of the four different impeller speeds. The same air flow rates were used for each impeller. Measurements were made at six different locations in the test cell as indicated in Figure 3 of Gorain e t al. [1].
Gas holdup measurement The gas holdup was measured using a modified version of the device used by Jameson and Allum [2]. It consisted of a 40 mm diameter copper cylinder with two plungers 150 mm apart attached to a central rod as shown in Figure 1. The plungers had O-rings for an air tight fitting when they moved inside the cylinder. The plungers were actuated by using a vacuum pump of about 20 mm Hg vacuum. The vacuum drew the plungers into the cylinder, encapsulating a volume of aerated pulp in the space between the plungers.
.ql--------Vacuum outlet
Cylinder ,h\\\\\\~ "q"-'-Handle to hold piston
.4~..._.-P1unger
piston rod
Plunger Fig.1 Gas holdup measurement equipment The pulp--air mixture encapsulated between the plungers was emptied into a measuring cylinder at which point the air escaped into the atmosphere. This process was repeated five times and the total volume of slurry contained in the five samples was determined. The volume of the space between the cylinders was determined by calibration with water. The volume of air in the aerated pulp was calculated as the difference between the measured volume of slurry and the calibrated volume of water; eg was then the volume fraction of air in each sample.
Reproducibility Table 1 shows the results of reproducibility tests carried out with the gas holdup measurement device. Four measurements were made at each of the six locations in the cell and the standard deviation of the measured values for each location was calculated. The reproducibility of the ~g measurements was found to be very good except for locations 2 and 4 which are in the turbulent zone of the ceil. At these locations the reproducibility was reasonably good, though not as good as at other locations.
An industrial scale flotation tank
1559
TABLE 1 Reproducibility of gas holdup measurements at different locations in test cell (All figures in %) Test 1
Test 2
Test 3
[ Test 4
Mean
S.D
10.08 8.51 6.38 12.7'9 7.49 7.88
0.75 1.20 0.28 1.23 0.28 0.77
I
Locatn Locatn Locatn Loeatn Locatn Locatn
1 2 3 4 5 6
10.13 7.88 6.63 13.26 7.68 8.07
9.72 9.27 5.98 10.83 7.52 8.99
11.26' 9.93 6.65 14.20 7.02 6.88
9.22' 6.95 6.25 12.88 7.73 7.57
RESULTS The results of all the £g values obtained during this investigation are listed in Tables 2a to 2d. Each Table presents, for a p;micular impeller type, eg in % measured at each of the six locations for each of the 16 operating conditions tested. For each operating condition, the arithmetic average of the values measured at the six locations is also given, as well as the measured power input in kW. As may be seen the values differ from impeller to impeller, and vary with air flow rate, impeller speed and location within t)ae flotation cell. Various patterns are discernible, and will be discussed below.
TABLE 2a Gas holdup measured at different locations and operating conditions (impeller speed, air flow rate): Pipsa impeller
Operating conditions
Gas holdup %
"Average gas holdup"
Power Input (kW)
Location
Location
Location
Location
Location
Location
1
2
3
4
5
6
4.00 6.18 12.83 13.05
3.97 4.12 11.66 1 !.97
3.29 4.23 11.23 12.23
6.43 8.06 13.63 14.98
4.38 5.12 4.87 4.73
5.18 6.25 5.28 5.06
4.54 5.66 9.92 10.34
1.73 1.70 1.69 1.67
5.67 7.22 10.22 16.18
4.39 6.05 9.75 15.88
4.68 5.17 10.13 15.27
7.75 9.98 12.96 18.65
5.11 6.28 6.77 8.25
5.67 7.35 8.24 9.85
5.55 7.01 9.68 14.01
2.95 2.77 2.70 2.55
7.80 7.83 11.15 13.88
6.42 6.18 10.35 11.75
5.88 6.22 10.22 10.62
9.94 12.58 14.75 20.83
6.03 7.63 8.37 9.96
7.77 8.95 9.98 11.12
7.31 8.23 10.80 13.03
4.46 4.15 3.94 3.81
7.83 8.29 9.98 14.23
6.58 7.11 7.89 11 .(30
6.70 7.03 8.55 I i. 11
11.65 13.88 16,85 23,18
6.77 8.88 10.83 13.88
8.78 10.35 12.65 15.93
8.05 9.26 11.13 14.89
5.55' 4.65 4.12 3.64
105 r p m 16.5 28.3 42.5 56.7
l/sec l/see I/see l/see
120 rpm 16.5 l/see 28.3 l/see 42.5 l/sec 56.7 I/see 140 r p m 16.5 I/see 28.3 l/see 42.5 l/see 56.7 l/see 160 r p m 16.5 l/see 28.3 l/see 42.5 l/see 56.7 l/see
B. K. Gorain et al.
1560 T A B L E 2b
Gas holdup measured at different locations and operating conditions (impeller speed, a i r flow rate): ChHe-X i m p e l l e r Gas holdup %
Operating conditions
"Average gas holdup"
Power input (kW)
location 1
Lo~lon 2
Location 3
Location 4
Location 5
Location 6
28.3 I/sec 42.5 I/sec 56.7 I/sec
3.52 5.89 12.83 18.13
3.28 3.29 14~82 19.25
2.00 4.83 11.88 16.11
7.30 9.18 13.81 15.83
2.05 6.20 5.96 5.18
3.70 7.40 7.07 6.23
3.64 6.13 11.06 13.46
2.82 2.70 2.62 2.55
160rpm 16.5 Fsec 28.3i/sec 42.5 I/sec 56.7 I/sec
5.95 9.23 12.22 17.89
4.68 8.05 ll.60 18.92
3.25 6.63 10.35 16.03
9.23 12.78 15.82 20.06
3.28 7.68 9.99 12.06
4.60 8.07 10.63 16.20
5.17 8.74 11.77 16.86
3.20 3.08 2.95 2.82
140 rpm 16.5 I/sec
"
il;[tlf,).m
m. . mm m.u. 200 rpm 16.5 I/sec 28.3 I/sec 42.5 I/sec 56.7 I/sec
5.69 8.88 14.16 ! 9.22
T A B L E 2c
mm m l m n
4.88 9.25 12.~9 15.'88
4.03 5.89 12.88 17.23
munro,
12.39 14.86 20.00 25.67
mm
.am
5.29 7.68 14.36 18.66
mm
6.48 9.72 16.67 23.61
6.46 9.38 15.06 20.05
4.67 4.49 4.32 4.18
Gas holdup measured at different locations and operating conditions (impeller speed, air flow rate): Dorr-Oliver impeller
Operating conditions
235 rpm 16.5 I/sec 28.3 I/sec 42.5 I/sec 56.7 l/see 255 rpm 16.5 I/sec 28.3 I/sec 42.5 I/sec 56.7 Fsec 275 rpm 16.5 Fsec 28.3 I/see 42.5 I/sec 56.7 I/sec 295 rpm 16.5 I/see 28.3 I/sec 42.5 I/sec 56.7 I/sec
Gas holdup %
"Average gas holdup"
Power input (kVO
Location 1
Location 2
Location 3
Loca~on 4
Location 5
Location 6
4.21 6.29 8.35 13.16
3.89 5.27 7.79 12.41
3.08 5.48 6.42 6.77
