The gas phase residence time distribution in flotation columns was investigated ... residence time of about 2-4(min), and the gas dispersion number was about ...
Minerals Engineering, Vo|. 7, Nos 2•3, pp. 333-344, 1994
0892--6875194 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
MEASUREMENT OF RESIDENCE TIME DISTRIBUTION OF THE GAS PHASE IN FLOTATION COLUMNS J.B. YIANATOS§, L.G. BERGH§, O.U. D U R ~ ~f, F.J. DIAZ~f and N.M. HERESI¢ Department of Chemical Engineering, University of Santa Mafia Valparafso, Chile t Department of Radioactive Tracer Applications Chilean Commission of Nuclear Energy, Santiago, Chile Heresi Consultants, Santiago, Chile (Received 6 July 1993; accepted 16 August 1993)
ABSTRACT The gas phase residence time distribution in flotation columns was investigated experimentally by the impulse response method using a radioactive tracer gas (Krypton-85). The experimental methodology consisted of introducing an impulse of radioactive gas inside the air sparger using a specially designed device, and on-line measurement of the transient response at various levels in the column. This technique wasfirst tested in a pilot column of l O(cm) diameter and 8(m) height. Tests were made at 0.8-0.9-1.0-1.5 (cm/s) superficial gas rates and 0-10-20 (ppm) frother dosage. Afterwards, the gas phase of an industrial column of O.91(m) diameter and 15(m) height, operating in a molybdenite cleaning circuit, was studied. Tests were made at 1.3-1.7-2.1 (cm/s) superficial gas rates and O. 7 (cm/s) superficial tailing rate. The transient response curves from pilot and imtustrial columns showed a typical gas residence time of about 2-4(min), and the gas dispersion number was about O.1 and O.4, respectively. The go4. phase in the froth zone behaved closer to a plug flow while operating at superficial gas rates lower than 1.5(cm/s) and superficial wash water rates of O.2-0. 4(cm/s). A significant gas short-circuiting into the tailing flowrate was clearly observed in the industrial column. Keywords Column flotation; gas RTD; gas dispersion; radiotracer; krypton-85
INTRODUCTION A flotation column consists of two principal zones: the collection (bubbling) zone and the cleaning (froth) zone. The pulp feed enters the collection zone below the interface and moves downward by gravity, thus contacting a bubble swarm generated from the bottom through a gas sparger. The froth zone is a mobile packed bubble bed that is contacted countercurrently with wash water (added near the lip level). Some of the wash water is recovered into the concentrate overflow, the remainder providing a net downward flowrate called a positive bias. 333
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J . B . YI^NATOS et al.
The gas phase, finely dispersed in small bubbles, is responsible for the selective recovery of the floatable hydrophobic mineral in the collection zone. The gas phase must also be able to transport the mineral throughout the cleaning zone up to the lip level. In the homogeneous bubble-flow regime, which is characteristic of a good column flotation operation, a size distribution of bubbles occurs. Hence, the bubbles rise with different velocities and their residence times in the system are different. Because of the presence of frother and collected solid, bubble coalescence is not significant in the collection zone and bubble flow is segregated in fast-rising large bubbles and slow-rising small bubbles. The small bubbles can even be entrained downwards in the presence of strong liquid circulation. On the other hand, the gas holdup almost doubles from the bottom to the interface level, mainly because of the gas expansion by decreasing the hydrostatic pressure. The RTD of the gas phase determines the driving force for mass transfer between the pulp and gas phases, as well as for mass transport from the collection zone to the froth zone, and thus is of direct importance for the recoveries and selectivities in flotation columns. Despite the important role of the gas phase in the column flotation process, only the gas flowrate is commonly measured and controlled (typical superficial gas velocities are 1-2 era/s), and sometimes the average gas holdup in the collection zone is also estimated. However, there is a lack of data on actual bubble size distributions in industrial columns, and to our knowledge no study has been reported concerning the RTD of the gas phase in flotation columns. There are only very limited data available on RTD of the gas in bubble columns operating in the range of interest for column flotation. In most eases, gas phase dispersion has been measured using distilled water or tap water without frother, as the liquid phase [2,3]. Under these conditions only large bubbles were typically generated using perforated plates with large hole sizes (several millimetres), and tests were developed at very high superficial gas rates 3-50 (era/s). The above conditions are far different from a flotation column operation, where bubbles are small (0.5-2.0 mm) because of the presence of frother, and where two distinct zones are present. In order to estimate the metallurgical recovery of the column flotation process the common approach is to use kinetic models which are related to the residence time and the degree of mixing of the different phases. In previous work [1], the residence time distributions of the actual liquid and mineral particles (size by size) have been measured. In the present work the objective was to study the gas phase transport in industrial flotation columns, under normal operating conditions of gas rates and bubble size. The effect of gas entrainment into the tails, was also investigated.
