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Industrial testing of a gas holdup sensor for flotation systems
Minerals Engineering 16 (2003) 493–501 This article is also available online at: www.elsevier.com/locate/mineng
Industrial testing of a gas holdup sensor for flotation systems C.O. Gomez, F. Cortes-L opez, J.A. Finch
*
Department of Mining, Metals and Materials Engineering, McGill University, 3610 University Street, Montreal, Que., Canada H3A 2B2 Received 6 November 2002; accepted 4 March 2003
Abstract The role of gas holdup in flotation has long been discussed but never demonstrated, arguably because a reliable measurement technique has not been available. Work was initiated to develop a gas holdup sensor for industrial operations based on the use of two so-called flow cells for measuring the conductivity of the pulp with and without air. These are the measurements required to estimate gas holdup using MaxwellÕs equation that relates conductivity to concentration of a dispersed non-conducting phase (i.e., bubbles) in a continuous liquid phase (pulp in this case). After a series of prototypes a unit robust enough for industrial use that continuously measures and delivers signals easily integrated into a plant PLC system has been developed. This communication describes the working principle along with some construction details. The experience of plant tests, ranging from paper to mineral pulps, and mechanical cells to columns, is reviewed. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Flotation; Gas holdup; Measurement; Conductivity
1. Introduction When gas is dispersed into a flotation machine a fraction of the pulp is displaced, the volumetric gas fraction or gas holdup, eg . The role of gas holdup as a variable in flotation has long been discussed, but no convincing relationship with metallurgical performance has been established (Dobby et al., 1985; Mankosa et al., 1990). A prime reason, we argue, is because a reliable industrial measurement technique has not been available. It is known that gas holdup is defined by bubble size, gas and liquid flow rate, mixing patterns, and physical properties of the pulp such as density and viscosity (Finch and Dobby, 1990). It is also known that these same factors in turn contribute to flotation performance; therefore, it is reasonable to hypothesise that flotation response is related to gas holdup. The methods of measurement in use remain largely manual and discontinuous based on extracting a sample of pulp and determining the volume before and after deaeration (Deglon et al., 2000; Yianatos et al., 2001). About 10 years ago, the Mineral Processing Group at
*
Corresponding author. E-mail address: jim.fi[email protected] (J.A. Finch).
McGill University initiated development of a gas holdup sensor for industrial flotation machines that could be automated and give continuous signals. Electric conductivity, an intensive property sensitive to the presence and concentration of a dispersed phase (bubbles in this case) was considered a suitable basis for the sensor. Models relating gas holdup to conductivity are available (Fan, 1989). The models require the conductivity of liquid with and without air. Testing of the approach in laboratory flotation columns proved successful (Gomez and Finch, 1993). Application to industrial units required conductivity cells allowing a dispersion to flow through while conductance was being measured. Of the cell designs explored, cylinders with ring electrodes flush mounted to the internal wall performed the best (Tavera, 1996). These cells were referred to as ‘‘flow’’ cells. Characterization studies enabled development of relationships between cell geometry and electrical behaviour (Tavera et al., 1998). Two cell types were created: a cylinder open at both ends allowing relatively unrestricted flow of aerated pulp––the open cell––and a cylinder with a conical bottom which excluded the bubbles––the syphon cell. Testing of a prototype in flotation columns was successful (Gomez et al., 1994; Tavera et al., 1996). Refinements were necessary to the syphon cell to avoid plugging with particles. Hydrodynamic studies resulted in a model which made possible
0892-6875/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0892-6875(03)00083-9
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the design of syphon cells able to perform under most conditions encountered in practice (Cortes-Lopez, 1999). The electronics to drive the sensor also underwent change. In the prototype the conductivity in each cell was manually recorded and gas holdup calculated offline. This has evolved into the current arrangement where measurements are made continuously and the gas holdup is delivered on-line as a signal that can be interfaced to a personal computer or integrated into a plant PLC system. This communication describes the working principle of the two-cell concept used in the development of the sensor. Some construction details are included where they aid commissioning and operation of the unit. The results of tests in mechanical cells and flotation columns processing mineral and paper pulps illustrate the potential of the sensor.
2. The conductivity gas holdup sensor 2.1. Principle Electrical conductivity is an intensive property that for an aqueous solution depends on temperature and ionic strength. If a non-conducting material is dispersed in the liquid in the form of particles, droplets or bubbles, the conductivity of the dispersion decreases as a function of the volumetric concentration of the dispersed phase. Maxwell (1954) modelled this system and derived a relationship between the volumetric concentration of the dispersed phase and the conductivity of the dispersion relative to that of the continuous phase. The model has proved sufficiently flexible to describe mineral systems. One application is determination of volumetric solids concentration in slurries (Gomez et al., 1998). The situation of interest here is the gas concentration (gas holdup) in a flotation pulp. The conductivities required are those of the dispersion kd and the air-free dispersion (pulp) kp where the appropriate form of MaxwellÕs model is: eg ¼ ð1 kd =kp Þ=ð1 þ 0:5kd =kp Þ
ð1Þ
2.2. Flow cells The measurement of conductivity requires a conductivity cell. An approach based on the use of so-called flow conductivity cells is utilized. A flow conductivity cell in this context is defined as one that allows a dispersion to flow through while the conductance is being measured. Two cells were designed: an open cell to measure the conductivity of the dispersion, and a syphon cell used to exclude bubbles and measure the conductivity of the pulp (Fig. 1). Their design presented
Fig. 1. Schematic representation of open and syphon cells during measurement.
