- Email: [email protected]
An automated data acquisition technique for settling tests
Minerals Engineering, Vol. 10, No. 10, pp. 1095-1105, 1997 © 1997 Elsevier Science Ltd
Pergamon
All rights reserved. Printed in Great Britain
S0892--6875(97)00096-4
0s92-6875/97 $17.00+0.00
AN AUTOMATED DATA ACQUISITION TECHNIQUE FOR SETTLING TESTS
J.M. VERGOUW, J. ANSON, R. DALHKE, Z. XU, C. GOMEZ and J.A. FINCH Department of Mining and Metallurgical Engineering, McGill University, Montreal, Caned.q= E-mail: [email protected] (Received 9 January 1997; accepted 27 May 1997)
ABSTRACT
An automated technique to measure the settling velocity of particle suspensions has been developed. The technique relies on the measurement of conductance as solids settle through a' conductivity cell. The cell consists of two electrodes mounted flush with the wall of a cylinder and separated by a set distance. The technique is illustrated by determining settling velocities for single and mixed sizes of silica, and as means of detecting particle agglomeration. © 1997 Elsevier Science Ltd
Keywordls Agglomeration; fine particle processing; process instrumentation
INTRODUCTION The sedimentatiorh or settling of solids in solid-liquid mixtures (slurries) is of great importance in many industries. Detailed information on particle settling behaviour is required for design of such diverse units as classifiers, pipelines, thickeners, and filters [1]. Laboratory characterization of settling is mostly performed manually. The conventional method to measure settling velocity is to fill a cylinder with slurry, agitate to completely disperse the particles, then time the descent of the so}Lid/liquid interface (or fron0 that forms. The technique is tedious and the accuracy is determined by how quickly the front is moving and how dearly it is seen. The latter points to an important limitation: the technique can not be used in opaque systems, encountered, for example, in the settling of one species in the background of another. Therefore there is considerable incentive to develop a technique capable of measming settling velocities in a non-visual manner. Methods which have been used for measuring settling velocity, include ones based on neutron and x-ray radiation, ultrasonics, radioactive and magnetic tracers, hydrometers and pressure sensors [2]. Although many of these techniques work well they tend to be expensive, complicated, invasive, model dependent or unable to perform on-line. A method that overcomes many of these objections has been developed by us based on measurenaent of conductance as the particles move through a conductivity cell. It has been used to study settling of solid particles [3,4] and the rising of bubble swarms [5]. In the original work the cell
1095
1096
J.M. Vergouw et al.
had grid electrodes, which were invasive and could interfere with particle motion. This problem has been circumvented by the use of ring electrodes. The new apparatus is described, the technique verified, and uses illustrated.
THEORY
Concept The conductivity of a slurry depends on solids content, hence this property can be used to detect changes as solids settle in a cylinder. Consider the simple case where a front forms with clear liquid above (Figure 1). As the particles settle, the front descends and the conductance rises (assuming the solids are nonconducting relative to the liquid which is usually the case in mineral systems [4]). This change in conductance with time is directly related to the settling of particles and can be used to determine the settling velocity. It is not necessary that the continuous phase be clear, it could be a suspension of slower settling particles, in which case the front is usually not visible. As will be shown, the technique is capable of monitoring descent of the front in this situation.
(b)
(a)
/A
l
L
(c)
o
TA ] Fig.1
[ liquid
TC ~
slurry
l
Time (s) settledsolids
Conceptional curve generated as front descends through conductivity cell, or "conduct~mce settling curve". (a) slurry dispersed and front above top electrode; (b) front is between electrodes; (c) front is below bottom electrode; A = cross-sectional area of cylinder, L = distance between two ring electrodes
Estimating settling velocity For present purposes, the simple case (Figure 1) is considered. --
F r o m TA a n d T c
From Figure 1, T A represents the time when the front just passes the top electrode, and T c the time when it just passes the bottom electrode. By knowing the distance between electrodes, L, and measuring T c - T A then the average settling velocity, v, is given by:
Data acquisition technique for settling tests
1097
L
V =
(r.
(1)
-
It is necessary to establish a reliable method of estimating T A and Tc (and to determine that they do correspond to the front passing the top and bottom electrode, respectively). In many cases, linear regression over periods (a) and (b), to give T A, and over periods (b) and (c), to give T C, can be used. From the curve
Assuming the shm'y conductance inside the cell during settling does not change then the system can be represented as a sianple electrical circuit consisting of two variable resistances in series (Figure 2).
f.__~A
ring electrode
T~x
~ R1 R=Rl+Rsl
L Rsl
F--q liquid
~
slurry
1
R
1 K
settledsolids
Fig.2 Conductance measurement represented as two resistances in series Uribe-Salas et al. [3] showed that x(t), the distance from the bottom electrode as a function of time, is related to K(t), the conductance as a function of time, by:
X(t)
1
A Kt Kst
L Kst
K(t)
(K l - Ksl)
K z - Ksl
-
(2)
where A is the cross-sectional area of the cylinder, and r t and ~:sl are the conductivity of the clear liquid and slurry, respectively. The two conductivities are estimated independently: in period (a), ~1 is given by:
L K.t = K,l
and in period (c),
(3)
1008
J.M. Vergouw et ol.
