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Operational characteristics of superconducting magnetic separators
PERCONDUCTINGMAGNETIC SEPARATION
Operational characteristics of superconducting magnetic separators High-gradient magnetic separation using superconducting magnetic separators is employed in the processing of kaolin clay. Here Jerry A Selvaggi and David Cottrell of Eriez Magnetics report on tests carried out using this technology to determine the optimum operational parameters for this application.
(B}
(A)
85
N
I
s4
~
~ .-
I
3
study of kaolin clay operating parameters was p u r s u e d using high-gradi('nt magnetic separation (HGMS). A Georgia clay, prepared by the supplier, was processed and returned to the supplier for (waluation. Parameters t h a t were investigated included canister length, magnetic field strength and flow velocity. Testing revolved flowing equal canister volumes of (-lay t h r o u g h canisters built at 5, 10, 15 and 20 inch lengths at a specified flow rate and variable field strength. The 'brightness' of the kaolin clay was used to determine the product quality. The canisters contained magnetic stainless-steel wool. The testing objective was to investigate lhe p a r a m e t e r s under which HGMS produees the best results with kaolin clay. ]'hese tests would ultimately give informa~ton to aid in the u n d e r s t a n d i n g and d e v e l o p m e n t of HGMSs in t h e kaolin industry. This article e x p l a i n s the test brocedure, and discusses results that were ,)btained from testing.
~
i
c
A
"--
20000
25000
30000 35090 40000 Field Strength (gauss)
45000
--
82
50000 20000
25000
30000 35000 40000 Field Strength (gauss)
(Cl
45000
50000
(0) 86
__t
85f
L 84
~ 83 m
82
--
---
8t
20000
25000
3000~ 35000 40000 Field Strer~gth (gauss)
45000
50000
80
20000
25000
30000 35000 40000 Field Strength (gauss)
45000
50000
Figure 1. Magnetic field strength ccymparisons fv~r (a) 5, (b) 10, (c) 15 a n d (el) 20 inch canLster lengtlus, f o r feed velocqties of O.5 cvn/s ( A ) , 1.75 em/s ( 0 ) and 3 c'm/s ( B ) .
Testing "lests were conducted using the Eriez 5 tesla I~aboratory Superconducting HGMS. Special ('anisters were c o n s t r u c t e d to c o m p a r e ( ' a n i s t e r l e n g t h s . C a n i s t e r s w e r e con~tructed of 2 inch internal-diameter PVC pipe. Some had sealed ends, to allow clay to he p m n p e d up t h r o u g h the canister at a ~',mtrolled rate. Other canisters were used ~ith the slurry in a free-fall mode. Canister i(,ngths were tested from 5 to 20 inches at :~ inch intervals. Magnetic field s t r e n g t h s were varied horn 2 to 5 tesla. A 75 ~m d i a m e t e r magnetic stainless-steel wool packed at a I~:msity of 5% filling was used as a m a t r i x to 4,'nerate the n e c e s s a r y gradient. Some ;misters were fed by p u m p i n g clay at a ,,mtrol]ed rate, using a peristaltic p u m p , I h r o u g h the canisters. O t h e r c a n i s t e r s ~'xperienced free-fall flow. Controlled feed rates were tested at 0.50, 1.75 and 3.0 cm/s. t'anister volumes were used to determine he a m o u n t of clay processed. One canister ~olume is defined as the volume of clay that w(mld fill a process canister. Larger canist (,rs would have a greater volume of clay and
Filtration & Separation
March 1995
more matrix, but the n u m b e r of m a t r i x wires per unit volume w i l l n o t change; t h u s the magnetic loading will remain the same. A fully dispersed clay was obtained from the clay producing supplier. Only slight agitation was required p r i o r to testing. Feed controlled testing required t h a t the fully mixed sample be p u m p e d t h r o u g h each canister length at a specified feed velocity. A volume of clay was used t h a t correlated to 3.7 volumes for each canister tested. After the non-magnetic fraction was collected, the magnetic field was t u r n e d off, and the retained magnetic fraction was flushed out of the matrix. Once tested, the s a m p l e s were p r e p a r e d for 'brightness' analysis. This required fully mixing each sample, and removing solids to dry in a microwave oven; each fraction was dried to completion. Brightness analysis was provided by the clay supplier using a GE brightness meter.