5.98 8.92 13.90 14.22
3.93 6.23 7.30 6.05
4.56 6.63 8.52 7.22
4.28 6.47 8.7 i 9.97
1.97 1.72 1.53 i .36
6.17 7.77 8.94 11.12
5.25 8.30 8.96 10.65
4.38 8.08 8.14 9.48
7.40 I l.l I 16.20 17.82
5.20 9.01 9.97 12.36
5.56 I I.I I 12.96 14.51
5.66 9.23 10.86 12.66
2.76 2.42 2.12 1.78
9.00 11.27 14.02 16.92
8.90 I 0.89 12.76 15.10
8.25 10.23 14.29 15.39
9.26 13.89 20.60 29.63
8.77 12.20 15.03 16.58
9.26 15.28 19.06 21.30
8.91 12.29 15.96 18.99
2.98 2.72 2.26 1.99
11.00
9.98
10.23
i1.11
10.88
14.75 15.30 17.97
14.44 13.07 15.66
14.73 15.09 15.88
16.05 22.53 30.56
15.25 17.05 18.68
I I. l I 16.98 20.11 24.54
10.72 15.37 17.19 20.55
3.32 2.93 2.50 2.19
An industrial scale flotation tank
1561
TABLE 2d Gas holdup measured at different locations and operating conditions (impeller speed, air flow rate): Outokumpu impeller Operating conditions
185 rpm 16.5 I/see 28.3 I/sec 42.5 l/sec 56.7 I/sec 205 r p m 16.5 I/see 28.3 I/sec 42.5 I/sec 56.7 I/sec 225 r p m 16.5 I/sec 28.3 I/see 42.5 I/see 56.7 l/see 245 r p m 16.5 I/sec 28.3 I/sec 42.5 I/see 56.7 I/s~,:
Gas holdup %
"Average gas holdup"
Power Input (kW)
Location
Location
Location
Location
Location
Location
1
2
3
4
5
6
8.32 11.05 14.14 15.27
7.67 10.68 13.67 14.24
7.88 10.13 10.66 12.10
11.01 14.81 18.52 22.22
8.93 11.87 12.27 14.73
10.55 13.65 14.73 18.75
9.06 12.03 13.92 16.22
4.31 3.29 2.96 2.64
8.77 12.20 14.11 19.77
7.89 11.80 14.17 16.53
8.36 9.99 10.03 14.39
12.89 15.62 18.52 23.61
9.67 11.95 13.55 16.98
11.89 14.81 16.90 20.83
9.91 12.73 14.55 18.69
5.47 4.34 3.57 3.28
10.68 13.16 17.24 20.63
10.77 12.25 15.40 16.29
8.90 il.93 16.77 17.89
13.99 15.97 19.91 24.07
9.90 13.89 17.92 18:99
12.11 15.28 19.91 22.22
11.06 13.75 17.86 20.02
6.80 5.35 4.50 3.90
11.75 13.89 23.12 26.28
10.89 13.07 21.18 23.23
12.04 13.69 22.00 24.18
13.50 15.71 27.28 33.28
11.88 14.28 20.12 23.07
12.60 15.98 23.15 30.07
12.11 14.44 22.81 26.69
8.41 6.54 5.56 4.58
Effect of air flow rate The variation of a:g with air flow rate at various impeller speeds, at each of the six measuring locations in the test cell, is shown in Figures 2a to 2d for the Pipsa, Chile-X, Dorr-Oliver and Outokumpu impellers, respectively. For all the impellers at all the locations, eg generally increased with increase in air flow rate, with the manner in which it increased depending on the impeller type and speed. At the lowest air rate employed (16.5 1/sec), the eg values varied from 2.05% to 13.99%, with over 60% of the values being greater than 6%. However, fewer than 25% of the cg values were greater than 10%, and of these nearly two-thirds were obtained with the Outokumpu impeller. At the highest air flow rate (56.7 l/sec), the values of Eg were between 4.73% and 33.28%, with over 60% of the values being greater than 15%. In the case of the Pipsa impeller (Figure 2a), there was a steady increase in eg with increase in air flow rate at each location, for each value of impeller speed, with the measured values approximately doubling on average in going from the lowest to the highest air flow rate. A notable exception was found at the lowest impeller speed (1105 rpm) where a sudden sharp increase in eg is clearly evident on going from 28.3 I/sec to 42.5 I/sec at locations 1 to 4; a corresponding (though much smaller) decrease is evident in locations 5 and 6, under the same conditions. This corresponds to the point at which the cell contents could be seen to be flooding with a boiling appearance near to the impeller, and the measured bubble sizes increased dramatically [1]. This indicates that under these conditions the Pipsa impeller was no longer able to disperse or pump air to the furthest points in the cell (locations 5 and 6) and that the air was rising in bulk vertically over the impeller zone. When the impeller speed was increased to 120 rpm, the sudden increase in £g at location:~ 1 to 4 occurred only at the highest air flow rate of 56.7 l/sec; additionally, the air was obviously being sufficiently dispersed in the cell for a corresponding (modest) increase in eg to be noted at locations 5 acd 6.
1562
B.K. Gorainet al.
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:::::::::::::::::::::
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O.
. . . .
flow
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(llaec)
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60
o
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4 '0
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Fig.2a Variation of gas holdup with air flow rate and impeller speed at each of the six measuring locations. Pipsa impeller.
(llaec)
5
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60
An industrial scale flotation tank
LOCATION
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1563
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----I--
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"-~
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160 rpm
--I-
180 Tpm
- -e. -
200 rpm
flow
;o rate
' 50 (I/sac)
"
60
0
2'0 Air
3'0 flow
4'0 rate
Fig.2b Variation of gas holdup with air flow rate and impeller speed at each of the six measuring locations. Chile-X impeller.
(llsec)
50
60
1564
B.K. Gomin et al.
LOCATION
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Fig.2c Variation of gas holdup with air flow.rate and impeller speed at each of the six measuring locations. Dorr--Oliver impeller.
(lilac)
5'0
60
An industrial scale flotation tank
LOCATION
ze Q. :l O .C m I
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1565
LOCATION 2
1
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Fi~g.2d Variation of gas holdup with air flow rate and impeller speed at each of the six measuring locations. Outokumpu impeller.
(llaac)
5O "
60
1566
B.K. Gorainet
al.
