USE OF SUPERCONDUCTING MAGNETS IN MAGNETIC SEPARATION

JOURNAL DE PHYSIQUE

Colloque C1, supplCment a u n o 1, Tome 45, janvier 1984 page C1-753

USE OF SUPERCONDUCTING MAGNETS IN MAGNETIC SEPARATION

M.R. Parker

Department of Physic's, University of SaZford, U.K.

Resum6 - Un aperGu des p o s s i b i l i t e s de l ' u t i l i s a t i o n des systemes d'aimants sup-aconducteurs 1 champs f o r t s pour l a separation magnetique sera donne.

Une a t t e n t i o n p a r t i c u l i e r e e s t accordge aux avantages de l a methode de l a b o i t e ?i mouvement a l t e r n a t i f pour l a separation magnetique dans des champs i g r a d i e n t s f o r t s , e t i l ' u s a g e d'aimants quadrupolaires pour l a separation magnetique en g r a d i e n t s ouverts.

A b s t r a c t

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A review i s given assessing t h e p o t e n t i a l f o r superconducting h i g h - f i e l d magnet systems i n magnetic separation. P a r t i c u l a r a t t e n t i o n i s

given t o t h e advantages o f t h e r e c i p r o c a t i n g c a n i s t e r approach t o h i g h g r a d i e n t magnetic separation and t o t h e use o f quadrupole magnets i n open g r a d i e n t magnetic separation.

I

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INTRODUCTION

Since about 1970 an i n c r e a s i n g and widespread i n t e r e s t has developed i n t h e design and working p r i n c i p l e s and i n t h e commercial development o f h i g h f i e l d magnetic separation. This i s commonly r e f e r r e d t o /1/ as ' h i g h g r a d i e n t magnetic s e p a r a t i o n ' (HGMS). Much o f t h e f o l l o w i n g i s concerned w i t h t h e development p o t e n t i a l f o r super- conducting (S/C) magnets i n t h a t technology. Other novel S/C magnet systems used i n magnetic separation a r e a l s o reviewed b r i e f l y .

A c r u c i a l f a c t o r i n any magnetic separation device i s t h e magnetic t r a c t i o n f o r c e a c t i n g upon p a r t i c l e s i n t h e separating zone. For many o f these though n o t a l l /2/

t h i s force, FM, i s a magnetic d i p o l a r i n t e r a c t i o n o f t h e approximate form /3/

where VB i s t h e s p a t i a l g r a d i e n t o f an e x t e r n a l i n d u c t i o n f i e l d , Bo, a t t h e l o c a t i o n o f a p a r e i c l e o f volume V p and o f magnetic volume s u s c e p t i b i l i t y

xp

i n a medium o f corresponding value x ( x =

x - x,).

I n Eq. ( 1 ) x and V are e x c l u s i v e l y p r o p e r t i e s o f t h e p a r t i c l e and A u i d medyum. The q u a n t i t y f, ; a l e a s u r e o f t h e f o r c e d e n s i t y on t h e p a r t i c l e s /4,5/ can be optimised by m a x i m i s ~ n g Bp and vB0 ( t h e magnetic f i e l d g r a d i e n t ) independently over s p a t i a l dimensions a p p r o p r i a t e t o t h e p a r t i c l e s . I n HGMS, t h e g r a d i e n t i n Bo i s achieved by t h e i n s e r t i o n i n the separating zone of a m a t r i x o f ferromagnetic f i b r e s ( o r expanded metal screens). Above a t h r e s h o l d value o f Bo t h e m a j o r i t y o f these f i b r e s , wires, g r i d s , etc., are magnetised t o s a t u r a t i o n i n a d i r e c t i o n transverse t o t h e i r axes, whereupon Bo approaches a l i m i t i n g value o f order MS/a /2,5/ where MS i s t h e s a t u r a t i o n magnetisation o f t h e f i b r e s and a i s t h e i r ( e f f e c t i v e ) r a d i u s o f curvature. I n HGMS systems, fm is , t y p i c a l l y , o f order 1011 ~ m - 3 and increases l i n e a r l y w i t h i n c r e a s i n g values o f Bo. In HGMS, t h e l a r g e values o f fm extend o n l y over l i m i t e d volumes o f space adjacent t o f i b r e surfaces i n t h e separating zone and have a f o r c e range, t h e r e f o r e , o f order a ( s 0.1 mm)

.