L KI = KI -A-
(4)
Equation (2) permits the conductance axis on the settling curve to be converted to distance and velocity is estimated from the slope. This approach is useful if there is curvature around the transition from (a) to (b) and/or to (c) which introduces uncertainty in the estimates of T A and T C.
EQUIPMENT Two Plexiglas (non-conducting) cylinders were used with the dimensions in Table 1. The two ring electrodes, made of stainless steel, were identical and mounted flush with the cylinder wall. The top was closed with a rubber stopper through which various probes could be inserted. In the present case pH of the slurry was monitored.
TABLE 1 Dimensions (cm) of cylinders used. Cylinder
Height
Diameter
Ring width
L
A
29.0
3.8
1.1
8.3
B
39.9
5.7
1.1
19.2
The electrodes were connected to a Tacussel conductivity meter (model CD 810). The meter runs at a potential of 0 to 1 volt and was set at 1000 Hz to avoid electrode polarization. The analog output of the meter was fed to a 12 bit A/D data acquisition board (DAS-8PGA) hosted in an IBM compatible computer which recorded all data. A computer interface program was developed in QBASIC to record the conductance as a function of time. Measurements were taken every 0.5 s and the conductance vs. time was plotted in real time on the monitor.
EXPERIMENTAL
Procedure for measuring settling velocity The slurry was conditioned for a given time then transferred to cylinder A or B (Table 1). The solids were suspended by subjecting the cylinder to a rhythmic rotation. After full dispersion had been obtained, judged by the conductance becoming approximately constant, the cylinder was stood vertically to allow the solids to settle. The "conductance settling curve" was generated and T A and T C were estimated by performing linear regression over sections (a), (b) and (c) of the curve, using the software "TBLcurve 2D" (Jandel Scientific).
Procedure for measuring zeta-potential and agglomeration Zeta potential was measured using the Laser Zee meter (Model 501, PenKem, Inc.). One-half gram of -38 gin solids were dispersed in 500 mL 10-3 M KCI and conditioned for 5 minutes. For acidic and alkaline measurements separate fresh samples were prepared. The pH modifiers used were HCI and NaOH. Agglomeration as a function of pH was inferred from the relative settling velocity. For single systems 2 %v/v (for mixed systems l%v/v of each mineral) of -38 gm solids was conditioned for 5 minutes in 275 mL 10-3 M KCI at the desired pH and then transferred to cylinder B (Table 1). In the case of single minerals, after settling, the zeta-potential was remeasured to verify the values.
Data acquisition technique for settling tests
1099
Material preparation Silica
Coarse (+75 lain) and fine (-75 Inn) silica products were purchased from Daubois Inc. The fine silica was wet screened from 75 to 25 lam. The coarse silica was dry screened from 150 to 75 ~na followed by impurity removal ,an a Mozley table. Several size classes were produced. B
Pyrite and sphalerite
Pyrite (Carthage, Tennessee) and sphalerite (Rico, Colorado) were obtained from Ward's Earth Science Establishment. Both minerals were ground and dry screened to obtain the - - 3 8 lam size class and stored in a fridge between experiments. For each experiment the sphalerite was used directly, while the pyrite was gently ground in a porcelain mortar to create fresh surface. This was found to give reproducible results. RESULTS AND DISCUSSION
Reproducibility and Accuracy Figure 3 shows a typical conductance settling curve; it is dearly similar to Figure 1. Table 2 demonstrates the reproducibility of the data. Five sedimentation tests on 13 %v/v silica of -105/+90 Inn were performed (one is shown in Figure 3). The relative standard deviation from this experience was ca. 1% ((0.004/0.34) x 100%).
1,600 1,550
(a)
l
(c)
~ 1,500 1,450 1,400 •~ 1,350
,../'T
0
r,,j 1,300 1,250 1,20030
I
,
,
40
liquid
I
50 TA
Fig.3
~ w ~4
,
I
,
~
60 Time (s) slurry
70
I
80
,
90
TC
I
settled solids
Conductance seRling curve for 13% v/v silica, -105/+90 Inn. Background electrolyte, 10--3 M KC1. (a) homogeneous dispersion; (b) settling front passes through cell; (c) settling front below bottom electrode
J. M. Vergouwa a/.
1 I(X)
TABLE 2 Reproducibility: Repeat tests on 13 % v/v sUiea, -105/+90 pro. Test
Settling velocity (cm/s)
1
0.341
2
0.336
3
0.333
4
0.345
5
0.341
Average
0.339
Standard deviation
0.004
The accuracy of the settling velocity was assessed by comparing the estimates from Eqs. (1) and (2) with a manual determination. Table 3 shows the conductivity technique is accurate.