The results obtained with the Chile-X impeller (Figure 2b) follow a very similar pattern to those obtained with the Pipsa impeller, though the Chile-X impeller in general produced slightly greater Eg values, except at the lowest air flow rate. At each location, the increase in Eg on going from the lowest to the highest air flow rate at constant impeller speed was on average threefold. As with the Pipsa impeller, an anomaly is observed especially at the lowest impeller speed (140 rpm) on going from 28.3 l/sec to 42.5 l/see; there was a very sharp increase in eg at locations 1 to 4, and an uncharacteristic decrease at locations 5 and 6. This again corresponds to a condition of flooding in the cell. Increasing the impeller speed to 160 rpm improved the air dispersion at locations 5 and 6 substantially, even at the highest air flow rate. The Dorr--Oliver impeller (Figure 2c) produced a much flatter profile of eg versus air flow rate than the Chile-X impeller, except at location 4 (the point at which air was introduced into the cell). The eg value at each location was on average doubled in going from the lowest to the highest air flow rate at constant impeller speed. The impeller began to flood only at the highest air flow rate (56.7 l/sec) at the lowest impeller speed (235 rpm). At this condition, there was a sharp increase in eg at locations 1 and 2 as expected (but not at locations 3 and 4), and a noticeable decrease at locations 5 and 6. In general, the Dorr-Oliver impeller produced greater air holdup in the cell than the Pipsa and Chile-X impellers, especially at the lower air flow rates. This would suggest that this impeller was able to disperse even small quantities of air evenly throughout the cell, a conclusion which is supported by the detection of small diameter air bubbles throughout the cell under all the operating conditions investigated [1]. A major feature of the results obtained with the Outokumpu impeller (Figure 2d) is the high values of eg produced at all locations under all conditions. In terms of air holdup, this impeller was clearly superior to the others at low impeller speeds. The eg values still increased on average by a factor of two on going from the lowest to the highest value of air flow rate at constant impeller speed, but the increases were most pronounced at the highest impeller speed (245 rpm), at practically all locations in the cell. No flooding was observed with this impeller, even at the lowest impeller speed and highest air flow rate; this improved air dispersion is substantiated by the very small air bubbles produced by this impeller under these conditions [1].
Effect of Impeller Speed As anticipated, ~g generally increased with increase in impeller speed, for all the impellers, except for the anomalies associated with flooding. For the Pipsa and Chile-X impellers, the increase in eg from the lowest to the highest impeller speed was not at all remarkable at locations 1, 2 and 3; however, at location 4, and especially at locations 5 and 6, the increase was dramatic, especially at the highest air flow rate. This indicates that these impellers require high impeller speeds to disperse large quantities of air to the outermost limits of the cell. For the Dorr-Oliver impeller, the increase in air holdup with increase in impeller speed was fairly steady. Values of eg increased on average about threefold in going from the lowest (235 rpm) to the highest (295 rpm) impeller speed at the lowest air flow rate (16.5 l/see), but the increase was approximately fourfold at the highest air flow rate (56.7 l/see). This latter increase is slightly exaggerated by the conditions of flooding prevailing in the cell at 235 rpm and 56.7 l/see, which cause the eg values at locations 5 and 6 to be unusually low. Increasing the speed of the Outokumpu impeller had hardly any influence on the measured values of eg at the lowest air flow rates investigated (16.5 Fsec and 28.3 l/see). At the higher air flow rates, there was a very big increase in eg in going from 225 rpm to 245 rpm (the highest impeller speed investigated) at all locations. This corresponded to a sudden increase in mean bubble size [1], a complete reversal of the trend of decreasing bubble size with increasing impeller speed, and was attributed to a dramatic increase in internal gas recireulation rate at the very high impeller speed. Flow patterns in the cell were not directly discernible from this work, but will be investigated in a glass cell of the same dimensions as the test flotation cell at the JKMRC in Brisbane.
An industrial scale flotationtank
1567
Effect of location It is quite evident from Tables 2a to 2d and Figures 2a to 2d that eg varies with location in the flotation cell. For all four impellers, Eg was greatest at location 4, which corresponds to the impeller tip, where air would be discharged into the cell. The value of Eg then typically decreased to location 6, which is at the wall of the cell (see Figure 3 of Gorain et al. [1]). In turn, the value at location 2, which is near to the impeller shaft and above the point where slurry would be drawn into the impeller, was typically lower than the value at location 6. There are some exceptions to these generalisations, mostly as a result of flooding in the cell, but the overwhelming majority of the results follow this pattern. In almost all cases, there was also a very clear decrease in eg in going from location 6 through location 5 to location 3. The path of decreasing eg values at locations 4--6-5-3 is part of the postulated circulation loop in the cell which is illustrated in Figure 3 of Gorain et al. [1]. The mean bubble size was generally found to increase along this path, a phenomenon previously attributed to coalescence [ 1]. These findings tend to support each other, as one would expect to measure smaller values of Eg in regions where bubble size is larger (as large bubbles rise more quickly than small ones, and would escape from the slurry). This suggests that the few exceptions to the finding that eg decreased along the location sequence 4--6-5-3 came about as a result of flooding (which would result in a high value of eg at location 3, vertically above the point of air addition into the cell) or a change in flow pattern. Exceptions of the first kind would be represented by the results obtained with the Dorr--Oliver impeller at 235 rpm and 56.7 l/sec air flow rate, the Chile-X impelle:r at 140 rpm and 160 rpm and 42.5 l/sec and 56.7 l/sec air rate, and the Pipsa impeller at 105 rpm, 120 rpm and 140 rpm, at 42.5 l/sec and 56.7 1/sec air flow rate. It is interesting to note that the cell had not previously been thought to be flooding at some of these conditions. Exceptions of the second kind are represented by the Outokumpu impeller at 245 rpm, and suggest the presence of a secondary flow loop within the outer circulation loop, or a different circulation loop completely. As mentioned above, this will be investigated in the 2.8 m 3 glass cell at the JKMRC.
Effect of impeller ~ype Figure 3 shows the variation of Eg with air flow rate and location in the cell for each of the four impellers tested. The values are those obtained when the impellers were being operated at the manufacturers' recommended speeds. As noted above, there is a general trend of decreasing air holdup along the flow profile of the cell fiom location 4 through location 6 to location 5 or 3; this increases again at location 1 and decreases once more at location 2. Whether the minimum occurs at locations 3 or 5 seems to depend on the impeller tyl~. and air flow rate. For all the impellers, the maximum eg value at each air flow rate occurred at location 4, which corresponded to the point of air addition to the cell. The shape of the eg versus "location" profile is indicative of how evenly the impeller was able to disperse the air in the cell - - pronounced minima or maxima would reveal that the dispersion was uneven. The magnitude of the eg values indicates how successfully the impeller was able to disperse the air into small bubbles (which hav,~ a slower rise velocity and hence a longer residence time in the cell, and consequently a greater holdup); alternatively, high values of eg may indicate flooding. Almost all the values obtained with the Pipsa impeller were between 5% and 10%, except at the very high air flow rate (and these were the result of flooding). The very sharp minimum at locations 5 and 6 at the highest air flow rate indicates that this impeller was not able to disperse this quantity of air evenly throughout the cell at the manufacturer's recommended speed. The situation was fairly similar with the Chile-X impeller, with most air holdup values being between 2.5% and 12.5%, supporting the finding that the bubbles produced were slightly smaller than those obtained with the Pipsa impeller [ 1]. With the Dorr-Oliver impeller, eg values were mostly between 5% and 12.5%, and no minimum occurred at locations 5 and 6. In fact, at the manufacturer's recommended speed, Eg values at locations 5 and 6 were, with only one exception, greater than the values at locations 1 and 2, indicating that this impeller was able to disperse air to the outer regions of the cell even at the highest flow rate investigated. The range of eg
1568
B.K. Gorain et al.
values suggests bubble sizes of the same order of magnitude as the Chile-X impeller, which was in fact found to be the case. The eg values obtained with the Outokumpu impeller were on the whole greater than those produced by the other impellers, ranging between 7.5% and 17.5%, and suggest that the bubbles were very small, which is supported by the experimental values obtained [1]. As no flooding was observed to occur with this impeller, the increase in eg values at the highest air flow rate has been attributed to internal gas recirculation in the ceil. An investigation of the flow patterns obtaining when using the different impellers will be carried out using the 2.8 m 3 glass cell at the JKMRC.
