Coal Desulfurization

8 Dry Table—Pyrite Removal from Coal D. C. WILSON

Downloaded by HARVARD UNIV on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch008

FMC Corp., Santa Clara, CA 95052

Particle segregation within a flowing granular bed during passage through material handling equipment or during transfer from one piece of equipment to another is commonplace. The most readily observable of these is size segregation. Usually such segregations are undesirable in a system because of the potential of unstable processing and an interruption of product continuity. Three factors determine the extent of particle segregation in a moving particle bed: the physical configuration of the material handling equipment, the forces which convey the particles through that equipment, and the differences between the particles in one or more of their physical properties (size, shape, bulk density, resiliency and surface roughness). The equipment described below, specifically designed as a separator for dry particulates, combines these factors to exploit the inherent segregation within moving particle beds. The equipment can be used to remove pyrite and other ash-forming minerals from coal; experimental results of cleaning bituminous and subbituminous coals with it are reported. Equipment D e s c r i p t i o n Figure 1 contains a p e r s p e c t i v e drawing o f the d r y t a b l e , being developed by FMC Corporation, and a c r o s s s e c t i o n through the u n i t i l l u s t r a t i n g the p a r t i c l e bed. The d r i v e u n i t f o r the t a b l e shown i s an electromechanical e x c i t e r o f the type used f o r v i b r a t i n g feeders. In f a c t , t h i s r e c e n t l y developed c o a l c l e a n ing u n i t i s a feeder, but with the f o l l o w i n g design d i f f e r e n c e s : (1) The deck surface i s short but very wide. (2) The c o a l i s i n s e r t e d a t one side o f the feeder's deck. (3) The conveying f o r c e i s reversed; i t feeds the m a t e r i a l i n t o the t a b l e ' s backwall. (4) The p a r t i c l e bed's net flow i s from one side o f the feeder t o the other s i d e . (5) The deck i s nonsymmetrical about the v e r t i c a l plane passing through i t s center o f g r a v i t y and the e x c i t e r ' s l i n e o f 101

In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

102

DESULFURIZATION


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Figure 1.

Dry table principle; schematic view


8.