TABLE 3
Accuracy: Comparison of different techniques to measure settling velocity on 13% v/v silica, -63/+53 pm. Method
Settling velocity (cm/s)
Equation (1)
0.1226
Equation (2)
0.1223
Manual
0.1227
Settling studies
Single size class of silica The data for silica are all at natural pH. (Exploratory work established that at natural pH silica was well dispersed as expected since the iso-electric point of silica is ca. pH 2-3.) Figure 3 shows the settling curve for one case. T A and T C were estimated and the settling velocity calculated using Eq. (1). The settling velocity for different size classes of silica as a function of solids holdup (% v/v solids) is shown in Figure 4. Two trends are apparent both of which were anticipated: settling velocity decreased with increasing holdup, and with decreasing particle size. The data in Figure 4 could be obtained manually, although it would be tedious. If a second size class is added the front produced by the coarser fraction settling in the background of the finer cannot be seen. The power of the conductivity technique then becomes evident.
Data acquisitiontechniquefor settlingtests
1101
Size
~
class
(um)
-! 50•+ 105 0
.
"--I--105/+90 #
8
-63/+53 -53•+45 E] -451+38
~0.2 13 2
('
,
I
,
4
I
,
I
6
=
I
8
10
~
,
t
12
1
~
14
I
16
18
Solids holdup(%v/v) Fig.4 Se,ttling velocities for several size classes of silica as a function of solids holdup Two size classes of silica Figure 5 shows ml example of a settling curve, which is clearly different than that for the single size class. Coarse particles settle faster than fine particles and consequently, two settling fronts develop: one
19O0
1800
17(10
1600
15()0 1400 150
I
I
I
I
I
I
i
I
175
200
225
250
275
300
325
350
TA Fig.5
TB
Tc
Time(,) TD
Conductance settling curve for mixture of 6.8% v/v coarse (-150/+106 iJm) and 9.6% v/v fine (-63/+53 IJm) silica. Background electrolyte, 10-3M KC1. (a) homogeneous dispersion of coarse and fine particles; (13) settling of coarse particles within the suspension of fines; (c) coarse solids front is below bottom electrode, conductance is that of fine slurry; (d) settling of fine particles with clear liquid above; (e) all solids have settled below bottom electro&~.
1102
J.M.
Vergouw
et al.
for coarse silica in a background of fines and a second for the slower descending front of line silica. By carefully selecting the conditions (relative particle size and volume fraction of the two classes) the two fronts can be "seen"; this is the case in Figure 5. In period (a), the fine and coarse particles are initially well dispersed to form a homogeneous slurry. At time T A the coarse particle front passes the top electrode and the conductance starts to increase, recording the descent of this front. At time T B the coarse front has passed the bottom ring. At this point and during period (c) the slower moving front of fine silica has not yet entered the conductivity cell; the conductance is constant and equal to that of line slurry alone. When the line silica front passes the top ring (time T C) the conductance starts to increase again, monitoring the descent of the line silica in a background of clear liquid (period (d)). At time T D, the fine front passes the bottom ring and the liquid conductance is measured (period (e)). The settling velocities are determined using Eq. (1) and times T A and T B for coarse in the background of fines, and times T C and T D for fines in the background of clear liquid. Again these times are estimated from the intercept of the linear regression lines applied to the relevant portions of the settling curve. Figure 6 shows the settling velocities of coarse silica alone and in the presence of line silica as a function of total solids holdup. In the mixture the percentage of coarse silica was kept constant (6.8 %v/v). The results reveal that the coarse particles settled faster in a fine slurry than in a coarse slurry of the same total solids holdup. This reflects the fact that the line particles settle slowly, and consequently, the velocity of the liquid displaced upward is low offering less hindrance to the settling of the coarse particles. As the fraction of lines relative to the coarse increases, i.e., with increasing total solids holdup, this difference between the mixed and single size systems is increased. While the effect of fines on the settling of coarse particles is known, relatively few demonstrations are in the literature; the technique introduced here makes experimentation possible.
0.8 coarse-fine coarse
"•0.7 ¢d
EO.6 ca
Q
~"
r¢~ ~
0.5 Fine (-63 pm + 5 3 pm) Coarse (-150 ~tm + 106 pm)
0.4 03.
~
6 Fig .6
I
8
L
I
,
I
,
I
10 12 14 Solids holdup (%v/v)
,
I
16
,
18
Settling velocity of coarse particles in mixed and coarse only systems. In the coarse-fine mixture, the coarse fraction was constant at 6.8% v/v and fines were added to the total holdup shown
Agglomeration studies --
Pyrite
Figure 7 shows the zeta-potential (a) and the settling rote (b) of pyrite as a function of pH. The maximum settling rate, and, therefore, maximum degree of agglomeration, is around pH 6-8.5. This corresponds reasonably to the pl-Iiep (~ 6), as expected.