Other h i g h - f i e l d separation devices are c h a r a c t e r i s e d by much s m a l l e r values of fm ( o f order 108 ~ m - ~ ) b u t by v e r y much l a r g e r (0.05 t o 0.1 m) f o r c e ranges. Such devices, which achieve separation by p a r t i c l e d e f l e c t i o n r a t h e r than by magnetic Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841153

JOURNAL DE PHYSIQUE

Av Fez03 extract~on eff 1%)

70 -

0 90 L 36

10 -

I

0 2 1 6 8 10 12 11

Canister volumes [no)

F i g . 1

-

Fe O3 r e c o v e r y a t v a r i o u s F i g . 2

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Dependence o f rc on Stokes values o f a

ti

.e.V / V ) . (Courtesy number ( K ) f o r v a r i o u s va7ues of Vm/Vo o f J.H.P. ats son 77/7 ( a f t e r R.P. Walker / 8 / ) .

entrapment a t a f i b r e s u r f a c e a r e n o r m a l l y r e f e r r e d t o c o l l e c t i v e l y as open g r a d i e n t magnetic s e p a r a t i o n (OGMS) systems /2/.

The b a s i c mechanisms o f magnetic p a r t i c l e entrapment and b u i l d u p on f i b r e s i n HGMS systems a r e now w e l l understood as i s t h e r e d u c t i o n i n t h e performance o f f i l t e r s w i t h i n c r e a s e d p a r t i c l e l o a d i n g /2,6/. I t i s , t h e r e f o r e , s u f f i c i e n t t o n o t e here t h a t , a p a r t from v a r i o u s f i l t e r constants,the r e c o v e r y o f magnetic p a r t i c l e s depends, i n s i m p l e terms upon a s i n g l e dimensionless v a r i a b l e rca. T h i s q u a n t i t y , a ( n o r m a l i s e d ) p a r t i c l e c a p t u r e c r o s s - s e c t i o n p e r u n i t l e n g t h o f f i b r e ( o r w i r e , g r i d , e t c . ) , can i t s e l f be expressed a p p r o x i m a t e l y as /2/

f o r paramagnetic p a r t i c l e s i n t h e l o n g i t u d i n a l /2/ c o n f i g u r a t i o n . Here, VoI i s t h e average f 1 ow v e l o c i t y o f t h e p a r t i c l e s , Vm = 2 ~ ~ ~ ~ ~ b ~ / 9 1 1 ~ ~ a i s t h e 'magnet, c v e l o c i t y ' / 6 / , b i s t h e p a r t i c l e r a d i u s and q i s t h e f l u i d v i s c o s i t y . I t f o l l o w s t h a t , t o a f i r s t approximation, rC and, t h e r e f o r e , f i l t e r performance depends o n l y on t h e r a t i o Vm/Vo. T h i s i s o f c o n s ~ l e r a b l e advantage i n h i g h - f i e l d HGMS systems where an i n c r e a s e i n magnetic f i e l d t o a h i g h e r v a l u e ( B ,) can be accompanied by a corresponding i n c r e a s e i n Vo (and, t h e r e f o r e , i n p r o a u c t i o n r a t e ) w i t h o u t l o s s o f performance. Some i n d i c a t i o n o f t h i s i s p r o v i d e d i n F i g . 1 where t h e r e c o v e r y o f i r o n o x i d e contamina- t i o n f r o m k a o l i n s l u r r i e s / 7 / i s shown f o r v a r i o u s values of Bo ( r a n g i n g from 1.17 t o 5.0 T) and f o r corresponding v a l u e s o f Vo a d j u s t e d t o m a i n t a i n a f i x e d v a l u e o f a

( i . e . o f Vm/Vo). A p a r t f r o m t h e low f i e l d d a t a (1.17 T) t h e f i l t e r performance i s seen t o be i n accordance w i t h t h e above-described t h e o r y . There are, o f course, o t h e r l i m i t s t o t h e r e l a t i o n s h i p s expressed i n Eq. ( 2 ) . F i g . 2 shows computed v a l u e s /8/ of rca as a f u n c t i o n o f l o g K where K i s t h e Stokes number. F o r values o f l o g K > -0.18, t h e s i m p l e r e l a t i o n s h i p f o r f i l t e r performance embodied i n E q . ( 2 ) s t a r t s t o break down. The Stokes number may be regarded as a measure o f t h e balance between i n e r t i a l and hydrodynamic f o r c e s and, a t h i g h e r K values, t h e former becomes i n c r e a s i n g l y