Results The initial feed brightness was 80 GEB. A 2 point brightness increase was the m i n i m u m
increase, with all other tests being significantly higher. The first set of g r a p h s (Figure 1) represents field strength comp a r i s o n s for each specific canister length. From the graphs it is seen t h a t as the field is increased, the brightness increases linearly. It is also seen that as the feed velocity is increased, a much larger field 0strength is required to maintain an equivalent brightness. I n s p e c t i o n of velocity c o m p a r i s o n g r a p h s (Figure 2) suppoJ%s the s t a t e m e n t t h a t as velocity is increased, brightness declines as a parabolic function. This shows t h a t to achieve m a x i m u m brightness in a clay, the velocity t h r o u g h the canister is slow. However, to slightly improve the b r i g h t n e s s , clay feed wdocities can be i n c r e a s e d d r a m a t i c a l l y w i t h o u t a large sacrifice to brightness. The g r a p h s in Figure 3 represent canis~ ter length c o m p a r i s o n s as a result of feed velocities. It can be seen from the g r a p h s t h a t as the matrix lengths are increased, the brightness increased. The increase was v e ~ linear from 5 inches to 15 inches, but the increase from 15 inches to 2() inches was
0015-1882/95/US$7.00 :~,, 1995 Elsevier Scierqce Lid
219
bERCONDUCTINGMAGNETIC SEPARATION (A)
(A)
{e)
87
87
86
88 :
85
85 ~ ' - . . . ~
I
i
82 - -
-
I
_
80 (3
0.5
1 1.5 2 Velocity (cm/sec)
25
3
.
i 0
0.5
1 15 2 Velocity (cm/sec)
25
3 I
0
5
10
15
20
Length (in) (c}
(S)
(D) 87 86 85 r
~ 85-~
~ 84-
~6a 82 81
0.5
1 1.5 2 Velocity (cm/sec)
25
3
80 0
0.5
1 15 2 Velocity (cmlsec)
25
0
Figure 2. Velocity camparisons for (a) 5, (b) 10, (c) 15 and (d) 20 inch canister l~ngths, for magnetic field strengths of 20 kgauss ( I ) , 35 kgauss ( 0 ) and 50 kgauss ( A ) .
minimal. This m a y s u g g e s t o n e of two possibilities: [] The p r o b a b i l i t y of m a g n e t i c p a r t i c l e c a p t u r e is g r e a t e s t at 15 inches, a n d a 20 inch c a n i s t e r is n o t necessary.
15
20
(c) 87 B6
Conclusions
85
[ ] This p a r t i c u l a r c l a y w a s n o t dirty e n o u g h to justify t h e additional cleaning volume.
220
lo Length (in)
t h e m a t r i x , or gravity flow. Here r e t e n t i o n t i m e control is lost. C o m p a r i s o n with Figure 3 clearly s h o w s a r e d u c t i o n in brightness.