Plpsa
L
0 .C
Impeller
Chlle-X
35
35
30
30-
25
25"
20
Impeller
20-
i
% %
15
15"
m I . . . .
10
_
10"
5
~
~---t---4r-
=
5"
0
i
i
I
i
i
4
6
5
3
1
0
l
4
I
6
location
Dorr-Oliver
I
I
l
5
3
1
2
Location
Outokumpu
Impeller
impeller
35
35
30-
30 o
25 "t
25" Q. =1
_.R
20"
0
era ii 0
15"
15 "t
- - - "~' ,., .,.', ,.,~
t
10"
D
-,,-
~"e"
..m.---
1ll
.---'*---,F
----
"':l~:m't'==:~ : - ' - :
50 0
i
i
i
I
4
6
5
3
location
--D-
16.5 I / x c
- -41, -
28.3 I/eec
- -I
-
42.5 I/No
- -e, -
56.7 I/see
!
1
2
0
4
6
5
3
Iocstlon
Fig.3 Variation of gas holdup with air flow rate and location for the four impellers at the manufacturer's recommended impeller speed (Pipsa 120 rpm, Chile-X 160 rpm, Don'-Oliver 255 rpm, Outokumpu 205 rpm).
1
2
An industrialscale flotationtank
1569
"Average air holdup" Another way of comparing the different types of impellers used in the test programme is on the basis of "average air holdup", calculated as the simple arithmetic average of the values measured at locations 1 to 6. These values are shown alongside the measured values in Tables 2a to 2d above. Although the values have no strict physical significance, they are an estimate of the "mean" air holdup in the cell at a particular combination of impeller type, speed and air flow rate, in the same way that "average bubble size" was calculated and used by the authors previously [1]. Interestingly, most of the values lie between the corresponding values for locations 3 and 4 (i.e. in the centre of the cell). Table 3 shows the "average air holdup" values for each of the four impellers at the recommended impeller speeds, at each of the four air flow rates investigated in this work. The values range from around 5% to nearly 19%. The Outokumpu impeller produced the greatest values of eg at each of the four air flow rates, a not unexpected finding as it also produced the smallest average mean bubble sizes [1]. No clear pattern emerges for the other three impellers; the Pipsa impeller produced the smallest values at the intermediate air flow rates (28.3 l/see and 42.5 Use,c), the Chile-X at the lowest air flow rate (16.5 l/see) and the Dorr--Oliver impeller at the highest (56.7 l/see).
TABLE 3 Gas holdup and power input at the manufacturers' recommended speed Qg=air flow rate, l/sec; I~g=air holdup, %; P=input power, kW
Impeller type Qg[
~p~
i
il
ChH~X
iiJRll lmammm laJ l
.~,'/~rlB
lr~al B
ireS,
Don-Olive
Outokumpu
N
It is also interesting to compare the relative performance of the impellers in producing air holdup in the cell on the basis of power input. The power drawn by the impeller in each of the tests carried out in this programme is given in the last column of Tables 2a to 2d. For each impeller, at each impeller speed, the power draw decreased steadily with increase in air flow rate. The power input values at the manufacturers' recommended impeller speeds are listed in Table 3: the Outokumpu impeller drew more power than any of the other impellers, and the Dorr-Oliver the least. To produce an average gas holdup value of approximately 15% at the maximum air flow rate of 56.7 l/see, the Pipsa impeller would have to be rotated at 160 rpm, requinng a power input of 3.64 kW. To do the same, the Chile-X impeller would need around 150 rpm and 2.68 kW, the Outokumpu 185 rpm and 2.64 kW, and the Dorr--Oliver impeller 265 rpm and only 1.88 kW. Th,~se values were obtained from Tables 2a to 2d by interpolation.
CONCLUSIONS Gas holdup measurements have been carried out in an industrial-scale test flotation cell, located at the head of a zinc cleaner circuit, using four typical impellers over a range of air flow rates and impeller speeds. The gas holdup was fcund to increase with increasing air flow rate at various locations in the cell, for all four impellers. The manner in which the increase took place depended on the impeller type and speed. In general the gas holdup values increased with increasing impeller speed for all the impeller types, as expected with a decrease in bubble size; except for the anomalies associated with flooding. With the Pipsa Ill I-IZ-I
1570
B.K. Gorainet al.
and Chile-X impellers, there was a very significant increase in air holdup near the cell wall with increase in impeller speed, reflecting the improved air dispersion. The Outokumpu impeller produced higher gas holdup compared to other impellers, thought to be associated with greater air circulation within the cell.
ACKNOWLEDGEMENTS The authors would like to thank Geoff Richmond, John Glen and other staff of Aberfoyle Resources Limited for their generous help throughout the period of the test work and for their constructive comments on this paper. The financial support of the Australian Mineral Industries Research Association (AMIRA Project P9K) and the Australian Research Council is also gratefully acknowledged.
REFERENCES .
.
Gorain, B.K., Franzidis, J.-P. & Manlapig, E.V., Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell. Part 1: Effect on bubble size distribution. Minerals Engineering, 8(6), 615-635 (1995). Jameson, G.J. & Allum, P., A survey of bubble sizes in industrial flotation cells, report prepared for AMIRA limited, (1984).
Pergamon 0897,-~7S(95)00118--2
0892-6875/95 $9.5o+o.oo
STUDIES ON IMPELLER TYPE, IMPELLER SPEED AND AIR FLOW RATE IN AN INDUSTRIAL SCALE FLOTATION CELL PART 2: EFFECT ON GAS HOLDUP
B. K . G O R A I N § , J . - P . F R A N Z I D I S ' ~ a n d E . V. M A N L A P I G §
§ Julius Kruttschnitt Mineral Research Centre, Isles Road, Indooroopilly, Australia 4068 1" I~.partment of Chemical Engineering, University of Cape Town, South Africa (Received 8 December 1994; accepted 24 July 1995)
ABSTRACT Gas holdup was measured at different locations in a 2.8 m3 portable industrial scale subaeration flotation cell, treating zinc cleaner feed at Hellyer Concentrator in Tasmania, Australia. The cell was fitted in turn with four different impeller-stator systems, and operated over a range of airflow rates and impeller speeds. The gas holdup was found to increase with increase in impeller speed as well as with increase in airflow rate, the manner in which it increased depended on the impeller type. Values ranged from 2% to 33%, with the greatest values produced by the Outokumpu impeller. Keywords Industrial flotation cell, Outokumpu impeller, Dorr--Oiiver impeller, Agitair Chile-X impelle:r, Agitair Pipsa impeller, Gas holdup, Aeration rate, Impeller speed
INTRODUCTION An investigation into the effect of impeller type, impeller speed and air flow rate on the bubble size (db) at different localions in an industrial scale flotation cell was recently reported by the authors [1]. The work was carried out in the zinc cleaner feed circuit at Hellyer Mine operated by Aberfoyle Resources Limited in Tasmania, Australia. The 2.8 m 3 test cell was fitted in turn with four different impellers commonly used in flotation cells, and operated at various combinations of air flow rates and impeller speeds, at values around those recommended by the manufacturers. This paper presents the results of a parallel investigation into the effect of the same variables (impeller type, speed and air flow rate) on gas holdup (eg) at the same six locations at which d b was measured. The investigations were carried out in April and May 1994. It should be emphasised that these experiments were not conducted to compare the performance or characteristics c,f the different impellers used. The impellers were not necessarily operated under optimum (recommended) conditions, or in a cell of optimum design, and were utilised solely to provide a range of hydrodynamic conditions for the study.