Data acquisition technique for settling tests
1103
The settling rate raay offer a means of quantifying agglomeration. The maximum difference in settling rate from most dispersed (pHs < 4 and > 11) to most agglomerated was about a factor of two, thus the settling velocity at any condition in between could be made relative to this difference.
(a)
40 "IL ,~
:20
m alml
'~
0
0
-20
-1,
",,,,,
",,,\ =,,,,
!
,~
-40
N -60
",=_
•
2
4
6
8
10
12
14
2
4
6
8
10
12
14
(b) rr~ 0.'20 *m
~0.18 o 0.16 1~0.14
om
--0.12 r~
0
pH Fig.7 Comparison of zeta-potential (a) and settling velocity (b) of pyrite as a function of pH - - Pyrite and spi~lerite
The zeta-potenti~d for the two minerals (Figure 8a) shows a pH range of ca. 4 to 6 where sphalerite is negatively charged and pyrite is positively charged. This is ideal for heterocoagulation and the settling rate strongly suggests this happens, the maximum occurring around pH 5-6 (Figure 8b). The conductance settling curves at two pHs are given in Figure 9. This serves to illustrate that the actual conductance values, which depend on electrolyte concentration, ions added, pH etc., are not important. The technique require, s only that the conductance change as the particles settle. The time T C - T A is clearly some 3 times shorter at pH 5.35 compared to pH 12.15, i.e. the settling rate is some 3 times higher as shown in Figure 8b.
CONCLUSIONS A conductivity-based technique has been established for automatically recording settling data. It involves a relatively simplie modification to a conventional settling cylinder. The usual initial manual dispersion is retained, but the automated data acquisition greatly facilitated testing, and proved capable of reproducible and accurate me~Lsurement of the settling velocity of a front, whether it was visible or not. The technique was illustrated by studies on single and mixed sizes of silica. The advantage of the technique is revealed in the latter where the coarse fraction settling among the fines could be followed.
1104
J.M.
(a)
Vergouwet
al.
40 sphalerite
20
,~
-llpyrite
~ 0 I=
\
0 -20 I
,., -40 M -6%
2
4
6
8
r~
(b)
//
0, "~
0.3
lm
@
lali
Ii
0.2
10
14
12
14
""1
It
B----
=
12
0.1
--
r,~
0
2
4
6
8
10
pH Fig.8
Comparison between zeta-potential of sphalerite and pyrite (a) ans settling velocity of a 50:50 mixture of the two (b) as a function of pH
785
855 0
0 0 ,5' Q3DO, sTJO0 ,:.,"O,,gX3~
A
E --..850
• •
410 o
e •lSImffMIBe
pH 5.35 =
845
m
pH 12.15
i
•
-
775 ~ N
770==. 765 •
Om
O L~
= 840
00
• •Do •l•ll (rob
0
i Qe
CO
i ~ e
e
= 835
m
• •,•.mm ,
CL' L,~3'
780 E
• lID •0 gee • O Z e D
IBiX~Oa3
760 ~
~
e ~
c
0
755 o ~
830
~
i
O ,
0
I
50
,
I 100 Time(s)
,
i 150
750
Fig.9 Conductance settling curves for the mixed pyrite--sphalerite system at pH 5.35 and pH 12.15 The second illustration was as a means to detect agglomeration. It is well appreciated that settling rate depends on the degree of agglomeration of a suspension, but the tedium o f manual measurement has restricted its use as a measure. Our device now makes settling tests for this purpose more accessible.
Data acquisitiontechniquefor settling tests
1105
The apparatus not only removes much of the tedium and chance for error associated with manual recording of settling data but it also opens up possibilities for fundamental and practical studies on mixed systems.
ACKNOWLEDGEMENTS Funding for this work was under the Collaborative Research and Development program of the Natural Sciences and Engineering Research Council of Canada with industrial sponsorship from Inco, Falconbridge, Cominco, Noranda, Hudson Bay Mining and Smelting, and Teck co-ordinated by CAMIRO.
REFERENCES .
2.
.
4.
5.
Rushton, A., Ward, A.S. & Holdich, R.G., Solid-liquid filtration and separation technology. VCH Weinheim, 1996, pp. 1-31 and 221-226. Williams, R.A., Xie, C.G., Bragg, R. & Amarasinghe, W.P.K., Experimental techniques for monitoring sedimentation in optically opaque suspensions. Colloids and Surfaces, 1990, 43(1), 1-32. Uribe-Snlas, A., Vermet, F. & Finch, J.A., Apparatus and technique to measure settling velocity and holdup of solids in water slurries. Chemical Engineering Science, 1992, 48(4), 815-819. Uribe-Sidas, A., Gomez, C.O. & Finch, J.A., A conductivity technique for gas and solids holdup determination in three-phase reactors. Chemical Engineering Science, 1994, 49 (1), 1-10. Shen, G. & Finch, J. A., Bubble swarm velocity in a column. Chemical Engineering Science, 1994, 51(14), 3665-3674.
