OPERATING CHARACTERISTICS OF A PLASMACAN TRANSFERRED-ARC FURNACE FOR THE MELTING OF FINE PARTICLES

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OPERATING CHARACTERISTICS OF A

PLASMACAN TRANSFERRED-ARC FURNACE FOR THE MELTING OF FINE PARTICLES

P. Parisi, W. Gauvin

To cite this version:

P. Parisi, W. Gauvin. OPERATING CHARACTERISTICS OF A PLASMACAN TRANSFERRED-

ARC FURNACE FOR THE MELTING OF FINE PARTICLES. Journal de Physique Colloques, 1990,

51 (C5), pp.C5-161-C5-168. �10.1051/jphyscol:1990520�. �jpa-00230827�

OPERATING CHARACTERISTICS OF A PLASMACAN TRANSFERRED-ARC FURNACE FOR THE MELTING OF FINE PARTICLES

P.J. PARISI*

and W.H. GAUVIN

Department of Chemical Engineering, McGilf University, Montreal, Que, ganada;

Electrotechnology Laboratory (LTEE), Hydro-Quebec, Shawinigan, Que, Canada

RBsume

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La p u i s s a n c e d e rayonnement d ' a r c s t r a n s f k r k s d ' a z o t e e t d ' a r g o n a k t k mesuree d a n s une e n c e i n t e c y l i n d r i q u e s u r l e s p a r o i s de l a q u e l l e de p e t i t e s p a r - t i c u l e s ( 1 0 0 A 290 microns de d i a m & t r e ) de c u i v r e , d'aluminium e t d e f e r o n t e t 6 p r o j e t B e s s o u s l ' e f f e t c e n t r i f u g e d ' u n v o r t e x i n d u i t d a n s l ' e n c e i n t e , pour f o r - mer une couche d e m e t a l fondu s ' k c o u l a n t d a n s un c r e u s e t s o u s - j a c e n t c o n t e n a n t l e b a i n a n o d i q u e .

A b s t r a c t - The r a d i a t i v e power o f a r g o n and n i t r o g e n t r a n s f e r r e d a r c s was mea- s u r e d i n a c y l i n d r i c a l e n c l o s u r e , on t h e w a l l s o f which s m a l l p a r t i c l e s ( 1 0 0 t o 290 microns i n d i a m e t e r ) o f c o p p e r , aluminum and i r o n were c e n t r i f u g a l l y pro- j e c t e d by means o f a v o r t e x i n d u c e d w i t h i n t h e e n c l o s u r e , i n o r d e r t o form a m o l t e n l a y e r o f m e t a l f l o w i n g i n t o t h e a n o d i c b a t h c o n t a i n e d i n t h e h e a r t h o f t h e f u r n a c e .

1

-

INTRODUCTION

The u s e o f t r a n s f e r r e d - a r c plasmas a s a s o u r c e o f e n e r g y i n i n d u s t r i a l m e t a l l u r g i c a l p r o c e s s e s h a s been s t e a d i l y i n c r e a s i n g d u r i n g t h e p a s t few y e a r s . A r e c e n t a r t i c l e / l / r e v i e w s t h e many d i f f e r e n t a p p l i c a t i o n s which t h i s t e c h n o l o g y makes p o s s i b l e i n t h i s f i e l d . S p u r r e d by t h e s e d e v e l o p m e n t s , t h e d e s i g n o f plasma r e a c t o r s s p e c i a l l y a d a p t e d t o t h e e f f i c i e n t u t i l i z a t i o n o f t h e e n e r g y c o n t a i n e d i n a t r a n s f e r r e d a r c h a s a l s o r e c e i v e d i n c r e a s e d a t t e n t i o n . One s u c h d e s i g n d e v e l o p e d d u r i n g r e c e n t y e a r s i s t h e Plasmacan f u r n a c e / 2 / . An e x p e r i m e n t a l v e r s i o n o f t h i s t y p e o f f u r n a c e , which was u s e d i n t h e p r e s e n t s t u d y , i s shown i n F i g . 1. B r i e f l y summarized, t h i s f u r n a c e i s c h a r a c t e r i z e d by a c y l i n d r i c a l , i n s u l a t e d s l e e v e which s u r r o u n d s t h e a r c o v e r a l a r g e p o r t i o n o f i t s l e n g t h . The f i n e m a t e r i a l t o b e p r o c e s s e d i s i n j e c t e d i n t o t h e s l e e v e i n s u c h a manner t h a t t h e c e n t r i f u g a l f o r c e s , g e n e r a t e d by a v o r t e x i n d u c e d w i t h i n t h e s l e e v e , p r o p e l t h e p a r t i c l e s o n t o t h e s l e e v e w a l l s , where t h e y a r e m e l t e d by t h e r a d i a t i o n emanating from t h e t r a n s f e r r e d a r c column ( t y p i c a l l y a t a t e m p e r a t u r e o f 18 OOOK n e a r t h e c a t h o d e , d e c r e a s i n g t o a b o u t 1 2 000 K j u s t above t h e a n o d i c m o l t e n p r o d u c t ) /3/. The f i l m o f m o l t e n m a t e r i a l on t h e s u r f a c e o f t h e s l e e v e c o n t i n u o u s l y f l o w s i n t o t h e b a t h o f p r o d u c t , and t h u s s e r v e s t o c a p t u r e a n i m p o r t e d f r a c t i o n o f t h e e n e r g y c o n t a i n e d i n t h e t r a n s f e r r e d a r c which, w i t h o u t t h e s l e e v e , would be r a d i a t e d t o t h e w a l l s o f t h e f u r n a c e . The n e t e f f e c t i s t o enhance t h e o v e r a l l e f f i c i e n c y o f e n e r g y u t i l i z a t i o n o f t h e f u r n a c e . A f r a c t i o n o f t h e f e e d i s a l l o w e d t o f a l l d i r e c t l y i n t o t h e m o l t e n b a t h , where t h e e n e r g y r e l e a s e d a t t h e anode by e l e c t r o n r e c o m b i n a t i o n / 4 / i s u t i l i z e d t o m e l t a n d / o r p r o c e s s t h i s a d d i t i o n a l f e e d m a t e r i a l .