1557
1558
B.K. Gorain et al.
EXPERIMENTAL Equipment and operating conditions The design of the test cell, a schematic diagram of the rig, and details of the impellers used in the testwork, are given in Gorain e t al. [1]. Feed to the cell was taken from the feed to the head of the zinc cleaner circuit; concentrates and tailings were returned to the same circuit. Details of the operating conditions prevailing during the testwork may also be found in the previous paper. For each of the four impellers tested, the air holdup was measured at four different flow rates at each of the four different impeller speeds. The same air flow rates were used for each impeller. Measurements were made at six different locations in the test cell as indicated in Figure 3 of Gorain e t al. [1].
Gas holdup measurement The gas holdup was measured using a modified version of the device used by Jameson and Allum [2]. It consisted of a 40 mm diameter copper cylinder with two plungers 150 mm apart attached to a central rod as shown in Figure 1. The plungers had O-rings for an air tight fitting when they moved inside the cylinder. The plungers were actuated by using a vacuum pump of about 20 mm Hg vacuum. The vacuum drew the plungers into the cylinder, encapsulating a volume of aerated pulp in the space between the plungers.
.ql--------Vacuum outlet
Cylinder ,h\\\\\\~ "q"-'-Handle to hold piston
.4~..._.-P1unger
piston rod
Plunger Fig.1 Gas holdup measurement equipment The pulp--air mixture encapsulated between the plungers was emptied into a measuring cylinder at which point the air escaped into the atmosphere. This process was repeated five times and the total volume of slurry contained in the five samples was determined. The volume of the space between the cylinders was determined by calibration with water. The volume of air in the aerated pulp was calculated as the difference between the measured volume of slurry and the calibrated volume of water; eg was then the volume fraction of air in each sample.
Reproducibility Table 1 shows the results of reproducibility tests carried out with the gas holdup measurement device. Four measurements were made at each of the six locations in the cell and the standard deviation of the measured values for each location was calculated. The reproducibility of the ~g measurements was found to be very good except for locations 2 and 4 which are in the turbulent zone of the ceil. At these locations the reproducibility was reasonably good, though not as good as at other locations.
An industrial scale flotation tank
1559
TABLE 1 Reproducibility of gas holdup measurements at different locations in test cell (All figures in %) Test 1
Test 2
Test 3
[ Test 4
Mean
S.D
10.08 8.51 6.38 12.7'9 7.49 7.88
0.75 1.20 0.28 1.23 0.28 0.77
I
Locatn Locatn Locatn Loeatn Locatn Locatn
1 2 3 4 5 6
10.13 7.88 6.63 13.26 7.68 8.07
9.72 9.27 5.98 10.83 7.52 8.99
11.26' 9.93 6.65 14.20 7.02 6.88
9.22' 6.95 6.25 12.88 7.73 7.57
RESULTS The results of all the £g values obtained during this investigation are listed in Tables 2a to 2d. Each Table presents, for a p;micular impeller type, eg in % measured at each of the six locations for each of the 16 operating conditions tested. For each operating condition, the arithmetic average of the values measured at the six locations is also given, as well as the measured power input in kW. As may be seen the values differ from impeller to impeller, and vary with air flow rate, impeller speed and location within t)ae flotation cell. Various patterns are discernible, and will be discussed below.
TABLE 2a Gas holdup measured at different locations and operating conditions (impeller speed, air flow rate): Pipsa impeller
Operating conditions
Gas holdup %
"Average gas holdup"
Power Input (kW)
Location
Location
Location
Location
Location
Location
1
2
3
4
5
6
4.00 6.18 12.83 13.05
3.97 4.12 11.66 1 !.97
3.29 4.23 11.23 12.23
6.43 8.06 13.63 14.98
4.38 5.12 4.87 4.73
5.18 6.25 5.28 5.06
4.54 5.66 9.92 10.34
1.73 1.70 1.69 1.67
5.67 7.22 10.22 16.18
4.39 6.05 9.75 15.88
4.68 5.17 10.13 15.27
7.75 9.98 12.96 18.65
5.11 6.28 6.77 8.25
5.67 7.35 8.24 9.85
5.55 7.01 9.68 14.01
2.95 2.77 2.70 2.55
7.80 7.83 11.15 13.88
6.42 6.18 10.35 11.75
5.88 6.22 10.22 10.62
9.94 12.58 14.75 20.83
6.03 7.63 8.37 9.96
7.77 8.95 9.98 11.12
7.31 8.23 10.80 13.03
4.46 4.15 3.94 3.81
7.83 8.29 9.98 14.23
6.58 7.11 7.89 11 .(30
6.70 7.03 8.55 I i. 11
11.65 13.88 16,85 23,18
6.77 8.88 10.83 13.88
8.78 10.35 12.65 15.93
8.05 9.26 11.13 14.89
5.55' 4.65 4.12 3.64
105 r p m 16.5 28.3 42.5 56.7
l/sec l/see I/see l/see
120 rpm 16.5 l/see 28.3 l/see 42.5 l/sec 56.7 I/see 140 r p m 16.5 I/see 28.3 l/see 42.5 l/see 56.7 l/see 160 r p m 16.5 l/see 28.3 l/see 42.5 l/see 56.7 l/see
