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Operating range of a flotation cell determined from gas holdup vs. gas rate
Minerals Engineering 18 (2005) 977–980 This article is also available online at: www.elsevier.com/locate/mineng
Technical note
Operating range of a flotation cell determined from gas holdup vs. gas rate R. Dahlke 1, C. Gomez, J.A. Finch
*
Department of Mining, Metals and Materials Engineering, McGill University, 3610 University Street, Montreal, Canada H3A 2B2 Received 15 October 2004; accepted 16 December 2004 Available online 13 February 2005
Abstract Flotation cells have an operating gas rate range. A method to define the range objectively using the relationship between gas holdup and gas rate (eg vs. Jg) is introduced. This is made tractable by the necessary sensors becoming available. Examples in three cell types at three plants illustrate the procedure. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation machines; Process instrumentation
1. Introduction A flotation machine has an operating range of gas (air) rate; too low and no concentrate is produced or sanding may occur, too high and ‘‘boiling’’ (breakthrough of large bubbles) and a general disturbed look ensues. Operations normally want to keep inside the range. This becomes important, for example, when trying to take advantage of setting a gas rate profile down a bank, which may mean each cell is at a different setting but all should be in their operating range (Cooper et al., 2004). Defining the range visually is subject to interpretation. A procedure is introduced where measurement of gas holdup vs. gas rate is used to provide an objective definition, in particular, of the upper end of the range. It is based on the approach used to characterize hydrodynamics in flotation columns (Finch and Dobby, 1990)
*
1
Corresponding author. Tel.: +1 514 398 1452; fax: +1 514 398 4492. E-mail address: jim.fi[email protected] (J.A. Finch). Now with Cambior.
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.12.013
and has been adapted to mechanical machines. The methodology is described and illustrated.
2. Gas holdup vs. gas rate 2.1. Methodology Gas holdup (eg) is the volumetric fraction (usually expressed in %) of gas in the gas–slurry mixture. It is measured here using a conductivity-based sensor, shown in Fig. 1 (Tavera et al., 1996; Gomez and Finch, 2002). The sensor comprises two flow cells, an open cell to measure the conductivity of aerated slurry (jslg) and a syphon cell to measure the conductivity of de-aerated slurry (jsl). The syphon cell has a tapered base (spigot) that acts to restrict bubble entry and establish a bulk density difference between the fluid inside and outside the cell. This drives a flow of slurry in through the top of the cell (hence the description ‘‘syphon’’), which both completes the elimination of air (the bubbles cannot enter against the outflow of slurry) and replenishes the cell contents. With the two conductivities, MaxwellÕs model (included in figure) is solved. Note, the equation
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R. Dahlke et al. / Minerals Engineering 18 (2005) 977–980
Fig. 1. Schematic of gas holdup sensor.
Fig. 2. (a) Schematic of gas rate sensor. (Note, tube contents emptying as air accumulates.) (b) Example pressure-variation curves.
involves only the ratio of the conductivities, the absolute slurry conductivity, or whether it varies during measurement (e.g., due to a change in % solids), is not a factor. Operation and validation of the sensor is described in the two references. Gas rate is expressed as the volumetric flow rate of gas (air) (Qg) per cross-sectional area of the cell (A), i.e., the gas superficial velocity (Jg = Qg/A). All the data reported here were measured using the on/off2 McGill Jg sensor (Fig. 2a) (Gomez and Finch, 2002; Gomez et al., 2003). This comprises a tube to collect bubbles by natural buoyancy and inferring Jg from the rate of increase in pressure once a valve is closed (specimen ‘‘pressure-
2 To distinguish from the continuous version (Torrealba-Vargas et al., 2004).
variation’’ curves are included, Fig. 2b). Again, the unit is well described in the references. In use, the two sensors are placed close to the same location in the cell below the pulp/froth interface. The location should avoid baffles, shrouds, booster cones or launders (as examples). The choice may reflect requirements. For example, if cells in a bank are to be compared then an accessible, geometrically similar location in each may be the target.
2.2. Example eg vs. Jg It was when exploring Jg profiles at Brunswick Mine (division of Noranda), that the need to know the operating range of the cell became evident (Dahlke et al., 2001). Fig. 3 shows two runs on the same Denver DR 100 cell (nominally 100 ft3) in the final (4th) Zn cleaner
R. Dahlke et al. / Minerals Engineering 18 (2005) 977–980
979
Fig. 3. Examples of gas holdup vs. gas rate curves: same Denver DR 100 cell on two occasions. Inset depicts sensor position relative to impeller; the cell is 70’’ wide and 60’’ long.