Pergamon
All rights reserved. Printed in Great Britain
S0892--6875(97)00096-4
0s92-6875/97 $17.00+0.00
AN AUTOMATED DATA ACQUISITION TECHNIQUE FOR SETTLING TESTS
J.M. VERGOUW, J. ANSON, R. DALHKE, Z. XU, C. GOMEZ and J.A. FINCH Department of Mining and Metallurgical Engineering, McGill University, Montreal, Caned.q= E-mail: [email protected] (Received 9 January 1997; accepted 27 May 1997)
ABSTRACT
An automated technique to measure the settling velocity of particle suspensions has been developed. The technique relies on the measurement of conductance as solids settle through a' conductivity cell. The cell consists of two electrodes mounted flush with the wall of a cylinder and separated by a set distance. The technique is illustrated by determining settling velocities for single and mixed sizes of silica, and as means of detecting particle agglomeration. © 1997 Elsevier Science Ltd
Keywordls Agglomeration; fine particle processing; process instrumentation
INTRODUCTION The sedimentatiorh or settling of solids in solid-liquid mixtures (slurries) is of great importance in many industries. Detailed information on particle settling behaviour is required for design of such diverse units as classifiers, pipelines, thickeners, and filters [1]. Laboratory characterization of settling is mostly performed manually. The conventional method to measure settling velocity is to fill a cylinder with slurry, agitate to completely disperse the particles, then time the descent of the so}Lid/liquid interface (or fron0 that forms. The technique is tedious and the accuracy is determined by how quickly the front is moving and how dearly it is seen. The latter points to an important limitation: the technique can not be used in opaque systems, encountered, for example, in the settling of one species in the background of another. Therefore there is considerable incentive to develop a technique capable of measming settling velocities in a non-visual manner. Methods which have been used for measuring settling velocity, include ones based on neutron and x-ray radiation, ultrasonics, radioactive and magnetic tracers, hydrometers and pressure sensors [2]. Although many of these techniques work well they tend to be expensive, complicated, invasive, model dependent or unable to perform on-line. A method that overcomes many of these objections has been developed by us based on measurenaent of conductance as the particles move through a conductivity cell. It has been used to study settling of solid particles [3,4] and the rising of bubble swarms [5]. In the original work the cell
1095
1096
J.M. Vergouw et al.
had grid electrodes, which were invasive and could interfere with particle motion. This problem has been circumvented by the use of ring electrodes. The new apparatus is described, the technique verified, and uses illustrated.
THEORY
Concept The conductivity of a slurry depends on solids content, hence this property can be used to detect changes as solids settle in a cylinder. Consider the simple case where a front forms with clear liquid above (Figure 1). As the particles settle, the front descends and the conductance rises (assuming the solids are nonconducting relative to the liquid which is usually the case in mineral systems [4]). This change in conductance with time is directly related to the settling of particles and can be used to determine the settling velocity. It is not necessary that the continuous phase be clear, it could be a suspension of slower settling particles, in which case the front is usually not visible. As will be shown, the technique is capable of monitoring descent of the front in this situation.
(b)
(a)
/A
l
L
(c)
o
TA ] Fig.1
[ liquid
TC ~
slurry
l
Time (s) settledsolids
Conceptional curve generated as front descends through conductivity cell, or "conduct~mce settling curve". (a) slurry dispersed and front above top electrode; (b) front is between electrodes; (c) front is below bottom electrode; A = cross-sectional area of cylinder, L = distance between two ring electrodes
Estimating settling velocity For present purposes, the simple case (Figure 1) is considered. --
F r o m TA a n d T c
From Figure 1, T A represents the time when the front just passes the top electrode, and T c the time when it just passes the bottom electrode. By knowing the distance between electrodes, L, and measuring T c - T A then the average settling velocity, v, is given by:
Data acquisition technique for settling tests
1097
L
V =
(r.
(1)
-
It is necessary to establish a reliable method of estimating T A and Tc (and to determine that they do correspond to the front passing the top and bottom electrode, respectively). In many cases, linear regression over periods (a) and (b), to give T A, and over periods (b) and (c), to give T C, can be used. From the curve
Assuming the shm'y conductance inside the cell during settling does not change then the system can be represented as a sianple electrical circuit consisting of two variable resistances in series (Figure 2).
f.__~A
ring electrode
T~x
~ R1 R=Rl+Rsl
L Rsl
F--q liquid
~
slurry
1
R
1 K
settledsolids
Fig.2 Conductance measurement represented as two resistances in series Uribe-Salas et al. [3] showed that x(t), the distance from the bottom electrode as a function of time, is related to K(t), the conductance as a function of time, by:
X(t)
1
A Kt Kst
L Kst
K(t)
(K l - Ksl)
K z - Ksl
-
(2)
where A is the cross-sectional area of the cylinder, and r t and ~:sl are the conductivity of the clear liquid and slurry, respectively. The two conductivities are estimated independently: in period (a), ~1 is given by:
L K.t = K,l
and in period (c),
(3)
1008
J.M. Vergouw et ol.