WILSON

Dry Table—Pyrite Removal from Coal

drive. C o a l i s f e d o n t o t h e l o n g e s t s i d e o f t h e u n i t , and t h e c o n v e y i n g f o r c e f r o m t h e d r i v e moves t h e p a r t i c l e s t o w a r d s t h e backwall. A l a r g e p i l e of p a r t i c l e s forms a g a i n s t the b a c k w a l l , f i l l i n g the e n t i r e trough. G r a v i t y moves t h e p a r t i c l e s on t h e p i l e s s u r f a c e down t h e open s l o p e as t h e c o n v e y i n g f o r c e c o n t i n u e s to d r i v e the u n d e r l y i n g m a t e r i a l a g a i n s t the b a c k w a l l . The r e s u l t i s t h e c o n t i n u o u s o v e r t u r n i n g o f t h e bed. The p r e s s u r e o f t h e i n c o m i n g f e e d f o r c e s t h e o v e r t u r n i n g bed t o f l o w a c r o s s t h e d e c k away f r o m t h e f e e d s i d e i n a h e l i c a l m o t i o n , and b e c a u s e t h e deck's l e n g t h d i m i n i s h e s (tapers) i n t h i s d i r e c t i o n , the toe of the p i l e i s being c o n t i n u o u s l y d i s c h a r g e d . S i m u l t a n e o u s l y , s i z e and b u l k d e n s i t y s e p a r a t i o n s a r e o c c u r r i n g i n t h e o v e r t u r n i n g bed. The l a r g e o r l o w - d e n s i t y p a r t i c l e s move i n t o a s p i r a l l i n g p a t h t h a t m i g r a t e s t o w a r d t h e t o e o f t h e p i l e ( S e c t i o n Α-A, F i g u r e 1) w h e r e a s t h e s m a l l o r h i g h - d e n s i t y p a r t i c l e s move i n t o a s m a l l e r s p i r a l and c o n c e n t r a t e t o w a r d s t h e backwall. Those p a r t i c l e s t h a t a r e b o t h l a r g e and o f low d e n s i t y ( c o a l ) a d v a n c e p a s t t h e l a r g e and h i g h - d e n s i t y p a r t i c l e s ( r o c k and p y r i t e ) and p r e v a i l i n o b t a i n i n g p o s i t i o n s a t t h e t o e o f t h e pile. A l s o s m a l l p a r t i c l e s of p y r i t e w i l l c o n c e n t r a t e a t the b a c k w a l l i n preference to s m a l l p a r t i c l e s of c o a l . The o v e r a l l r e s u l t i n g d i s c h a r g e f r o m t h e h o r i z o n t a l d e c k p o r t i o n of the u n i t i s a s e r i e s of staggered p a r t i c l e s i z e g r a d a t i o n s o f d i f f e r e n t d e n s i t i e s (when b u l k d e n s i t i e s a r e d i r e c t l y r e l a t e d to apparent d e n s i t i e s ) . To a v o i d t h i s o v e r l a p p i n g o f t h e s i z e g r a d a t i o n s o f t h e r o c k and p y r i t e w i t h t h e c o a l , t h e f e e d t o t h e u n i t i s p r e s i z e d t o d e f i n i t e s i z e r a n g e s . F o r t h e c o a l , r o c k , and p y r i t e s e p a r a t i o n , the g e n e r a l i z e d top s i z e t o bottom s i z e of the f e e d p a r t i c l e s i n any one p a s s i s a 4 - t o - l r a t i o (8 χ 2 i n . , 2 χ 1/2 i n . , e t c . ) . The optimum ratio» however, i s a f u n c t i o n o f b u l k d e n s i t y , s h a p e , r e s i l i e n c y , and s u r f a c e r o u g h n e s s d i f f e r e n c e s b e t w e e n t h e c o a l and r o c k - p y r i t e f o r a p a r t i c u l a r c o a l seam and mining c o n d i t i o n s . The p a r t i c l e s d i s c h a r g e f r o m t h e n e a r l y h o r i z o n t a l d e c k o n t o an a t t a c h e d downward s l o p i n g s u r f a c e r e f e r r e d t o as t h e " d i s c h a r g e lip". T h i s l i p c a n make f u r t h e r s e p a r a t i o n s b a s e d on p a r t i c l e s h a p e , r e s i l i e n c y , and s u r f a c e r o u g h n e s s i f d e s i r e d . The shape s e p a r a t i o n i s b a s e d on t h e c u b i c a l c o a l p a r t i c l e s b e i n g u n s t a b l e on t h e d i s c h a r g e l i p and t h e n e a r t a b u l a r r o c k and p y r i t e p a r t i c l e s b e i n g s t a b l e when t h e u n i t i s v i b r a t i n g . The u n s t a b l e c o a l w i l l t h u s be d i s c h a r g e d by r o l l i n g o f f t h e l i p w h i l e t h e t a b u l a r r o c k and p y r i t e a r e c o n v e y e d b a c k up t h e l i p i n t o t h e p i l e . The s u r f a c e roughness of the h i g h l y m i n e r a l i z e d p a r t i c l e s i s g r e a t e r than that f o r the c l e a n c o a l p a r t i c l e s . T h i s a d d i t i o n a l roughness h e l p s t o c o n v e y t h e r o c k and p y r i t e b a c k i n t o t h e deep p a r t i c l e bed w h e r e a s t h e s l i c k c o a l t e n d s t o s l i p o f f t h e l i p . Generally, the r e s i l i e n c y of the c o a l i s g r e a t e r than t h a t of the r o c k particles. The c o n v e y i n g v i b r a t i o n s c a u s e s t h e more r e s i l i e n t c o a l p a r t i c l e s t o bounce and a s s u r e t h e i r u n s t a b i l i t y on t h e 1


