Coal Desulfurization - ACS Publications

6 Use of the Flotation Process for Desulfurization of Coal

Downloaded by UNIV LAVAL on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch006

F. F. APLAN Mineral Processing Section, Pennsylvania State University, University Park, PA 16802

The flotation process is a method for desulfurizing fine coal whose time has come. The process has proven to be a great success in the beneficiation of a wide variety of metalliferous ores and industrial minerals, and currently, the United States has an installed capacity to treat about 1.7 million tons of ore per day. Unfortunately, coal processing has not participated in this growth in any major way. In the U.S. in 1973 only 14 million tons of clean coal were produced by the flotation process (1) . By comparison, in the same year approximately 400 million tons of coal were treated in preparation plants to produce 300 million tons of clean coal. This amount of coal produced by washing techniques was about half of the 600 million tons of the marketable coal produced in the U.S. that year. There are many reasons why coal flotation has not made a greater impact on the coal industry, but chief among them are the effectiveness of coarse cleaning processes (which treat particles of 1-5 in. top size), the low value of a ton of coal until recent years, the lack of an adequate research and development program to evaluate the process thoroughly, and, to some extent, inertia. To these reasons should be added the following shortcomings of the coal flotation process enumerated by Mitchell (2) in 1948: (1) (2) (3) (4) (5)

M a r k e t i n g problems w i t h t h e f i n e s , Cost of dewatering, I n d i f f e r e n t t e s t r e s u l t s i n removing s u l f u r , I n a b i l i t y t o make c l e a n s e p a r a t i o n s w i t h t h e f i n e r s i z e s , I n a b i l i t y t o clean s l u r r i e s containing a high percentage of c l a y . Two new f a c t o r s , w h i c h have come t o t h e f o r e w i t h i n t h e p a s t d e c a d e , have c a u s e d a s e r i o u s r e - e v a l u a t i o n o f t h e p r o c e s s i n c o a l preparation: t h e s h a r p l y i n c r e a s e d market value o f a t o n o f c o a l and e n v i r o n m e n t a l c o n s i d e r a t i o n s . The i n c r e a s e d c o a l p r i c e h a s l e d t o a g r e a t l y i n c r e a s e d e m p h a s i s on f i n e c o a l c l e a n i n g i n o r d e r to achieve a g r e a t e r y i e l d of combustible m a t e r i a l from a t o n o f raw c o a l . On t h e e n v i r o n m e n t a l f r o n t c o a l f l o t a t i o n h e l p s t o c l a r i f y t h e l a r g e q u a n t i t i e s o f w a t e r u s e d and r e c y c l e d i n t h e

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


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Flotation Process for Coal Desulfurization

