SUPERCONDUCTING MAGNET FOR 60 TONNE/HOUR MINERAL SEPARATOR WITH CLOSED CYCLE 4 KELVIN REFRIGERATION

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SUPERCONDUCTING MAGNET FOR 60 TONNE/HOUR MINERAL SEPARATOR WITH

CLOSED CYCLE 4 KELVIN REFRIGERATION

J. Good, K. White

To cite this version:

J. Good, K. White. SUPERCONDUCTING MAGNET FOR 60 TONNE/HOUR MINERAL SEPA-

RATOR WITH CLOSED CYCLE 4 KELVIN REFRIGERATION. Journal de Physique Colloques,

1984, 45 (C1), pp.C1-759-C1-761. �10.1051/jphyscol:19841154�. �jpa-00223627�

JOURNAL DE PHYSIQUE

Colloque C1, s u p p l i m e n t a u n o 1, T o m e 45, janvier 1984 p a g e C1-759

S U P E R C O N D U C T I N G M A G N E T FOR 6 0 T O N N E / H O U R M I N E R A L SEPARATOR

W I T H

CLOSED CYCLE 4 KELVIN REFRIGERATION

J . A . Good and K. White

Cryogenic Consultants Limited, Metrostore Building, 231 The Vale, Acton, London, W 3 7Q5, U . K .

Resume

-

Cryogenic C o n s u l t a n t s Limited a c o n s t r u i t un systame 5 aimant supraconducteur pour s e p a r a t i o n magnetique q u i e s t c o n s t i t u e d ' u n d i - pBle de t r o i s mstres de long r e f r o i d i p a r un r d f r i g e r a t e u r 2 c y c l e f e r -

me.

Cet a r t i c l e e s t r e l a t i f a u p r o j e t e t 2 l a c o n s t r u c t i o n de l ' a i m a n t en r e l a t i o n avec une e x p r e s s i o n t h e o r i q u e s u r l a c a p a c i t e du p r o c e s s u s . A b s t r a c t

-

Cryogenic C o n s u l t a n t s Limited h a s c o n s t r u c t e d a superconducting m a g n e t s y s t e m f o r m a g n e t i c separation, with a t h r e e m e t r e long dipole m a g n e t cooled by a closed-cycle refrigerator. This p a p e r considers t h e design and construction of t h e m a g n e t s y s t e m in relation t o a t h e o r e t i c a l expression f o r processing capacity.

Magnetic s e p a r a t i o n of mineral o r e s is a widespread technique using powerful e l e c t r o magnets. C C L h a s b e e n involved f o r a number of y e a r s in t h e development of cryogenic machines which u s e superconducting m a g n e t s t o provide high fields and g r a d i e n t s f o r mineral separation. In t h e m a g n e t i c s e p a r a t o r described h e r e t h e field is used t o d i v e r t mineral f r o m i t s original p a t h r a t h e r than t o c a p t u r e i t a s is usual in m a n y o t h e r separators.

111.

F o r t h i s purpose a s t r o n g and relatively uniform g r a d i e n t is required o v e r a l a r g e working volume.

C C L originally developed a c i r c u l a r g e o m e t r y based on a quadropole and t h i s machine was used f o r both w e t and dry processing 121, As a result of t h e initial dry processing work, which showed considerable promise, an improved s e p a r a t o r w a s developed using closed c y c l e 4 Kelvin refrigeration. This c o n s i s t e d of a r e v e r s e pair winding providing a s t r o n g a t t r a c t i v e field around t h e outside of a cylindrical cryostat. The c r y o s t a t d i a m e t e r w a s 365mm and t h e field on t h e s u r f a c e of t h e c r y o s t a t 3.2 T e s l a with g r a d i e n t s typically in t h e region of 0.8 Teslalcm. T h e r e f r i g e r a t o r w a s built into t h e t o p of t h e c r y o s t a t s o t h a t t h e whole s y s t e m could b e o p e r a t e d entirely without cryogens. F r o m switching on approximately 36 hours w e r e required f o r t h e m a g n e t t o r e a c h operating t e m p e r a t u r e . O n e of t h e s e m a c h i n e s w a s sold t o t h e Foskor Corporation in South A f r i c a f o r t h e development of dry m a g n e t i c s e p a r a t i o n of phosphate minerals. R e s e a r c h work indicated t h a t successful s e p a r a t i o n could b e obtained b u t t h a t t o e n s u r e a n e c o n o m i c p e r f o r m a n c e considerable i m p r o v e m e n t s in t h e c o s t b e n e f i t r a t i o w e r e required.

T h e o r e t i c a l s t u d i e s by Professor Kopp a t Wits University, and one of t h e a u t h o r s indicated a relationship b e t w e e n t h e processing c a p a c i t y of t h e s e p a r a t o r and m a g n e t i c f i e l d intensity and volume /4,5/. T h e c a p a c i t y p e r m e t r e l e n g t h of t h e m a g n e t is described by t h e following a p p r o x i m a t e equation:-