B. K. Gorain et al.
1560 T A B L E 2b
Gas holdup measured at different locations and operating conditions (impeller speed, a i r flow rate): ChHe-X i m p e l l e r Gas holdup %
Operating conditions
"Average gas holdup"
Power input (kW)
location 1
Lo~lon 2
Location 3
Location 4
Location 5
Location 6
28.3 I/sec 42.5 I/sec 56.7 I/sec
3.52 5.89 12.83 18.13
3.28 3.29 14~82 19.25
2.00 4.83 11.88 16.11
7.30 9.18 13.81 15.83
2.05 6.20 5.96 5.18
3.70 7.40 7.07 6.23
3.64 6.13 11.06 13.46
2.82 2.70 2.62 2.55
160rpm 16.5 Fsec 28.3i/sec 42.5 I/sec 56.7 I/sec
5.95 9.23 12.22 17.89
4.68 8.05 ll.60 18.92
3.25 6.63 10.35 16.03
9.23 12.78 15.82 20.06
3.28 7.68 9.99 12.06
4.60 8.07 10.63 16.20
5.17 8.74 11.77 16.86
3.20 3.08 2.95 2.82
140 rpm 16.5 I/sec
"
il;[tlf,).m
m. . mm m.u. 200 rpm 16.5 I/sec 28.3 I/sec 42.5 I/sec 56.7 I/sec
5.69 8.88 14.16 ! 9.22
T A B L E 2c
mm m l m n
4.88 9.25 12.~9 15.'88
4.03 5.89 12.88 17.23
munro,
12.39 14.86 20.00 25.67
mm
.am
5.29 7.68 14.36 18.66
mm
6.48 9.72 16.67 23.61
6.46 9.38 15.06 20.05
4.67 4.49 4.32 4.18
Gas holdup measured at different locations and operating conditions (impeller speed, air flow rate): Dorr-Oliver impeller
Operating conditions
235 rpm 16.5 I/sec 28.3 I/sec 42.5 I/sec 56.7 l/see 255 rpm 16.5 I/sec 28.3 I/sec 42.5 I/sec 56.7 Fsec 275 rpm 16.5 Fsec 28.3 I/see 42.5 I/sec 56.7 I/sec 295 rpm 16.5 I/see 28.3 I/sec 42.5 I/sec 56.7 I/sec
Gas holdup %
"Average gas holdup"
Power input (kVO
Location 1
Location 2
Location 3
Loca~on 4
Location 5
Location 6
4.21 6.29 8.35 13.16
3.89 5.27 7.79 12.41
3.08 5.48 6.42 6.77
5.98 8.92 13.90 14.22
3.93 6.23 7.30 6.05
4.56 6.63 8.52 7.22
4.28 6.47 8.7 i 9.97
1.97 1.72 1.53 i .36
6.17 7.77 8.94 11.12
5.25 8.30 8.96 10.65
4.38 8.08 8.14 9.48
7.40 I l.l I 16.20 17.82
5.20 9.01 9.97 12.36
5.56 I I.I I 12.96 14.51
5.66 9.23 10.86 12.66
2.76 2.42 2.12 1.78
9.00 11.27 14.02 16.92
8.90 I 0.89 12.76 15.10
8.25 10.23 14.29 15.39
9.26 13.89 20.60 29.63
8.77 12.20 15.03 16.58
9.26 15.28 19.06 21.30
8.91 12.29 15.96 18.99
2.98 2.72 2.26 1.99
11.00
9.98
10.23
i1.11
10.88
14.75 15.30 17.97
14.44 13.07 15.66
14.73 15.09 15.88
16.05 22.53 30.56
15.25 17.05 18.68
I I. l I 16.98 20.11 24.54
10.72 15.37 17.19 20.55
3.32 2.93 2.50 2.19
An industrial scale flotation tank
1561
TABLE 2d Gas holdup measured at different locations and operating conditions (impeller speed, air flow rate): Outokumpu impeller Operating conditions
185 rpm 16.5 I/see 28.3 I/sec 42.5 l/sec 56.7 I/sec 205 r p m 16.5 I/see 28.3 I/sec 42.5 I/sec 56.7 I/sec 225 r p m 16.5 I/sec 28.3 I/see 42.5 I/see 56.7 l/see 245 r p m 16.5 I/sec 28.3 I/sec 42.5 I/see 56.7 I/s~,:
Gas holdup %
"Average gas holdup"
Power Input (kW)
Location
Location
Location
Location
Location
Location
1
2
3
4
5
6
8.32 11.05 14.14 15.27
7.67 10.68 13.67 14.24
7.88 10.13 10.66 12.10
11.01 14.81 18.52 22.22
8.93 11.87 12.27 14.73
10.55 13.65 14.73 18.75
9.06 12.03 13.92 16.22
4.31 3.29 2.96 2.64
8.77 12.20 14.11 19.77
7.89 11.80 14.17 16.53
8.36 9.99 10.03 14.39
12.89 15.62 18.52 23.61
9.67 11.95 13.55 16.98
11.89 14.81 16.90 20.83
9.91 12.73 14.55 18.69
5.47 4.34 3.57 3.28
10.68 13.16 17.24 20.63
10.77 12.25 15.40 16.29
8.90 il.93 16.77 17.89
13.99 15.97 19.91 24.07
9.90 13.89 17.92 18:99
12.11 15.28 19.91 22.22
11.06 13.75 17.86 20.02
6.80 5.35 4.50 3.90
11.75 13.89 23.12 26.28
10.89 13.07 21.18 23.23
12.04 13.69 22.00 24.18
13.50 15.71 27.28 33.28
11.88 14.28 20.12 23.07
12.60 15.98 23.15 30.07
12.11 14.44 22.81 26.69
8.41 6.54 5.56 4.58
Effect of air flow rate The variation of a:g with air flow rate at various impeller speeds, at each of the six measuring locations in the test cell, is shown in Figures 2a to 2d for the Pipsa, Chile-X, Dorr-Oliver and Outokumpu impellers, respectively. For all the impellers at all the locations, eg generally increased with increase in air flow rate, with the manner in which it increased depending on the impeller type and speed. At the lowest air rate employed (16.5 1/sec), the eg values varied from 2.05% to 13.99%, with over 60% of the values being greater than 6%. However, fewer than 25% of the cg values were greater than 10%, and of these nearly two-thirds were obtained with the Outokumpu impeller. At the highest air flow rate (56.7 l/sec), the values of Eg were between 4.73% and 33.28%, with over 60% of the values being greater than 15%. In the case of the Pipsa impeller (Figure 2a), there was a steady increase in eg with increase in air flow rate at each location, for each value of impeller speed, with the measured values approximately doubling on average in going from the lowest to the highest air flow rate. A notable exception was found at the lowest impeller speed (1105 rpm) where a sudden sharp increase in eg is clearly evident on going from 28.3 I/sec to 42.5 I/sec at locations 1 to 4; a corresponding (though much smaller) decrease is evident in locations 5 and 6, under the same conditions. This corresponds to the point at which the cell contents could be seen to be flooding with a boiling appearance near to the impeller, and the measured bubble sizes increased dramatically [1]. This indicates that under these conditions the Pipsa impeller was no longer able to disperse or pump air to the furthest points in the cell (locations 5 and 6) and that the air was rising in bulk vertically over the impeller zone. When the impeller speed was increased to 120 rpm, the sudden increase in £g at location:~ 1 to 4 occurred only at the highest air flow rate of 56.7 l/sec; additionally, the air was obviously being sufficiently dispersed in the cell for a corresponding (modest) increase in eg to be noted at locations 5 acd 6.
1562
B.K. Gorainet al.
-_jl
LOCATION
30 25
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2
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flew
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50
60
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(ll-ec)
LOCATION
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Air
3
flew
4
rate
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LOCATION
35
35"
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Air ----B--
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:::::::::::::::::::::
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flew
LOCATION
5 35"
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3
Air
m a
(~
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(l/sac)
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O.
. . . .
flow
40
rate
(llaec)
50
60
o
0
2
'o Air
3
'o
flow
4 '0
rate
Fig.2a Variation of gas holdup with air flow rate and impeller speed at each of the six measuring locations. Pipsa impeller.
(llaec)
5
'o
60
An industrial scale flotation tank
LOCATION
O. =1
"3 J¢
LOCATION
1
35
35
30
30"
25
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20
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1563
40'
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LOCATION
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30
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"-~
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160 rpm
--I-
180 Tpm
- -e. -
200 rpm
flow
;o rate
' 50 (I/sac)
"
60
0
2'0 Air
3'0 flow
4'0 rate
Fig.2b Variation of gas holdup with air flow rate and impeller speed at each of the six measuring locations. Chile-X impeller.
(llsec)
50
60
1564
B.K. Gomin et al.