EXPERIMENTAL A preliminary study was developed in a pilot column 10(cm) diameter and 8(m) height, fabricated from "fiber-glass" in sections of 0.5, 1.0 and 2.0(m) height, and provided with a single filter cloth sparger. Two sections of "Plexiglas', located at the gas sparger level and at the liquid-froth interface level, allow visual inspection of the gas sparger operation and level control. The column was operated under automatic level control by manipulating the tailing flowrate with a pinch valve. The objective of the pilot scale work was to develop a methodology for manipulation, injection and detection of the radioactive gas at different levels in the column, and also, to get a first insight into the effect of superficial gas rate and frother dosage on the residence time distribution of the gas in the column. Although the presence of mineral particles is important in studying the gas phase hydrodynamics, the scope of the investigation at pilot scale was limited to gas-in-liquid dispersions.
Measurement of residence time distribution
335
The industrial study was developed in a column 0.91(m) diameter and 15(m) height, operating as a 3rd cleaner in a molybdenite circuit. The column was operated under automatic level control by manipulating the wash water flowrate, while the tailing flowrate was automatically manipulated with a pinch valve in order to maintain a positive difference (bias) between feed and tailing flowrate. The objective of this work was to get a direct measurement of the gas residence time distribution in an industrial flotation column operating under normal conditions. Also, the effect of gas flowrate on gas dispersion and entrainment of gas into the tails were explored. The experimental methodology consists of introducing an impulse of radioactive gas into the air sparger using a specially designed device, together with on-line measurement of the transient response at various levels in the column, from the air entrance up to the air exit at the top of the column. Tracer Selection The RTD of the gas in bubble columns has been estimated using different inert gases. For instance, Molerus and Kurtin [2] used Argon as the gaseous tracer to eliminate absorption effects in water. The tracer injection was performed using a solenoid valve, and the concentration of tracer gas exiting at the column head was measured by means of a mass spectrometer. Wachi and Nojima [3] used Freon 12 as the gaseous tracer, and the gas was injected into a gas nozzle by manually opening a ball valve. To observe the concentration response of the gaseous tracer, a part of the exhaust gas was continuously sampled, and was introduced into a FID gas-chromatograph. Alternatively, the use of a radioactive gas tracer should be more efficient because the volume of tracer required is very small, thus it can be diluted into a small amount of carrier gas and injection is almost instantaneous, without disturbing the system. Furthermore, the presence of tracer in the overall gas flowrate can be detected on-line at any level in the column, without sampling errors. In this study, a radioactive gaseous tracer Krypton-85 was selected. Krypton-85 is a beta emitter and also has a low gamma radiation emission (514 KeV, 0.41%). Another important property of the Kr-85 is the large half life (10.7 year), which allows for a large storage time. On the other hand, the disadvantage is that after the tracer discharges to the atmosphere it has a slow decay. Fortunately, this is not critical because the amount of tracer used for process testing is very small and there is a large dilution into the air. This aspect was also quantified. Estimation of Radioactive Tracer Activity Figure 1 shows the location of the gamma radiation sensors in the pilot and industrial columns. According to these arrangements and considering the column characteristics shown in Table 1, activities were calculated in order to measure the tracer presence from outside the column at different sensor locations. Calculations were made considering that the radiation sensors, located in front of the column wall, are colimated by means of a lead shell. It was assumed that the response signal should be 10 times the background noise. Results showed an activity requirement in the order of 20(mCi) per injection for the pilot column and 300(mCi) per injection for the industrial column. Gas Injection System Pilot Column. In order to inject the gaseous tracer into the gas sparger, the air line was split into two parallel lines, one of them including a stainless steel cylinder of special design, (see Figure 2). The Kr-85 gas, previously diluted with nitrogen and stored under vacuum, was first transferred from the gas storage tank into the cylinder isolated under vacuum. Afterwards, the gaseous tracer was injected into the gas sparger by an instantaneous switch of the air flowrate through the line containing the tracer sample. The gas residence time in the sparger was about 5-10(s).