challenges in two areas: hydrodynamics and electrical performance. The open cell is a vertical cylinder open at both ends to allow relatively free flow of bubbles and pulp. The syphon cell, also a vertical cylinder open at the top, ends in a conical bottom with a small orifice. This design retards entry of bubbles and the cell fills with pulp that, being denser than the dispersion outside the cell, causes the cell contents to flow out through the orifice. A steady state is reached when the pressure head at the orifice is the same as that outside the cell, achieved when a velocity within the cell that exactly compensates the hydrostatic pressure difference is established. This downward pulp velocity, largest at the orifice, completes the exclusion of bubbles from the cell. Operation requires that the pulp velocity at the top of the cell be lower than the terminal velocity of the smallest size bubble present in the flotation system, otherwise air bubbles would be drawn (entrained) into the cell. The design model for the syphon cell includes these features as a function of orifice size (Cortes-Lopez, 1999). In the case of electrical performance, the most important feature of a conductivity cell is the cell constant which permits the measurement, namely conductance K, to be converted to the property, conductivity k, needed in MaxwellÕs model. The cell constant depends on the dimensions and geometry of the electrodes used to transfer the electric energy and is defined as the ratio of the effective surface area A (normal to the flux of electric current) to the distance L between the electrodes. K ¼ ðA=LÞk
ð2Þ
In an ideal conductivity cell A=L is independent of the conductivity of the liquid. In our case, the open and syphon cells have three internal ring electrodes flush mounted to the wall. The three-ring arrangement is used to limit the electric field to the volume in between the
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top and bottom rings: by maintaining these two electrodes at the same potential and the central electrode at opposite polarity, a current path outside this volume is avoided. This arrangement renders a cell constant that depends slightly on the conductivity of the material being measured. This variation is characterized by calibration as a polynomial that relates cell constant to the conductance measured. Design criteria have been developed to define ring dimensions and their separation to achieve a specific cell constant (Tavera et al., 1998; Cortes-Lopez, 1999). 2.3. Electronics The sensor measures the conductance in the two cells, open and syphon, immersed side by side in the flotation pulp. Prototypes used a single-channel conductivity meter and a computer to sequentially connect each cell through a relay board and digitize the output using an analog-to-digital converter. Attempts to use two singlechannel conductivity meters simultaneously to provide a continuous measurement failed because of electrical interference. This was solved with a two-channel conductivity meter (Yokogawa, model RS402). The unit provides two 4–20 mA loops that can be programmed to deliver either of the conductivities, their ratio, or directly the gas holdup, which facilitates integration of the sensor to plant PLC systems. The unit provides a continuous signal. The sensor has been used to date primarily to demonstrate accuracy and response to process changes. The unit is driven from a laptop computer to collect data for off-line analysis. A box has been assembled using commercial components to deliver digitized signals through serial communication. 2.4. Sensor specifications and commissioning The sensor shell is made of PVC and the electrodes from stainless steel. A typical design has an open cell 0.1 m in diameter and 0.5 m long, and a syphon cell 0.075 m in diameter and 0.45 m long, attached to a supporting 00 bar. By screwing a 1/2 threaded (NPT) tube into this bar the sensor can be firmly and easily inserted from the top of a flotation machine (column or mechanical) to a desired location. Operation mainly depends on the syphon cell: it must not plug with solids or entrain bubbles. To ensure this, the orifice has been made removable (Fig. 2) to allow installation of the most effective size for the prevailing conditions. Low pulp densities and gas holdups call for a larger orifice while large pulp densities and gas holdups require a smaller one. One of three orifices, 0.0010, 0.0012 and 0.0014 m diameter, have proven effective in the industrial testing completed to this point.
Fig. 2. Details of gas holdup probe: open and syphon cell showing removable orifice tip.
Commissioning a unit involves calibration of the flow cell/meter combination, and programming the meter to define values for the cell constant and range of channel output signal (4–20 mA). Cell constants can be defined by the user, or calculated automatically and stored in the conductivity meter following input of the conductivity of the liquid being tested determined with a reference conductivity meter. Calibration is accomplished through measurements in water of known conductivity varied by addition of a salt (usually sodium or potassium chloride). Calibration includes the cable connection to the conductivity meter (a four-wire cable up to 100 ft long) which adds a 1–2 X resistance to the circuit (in most cases negligible as the resistance in the cells for a liquid of conductivity 1 mS/cm is in the order of 200 X). To illustrate the calibration procedure results from one exercise are included in Fig. 3. Actual conductivity was measured with a reference conductivity meter (Radiometer, model CDM210) using a standard two-pole probe (Radiometer, model CDC641T, cell constant 0.75 cm1 ). Standard conductivity liquids were used to calibrate the reference conductivity meter/sensor combination. When the sensor was immersed in a sodium chloride solution of conductivity 3.04 mS/cm at 16.2 °C, calculated cell constants were 0.388 and 0.412 cm1 for the syphon cell and open cell, respectively. The output for both cells was the same as the inputted value of 3.04 mS/cm and constant. As the conductivity of the liquid was changed, the readings from both cells deviated from the value measured with the reference meter, reflecting
C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501 20
7 Gas holdup
GAS HOLDUP, %
CELL CONDUCTIVITY, mS/cm
6
4
2 Open
15
10
5 Syphon cell
5
4 Open cell
Syphon 0 15:00
0 0
2
4
6
15:10
3 15:30
15:20
TIME
ACTUAL CONDUCTIVITY, mS/cm
Fig. 3. Typical results of measurements for cell constant determination.
the non-ideal behaviour of the flow cells. However, the drift was small and practically the same for both cells; fitting to a straight line through the origin accounts for the drift in the gas holdup calculation. A possible source of error comes from compensating for temperature variations during measurements. Conductivity cells often include a temperature measurement to correct the conductivity to a reference temperature (normally 25 °C). In our experience temperature compensation has not proven necessary. The objective here is to measure the ratio between the conductivities of aerated and bubble-free pulp. This ratio will be the same no matter what the temperature provided both cells are at the same temperature and the cell constant in both cells is not affected differently. Measurements in several standard liquids at three temperatures demonstrated performance of the flow cell (cell constant) was not affected by temperature (Fig. 4). The conductivity meter can deliver up to two signals. We generally use: (i) for on-line plant monitoring, the gas holdup and the conductivity in the syphon cell (used
Fig. 5. Effect of temperature on open cell conductivity measurements.
to detect orifice plugging); and (ii) for off-line diagnosis, the conductivity in the open and syphon cells. During calibration conductivity readings are steady. In use when air bubbles are present, the readings fluctuate as a consequence of the variable number of bubbles. To illustrate, Fig. 5 gives the conductivity data associated with the gas holdup; the process noise is evident for the open cell through which the bubbles pass. Error propagation increases the noise on the gas holdup signal. Gas holdup values (and conductivity) are normally averaged over a period of time (e.g., 15 min in Fig. 5). To illustrate sensor performance the unit was immersed about 0.5 m below the water level in a pilot column bubbling air through water with 50 ppm of Dowfroth 250C. As gas flow rate was increased (Fig. 6) the desired behaviour was found: (i) a constant value reported by the syphon cell, indicating that no bubbles are entrained, and (ii) a decreasing value reported by the open cell reflecting the increase in gas holdup. Both cells reported the same value when no air was present (gas velocity zero).
1.2
CELL CONDUCTIVITY, mS/cm
8
OPEN CELL, mS/cm
6
Average
CELL CONDUCTIVITY, mS/cm
496
6
4 T=10 C 2
T=20 C T=30 C
1.0
0.8 Open Syphon 0.6 0.0
0 0
2
4
6
8
0.5
1.0
1.5
2.0
GAS VELOCITY, cm /s
ACTUAL CONDUCTIVITY, mS/cm
Fig. 4. Cell conductivity variations as a function of gas holdup.
Fig. 6. Typical results collected during a stable column operation period: (a) gas holdup results and (b) cell conductivity measurements.
C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501
the opportunity to test the effect on gas holdup of changing the number of spargers (anticipated to modify bubble size). In the case of mineral pulps, the experiments included: (i) step change in the gas flow rate to a flotation column; (ii) long-term (17 days) monitoring in a column; and (iii) mapping of gas holdup distribution in a mechanical machine.