Fig. 5. Gas holdup vs. gas rate for three cells types, Galigher (Matagami), OK 50 (Red Dog) and Denver DR 100 (Brunswick, average of data in Fig. 2).
bank. The relationship is similar to that found in flotation columns (Finch and Dobby, 1990): a range in Jg over which eg responds consistently, almost linearly, above which eg varies erratically. In the present case the transition3 occurs at ca. Jg = 2.5 cm/s, which is identified with the maximum Jg of the operating range. Sometimes this upper limit is indicated by ‘‘boiling’’, but not always. The result at low Jg is interesting: instead of trending to eg = 0, with the valve closed there remained a finite gas holdup (and gas rate). This is either a leaking valve or gas being entrained in the slurry from previous cell in the bank. The low limit was established as that giving some concentrate flow (i.e., a ‘‘practical’’ limit) and corresponded approximately with the deviation from the linear response at ca. 0.5 cm/s (there was no sanding issue with these cells). Fig. 4 is taken from Cooper et al. (2004) and shows examples of three gas rate profiles set along the bank: the operating range in Fig. 3 (0.5–2.5 cm/s) is clearly respected.
2.3. Other experiences This technique of establishing the operating range is routinely used in plant campaigns conducted by McGill teams. Three examples of operating range, with cells ranging from a 4 ft3 Galigher–Agitair cell at Matagami Mines (division of Noranda) to a 50 m3 OK 50 cell at Teck ComincoÕs Red Dog Operation, are given in Fig. 5. The trends (and ranges) in each example are different. There are a number of reasons. The eg vs. Jg response not only reflects cell mechanism but solution chemistry (particularly frother dosage and possibly salt concentration) and slurry properties (particle size, % solids, etc.). In this early work the gas rate was taken as the slope of the pressure-variation curve (Fig. 2b), which incurs a bias compared to using the full model (Torrealba-Vargas et al., 2004). This does not affect establishing the operating range but must be considered when comparing data. Along the same lines, to compare gas holdup and gas rate data must be corrected to the same conditions (pressure and temperature). Not all cells investigated have shown such a consistent eg vs. Jg trend, some revealing a very narrow operating range. As the database expands it may be possible to determine the reason(s).
3. Conclusion
Fig. 4. Examples of three air distribution (Jg) profiles (after Cooper et al., 2004).
The use of gas holdup vs. gas rate (eg vs. Jg) is introduced as an objective way to define the operating gas rate range of a flotation cell. Difficult to determine before, the necessary sensors (for eg and Jg) are now available (the ones used here are briefly described but others would suffice). Examples at three plants illustrated the technique.
980
R. Dahlke et al. / Minerals Engineering 18 (2005) 977–980
Acknowledgments Funding for this work was sponsored initially by Inco, Teck Cominco, Falconbridge and Noranda and now including Corem and SGS Lakefield Research under the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Research and Development (CRD) program. Since 2001, the work is also supported under a NSERC CRD sponsored by the Amira P9 project. References Cooper, M., Scott, D., Dahlke, R., Finch, J.A., Gomez, C.O., 2004. AImpact of Air Distribution Profile on Banks in a Zn Cleaning Circuit@. In: Proceedings 36th Annual Meeting of the Canadian Mineral Processors of CIM, January 20–22, pp. 525–540.
Dahlke, R., Scott, D., Leroux, D., Gomez, C.O., Finch, J.A., 2001. Trouble Shooting Flotation Cell Operation Using Gas Velocity Measurements. In: Proceedings-33rd Annual Meeting of the Canadian Mineral Processors (division of CIM), January 23–25, pp. 359–370. Finch, J.A., Dobby, G.S., 1990. Column Flotation. Pergamon Press, p. 180. Gomez, C.O., Finch, J.A., 2002. Gas dispersion measurements in flotation machines. CIM Bull. 95 (1), 73–78. Gomez, C., Torrealba-Vargas, J.A., Dahlke, R., Finch, J.A., 2003. Measurement of Gas Velocity in Industrial Flotation Cells. In: Lorenzen, L., Bradshaw, D.J. (Eds.), XXII International Mineral Processing Congress, Cape Town, September 29–October 3, vol. 3. S. African Inst. Min. Metall. pp. 1703–1710. Tavera, F., Gomez, C., Finch, J.A., 1996. A gas holdup sensor for slurry–air systems. Trans. IMM Sect. C. 105, C99–104. Torrealba-Vargas, J.A., Gomez, C.O., Finch, J.A., 2004. Model for the JK and On-off McGill Gas Velocity Sensor. Final Report AMIRA Project P9M Volume II Flotation Module, AMIRA International. pp. 47–70.