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p r e p a r a t i o n p r o c e s s and i n t h e r e m o v a l o f p y r i t i c s u l f u r f r o m t h e coal. While f o r coarse c o a l , g r a v i t y concentration processes are very e f f i c i e n t , t h e i r f i n e c l e a n i n g counterparts are not n e a r l y as e f f e c t i v e i n e i t h e r c o a l r e c o v e r y o r i n p y r i t e r e m o v a l . The c o a l f l o t a t i o n process o f f e r s the p o t e n t i a l of overcoming both of t h e s e d e f e c t s f o r t h e t r e a t m e n t o f t h e f i n e , -28 mesh c o a l . In t h i s r e s p e c t i t c a n r e d u c e t h e s u l f u r c o n t e n t o f some c o a l s t o EPAa c c e p t a b l e l e v e l s o r , f o r h i g h e r s u l f u r c o a l s , can reduce the o v e r a l l s u l f u r c o n t e n t o f t h e c o a l s u c h t h a t t h e n e e d f o r f l u e gas d e s u l f u r i z a t i o n a t t h e power p l a n t c a n be m i n i m i z e d . Where a p p l i c a b l e , c o a l p r e p a r a t i o n t e c h n i q u e s c a n remove p y r i t i c s u l f u r f o r a f r a c t i o n o f t h e c o s t o f s u l f u r r e m o v a l by a f l u e gas s c r u b b i n g system. I n a r e c e n t a r t i c l e H o f f m a n e t a l . C3), have shown t h e e c o n o m i c p o t e n t i a l o f s u c h c o m b i n a t i o n c o a l p r e p a r a t i o n and f l u e gas d e s u l f u r i z a t i o n t e c h n i q u e s . The c o a l f l o t a t i o n p r o c e s s has b e e n d e t a i l e d i n t h e l i t e r a t u r e i n r e v i e w a r t i c l e s by A p l a n ( 4 ) , Brown ( 5 ) , and Zimmerman ( 6 ) . U n f o r t u n a t e l y , s i n c e the h i s t o r i c a l purpose of c o a l p r e p a r a t i o n has b e e n t o remove t h e h i g h - a s h c o n s t i t u e n t s , t h e l i t e r a t u r e does n o t c o n t a i n a g r e a t d e a l o f i n f o r m a t i o n on s u l f u r r e d u c t i o n by flotation. F o r t h i s r e a s o n , t h e M i n e r a l P r o c e s s i n g S e c t i o n a t The P e n n s y l v a n i a S t a t e U n i v e r s i t y s e v e r a l y e a r s ago embarked on a c o m p r e h e n s i v e p r o g r a m aimed a t u n d e r s t a n d i n g t h e f u n d a m e n t a l s o f the process, d e l i n e a t i n g the best c o n d i t i o n s f o r r e j e c t i n g p y r i t i c s u l f u r d u r i n g c o a l f l o t a t i o n , and e s t a b l i s h i n g a d a t a b a s e . T h i s c h a p t e r w i l l e m p h a s i z e t h a t work. G e n e r a l Methods of P y r i t e Removal T h e r e a r e t h r e e g e n e r a l methods t h a t may be u s e d t o r e j e c t p y r i t i c s u l f u r during coal f l o t a t i o n : (1) m u l t i p l e c l e a n i n g , (2) p y r i t e d e p r e s s i o n , and (3) c o a l d e p r e s s i o n w i t h c o n c u r r e n t f l o t a t i o n of the p y r i t e . The f i r s t p r o c e s s i s b a s e d on t h e a s s u m p t i o n t h a t c o a l i s e a s i l y f l o a t a b l e whereas p y r i t e i s not. I n t h i s c a s e any p y r i t e w h i c h f l o a t s w o u l d be c a u s e d by m e c h a n i c a l e n t r a p m e n t w i t h t h e floating coal. I t c o u l d t h u s be removed by r e p u l p i n g t h e f l o a t e d c o a l and r e p e a t i n g t h e f r o t h f l o t a t i o n p r o c e s s s e v e r a l t i m e s ; e a c h t i m e r e j e c t i n g some of t h e m e c h a n i c a l l y e n t r a p p e d p y r i t e p a r t i c l e s . The a s s u m p t i o n t h a t p y r i t e i s n o t f l o a t a b l e i s t r u e o n l y as a gross g e n e r a l i z a t i o n , s i n c e , i n p r a c t i c e , f i n e p y r i t i c s u l f u r o f t e n f l o a t s w i t h the c o a l . The f l o t a t i o n o f p y r i t e o c c u r s n o t o n l y by i t s e l f , b u t i t s f l o t a t i o n i s o f t e n e n c o u r a g e d by t h e e x i s t a n c e o f l o c k e d p y r i t e - c o a l p a r t i c l e s o r by t h e u s e o f an o i l y c o l l e c t o r f o r the f l o t a t i o n of the c o a l . Early-day ore f l o t a t i o n p r o c e s s e s , c i r c a 1905, u s e d f u e l o i l i n t h e f l o t a t i o n o f s u l f i d e minerals including pyrite. S t u d i e s by M i l l e r e t a l . (7) and more r e c e n t l y by Im (8) show t h a t t h e m u l t i p l e c l e a n i n g method i s n o t o n l y l a b o r i o u s but o f t e n not p a r t i c u l a r l y e f f e c t i v e . The f r o t h s p r i n k l i n g method, a l s o e v a l u a t e d by M i l l e r (9) i s a m u t a t i o n o f