30
GAS HOLDUP (Cond), %
497
20
10
3.1. Measurements in paper pulps 0 10
20
30
GAS HOLDUP (Pressure), %
Fig. 7. Comparison of gas holdup estimates from conductivity and pressure measurements in an idle industrial column filled with water.
The accuracy of the sensor was also tested in the same two-phase setup. Two portable, submersible pressure transmitters were installed 0.5 m above and below the center ring of the open cell. Gas holdup was manipulated by changing gas rate and measured at various depths (the sensor can be reeled up and down easily). The results showed good agreement between the conductivity- and pressure-based gas holdups (Fig. 7). One source of discrepancy is that gas holdup is not uniformly distributed in a column and the conductivity sensor gives a more localized measurement than pressure, which is the average for the entire column volume between the two pressure transducers. In the case of three-phase systems verification is more difficult. The approach has been demonstrated in the laboratory (Tavera, 1996), but validation in mineral flotation plants has not been accomplished. By using paper pulps, however, plant verification test work can be accomplished, as described later.
3. Plant testing A testing program was devised to examine sensor performance in plant and its sensitivity to gas holdup variations resulting from operational changes (CortesLopez, 1999). A sample of the results is described here, namely: testing on a pilot flotation column in a paper recycling mill (Bowater, De-Inking Plant, Gatineau, Quebec); in a flotation column at IncoÕs Matte Separation Plant (Sudbury, Ontario), and in one of the mechanical cells at NorandaÕs Brunswick Mine (Bathurst, New Brunswick). The tests in paper pulps offered a means of checking the accuracy of the gas holdup measurement. Paper pulp feeds to flotation have a solids content (consistency) of about 1% w/w so the density remains close to that of water. Therefore, gas holdup from pressure is, for practical purposes, accurate. The column also presented
BowaterÕs de-inking plant has two banks of eight Voigt cells in a rougher/cleaner circuit (six and two cells, respectively). A pilot flotation column (0.5 m 5 m) was run continuously with pulp diverted from the feed to the flotation circuit. The column was constructed from five flanged PVC sections (0.5 m in diameter, 1 m high) (Watson et al., 1996; Leichtle, 1998). It was fully instrumented and run automatically with level control by manipulation of feed rate. Air was delivered using a mass flow meter/controller (MKS, model 1162B). Gas holdup was also calculated from pressure readings collected with a differential pressure transmitter (Bailey, model PTS-DDD). The unit could be operated with up to five stainless steel porous rigid spargers. 3.1.1. Gas holdup measurement using pressure and conductivity Gas holdup was varied by gas flow rate adjustments and selection of number of spargers. The column normally stabilized (stable traces for the pressure and conductivity signals) in less than 10 min after a set of conditions was selected; pressure and conductivity data were collected for 15 min after stabilization. A total of 33 conditions were tested; average values for the gas holdup calculated from pressure and conductivity are included in Fig. 8. Accuracy was considered acceptable although at lower gas holdup the conductivity value tended to be 20
GAS HOLDUP (Cond), %
0
15
10
5
0 0
5
10
15
20
GAS HOLDUP (Pressure), %
Fig. 8. Comparison of gas holdup estimates from conductivity and pressure measurements in a pilot column processing a paper pulp.
C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501
less than for pressure. Subsequent work established that micro-bubbles trapped in the pulp fibers, which were accounted for by the pressure technique, were not picked up by conductivity because they were present in the ‘‘non-aerated’’ pulp in the syphon cell. This argument is consistent with the error being practically constant (1.5% gas holdup) and in the same range as the micro-bubble content over the measurement range. To test if the difference was significant the Student t-test was applied; for paired samples, the difference must satisfy 2:00 < tð¼ 1:61Þ < 2:00, where 2.00 is for 95% confidence and 62 degrees of freedom (t distribution). From this analysis we concluded that gas holdup from conductivity was not different from the value given by pressure. 3.1.2. Sensor response to changes in number of spargers The column was started with five stainless steel spargers delivering a total gas flow rate of 300 l/min (Jg ¼ 2:5 cm/s). After stabilization, data were collected for a 15-min period; the procedure was repeated with four and three spargers always delivering the same total gas flow rate of 300 l/min. The results (Fig. 9) revealed an effect: average gas holdups of 14.4%, 11.7% and 8.3% were measured for the five, four and three sparger combinations, respectively. Lower gas holdups for the same gas flow rate are consistent with larger bubbles, being generated as the volume of air per unit area of sparger surface is increased (Xu and Finch, 1989). Traces for the three conditions showed a tendency for signal noise to increase as the number of spargers was reduced. The increased signal noise suggests a wider range of bubbles sizes, again consistent with increased air per unit sparger area. 3.2. Measurements in mineral pulps
plied by MinnoveEX. Tests were performed in columns 2 and 3, both 1.77 m in diameter and 12 m high. Column 3 was used for measuring sensor response to step changes in gas flow rate, and column 2 for long-term continuous monitoring. 3.2.1. Sensor response to step changes in gas flow rate The objective was to test the sensitivity of the sensor and the dynamic response to changes in gas flow rate. A 10-h test was devised based on step changes of about 10% in gas flow rate: the column was run with the normal gas flow rate (Jg ¼ 1:8 cm/s) for three hours, then increased to Jg ¼ 2:0 cm/s for 4 h, and then decreased to a Jg ¼ 1:7 cm/s for 3 h before returning to the normal gas flow rate. The results are presented in Fig. 10a which illustrates gas flow and gas holdup, and Fig. 10b which shows the corresponding conductivity traces. The sensor was located 4 m from the top of the column. The gas holdup trace clearly shows a response to both step changes, gas holdup varying from an average of 5.9–7.2% during the initial step, and from 7.2% to 5.8% during the second step. Even the gas holdup difference between initial and final setting (5.9–5.8%) appears to detect the small difference in gas rate 1.8–1.7 cm/s. In terms of process dynamics, the results showed a slow reaction to reach steady state after the gas flow increase
15
(a) Jg=2.0 cm /s
GAS HOLDUP, %
498
5 spargers 10 4 spargers 3 spargers
5
0 15:00
15:30
16:00
TIME Fig. 9. Effect of sparger number on gas holdup in a pilot column processing a paper pulp.
9
6
9:00
12:00
15:00
18:00
21:00
0:00
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CELL CONDUCTIVITY, mS/cm
GAS HOLDUP, %
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Jg=1.8 cm /s Jg=1.7 cm /s
3 6:00
IncoÕs Matte Separation plant has five flotation columns retrofitted with variable-gap jetting spargers sup20
12
2.0
(b) 1.5 Syphon 1.0
Open
0.5
0.0 6:00
9:00
12:00
15:00
18:00
21:00
0:00
TIME Fig. 10. Gas holdup measurement during a step variation of gas flow rate in an industrial flotation column processing a mineral pulp.
