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Performance characteristics within a modified hydrocyclone
Minerals Engineering, Vol. 6, No. 7, pp. 743-751, 1993
0892-6875/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
PERFORMANCE CHARACTERISTICS WITHIN A MODIFIED HYDROCYCLONE
M.S. LEE and R.A. WILLIAMS Department Of Chemical Engineering University Of Manchester Institute Of Science and Technology PO Box 88, Manchester, M60 1QD, UK. (Received 21 December 1992; accepted 23 February 1993)
ABSTRACT It has been reported by Xu et al (1990) that the suppression of the axial air core within a hydrocyclone could improve process performance. This work presents the results of an extensive series of experiments to test this postulation by measuring the classification and separation efficiency of conventional hydrocyclones compared with modified hydrocyclones, in which the air core is replaced by an axial steel rod insert. The influence of rods of various dimensions on a number of design parameters (e.g. pressure drop, volumetric flowrate and cut size) is reported. The insertion of a 3 mm diameter 377 mm long rod into the hydrocyclone, 8 mm vortex finder diameter and 5 mm spigot diameter, to remove the central air core resulted in a maximum increase of 44 ~ in the volumetric throughput for the same pressure drop. But in all cases the sharpness of classification, fineness of cut-size and overall separation efficiency was inferior. Since the insertion of a rod into the hydrocyclone does not appear to improve the separation efficiency other approaches, e.g. hydrocyclone networks, must be employed.
Keywords Hydrocyclones; classification efficiency
INTRODUCTION Hydrocyclones are widely employed in the minerals industry in single and multiple banks [1-3]. Their applications are numerous and include classification, clarification and thickening, but the design of operations involving hydrocyclone separators and associated processing plant is still at a fairly rudimentary level, since process models are generally based on system-specific empirical modelling methods [3]. Recently significant advances in the application of more generic fundamental models have been proposed, based on adapting conventional fluid flow approaches to incorporate the viscosity-modifying effects of concentrated particulate suspensions [4]. This contribution addresses the possibility of modifying the performance of small diameter hydrocyclones used for the particle separation by radical changes to the way in which the unit is operated. The conventional empirical models and two-dimensional fluid flow simulations recognise that hydrocyclones exhibit certain inherent inefficiencies, and limitations exist in terms of sharpness of cut and the range of operating cut size. Attempts have been made to improve separation performance, an example being the variation of hydrocyclone geometries until the desired separation is obtained. Such practices are utilised by hydrocyclone manufacturers. 743
744
M . S . LEE and R. A. WILLIAMS
To understand the fundamental separation principles demands some appreciation of the microhydrodynamic behaviour within the hydroeyclone. It is expected that the separation efficiency may be improved by increasing the magnitude of the tangential velocities within the body. This would increase the centrifugal force acting on the solid particles within the hydrocyclone, thus permitting the separation of finer solid particles. Low radial velocities would also be advantageous for recovery of free particles because the drag forces which act towards the centre are smaller. Xu et al. [5] proposed that by inserting a solid rod within the hydrocyclone, to replace the air core, an increase in tangential and a reduction in radial velocities would be apparent. This being so, it is therefore expected that an increase in separation efficiency is obtained. This paper examines the effects on the separation efficiency, when a solid rod is inserted into the hydrocyclone body in an attempt to verify by Xu et al's postulate. Liquid only and solid/liquid systems are considered. Theeffect ofdifferent diameters forvortex finder and spigoton the separation efficiency, and the influence of the rod diameter and length will be reported.
EXPERIMENTAL ARRANGEMENT A semi-automated test-rig was constructed for the purpose of this work, Figure 1. For the results presented here, a 44 mm body diameter polyurethane hydrocyclone (Richard Mozley, Ltd) was employed and placed into a closed circuit. The hydrocyclone could be fitted with one of three interchangeable vortex finder and one of three spigot caps making a total of nine possible combinations. The corresponding diameters of the exit orifices are shown in Table 1.
Flowsheet for the test rig Size
Mass flowmeter Overflow T ~ ~ I I I I
Sizl
Size distribution from loser porticle onolyzer
Prenucu Ironldt~et[ Underflow~
l~/-passline F ~
ToCoo~i9n~ i l ~ ~ ~
' '
-]
F control Monopump
Fig. 1 Schematic diagram of the flow test rig and associated instrumentation.