72

C O A L DESULFURIZATION

the m u l t i p l e c l e a n i n g p r o c e s s . P y r i t e d e p r e s s i o n d u r i n g c o a l f l o t a t i o n w o u l d a p p e a r t o be t h e most l o g i c a l a p p r o a c h t o t h e p r o b l e m e s p e c i a l l y i n v i e w o f t h e f a c t t h a t p y r i t e d e p r e s s i o n t e c h n i q u e s d u r i n g the f l o t a t i o n of c o p p e r , l e a d , n i c k e l , and z i n c s u l f i d e s a r e w e l l known ( 1 0 ) . In c o a l f l o t a t i o n use has been made of pH c o n t r o l , l i m e , s o d i u m c y a n i d e , s o d i u m s u l f i d e , h y d r o s u l f i d e and s u l f i t e , p o t a s s i u m d i c h r o m a t e , p o t a s s i u m p e r m a n g a n a t e , and f e r r o u s and f e r r i c s u l f a t e t o r e j e c t p y r i t e ( 4 ) . The c l a s s i c w o r k i n t h i s a r e a i s t h a t o f Y a n c e y and T a y l o r ( 1 1 ) , and more r e c e n t w o r k has b e e n done by M i l l e r (12). Zimmerman (13) d e m o n s t r a t e d t h a t t h e s u l f u r c o n t e n t o f c l e a n c o a l p r o d u c e d by f l o t a t i o n o f P i t t s b u r g h Seam c o a l d e c r e a s e s as t h e pH i s r a i s e d f r o m 4 t o 11. U n f o r t u n a t e l y , the r e c o v e r y o f c l e a n c o a l d e c r e a s e s as t h e pH i s i n c r e a s e d above pH 7. The p r a c t i c a l i t y o f t h e s i t u a t i o n i s t h a t none o f t h e s e t e c h n i q u e s a r e u s e d i n d u s t r i a l l y , and l a b o r a t o r y t e s t i n g o f t e n shows m e d i o c r e r e s u l t s a t b e s t . T r u l y , t h i s i s an a r e a where an expanded r e s e a r c h and d e v e l o p m e n t e f f o r t i s c l e a r l y n e e d e d . The t h i r d method o f p y r i t e r e j e c t i o n i s t h a t o f c o a l d e p r e s s i o n and has r e c e n t l y been t e s t e d by t h e U.S. B u r e a u o f M i n e s (14) i n a process c a l l e d the two-stage or reverse f l o t a t i o n process. I t i n v o l v e s a f i r s t - s t a g e (rougher) f l o t a t i o n of the c o a l to r e j e c t t h e b u l k o f t h e a s h - c o n t a i n i n g c o n s t i t u e n t s i n c l u d i n g much o f t h e pyrite. The r o u g h e r f r o t h i s t h e n s e n t t o a c l e a n e r c i r c u i t where a p r o p r i e t a r y d e p r e s s a n t ( A m e r i c a n Cyanamid R e a g e n t 633) i s added t o d e p r e s s t h e c o a l w h i l e a x a n t h a t e c o l l e c t o r and a f r o t h e r a r e added t o f l o a t p y r i t e f r o m t h e c o a l . The p r o c e s s has r a t h e r s t r i k i n g l y reduced the s u l f u r l e v e l of the c l e a n c o a l a l t h o u g h the p r o b l e m s o f e x c e s s i v e r e a g e n t u s e and c o s t and i n a d e q u a t e r e c o v e r y of c l e a n c o a l w i l l r e q u i r e a d d i t i o n a l study. T h e r e i s a need t o e v a l u a t e c o a l d e p r e s s i n g a g e n t s , and t h e s e s t u d i e s a r e p r e s e n t l y in progress (8). Response of P y r i t e i n C o a l F l o t a t i o n Systems Because of the d e a r t h of d a t a c o n c e r n i n g the response of p y r i t e i n a c o a l f l o t a t i o n c i r c u i t , t h e c u r r e n t p r o g r a m a t Penn S t a t e has f o c u s e d on e s t a b l i s h i n g a p r o p e r d a t a b a s e . F a c t o r s t o be c o n s i d e r e d i n a d d i t i o n t o t h e n a t u r e o f t h e c o a l a r e t h e amount and k i n d o f f r o t h e r , q u a n t i t y and t y p e o f c o l l e c t o r ( i f a n y ) , t y p e o f f l o t a t i o n c e l l , and o p e r a t i n g c o n d i t i o n s . The f o l l o w i n g r e p o r t on t h e e x p e r i m e n t a l p r o g r a m i n p r o g r e s s e v a l u a t e s many o f t h e s e factors. E x p e r i m e n t a l Methods. A l l of the work r e p o r t e d h e r e , e x c e p t t h a t i n t h e f i r s t f i g u r e , has b e e n done w i t h c l e a n c o a l f r o m t h e Lower K i t t a n n i n g o r P i t t s b u r g h Seam t o w h i c h 5% p u r i f i e d p y r i t e has b e e n added. R e c e n t w o r k has shown (15) t h a t p y r i t e s a m p l e s from d i f f e r e n t c o a l sources possess d i f f e r e n t f l o a t a b i l i t i e s (which, i n t u r n , are d i f f e r e n t from the f l o a t a b i l i t y of ore p y r i t e ) .