LOCATION
=e D.
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"1 l
275 rpm
flow
4 '0
rate
(llsec)
50 '
0
10
2 '0
3 '0
Air
flow
4 '0
rate
Fig.2c Variation of gas holdup with air flow.rate and impeller speed at each of the six measuring locations. Dorr--Oliver impeller.
(lilac)
5'0
60
An industrial scale flotation tank
LOCATION
ze Q. :l O .C m I
r3
1565
LOCATION 2
1
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35
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30
40
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60 Air
(I/sac)
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LOCATION 3 35
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50
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5=0
LOCATION 6
35
35
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ze
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185 rpm
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205 q0nl
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225rpm
"~-
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0 60
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' 20
' 30 Air
flow
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Fi~g.2d Variation of gas holdup with air flow rate and impeller speed at each of the six measuring locations. Outokumpu impeller.
(llaac)
5O "
60
1566
B.K. Gorainet
al.
The results obtained with the Chile-X impeller (Figure 2b) follow a very similar pattern to those obtained with the Pipsa impeller, though the Chile-X impeller in general produced slightly greater Eg values, except at the lowest air flow rate. At each location, the increase in Eg on going from the lowest to the highest air flow rate at constant impeller speed was on average threefold. As with the Pipsa impeller, an anomaly is observed especially at the lowest impeller speed (140 rpm) on going from 28.3 l/sec to 42.5 l/see; there was a very sharp increase in eg at locations 1 to 4, and an uncharacteristic decrease at locations 5 and 6. This again corresponds to a condition of flooding in the cell. Increasing the impeller speed to 160 rpm improved the air dispersion at locations 5 and 6 substantially, even at the highest air flow rate. The Dorr--Oliver impeller (Figure 2c) produced a much flatter profile of eg versus air flow rate than the Chile-X impeller, except at location 4 (the point at which air was introduced into the cell). The eg value at each location was on average doubled in going from the lowest to the highest air flow rate at constant impeller speed. The impeller began to flood only at the highest air flow rate (56.7 l/sec) at the lowest impeller speed (235 rpm). At this condition, there was a sharp increase in eg at locations 1 and 2 as expected (but not at locations 3 and 4), and a noticeable decrease at locations 5 and 6. In general, the Dorr-Oliver impeller produced greater air holdup in the cell than the Pipsa and Chile-X impellers, especially at the lower air flow rates. This would suggest that this impeller was able to disperse even small quantities of air evenly throughout the cell, a conclusion which is supported by the detection of small diameter air bubbles throughout the cell under all the operating conditions investigated [1]. A major feature of the results obtained with the Outokumpu impeller (Figure 2d) is the high values of eg produced at all locations under all conditions. In terms of air holdup, this impeller was clearly superior to the others at low impeller speeds. The eg values still increased on average by a factor of two on going from the lowest to the highest value of air flow rate at constant impeller speed, but the increases were most pronounced at the highest impeller speed (245 rpm), at practically all locations in the cell. No flooding was observed with this impeller, even at the lowest impeller speed and highest air flow rate; this improved air dispersion is substantiated by the very small air bubbles produced by this impeller under these conditions [1].
Effect of Impeller Speed As anticipated, ~g generally increased with increase in impeller speed, for all the impellers, except for the anomalies associated with flooding. For the Pipsa and Chile-X impellers, the increase in eg from the lowest to the highest impeller speed was not at all remarkable at locations 1, 2 and 3; however, at location 4, and especially at locations 5 and 6, the increase was dramatic, especially at the highest air flow rate. This indicates that these impellers require high impeller speeds to disperse large quantities of air to the outermost limits of the cell. For the Dorr-Oliver impeller, the increase in air holdup with increase in impeller speed was fairly steady. Values of eg increased on average about threefold in going from the lowest (235 rpm) to the highest (295 rpm) impeller speed at the lowest air flow rate (16.5 l/see), but the increase was approximately fourfold at the highest air flow rate (56.7 l/see). This latter increase is slightly exaggerated by the conditions of flooding prevailing in the cell at 235 rpm and 56.7 l/see, which cause the eg values at locations 5 and 6 to be unusually low. Increasing the speed of the Outokumpu impeller had hardly any influence on the measured values of eg at the lowest air flow rates investigated (16.5 Fsec and 28.3 l/see). At the higher air flow rates, there was a very big increase in eg in going from 225 rpm to 245 rpm (the highest impeller speed investigated) at all locations. This corresponded to a sudden increase in mean bubble size [1], a complete reversal of the trend of decreasing bubble size with increasing impeller speed, and was attributed to a dramatic increase in internal gas recireulation rate at the very high impeller speed. Flow patterns in the cell were not directly discernible from this work, but will be investigated in a glass cell of the same dimensions as the test flotation cell at the JKMRC in Brisbane.
An industrial scale flotationtank
1567
Effect of location It is quite evident from Tables 2a to 2d and Figures 2a to 2d that eg varies with location in the flotation cell. For all four impellers, Eg was greatest at location 4, which corresponds to the impeller tip, where air would be discharged into the cell. The value of Eg then typically decreased to location 6, which is at the wall of the cell (see Figure 3 of Gorain et al. [1]). In turn, the value at location 2, which is near to the impeller shaft and above the point where slurry would be drawn into the impeller, was typically lower than the value at location 6. There are some exceptions to these generalisations, mostly as a result of flooding in the cell, but the overwhelming majority of the results follow this pattern. In almost all cases, there was also a very clear decrease in eg in going from location 6 through location 5 to location 3. The path of decreasing eg values at locations 4--6-5-3 is part of the postulated circulation loop in the cell which is illustrated in Figure 3 of Gorain et al. [1]. The mean bubble size was generally found to increase along this path, a phenomenon previously attributed to coalescence [ 1]. These findings tend to support each other, as one would expect to measure smaller values of Eg in regions where bubble size is larger (as large bubbles rise more quickly than small ones, and would escape from the slurry). This suggests that the few exceptions to the finding that eg decreased along the location sequence 4--6-5-3 came about as a result of flooding (which would result in a high value of eg at location 3, vertically above the point of air addition into the cell) or a change in flow pattern. Exceptions of the first kind would be represented by the results obtained with the Dorr--Oliver impeller at 235 rpm and 56.7 l/sec air flow rate, the Chile-X impelle:r at 140 rpm and 160 rpm and 42.5 l/sec and 56.7 l/sec air rate, and the Pipsa impeller at 105 rpm, 120 rpm and 140 rpm, at 42.5 l/sec and 56.7 1/sec air flow rate. It is interesting to note that the cell had not previously been thought to be flooding at some of these conditions. Exceptions of the second kind are represented by the Outokumpu impeller at 245 rpm, and suggest the presence of a secondary flow loop within the outer circulation loop, or a different circulation loop completely. As mentioned above, this will be investigated in the 2.8 m 3 glass cell at the JKMRC.