TABLE 1 Range of diameters of vortex finder and spigot orifices used in 44mm diameter hydrocydone tests. iIi
Vortex finder outlet diameter (nun)
$
11
Spigot outlet diameter (ram)
5
6.5
14
Hydrocyclone performance characteristics
745
A positive displacement pump (Mono pumps) was employed to deliver the liquid to the hydrocyclone using a control valve on the by-pass line to adjust the flowrate, (see Figure 1). A 0-680 KPa pressure gauge and a piezoelectric pressure transmitter 0Atika 891/14/520)was incorporated onto the feed stream. Volumetric flowrate, mass flowrate, density, temperature and solids concentration were measured on-line using a mass flowmeter, based on the Coriolis principle (Foxboro CFT-10). Electrical signals from the pressure and flow measuring devices were sent to a measurement and control system (MT-1000, Measurement Technology Corp.) This interface unit converts the 4-20 mA signals from the measuring instruments, and transmits a 0-1 V output to a PC hosting a process management and control software (Paragon Control), for data logging purposes. The underflow and overflow streams were measured by collecting a known volume of slurry over a set time interval and then measuring the gravimetric density of a slurry sample directly. Typically using this method density and flowrates were measured to an accuracy of 1% and 2 % respectively. The particle size distribution of the feed, underflow and overflow, were measured using a scanning laser microscope (SLM) particle size analyzer (Lasentec Corp.). This measurement instrument displays the complete size distributions in the size range 1.8 - 1000 microns, of the three sampled streams, as described in detail elsewhere [6-8]. The rig could also modified to incorporate the SLM to take on-line measurements, but this would result in the measurement of a single stream only at any one time. These data together with mass flow measurements were reconciled using a Fortran code HYDRAS and used to compute the classification function curves [3,8]. Steel rods, of various diameters and lengths (Table 2), were inserted along the central axis of the hydrocyclone, to replace the air core. Thus a possible total of 54 rod-spigot-vortex finder combinations could be tested. TABLE 2 Dimensions of the steel rods used as insert in the 44 mm diameter hydrocyclone. ROd diameter (ram) ROd length (ram)
3
3
4
5
8
311
377
311
311
320
The rod was positioned along the central axis aided by vortex finder and body supports. The vortex finder support consisted of two discs, each 32 mm diameter and 8 mm high. A 15 nun diameter hole was bored into the centre to allow upward flowing liquid to leave through the overflow. Three smaller 3 mm holes surround the central hole and are threaded so that the two discs can be screwed together. Situated at the top of the rod are three projections, or arms, see Figure 2, centred on the rod and set 120 degrees apart. The two discs grip the arms and are then lowered into the vortex finder unit of the hydrocyclone. The body supports, also seen in Figure 2, were used hold the rod centrally and reduce excessive vibration of the rod at lower parts of the hydrocyclone.
EXPERIMENTAL PROGRAMME Experiments were performed, both with and without the insertion of a solid rod into the hydrocyclone. When using a water-only system the influence of the rod on the volumetric flowrate and the flow-ratio (the ratio of the mass flow in the underflow to that in the feed) was investigated for varying values of pressure drop across the hydrocyclone. A total of 47 different hydrocyelone-rod configurations consider the behaviour of the unit treating a wateronly feed. Of these a further 14 were repeated but using a calcium silicate slurry for the feed. A slurry 1% by weight solids concentration was used, resulting in a mixture density of 1004 Kg/m3. This mixture was continuously agitated and recirculated, for at least an hour, through the system prior to testing to
746
M . S . LEE and R. A. WILLIAMS
ensure a homogenous feed stock had been prepared before any sampling was performed. The (volume) average mean size of the calcium silicate was measured by SLM to be 21 microns. The density of the solid was 2910 Kg/m 3.
V°~;:r:inder 311 rnrn rod within the hydrocyclone.
F',- ;J
I
377 mm (extended) rod within the hydrocyclone.
b)
Body support
_J Fig.2 (Left) Rod inserts and associated support structures. (Right) ROd position within the hydrocyclone.