Results of t e s t i n g s h o w t h a t for similar c a n i s t e r v o l u m e s of clay, a longer c a n i s t e r p e r f o r m s better. A long m a t r i x (15 or 20 Either of t h e s e two h y p o t h e s e s s u g g e s t s t h a t inch) p e r f o r m s better t h a n a s h o r t m a t r i x t h e c a n i s t e r length is i m p o r t a n t to m a x (5 inch). However, m a g n e t i c field s t r e n g t h imise t h e clay brightness. Figure 4 s h o w s alone is n o t t h e only criterion. The m a t r i x r e s u l t s u n d e r free-flow conditions t h r o u g h l e n g t h a n d velocity, w h i c h dictate r e t e n t i o n time, also have m a j o r effects on t h e brightness. The lower t h e flow velocity, t h e g r e a t e r t h e probability of p a r t i c l e c a p t u r e by t h e m a t r i x , r e s u l t i n g in i m p r o v e d s e p a r a t i o n . C o n c l u s i o n s t h a t c a n be d r a w n from t e s t i n g lead u s to believe t h a t t h e ,...._._.. ~ ~ =::==~ c a n i s t e r s h a p e does play a n imporJ t a n t role in separation. Results from J I gravity flow t e s t s s h o w e d significant j J brightening, b u t n o t to t h e e x t e n t of t h e controlled velocity tests. It s h o u l d be r e m e m b e r e d t h a t d a t a o b t a i n e d for t h e clay are n o t necessarily t r a n s f e r a b l e to all kaolin clays. All clays b e h a v e differently, and have different types and a m o u n t s of m a g n e t i c c o n t a m i n a t i o n . T h e s e r e s u l t s are, however, a valuable tool to aid in t h e under20000 25000 30000 35000 40000 45(~00 50000 s t a n d i n g of m a g n e t i c s e p a r a t i o n Field Strength (gaus ,~) u s i n g h i g h - g r a d i e n t m a g n e t i c sep a r a t o r s . T h i s work, a l o n g w i t h Figure 4. Comparison of canister lengths plotted f u r t h e r r e s e a r c h in t h e theoretical against magnetic field strength, for free-flow velocity: a n d e x p e r i m e n t a l areas, s h o u l d help 5inch ( l ) , lOinch ( 0 ) , 15 inch ( A ) and2Oinch with t h e design of higher-capacity ( . ) canister lengths. HGMSs. []
5
84
~, 83 82 81 80 0
5
10 Length (in)
15
20
Figure 3. Canister length comparisons as a result of feed velocities of (a) 3.0 cm/s, (b) 1.75 cm/s and (e) 0.5 cm/s, at magnetic field strengths of 20 kgauss ( l ) , 35 kgauss ( 0 ) and 50 kgauss ( A ) .
i
March
1995
Filtration
& Separation
Operational characteristics of superconducting magnetic separators High-gradient magnetic separation using superconducting magnetic separators is employed in the processing of kaolin clay. Here Jerry A Selvaggi and David Cottrell of Eriez Magnetics report on tests carried out using this technology to determine the optimum operational parameters for this application.
(B}
(A)
85
N
I
s4
~
~ .-
I
3
study of kaolin clay operating parameters was p u r s u e d using high-gradi('nt magnetic separation (HGMS). A Georgia clay, prepared by the supplier, was processed and returned to the supplier for (waluation. Parameters t h a t were investigated included canister length, magnetic field strength and flow velocity. Testing revolved flowing equal canister volumes of (-lay t h r o u g h canisters built at 5, 10, 15 and 20 inch lengths at a specified flow rate and variable field strength. The 'brightness' of the kaolin clay was used to determine the product quality. The canisters contained magnetic stainless-steel wool. The testing objective was to investigate lhe p a r a m e t e r s under which HGMS produees the best results with kaolin clay. ]'hese tests would ultimately give informa~ton to aid in the u n d e r s t a n d i n g and d e v e l o p m e n t of HGMSs in t h e kaolin industry. This article e x p l a i n s the test brocedure, and discusses results that were ,)btained from testing.
~
i
c
A
"--
20000
25000
30000 35090 40000 Field Strength (gauss)
45000
--
82
50000 20000
25000
30000 35000 40000 Field Strength (gauss)
(Cl
45000
50000
(0) 86
__t
85f
L 84
~ 83 m
82
--
---
8t
20000
25000
3000~ 35000 40000 Field Strer~gth (gauss)
45000
50000
80
20000
25000
30000 35000 40000 Field Strength (gauss)
45000
50000
Figure 1. Magnetic field strength ccymparisons fv~r (a) 5, (b) 10, (c) 15 a n d (el) 20 inch canLster lengtlus, f o r feed velocqties of O.5 cvn/s ( A ) , 1.75 em/s ( 0 ) and 3 c'm/s ( B ) .
