Bubble formation at nozzles in pig iron
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T H E i n j e c t i o n of gas f r o m s u b m e r g e d n o z z l e s or o r i f i c e s into liquid m e t a l baths is an i n t e g r a l f e a t u r e of many m e t a l l u r g i c a l p r o c e s s i n g and r e f i n i n g o p e r a t i o n s . F a m i l i a r e x a m p l e s include the m i x i n g of liquid m e t a l s by i n e r t gas b u b b l i n g through p o r o u s plugs or s u b m e r g e d lances, as well a s t u y e r e - b a s e d i n j e c t i o n p r o c e s s e s for p r o d u c i n g s t e e l , b l i s t e r copper o r B e s s e m e r matte. Any s u c c e s s f u l u n d e r s t a n d i n g of p r o c e s s k i n e t i c s u n d e r such c i r c u m s t a n c e s r e q u i r e s a d e t a i l e d knowledge of the fluid m e c h a n i c p h e n o m e n a involved. The p r e s e n t p a p e r is c o n c e r n e d s p e c i f i c a l l y with a study of those f a c t o r s d e t e r m i n i n g bubble s i z e s when the gas is i n j e c t e d at low and m o d e r a t e flow r a t e s through s u b m e r g e d n o z z l e s . Although much data have b e e n a c c u m u l a t e d on aqueous s y s t e m s , d i r e c t e x t r a p o l a t i o n to liquid m e t a l s y s t e m s can often be m i s l e a d i n g and yield e r r o n e o u s c o n c l u s i o n s . Consequently the work now d e s c r i b e d has b e e n l a r g e l y e x p e r i m e n t a l , due to the paucity of p h e n o m e n o l o g i c a l data on liquid m e t a l s y s t e m s . Such s t u d i e s should p r e c e d e the d e v e l o p m e n t of a p p r o p r i a t e t h e o r e t i c a l m o d e l s , which a r e n e c e s s a r i l y m e c h a n i s t i c in approach. The e x p e r i m e n t a l s y s t e m chosen involved g r a p h i t e l a n c e s s u b m e r g e d in c a r b o n - s a t u r a t e d pig i r o n at 1250~ It should a l s o be noted that the study made use of p i l o t - s c a l e , i n d u c t i o n - m e l t i n g f a c i l i t i e s , and this enabled r e a s o n a b l y l a r g e gas flow r a t e s and nozzle d i m e n s i o n s to be used in the work.
these a r e t r e a t e d below, m a n y o b s e r v a t i o n s s t i l l lack q u a n t i t a t i v e explanations, p a r t i c u l a r l y those at high flow r a t e s . C o n s t a n t Volume R e g i m e It has b e e n found that at low flow r a t e s of gas through s u b m e r g e d n o z z l e s , the v o l u m e of the bubble at r e l e a s e , Vbm , is d e t e r m i n e d by a b a l a n c e b e t w e e n its buoyancy f o r c e i n the liquid and those s u r f a c e t e n s i o n f o r c e s c o n s t r a i n i n g it to the i n n e r c i r c u m f e r ence of the nozzle, 1'2 hence Vbm = 7rDnia. Pg
T h e s e p r e d i c t e d bubble v o l u m e s apply to a q u e o u s - t y p e s y s t e m s in which the liquid wets the nozzle m a t e r i a l . However, for nonwetting, m e t a l l i c - t y p e s y s t e m s , the b u b b l e s g e n e r a l l y tend to f o r m at the o u t e r c i r c u m -
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NON-WETTING
THEORY Although s e v e r a l a s p e
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