RESULTS Pressure drop - volumetric throughput performance Figure 3a shows that with the smallest vortex finder diameter (8 ram) and for a fixed pressure drop, the volumetric flowrate increased with the insertion of a solid rod (especially the 3 nun diameter rod). This effect was still observed when the spigot diameter was increased from 5 to 8 mm (Figure 3b), but the larger diameter rods had a reduced effect. However, this does not appear to be the case with the longer 3 mm diameter rod, (which extends through the spigot orifice outside the hydrocyclone) which maintained a higher volumetric throughput value. The ability of the use of a rigid cylindrical rod to increase the flow, for the same value of pressure drop, was also reduced when the vortex finder diameter was increased. Effect on How-ratio The insertion of a solid rod into the hydrocyclone appeared to have little effect on the flow ratio as illustrated by Figures 4a and 4b. In figure 4a no significant trends are evident which relate the value of the flow ratio to the rod dimensions. It might be expected that the flow ratio would be dependant on the size of the spigot opening and hence the insertion of a rod would change the outlet area. This is summarised in Table 3. In the case of the 5 mm spigot, use of a 4 nun diameter
Hydrocyclone
performance
747
characteristics
Volumetric flowrate (litres/s)
0.8
o.7i
0.6
0.5
0 . 4 I ........... 0.3
..
E
0.2 ~.........................................
t t Ii 5 r a m rod
4~rod n o rod
0.1 . . . . . . . . . . . . . . . . . . . .
~
3 r a m rod
---X-- E x t e n d e d 3 r a m r o d
0
0
I
I
J
L
I
2
3
4
5
P r e s s u r e drop (Bar)
Fig.3a Pressure drop - volumetric (water) throughput relationship for conventional hydrocyclone operation and in the presence of rod inserts of various diameters. 8 mm vortex finder and 5 mm spigot.
O.a
Volumetric flowrate (litres/s)
i I
0.7
..................................................................................
o4/¸¸¸ .. 013 1. . . . . . . . . . . . 0.2
•. . . . . .
•
] ~ --~
i
5 m m rod 4 r a m rod 3 r a m rod
O. 1
. . . . . . . . . .
r
i
0.5
I Pressure
--)~ n o rod ")<-- E x t e n d e d 3 r a m rod 1.5
2
I
I
I
2.5
drop ( B a r )
Fig.3b Pressure drop - volumetric (water) throughput relationship for conventional hydra, cyclone operation and in the presence of rod inserts of various diameters. 14 mm vortex finder and 8 mm spigot.
748
M.S. LEE and R. A. WILLIAMS
rod produced a small reduction in proportion of flow reporting to underflow for pressures below 200 KPa. However in general a slight enhancement in flow ratio was observed. For the 8 nun spigot (with 14 mm vortex finder), Figure 4b shows that the flow ratio increases with pressure drop in all cases when the rod is present, whereas, under conventional operation the flow ratio decreases slightly with operating pressure. Hence it may be concluded that the presence of the rod is having a profound effect on flow characteristics of the process. Flow ratio
0.8 5ram rod 4ram rod
0.7 -~-
3turn rod
--~ no rod 0.6
Extended 3ram rod
0.5
0.4
\
0.3
-
0.2
-
0.1
0 0
0.5
1
1.5 drop
3
2.5
2
Pressure
3.5
(bar)
Fig.4a Pressure drop - flow ratio relationship for conventional hydrocyclone operation and in the presence of rod inserts of various diameters. 8 mm vortex finder and 5 mm spigot. Flow ratio
O.B
J
/
0.7 . . . . . .
0.6 ..........
~
t
5mm rod
I --r'-
4ram rod
J
Omm rod
"-~
No r o d
- X¢ -
Extended 3mm rod
0.5
0.3-
0.2
0.1
0 0
i
r
i
I
i
0.5
1
1.5
2
2.5
Pressure
drop
3
3.5
(Bar)
Fig.4b Pressure drop - flow ratio relationship for conventional hydrocyclone operation and in the presence of rod inserts of various diameters. 14 nun vortex finder and 8 nun spigot.
Hydrocyclone performance characteristics
749
TABLE 3 Overflow outlet areas (x10-4 m2) with and without the insertion of the solid rod.
VORTEX FINDER DIAMETER
no rod
3 mm
4 mm
5 mm
8 mm
14 mm
1.54
1.47
1.41
1.34
1.04
11 mm
0.95
0.88
0.83
0.75
0.45
8 nun
0.50
0.43
0.38
0.31
Effect of solid rod on particle classification performance Experiments using calcium silicate slurry showed that an increase in the particle recovery occurred when pressure was increased [8]. However, for the same outlet orifice dimensions insertion of a solid rod into the body of the hydrocyclone depressed the recovery to the underflow, as shown in Figures 5a and 5b. Table 4 shows the corresponding ds0 cut size, gross efficiency of solids recovery and corrected efficiency (after subtraction of underflow by-pass effects). This suggests a lower separation efficiency and clearly shows that, unfortunately, the anticipated enhancement of performance [5,9] does not occur in practice.