STEEL RETURN i m p o r t a n t . One consequence o f t h i s i s t h a t magnetic gas f i l t r a t i o n which r e q u i r e s low p a c k i n g d e n s i t y f i l t e r s (and, c o r r e s - p o n d i n g l y , h i g h v a l u e s o f r c a ) i s l i k e l y t o b e n e f i t l e s s f r o m t h e use o f v e r y h i g h f i e l d s .

I 1

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HIGH GRADIENT MAGNETIC SEPARATION (HGMS)

ED I n t h e l i g h t o f t h e above a n a l y s i s i t may

seem a l i t t l e odd t h a t , w i t h t h e i r h i g h - f i e l d advantage, S/C magnets have, s o f a r , made e s s e n t i a l l y no commercial impact F i g . 3

-

Conventional HGMS magnet system upon HGMS.

he&

a r e s e v e r a l reasons f o r ( a f t e r Gerber / 5 / ) . t h i s n o t t h e l e a s t o f which i s t h e

e f f e c t i v e n e s s o f t h e c o n v e n t i o n a l HGMS c y c l i c system shown s c h e m a t i c a l l y i n F i g . 3. L a r g e - s c a l e v e r s i o n s o f t h i s d e v i c e , w i t h values o f d > 2.0 m and w i t h f i e l d s o f up t o 2.0 T have been produced w i d e l y on a commercial b a s i s . The magnetic c i r c u i t and d e s i g n o f t h i s d e v i c e has been des- c r i b e d i n d e t a i l elsewhere / 5 / and o n l y a b r i e f mention i s g i v e n here. The main advantages o f t h i s d e v i c e i n c l u d e i t s r o b u s t c o n s t r u c t i o n (which i s o f c o n s i d e r a b l e importance i n t h e t y p i c a l l y h o s t i l e environments found i n m i n e r a l s p r o c e s s i n g ) , i t s r e l i a b i l i t y (founded on c o n s e r v a t i v e e n g i n e e r i n g p r i n c i p l e s ) and t h e comparative ease w i t h which t h e d e s i g n may be s c a l e d upwards i n t h e dimension d o f F i g . 3.

However, t h e r e a r e m a j o r disadvantages a s s o c i a t e d w i t h t h e s e d e v i c e s i n c l u d i n g com- p a r a t i v e l y h i g h c a p i t a l c o s t s and o p e r a t i n g power c o s t s and an unfavourably l o n g (- 200 s) 'dead' t i m e ( a s s o c i a t e d w i t h magnetic f i e l d removal, f o l l o w e d by washing f o l l o w e d by f i e l d r e s t o r a t i o n ) . They a r e a l s o l i a b l e t o l o n g - t e r m m a t r i x c o n t a m i n a t i o n by u l t r a - f i n e p a r t i c l e s .

The p r o d u c t i o n r a t e , P, o f a c y c l i c magnetic s e p a r a t o r may be expressed /6/ as a p r o d u c t o f mass f l u x p vo(*d2) and o f t h e d u t y c y c l e n o ~ / p o v + nrT + D) where no i s

t h e number o f e q u i v a l e n t c a n i s t e r s o f t h e f e e d stream processed b e f o r e t h e r e c o v e r y o f t h e magnetic component becomes unaccept- a b l y low. When t h e f l o w i s h a l t e d by a p p r o p r i a t e heavy d u t y v a l v e o p e r a t i o n t h e m a t r i x i s r i n s e d w i t h nr c a n i s t e r s of water ( a t speed Vo and a t f u l l f i e l d ) t o remove e s s e n t i a l l y a1 1 o f t h e non-magnetic f r a c t i o n f r o m t h e m a t r i x . T y p i c a l l y n r z 1.