336
J . B . YIANATOS et al.
GAS EXIT WASH WATER
CONCENTRATE
INTERFACE LEVEL FEED
@ /
AIR
Fig. 1 Sensors Location.
TABLE 1 Pilot and Industrial Column Characteristics
Pilot Column
Industrial Column
Glass Fiber
Iron
10
91
Height (cm)
800
1500
Wall thickness (cm)
0.6
0.9
Collection Zone Density (g/mL)
0.8-0.9
0.9-1.0
Froth Zone Density (g/mL)
0.1-0.3
0.2-0.3
Superficial Air Rate (cm/s)
0.5-3.0
0.5-3.0
30-40
30-40
Material Diameter (cm)
Air Line Pressure (psi)
Measurement of residence time distribution
337
injection cilinder (~
AIR
1
vacuum pump
storage Kr-85
Fig.2 Gas Injection System in Pilot Column.
Industrial Column The gas sparger consists of 8 parallel rubber tubes. The Kr-85 gas injection system, Figure 3, was connected into the air line entering one central rubber tube, from the air manifold. The tracer was first transferred from the storage tank to the injection cylinder under vacuum. The gaseous tracer was then diluted with air until it reached the pressure of the air line in the cylinder. Finally, the Kr- 85 was instantaneously injected into the gas sparger by means of a nitrogen overpressure. The gas residence time in the sparger was about 3-6(s).
INDUSTRIAL COLUMN I
AIR
~)~aouum pump
--@
N2 storage Kr-85 injection cilinder Fig.3 Gas Injection System in Industrial Column.
338
J.B. YIANATOSet
al.
Security In order to insure the proper removal and dilution of the gaseous tracer from the top of the column~, in extraction unit was installed above each column. The system was arranged like an inverted funnel and was made of polyethylene and provided with a gas extractor to discharge the gaseous tracer outside the building. All the system was on-line monitored with portable radiation sensors during tests.
Radioactive Gas Sensor Locations The radiation from the gaseous Kr-85 was detected by using scintillating sensors of NaI(TI) of l ' x l . 5 " , Saphymo Srat, with their associated electronics. According to Figure 1, sensor 1 was located in front of the gas sparger to register the tracer input into the bottom of the collection zone. Sensor 2 was located in the collection zone of the pilot column, near the interface level, at 135(cm) from the top of the column. In the industrial column, sensor 2 was located in the froth 65(cm) below the lip level. Sensor 3 was located 40(cm) below the top of the froth in the pilot column, while sensor 3 was located 15(cm) above the top of the froth in the industrial column. In both columns sensor 4 was located in front of the tailings line, to register the gas entrainment.
RESULTS AND DISCUSSION Pilot Column Study Table 2 shows a summary of experimental conditions used during tests. Solutions of 0, 10 and 20 ppm "Aerofroth-65" (a commercial frother produced by Cyanamid) in tap water at 290K were used. Sensor 1 showed that tracer injection was closer to an impulse, before the signal became contaminated by gas circulation in the column. TABLE 2 Pilot Column: Operating Conditions
(cm/s)
Frother Aerofrother-65 (ppm)
1.5
110
10
0.76
1.5
110
10
0.4
1.00
0.9
110
10
1.20
0.4
1.55
0.8
110
10
0.70
0.2
0.83
0.8
110
10
0.70
0.4
0.72
1.0
70
20
1.20
0.4
1.45
1.0
110
20
0.00
0.0
0.00
1.0
0
0
0.00
0.0
0.00
1.5
0
0
Wash Water rate(cn~s)
Tails rate
Air rate
(cm/s)
(cm/s)
0.60
0.3
0.70
2
0.65
0.3
3
0.70
4
Test #
Feed rate
(cm/s)
9
Froth Depth
Figure 4 shows the time response curves, test 1, observed after the impulse injection, near the top of the collection zone (sensor 2) and near the top of the froth zone (sensor 3). Both curves are similar in shape,
Measurement of residence time distribution
339
but the response from sensor 2 shows a relatively longer tail, related to the higher gas circulation in the collection zone, while the response from sensor 3 is clearly shifted by a time delay. These results show that the significant decrease (about 80~) of the interstitial velocity of the bubbles entering the froth zone aim decreases the gas dispersion, hence the gas in the froth behaves closer to plug flow. This observation agrees with the study on froth transport in a laboratory scale column [5].