L KI = KI -A-
(4)
Equation (2) permits the conductance axis on the settling curve to be converted to distance and velocity is estimated from the slope. This approach is useful if there is curvature around the transition from (a) to (b) and/or to (c) which introduces uncertainty in the estimates of T A and T C.
EQUIPMENT Two Plexiglas (non-conducting) cylinders were used with the dimensions in Table 1. The two ring electrodes, made of stainless steel, were identical and mounted flush with the cylinder wall. The top was closed with a rubber stopper through which various probes could be inserted. In the present case pH of the slurry was monitored.
TABLE 1 Dimensions (cm) of cylinders used. Cylinder
Height
Diameter
Ring width
L
A
29.0
3.8
1.1
8.3
B
39.9
5.7
1.1
19.2
The electrodes were connected to a Tacussel conductivity meter (model CD 810). The meter runs at a potential of 0 to 1 volt and was set at 1000 Hz to avoid electrode polarization. The analog output of the meter was fed to a 12 bit A/D data acquisition board (DAS-8PGA) hosted in an IBM compatible computer which recorded all data. A computer interface program was developed in QBASIC to record the conductance as a function of time. Measurements were taken every 0.5 s and the conductance vs. time was plotted in real time on the monitor.
EXPERIMENTAL
Procedure for measuring settling velocity The slurry was conditioned for a given time then transferred to cylinder A or B (Table 1). The solids were suspended by subjecting the cylinder to a rhythmic rotation. After full dispersion had been obtained, judged by the conductance becoming approximately constant, the cylinder was stood vertically to allow the solids to settle. The "conductance settling curve" was generated and T A and T C were estimated by performing linear regression over sections (a), (b) and (c) of the curve, using the software "TBLcurve 2D" (Jandel Scientific).
Procedure for measuring zeta-potential and agglomeration Zeta potential was measured using the Laser Zee meter (Model 501, PenKem, Inc.). One-half gram of -38 gin solids were dispersed in 500 mL 10-3 M KCI and conditioned for 5 minutes. For acidic and alkaline measurements separate fresh samples were prepared. The pH modifiers used were HCI and NaOH. Agglomeration as a function of pH was inferred from the relative settling velocity. For single systems 2 %v/v (for mixed systems l%v/v of each mineral) of -38 gm solids was conditioned for 5 minutes in 275 mL 10-3 M KCI at the desired pH and then transferred to cylinder B (Table 1). In the case of single minerals, after settling, the zeta-potential was remeasured to verify the values.
Data acquisition technique for settling tests
1099
Material preparation Silica
Coarse (+75 lain) and fine (-75 Inn) silica products were purchased from Daubois Inc. The fine silica was wet screened from 75 to 25 lam. The coarse silica was dry screened from 150 to 75 ~na followed by impurity removal ,an a Mozley table. Several size classes were produced. B
Pyrite and sphalerite
Pyrite (Carthage, Tennessee) and sphalerite (Rico, Colorado) were obtained from Ward's Earth Science Establishment. Both minerals were ground and dry screened to obtain the - - 3 8 lam size class and stored in a fridge between experiments. For each experiment the sphalerite was used directly, while the pyrite was gently ground in a porcelain mortar to create fresh surface. This was found to give reproducible results. RESULTS AND DISCUSSION
Reproducibility and Accuracy Figure 3 shows a typical conductance settling curve; it is dearly similar to Figure 1. Table 2 demonstrates the reproducibility of the data. Five sedimentation tests on 13 %v/v silica of -105/+90 Inn were performed (one is shown in Figure 3). The relative standard deviation from this experience was ca. 1% ((0.004/0.34) x 100%).
1,600 1,550
(a)
l
(c)
~ 1,500 1,450 1,400 •~ 1,350
,../'T
0
r,,j 1,300 1,250 1,20030
I
,
,
40
liquid
I
50 TA
Fig.3
~ w ~4
,
I
,
~
60 Time (s) slurry
70
I
80
,
90
TC
I
settled solids
Conductance seRling curve for 13% v/v silica, -105/+90 Inn. Background electrolyte, 10--3 M KC1. (a) homogeneous dispersion; (b) settling front passes through cell; (c) settling front below bottom electrode
J. M. Vergouwa a/.
1 I(X)
TABLE 2 Reproducibility: Repeat tests on 13 % v/v sUiea, -105/+90 pro. Test
Settling velocity (cm/s)
1
0.341
2
0.336
3
0.333
4
0.345
5
0.341
Average
0.339
Standard deviation
0.004
The accuracy of the settling velocity was assessed by comparing the estimates from Eqs. (1) and (2) with a manual determination. Table 3 shows the conductivity technique is accurate.