Some o f t h e a p p l i c a t i o n s which have r e c e n t l y been s t u d i e d e x p e r i m e n t a l l y by t h e McGill and t h e Hydro-Quebec LTEE g r o u p s i n t h i s t y p e o f f u r n a c e a r e l i s t e d i n r e f e r e n c e s /5/ t o / 9 / . Also p e r t i n e n t a r e an i n t e g r a t e d o r e - p r e r e d u c t i o n and plasma s m e l t i n g

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990520

COLLOQUE DE PHYSIQUE

Feed

l

~ C a l h o d e a6samblv T r a n s f e r r e d - a r o ~ ~ l l ~ / plasma column Tangentlal gas

'

Wtndow porl

F '=

Exhust port

-fi

Coolant In-out

+

,,,A L\ l I I

11

Molten, metal

<C. U U Hh Graphlte electrode

51 cm T.C. : thermocouple

COollng water In

1

trlc connector

Coollng water out

36.8 cm

Teilon Insulator Copper Brass

F i g . 1 E x p e r i m e n t a l F u r n a c e F i g . 2 Cathode Assembly p r o c e s s f o r i r o n a n d f e r r o m a n g a n e s e p r o d u c t i o n / 1 0 / , t h e o p e r a t i o n o f a 1 - M W r e a c t o r f o r t h e m e l t i n g of s t e e l by BHP i n N e w c a s t l e , A u s t r a l i a / I l l , and o f a 5-MW r e a c t o r f o r f e r r o a l l o y s p r o d u c t i o n by Plasma Arc L i m i t e d , a l s o i n A u s t r a l i a / 1 2 / . The l a s t t h r e e r e a c t o r s u s e t h e same d e s i g n a s t h e Plasmacan f u r n a c e and were c o n s t r u c t e d and commissioned by Davy McKee.

The p r e s e n t s t u d y forms p a r t o f a c o n t i n u i n g program a t t h i s U n i v e r s i t y t o p r o v i d e e n g i n e e r i n g i n f o r m a t i o n f o r t h e d e s i g n and s c a l e - u p o f t h e s e f u r n a c e s .

2

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EXPERIMENTAL METHODS

The e x p e r i m e n t a l f u r n a c e i s shown i n F i g . 1 and d e t a i l s o f t h e c a t h o d e assembly a r e g i v e n i n F i g . 2 . Not shown on F i g . 1 i s a c r u c i b l e r e s t i n g on t h e graphite a n o d i c e l e c - t r o d e s t o c o l l e c t t h e m o l t e n m e t a l from t h e t e s t s . D e t a i l s o f t h e p e r i p h e r a l equipment ( r e c t i f i e r , i n s t r u m e n t a t i o n , e t c ) a r e s t a n d a r d and c a n be found i n a p r e v i o u s s t u d y / 1 3 / . The s l e e v e s u s e d were r e f r a c t o r y - l i n e d i n t e r n a l l y and w a t e r - c o o l e d e x t e r n a l l y . I n t e r n a l s l e e v e d i a m e t e r s ( D ) o f 7 . 5 and 1 0 cm were u s e d . T h e i r i n s i d e l e n g t h was 1 4 cm i n a l l c a s e s . The c a t h o d e t i p was p o s i t i o n e d 4 cm below t h e r o o f o f t h e s l e e v e t o p r e v e n t p l u g g i n g o f t h e f e e d p o r t s a s a r e s u l t o f s i n t e r i n g o f t h e f e e d p a r t i c l e s due t o t h e i n t e n s e a r c r a d i a t i o n . Thus t h e a r c l e n g t h ( L ) w i t h i n t h e s l e e v e was 1 0 cm,. The f e e d p o r t s l o c a t e d on t h e r o o f o f t h e s l e e v e c o n s i s t e d o f two 6-mm i . d . t u b e s s y m m e t r i c a l l y l o c a t e d i n one c a s e 8 mm and i n a s e c o n d s l e e v e 1 2 . 5 cm from t h e s l e e v e w a l l . The m e t a l l i c powders u s e d a s f e e d m a t e r i a l s a r e d e s c r i b e d i n T a b l e I . They were f e d i n t o t h e s l e e v e v i a two c a l i b r a t e d s c r e w f e e d e r s and conveyed pneumatic- a l l y t o t h e f e e d p o r t s . T h i s a r r a n g e m e n t p e r m i t t e d t o c o n t r o l t h e v e r t i c a l i n j e c t i o n v e l o c i t y ( V a ) o f t h e p a r t i c l e s i n t o t h e s l e e v e . The maximum f e e d r a t e a t t a i n a b l e was 48 kg o f powder p e r h o u r . N i t r o g e n was used t o g e n e r a t e t h e plasma a r c and t h e v o r t e x , and a l s o t o convey t h e f e e d p a r t i c l e s . Argon was used i n s t e a d o f n i t r o g e n w i t h alurninum p a r t i c l e s .