C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501
(more than 2 h), perhaps because of limitations in the gas delivery system, but a fast response when the gas flow was decreased. The conductivity data (Fig. 10b) demonstrated the sensor performed even though the pulp conductivity was continuously changing (presumably indicative of variations in the solids content and/or chemical environment). For example, the same gas holdup was measured for the first 2–3 h of the test when pulp conductivity changes of about 20% were occurring, and a sudden decrease in gas holdup was measured around the second step when the pulp conductivity remained constant. 3.2.2. Long-term continuous monitoring The motivation was primarily to test sensor reliability, especially whether the syphon cell design resisted plugging of the orifice, and secondarily to demonstrate remote data retrieval (the signals were processed at McGill). Fig. 11 shows the results obtained over the 17day period for the sensor located about 3.75 m below the column lip (Fig. 11a gives gas holdup and Fig. 11b shows the conductivity in both cells). The test was successful: the sensor ran with no interruptions during the testing period and daily access from McGill was ac-
12
(a)
GAS HOLDUP, %
9
6 D 3 A
B
0 13-Mar
C
20-Mar
27-Mar
DATE
CELL CONDUCTIVITY, mS/cm
2.0
(b) 1.5
1.0
0.5 B
C
D
A 0.0 13-Mar
20-Mar
27-Mar
DATE
Fig. 11. Long-term sensor testing in an industrial flotation column processing a mineral pulp: (a) gas holdup results; and (b), cell conductivity measurements (note, upper trace is syphon cell).
499
complished without affecting the performance of the unit. From the perspective of operations, gas holdup slowly decreased from 8% to 4% during the first 12 days (Fig. 10a). One day after the shutdown on March 23 (C), the gas holdup started to recover but at the end of March 27 it went down again to about 6%. A change was made late on March 27 (D) that rapidly moved gas holdup to about 8% where it remained stable for the last three days of the test. The sudden nature of the changes suggests interruption in air delivery. The conductivity results (Fig. 11b) help diagnose the response. For example, the low gas holdup at the end of March 16 (A in the figure) corresponds to a similar and low conductivity in both open and syphon cells. This is consistent with a gas shutdown and the level quickly dropping (as the bubbles leave the pulp) and stabilizing below the sensor in this case, i.e., the sensor is immersed in the froth. Both cells then simultaneously report the same conductivity at about half of the value measured for the pulp, a result typical for a sensor located in the froth. This was different from the low gas holdup case early on March 19 (B). Again, as a consequence of a gas shutdown the level rapidly moved down but now the level appeared to stabilize above the sensor, the two cells registering the same conductivity this time indicative of non-aerated pulp. The fact that the sensor returned to operation after these incidents indicates a robust design. Previous versions plugged when the air was shut off which, by cutting out the syphon (flow) action, encourages particle settling in the conical bottom. It is notable that during the stable period starting late March 27 (D), the conductivity was changing substantially but this did not disturb the gas holdup result. The conductivity traces show a series of regular peaks at a frequency of about half a day; this may correspond to shift changes. After the test, the sensor calibration was checked; the results indicated that cell constants had not changed. 3.2.3. Mapping of gas holdup distribution in mechanical cells As part of a program to characterize cell performance at NorandaÕs Brunswick Mine (Dahlke et al., 2001), the sensor was used to try to identify a location as a cell sampling point. The results obtained in one of the zinc cleaning circuit Denver DR-100 machines (Fig. 12) demonstrated a distribution of gas holdup over the cross-section (depth was constant at 0.5 m). A consistent decrease from the near the impeller to half way to the wall occurred when the value stabilized. It was not possible to sample over the entire cross-section, as access was restricted to where cover plates were removable. These results demonstrate that the sensor functions equally well in agitated tanks as in the more quiescent columns. In terms of locating the sensor the
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C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501 Impeller
Gas holdup 16.6% 12.0% Baffle
12.0% 11.0%
inking Plant, and of NorandaÕs Brunswick Mine for their constant, enthusiastic support during the plant tests. Funding was under the Natural Sciences and Engineering Research Council (NSERC) Collaborative Research and Development program with sponsorship from Inco, Falconbridge, Noranda and Teck-Cominco.
10.3% 10.7%
PULP INLET
10.8%
References
10.3% 10.5%
Removable cell cover plates
Fig. 12. Gas holdup horizontal mapping in an industrial mechanical cell (Denver DR-100).
observations indicate gas is not distributed evenly; cell geometry and gas dispersion distributions must be understood to define a sampling point with the characteristics required for the purpose of the measurement. This is particularly true if different machines are to be compared.
4. Conclusion and future work A gas holdup sensor based on conductivity capable of performing in industrial environments has been created. The use of a commercial two-channel conductivity meter to drive the sensor facilitates commissioning and integration of the unit to plant control systems. Tests on paper pulps verified the accuracy of the measurement by comparing with gas holdup estimated from pressure. The sensor demonstrated sensitivity to gas holdup differences resulting from bubble size changes. Tests in mineral pulps showed a sensitive response to step changes in gas flow rate. A long-term test demonstrated the unit was reliable, the syphon cell remaining operational even when the air was unexpectedly cut off. The sensor detected radial gas holdup variation in a mechanical flotation machine, higher around the impeller decreasing towards the wall. Future use will be in machine hydrodynamic characterization and diagnosis, augmenting the use of parallel developments in gas velocity and bubble size sensors (Torrealba-Vargas et al., 2002; HernandezAguilar et al., 2002). One target will be to relate to metallurgical performance to aid development of process optimization strategies (Finch, 2002).