Effect of impeller ~ype Figure 3 shows the variation of Eg with air flow rate and location in the cell for each of the four impellers tested. The values are those obtained when the impellers were being operated at the manufacturers' recommended speeds. As noted above, there is a general trend of decreasing air holdup along the flow profile of the cell fiom location 4 through location 6 to location 5 or 3; this increases again at location 1 and decreases once more at location 2. Whether the minimum occurs at locations 3 or 5 seems to depend on the impeller tyl~. and air flow rate. For all the impellers, the maximum eg value at each air flow rate occurred at location 4, which corresponded to the point of air addition to the cell. The shape of the eg versus "location" profile is indicative of how evenly the impeller was able to disperse the air in the cell - - pronounced minima or maxima would reveal that the dispersion was uneven. The magnitude of the eg values indicates how successfully the impeller was able to disperse the air into small bubbles (which hav,~ a slower rise velocity and hence a longer residence time in the cell, and consequently a greater holdup); alternatively, high values of eg may indicate flooding. Almost all the values obtained with the Pipsa impeller were between 5% and 10%, except at the very high air flow rate (and these were the result of flooding). The very sharp minimum at locations 5 and 6 at the highest air flow rate indicates that this impeller was not able to disperse this quantity of air evenly throughout the cell at the manufacturer's recommended speed. The situation was fairly similar with the Chile-X impeller, with most air holdup values being between 2.5% and 12.5%, supporting the finding that the bubbles produced were slightly smaller than those obtained with the Pipsa impeller [ 1]. With the Dorr-Oliver impeller, eg values were mostly between 5% and 12.5%, and no minimum occurred at locations 5 and 6. In fact, at the manufacturer's recommended speed, Eg values at locations 5 and 6 were, with only one exception, greater than the values at locations 1 and 2, indicating that this impeller was able to disperse air to the outer regions of the cell even at the highest flow rate investigated. The range of eg
1568
B.K. Gorain et al.
values suggests bubble sizes of the same order of magnitude as the Chile-X impeller, which was in fact found to be the case. The eg values obtained with the Outokumpu impeller were on the whole greater than those produced by the other impellers, ranging between 7.5% and 17.5%, and suggest that the bubbles were very small, which is supported by the experimental values obtained [1]. As no flooding was observed to occur with this impeller, the increase in eg values at the highest air flow rate has been attributed to internal gas recirculation in the ceil. An investigation of the flow patterns obtaining when using the different impellers will be carried out using the 2.8 m 3 glass cell at the JKMRC.
Plpsa
L
0 .C
Impeller
Chlle-X
35
35
30
30-
25
25"
20
Impeller
20-
i
% %
15
15"
m I . . . .
10
_
10"
5
~
~---t---4r-
=
5"
0
i
i
I
i
i
4
6
5
3
1
0
l
4
I
6
location
Dorr-Oliver
I
I
l
5
3
1
2
Location
Outokumpu
Impeller
impeller
35
35
30-
30 o
25 "t
25" Q. =1
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20"
0
era ii 0
15"
15 "t
- - - "~' ,., .,.', ,.,~
t
10"
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1ll
.---'*---,F
----
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50 0
i
i
i
I
4
6
5
3
location
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16.5 I / x c
- -41, -
28.3 I/eec
- -I
-
42.5 I/No
- -e, -
56.7 I/see
!
1
2
0
4
6
5
3
Iocstlon
Fig.3 Variation of gas holdup with air flow rate and location for the four impellers at the manufacturer's recommended impeller speed (Pipsa 120 rpm, Chile-X 160 rpm, Don'-Oliver 255 rpm, Outokumpu 205 rpm).
1
2
An industrialscale flotationtank
1569
"Average air holdup" Another way of comparing the different types of impellers used in the test programme is on the basis of "average air holdup", calculated as the simple arithmetic average of the values measured at locations 1 to 6. These values are shown alongside the measured values in Tables 2a to 2d above. Although the values have no strict physical significance, they are an estimate of the "mean" air holdup in the cell at a particular combination of impeller type, speed and air flow rate, in the same way that "average bubble size" was calculated and used by the authors previously [1]. Interestingly, most of the values lie between the corresponding values for locations 3 and 4 (i.e. in the centre of the cell). Table 3 shows the "average air holdup" values for each of the four impellers at the recommended impeller speeds, at each of the four air flow rates investigated in this work. The values range from around 5% to nearly 19%. The Outokumpu impeller produced the greatest values of eg at each of the four air flow rates, a not unexpected finding as it also produced the smallest average mean bubble sizes [1]. No clear pattern emerges for the other three impellers; the Pipsa impeller produced the smallest values at the intermediate air flow rates (28.3 l/see and 42.5 Use,c), the Chile-X at the lowest air flow rate (16.5 l/see) and the Dorr--Oliver impeller at the highest (56.7 l/see).
TABLE 3 Gas holdup and power input at the manufacturers' recommended speed Qg=air flow rate, l/sec; I~g=air holdup, %; P=input power, kW
Impeller type Qg[
~p~
i
il
ChH~X
iiJRll lmammm laJ l
.~,'/~rlB
lr~al B
ireS,
Don-Olive
Outokumpu
N
It is also interesting to compare the relative performance of the impellers in producing air holdup in the cell on the basis of power input. The power drawn by the impeller in each of the tests carried out in this programme is given in the last column of Tables 2a to 2d. For each impeller, at each impeller speed, the power draw decreased steadily with increase in air flow rate. The power input values at the manufacturers' recommended impeller speeds are listed in Table 3: the Outokumpu impeller drew more power than any of the other impellers, and the Dorr-Oliver the least. To produce an average gas holdup value of approximately 15% at the maximum air flow rate of 56.7 l/see, the Pipsa impeller would have to be rotated at 160 rpm, requinng a power input of 3.64 kW. To do the same, the Chile-X impeller would need around 150 rpm and 2.68 kW, the Outokumpu 185 rpm and 2.64 kW, and the Dorr--Oliver impeller 265 rpm and only 1.88 kW. Th,~se values were obtained from Tables 2a to 2d by interpolation.
CONCLUSIONS Gas holdup measurements have been carried out in an industrial-scale test flotation cell, located at the head of a zinc cleaner circuit, using four typical impellers over a range of air flow rates and impeller speeds. The gas holdup was fcund to increase with increasing air flow rate at various locations in the cell, for all four impellers. The manner in which the increase took place depended on the impeller type and speed. In general the gas holdup values increased with increasing impeller speed for all the impeller types, as expected with a decrease in bubble size; except for the anomalies associated with flooding. With the Pipsa Ill I-IZ-I
1570
B.K. Gorainet al.
and Chile-X impellers, there was a very significant increase in air holdup near the cell wall with increase in impeller speed, reflecting the improved air dispersion. The Outokumpu impeller produced higher gas holdup compared to other impellers, thought to be associated with greater air circulation within the cell.
ACKNOWLEDGEMENTS The authors would like to thank Geoff Richmond, John Glen and other staff of Aberfoyle Resources Limited for their generous help throughout the period of the test work and for their constructive comments on this paper. The financial support of the Australian Mineral Industries Research Association (AMIRA Project P9K) and the Australian Research Council is also gratefully acknowledged.
REFERENCES .
.
Gorain, B.K., Franzidis, J.-P. & Manlapig, E.V., Studies on impeller type, impeller speed and air flow rate in an industrial scale flotation cell. Part 1: Effect on bubble size distribution. Minerals Engineering, 8(6), 615-635 (1995). Jameson, G.J. & Allum, P., A survey of bubble sizes in industrial flotation cells, report prepared for AMIRA limited, (1984).