To s t a r t a n e x p e r i m e n t , a c h a r g e of a b o u t 5 kg o f p a r t i c l e s was p l a c e d i n t h e c r u c i b l e . The a r c was t h e n i n i t i a t e d and a d j u s t e d t o i t s f i n a l l e n g t h o f 1 0 cm i n s i d e t h e s l e e v e . The r e a c t o r a n d s l e e v e were p r e h e a t e d u n t i l t h e s l e e v e w a l l t e m p e r a t u r e a p p r o a c h e d t h o s e e x p e c t e d w h i l e f e e d i n g ( 1 5 m i n u t e s ) , d u r i n g which t i m e t h e c h a r g e i n t h e c r u c i b l e was c o m p l e t e l y m e l t e d . A t t h i s p o i n t , t h e t a n g e n t i a l g a s r a t e t h r o u g h t h e two p o r t s shown on F i g . 1 was s e t a t t h e d e s i r e d r a t e , a l l i n i t i a l t e m p e r a t u r e s were n o t e d and t h e f e e d i n g o f t h e p a r t i c l e s commenced. A n o t h e r 10 t o 15 m i n u t e s were r e q u i r e d t o a t t a i n s t e a d y s t a t e , a s i n d i c a t e d by t h e t e m p e r a t u r e r e a d i n g s , and t h e d a t a r e c o r d e d . The r a p i d i t y w i t h which t h e v a r i o u s p r e p a r a t o r y s t a g e s were e f f e c t e d and a f i n a l s t e a d y s t a t e a t t a i n e d i s w o r t h n o t i n g and i s due t o t h e v e r y h i g h t e m p e r a t u r e d r i v i n g f o r c e e x e r t e d by t h e r a d i a t i v e power o f t h e a r c column.

H" i s t h e e n t h a l p y o f t h e m o l t e n powder a t i t s m e l t i n g p o i n t , c a l c u l a t e d above 298 K and i n c l u d e s t h e l a t e n t h e a t o f f u s i o n .

3

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METHOD OF PARTICLES FEEDING

Workers i n t h e f i e l d c a n t e s t i f y t o t h e d i f f i c u l t y o f f e e d i n g a f i n e m e t a l l i c powder i n t o a r e a c t o r a t s t e a d y s t a t e and u n d e r c o n t r o l l e d c o n d i t i o n s . These c r i t e r i a were n e c e s s a r y i n t h e p r e s e n t work i n o r d e r t o form a u n i f o r m d e p o s i t o f m o l t e n m e t a l on t h e s l e e v e . P r e l i m i n a r y t e s t s showed t h a t conveying t h e f e e d p a r t i c l e s w i t h t h e v o r t e x g a s q u i c k l y formed lumps o r s k u l l s o f a m i x t u r e o f molten and s i n t e r e d m e t a l on t h e s l e e v e w a l l s and r o o f which q u i c k l y grew and f i n a l l y smothered t h e a r c column. T h i s s i t u a t i o n c o u l d n o t be c o r r e c t e d by i n c l i n i n g t h e v o r t e x g a s i n l e t t o 45O w i t h t h e h o r i z o n t a l o r by u s i n g m u l t i p l e h o r i z o n t a l i n l e t s . The problem was s o l v e d e m p i r i c a l l y by i n j e c t i n g t h e powder v e r t i c a l l y downward t h r o u g h t h e two f e e d t u b e s shown on F i g . 1 . Depending on t h e d e n s i t y and t h e p a r t i c l e s i z e , a v a r i e t y o f o p e r a t i n g c o n d i t i o n s were u s e d : V ( m / s ) t h e p a r t i c l e a x i a l i n j e c t i o n v e l o c i t y ; V ( m / s ) t h e t a n g e n t i a l v e l o c i - t y o f th% v o r t e x g a s a t t h e p o r t e x i t ; and t h e l o c a t i o i g o f t h e a x i a l f e e d i n j e c t i o n , i n m m , from t h e s l e e v e w a l l . The c o m b i n a t i o n o f t h e s e v a r i a b l e s l e a d i n g t o good d e p o s i t f o r m a t i o n i s shown i n T a b l e 1 1 , from which c e r t a i n t r e n d s c a n be i d e n t i f i e d . For t h e s m a l l e r p a r t i c l e s (Cu-B and F e ) a s m a l l e r v a l u e o f V i s p r e f e r a b l e a s w e l l a s an i n - j e c t i o n p o i n t c l o s e r t o t h e s l e e v e a x i s . F o r a l a r g z r p a r t i c l e (Cu-A), no p a r t i c u l a r s e n s i t i v i t y was o b s e r v e d o v e r a wide r a n g e o f c o n d i t i o n s . F o r aluminum p a r t i c l e s , which a r e b o t h l a r g e r and l i g h t e r and w i t h a l o w e r m e l t i n g p o i n t , a s m a l l e r l o a d i n g (mass flow r a t e r a t i o o f p a r t i c l e s t o t a n g e n t i a l g a s , k g / k g ) had t o be u s e d . A h i g h e r c u r r e n t was a l s o n e c e s s a r y t o compensate f o r t h e l o w e r r a d i a t i v e power o f a r g o n ( u s e d t o p r e v e n t n i t r i d e f o r m a t i o n ) compared w i t h t h a t o f n i t r o g e n . No d e p o s i t a s s u c h was o b t a i n e d ; i n s t e a d , a l a r g e number o f n e a r l y s p h e r i c a l d r o p s , a few m i l l i m e t e r s i n d i a m e t e r , were formed,which however d i d n o t p r e v e n t a good aluminum m e l t from b e i n g o b t a i n e d i n t h e c r u c i b l e . T h i s phenomenon was u n d o u b t e d l y c a u s e d by t h e well-known i n a b i l i t y o f aluminum f i l m s t o wet c e r t a i n r e f r a c t o r y s u r f a c e s . F i n a l l y , no problem was e n c o u n t e r e d w i t h i r o n , which behaved l i k e Cu-B, a l t h o u g h a h i g h e r c u r r e n t was u s e d b e c a u s e of i r o n ' s h i g h e r m e l t i n g p o i n t .