0.60 13 13 "o -o ID -4--
N
0.40 0.30
o
to 0.20
z
0"101~ ~ 0.000
2
4 6 Time[mini
8
10
Fig.4 Time Response Curves from Pilot Column Table 3 shows a summary of estimated average residence time and dispersion number Nd, corresponding to sensors 2 and 3. In both cases observations were made inside the c o l u m before the gas exit. Thus, the signals are contaminated by internal gas circulation, particularly sensor 2, located in the collection zone. Even so, the response curve from sensor 3 is closer to the actual gas residence time distribution because of the smaller gas dispersion in the froth zone.
TABLE 3 Pilot Column: Gas Residence Time and Dispersion Number Test No.
Sensor 2
Sensor 3
m
Nd
Tan (rain)
Nd
2.35
0.110
2.73
0.070
2
2.51
0.110
3.00
0.060
3
2.60
0.110
3.90
0.090
4
3.31
0.159
4.56
0.110
5
2.86
0.119
4.23
0.080
6
3.21
0.179
3.58
0.129
7
3.59
0.070
4.31
0.040
8
1.96
2.33
0.159"
.
Tan (rain) [
9 1.44 1.67 0.249* * model fit is nol good for tests without frother
340
J.B. YIANATOSet
al.
The dispersion number Nd, corresponding to the axial dispersion model, was estimated using a numerical integration routine proposed by Xu and Finch [6]. According to the experimental arrangement, the open vessel assumption more closely approximates the gas input/output conditions. Thus, the open-open boundary condition was used with a cut-off point on RTD tail greater than 3 residence times. Tests 8 and 9, without frother, showed a different response curve. In these cases the curve consists of a dead time followed by a sharp peak, showing the faster transport of larger bubbles, and a longer tail accounting for a highly mixed system. These types of curve can not be fitted properly using the axial dispersion model.
Plant Column Study Again, sensor 1 showed that tracer injection was closer to an impulse, before the signal became contaminated by internal gas circulation in the column. Table 4 shows a summary of the column operating conditions for industrial tests. Estimates of average gas holdup in the collection zone, ego, and average gas holdup in the froth zone, egf, were calculated from independent measurements of pressure profiles along the column, using a portable submergible pressure sensor.
TABLE 4 Industrial Column: Operating Conditions
Test #
Feed rate
(cm/s)
Wash Water rate (cm/s)
Tails rate
Air rate
(cm/s)
(cm/s)
Froth depth (cm)
Gas holdup ego
I
] egf
0.60
0.20
0.70
1.3
250
0.13
0.73
0.59
0.18
0.69
1.7
250
0.15
0.75
0.60
0.28
0.70
2.1
250
0.22
0.75
Figure 5 shows the time response curves from test 2, observed after the impulse injection at time zero. Sensors 2, in the froth, and sensor 3, at the gas exit, show the dispersion of tracer in the froth zone and leaving the froth zone are similar with a time delay. This observation agrees with results from pilot scale showing that gas dispersion in the froth zone is less significant, hence the gas behaves closer to a plug flow for lower superficial gas rates. Sensor 3 typically showed a pulsating response, about 2 (min) period. This behaviour can be related to low frequency oscillations of interface level, which allow the accumulation or release of a gas volume, thus enhancing or decreasing the average gas flowrate. This effect is more important at lower gas rates. Table 5 shows a summary of estimated average residence time and dispersion number Nd, corresponding to sensor 2, located in the froth, and sensor 3 located just above the top of the froth at the air exit. Thus, the observation from sensor 3 corresponds to the overall gas residence time distribution RTD. Furthermore, an independent estimate of the gas residence time was developed from direct measurements of gas flowrate and gas holdup. These results show a reasonable agreement which validates the RTD data.