100
% wt to underflow
80
60
ing pressure . T . ' n l . ea.s.lngl3artlole recovery
.......
40
20
0
I
0
I
20
•
150 KPa
I
I
40 60 80 Particle size (micrens) I
200 KPa
~
250 KPa
I
100
120
D 300 KPa
Fig.5a Measured classification curves as a function of pressure drop for conventional hydrocyclone operation in the presence of an air vortex It may also be seen that the recovery of smaller particle sizes ( < 20 microns) was enhanced in the presence of the rod. This might have some benefit if the hydrocyclone was to be used for clarification duties (i.e. to produce a clean overflow stream) but is not beneficial for sharp size classification. In all the experimental results a distinct "fish-hook" was present in the classification curves [3,10]. Since the results have been rigorously reconciled this phenomena is believed to be a real and important effect, for which some detailed explanations are been proposed elsewhere [1 I].
750
M.S. LEE and R. A. WILLIAMS % wt to underflow 100
80
60
40
20
I
0 0
20
•
106 K P a
I
__
40 60 80 Particle size (microns) I
135 K P a
•
163 K P a
100
[]
120
199 K P a
Fig.Sb Measured classification curves as a function of pressure drop for conventional hydrocyclone operation in the absence of an air vortex
TABLE 4 Values of efficiency and cut size for different pressures operating the hydrocyclone in conventional manner and with the air vortex replaced by a solid rod. TABLE 4
P (KPa)
ds0 (microns)
Gross efficiency
Corrected efficiency
(~)
(~)
Conventional
1.5 2.0
18.7 24.9
48.9 65.5
10.8 34.1
No air-vortex (with 3ram wide and 377mm long rod)
1.35 1.63 1.99
65.7 36.8 36.8
47.7 56.0 59.7
16.4 21.5 33.2
CONCLUSION Contrary to previous indications in the literature, this work has shown that the insertion of a solid rod into the hydrocyclone does not improve the particle separation. Although an increase in tangential velocity and a reduction in radial velocity should aid the separation, in theory, the rod appears to hinder it. This could be the result of a reduced overflow area, due to the presence of the rod in the hydrocyclone, causing a greater pressure gradient across the inlet and outlet thus increasing the short circuit flow and increasing the inefficiency. Significant fish-hooks in the classification curves were encountered on numerous occasions and this is an important deleterious effect on the sharpness of classification.
Hydrocycloneperformancecharacteristics
751
ACKNOWLEDGEMENTS This work was supported by the European Coal and Steel Community under contract 7220/EA/829.
REFERENCES lo
2. 3. 4.
.
6. 7. 8.
9. 10. 11.
ME 6/7--E
Flinthoff, B.C., Plitt, L.R. & Turak, A.A., Cyclone modelling: a review of present technology, CIM Bull., 69, 39-50 (1987). Cilliers, J.J. • Hinde, A.L., An improved hydrecyclonemodel for backfill preparation, Minerals Eng., 4/4, 683-693 (1991). Roldan-Villasana, E.J., Modelling and simulation of hydrocyclone networks for fine particle processing, Ph.D. Thesis, University of Manchester Institute of Science and Technology, (1992). Rajamani, R.K. & Milin, L., Fluid-flow model of the hydrocyclone for concentrated slurry classification, Hydrocyclones - Analysis and Applications, Fluid Mechanics And It's Applications Volume 2, 95-108, Kluwer Academic Publishers, Oxford (1992). Xu, J., Luo, Q. & Qui, J., Studying the flow field in a hydrocyclonewith no forced vortex. Part I, average velocity, Filtration & Separation, July-Aug, 27, 181-182 (1990). Williams, R.A., Peng, S.J. & Naylor, A., In-situ measurement of particle aggregation and breakage kinetics in a concentrated suspension, Powder Technology, 73, 75-83 (1992). Peng, S.J. & Williams. R.A., Control and optimisation of mineral flocculation and transport processes using on-line particle size analysis, Mineral Engineering, 6/2, 133-153 (1993). Lee, M.S., Performance characteristics within a modified hydrocyclone, MSc Thesis, University of Manchester Institute of Science and Technology, (1992). Dyakowski, T. & Williams, R.A., Modelling turbulent flow within a small diameter hydrocyclone, Chem. Eng. Sci., 47, (1992), in press. Finch, J.A., Modelling a fish-hook in hydrocyclone selectivity curves, Powder Technology, 36, 127-129 (1983). Roldan-Villasana, E.J. & Williams, R.A., The origin of the fish-hook effect in hydrocyclone separators, submitted for publication 1992.