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73

T h e r e f o r e , t h e p y r i t e u s e d i n e a c h t e s t came f r o m t h e same seam as d i d t h e c o a l . T h i s p r o c e d u r e e l i m i n a t e s l o c k e d p a r t i c l e s so t h a t t h e p a t h o f t h e p y r i t e d u r i n g f l o t a t i o n c a n be a c c u r a t e l y t r a c e d , u n c o m p l i c a t e d by t h e i n t e r m e d i a t e r e s p o n s e o f p a r t i c l e s t h a t a r e p a r t c o a l and p a r t p y r i t e . O t h e r s t u d i e s a r e underway t o e v a l u a t e the response of l o c k e d p a r t i c l e s i n c o a l f l o t a t i o n systems. C o a l f r o m t h e Lower K i t t a n n i n g seam ( C a m b r i a C o u n t y , PA.) o r t h e P i t t s b u r g h seam ( F a y e t t e C o u n t y , PA.) was p u l v e r i z e d t o 100% -14 mesh, 90% -28 mesh, t o w h i c h -28 mesh p u r i f i e d p y r i t e f r o m e a c h r e s p e c t i v e seam was added. C o a r s e r p y r i t e was n o t added s i n c e R a s t o g i and A p l a n (16) have d e m o n s t r a t e d t h a t +28 mesh p y r i t e w i l l not f l o a t under the c o n d i t i o n s n o r m a l l y encountered i n f r o t h flotation. To i n s u r e t h a t s u r f a c e o x i d a t i o n d i d n o t i n f l u e n c e t h e r e s u l t s , t h e c o a l and p y r i t e s a m p l e s w e r e g r o u n d s h o r t l y b e f o r e flotation. The o l d e r s t y l e F a g e r g r e n l a b o r a t o r y f l o t a t i o n c e l l was u s e d , and t h e p u l p c o n c e n t r a t i o n was a b o u t 17% s o l i d s . The s p e c i f i c d a t a g i v e n i n t h i s r e p o r t w e r e drawn f r o m s t u d i e s on two d i f f e r e n t c o a l seams and t h u s a r e i n t e n d e d t o i l l u s t r a t e g e n e r a l p r i n c i p l e s r a t h e r t h a n t o s e r v e as a b a s i s f o r a f l o t a t i o n scheme f o r a g i v e n c o a l sample. I n u s i n g t h e d a t a , one c a u t i o n s h o u l d be n o t e d : s i n c e 95% c l e a n c o a l and 5% p y r i t e were u s e d , t h e maximum y i e l d o f c o m b u s t i b l e m a t e r i a l i s 95%. Y i e l d s a p p r o a c h i n g and e x c e e d i n g t h i s v a l u e a r e most c e r t a i n l y a c c o m p a n i e d by t h e f l o t a t i o n o f much l i b e r a t e d p y r i t i c s u l f u r (SLP). Coal F l o t a t i o n K i n e t i c s . H i s t o r i c a l l y , the r e s u l t s of s o f t c o a l p r e p a r a t i o n h a v e been most o f t e n a n a l y z e d i n t e r m s o f y i e l d and o f p r o d u c t a s h and, more r e c e n t l y , s u l f u r . As a r e s u l t o f o u r e x t e n s i v e s t u d i e s on t h e n a t u r e o f c o a l f l o t a t i o n , h o w e v e r , we b e l i e v e t h a t t h e r a t e o f c o a l f l o t a t i o n s h o u l d be added t o t h e l i s t of e v a l u a t i o n c r i t e r i a used f o r l a b o r a t o r y s t u d i e s . I t has b e e n amply d e m o n s t r a t e d (16) t h a t d i f f e r e n t f r a c t i o n s i n t h e raw c o a l f l o a t a t d i f f e r e n t r a t e s , and t h u s t h e c o a l f l o t a t i o n r a t e i s a v a r i a b l e t h a t c a n be u s e d t o a c h i e v e b e t t e r s e p a r a t i o n s , most p a r t i c u l a r l y b e t w e e n c o a l and p y r i t e . Coal f l o t a t i o n f o l l o w s a f i r s t - o r d e r r a t e equation (16): - £ = k C at

(1)

where C = c o n c e n t r a t i o n o f f l o a t a b l e m a t e r i a l ; t = t i m e , s e c ; k = r a t e constant, sec""l. This agrees w i t h the f i n d i n g s of B u s h e l l f o r the f l o t a t i o n of q u a r t z w i t h an amine (17) and t h e f i n d i n g s o f numerous o t h e r i n v e s t i g a t i o n s f o r ores (18). The e x p e r i m e n t a l a p p r o a c h was t o use a b a t c h f l o t a t i o n method and t o a p p l y t h e a n a l y s i s p r o p o s e d by B u s h e l l (17). F i g u r e 1 f o r t h e f l o t a t i o n o f P i t t s b u r g h Seam c o a l w i t h MIBC, shows t h a t c o a l f l o t a t i o n f o l l o w s t h e c u r v e AB3C3, and t h i s c u r v e i s a s t r a i g h t l i n e , and h e n c e f o l l o w s E q u a t i o n 1, o v e r t h e segment A-B3. T h i s c o r r e s p o n d s t o t h e f l o t a t i o n o f 80% o f t h e


COAL

DESULFURIZATION


100

TIME, MINUTES American Institute of Mining, Metallurgical and Petroleum Engineers

Figure 1. The rate of flotation of Pittsburgh seam coal using 0.84 lb/ton pine oil. Percentage remaining in the flotation cell as a function of time (16).


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c o a l , and, i n f a c t , there i s o n l y s l i g h t d e v i a t i o n from l i n e a r i t y f o r t h e f l o t a t i o n o f 90% o f t h e c o a l . The f i r s t - o r d e r r a t e c o n s t a n t , i n s e c ~ l , may be d e t e r m i n e d g r a p h i c a l l y f r o m p l o t s s u c h a s t h o s e g i v e n i n F i g u r e 1 o r may be c a l c u l a t e d o v e r t h e s t r a i g h t l i n e p o r t i o n o f t h e c u r v e by t h e f o l l o w i n g equation: log k = 2.303