Thus s i n c e Vo

z

&/I-, P can be expressed i n terms o f t h e s e p a r a t i n g z o n e g e o m e t r y as

P = pAano/[ (no + n,)~ + D l ( 3 P then c l e a r l y depends l a r g e l y on t h e v a l u e of D. F o r c o n v e n t i o n a l ( c ) HGMS, Dc = 200 s i s n o t unreasonable /6/. I n l a r g e - s c a l e S/C magnets t h e corresponding v a l u e would be somewhere between 300 s and

+ Mlddllngs 600 ^S. Under such circumstances a1 1 o f t h e

,

b e n e f i t o f reduced r e s i d e n c e t i m e r S ( = r c / h ) i n Eq. ( 3 ) f r o m t h e magnetic f i e l d r a t i o h (= Bos/Bo ) would be l o s t . T h i s unfavour- a b l e s~ tu a f i o n i s r e t r i e v e d i n S/C s o l e n o i d s F i g . 4

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Schematic o f c r y o g e n i c RC system by t h e use o f r e c i p r o c a t i n g c a n i s t e r (R.C.)

( a f t e r R i l e y and Hocking / 9 / ) . systems. These were developed o r i g i n a l l y b y s c i e n t i s t s and engineers a t E n g l i s h China Clays (ECLP) and based upon designs

C1-756 _JOURNAL DE PHYSIQUE

A: Active filter an i s o l a t e d Y c a n i s t e r from a large-bore '

solenoid a t f u l l f i e l d i s immense. Watson

0: Dummy

filter

e t a1

/ l o /

e s t i m a t e somewhere between 300 t

s l c solenoid

and p a t e n t s 0 f Z . J . J . S t e k l y / 9 / . A s c h e m a t i c i l l u s t r a t i o n o f an R.C. system i s shown i n F i g . 4. I n t h i s design, a f u l l y loaded c a n i s t e r i s e x p e l l e d from t h e S/C magnetat

Fig. 5

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R.C. t r a i n ( a f t e r Watson and 5 0 0 t f o r a c a n i s t e r o f 2 . 0 m l e n g t h and

e t a1 / l o / ) . 1.0mdiameter taken froma s o l e n o i d a t 8 . 0 T .

The same authors have c a r r i e d o u t a computer s i m u l a t i o n o f t h e r e s i d u a l h y d r a u l i c f o r c e requirements f o r a c a n i s t e r t r a i n o f t h e t y p e

ps

/PC

shownin Fig. 5. Here,the c a n i s t e r t r a i n

comprisestwo a c t i v e (A) f i l t e r s interspaced

T ~ = ~ O S

a l t e r n a t e l y w i t h t h r e e dummy (D) c a n i s t e r s

6 -

i n an i n f i n i t e t r a i n o f constant suscepti- b i l i t y . P e r i o d i c r e s i d u a l f o r c e s o f a m p l i t u d e

5,0.15 -

⁵ x 103 N a c t i n g on a t r a i n o f c a n i s t e r s w i t h t h e dimensions given e a r l i e r a r e found a s a f u n c t i o n o f a x i a l p o s i t i o n ( w i t h assumed c a n i s t e r r e t r a c t i o n speeds o f

-

^{1 ms-1).}

The requirements o f t h e h y d r a u l i c r a m system a r e t h e r e f o r emodest. Asecondmajor advantage

3~0.15

o f a compensated c a n i s t e r system i s a g r e a t l y reduced f o r c e on t h e S/C magnet c o i 1 s them- selves. Watson e t a1

/ l o /

have describedan

2,015

ingenious c o i l p r o t e c t i o n device i n which, as a l i v e c a n i s t e r i s r e t r a c t e d from t h e magnet bore, t h e movement o f t h e c o i l s i s sensed (say) by s t r a i n gauges whereupon a

1 -

c l a s s i c a l f i e l d c o i l , adjacent t o t h e end o f (and c o - a x i a l w i t h ) t h e c r y o s t a t , i s energised i n o p p o s i t i o n t o t h e S/C magnet

0 1

^{I I I I I} f i e l d . A t a (main) f i e l d value o f 5.0 T approximately 103 ampere-turns a r e r e q u i r e d

0 2 4 6 8 lo "0

i n t h e comoensatina c o i l s t o r e s t r a i n f u l l v t h e magnets d u r i n g E a n i s t e r withdrawal

.

^{~ i i h}

Fig. 6 - P r o d u c t i o n r a t e r a t i o f o r i d e n t i c a l this type of safeguard a

solenoidcan be superconducting ( s ) and f c ) operated safely i t h e persistent mode wi t h HGMS systems ( D c = ^{s y} D ~ = 3 0 16/' consequent r e d u c t i o n o f the helium r e q u i r e - ment o f the r e f r i g e r a t o r (- l a h - l f o r 300 A leads)

/lo/.

