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Estimating true level in a thickener using a conductivity probe
Minerals Engineering 17 (2004) 87–88 This article is also available online at: www.elsevier.com/locate/mineng
Technical Note
Estimating true level in a thickener using a conductivity probe J. Vergouw, C.O. Gomez, J.A. Finch
*
Department of Mining, Metals and Materials Engineering, Wong Building, McGill University, 3610 University Street, Montreal, QC, Canada H3A 2B2 Received 23 July 2003; accepted 12 October 2003
Abstract Level in a thickener can be estimated from the conductance profile (conductance vs depth). Using a conductivity probe the original 0–1 level scale has been calibrated to read ‘‘true’’ level as defined by local practice. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Thickening; On-line analysis
1. Introduction
0
-100 -150 -200 -250 -300 -350 -400 0.00
4.00
8.00
12.00
16.00
CONDUCTANCE (mS)
Fig. 1. An example of a conductance profile: conductance is high in liquid and decreases when entering the solids bed (ca. 225 cm from top of thickener in this case).
DISTANCE FROM TOP
In previous communications a ‘‘stationary’’ conductivity probe for level detection, percent solids and inventory determination in thickeners was described (Xu et al., 1994; Gomez et al., 1998). The probe consisted of a series of electrode rings forming conductivity cells mounted on the exterior of a tube of non-conducting material. An alternative approach is to reel up and down a single cell probe (Schoenmann et al., 2002). Either way the conductance is measured as a function of depth to generate a ‘‘conductance profile’’ (Fig. 1). The profile locates the level (solid/water interface) by the decrease in conductance (most solids of interest in mineral processing are non-conducting relative to process waters). In application, estimating the level automatically proved difficult due to the inevitable signal noise. Gomez et al. (1998) suggested an approach. The profile was divided into three zones, viz. liquid, interface and solids (Fig. 2), and the average conductance in each zone, K, was computed. This way the noise associated with individual cells was damped. An ‘‘interface’’ I was defined as shown in Fig. 2, which gives level on a 0–1 scale (low level, I ! 0; high level, I ! 1). The scale was not readily accepted in operations, the preference being for a ‘‘true’’ level (i.e., in length units) as defined by local practice. In this communication we show that the interface scale can be calibrated to give the true level.
DEPTH (cm)
-50
liquid interface
“Interface” I =
KL - KI K L - KS
solids CONDUCTANCE
* Corresponding author. Tel.: +1-514-398-1452; fax: +1-514-3984492. E-mail address: jfi[email protected] (J.A. Finch).
0892-6875/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.10.006
Fig. 2. The definition of ‘‘interface’’ introduced by Gomez et al. (1998): K is average conductance for zone; subscript L is liquid zone, S solids zone, and I the interface zone.
LEVEL (cm, from top of thickener)
88
J. Vergouw et al. / Minerals Engineering 17 (2004) 87–88
sensitivity (the slope of the lines is much the same). The choice of boundary could reflect local definitions of high and low level; i.e., I ! 1 could trigger a ‘‘high’’ alarm, and I ! 0 could trigger a ‘‘low’’ alarm.
15-27 18-27 15-25
100
21-28
3. Conclusion
50
0 0.0
0.2
0.4 0.6 “INTERFACE”
0.8
1.0
Fig. 3. Calibration: True level, L, (distance from top of thickener) as a function of interface, I, for different zone boundary choices (e.g., 15– 25, means the water/interface boundary is set, from the top, at ring 15 and the interface/solids boundary is at ring 25). Key: 15–25, squares; 15–27, circles; 18–27, triangles; 21–28, diamonds.
2. Calibration Local practice is to measure level from the top of the thickener as judged by lowering a weighted rope till it slackens indicating the top of the bed. This defines the true depth, L. This estimate of L and the inferred interface, I, using a 33-ring stationary probe were measured simultaneously over a few weeks of normal operation. To calculate I, the two boundaries, liquid/ interface and interface/solids, have to be set. Different settings were used designated by the cell number from the top of the probe (e.g., 15–25 means the liquid/ interface boundary is at ring 15 and the interface/solids boundary is at ring 25). Fig. 3 shows the true level (in cm from the top of the thickener) vs interface. Regardless of the boundary choices a linear calibration was obtained with similar
The inferred level in a thickener using a conductivity probe has been converted from the original 0–1 scale to true depth by calibration against estimates made by accepted local practice.
Acknowledgements Funding was under the Natural Sciences and Engineering Research Council of Canada Collaborative Research and Development program with industrial sponsorship from Inco, Teck Cominco, Falconbridge and Noranda (and now including Corem and SGS Lakefield Research) forming an Industry Chair in Mineral Processing. References Gomez, C.O., Probst, A., Finch, J.A., Moores, N., 1998. Monitoring thickener operation using a conductivity probe. Miner. Metall. Process. 15 (4), 9–15. Schoenmann, F., Hales, L., Bedell, D., 2002. Strategies for instrumentation and control of thickeners and other solid–liquid separation circuits. In: Mular, A.L., Halbe, D.N., Barratt, D.J. (Eds.), Mineral Processing Plant Design, Practice, and Control, vol. 2. SME, Littleton, CO, pp. 2164–2173. Xu, M., Probst, A., Finch, J.A., 1994. Level and solids profile detection in thickeners using conductivity. C.I.M. Bull. 87 (985), 46–52.