Measurement of residence time distribution
0.60
341
--IISensor 2 []
0.50
Sensor 3 0
u 0.40
"0
"15 0
%
N 0.30 0
E ,0.20 0
"7
0.10
0.00
0
2
4 6 Time [min]
8
10
Fig.5 Time Response Curves from Industrial Column
TABLE 5 Industrial Column: Gas Residence Time and Dispersion Number
Test #
Sensor 2
Tan (min)*
Sensor 3
m
Tan (rain) I
Nd
Tan (rain)
Nd
1
3.54
0.328
4.60
0.338
4.9
2
3.29
0.299
4.13
0.378
4.1
3
3.33
0.219
4.39
0.299
4.1
* estimated from independent measurements
Effect of Gas Flowrate on Gas Residence Time From pilot column results, Table 3, it can be seen that average gas residence time increases slightly by decreasing the superficial gas rate (tests 1-3). From industrial observations, Table 5, it can be seen that average gas residence time does not change significantly while varying the superficial gas rate from 1.3 to 2. l(cm/s). This is because the increase in superficial gas rate is partially compensated by an increase in the collection zone gas holdup. However, what is important to note is that an increase of superficial gas rate dramatically decreases the gas residence time in the froth zone, because changes in the gas holdup of the froth are less significant. It is well known that mineral recovery is strongly dependent on the mineral residence time in the froth. According to this, complementary experiments were done using radioactive floatable minerals [4]. It was found that transport of floatable minerals along the froth was very similar to that of the gas, showing a similar dispersion and time delay. On the other hand, the residence time distribution of the floatable minerals reporting to the tails showed a behaviour similar to that of the gengue.
342
J . B . YIANATOS et al.
Effect of Frother Dosage on Gas Residence Time In the pilot column, Table 3, the average gas residence time increases by increasing the frotber dosage from 0 to 20 ppm Aerofroth-65 (tests 9-3-8), because of the increase in gas holdup relst_~_ to the bubble size decreasing with frother dosage. Effect of Gas Flowrate on Gas Dispersion Results from the pilot column, Table 3, show that gas dispersion did not change significantly by increasing the superficial gas rate from 0. 8-1.5(cm/s) in tests 1,2,3,5, despite the change in supzdicial wash water rates. However, tests 8-9 show a more significant increase in gas dispersion with gas rate, while operating without frother. Increasing the superficial feed rate from 0.7-1.2(cm/s) also increases the gas dispersion (tests 3,4). From the industrial column, Figure 6, the gas dispersion shows a slight trend to decrease by increasing the superficial gas rate from 1.3-2. l(em/s). Also, from Figure 6 it can be seen that increasing superficial gas rate above 1.5(cm/s) the difference between the gas dispersion from sensor 2 and sensor 3 became significant. Otherwise, the gas mixing in the froth increased by increasing the gas rate above 1.3(cm/s). This result is in good agreement with studies on the froth cleaning action [1,7], where it was shown that increasing superficial gas velocities above 1.5(em/s), the liquid and free particles entrainment into the froth increases strongly, thus decreasing the cleaning action.
0.50 --11-Sensor
0.40
3
Sensor 2
cO 0
o.3o
o 0.20 0
0
0.10
0.00
E
.0
r
1.5 2.0 Superficial Gas Rate [cm/s]
2.5
Fig.6 Effect of Superficial Gas Rate on Gas Dispersion.
Gas Entrainment into the Tailings Data obtained from sensor 4, located in front of the tailing flow, are shown in Figure 7. Here, it can be seen that in the pilot column there was no gaseous tracer reported into the tails, while the industrial operation showed a significant presence of gaseous tracer in the tailings stream. The difference can be attributed in part to the higher viscosity of the pulp and mainly to the geometry of the system. For instance, in the pilot column the distance from the gas sparger to the bottom pipe was about 2 column diameters, and in this zone the liquid velocity increases about 40 times. In the industrial column, however, the distance from the gas sparger to the bottom pipe was about 0.5 column diameter and the pulp velocity increases about 80 times. Thus, the industrial column design favours the entrainment of finer bubbles from the gas sparger to the bottom exit pipe.