4

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CALCULATIONS OF RADIANT ENERGY TRANSFER

I n a p r e v i o u s p a p e r / 1 3 / , t h e power t r a n s f e r r e d from a n a r c column t o a w a t e r - c o o l e d , u n i n s u l a t e d m e t a l l i c s l e e v e , w i t h o u t p a r t i c u l a t e f e e d , was e x p e r i m e n t a l l y measured from t h e f o l l o w i n g b a l a n c e ( s e e d i a g r a m ) :

P = P ~ + P o + P a - ' c ( 1 )

where P = power t r a n s f e r r e d t o t k sleeve c o o l i n g w a t e r

pS = power r a d i a t e d t o t h e s l e e v e by t h e a r c l e n g t h w i t h i n t h e s l e e v e pr = power r a d i a t e d t o t h e s l e e v e by t h e a r c l e n g t h

below

t h e s l e e v e P

: = power r a d i a t e d t o t h e s l e e v e by t h e m o l t e n m e t a l i n t h e c r u c i b l e P = power l o s t by t h e s l e e v e by c o n v e c t i o n t o t h e v o r t e x g a s

COLLOQUE DE PHYSIQUE

TABLE I1

TESTS RESULTING I N DEPOSIT FORMATION

T e s t S l e e v e I Feed Feed V V I n j . Loading

Number D,cm Amp kg/h m a t ' l

m 8 s

^{P t ,} mm kg/kg

CU-A CU-A CU-A CU-A CU-A CU-A CU-B CU-B CU-B Cu- A A 1 A 1 A 1 Fe

P and P were found t o be q u i t e s m a l l and c o u l d be neg- l g c t e d i$ t h e p r e s e n t s t u d y . P and P were e q u a l t o t h e

-M J o u l e h e a t ( t h e p r o d u c t of t h e r c u r r e n ? I , t h e e l e c t r i c a l f i e l d o r v o l t a g e g r a d i e n t V and t h e a r c l e n g t h ) d i s s i p a t - i n t h e i r r e s p e c t i v e l e n g t h s g o f a r c ( L and L . ) , m u l t i p l i e d by t h e i r r e s p e c t i v e e f f i c i e n c y f a c t o r s , ,Oand

To,

^which

were d e t e r m i n e d e x p e r i m e n t a l l y . Thus,

P V L . . . . 2 and P = ^ ( L o ( I V L )

....

( 3 )

g go 0

and were c o r r e l a t e d g r a p h i c a l l y a s f u n c t i o n s o f (L/D)

)

^{k d} o f (Lo/D), i n which D i s t h e s l e e v e d i a m e t e r . T h e i r va-

Molln &I* barn l u e s a r e shown i n Appendices I and 11. D i f f e r e n t c u r v e s ob- r)

-

t a i n e d f o r a r g o n and f o r n i t r o g e n . I n t h e p r e v i o u s work, L.

P*.Pr'Po*Pa-PS

was k e p t c o n s t a n t a t 7 cm, and t h e c o r r e l a t i o n b e t w e e n q o and (Lo/D) was d e r i v e d f o r t h i s v a l u e o n l y . I n t h e p r e s e n t s t u d y , t h e s l e e v e was

still

w a t e r - c o o l e d b u t was now i n s u l a t e d w i t h a c e r l i t e r e f r a c t o r y on i t s i n t e r n a l s u r f a c e . Although a f e e d o f p a r t i c l e s was now u s e d , t h e c o n t r i b u t i o n s o f r a d i a n t e n e r g y t r a n s f e r s remained n e a r l y t h e same a s above.

Thg m e t a l l i c f i l m on t h e s l e e v e r e q u i r e d a power Pf t o m e l t a t s t e a d y s t a t e , and a new b a l a n c e c a n now be w r i t t e n :

I n t h i s s i t u a t i o n L, was no l o n g e r c o n s t a n t a t 7 cm, b u t v a r i e d between 4 and 6 cm. Hen- c e , u s e of t h e p l o t f o r v p r e v i o u s l y o b t a i n e d c r e a t e d an e r r o r o f a b o u t 1 0 % i n t h e v a l u ? o f P b u t o n l y 2 % i n t h e v a l u e o f ( P p + P O ) .