TABLE 3
Accuracy: Comparison of different techniques to measure settling velocity on 13% v/v silica, -63/+53 pm. Method
Settling velocity (cm/s)
Equation (1)
0.1226
Equation (2)
0.1223
Manual
0.1227
Settling studies
Single size class of silica The data for silica are all at natural pH. (Exploratory work established that at natural pH silica was well dispersed as expected since the iso-electric point of silica is ca. pH 2-3.) Figure 3 shows the settling curve for one case. T A and T C were estimated and the settling velocity calculated using Eq. (1). The settling velocity for different size classes of silica as a function of solids holdup (% v/v solids) is shown in Figure 4. Two trends are apparent both of which were anticipated: settling velocity decreased with increasing holdup, and with decreasing particle size. The data in Figure 4 could be obtained manually, although it would be tedious. If a second size class is added the front produced by the coarser fraction settling in the background of the finer cannot be seen. The power of the conductivity technique then becomes evident.
Data acquisitiontechniquefor settlingtests
1101
Size
~
class
(um)
-! 50•+ 105 0
.
"--I--105/+90 #
8
-63/+53 -53•+45 E] -451+38
~0.2 13 2
('
,
I
,
4
I
,
I
6
=
I
8
10
~
,
t
12
1
~
14
I
16
18
Solids holdup(%v/v) Fig.4 Se,ttling velocities for several size classes of silica as a function of solids holdup Two size classes of silica Figure 5 shows ml example of a settling curve, which is clearly different than that for the single size class. Coarse particles settle faster than fine particles and consequently, two settling fronts develop: one
19O0
1800
17(10
1600
15()0 1400 150
I
I
I
I
I
I
i
I
175
200
225
250
275
300
325
350
TA Fig.5
TB
Tc
Time(,) TD
Conductance settling curve for mixture of 6.8% v/v coarse (-150/+106 iJm) and 9.6% v/v fine (-63/+53 IJm) silica. Background electrolyte, 10-3M KC1. (a) homogeneous dispersion of coarse and fine particles; (13) settling of coarse particles within the suspension of fines; (c) coarse solids front is below bottom electrode, conductance is that of fine slurry; (d) settling of fine particles with clear liquid above; (e) all solids have settled below bottom electro&~.
1102
J.M.
Vergouw
et al.
for coarse silica in a background of fines and a second for the slower descending front of line silica. By carefully selecting the conditions (relative particle size and volume fraction of the two classes) the two fronts can be "seen"; this is the case in Figure 5. In period (a), the fine and coarse particles are initially well dispersed to form a homogeneous slurry. At time T A the coarse particle front passes the top electrode and the conductance starts to increase, recording the descent of this front. At time T B the coarse front has passed the bottom ring. At this point and during period (c) the slower moving front of fine silica has not yet entered the conductivity cell; the conductance is constant and equal to that of line slurry alone. When the line silica front passes the top ring (time T C) the conductance starts to increase again, monitoring the descent of the line silica in a background of clear liquid (period (d)). At time T D, the fine front passes the bottom ring and the liquid conductance is measured (period (e)). The settling velocities are determined using Eq. (1) and times T A and T B for coarse in the background of fines, and times T C and T D for fines in the background of clear liquid. Again these times are estimated from the intercept of the linear regression lines applied to the relevant portions of the settling curve. Figure 6 shows the settling velocities of coarse silica alone and in the presence of line silica as a function of total solids holdup. In the mixture the percentage of coarse silica was kept constant (6.8 %v/v). The results reveal that the coarse particles settled faster in a fine slurry than in a coarse slurry of the same total solids holdup. This reflects the fact that the line particles settle slowly, and consequently, the velocity of the liquid displaced upward is low offering less hindrance to the settling of the coarse particles. As the fraction of lines relative to the coarse increases, i.e., with increasing total solids holdup, this difference between the mixed and single size systems is increased. While the effect of fines on the settling of coarse particles is known, relatively few demonstrations are in the literature; the technique introduced here makes experimentation possible.
0.8 coarse-fine coarse
"•0.7 ¢d
EO.6 ca
Q
~"
r¢~ ~
0.5 Fine (-63 pm + 5 3 pm) Coarse (-150 ~tm + 106 pm)
0.4 03.
~
6 Fig .6
I
8
L
I
,
I
,
I
10 12 14 Solids holdup (%v/v)
,
I
16
,
18
Settling velocity of coarse particles in mixed and coarse only systems. In the coarse-fine mixture, the coarse fraction was constant at 6.8% v/v and fines were added to the total holdup shown
Agglomeration studies --
Pyrite
Figure 7 shows the zeta-potential (a) and the settling rote (b) of pyrite as a function of pH. The maximum settling rate, and, therefore, maximum degree of agglomeration, is around pH 6-8.5. This corresponds reasonably to the pl-Iiep (~ 6), as expected.