Measurement of residence time distribution
0.60
343
--IsIndustrlal
0.50
o Pilot
0
.,tO
13
0.40
"0 (1)
N 0.30 0
E
0.20
Z
0.10 0.00
---._,,_ 0
2
4 6 Time [min]
8
10
Fig.7 Gas Entrainment into the Tailings
Comparison of Liquid, Solid and Gas Residence Time Using radioactive tracer techniques previously developed [4], the liquid residence time, as well the floatable and non-floatable solid residence time was also measured in the industrial column. Comparative results are shown in Table 6.
TABLE 6 Industrial Column: Liquid, Solid and Gas Residence Time Tracer
Floatable Solid
Average Time (mln) Concentrate
Tailings
6.1
14.9
Non-floatable (Gangue)
16.1
Liquid
25.0
Gas
4.1
2.7
From Table 6 it can be observed that the floatable solid reporting to the tails has a residence time similar to the gangue. The solid residence time is significantly smaller than the liquid, as was shown previously [1]. The floatable solid reporting to the concentrate shows an average residence time similar to that of the gas, despite the internal solid circulating load (dropback from the froth to the pulp). These results show that the main part of the floatable mineral was collected very fast after entering the column and was removed and transported into the concentrate with an average velocity similar to the gas. Furthermore, it is important to note that under typical industrial operating conditions, more than 30(%) of the gas residence time is spent in the froth, which helps to improve the mineral grade (froth cleaning and selectivity) but decreases recovery (dropback).
344
J.B. YIANATOSet ai. CONCLUSIONS Average gas residence time from the industrial flotation column was about 4-5 (rain). Typically, more than 30(%) of the gas residence time was spent in the froth zone. Average gas residence time estimated from RTD was in good agreement with independent estimates based on gas holdup and gas flowrate measurements. Gas residence time did not change significantly while varying the superficial gas rate from 12(cm/s), because of the increase in the collection zone gas holdup. Gas dispersion numbers of about 0.3-0.4 were estimated from testing an industrial flotation column, 0.91(m) diameter and 15(m) height, for a range of 1-2(cm/s) superficial gas rate. The gas transport along the froth was closer to plug flow for superficial gas rates lower than 1.5(cm/s). At higher gas rates the gas mixing in the froth became significant. The same result was observed from the pilot and industrial columns. A significant amount of gas entrainment into the tails was observed in all tests at industrial scale. RTD of floatable solid reporting to the concentrate was similar to the RTD of the gas, while the floatable solid reporting to the tails showed an RTD closer to that of the gangue.
Progress in the field of modelling and design of flotation columns can be achieved if fundamental hydrodynamic relationships of each phase of this process are better known and understood.
ACKNOWLEDGEMENTS The authors are grateful to the Divisi6n El Teniente of CODELCO, for providing access to its flotation columns and for assistance in the experimental work. Funding for the column flotation research was provided by Projects FONDECYT 249/91, FONDEF MI-17, Chile.
REFERENCES I.
2. 3. 4. . .
7.
Yianatos, J.B. & Bergh, L.G., Int. J. Miner. Process., 36, 81-91 (1992). Molerus, O. & Kurtin, M., Chem. Engng. Sci., 41(10), 2693-2698 (1986). Wachi, Sh. & Nojima, Y., Chem. Engng. 8ci., 45(4), 901-905 (1990). Dur(m, O., Dfaz, F. & Heresi, N., C.Ch.E.N. Internal Repo~% Project #249/91, FONDECYT, Chile, March (1993). Pereira, M. Joao, M. Sc. Thesis, Institute Superior Tecnico, Universidade de Lisboa, Portugal, (June 1993). Xu, M. & Finch, J.A., Minerals Engineering, 4, 5/6, 553-562 (1991). Yianatos, J.B., Finch, J.A. & Laplante, A.R., Trans. Inst. Min. MetalL, 96, Sect. C, C199-C205 (1987).