I n E q u a t i o n ( ' I ) , PS was measured c a l o r i m e t r i c a l l y by t h e f l o w r a t e of c o o l i n g w a t e r and i t s t e m p e r a t u r e r i s e i n t h e e x t e r n a l c o o l i n g c o i l . P, and PO were c a l c u l a t e d from Equa- t i o n s ( 2 ) and

( 3 ) ,

i n which t h e f o l l o w i n g v a l u e s o f t h e a r c v o l t a g e g r a d i e n t s , o b t a i n e d e x p e r i m e n t a l l y i n t h e p r e v i o u s s t u d y , were used:

I n t h e above T a b l e , i t i s assumed t h a t Vg i s i n d e p e n d e n t o f t h e c u r r e n t . T h i s i s n o t s t r i c t l y t r u e . Thus, f o r n i t r o g e n , Vg d $ c r e a s e s from 3 . 4 V/cm a t 200 Amp t o 3 . 3 V/cm a t 300 Amp. On t h e o t h e r hand, f o r a r g o n , i t i s r e p o r t e d /10/ t h a t up t o 1 500 Amp, Vg i s r e l a t i v e l y i n s e n s i t i v e t o a r c c u r r e n t . I t i s p r o b a b l e t h a t t h e v a l u e s g i v e n a - bove a r e a c c u r a t e w i t h i n 3 o r 4%.

A s i n t h e p r e v i o u s s t u d y , i t s h o u l d be n o t e d t h a t t h e c o n s t r i c t i n g e f f e c t o f t h e vor- t e x (which a t c o n s t a n t c u r r e n t i n c r e a s e s t h e t e m p e r a t u r e and e l e c t r i c a l c o n d u c t i v i t y o f t h e a r c , and hence d e c r e a s e s V g ) d i s a p p e a r s when t h e a r c emerges from t h e s l e e v e , s i n c e t h e v o r t e x i s now d e s t r o y e d . Hence Vgo, t h e v o l t a g e g r a d i e n t o f t h e a r c below t h e s l e e v e , assumes t h e v a l u e f o r z e r o v o r t e x i n t h e above t a b l e .

The e x p e r i m e n t a l v a l u e o f Pf c a n now be c a l c u l a t e d from:

Pf = P, + PO

-

^PS

...

^{( 5 )}

I f m i s t h e mass f e e d r a t e o f p a r t i c l e s i n kg/h and X i s t h e f r a c t i o n of t h e f e e d m e l t - on t h e s l e e v e :

...

P = xmH ( 6 )

T h i s e q u a t i o n assumes t h a t t h e m o l t e n f e e d on t h e s l e e v e r e m a i n s a t t h e m e l t i n g p o i n t , f i n which c a s e t h e v a l u e s o f H g i v e n i n T a b l e I c a n be used.

F i n a l l y , a u s e f u l c r i t e r i o n o f t h e e f f i c i e n c y of t h e m e l t i n g o p e r a t i o n i s t h e r a t i o P f / ( P r + P O ) , d e n o t e d by and e x p r e s s e d a s a p e r c e n t a g e .

F o r e a c h of t h e t e s t l i s t e d i n T a b l e I1 t h e e x p e r i m e n t a l r a t e o f c o o l i n g o f t h e s l e e v e , PS, e x p r e s s e d i n kW, was measured. Based on t h e i n t e r n a l d i a m e t e r o f t h e t u b e s f e e d i n g t h e t a n g e n t i a l g a s i n t o t h e s l e e v e , t h e v e l o c i t y o f t h e l a t t e r , Vtg shown i n T a b l e 11, was c o n v e r t e d i n t o L/min f o r e a c h r u n a s a measure o f t h e v o r t e x s t r e n g t h . I n a l l c a - s e s , t h e flow r a t e o f n i t r o g e n o r a r g o n f e d t o t h e c a t h o d e a s plasma g a s was m a i n t a i n - e d c o n s t a n t a t 1 4 L/min (NTP). F o r n i t r o g e n , t h e t o t a l a r c v o l t a g e v a r i e d between 80 and 1 2 0 V o l t s .

Using t h e o p e r a t i n g c o n d i t i o n s g i v e n i n T a b l e I1 and E q u a t i o n s ( 1 ) t o ( 6 ) , t h e v a l u e s f o r e a c h r u n o f ( I V g L ) , (Pr + P O ) ,

?p

and X were c a l c u l a t e d , a s shown i n T a b l e 111.