0892-6875/93 $6.00+0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain
PERFORMANCE CHARACTERISTICS WITHIN A MODIFIED HYDROCYCLONE
M.S. LEE and R.A. WILLIAMS Department Of Chemical Engineering University Of Manchester Institute Of Science and Technology PO Box 88, Manchester, M60 1QD, UK. (Received 21 December 1992; accepted 23 February 1993)
ABSTRACT It has been reported by Xu et al (1990) that the suppression of the axial air core within a hydrocyclone could improve process performance. This work presents the results of an extensive series of experiments to test this postulation by measuring the classification and separation efficiency of conventional hydrocyclones compared with modified hydrocyclones, in which the air core is replaced by an axial steel rod insert. The influence of rods of various dimensions on a number of design parameters (e.g. pressure drop, volumetric flowrate and cut size) is reported. The insertion of a 3 mm diameter 377 mm long rod into the hydrocyclone, 8 mm vortex finder diameter and 5 mm spigot diameter, to remove the central air core resulted in a maximum increase of 44 ~ in the volumetric throughput for the same pressure drop. But in all cases the sharpness of classification, fineness of cut-size and overall separation efficiency was inferior. Since the insertion of a rod into the hydrocyclone does not appear to improve the separation efficiency other approaches, e.g. hydrocyclone networks, must be employed.
Keywords Hydrocyclones; classification efficiency
INTRODUCTION Hydrocyclones are widely employed in the minerals industry in single and multiple banks [1-3]. Their applications are numerous and include classification, clarification and thickening, but the design of operations involving hydrocyclone separators and associated processing plant is still at a fairly rudimentary level, since process models are generally based on system-specific empirical modelling methods [3]. Recently significant advances in the application of more generic fundamental models have been proposed, based on adapting conventional fluid flow approaches to incorporate the viscosity-modifying effects of concentrated particulate suspensions [4]. This contribution addresses the possibility of modifying the performance of small diameter hydrocyclones used for the particle separation by radical changes to the way in which the unit is operated. The conventional empirical models and two-dimensional fluid flow simulations recognise that hydrocyclones exhibit certain inherent inefficiencies, and limitations exist in terms of sharpness of cut and the range of operating cut size. Attempts have been made to improve separation performance, an example being the variation of hydrocyclone geometries until the desired separation is obtained. Such practices are utilised by hydrocyclone manufacturers. 743
744
M . S . LEE and R. A. WILLIAMS
To understand the fundamental separation principles demands some appreciation of the microhydrodynamic behaviour within the hydroeyclone. It is expected that the separation efficiency may be improved by increasing the magnitude of the tangential velocities within the body. This would increase the centrifugal force acting on the solid particles within the hydrocyclone, thus permitting the separation of finer solid particles. Low radial velocities would also be advantageous for recovery of free particles because the drag forces which act towards the centre are smaller. Xu et al. [5] proposed that by inserting a solid rod within the hydrocyclone, to replace the air core, an increase in tangential and a reduction in radial velocities would be apparent. This being so, it is therefore expected that an increase in separation efficiency is obtained. This paper examines the effects on the separation efficiency, when a solid rod is inserted into the hydrocyclone body in an attempt to verify by Xu et al's postulate. Liquid only and solid/liquid systems are considered. Theeffect ofdifferent diameters forvortex finder and spigoton the separation efficiency, and the influence of the rod diameter and length will be reported.
EXPERIMENTAL ARRANGEMENT A semi-automated test-rig was constructed for the purpose of this work, Figure 1. For the results presented here, a 44 mm body diameter polyurethane hydrocyclone (Richard Mozley, Ltd) was employed and placed into a closed circuit. The hydrocyclone could be fitted with one of three interchangeable vortex finder and one of three spigot caps making a total of nine possible combinations. The corresponding diameters of the exit orifices are shown in Table 1.
Flowsheet for the test rig Size
Mass flowmeter Overflow T ~ ~ I I I I
Sizl
Size distribution from loser porticle onolyzer
Prenucu Ironldt~et[ Underflow~
l~/-passline F ~
ToCoo~i9n~ i l ~ ~ ~
' '
-]
F control Monopump
Fig. 1 Schematic diagram of the flow test rig and associated instrumentation.