Second, and o f g r e a t e r im- portance, i s t h a t t h e magnet can be mounted s a f e l y on a modest support s t r u c t u r e w i t h an estimated helium l o s s saving o f o r d e r 3 t o 4 l h - 1 i n t h i s type o f system.

These savings i n helium consumption a r e a l s o r e f l e c t e d i n savings i n c a p i t a l c o s t s o f the l i q u e f i e r and power costs o f t h e r e f r i g e r a t o r .

f u l l f i e l d b y a h y d r a u l i c r a m and i s replaced meanwhile by a f r e s h c a n i s t e r . As F i g . 4

A number o f advantages accrue from t h e R.C. system. As F i g . 4 indicates,the withdrawn c a n i s t e r may be s h i e l d e d m a g n e t i c a l l y and de-gaussed d u r i n g washing. The wash p e r i o d i t s e l f can be increased t o a value approaching t h e residence time ( n ~ , / h = n,~,) o f t h e second a c t i v e c a n i s t e r i n t h e S/C magnet. Nevertheless, by f a r tRe g r e a t e s t advantage o f the R.C. system i s i n t h e r e d u c t i o n o f D t o a value (D,) which i s j u s t t h e withdrawal ( o n l y ) t i m e o f s t h e loaded c a n i s t e r from t h e system. Watson /11/ has r e c e n t l y d e s c r i b e d a d e s i g n s p e c i f i c a t i o n o f 1 0 s f o r a l a r g e b o r e (dl=0.30m)5T solenoid.

shows, t h e spent and fresh c a n i s t e r s maybe

Ezl

p a r t o f a l i n e a r c a n i s t e r t r a i n . There are several a d v a n t a g e s t o t h i s design. F i r s t , the t o t a l h v d r a u l i c f o r c e r e a u i r e d t o o u l l

0

Fig. 6 shows production r a t e (P) comparisons between l a r g e - s c a l e superconducting ( s ) and conventional ( c ) HGMS systems o f i d e n t i c a l s i z e as a f u n c t i o n o f no. Here, ^T~ i s f i x e d a t 5 0 s a n d t h e value o f t h e f i e l d r a t i o h ( = BoS/Boc) and the dead-time r a t i o

A

D A D

k ( = Ds/Dc) a r e l i s t e d i n t h e s e t o f c u r v e s A t l o w v a l u e s o f nn, Pc/P, shows, i n a d d i -

I

t i o n t o t h e b e n e f i i s c? h;'gh f i e l d s a

I

vibrating feeder c l e a r e n g i n e e r i n g advantage. When Ds f a l l s

! t o 1 0 s t h i s advantaqebecomes c o n s i d e r a b l e .

I n a d d i t i o n t o t h i s - p r o d u c t i o n r a t e ad- vantage, t h e r e a r e c l e a r c a p i t a l c o s t a n d o p e r a t i n g c o s t advantages f o r t h e R.C.

system. The f o r m e r comes f r o m t h e reduced p h y s i c a l s c a l e o f theR.C. system compared t o i t s c o n v e n t i o n a l c o u n t e r p a r t . I t has been e s t i m a t e d /11/ t h a t a 5TR.C. system, w i t h a p r o d u c t i o n r a t e e q u i v a l e n t t o a 2.1 m b o r e 2 T c o n v e n t i o n a l system, w i l l have a r e f r i g e r a t i o n power consumption o f around 20 KW.

separation boxes

C l e a r l y , problems do e x i s t f o r R.C. c r y o - g e n i c systems. Economic and t e c h n i c a l

U ~ I a

reasons p u t an upper l i m i t on t h e f i e l d r a t i o , h o f around 4 /11/. Cryoqenic s o l e n o i d s o p e r a t e comfortably- up t o about 8 . 0 T w i t h c o n v e n t i o n a l Nb-Ti w i r e . Above F i g . 7

-

Schematic o f S/C s p l i t - p a i r d r y OGMS t h i s f i e l d value costs escalate.

system. ( C o u r t e s y o f H . K . C o l l a n / 3 / ) a d d i t i o n , i n l a r g e - b o r e ( d : 0.3m) S/C