Because o f t h e e x p l o r a t o r y n a t u r e o f t h e work, a v a r i e t y o f c u r r e n t s , f e e d r a t e s , v o r - t e x s t r e n g t h s , f e e d l o c a t i o n a n d s l e e v e d i a m e t e r s were t r i e d . I n r e t r o s p e c t , however, i t i s e v i d e n t from T a b l e I11 t h a t t h e r a t e o f c o o l i n g

,

P,, was e x c e s s i v e , a s demonst- . r a t e d by t h e s m a l l d i f f e r e n c e between PS-,and t h e r a t e o f r a d i a n t h e a t i n g (Pr +

g

) . I n a n i n d u s t r i a l f u r n a c e , l i t t l e o r no c o o l i n g o f t h e s l e e v e s h o u l d be u s e d , t h u s p e r - m i t t i n g an i n c r e a s e i n t h e f e e d r a t e and a b e t t e r u t i l i z a t i o n o f t h e r a d i a n t power o f t h e a r c . T h i s w i l l a l l o w a h i g h e r f r a c t i o n o f t h e f e e d ( 1

-

X ) t o d r o p d i r e c t l y i n t o t h e b a t h , i n o r d e r t o u t i l i z e f u l l y t h e e l e c t r o n r e c o m b i n a t i o n e n e r g y r e l e a s e d i n t h e l a t t e r by t h e a r c . T h i s f r a c t i o n w i l l depend on t h e l e n g t h o f t h e a r c which i n t u r n g o v e r n s t h e t o t a l r a d i a t i v e power o f t h e a r c . It w i l l assume a v a l u e c l o s e t o 0 . 5 i n s m a l l f u r n a c e s / 3 / , b u t w i l l be h i g h e r i n i n d u s t r i a l f u r n a c e s which u s e l o n g - e r a r c s .

I n t h e p r e s e n t work, i t was v e r i f i e d t h a t t h e s u r f a c e t e m p e r a t u r e o f t h e m o l t e n b a t h , a s measured w i t h a h i g h - r e s o l u t i o n o p t i c a l p y r o m e t e r aimed t h r o u g h t h e o b s e r v a t i o n window, was s u f f i c i e n t l y low t o p r e v e n t a p p r e c i a b l e v a p o r i z a t i o n o f t h e m o l t e n m e t a l . F o r c o p p e r , f o r example, t h e t e m p e r a t u r e r e c o r d e d was always w e l l below 1 200°b, b e i n g c o n t r o l l e d by t h e h i g h f r a c t i o n (1

-

X ) o f p a r t i c l e s f a l l i n g d i r e c t l y i n t o t h e b a t h and c o n s e q u e n t h i g h power demand f o r t h e i r m e l t i n g . S i m i l a r l y , n o e n t r a i n e d powder c o u l d b e d e t e c t e d i n t h e g a s s c r u b b e r c o n n e c t e d t o t h e two 2-cm d i a m e t e r e x h a u s t p o r t s ( o n l y one o f which i s shown on F i g . 1 ) on t o p o f t h e f u r n a c e .

COLLOQUE DE PHYSIQUE

TABLE I11 TESTS RESULTS

- - P -Pp

T e s t V o r t e x IVgL P, + PO Pf

N O .

:I

~ / m i n LW k~ k~

Ye%

^X

*

Value o f P u n r e l i a b l e due t o f a u l t y thermocouples

5 - CONCLUSIONS

The f i r s t o b j e c t i v e o f t h i s s t u d y was t o d e v e l o p a method o f f e e d i n g f i n e p a r t i c l e s (much f i n e r t h a n p r e s e n t l y h a n d l e d by commercial r e a c t o r s o f t h i s t y p e ) i n t o t h e s l e e v e o f a Plasmacan f u r n a c e u n d e r c o n t r o l l e d c o n d i t i o n s , s o t h a t a d e s i r e d f r a c t i o n , X , o f t h e f e e d be d e p o s i t e d on t h e s l e e v e t o u t i l i z e t h e r a d i a t i v e power of t h e t r a n s f e r - - r e d a r c , w h i l e t h e r e m a i n i n g f r a c t i o n (1 - X ) be a l l o w e d t o f a l l d i r e c t l y i n t o t h e m o l t e n b a t h t o u t i l i z e t h e power r e l e a s e d a t t h e anode.

I n o r d e r t o p r e d i c t a n a l y t i c a l l y t h e motion o f p a r t i c l e s e n t r a i n e d i n t h e v o r t e x i n s i d e t h e s l e e v e o f a Plasmacan f u r n a c e , i t i s n e c e s s a r y t o know t h e v o r t e x f l o w f i e l d /14/.

A number o f f a c t o r s , however, p r e c l u d e s u c h a n a p p r o a c h . F i r s t o f a l l , t h e flow f i e l d o f t h e v o r t e x i s m o d i f i e d by t h e p r e s e n c e o f t h e plasma column i n t h e c e n t e r o f t h e s l e e v e . I n a d d i t i o n , t h e a r c a s p i r a t e s g a s a t t h e c a t h o d e t i p due t o t h e Maecker E f f e c t and a l s o e n t r a i n s g a s a l o n g t h e l e n g t h o f t h e a r c column / 3 / . F i n a l l y , t h e h o t s l e e v e w a l l h e a t s up t h e v o r t e x g a s f l o w i n g a l o n g i t s s u r f a c e t o a s m a l l e x t e n t b u t i n t r o d u c e s buoyancy e f f e c t s , due t o n a t u r a l c o n v e c t i o n , i n t o t h e o v e r a l l v o r t e x flow. S u i t a b l e models o f v o r t e x f l o w t h a t t a k e t h e s e e f f e c t s i n t o a c c o u n t have y e t t o be d e v e l o p e d . I n view o f t h i s complex s i t u a t i o n , t h e e m p i r i c a l a p p r o a c h d e s c r i b e d i n t h e e x p e r i m e n t a l s e c t i o n was u s e d .