The f i r s t p a r t of t h e equation r e l a t e s t o t h e m a g n e t design, (b) is t h e e f f e c t i v e height of t h e m a g n e t i c field, (H) is t h e m a g n e t i c field. T h e second p a r t c o n t a i n s t h e mineral p a r a m e t e r s ,

b)

is t h e density of t h e mineral,

(x)

t h e susceptibility, (C2) t h e c o n c e n t r a t i o n of magnetics, (r) t h e radius of t h e mineral particles. The q u a n t i t y (h) is t h e e f f e c t i v e height f r o m which t h e mineral falls b e f o r e entering t h e high field region. The c a p a c i t y is n o t strongly influenced by this f a c t o r which m u s t be non-zero in this approximation. In any p a r t i c u l a r s e p a r a t i o n t h e mineral p a r a m e t e r s a r e largely fixed by t h e r e q u i r e m e n t f o r a d e q u a t e liberation.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841154

C1-760 JOURNAL DE

PHYSIQUE

The magnet designer has t o provide the best possible magnet performance f o r a given capital cost. F o r any winding geometry the separation power i s proportional directly t o the current i n the windings, as both f i e l d and f i e l d gradient are directly proportional t o current. Further- more, increasing the magnetic f i e l d increases the cost of the magnet i n a non-linear fashion.

Using niobium titanium the costs increase strongly once the field on the winding exceeds 6 Tesla. F o r this reason it is generally preferable t o generate larger volumes o f f i e l d rather than greater magnetic intensity. The new magnet was, therefore, designed t o have the same magnetic f i e l d as the previous prototype but t o generate the f i e l d i n a larger volume.

The magnet was designed as a linear dipole i n a rectangular cross-section cryostat. The ad- vantages are that w i t h the linear geometry there are t w o separating zones both o f which are against f l a t surfaces. As the stream o f ore drops past the magnet i t i s necessary t o have an adjustable s p l i t t e r between the magnetic and non-magnetic fractions. I n a linear geometry this can be a straight edge which can be moved closer t o or further f r o m the magnet. With a circular geometry radial movement o f the splitter i s difficult; a disadvantage o f the original prototype separator. The new magnet has a t o t a l separation length o f some 6 metres, since both sides o f the 3 m e t r e long dipole can be used f o r mineral processing. Further technical details are given i n the Table below.

The cross section o f the linear cryomagnet is shown i n the Figure. The magnet i s cooled by liquid helium passing through heat exchangers a t the top and b o t t o m o f the magnet. The radiation shields are cooled by helium gas. Refrigeration f o r the magnet i s provided by an improved version o f the CCL R.700 refrigerator which i s b u i l t on t o one end o f the cryomagnet.

The magnet consists o f t w o windings supported by a stainless steel yoke. The yoke passes between the windings which are placed as close t o the surface o f the cryostat as practical.

The windings are surrounded by t w o shields, one a t about 16 K e l v i n and the other a t about 60 Kelvin. To support the f l a t sides o f the cryostat, pillars at room temperature pass through the centre o f the magnet a t several points. The cryostat outer parts are formed f r o m alu- minium and stainless steel. The whole cryostat is assembled w i t h O-rings so t h a t the interior can be inspected without breaking welds.

Since building the magnet presented a considerable technical challenge it was decided t o carry out a test programme on a half-metre long dipole. The design o f the half-metre dipole was such t h a t the forces per metre length on the winding and the supporting structure were as similar as possible t o the requirements of the f u l l length magnet.

These tests showed t h a t considerable care was needed i n the design o f the support structure t o avoid fracture or movement o f the coils during either cooldown t o operating temperature or energisation. The forces on the c o i l are given i n the Table below. Several methods of clamping were investigated. I n i t i a l tests showed t h a t there could be substantial training.

The solution chosen was a set o f stainless steel clamps bolted f r o m top t o b o t t o m o f the coils w i t h separate spacers t o maintain the separation between the coils. I n order t o have suffic- ient proof stress f o r repeated cycling o f the magnetic f i e l d the stainless steel Type 316.LN was 50% hard-worked during the fabrication process. With the f i n a l configuration it was possible t o reach the guaranteed rated f i e l d without training and t o considerably exceed this a f t e r one o r t w o training quenches, achieving over 90% o f short-sample performance.

/1/ ^Watson J.H.P., Magnetic Filtration, Journal o f Applied Physics (1973)

44

(1973) /2/ Cohen H.E. and Good J.A., Industrial Applications o f Magnetic Separation,

IEEE Publication 78 C H 1447-2 74.

/3/ Watson J.H.P., Superconducting High Gradient Magnetic Separation, Mining Magazine August (1983) 121.

141 Kopp J. and Good J.A., IEEE Trans. Mag/ MAG-18 N0.3 (1982) 833 /5/ Kopp J., I n s t i t u t e Journal o f Mining Processes

I;E!

(1983) 297.

TABLE o f Technical Specification Superconductins material

~ d ~ ~ e r / s u ~ e r c o ~ d u c t o r r a t i o Guaranteed operating current Weight o f superconductor Weight o f magnet

Magnetic force vertical (expansive) Magnetic force horizontal (compressive) Magnet length

Magnet width Magnet height Cryostat length Cryostat width Cryostat height

Computed heat loads i n watts

-

Radiation

-

Support structure

-

Gas Exchange

(JT)

-

Current Leads

-

Sundry

-

Total

-

Available Power

F.60 dia. 0.5 and 0.4mrn

60 k g 110 kg

88 tonneslmetre 47 tonneslmetre 3,000mm Less than 67mm 122mm

3,850rnm 85mm 446rnrn

Temp. - 4.7K

7.8 0.14

6 0 K s h i e l d

1 6 K s h i e l d

4K magnet c o o l i n g Magnet support

Cryos t a t w a l l support

Magnet C r y o s t a t w a l l

O-ring