C

- log C Ν — ~

(2)

where = concentration of material at t ^ ; C = concentration of material at t . From F i g u r e 1 i t may be s e e n t h a t t h e f l o t a t i o n o f t h e a s h b e a r i n g c o n s t i t u e n t s and t h e l i b e r a t e d p y r i t i c s u l f u r (SLP) a l s o f o l l o w s the f i r s t - o r d e r r a t e law over the f i r s t minute t o minute and o n e - h a l f o f f l o t a t i o n . This period corresponds to the f l o t a t i o n o f n e a r l y a l l o f t h e c o a l p r e s e n t . Numerous o t h e r f l o t a t i o n t e s t s w i t h a v a r i e t y of c o a l samples, f r o t h e r s , c o l l e c t o r s , p a r t i c l e s i z e s , and o p e r a t i n g c o n d i t i o n s c o n f i r m t h e a c c u r a c y o f t h i s i n t e r p r e t a t i o n ( 1 5 , 1 6 , 1 9 , 2 0 ) . The s l o w f l o a t i n g f r a c t i o n t h a t f l o a t i n g a f t e r ^ 90 s e c - i s composed o f c o a r s e r p a r t i c l e s , t h o s e whose s u r f a c e i s s l i g h t l y o x i d i z e d , l o w e r r a n k and h i g h a s h f r a c t i o n s s u c h a s f u s i n i t e , a n d , i n raw c o a l s y s t e m s , l o c k e d particles. As w i l l be d e m o n s t r a t e d l a t e r , k i n e t i c a n a l y s i s i s a v a l u a b l e a i d i n e v a l u a t i n g t h e c o m p l i c a t e d i n t e r a c t i o n s between c o a l t y p e , f r o t h e r s y s t e m , and f l o t a t i o n o p e r a t i n g c o n d i t i o n s . 2


2

E f f e c t o f F r o t h e r Systems. A comparison o f f r o t h e r s i s g i v e n i n T a b l e I (15) s h o w i n g t h e y i e l d ( a f t e r 7.75 m i n o f f l o t a t i o n t i m e ) , p e r c e n t a g e o f l i b e r a t e d p y r i t i c s u l f u r (% S L P ) , and t h e amount o f c o a r s e , 14 χ 28 mesh, c o a l f l o a t e d . An e q u i m o l a r c o n c e n t r a t i o n o f e a c h f r o t h e r was c h o s e n a s t h e b a s i s o f c o m p a r i s o n . TABLE I .

F r o t h e r C o m p a r i s o n f o r t h e F l o t a t i o n o f Lower K i t t a n n i n g and P i t t s b u r g h Seam C o a l s ( 1 5 ) .

C o a l Type

Lower Kittanning Pittsburgh Seam

Frother

MIBC^ Pine O i l Cresol C

MIBC Pine O i l Cresol

Concentration mol lb. L. ton

Floated mat'l. Yield, %

h

SLP

+28 mesh

-4 2.4x10" 2.4x10"-4 2.4x10"-4

0.28 0.44 0.29

94 96 69

—

92 91 24

-4 2.4x10" 2.4x10"-4 2.4x10"-4

0.28 0.44 0.29

84 89 19

16 21 2

29 48 2

20 33

C o a l y i e l d a f t e r 7.75 m i n o f f l o t a t i o n . b SLP = L i b e r a t e d p y r i t i c s u l f u r . ° MIBC = M e t h y l i s o b u t y l c a r b i n o l , 4 - m e t h y l - 2 - p e n t a n o l .


%


76


From t h i s t a b l e s e v e r a l c o n c l u s i o n s emerge: (1) MIBC and p i n e o i l a r e r o u g h l y e q u i v a l e n t f r o t h e r s on an e q u i m o l a r b a s i s , b u t MIBC i s s u p e r i o r on a pounds p e r t o n b a s i s (2) C r e s o l i s a much i n f e r i o r f r o t h e r t o e i t h e r MIBC o r p i n e oil (3) P i n e o i l t e n d s t o f l o a t s l i g h t l y more p y r i t e t h a n does MIBC (4) P i n e o i l i s a s u p e r i o r f r o t h e r f o r the f l o t a t i o n of c o a r s e r c o a l p a r t i c l e s , e s p e c i a l l y f o r an i n t e r m e d i a t e f l o a t i n g c o a l s u c h as t h e P i t t s b u r g h seam (5) The Lower K i t t a n n i n g c o a l i s a f a s t e r f l o a t i n g c o a l t h a n i s t h a t f r o m t h e P i t t s b u r g h seam T h e s e a r e o n l y g e n e r a l c o n c l u s i o n s , and t h e n a t u r e o f t h e c o a l and t h e n a t u r e and c o n c e n t r a t i o n o f t h e f r o t h e r may w e l l change t h e o r d e r o f t h i n g s . Most c r i t i c a l i s the problem of i m p u r i t i e s w h i c h t e n d t o f l o a t p y r i t e . The p r o b l e m o f i m p u r e c r e s o l s i s p a r t i c u l a r l y c r i t i c a l ; t h e o i l y i m p u r i t i e s a c t as c o l l e c t o r s and, h e n c e , f a v o r t h e f l o t a t i o n o f c o a r s e and o t h e r d i f f i c u l t - t o - f l o a t p a r t i c l e s — b u t a t the expense of encouraging the f l o t a t i o n of u n d e s i r a b l e p y r i t e . O i l has l o n g b e e n u s e d t o enhance t h e f l o t a t i o n o f c o a l p a r t i c l e s otherwise d i f f i c u l t to f l o a t w i t h a f r o t h e r only. The i n t r o d u c t i o n o f an o i l y c o l l e c t o r i s , h o w e v e r , a c c o m p a n i e d by t h e f l o t a t i o n o f much u n d e s i r a b l e p y r i t e . T h i s i s c l e a r l y i l l u s t r a t e d i n T a b l e I I (21) w h e r e t h e i n t r o d u c t i o n o f 0.46 l b . / t o n o f f u e l TABLE I I .