8 0

- ,

^{I I I I I}

,

s o l e n o i d s , c o i l s t r e s s e s become s i g n i f i - c a n t i n terms o f d e s i g n and f a b r i c a t i o n

'=loo* c o s t s a t f i e l d s g r e a t e r t h a n 5.0 T. T h i s i s a l s o an approximate upper f i e l d l i m i t a t which d i s p l a c e m e n t s o f f i n e m a t r i c e s i n f i e l d g r a d i e n t s a t t h e e x t r e m i t i e s o f s o l e n o i d s d u r i n g c a n i s t e r w i t h d r a w a l cease t o be r e v e r s i b l e /11/.

I 1 1

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OPEN GRADIENT MAGNETIC SEPARATION ( OGMS)

z(mm) T h i s s e p a r a t i o n method c o n t r a s t s s h a r p l y F i g . 8 - R a d i a l f o r c e d e l l s i t y p a t t e r n ( i n u n i t s with.HGMS i n i t s u s e o f m u c h s m a l l e r f o r c e o f ~2 m-1). ( C o u r t e s y o f H. K. C o l l an / 3 / ) d e n s i t i e s (- 108 Nme3), generated b y

s p e c i a l l y designed magnet c o i l s , and o f much l a r g e r f o r c e ranges ( u p t o 0 . l m ) t o d e f l e c t p a r t i c l e streams s e l e c t i v e l y and c o n t i n u o u s l y towards c o l l e c t i o n p o i n t s a c c o r d i n g t o magnetic s u s c e p t i b i l i t y . Such d e v i c e s a r e a l s o c h a r a c t e r i s e d by t h e absence b o t h o f a m a t r i x and o f a d u t y c y c l e r e l a t i n g t o c y c l i c b e h a v i o u r .

E a r l y designs o f OGMS systems /12,13/ appear t o have f a v o u r e d t h e use o f a n n u l a r quadrupole magnets. Here t h e s e p a r a t i n g zone has a p r e d o m i n a n t l y r a d i a l f i e l d g r a d i e n t and f o r c e d e n s i t y p a t t e r n . One c l a s s i c a l magnet system o f t h i s t y p e used as l o n g ago as 1968 by Kolm /2/ f o r t h e u p g r a d i n g o f molybdenum o r e s had t h e wet f e e d stream moving a x i a l l y i n a p a i r o f c o n c e n t r i c p i p e s connected by p e r f o r a t i o n c o v e r i n g t h e s u r f a c e o f t h e i n n e r p i p e . Magnetic p a r t i c l e s i n t h e i n n e r p i p e d i f f u s e r a d i a l l y o u t - wards t h r o u g h t h e p e r f o r a t i o n s under t h e i n f l u e n c e o f t h e r a d i a l f i e l d g r a d i e n t s . A t t h e end o f t h e magnet t h e magnetic and non-magnetic f r a c t i o n s e x i t s e p a r a t e l y . A number o f papers were p u b l i s h e d by Cohen and Goqd /12/ on t h e use o f a S/C a n n u l a r quadrupole f o r OGMS s e p a r a t i o n on dense m i n e r a l s l u r r i e s . Here, t h e f i e l d and f o r c e d e n s i t y decrease s h a r p l y across t h e w i d t h o f t h e p i p e and s e p a r a t i o n o f t h e mixed s l u r r y i s enhanced by p r o m o t i n g o r b i t a l m o t i o n o f t h e s l u r r y t o a l l o w a l l p a r t i c l e s t o e n t e r t h e s t r o n g e s t p a r t o f t h e f i e l d . A s l o w l y moving dense s l u r r y o f magnetic p a r t i c l e s forms c l o s e t o t h e c r y o s t a t w a l l and i s d i s c h a r g e d s e p a r a t e l y a t a s p l i t t e r a t t h e e x i t p o i n t o f t h e system.