103



104

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discharge l i p . The above d e s c r i b e s t h e m a j o r s e p a r a t i o n s o c c u r r i n g a t s p e c i f i e d l o c a t i o n s on t h e d r y t a b l e . Even t h o u g h p a r t i c l e s h a p e , r e s i l i e n c y , and s u r f a c e r o u g h n e s s d i f f e r e n c e s a r e n o t i n c l u d e d i n t h e d e s c r i p t i o n o f t h e o v e r t u r n i n g p a r t i c l e p i l e , t h e y do c o n t r i bute t o minor s e p a r a t i o n s w i t h i n the p i l e . Likewise, particle b u l k d e n s i t y and s i z e d i f f e r e n c e s a r e i n v o l v e d i n t h e s e p a r a t i o n on t h e l i p , b u t t h e s e a r e o f m i n o r i m p o r t a n c e when compared t o t h e d i f f e r e n c e s i n p a r t i c l e s h a p e , r e s i l i e n c y and s u r f a c e - r o u g h n e s s . W i t h t h e s e p a r a t i o n d r i v i n g f o r c e s o f t h e f i v e known p h y s i c a l property differences operating simultaneously, i t i s highly u n l i k e l y that a l l f i v e are working together to a r r i v e a t the d e s i r e d c o a l c l e a n i n g . T h e r e f o r e , most c o a l c l e a n i n g s e p a r a t i o n s a r e a compromise i n w h i c h t h e d r y t a b l e ' s o p e r a t i n g p a r a m e t e r s a r e a d j u s t e d t o enhance t h e d e s i r e d p h y s i c a l p r o p e r t y s e p a r a t i o n s and depress the undesirable separations. The f o l l o w i n g p h y s i c a l p r o p e r t y d i f f e r e n c e s l i s t d e s c r i b e s how t h e d r y t a b l e p a r a m e t e r c o n t r o l s i n f l u e n c e t h e d e g r e e o f t h e individual separations. Size. For c o a l c l e a n i n g , the feed t o the d r y t a b l e i s pre-screened i n t o s i z e ranges t o minimize the n a t u r a l l y strong s i z e s e g r e g a t i o n e f f e c t s i n a m o v i n g p a r t i c l e b e d . The n a r r o w e r t h e s i z e r a n g e , t h e l e s s s i z e s e g r e g a t i o n e f f e c t on t h e o v e r a l l separation. Systems c a n be d e v i s e d f o r p o s t - s c r e e n i n g b u t a t a s a c r i f i c e of the d r y t a b l e ' s throughput r a t e . Bulk Density. F o r c o a l c l e a n i n g a m o d e r a t e l y deep p i l e i s b e s t , as d e s c r i b e d below. I f t h e f r e q u e n c y and/or a m p l i t u d e i s i n c r e a s e d above a n o r m a l l e v e l , t h e n t h e p a r t i c l e bed becomes q u i t e p o r o u s and a l l p a r t i c l e s w i l l e x h i b i t a l i g h t e r b u l k d e n s i t y , thus reducing the e f f e c t i v e bulk d e n s i t y d i f f e r e n c e s . Shape. F o r w a r d o r b a c k w a r d t i l t i n g o f t h e d r y t a b l e c a u s e s t h e d i s c h a r g e l i p t o be a t d i f f e r e n t a n g l e s r e l a t i v e t o t h e h o r i z o n t a l w h i c h d e t e r m i n e s what s h a p e s a r e r e j e c t e d o r r e t a i n e d o n the t a b l e . A d i s c h a r g e l i p a n g l e o f -12° i s g e n e r a l l y u s e d f o r the s e p a r a t i o n o f c u b i c c o a l from t a b u l a r r o c k - p y r i t e . I f t h e r o c k - p y r i t e i s present as round p a r t i c l e s , then a l e s s steep d i s charge l i p i s used. Resiliency. By i n c r e a s i n g t h e u n i t ' s f r e q u e n c y a n d / o r s t r o k e above t h e n o r m a l l e v e l , t h e more r e s i l i e n t p a r t i c l e s i n t h e bed w i l l e x h i b i t a l o w e r b u l k d e n s i t y i n t h e p i l e s e c t i o n and s i m u l t a n e o u s l y become more u n s t a b l e on t h e l i p s e c t i o n . S u r f a c e R o u g h n e s s . The t y p e o f s u r f a c e f i n i s h on t h e d i s c h a r g e l i p combined w i t h t h e p a r t i c l e ' s own s u r f a c e r o u g h n e s s w i l l d e t e r m i n e i f i t i s d i s c h a r g e d o r r e t a i n e d on t h e t a b l e . A very hard s l i c k s u r f a c e w i l l discharge n e a r l y a l l p a r t i c l e types w h i l e