E f f e c t o f F u e l O i l on I n c r e a s i n g t h e F l o t a t i o n o f P y r i t i c S u l f u r D u r i n g t h e F l o t a t i o n o f Lower K i t t a n n i n g Coal (21). 3 Flotation

MIBC lb./ton 0.17 0.17

Fuel O i l lb./ton 0 0.46

r a t e χ 10

,sec

-1 Yield,

k

C

37.5 55

k

SLP 2.3 12.4

a

SLP

floated,

%

kc/^SLP

90 97

16 4

% 13 56

C o a l y i e l d a f t e r 7.75 min o f f l o t a t i o n . k£ = F l o t a t i o n r a t e c o n s t a n t f o r c o a l . o i l t o a c o a l f l o t a t i o n s y s t e m u s i n g MIBC as f r o t h e r i n c r e a s e d the amount o f t h e l i b e r a t e d p y r i t i c s u l f u r (SLP) w h i c h f l o a t e d f r o m 13 t o 5 6 % ! ! ! I n t h i s c a s e t h e k i n e t i c a n a l y s i s i s h e l p f u l . Numerous o t h e r d a t a (15,16,19) have e s t a b l i s h e d t h a t c o a l g e n e r a l l y f l o a t s a b o u t 10-30 t i m e s f a s t e r t h a n d o e s p y r i t e ; t h e e x a c t v a l u e o f t h e r a t i o depends on t h e s p e c i f i c c o a l and p y r i t e and on t h e f r o t h e r s y s t e m and f l o t a t i o n o p e r a t i n g c o n d i t i o n s . An i m p o r t a n t e v a l u a t i o n c r i t e r i o n i s thus to l o o k at the r a t i o of the f l o t a t i o n r a t e c o n s t a n t of c o a l to t h a t of p y r i t e , k / k . As t h e v a l u e o f p

q T p


6.


APLAN

77

t h i s r a t i o d i f f e r s f r o m an a v e r a g e v a l u e o f , s a y , 16, t h o s e f l o t a t i o n c o n d i t i o n s e i t h e r f a v o r a b l e or unfavorable f o r p y r i t e r e j e c t i o n may be r e a d i l y d i s c e r n e d . I t c a n be s e e n f r o m T a b l e I I t h a t w i t h the a d d i t i o n of f u e l o i l , the c o a l f l o t a t i o n r a t e i n c r e a s e d somewhat b u t t h a t t h e p y r i t e f l o t a t i o n r a t e i n c r e a s e d f i v e - f o l d ; t h e k ç / k s p r a t i o d r o p p e d t o 4. O i l encourages the f l o t a t i o n o f p y r i t e much more t h a n i t d o e s t h a t o f c o a l i n t h i s instance.


L

E f f e c t of F r o t h e r Concentration. F i g u r e s 2 and 3 show t h e e f f e c t o f an i n c r e a s i n g amount o f t h e f r o t h e r , p i n e o i l , on c o a l y i e l d and t h e amount of l i b e r a t e d p y r i t i c s u l f u r (SLP) f l o a t e d as a f u n c t i o n of f l o t a t i o n time (22). With respect to y i e l d (Figure 2 ) , t h e r e i s a t h r e s h o l d amount o f f r o t h e r needed t o o b t a i n good coal yields. Beyond t h i s i n c i p i e n t amount o f f r o t h e r , h o w e v e r , a d d i t i o n a l f r o t h e r does n o t f u r t h e r i n c r e a s e y i e l d w i t h t h i s c o a l . The same p a t t e r n h o l d s f o r a f l o t a t i o n t i m e o f 0.25 o r o f 7.75 min. A 1-min f l o t a t i o n t i m e i s s u f f i c i e n t t o a c h i e v e n e a r l y c o m p l e t e y i e l d f o r f r o t h e r c o n c e n t r a t i o n s o f 0.5 l b . / t o n o r more. However, w i t h the f l o t a t i o n of p y r i t e (Figure 3 ) , a d i f f e r e n t p a t t e r n emerges: b o t h an e x c e s s f r o t h e r d o s a g e and e x t e n d e d f l o t a t i o n t i m e s l e a d t o t h e f l o t a t i o n o f an e v e r - i n c r e a s i n g amount o f p y r i t e . The a d v a n t a g e o f u s i n g s h o r t f l o t a t i o n t i m e s and a low f r o t h e r concentration i s obvious.

three

E f f e c t of F l o t a t i o n Operating Conditions. T a b l e I I I compares l e v e l s o f a i r r a t e and f l o t a t i o n c e l l i m p e l l e r s p e e d ( 1 6 ) .