TABLE 1 Range of diameters of vortex finder and spigot orifices used in 44mm diameter hydrocydone tests. iIi
Vortex finder outlet diameter (nun)
$
11
Spigot outlet diameter (ram)
5
6.5
14
Hydrocyclone performance characteristics
745
A positive displacement pump (Mono pumps) was employed to deliver the liquid to the hydrocyclone using a control valve on the by-pass line to adjust the flowrate, (see Figure 1). A 0-680 KPa pressure gauge and a piezoelectric pressure transmitter 0Atika 891/14/520)was incorporated onto the feed stream. Volumetric flowrate, mass flowrate, density, temperature and solids concentration were measured on-line using a mass flowmeter, based on the Coriolis principle (Foxboro CFT-10). Electrical signals from the pressure and flow measuring devices were sent to a measurement and control system (MT-1000, Measurement Technology Corp.) This interface unit converts the 4-20 mA signals from the measuring instruments, and transmits a 0-1 V output to a PC hosting a process management and control software (Paragon Control), for data logging purposes. The underflow and overflow streams were measured by collecting a known volume of slurry over a set time interval and then measuring the gravimetric density of a slurry sample directly. Typically using this method density and flowrates were measured to an accuracy of 1% and 2 % respectively. The particle size distribution of the feed, underflow and overflow, were measured using a scanning laser microscope (SLM) particle size analyzer (Lasentec Corp.). This measurement instrument displays the complete size distributions in the size range 1.8 - 1000 microns, of the three sampled streams, as described in detail elsewhere [6-8]. The rig could also modified to incorporate the SLM to take on-line measurements, but this would result in the measurement of a single stream only at any one time. These data together with mass flow measurements were reconciled using a Fortran code HYDRAS and used to compute the classification function curves [3,8]. Steel rods, of various diameters and lengths (Table 2), were inserted along the central axis of the hydrocyclone, to replace the air core. Thus a possible total of 54 rod-spigot-vortex finder combinations could be tested. TABLE 2 Dimensions of the steel rods used as insert in the 44 mm diameter hydrocyclone. ROd diameter (ram) ROd length (ram)
3
3
4
5
8
311
377
311
311
320
The rod was positioned along the central axis aided by vortex finder and body supports. The vortex finder support consisted of two discs, each 32 mm diameter and 8 mm high. A 15 nun diameter hole was bored into the centre to allow upward flowing liquid to leave through the overflow. Three smaller 3 mm holes surround the central hole and are threaded so that the two discs can be screwed together. Situated at the top of the rod are three projections, or arms, see Figure 2, centred on the rod and set 120 degrees apart. The two discs grip the arms and are then lowered into the vortex finder unit of the hydrocyclone. The body supports, also seen in Figure 2, were used hold the rod centrally and reduce excessive vibration of the rod at lower parts of the hydrocyclone.
EXPERIMENTAL PROGRAMME Experiments were performed, both with and without the insertion of a solid rod into the hydrocyclone. When using a water-only system the influence of the rod on the volumetric flowrate and the flow-ratio (the ratio of the mass flow in the underflow to that in the feed) was investigated for varying values of pressure drop across the hydrocyclone. A total of 47 different hydrocyelone-rod configurations consider the behaviour of the unit treating a wateronly feed. Of these a further 14 were repeated but using a calcium silicate slurry for the feed. A slurry 1% by weight solids concentration was used, resulting in a mixture density of 1004 Kg/m3. This mixture was continuously agitated and recirculated, for at least an hour, through the system prior to testing to
746
M . S . LEE and R. A. WILLIAMS
ensure a homogenous feed stock had been prepared before any sampling was performed. The (volume) average mean size of the calcium silicate was measured by SLM to be 21 microns. The density of the solid was 2910 Kg/m 3.
V°~;:r:inder 311 rnrn rod within the hydrocyclone.
F',- ;J
I
377 mm (extended) rod within the hydrocyclone.
b)
Body support
_J Fig.2 (Left) Rod inserts and associated support structures. (Right) ROd position within the hydrocyclone.
RESULTS Pressure drop - volumetric throughput performance Figure 3a shows that with the smallest vortex finder diameter (8 ram) and for a fixed pressure drop, the volumetric flowrate increased with the insertion of a solid rod (especially the 3 nun diameter rod). This effect was still observed when the spigot diameter was increased from 5 to 8 mm (Figure 3b), but the larger diameter rods had a reduced effect. However, this does not appear to be the case with the longer 3 mm diameter rod, (which extends through the spigot orifice outside the hydrocyclone) which maintained a higher volumetric throughput value. The ability of the use of a rigid cylindrical rod to increase the flow, for the same value of pressure drop, was also reduced when the vortex finder diameter was increased. Effect on How-ratio The insertion of a solid rod into the hydrocyclone appeared to have little effect on the flow ratio as illustrated by Figures 4a and 4b. In figure 4a no significant trends are evident which relate the value of the flow ratio to the rod dimensions. It might be expected that the flow ratio would be dependant on the size of the spigot opening and hence the insertion of a rod would change the outlet area. This is summarised in Table 3. In the case of the 5 mm spigot, use of a 4 nun diameter
Hydrocyclone
performance
747
characteristics
Volumetric flowrate (litres/s)
0.8
o.7i
0.6
0.5
0 . 4 I ........... 0.3
..