Acknowledgements The authors thank the staff and crew of the IncoÕs Matte Separation Plant, of BowaterÕs Waste Paper De-
Cortes-Lopez, F., 1999. Design of a gas holdup sensor for flotation diagnosis. M. Eng. Thesis, McGill University, Canada. Dahlke, R., Scott, D., Leroux, D., Gomez, C.O., Finch, J.A., 2001. Trouble shooting flotation cell operation using gas velocity measurements. Proceedings of the 33rd Annual Meeting of the Canadian Mineral Processors (Division of CIM), 23–25 Jan., pp. 359–370. Deglon, D.A., Egya-Mensah, D., Franzidis, J.P., 2000. Review of hydrodynamics and gas dispersion in flotation cells on S. African platinum concentrators. Minerals Engineering 13 (2), 235–244. Dobby, G., Amelunxen, R., Finch, J.A., 1985. Column flotation: some plant experience and model development. Symposium on Automation for Mineral Resource Development, Brisbane, Australia, 9–11 July, Aus. IMM., pp. 259–264. Fan, L.-S., 1989. Gas–Solid–Liquid Fluidization Engineering. Butterworths., pp. 47–52. Finch, J.A., Dobby, G., 1990. Column Flotation. Pergamon Press, Oxford. Finch, J.A., 2002. Gas dispersion management. Plenary Lecture, S. African IMM, Western Cape Branch Annual Meeting, 1–2 Aug. Gomez, C.O., Finch, J.A., 1993. An on-line sensor for gas holdup in flotation columns. Presented at the 32nd Annual Conference of Metallurgists, CIM, Quebec City. Gomez, C.O., Tavera, F., Finch, J.A., Perez, R., del Villar, R., 1994. Continuous gas holdup measurements in flotation columns. In: Castro, S., Alvarez, J. (Eds.), Proceedings of the IV Meeting of the Southern Hemisphere on Mineral Technology, University of Concepcion, vol. 2, (Flotation) pp. 123–136. Gomez, C.O., Probst, A., Finch, J.A., Moores, N., 1998. Monitoring thickener operation using a conductivity probe. Minerals and Metallurgical Processing 15 (4), 9–15. Hernandez-Aguilar, J.R., Gomez, C.O., Finch, J.A., 2002. A technique for the direct measurement of bubble size distribution in industrial flotation cells. Proceedings 34th Annual Meeting of the Canadian Mineral Processors (Division of CIM), 22–24 Jan., pp. 389–402. Leichtle, G.F., 1998. Analysis of bubble generating devices in a deinking column. M. Eng. Thesis, McGill University, Canada. Mankosa, M.J., Adel, G.T., Luttrell, G.H., Yoon, R.H., 1990. Modeling of column flotation with a view toward scale-up and control. In: Rajamani, R.K., Herbst, J.A. (Eds.), Control Õ90–– Mineral and Metallurgical Processing, AIME, pp. 35–41. Maxwell, J.C., 1954. A Treatise of Electricity and Magnetism, third ed., vol. 1, Part 11. Oxford University Press, London, pp. 435–449 (chapter IX). Tavera, F., 1996. Flow cells to measure electrical conductivity: use in estimating gas holdup in flotation systems. Ph.D. Thesis, McGill University, Canada. Tavera, F.J., Gomez, C.O., Finch, J.A., 1998. Conductivity flow cells for measurement on dispersions. Canadian Metallurgical Quarterly 37 (1), 19–25. Tavera, F., Gomez, C.O., Finch, J.A., 1996. Novel gas holdup probe and application in flotation columns. Transactions of the Institution of Mining and Metallurgy Section C: Mineral Processing and Extractive Metallurgy 105, 99–104.
C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501 Torrealba-Vargas, J., Gomez, C.O., Finch, J.A., 2002. Measurement of gas velocity in industrial flotation cells. Presentation at AMIRA P9 Meeting, Montreal, 18–30 Jan., 2002. Watson, J.A., Gomez, C.O., Finch, J.A., 1996. Deinking recycled paper using column flotation. In: Gomez, C.O., Finch, J.A. (Eds.), Column Õ96. Met. Soc. CIM, Montreal, pp. 195–208.
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Xu, M., Finch, J.A., 1989. Effect of sparger surface area on bubble diameter in flotation columns. Canadian Metallurgical Quarterly 28 (1), 1–6. Yianatos, J., Bergh, L., Condori, P., Aguilera, J., 2001. Hydrodynamic and metallurgical characterization of industrial flotation banks for control purposes. Minerals Engineering 14 (4), 1033–1040.
Industrial testing of a gas holdup sensor for flotation systems C.O. Gomez, F. Cortes-L opez, J.A. Finch
*
Department of Mining, Metals and Materials Engineering, McGill University, 3610 University Street, Montreal, Que., Canada H3A 2B2 Received 6 November 2002; accepted 4 March 2003
Abstract The role of gas holdup in flotation has long been discussed but never demonstrated, arguably because a reliable measurement technique has not been available. Work was initiated to develop a gas holdup sensor for industrial operations based on the use of two so-called flow cells for measuring the conductivity of the pulp with and without air. These are the measurements required to estimate gas holdup using MaxwellÕs equation that relates conductivity to concentration of a dispersed non-conducting phase (i.e., bubbles) in a continuous liquid phase (pulp in this case). After a series of prototypes a unit robust enough for industrial use that continuously measures and delivers signals easily integrated into a plant PLC system has been developed. This communication describes the working principle along with some construction details. The experience of plant tests, ranging from paper to mineral pulps, and mechanical cells to columns, is reviewed. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Flotation; Gas holdup; Measurement; Conductivity
1. Introduction When gas is dispersed into a flotation machine a fraction of the pulp is displaced, the volumetric gas fraction or gas holdup, eg . The role of gas holdup as a variable in flotation has long been discussed, but no convincing relationship with metallurgical performance has been established (Dobby et al., 1985; Mankosa et al., 1990). A prime reason, we argue, is because a reliable industrial measurement technique has not been available. It is known that gas holdup is defined by bubble size, gas and liquid flow rate, mixing patterns, and physical properties of the pulp such as density and viscosity (Finch and Dobby, 1990). It is also known that these same factors in turn contribute to flotation performance; therefore, it is reasonable to hypothesise that flotation response is related to gas holdup. The methods of measurement in use remain largely manual and discontinuous based on extracting a sample of pulp and determining the volume before and after deaeration (Deglon et al., 2000; Yianatos et al., 2001). About 10 years ago, the Mineral Processing Group at
*
Corresponding author. E-mail address: jim.fi[email protected] (J.A. Finch).
McGill University initiated development of a gas holdup sensor for industrial flotation machines that could be automated and give continuous signals. Electric conductivity, an intensive property sensitive to the presence and concentration of a dispersed phase (bubbles in this case) was considered a suitable basis for the sensor. Models relating gas holdup to conductivity are available (Fan, 1989). The models require the conductivity of liquid with and without air. Testing of the approach in laboratory flotation columns proved successful (Gomez and Finch, 1993). Application to industrial units required conductivity cells allowing a dispersion to flow through while conductance was being measured. Of the cell designs explored, cylinders with ring electrodes flush mounted to the internal wall performed the best (Tavera, 1996). These cells were referred to as ‘‘flow’’ cells. Characterization studies enabled development of relationships between cell geometry and electrical behaviour (Tavera et al., 1998). Two cell types were created: a cylinder open at both ends allowing relatively unrestricted flow of aerated pulp––the open cell––and a cylinder with a conical bottom which excluded the bubbles––the syphon cell. Testing of a prototype in flotation columns was successful (Gomez et al., 1994; Tavera et al., 1996). Refinements were necessary to the syphon cell to avoid plugging with particles. Hydrodynamic studies resulted in a model which made possible
0892-6875/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0892-6875(03)00083-9
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the design of syphon cells able to perform under most conditions encountered in practice (Cortes-Lopez, 1999). The electronics to drive the sensor also underwent change. In the prototype the conductivity in each cell was manually recorded and gas holdup calculated offline. This has evolved into the current arrangement where measurements are made continuously and the gas holdup is delivered on-line as a signal that can be interfaced to a personal computer or integrated into a plant PLC system. This communication describes the working principle of the two-cell concept used in the development of the sensor. Some construction details are included where they aid commissioning and operation of the unit. The results of tests in mechanical cells and flotation columns processing mineral and paper pulps illustrate the potential of the sensor.