C1-7.58 JOURNAL DE PHYSIQUE

I n more r e c e n t t i m e s S/C l i n e a r m u l t i p o l e OGMS systems appear t o have c o m p l e t e l y r e - p l a c e d t h e a n n u l a r systems f o r b o t h wet and d r y s e p a r a t i o n . The f i r s t r e p o r t e d system o f t h i s t y p e was t h e ( w e t ) s o l e n o i d p i l e , s e p a r a t o r d e s c r i b e d by Schonert e t a1 /13/

i n 1977. More r e c e n t l y , t h r e e systems have been r e p o r t e d which reduce t h i s l i n e a r m u l t i p o l e t o a S/C s p l i t p a i r d r i v e n i n t h e cusp mode. The f i r s t o f t h e s e i s a p i l o t - s c a l e d r y OGMS used s u c c e s s f u l l y by Holman and H i s e /14/ f o r t h e b e n e f i c a t i o n o f d r y powdered c o a l s . Here, t h e coal i s f e d under g r a v i t y ( i n a f a s h i o n n o t u n l i k e t h a t of Schonert e t a l ) as an a n n u l a r c u r t a i n a d j a c e n t t o t h e i n n e r s u r f a c e s o f a v e r t i c a l a x i s S/C s p l i t p a i r i n a warm-bore c r y o s t a t o f i n n e r diameter 90mm.The f o r c e d e n s i t y a t a r a d i a l d i s t a n c e o f 30mm f r o m t h e c o i l s i s around 2.3 x

l o 7

~ m - 3 . The second two systems /3,15/ p r e f e r t o c a r r y o u t s e p a r a t i o n by a l l o w i n g powdered m i n e r a l t o f a l l under g r a v i t y as an a n n u l a r c u r t a i n ( F i g . 7 ) i n c l o s e p r o x i m i t y t o t h e c y l i n d r i c a l c r y o s t a t w a l l . The f o r c e d e n s i t y p a t t e r n o f one o f t h e s e /3/ i n u n i t s o f ~ ~ m - 1 as a f u n c t i o n o f t h e v e r t i c a l d i s t a n c e f r o m t h e h o r i z o n t a l p l a n e o f symmetry o f t h e s p l i t p a i r i s shown i n F i g . 8 f o r v a r i o u s r a d i a l d i s t a n c e s ( s ) f r o m t h e c r y o s t a t w a l l w i t h a peak v a l u e o f about 5.0 x

l o 7

~ m - 3 a t s = 0. An a n a l y s i s o f t h e f o r c e d e n s i t y p a t t e r n s f o r S/C s p l i t p a i r system o f v a r i o u s s i z e s i n d i c a t e s t h a t , u n l i k e t h e s o l e n o i d systems o f F i g s . 3 and 4, t h e r e i s e s s e n t i a l l y no p r o d u c t i o n r a t e advantage i n s c a l i n g t h e s e systems upwards i n s i z e . A g a i n s t t h a t an OGMS system such as MASU 3 /3/ can o p e r a t e i n t h e p e r s i s t a n t mode w i t h minimal h e l i u m l o s s ( u s i n g c r y o c o o l e r s ) and can process d r y m i n e r a l s a t r a t e s approaching 1 t h-1. I t i s p r o b a b l e t h a t f u r t h e r development w i l l i n d i c a t e an optimum c o i l diameter e n a b l i n g ( w i t h a warm bore) use o f b o t h t h e i n n e r and o u t e r s u r f a c e s o f t h e c o i l s .

I V - LONG-TERM DEVELOPMENTS

No r e v i e w o f t h i s t y p e would be complete w i t h o u t some p r e d i c t i o n s o f f u t u r e S/C sepa- r a t o r development. By f a r t h e most p r o m i s i n g o f these i s t h e c r y o g e n i c drum s e p a r a t o r , a S/C m u l t i o o l e d e v i c e f i r s t conceived and n a t e n t e d by Ries e t a1 /16/ i n 1978. An i n d u s t r i a l p r o t o t y p e o f t h i s d e v i c e o f approximate dimensions l . 0 m l e n g t h and 1.0m d i a m e t e r has r e c e n t l y been under i n d u s t r i a l t e s t . C l e a r l y , a d e v i c e o f t h i s t y p e combine t h e m e r i t s o f OGMS w i t h o v e r 100 y e a r s o f e n g i n e e r i n g experience i n drum technology. Another p o t e n t i a l development which combines t h e h i g h - f i e l d advantages o f magnetic f l o c c u l a t i o n w i t h those o f m a g n e t o - h y d r o s t a t i c s e p a r a t i o n (MHS) i s des- c r i b e d i n d e t a i l elsewhere i n t h e s e Proceedings.

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