TABLE I I I .

E f f e c t o f A i r F l o w R a t e and I m p e l l e r Speed on F l o t a t i o n o f P i t t s b u r g h Seam C o a l ( 1 6 ) .

C e l l Operating Conditions air rate speed (cfm) (rpm)

3 -1 F l o t . r a t e χ 10 , sec

Flotation

the

Data^

a

0.535 0.233 0.136

2540 1660 1380

.

h k

73 25.5 15.3

SLP

4.5

15.9

0.2

76

Yield

% SLP floated

92.4 79.1 67.8

27.9 17.7 1.4

C

% + 28M floated 70.3 29.8 15.0

MIBC 0.57 l b . / t o n . b kç = F l o t a t i o n r a t e c o n s t a n t f o r c o a l . ° SLP = L i b e r a t e d p y r i t i c s u l f u r ^ One m i n u t e f l o t a t i o n t i m e . I n t e n s e f l o t a t i o n c o n d i t i o n s ( h i g h a e r a t i o n r a t e s and h i g h i m p e l l e r speed) f a v o r t h e f a s t f l o t a t i o n o f c o a l , t h e f l o t a t i o n o f c o a r s e p a r t i c l e s , and t h e f l o t a t i o n o f p y r i t e . A g a i n , u s i n g t h e r a t i o o f t h e f l o t a t i o n r a t e c o n s t a n t s f o r c o a l and p y r i t e , k ç / k g L p , t h e maximum r e j e c t i o n o f p y r i t e o c c u r s u n d e r g e n t l e o p e r a t i n g


DESULFURIZATION


COAL

American Institute of Mining, Metallurgical and Petroleum Engineers

Figure 2. (top) Flotation yield of Lower Kittanning coal as a function of pine oil concentration and time (22) American Institute of Mining, Metallurgical and Petroleum Engineers

Figure 3. (bottom) Effect of pine oil dosage on the amount of liberated pyritic sulfur floated. Lower Kittanning coal (22).


6.

APLAN


79


conditions. Long f l o t a t i o n r e s i d e n c e t i m e s f a v o r t h e f l o t a t i o n o f b o t h c o a r s e p a r t i c l e s and p y r i t e . R a s t o g i and A p l a n (4,16) have d e m o n s t r a t e d t h a t t h e a i r r a t e alone i s r e s p o n s i b l e f o r the bulk of the increase i n c o a l f l o t a tion. The c o n t r i b u t i o n o f i m p e l l e r speed i s modest. T e s t E v a l u a t i o n . A s t u d y o f numerous f l o t a t i o n t e s t s shows t h a t t h e r e i s a r e l a t i o n s h i p b e t w e e n y i e l d and t h e amount o f p y r i t e w h i c h f l o a t s ( F i g u r e 4) ( 4 , 2 3 ) . This curve i s s i m i l a r to t h e g r a d e - r e c o v e r y c u r v e s o f t e n u s e d i n o r e d r e s s i n g s t u d i e s (24) and t o c o a l - s u l f u r d a t a r e p o r t e d by M i l l e r ( 1 2 ) . E a c h p a r t i c u l a r f l o t a t i o n s y s t e m shows a s i m i l a r f a m i l y o f c u r v e s . A s h , t o t a l s u l f u r , o r f l o t a t i o n r a t e may a l s o be u s e d as t h e a b s c i s s a and s i m i l a r l y shaped c u r v e s r e s u l t ( 4 , 2 3 ) . T h i s method may be u s e d to save c o n s i d e r a b l e time i n f l o t a t i o n t e s t i n g s i n c e s u l f u r v a l u e s a t i n t e r m e d i a t e y i e l d s may be r e a d i l y e s t i m a t e d w i t h o u t t h e need t o r e s o r t to s t i l l another t e s t . The b u l k o f t h e f r e e p y r i t e t h a t f l o a t s a c c o m p a n i e s t h e l a s t 20% o r so o f y i e l d . T h i s f a c t has b e e n u s e d t o d e v e l o p a c o a l f l o t a t i o n p r o c e d u r e c a l l e d t h e " g r a b and run" technique. The "Grab and Run" P r o c e s s . I n t h i s method a p p r o x i m a t e l y one-half to t w o - t h i r d s of the u l t i m a t e c o a l y i e l d i s f l o a t e d i n the rougher c e l l under g e n t l e o p e r a t i n g c o n d i t i o n s ( s t a r v a t i o n q u a n t i t i e s o f a g e n t l e f r o t h e r s u c h as MIBC, l o w a i r r a t e , l o w i m p e l l e r s p e e d , s h o r t f l o t a t i o n r e s i d e n c e t i m e , e t c . ) t o remove a f r a c t i o n low i n p y r i t e . U n d e r most c o n d i t i o n s s t u d i e d so f a r , t h i s f a s t - f l o a t i n g f r a c t i o n c a n go d i r e c t l y t o t h e c l e a n c o a l p r o d u c t ; hence t h e name " g r a b and r u n . " The b a l a n c e o f t h e r e c o v e r a b l e c o a l i s removed i n a s c a v e n g e r c i r c u i t u n d e r more i n t e n s e o p e r a t i n g c o n d i t i o n s . As t h e u l t i m a t e y i e l d i s a p p r o a c h e d , t h e amount o f p y r i t i c s u l f u r t h a t f l o a t s i n c r e a s e s g r e a t l y . The scavenger c o n c e n t r a t e , which i s h i g h e r i n p y r i t i c s u l f u r than the rougher c o n c e n t r a t e , i s sent to c l e a n e r f l o t a t i o n . H e r e two options are a v a i l a b l e : d e p r e s s p y r i t e and f l o a t c o a l o r v i c e versa. In the l a t t e r o p t i o n the scavenger c l e a n e r c i r c u i t r e s e m b l e s t h e U.S. B u r e a u o f M i n e s r e v e r s e f l o t a t i o n p r o c e s s e x c e p t t h a t o n l y a f r a c t i o n o f t h e t o t a l t o n n a g e n e e d be t r e a t e d . D e t a i l s o f t h i s p r o c e s s w i l l be r e p o r t e d s o o n ( 2 5 ) . Summary. P y r i t e i s b e s t r e j e c t e d u n d e r g e n t l e f r o t h i n g and o p e r a t i n g c o n d i t i o n s u s i n g short residence times i n the f l o t a t i o n cell. The u s e o f e x c e s s f r o t h e r and, e s p e c i a l l y an o i l y f r o t h e r and/or c o l l e c t o r , i s p a r t i c u l a r l y c o u n t e r - p r o d u c t i v e t o p y r i t i c sulfur rejection. The l a s t i n c r e m e n t f o r t h e r e c o v e r y o f c o a r s e o r o t h e r h a r d - t o - f l o a t p a r t i c l e s i s i n v a r i a b l y a c c o m p a n i e d by t h e f l o t a t i o n o f much p y r i t i c s u l f u r .