Data acquisition technique for settling tests
1103
The settling rate raay offer a means of quantifying agglomeration. The maximum difference in settling rate from most dispersed (pHs < 4 and > 11) to most agglomerated was about a factor of two, thus the settling velocity at any condition in between could be made relative to this difference.
(a)
40 "IL ,~
:20
m alml
'~
0
0
-20
-1,
",,,,,
",,,\ =,,,,
!
,~
-40
N -60
",=_
•
2
4
6
8
10
12
14
2
4
6
8
10
12
14
(b) rr~ 0.'20 *m
~0.18 o 0.16 1~0.14
om
--0.12 r~
0
pH Fig.7 Comparison of zeta-potential (a) and settling velocity (b) of pyrite as a function of pH - - Pyrite and spi~lerite
The zeta-potenti~d for the two minerals (Figure 8a) shows a pH range of ca. 4 to 6 where sphalerite is negatively charged and pyrite is positively charged. This is ideal for heterocoagulation and the settling rate strongly suggests this happens, the maximum occurring around pH 5-6 (Figure 8b). The conductance settling curves at two pHs are given in Figure 9. This serves to illustrate that the actual conductance values, which depend on electrolyte concentration, ions added, pH etc., are not important. The technique require, s only that the conductance change as the particles settle. The time T C - T A is clearly some 3 times shorter at pH 5.35 compared to pH 12.15, i.e. the settling rate is some 3 times higher as shown in Figure 8b.
CONCLUSIONS A conductivity-based technique has been established for automatically recording settling data. It involves a relatively simplie modification to a conventional settling cylinder. The usual initial manual dispersion is retained, but the automated data acquisition greatly facilitated testing, and proved capable of reproducible and accurate me~Lsurement of the settling velocity of a front, whether it was visible or not. The technique was illustrated by studies on single and mixed sizes of silica. The advantage of the technique is revealed in the latter where the coarse fraction settling among the fines could be followed.
1104
J.M.
(a)
Vergouwet
al.
40 sphalerite
20
,~
-llpyrite
~ 0 I=
\
0 -20 I
,., -40 M -6%
2
4
6
8
r~
(b)
//
0, "~
0.3
lm
@
lali
Ii
0.2
10
14
12
14
""1
It
B----
=
12
0.1
--
r,~
0
2
4
6
8
10
pH Fig.8
Comparison between zeta-potential of sphalerite and pyrite (a) ans settling velocity of a 50:50 mixture of the two (b) as a function of pH
785
855 0
0 0 ,5' Q3DO, sTJO0 ,:.,"O,,gX3~
A
E --..850
• •
410 o
e •lSImffMIBe
pH 5.35 =
845
m
pH 12.15
i
•
-
775 ~ N
770==. 765 •
Om
O L~
= 840
00
• •Do •l•ll (rob
0
i Qe
CO
i ~ e
e
= 835
m
• •,•.mm ,
CL' L,~3'
780 E
• lID •0 gee • O Z e D
IBiX~Oa3
760 ~
~
e ~
c
0
755 o ~
830
~
i
O ,
0
I
50
,
I 100 Time(s)
,
i 150
750
Fig.9 Conductance settling curves for the mixed pyrite--sphalerite system at pH 5.35 and pH 12.15 The second illustration was as a means to detect agglomeration. It is well appreciated that settling rate depends on the degree of agglomeration of a suspension, but the tedium o f manual measurement has restricted its use as a measure. Our device now makes settling tests for this purpose more accessible.
Data acquisitiontechniquefor settling tests
1105
The apparatus not only removes much of the tedium and chance for error associated with manual recording of settling data but it also opens up possibilities for fundamental and practical studies on mixed systems.
ACKNOWLEDGEMENTS Funding for this work was under the Collaborative Research and Development program of the Natural Sciences and Engineering Research Council of Canada with industrial sponsorship from Inco, Falconbridge, Cominco, Noranda, Hudson Bay Mining and Smelting, and Teck co-ordinated by CAMIRO.
REFERENCES .
2.
.
4.
5.
Rushton, A., Ward, A.S. & Holdich, R.G., Solid-liquid filtration and separation technology. VCH Weinheim, 1996, pp. 1-31 and 221-226. Williams, R.A., Xie, C.G., Bragg, R. & Amarasinghe, W.P.K., Experimental techniques for monitoring sedimentation in optically opaque suspensions. Colloids and Surfaces, 1990, 43(1), 1-32. Uribe-Snlas, A., Vermet, F. & Finch, J.A., Apparatus and technique to measure settling velocity and holdup of solids in water slurries. Chemical Engineering Science, 1992, 48(4), 815-819. Uribe-Sidas, A., Gomez, C.O. & Finch, J.A., A conductivity technique for gas and solids holdup determination in three-phase reactors. Chemical Engineering Science, 1994, 49 (1), 1-10. Shen, G. & Finch, J. A., Bubble swarm velocity in a column. Chemical Engineering Science, 1994, 51(14), 3665-3674.