8.

WILSON

Dry

Table—Pyrite Removal from Coal

105

a very hard rough s u r f a c e w i l l r e t a i n n e a r l y a l l p a r t i c l e types. The t h r o u g h p u t r a t e and bed r e t e n s i o n t i m e a r e c o n t r o l l e d by a l t e r i n g the c r o s s - s l o p e ( s i d e t o s i d e of u n i t ) . I n c r e a s i n g the c r o s s - s l o p e , by l o w e r i n g t h e f e e d s i d e o f t h e u n i t w i t h r e s p e c t t o i t s o p p o s i t e s i d e , w i l l c a u s e l o w e r t h r o u g h p u t r a t e s and l a r g e bed r e t e n s i o n times. There i s a c l o s e r e l a t i o n s h i p between the d r y t a b l e ' s v e r t i c a l s t r o k e component and f r e q u e n c y , w i t h some l i m i t a t i o n s d i c t a t e d by t h e p a r t i c l e s i z e s b e i n g s e p a r a t e d and t h e o p t i m i z a t i o n o f throughput r a t e s . Of o b v i o u s i m p o r t a n c e i s t h e h o r i z o n t a l c o n v e y ance of p a r t i c l e s t o form the o v e r t u r n i n g p i l e . The d e p t h o f t h a t p i l e i s t h e n , t o a l a r g e e x t e n t , d e t e r m i n e d by t h e v e r t i c a l c o n v e y a n c e . And t h e p i l e d e p t h i s i m p o r t a n t b e c a u s e v e r y deep p i l e s favor s i z e s e g r e g a t i o n w h i l e p i l e s of moderate depth favor b u l k density segregations. For c o a l c l e a n i n g , t h e r e f o r e , a f i x e d d r i v e a n g l e o f 30° was s e l e c t e d as t h e b e s t c o m b i n a t i o n o f h o r i z o n t a l and v e r t i c a l c o n v e y a n c e s i n f o r m i n g t h e d e s i r e d p i l e s o f m o d e r a t e d e p t h . The f r e q u e n c y i s r e l a t e d t o t h e s t r o k e ; w i t h i n c r e a s i n g s t r o k e l e n g t h , t h e f r e q u e n c y must be r e d u c e d t o m a i n t a i n t h e d e s i r e d t a b l i n g a c t i v i t y a t the s e l e c t e d d r i v e angle: 2 Stroke

(Frequency)

= Constant

The s t r o k e s h o u l d n o t be l a r g e r t h a n t h e s m a l l e s t p a r t i c l e d i a m e t e r t o be s e p a r a t e d o r p a r t i c l e m i x i n g becomes a f a c t o r i n t h e separation. However, u s i n g t h e l a r g e s t s i z e s t r o k e p o s s i b l e i s d e s i r a b l e because the throughput r a t e i n c r e a s e s w i t h i n c r e a s i n g stroke: 2 Stroke

Coal Desulfurization

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