Technical Note
Estimating true level in a thickener using a conductivity probe J. Vergouw, C.O. Gomez, J.A. Finch
*
Department of Mining, Metals and Materials Engineering, Wong Building, McGill University, 3610 University Street, Montreal, QC, Canada H3A 2B2 Received 23 July 2003; accepted 12 October 2003
Abstract Level in a thickener can be estimated from the conductance profile (conductance vs depth). Using a conductivity probe the original 0–1 level scale has been calibrated to read ‘‘true’’ level as defined by local practice. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Thickening; On-line analysis
1. Introduction
0
-100 -150 -200 -250 -300 -350 -400 0.00
4.00
8.00
12.00
16.00
CONDUCTANCE (mS)
Fig. 1. An example of a conductance profile: conductance is high in liquid and decreases when entering the solids bed (ca. 225 cm from top of thickener in this case).
DISTANCE FROM TOP
In previous communications a ‘‘stationary’’ conductivity probe for level detection, percent solids and inventory determination in thickeners was described (Xu et al., 1994; Gomez et al., 1998). The probe consisted of a series of electrode rings forming conductivity cells mounted on the exterior of a tube of non-conducting material. An alternative approach is to reel up and down a single cell probe (Schoenmann et al., 2002). Either way the conductance is measured as a function of depth to generate a ‘‘conductance profile’’ (Fig. 1). The profile locates the level (solid/water interface) by the decrease in conductance (most solids of interest in mineral processing are non-conducting relative to process waters). In application, estimating the level automatically proved difficult due to the inevitable signal noise. Gomez et al. (1998) suggested an approach. The profile was divided into three zones, viz. liquid, interface and solids (Fig. 2), and the average conductance in each zone, K, was computed. This way the noise associated with individual cells was damped. An ‘‘interface’’ I was defined as shown in Fig. 2, which gives level on a 0–1 scale (low level, I ! 0; high level, I ! 1). The scale was not readily accepted in operations, the preference being for a ‘‘true’’ level (i.e., in length units) as defined by local practice. In this communication we show that the interface scale can be calibrated to give the true level.
DEPTH (cm)
-50
liquid interface
“Interface” I =
KL - KI K L - KS
solids CONDUCTANCE
* Corresponding author. Tel.: +1-514-398-1452; fax: +1-514-3984492. E-mail address: jfi[email protected] (J.A. Finch).
0892-6875/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.10.006
Fig. 2. The definition of ‘‘interface’’ introduced by Gomez et al. (1998): K is average conductance for zone; subscript L is liquid zone, S solids zone, and I the interface zone.
LEVEL (cm, from top of thickener)
88
J. Vergouw et al. / Minerals Engineering 17 (2004) 87–88
sensitivity (the slope of the lines is much the same). The choice of boundary could reflect local definitions of high and low level; i.e., I ! 1 could trigger a ‘‘high’’ alarm, and I ! 0 could trigger a ‘‘low’’ alarm.
15-27 18-27 15-25
100
21-28
3. Conclusion
50
0 0.0
0.2
0.4 0.6 “INTERFACE”
0.8
1.0
Fig. 3. Calibration: True level, L, (distance from top of thickener) as a function of interface, I, for different zone boundary choices (e.g., 15– 25, means the water/interface boundary is set, from the top, at ring 15 and the interface/solids boundary is at ring 25). Key: 15–25, squares; 15–27, circles; 18–27, triangles; 21–28, diamonds.
2. Calibration Local practice is to measure level from the top of the thickener as judged by lowering a weighted rope till it slackens indicating the top of the bed. This defines the true depth, L. This estimate of L and the inferred interface, I, using a 33-ring stationary probe were measured simultaneously over a few weeks of normal operation. To calculate I, the two boundaries, liquid/ interface and interface/solids, have to be set. Different settings were used designated by the cell number from the top of the probe (e.g., 15–25 means the liquid/ interface boundary is at ring 15 and the interface/solids boundary is at ring 25). Fig. 3 shows the true level (in cm from the top of the thickener) vs interface. Regardless of the boundary choices a linear calibration was obtained with similar
The inferred level in a thickener using a conductivity probe has been converted from the original 0–1 scale to true depth by calibration against estimates made by accepted local practice.
Acknowledgements Funding was under the Natural Sciences and Engineering Research Council of Canada Collaborative Research and Development program with industrial sponsorship from Inco, Teck Cominco, Falconbridge and Noranda (and now including Corem and SGS Lakefield Research) forming an Industry Chair in Mineral Processing. References Gomez, C.O., Probst, A., Finch, J.A., Moores, N., 1998. Monitoring thickener operation using a conductivity probe. Miner. Metall. Process. 15 (4), 9–15. Schoenmann, F., Hales, L., Bedell, D., 2002. Strategies for instrumentation and control of thickeners and other solid–liquid separation circuits. In: Mular, A.L., Halbe, D.N., Barratt, D.J. (Eds.), Mineral Processing Plant Design, Practice, and Control, vol. 2. SME, Littleton, CO, pp. 2164–2173. Xu, M., Probst, A., Finch, J.A., 1994. Level and solids profile detection in thickeners using conductivity. C.I.M. Bull. 87 (985), 46–52.