2. The conductivity gas holdup sensor 2.1. Principle Electrical conductivity is an intensive property that for an aqueous solution depends on temperature and ionic strength. If a non-conducting material is dispersed in the liquid in the form of particles, droplets or bubbles, the conductivity of the dispersion decreases as a function of the volumetric concentration of the dispersed phase. Maxwell (1954) modelled this system and derived a relationship between the volumetric concentration of the dispersed phase and the conductivity of the dispersion relative to that of the continuous phase. The model has proved sufficiently flexible to describe mineral systems. One application is determination of volumetric solids concentration in slurries (Gomez et al., 1998). The situation of interest here is the gas concentration (gas holdup) in a flotation pulp. The conductivities required are those of the dispersion kd and the air-free dispersion (pulp) kp where the appropriate form of MaxwellÕs model is: eg ¼ ð1 kd =kp Þ=ð1 þ 0:5kd =kp Þ
ð1Þ
2.2. Flow cells The measurement of conductivity requires a conductivity cell. An approach based on the use of so-called flow conductivity cells is utilized. A flow conductivity cell in this context is defined as one that allows a dispersion to flow through while the conductance is being measured. Two cells were designed: an open cell to measure the conductivity of the dispersion, and a syphon cell used to exclude bubbles and measure the conductivity of the pulp (Fig. 1). Their design presented
Fig. 1. Schematic representation of open and syphon cells during measurement.
challenges in two areas: hydrodynamics and electrical performance. The open cell is a vertical cylinder open at both ends to allow relatively free flow of bubbles and pulp. The syphon cell, also a vertical cylinder open at the top, ends in a conical bottom with a small orifice. This design retards entry of bubbles and the cell fills with pulp that, being denser than the dispersion outside the cell, causes the cell contents to flow out through the orifice. A steady state is reached when the pressure head at the orifice is the same as that outside the cell, achieved when a velocity within the cell that exactly compensates the hydrostatic pressure difference is established. This downward pulp velocity, largest at the orifice, completes the exclusion of bubbles from the cell. Operation requires that the pulp velocity at the top of the cell be lower than the terminal velocity of the smallest size bubble present in the flotation system, otherwise air bubbles would be drawn (entrained) into the cell. The design model for the syphon cell includes these features as a function of orifice size (Cortes-Lopez, 1999). In the case of electrical performance, the most important feature of a conductivity cell is the cell constant which permits the measurement, namely conductance K, to be converted to the property, conductivity k, needed in MaxwellÕs model. The cell constant depends on the dimensions and geometry of the electrodes used to transfer the electric energy and is defined as the ratio of the effective surface area A (normal to the flux of electric current) to the distance L between the electrodes. K ¼ ðA=LÞk
ð2Þ
In an ideal conductivity cell A=L is independent of the conductivity of the liquid. In our case, the open and syphon cells have three internal ring electrodes flush mounted to the wall. The three-ring arrangement is used to limit the electric field to the volume in between the
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top and bottom rings: by maintaining these two electrodes at the same potential and the central electrode at opposite polarity, a current path outside this volume is avoided. This arrangement renders a cell constant that depends slightly on the conductivity of the material being measured. This variation is characterized by calibration as a polynomial that relates cell constant to the conductance measured. Design criteria have been developed to define ring dimensions and their separation to achieve a specific cell constant (Tavera et al., 1998; Cortes-Lopez, 1999). 2.3. Electronics The sensor measures the conductance in the two cells, open and syphon, immersed side by side in the flotation pulp. Prototypes used a single-channel conductivity meter and a computer to sequentially connect each cell through a relay board and digitize the output using an analog-to-digital converter. Attempts to use two singlechannel conductivity meters simultaneously to provide a continuous measurement failed because of electrical interference. This was solved with a two-channel conductivity meter (Yokogawa, model RS402). The unit provides two 4–20 mA loops that can be programmed to deliver either of the conductivities, their ratio, or directly the gas holdup, which facilitates integration of the sensor to plant PLC systems. The unit provides a continuous signal. The sensor has been used to date primarily to demonstrate accuracy and response to process changes. The unit is driven from a laptop computer to collect data for off-line analysis. A box has been assembled using commercial components to deliver digitized signals through serial communication. 2.4. Sensor specifications and commissioning The sensor shell is made of PVC and the electrodes from stainless steel. A typical design has an open cell 0.1 m in diameter and 0.5 m long, and a syphon cell 0.075 m in diameter and 0.45 m long, attached to a supporting 00 bar. By screwing a 1/2 threaded (NPT) tube into this bar the sensor can be firmly and easily inserted from the top of a flotation machine (column or mechanical) to a desired location. Operation mainly depends on the syphon cell: it must not plug with solids or entrain bubbles. To ensure this, the orifice has been made removable (Fig. 2) to allow installation of the most effective size for the prevailing conditions. Low pulp densities and gas holdups call for a larger orifice while large pulp densities and gas holdups require a smaller one. One of three orifices, 0.0010, 0.0012 and 0.0014 m diameter, have proven effective in the industrial testing completed to this point.
Fig. 2. Details of gas holdup probe: open and syphon cell showing removable orifice tip.
Commissioning a unit involves calibration of the flow cell/meter combination, and programming the meter to define values for the cell constant and range of channel output signal (4–20 mA). Cell constants can be defined by the user, or calculated automatically and stored in the conductivity meter following input of the conductivity of the liquid being tested determined with a reference conductivity meter. Calibration is accomplished through measurements in water of known conductivity varied by addition of a salt (usually sodium or potassium chloride). Calibration includes the cable connection to the conductivity meter (a four-wire cable up to 100 ft long) which adds a 1–2 X resistance to the circuit (in most cases negligible as the resistance in the cells for a liquid of conductivity 1 mS/cm is in the order of 200 X). To illustrate the calibration procedure results from one exercise are included in Fig. 3. Actual conductivity was measured with a reference conductivity meter (Radiometer, model CDM210) using a standard two-pole probe (Radiometer, model CDC641T, cell constant 0.75 cm1 ). Standard conductivity liquids were used to calibrate the reference conductivity meter/sensor combination. When the sensor was immersed in a sodium chloride solution of conductivity 3.04 mS/cm at 16.2 °C, calculated cell constants were 0.388 and 0.412 cm1 for the syphon cell and open cell, respectively. The output for both cells was the same as the inputted value of 3.04 mS/cm and constant. As the conductivity of the liquid was changed, the readings from both cells deviated from the value measured with the reference meter, reflecting
C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501 20
7 Gas holdup
GAS HOLDUP, %
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6
4
2 Open
15
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5 Syphon cell
5
4 Open cell
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2
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6
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Fig. 3. Typical results of measurements for cell constant determination.