Testing "lests were conducted using the Eriez 5 tesla I~aboratory Superconducting HGMS. Special ('anisters were c o n s t r u c t e d to c o m p a r e ( ' a n i s t e r l e n g t h s . C a n i s t e r s w e r e con~tructed of 2 inch internal-diameter PVC pipe. Some had sealed ends, to allow clay to he p m n p e d up t h r o u g h the canister at a ~',mtrolled rate. Other canisters were used ~ith the slurry in a free-fall mode. Canister i(,ngths were tested from 5 to 20 inches at :~ inch intervals. Magnetic field s t r e n g t h s were varied horn 2 to 5 tesla. A 75 ~m d i a m e t e r magnetic stainless-steel wool packed at a I~:msity of 5% filling was used as a m a t r i x to 4,'nerate the n e c e s s a r y gradient. Some ;misters were fed by p u m p i n g clay at a ,,mtrol]ed rate, using a peristaltic p u m p , I h r o u g h the canisters. O t h e r c a n i s t e r s ~'xperienced free-fall flow. Controlled feed rates were tested at 0.50, 1.75 and 3.0 cm/s. t'anister volumes were used to determine he a m o u n t of clay processed. One canister ~olume is defined as the volume of clay that w(mld fill a process canister. Larger canist (,rs would have a greater volume of clay and
Filtration & Separation
March 1995
more matrix, but the n u m b e r of m a t r i x wires per unit volume w i l l n o t change; t h u s the magnetic loading will remain the same. A fully dispersed clay was obtained from the clay producing supplier. Only slight agitation was required p r i o r to testing. Feed controlled testing required t h a t the fully mixed sample be p u m p e d t h r o u g h each canister length at a specified feed velocity. A volume of clay was used t h a t correlated to 3.7 volumes for each canister tested. After the non-magnetic fraction was collected, the magnetic field was t u r n e d off, and the retained magnetic fraction was flushed out of the matrix. Once tested, the s a m p l e s were p r e p a r e d for 'brightness' analysis. This required fully mixing each sample, and removing solids to dry in a microwave oven; each fraction was dried to completion. Brightness analysis was provided by the clay supplier using a GE brightness meter.
Results The initial feed brightness was 80 GEB. A 2 point brightness increase was the m i n i m u m
increase, with all other tests being significantly higher. The first set of g r a p h s (Figure 1) represents field strength comp a r i s o n s for each specific canister length. From the graphs it is seen t h a t as the field is increased, the brightness increases linearly. It is also seen that as the feed velocity is increased, a much larger field 0strength is required to maintain an equivalent brightness. I n s p e c t i o n of velocity c o m p a r i s o n g r a p h s (Figure 2) suppoJ%s the s t a t e m e n t t h a t as velocity is increased, brightness declines as a parabolic function. This shows t h a t to achieve m a x i m u m brightness in a clay, the velocity t h r o u g h the canister is slow. However, to slightly improve the b r i g h t n e s s , clay feed wdocities can be i n c r e a s e d d r a m a t i c a l l y w i t h o u t a large sacrifice to brightness. The g r a p h s in Figure 3 represent canis~ ter length c o m p a r i s o n s as a result of feed velocities. It can be seen from the g r a p h s t h a t as the matrix lengths are increased, the brightness increased. The increase was v e ~ linear from 5 inches to 15 inches, but the increase from 15 inches to 2() inches was
0015-1882/95/US$7.00 :~,, 1995 Elsevier Scierqce Lid
219
bERCONDUCTINGMAGNETIC SEPARATION (A)
(A)
{e)
87
87
86
88 :
85
85 ~ ' - . . . ~
I
i
82 - -
-
I
_
80 (3
0.5
1 1.5 2 Velocity (cm/sec)
25
3
.