E
0.2 ~.........................................
t t Ii 5 r a m rod
4~rod n o rod
0.1 . . . . . . . . . . . . . . . . . . . .
~
3 r a m rod
---X-- E x t e n d e d 3 r a m r o d
0
0
I
I
J
L
I
2
3
4
5
P r e s s u r e drop (Bar)
Fig.3a Pressure drop - volumetric (water) throughput relationship for conventional hydrocyclone operation and in the presence of rod inserts of various diameters. 8 mm vortex finder and 5 mm spigot.
O.a
Volumetric flowrate (litres/s)
i I
0.7
..................................................................................
o4/¸¸¸ .. 013 1. . . . . . . . . . . . 0.2
•. . . . . .
•
] ~ --~
i
5 m m rod 4 r a m rod 3 r a m rod
O. 1
. . . . . . . . . .
r
i
0.5
I Pressure
--)~ n o rod ")<-- E x t e n d e d 3 r a m rod 1.5
2
I
I
I
2.5
drop ( B a r )
Fig.3b Pressure drop - volumetric (water) throughput relationship for conventional hydra, cyclone operation and in the presence of rod inserts of various diameters. 14 mm vortex finder and 8 mm spigot.
748
M.S. LEE and R. A. WILLIAMS
rod produced a small reduction in proportion of flow reporting to underflow for pressures below 200 KPa. However in general a slight enhancement in flow ratio was observed. For the 8 nun spigot (with 14 mm vortex finder), Figure 4b shows that the flow ratio increases with pressure drop in all cases when the rod is present, whereas, under conventional operation the flow ratio decreases slightly with operating pressure. Hence it may be concluded that the presence of the rod is having a profound effect on flow characteristics of the process. Flow ratio
0.8 5ram rod 4ram rod
0.7 -~-
3turn rod
--~ no rod 0.6
Extended 3ram rod
0.5
0.4
\
0.3
-
0.2
-
0.1
0 0
0.5
1
1.5 drop
3
2.5
2
Pressure
3.5
(bar)
Fig.4a Pressure drop - flow ratio relationship for conventional hydrocyclone operation and in the presence of rod inserts of various diameters. 8 mm vortex finder and 5 mm spigot. Flow ratio
O.B
J
/
0.7 . . . . . .
0.6 ..........
~
t
5mm rod
I --r'-
4ram rod
J
Omm rod
"-~
No r o d
- X¢ -
Extended 3mm rod
0.5
0.3-
0.2
0.1
0 0
i
r
i
I
i
0.5
1
1.5
2
2.5
Pressure
drop
3
3.5
(Bar)
Fig.4b Pressure drop - flow ratio relationship for conventional hydrocyclone operation and in the presence of rod inserts of various diameters. 14 nun vortex finder and 8 nun spigot.
Hydrocyclone performance characteristics
749
TABLE 3 Overflow outlet areas (x10-4 m2) with and without the insertion of the solid rod.
VORTEX FINDER DIAMETER
no rod
3 mm
4 mm
5 mm
8 mm
14 mm
1.54
1.47
1.41
1.34
1.04
11 mm
0.95
0.88
0.83
0.75
0.45
8 nun
0.50
0.43
0.38
0.31
Effect of solid rod on particle classification performance Experiments using calcium silicate slurry showed that an increase in the particle recovery occurred when pressure was increased [8]. However, for the same outlet orifice dimensions insertion of a solid rod into the body of the hydrocyclone depressed the recovery to the underflow, as shown in Figures 5a and 5b. Table 4 shows the corresponding ds0 cut size, gross efficiency of solids recovery and corrected efficiency (after subtraction of underflow by-pass effects). This suggests a lower separation efficiency and clearly shows that, unfortunately, the anticipated enhancement of performance [5,9] does not occur in practice.
100
% wt to underflow
80
60
ing pressure . T . ' n l . ea.s.lngl3artlole recovery
.......