the non-ideal behaviour of the flow cells. However, the drift was small and practically the same for both cells; fitting to a straight line through the origin accounts for the drift in the gas holdup calculation. A possible source of error comes from compensating for temperature variations during measurements. Conductivity cells often include a temperature measurement to correct the conductivity to a reference temperature (normally 25 °C). In our experience temperature compensation has not proven necessary. The objective here is to measure the ratio between the conductivities of aerated and bubble-free pulp. This ratio will be the same no matter what the temperature provided both cells are at the same temperature and the cell constant in both cells is not affected differently. Measurements in several standard liquids at three temperatures demonstrated performance of the flow cell (cell constant) was not affected by temperature (Fig. 4). The conductivity meter can deliver up to two signals. We generally use: (i) for on-line plant monitoring, the gas holdup and the conductivity in the syphon cell (used
Fig. 5. Effect of temperature on open cell conductivity measurements.
to detect orifice plugging); and (ii) for off-line diagnosis, the conductivity in the open and syphon cells. During calibration conductivity readings are steady. In use when air bubbles are present, the readings fluctuate as a consequence of the variable number of bubbles. To illustrate, Fig. 5 gives the conductivity data associated with the gas holdup; the process noise is evident for the open cell through which the bubbles pass. Error propagation increases the noise on the gas holdup signal. Gas holdup values (and conductivity) are normally averaged over a period of time (e.g., 15 min in Fig. 5). To illustrate sensor performance the unit was immersed about 0.5 m below the water level in a pilot column bubbling air through water with 50 ppm of Dowfroth 250C. As gas flow rate was increased (Fig. 6) the desired behaviour was found: (i) a constant value reported by the syphon cell, indicating that no bubbles are entrained, and (ii) a decreasing value reported by the open cell reflecting the increase in gas holdup. Both cells reported the same value when no air was present (gas velocity zero).
1.2
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8
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6
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6
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Fig. 4. Cell conductivity variations as a function of gas holdup.
Fig. 6. Typical results collected during a stable column operation period: (a) gas holdup results and (b) cell conductivity measurements.
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the opportunity to test the effect on gas holdup of changing the number of spargers (anticipated to modify bubble size). In the case of mineral pulps, the experiments included: (i) step change in the gas flow rate to a flotation column; (ii) long-term (17 days) monitoring in a column; and (iii) mapping of gas holdup distribution in a mechanical machine.
30
GAS HOLDUP (Cond), %
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20
10
3.1. Measurements in paper pulps 0 10
20
30
GAS HOLDUP (Pressure), %
Fig. 7. Comparison of gas holdup estimates from conductivity and pressure measurements in an idle industrial column filled with water.
The accuracy of the sensor was also tested in the same two-phase setup. Two portable, submersible pressure transmitters were installed 0.5 m above and below the center ring of the open cell. Gas holdup was manipulated by changing gas rate and measured at various depths (the sensor can be reeled up and down easily). The results showed good agreement between the conductivity- and pressure-based gas holdups (Fig. 7). One source of discrepancy is that gas holdup is not uniformly distributed in a column and the conductivity sensor gives a more localized measurement than pressure, which is the average for the entire column volume between the two pressure transducers. In the case of three-phase systems verification is more difficult. The approach has been demonstrated in the laboratory (Tavera, 1996), but validation in mineral flotation plants has not been accomplished. By using paper pulps, however, plant verification test work can be accomplished, as described later.
3. Plant testing A testing program was devised to examine sensor performance in plant and its sensitivity to gas holdup variations resulting from operational changes (CortesLopez, 1999). A sample of the results is described here, namely: testing on a pilot flotation column in a paper recycling mill (Bowater, De-Inking Plant, Gatineau, Quebec); in a flotation column at IncoÕs Matte Separation Plant (Sudbury, Ontario), and in one of the mechanical cells at NorandaÕs Brunswick Mine (Bathurst, New Brunswick). The tests in paper pulps offered a means of checking the accuracy of the gas holdup measurement. Paper pulp feeds to flotation have a solids content (consistency) of about 1% w/w so the density remains close to that of water. Therefore, gas holdup from pressure is, for practical purposes, accurate. The column also presented
BowaterÕs de-inking plant has two banks of eight Voigt cells in a rougher/cleaner circuit (six and two cells, respectively). A pilot flotation column (0.5 m 5 m) was run continuously with pulp diverted from the feed to the flotation circuit. The column was constructed from five flanged PVC sections (0.5 m in diameter, 1 m high) (Watson et al., 1996; Leichtle, 1998). It was fully instrumented and run automatically with level control by manipulation of feed rate. Air was delivered using a mass flow meter/controller (MKS, model 1162B). Gas holdup was also calculated from pressure readings collected with a differential pressure transmitter (Bailey, model PTS-DDD). The unit could be operated with up to five stainless steel porous rigid spargers. 3.1.1. Gas holdup measurement using pressure and conductivity Gas holdup was varied by gas flow rate adjustments and selection of number of spargers. The column normally stabilized (stable traces for the pressure and conductivity signals) in less than 10 min after a set of conditions was selected; pressure and conductivity data were collected for 15 min after stabilization. A total of 33 conditions were tested; average values for the gas holdup calculated from pressure and conductivity are included in Fig. 8. Accuracy was considered acceptable although at lower gas holdup the conductivity value tended to be 20
GAS HOLDUP (Cond), %
0
15
10
5
0 0
5
10
15
20
GAS HOLDUP (Pressure), %
Fig. 8. Comparison of gas holdup estimates from conductivity and pressure measurements in a pilot column processing a paper pulp.
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less than for pressure. Subsequent work established that micro-bubbles trapped in the pulp fibers, which were accounted for by the pressure technique, were not picked up by conductivity because they were present in the ‘‘non-aerated’’ pulp in the syphon cell. This argument is consistent with the error being practically constant (1.5% gas holdup) and in the same range as the micro-bubble content over the measurement range. To test if the difference was significant the Student t-test was applied; for paired samples, the difference must satisfy 2:00 < tð¼ 1:61Þ < 2:00, where 2.00 is for 95% confidence and 62 degrees of freedom (t distribution). From this analysis we concluded that gas holdup from conductivity was not different from the value given by pressure. 3.1.2. Sensor response to changes in number of spargers The column was started with five stainless steel spargers delivering a total gas flow rate of 300 l/min (Jg ¼ 2:5 cm/s). After stabilization, data were collected for a 15-min period; the procedure was repeated with four and three spargers always delivering the same total gas flow rate of 300 l/min. The results (Fig. 9) revealed an effect: average gas holdups of 14.4%, 11.7% and 8.3% were measured for the five, four and three sparger combinations, respectively. Lower gas holdups for the same gas flow rate are consistent with larger bubbles, being generated as the volume of air per unit area of sparger surface is increased (Xu and Finch, 1989). Traces for the three conditions showed a tendency for signal noise to increase as the number of spargers was reduced. The increased signal noise suggests a wider range of bubbles sizes, again consistent with increased air per unit sparger area. 3.2. Measurements in mineral pulps
plied by MinnoveEX. Tests were performed in columns 2 and 3, both 1.77 m in diameter and 12 m high. Column 3 was used for measuring sensor response to step changes in gas flow rate, and column 2 for long-term continuous monitoring. 3.2.1. Sensor response to step changes in gas flow rate The objective was to test the sensitivity of the sensor and the dynamic response to changes in gas flow rate. A 10-h test was devised based on step changes of about 10% in gas flow rate: the column was run with the normal gas flow rate (Jg ¼ 1:8 cm/s) for three hours, then increased to Jg ¼ 2:0 cm/s for 4 h, and then decreased to a Jg ¼ 1:7 cm/s for 3 h before returning to the normal gas flow rate. The results are presented in Fig. 10a which illustrates gas flow and gas holdup, and Fig. 10b which shows the corresponding conductivity traces. The sensor was located 4 m from the top of the column. The gas holdup trace clearly shows a response to both step changes, gas holdup varying from an average of 5.9–7.2% during the initial step, and from 7.2% to 5.8% during the second step. Even the gas holdup difference between initial and final setting (5.9–5.8%) appears to detect the small difference in gas rate 1.8–1.7 cm/s. In terms of process dynamics, the results showed a slow reaction to reach steady state after the gas flow increase
15
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GAS HOLDUP, %
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5 spargers 10 4 spargers 3 spargers
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0 15:00
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IncoÕs Matte Separation plant has five flotation columns retrofitted with variable-gap jetting spargers sup20
12
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(b) 1.5 Syphon 1.0
Open
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0.0 6:00
9:00
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TIME Fig. 10. Gas holdup measurement during a step variation of gas flow rate in an industrial flotation column processing a mineral pulp.