Technical note
Operating range of a flotation cell determined from gas holdup vs. gas rate R. Dahlke 1, C. Gomez, J.A. Finch
*
Department of Mining, Metals and Materials Engineering, McGill University, 3610 University Street, Montreal, Canada H3A 2B2 Received 15 October 2004; accepted 16 December 2004 Available online 13 February 2005
Abstract Flotation cells have an operating gas rate range. A method to define the range objectively using the relationship between gas holdup and gas rate (eg vs. Jg) is introduced. This is made tractable by the necessary sensors becoming available. Examples in three cell types at three plants illustrate the procedure. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Flotation machines; Process instrumentation
1. Introduction A flotation machine has an operating range of gas (air) rate; too low and no concentrate is produced or sanding may occur, too high and ‘‘boiling’’ (breakthrough of large bubbles) and a general disturbed look ensues. Operations normally want to keep inside the range. This becomes important, for example, when trying to take advantage of setting a gas rate profile down a bank, which may mean each cell is at a different setting but all should be in their operating range (Cooper et al., 2004). Defining the range visually is subject to interpretation. A procedure is introduced where measurement of gas holdup vs. gas rate is used to provide an objective definition, in particular, of the upper end of the range. It is based on the approach used to characterize hydrodynamics in flotation columns (Finch and Dobby, 1990)
*
1
Corresponding author. Tel.: +1 514 398 1452; fax: +1 514 398 4492. E-mail address: jim.fi[email protected] (J.A. Finch). Now with Cambior.
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2004.12.013
and has been adapted to mechanical machines. The methodology is described and illustrated.
2. Gas holdup vs. gas rate 2.1. Methodology Gas holdup (eg) is the volumetric fraction (usually expressed in %) of gas in the gas–slurry mixture. It is measured here using a conductivity-based sensor, shown in Fig. 1 (Tavera et al., 1996; Gomez and Finch, 2002). The sensor comprises two flow cells, an open cell to measure the conductivity of aerated slurry (jslg) and a syphon cell to measure the conductivity of de-aerated slurry (jsl). The syphon cell has a tapered base (spigot) that acts to restrict bubble entry and establish a bulk density difference between the fluid inside and outside the cell. This drives a flow of slurry in through the top of the cell (hence the description ‘‘syphon’’), which both completes the elimination of air (the bubbles cannot enter against the outflow of slurry) and replenishes the cell contents. With the two conductivities, MaxwellÕs model (included in figure) is solved. Note, the equation
978
R. Dahlke et al. / Minerals Engineering 18 (2005) 977–980
Fig. 1. Schematic of gas holdup sensor.
Fig. 2. (a) Schematic of gas rate sensor. (Note, tube contents emptying as air accumulates.) (b) Example pressure-variation curves.
involves only the ratio of the conductivities, the absolute slurry conductivity, or whether it varies during measurement (e.g., due to a change in % solids), is not a factor. Operation and validation of the sensor is described in the two references. Gas rate is expressed as the volumetric flow rate of gas (air) (Qg) per cross-sectional area of the cell (A), i.e., the gas superficial velocity (Jg = Qg/A). All the data reported here were measured using the on/off2 McGill Jg sensor (Fig. 2a) (Gomez and Finch, 2002; Gomez et al., 2003). This comprises a tube to collect bubbles by natural buoyancy and inferring Jg from the rate of increase in pressure once a valve is closed (specimen ‘‘pressure-
2 To distinguish from the continuous version (Torrealba-Vargas et al., 2004).
variation’’ curves are included, Fig. 2b). Again, the unit is well described in the references. In use, the two sensors are placed close to the same location in the cell below the pulp/froth interface. The location should avoid baffles, shrouds, booster cones or launders (as examples). The choice may reflect requirements. For example, if cells in a bank are to be compared then an accessible, geometrically similar location in each may be the target.
2.2. Example eg vs. Jg It was when exploring Jg profiles at Brunswick Mine (division of Noranda), that the need to know the operating range of the cell became evident (Dahlke et al., 2001). Fig. 3 shows two runs on the same Denver DR 100 cell (nominally 100 ft3) in the final (4th) Zn cleaner
R. Dahlke et al. / Minerals Engineering 18 (2005) 977–980
979
Fig. 3. Examples of gas holdup vs. gas rate curves: same Denver DR 100 cell on two occasions. Inset depicts sensor position relative to impeller; the cell is 70’’ wide and 60’’ long.