i 0
0.5
1 15 2 Velocity (cm/sec)
25
3 I
0
5
10
15
20
Length (in) (c}
(S)
(D) 87 86 85 r
~ 85-~
~ 84-
~6a 82 81
0.5
1 1.5 2 Velocity (cm/sec)
25
3
80 0
0.5
1 15 2 Velocity (cmlsec)
25
0
Figure 2. Velocity camparisons for (a) 5, (b) 10, (c) 15 and (d) 20 inch canister l~ngths, for magnetic field strengths of 20 kgauss ( I ) , 35 kgauss ( 0 ) and 50 kgauss ( A ) .
minimal. This m a y s u g g e s t o n e of two possibilities: [] The p r o b a b i l i t y of m a g n e t i c p a r t i c l e c a p t u r e is g r e a t e s t at 15 inches, a n d a 20 inch c a n i s t e r is n o t necessary.
15
20
(c) 87 B6
Conclusions
85
[ ] This p a r t i c u l a r c l a y w a s n o t dirty e n o u g h to justify t h e additional cleaning volume.
220
lo Length (in)
t h e m a t r i x , or gravity flow. Here r e t e n t i o n t i m e control is lost. C o m p a r i s o n with Figure 3 clearly s h o w s a r e d u c t i o n in brightness.
Results of t e s t i n g s h o w t h a t for similar c a n i s t e r v o l u m e s of clay, a longer c a n i s t e r p e r f o r m s better. A long m a t r i x (15 or 20 Either of t h e s e two h y p o t h e s e s s u g g e s t s t h a t inch) p e r f o r m s better t h a n a s h o r t m a t r i x t h e c a n i s t e r length is i m p o r t a n t to m a x (5 inch). However, m a g n e t i c field s t r e n g t h imise t h e clay brightness. Figure 4 s h o w s alone is n o t t h e only criterion. The m a t r i x r e s u l t s u n d e r free-flow conditions t h r o u g h l e n g t h a n d velocity, w h i c h dictate r e t e n t i o n time, also have m a j o r effects on t h e brightness. The lower t h e flow velocity, t h e g r e a t e r t h e probability of p a r t i c l e c a p t u r e by t h e m a t r i x , r e s u l t i n g in i m p r o v e d s e p a r a t i o n . C o n c l u s i o n s t h a t c a n be d r a w n from t e s t i n g lead u s to believe t h a t t h e ,...._._.. ~ ~ =::==~ c a n i s t e r s h a p e does play a n imporJ t a n t role in separation. Results from J I gravity flow t e s t s s h o w e d significant j J brightening, b u t n o t to t h e e x t e n t of t h e controlled velocity tests. It s h o u l d be r e m e m b e r e d t h a t d a t a o b t a i n e d for t h e clay are n o t necessarily t r a n s f e r a b l e to all kaolin clays. All clays b e h a v e differently, and have different types and a m o u n t s of m a g n e t i c c o n t a m i n a t i o n . T h e s e r e s u l t s are, however, a valuable tool to aid in t h e under20000 25000 30000 35000 40000 45(~00 50000 s t a n d i n g of m a g n e t i c s e p a r a t i o n Field Strength (gaus ,~) u s i n g h i g h - g r a d i e n t m a g n e t i c sep a r a t o r s . T h i s work, a l o n g w i t h Figure 4. Comparison of canister lengths plotted f u r t h e r r e s e a r c h in t h e theoretical against magnetic field strength, for free-flow velocity: a n d e x p e r i m e n t a l areas, s h o u l d help 5inch ( l ) , lOinch ( 0 ) , 15 inch ( A ) and2Oinch with t h e design of higher-capacity ( . ) canister lengths. HGMSs. []
5
84
~, 83 82 81 80 0
5
10 Length (in)
15
20
Figure 3. Canister length comparisons as a result of feed velocities of (a) 3.0 cm/s, (b) 1.75 cm/s and (e) 0.5 cm/s, at magnetic field strengths of 20 kgauss ( l ) , 35 kgauss ( 0 ) and 50 kgauss ( A ) .
i
March
1995
Filtration
& Separation