C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501
(more than 2 h), perhaps because of limitations in the gas delivery system, but a fast response when the gas flow was decreased. The conductivity data (Fig. 10b) demonstrated the sensor performed even though the pulp conductivity was continuously changing (presumably indicative of variations in the solids content and/or chemical environment). For example, the same gas holdup was measured for the first 2–3 h of the test when pulp conductivity changes of about 20% were occurring, and a sudden decrease in gas holdup was measured around the second step when the pulp conductivity remained constant. 3.2.2. Long-term continuous monitoring The motivation was primarily to test sensor reliability, especially whether the syphon cell design resisted plugging of the orifice, and secondarily to demonstrate remote data retrieval (the signals were processed at McGill). Fig. 11 shows the results obtained over the 17day period for the sensor located about 3.75 m below the column lip (Fig. 11a gives gas holdup and Fig. 11b shows the conductivity in both cells). The test was successful: the sensor ran with no interruptions during the testing period and daily access from McGill was ac-
12
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Fig. 11. Long-term sensor testing in an industrial flotation column processing a mineral pulp: (a) gas holdup results; and (b), cell conductivity measurements (note, upper trace is syphon cell).
499
complished without affecting the performance of the unit. From the perspective of operations, gas holdup slowly decreased from 8% to 4% during the first 12 days (Fig. 10a). One day after the shutdown on March 23 (C), the gas holdup started to recover but at the end of March 27 it went down again to about 6%. A change was made late on March 27 (D) that rapidly moved gas holdup to about 8% where it remained stable for the last three days of the test. The sudden nature of the changes suggests interruption in air delivery. The conductivity results (Fig. 11b) help diagnose the response. For example, the low gas holdup at the end of March 16 (A in the figure) corresponds to a similar and low conductivity in both open and syphon cells. This is consistent with a gas shutdown and the level quickly dropping (as the bubbles leave the pulp) and stabilizing below the sensor in this case, i.e., the sensor is immersed in the froth. Both cells then simultaneously report the same conductivity at about half of the value measured for the pulp, a result typical for a sensor located in the froth. This was different from the low gas holdup case early on March 19 (B). Again, as a consequence of a gas shutdown the level rapidly moved down but now the level appeared to stabilize above the sensor, the two cells registering the same conductivity this time indicative of non-aerated pulp. The fact that the sensor returned to operation after these incidents indicates a robust design. Previous versions plugged when the air was shut off which, by cutting out the syphon (flow) action, encourages particle settling in the conical bottom. It is notable that during the stable period starting late March 27 (D), the conductivity was changing substantially but this did not disturb the gas holdup result. The conductivity traces show a series of regular peaks at a frequency of about half a day; this may correspond to shift changes. After the test, the sensor calibration was checked; the results indicated that cell constants had not changed. 3.2.3. Mapping of gas holdup distribution in mechanical cells As part of a program to characterize cell performance at NorandaÕs Brunswick Mine (Dahlke et al., 2001), the sensor was used to try to identify a location as a cell sampling point. The results obtained in one of the zinc cleaning circuit Denver DR-100 machines (Fig. 12) demonstrated a distribution of gas holdup over the cross-section (depth was constant at 0.5 m). A consistent decrease from the near the impeller to half way to the wall occurred when the value stabilized. It was not possible to sample over the entire cross-section, as access was restricted to where cover plates were removable. These results demonstrate that the sensor functions equally well in agitated tanks as in the more quiescent columns. In terms of locating the sensor the
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C.O. Gomez et al. / Minerals Engineering 16 (2003) 493–501 Impeller
Gas holdup 16.6% 12.0% Baffle
12.0% 11.0%
inking Plant, and of NorandaÕs Brunswick Mine for their constant, enthusiastic support during the plant tests. Funding was under the Natural Sciences and Engineering Research Council (NSERC) Collaborative Research and Development program with sponsorship from Inco, Falconbridge, Noranda and Teck-Cominco.
10.3% 10.7%
PULP INLET
10.8%
References
10.3% 10.5%
Removable cell cover plates
Fig. 12. Gas holdup horizontal mapping in an industrial mechanical cell (Denver DR-100).
observations indicate gas is not distributed evenly; cell geometry and gas dispersion distributions must be understood to define a sampling point with the characteristics required for the purpose of the measurement. This is particularly true if different machines are to be compared.
4. Conclusion and future work A gas holdup sensor based on conductivity capable of performing in industrial environments has been created. The use of a commercial two-channel conductivity meter to drive the sensor facilitates commissioning and integration of the unit to plant control systems. Tests on paper pulps verified the accuracy of the measurement by comparing with gas holdup estimated from pressure. The sensor demonstrated sensitivity to gas holdup differences resulting from bubble size changes. Tests in mineral pulps showed a sensitive response to step changes in gas flow rate. A long-term test demonstrated the unit was reliable, the syphon cell remaining operational even when the air was unexpectedly cut off. The sensor detected radial gas holdup variation in a mechanical flotation machine, higher around the impeller decreasing towards the wall. Future use will be in machine hydrodynamic characterization and diagnosis, augmenting the use of parallel developments in gas velocity and bubble size sensors (Torrealba-Vargas et al., 2002; HernandezAguilar et al., 2002). One target will be to relate to metallurgical performance to aid development of process optimization strategies (Finch, 2002).
Acknowledgements The authors thank the staff and crew of the IncoÕs Matte Separation Plant, of BowaterÕs Waste Paper De-
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