The most i m p o r t a n t p a r a m e t e r o f t h e f e e d i n g a r r a n g e m e n t was found t o be t h e d i s t a n c e o f t h e i n j e c t i o n p o i n t f o r t h e v e r t i c a l f e e d from t h e s l e e v e w a l l . F i n e powders ( s u c h a s Cu-B o r i r o n , w i t h median d i a m e t e r s o f 1 0 0 t o 1 0 5 m i c r o n s ) r e q u i r e d a d i s t a n c e o f 1 2 . 5 mm from t h e s l e e v e w a l l o r a p p r o x i m a t e l y 25% o f t h e r a d i u s o f t h e s l e e v e ( 5 0 mm i n most t e s t s ) . T h i s i s a r e g i o n where t h e t a n g e n t i a l v e l o c i t y i s a b o u t 125% of t h e i n l e t v e l o c i t y o f t h e t a n g e n t i a l v o r t e x g a s /14/ and a d e q u a t e t o convey t h e e x p a n d i n g s p r a y o f p a r t i c l e s o r i g i n a t i n g from t h e v e r t i c a l f e e d t u b e . I t i s t h e r e f o r e r e a s o n a b l e t o i n f e r t h a t t h e f i n e r t h e a v e r a g e s i z e o f t h e f e e d p a r t i c l e s , t h e g r e a t e r s h o u l d be t h e d i s t a n c e o f t h e i n j e c t i o n p o i n t from t h e s l e e v e w a l l t o p r o v i d e a wide s p r e a d o f d e p o s i t e d p a r t i c l e s on t h e s l e e v e . Attempts t o f e e d e v e n f i n e r c o p p e r powders (me- d i a m e t e r o f 60 m i c r o n s ) f a i l e d b e c a u s e o f t h e p h y s i c a l i m p o s s i b i l i t y o f p r o v i d i n g a n

X o r ( 1

-

X ) i n d i v i d u a l l y c o u l d be f o u n d . Although t h e e x e r c i s e was l a r g e l y q u a l i t a - t i v e and i n c l u d e d a f a i r number o f a s s u m p t i o n s ( a c c e p t a b l e however f o r a f i r s t a p p r o x i - m a t i o n ) i t was i n s t r u c t i v e i n i l l u s t r a t i n g t h e d e t r i m e n t a l e f f e c t o f e x c e s s i v e e x t e r n a l c o o l i n g of t h e s l e e v e on t h e m e l t i n g c a p a c i t y o f t h e s l e e v e . A l a r g e amount o f t h e r a d i a t i v e power o f t h e a r c (Pr + P O ) was t h u s l o s t t o t h e c o o l i n g w a t e r , a s measured by P S , which c o u l d have been u s e d t o m e l t a much l a r g e r q u a n t i t y o f f e e d . I n p r a c t i c e , i n a w e l l d e s i g n e d s y s t e m , l i t t l e o r no c o o l i n g s h o u l d be u s e d , t h u s p e r m i t t i n g t h e h a n d l i n g o f a much l a r g e r f e e d r a t e o f p a r t i c l e s . T h i s a p o r o a c h c o u l d n o t be i n v e s t i g - a t e d i n t h e e x i s t i n g e q u i p m e n t , however, b e c a u s e o f t h e l i m i t e d c a p a c i t i e s o f b o t h t h e f e e d i n g s y s t e m and o f t h e c r u c i b l e i n which t h e m o l t e n m e t a l was c o l l e c t e d .

Although t h i s s t u d y was a d m i t t e d l y v e r y l i m i t e d i n s c o p e , i t i s f e l t t h a t i t was s u c c e s - s f u l i n i d e n t i f y i n g some o f t h e f a c t o r s i n v o l v e d i n t h e e n g i n e e r i n g d e s i g n a n d s c a l e - up o f t h i s t y p e o f f u r n a c e . I t i s a l s o b e l i e v e d t h a t i t d e m o n s t r a t e d t h e l a t t e r ' s a b i l i t y t o p r o c e s s a f e e d o f f i n e p a r t i c l e s .

6

-

REFERENCES

/ l / G A U V I N , W. H., DROUET, M . G. and MUNZ, R . J . , "Developments i n Plasma P r o c e s s e s f o r E x t r a c t i v e M e t a l l u r g y " , J o u r n a l o f M e t a l s ,

9 ,

No. 1 2 ( 1 9 8 7 ) 14-17.

/ 2 / GAUVIN, W. H. and KUBANEK, G. R . , "A Novel T r a n s f e r r e d - A r c Plasma R e a c t o r f o r Chem- i c a l a n d M e t a l l u r g i c a l A p p l i c a t i o n s " , U. S . P a t e n t s No. 4 , 4 6 6 , 8 2 4 and No. 4 , 5 1 9 , 8 3 5 ( 1 9 8 5 ) ; Canada No. 1 , 1 7 3 , 784 ( 1 9 8 4 ) , e t c .

/ 3 / G A U V I N , W. H., "Some C h a r a c t e r i s t i c s o f T r a n s f e r r e d - A r c P l a s m a s " , Plasma C h e m i s t r y and Plasma P r o c e s s i n g ,

9 ,

No.1 ( S u p p ) ( 1 9 8 9 ) 65s-84s.