Fig. 5. Gas holdup vs. gas rate for three cells types, Galigher (Matagami), OK 50 (Red Dog) and Denver DR 100 (Brunswick, average of data in Fig. 2).
bank. The relationship is similar to that found in flotation columns (Finch and Dobby, 1990): a range in Jg over which eg responds consistently, almost linearly, above which eg varies erratically. In the present case the transition3 occurs at ca. Jg = 2.5 cm/s, which is identified with the maximum Jg of the operating range. Sometimes this upper limit is indicated by ‘‘boiling’’, but not always. The result at low Jg is interesting: instead of trending to eg = 0, with the valve closed there remained a finite gas holdup (and gas rate). This is either a leaking valve or gas being entrained in the slurry from previous cell in the bank. The low limit was established as that giving some concentrate flow (i.e., a ‘‘practical’’ limit) and corresponded approximately with the deviation from the linear response at ca. 0.5 cm/s (there was no sanding issue with these cells). Fig. 4 is taken from Cooper et al. (2004) and shows examples of three gas rate profiles set along the bank: the operating range in Fig. 3 (0.5–2.5 cm/s) is clearly respected.
2.3. Other experiences This technique of establishing the operating range is routinely used in plant campaigns conducted by McGill teams. Three examples of operating range, with cells ranging from a 4 ft3 Galigher–Agitair cell at Matagami Mines (division of Noranda) to a 50 m3 OK 50 cell at Teck ComincoÕs Red Dog Operation, are given in Fig. 5. The trends (and ranges) in each example are different. There are a number of reasons. The eg vs. Jg response not only reflects cell mechanism but solution chemistry (particularly frother dosage and possibly salt concentration) and slurry properties (particle size, % solids, etc.). In this early work the gas rate was taken as the slope of the pressure-variation curve (Fig. 2b), which incurs a bias compared to using the full model (Torrealba-Vargas et al., 2004). This does not affect establishing the operating range but must be considered when comparing data. Along the same lines, to compare gas holdup and gas rate data must be corrected to the same conditions (pressure and temperature). Not all cells investigated have shown such a consistent eg vs. Jg trend, some revealing a very narrow operating range. As the database expands it may be possible to determine the reason(s).
3. Conclusion
Fig. 4. Examples of three air distribution (Jg) profiles (after Cooper et al., 2004).
The use of gas holdup vs. gas rate (eg vs. Jg) is introduced as an objective way to define the operating gas rate range of a flotation cell. Difficult to determine before, the necessary sensors (for eg and Jg) are now available (the ones used here are briefly described but others would suffice). Examples at three plants illustrated the technique.
980
R. Dahlke et al. / Minerals Engineering 18 (2005) 977–980
Acknowledgments Funding for this work was sponsored initially by Inco, Teck Cominco, Falconbridge and Noranda and now including Corem and SGS Lakefield Research under the Natural Sciences and Engineering Research Council of Canada (NSERC) Collaborative Research and Development (CRD) program. Since 2001, the work is also supported under a NSERC CRD sponsored by the Amira P9 project. References Cooper, M., Scott, D., Dahlke, R., Finch, J.A., Gomez, C.O., 2004. AImpact of Air Distribution Profile on Banks in a Zn Cleaning Circuit@. In: Proceedings 36th Annual Meeting of the Canadian Mineral Processors of CIM, January 20–22, pp. 525–540.
Dahlke, R., Scott, D., Leroux, D., Gomez, C.O., Finch, J.A., 2001. Trouble Shooting Flotation Cell Operation Using Gas Velocity Measurements. In: Proceedings-33rd Annual Meeting of the Canadian Mineral Processors (division of CIM), January 23–25, pp. 359–370. Finch, J.A., Dobby, G.S., 1990. Column Flotation. Pergamon Press, p. 180. Gomez, C.O., Finch, J.A., 2002. Gas dispersion measurements in flotation machines. CIM Bull. 95 (1), 73–78. Gomez, C., Torrealba-Vargas, J.A., Dahlke, R., Finch, J.A., 2003. Measurement of Gas Velocity in Industrial Flotation Cells. In: Lorenzen, L., Bradshaw, D.J. (Eds.), XXII International Mineral Processing Congress, Cape Town, September 29–October 3, vol. 3. S. African Inst. Min. Metall. pp. 1703–1710. Tavera, F., Gomez, C., Finch, J.A., 1996. A gas holdup sensor for slurry–air systems. Trans. IMM Sect. C. 105, C99–104. Torrealba-Vargas, J.A., Gomez, C.O., Finch, J.A., 2004. Model for the JK and On-off McGill Gas Velocity Sensor. Final Report AMIRA Project P9M Volume II Flotation Module, AMIRA International. pp. 47–70.