40
20
0
I
0
I
20
•
150 KPa
I
I
40 60 80 Particle size (micrens) I
200 KPa
~
250 KPa
I
100
120
D 300 KPa
Fig.5a Measured classification curves as a function of pressure drop for conventional hydrocyclone operation in the presence of an air vortex It may also be seen that the recovery of smaller particle sizes ( < 20 microns) was enhanced in the presence of the rod. This might have some benefit if the hydrocyclone was to be used for clarification duties (i.e. to produce a clean overflow stream) but is not beneficial for sharp size classification. In all the experimental results a distinct "fish-hook" was present in the classification curves [3,10]. Since the results have been rigorously reconciled this phenomena is believed to be a real and important effect, for which some detailed explanations are been proposed elsewhere [1 I].
750
M.S. LEE and R. A. WILLIAMS % wt to underflow 100
80
60
40
20
I
0 0
20
•
106 K P a
I
__
40 60 80 Particle size (microns) I
135 K P a
•
163 K P a
100
[]
120
199 K P a
Fig.Sb Measured classification curves as a function of pressure drop for conventional hydrocyclone operation in the absence of an air vortex
TABLE 4 Values of efficiency and cut size for different pressures operating the hydrocyclone in conventional manner and with the air vortex replaced by a solid rod. TABLE 4
P (KPa)
ds0 (microns)
Gross efficiency
Corrected efficiency
(~)
(~)
Conventional
1.5 2.0
18.7 24.9
48.9 65.5
10.8 34.1
No air-vortex (with 3ram wide and 377mm long rod)
1.35 1.63 1.99
65.7 36.8 36.8
47.7 56.0 59.7
16.4 21.5 33.2
CONCLUSION Contrary to previous indications in the literature, this work has shown that the insertion of a solid rod into the hydrocyclone does not improve the particle separation. Although an increase in tangential velocity and a reduction in radial velocity should aid the separation, in theory, the rod appears to hinder it. This could be the result of a reduced overflow area, due to the presence of the rod in the hydrocyclone, causing a greater pressure gradient across the inlet and outlet thus increasing the short circuit flow and increasing the inefficiency. Significant fish-hooks in the classification curves were encountered on numerous occasions and this is an important deleterious effect on the sharpness of classification.
Hydrocycloneperformancecharacteristics
751
ACKNOWLEDGEMENTS This work was supported by the European Coal and Steel Community under contract 7220/EA/829.
REFERENCES lo
2. 3. 4.
.
6. 7. 8.
9. 10. 11.
ME 6/7--E
Flinthoff, B.C., Plitt, L.R. & Turak, A.A., Cyclone modelling: a review of present technology, CIM Bull., 69, 39-50 (1987). Cilliers, J.J. • Hinde, A.L., An improved hydrecyclonemodel for backfill preparation, Minerals Eng., 4/4, 683-693 (1991). Roldan-Villasana, E.J., Modelling and simulation of hydrocyclone networks for fine particle processing, Ph.D. Thesis, University of Manchester Institute of Science and Technology, (1992). Rajamani, R.K. & Milin, L., Fluid-flow model of the hydrocyclone for concentrated slurry classification, Hydrocyclones - Analysis and Applications, Fluid Mechanics And It's Applications Volume 2, 95-108, Kluwer Academic Publishers, Oxford (1992). Xu, J., Luo, Q. & Qui, J., Studying the flow field in a hydrocyclonewith no forced vortex. Part I, average velocity, Filtration & Separation, July-Aug, 27, 181-182 (1990). Williams, R.A., Peng, S.J. & Naylor, A., In-situ measurement of particle aggregation and breakage kinetics in a concentrated suspension, Powder Technology, 73, 75-83 (1992). Peng, S.J. & Williams. R.A., Control and optimisation of mineral flocculation and transport processes using on-line particle size analysis, Mineral Engineering, 6/2, 133-153 (1993). Lee, M.S., Performance characteristics within a modified hydrocyclone, MSc Thesis, University of Manchester Institute of Science and Technology, (1992). Dyakowski, T. & Williams, R.A., Modelling turbulent flow within a small diameter hydrocyclone, Chem. Eng. Sci., 47, (1992), in press. Finch, J.A., Modelling a fish-hook in hydrocyclone selectivity curves, Powder Technology, 36, 127-129 (1983). Roldan-Villasana, E.J. & Williams, R.A., The origin of the fish-hook effect in hydrocyclone separators, submitted for publication 1992.