/ 4 / DINULESCU, H. A. and PFENDER, E . , "The Anode Boundary Layer o f High I n t e n s i t y Arcs"

J . Appl. P h y s . ,

51,

No. 6 ( 1 9 8 0 ) 3149-3157

/5/ PARISI, P. J . , GAUVIN, W. H. and DROUET, M . G., "The P r o d u c t i o n o f Ferrovanadium i n a Plasmacan F u r n a c e " , P r o c e e d i n g s o f t h e 2 7 t h Annual C o n f e r e n c e o f M e t a l l u r g i s t s (CIM) ( M o n t r e a l , Canada, 1 9 8 8 ) p a p e r 43.16.

/ 6 / DROUET, M . G . , HANDFIELD, M . , PARISI, P. J . , HARRIS, M . and G A U V I N , W. H., "Study o f t h e I n j e c t i o n and T r e a t m e n t o f E l e c t r i c Arc F u r n a c e s Dusts i n a Plasmacan R e a c t o r "

Proceedings o f t h e 2 7 t h Annual C o n f e r e n c e o f M e t a l l u r g i s t s (CIM), ( M o n t r e a l , Canada, 1 9 8 8 ) p a p e r 3 4 . 1 .

/ 7 / DROUET, M . G . , PARISI, P. J . , HANDFIELD, M . H. and GAUVIN, W. H . , "The Plasma T r e a t m e n t o f Baghouse F i n e s from S t e e l E l e c t r i c F u r n a c e s " , P r o c e e d i n g s o f t h e 8 t h Sym- posium on Plasma C h e m i s t r y (Tokyo, J a p a n , 1 9 8 7 ) 1874-1880.

/8/ CHIN, E. J . , MUNZ, R. J . and GAUVIN, W. H., " K i n e t i c s o f C a r t h e r m i c R e d u c t i o n o f Niobium P e n t o x i d e and P y r o c h l o r e i n a T r a n s f e r r e d Arc Plasma", P r o c e e d i n g s o f 2 7 t h An- n u a l C o n f e r e n c e o f M e t a l l u r g i s t s ( C I M ) ( M o n t r e a l , Canada, 1 9 8 8 ) P a p e r 43.15.

/ 9 / HILBORN, M . M., MUNZ, E. J . , BERK, C . and DROUET, M . G . , " E v a l u a t i o n o f F e r r o n i o - bium C a r b i d e P r o d u c t i o n i n a L a b o r a t o r y - s c a l e Plasmacan F u r n a c e " , Can. Met. Q u a t e r l y , 2 8 , 2 , ( 1 9 8 9 ) 101-107.

-

/10/ NADEN, D . , KERSHAW, P. and PARKIN, K . , "Davy McKee Development o f I n t e g r a t e d Ore P r e - R e d u c t i o n and D. C. T r a n s f e r r e d - A r c Plasma S m e l t i n g Technology f o r I r o n and F e r r o - a l l o y s P r o d u c t i o n " , P r o c e e d i n g s o f t h e E x t r a c t i o n M e t a l l u r g y Symposium 1 9 8 5 , I n s t i t u - t i o n o f Min. and Met., (London, E n g l a n d , 1 9 8 5 ) 335-353.

/11/ HART, A. T . , C e n t r a l R e s e a r c h L a b o r a t o r i e s , BHP, A u s t r a l i a ( P r i v a t e Communication Nov. 1 6 , 1 9 8 8 ) .

/12/ KERSHAW, P . , MOSS, D . J . , GILL, M . E. and HEGGIE, J . G . , "A 5-MW Plasma Arc Fur- n a c e f o r t h e P r o d u c t i o n o f F e r r o a l l o y s " , P r o c e e d i n g s o f E x t r a c t i o n M e t a l l u r g y 1 9 8 9 , I n s t i t u t i o n o f Min. and Met., (London, E n g l a n d , 1 9 8 9 ) 197-208.

CQLLOQUE DE PHYSIQUE

/13/ PARISI, P. J . and GAUVIN, W. H . , "Heat T r a n s f e r from a T r a n s f e r r e d - A r c Plasma t o a C y l i n d r i c a l E n c l o s u r e " , Plasma Chemistry and Plasma P r o c e s s i n g ,

10,

4 ( 1 9 9 0 ) . /14/ REYDON, R. and GAUVIN, W. H . , " T h e o r e t i c a l and E x p e r i m e n t a l S t u d i e s o f C o n f i n e d Vortex Flow", Can. J . o f Chem. Eng.,

3

(1981)14-23.

APPENDICES

EFFICIENCY FACTORS QgAND ??,,AS FUNCTIONS OF DIMENSIONLESS ARC LENGTHS

0 1 8 1 1 : 8 1 j J

-

1 3 4

Dimenslonless arc length (m)

ACKNOWLEDGMENTS

The a u t h o r s wish t o e x p r e s s t h e i r g r a t i t u d e t o t h e members o f t h e McGill Plasma Group, i n p a r t i c u l a r P r o f . R. J. Munz, and o f t h e Plasmas I n d u s t r i e l s Group o f Hydro-Quebec LTEE, p a r t i c u l a r l y D r . M. G. D r o u e t , f o r t h e i r many u s e f u l s u g g e s t i o n s . We a r e a l s o i n d e b t e d t o D r . Drouet f o r p r o v i d i n g l a b o r a t o r y s p a c e , equipment and f i n a n c i a l a s s i s - t a n c e .