80



100

70h ι

0

•

10

20

30

ι

I

40

50

PERCENT SLP FLOATED American Institute of Mining, Metallurgical and Petroleum Engineers

Figure 4. Coal yield vs. liberated pyritic sul fur (SLP) recovery for Lower Kittanning coal

(4, 23j


6.

APLAN


81

Acknowledgements The a u t h o r w i s h e s t o t h a n k t h e A m e r i c a n I r o n a n d S t e e l I n s t i t u t e , The P e n n s y l v a n i a S c i e n c e and E n g i n e e r i n g F o u n d a t i o n , and t h e A p p a l a c h i a n R e g i o n a l C o m m i s s i o n f o r f i n a n c i a l s u p p o r t i n conducting these s t u d i e s . Acknowledgement i s made t o t h e f o l l o w i n g f o r m e r a n d p r e s e n t g r a d u a t e s t u d e n t s i n The M i n e r a l P r o c e s s i n g S e c t i o n , The P e n n s y l v a n i a S t a t e U n i v e r s i t y , who have c o n t r i b u t e d d a t a f r o m w h i c h t h i s r e p o r t was drawn: C. M. B o n n e r , V. C h o u d h r y , W. C. H i r t , C. J . Im, T. J . O l s o n and R. C. R a s t o g i .


Literature Cited

1. Miner. Yearbk., 1973 (1975). 2. Mitchell, D. R., Trans. AIME (1948), 177, 315. 3. Hoffman, L., Aresco, S. J., Holt, E. C , Deurbrouck, A. W., "Engineering/Economic Analysis of Coal Preparation with SO2 Clean-up Process for Keeping Higher Sulfur Coals in the Energy Market," in Second Symposium on Coal Preparation, J. F. Boyer, Jr., Ed., Bituminous Coal Research Inc., Monroeville, Pa. (1976). 4. Aplan, F. F., "Coal Flotation," in A. M. Gaudin Memorial International Flotation Symposium, M. C. Fuerstenau, Ed., AIME, New York (1976). 5. Brown, D. J., "Coal Flotation," in Froth Flotation - 50th Anniversary Volume, D. W. Fuerstenau, Ed., AIME, New York (1962). 6. Zimmerman, R. E., "Flotation," in Coal Preparation, J. W. Leonard and D. R. Mitchell, Eds., 3rd ed., AIME, New York (1968). 7. Miller, F. G., Podgursky, J. M., Aikman, R. P., "Study of the Mechanism of Coal Flotation and its Role in a System for Processing Fine Coal," Trans. AIME (1967), 238, 276. 8. Im, Chang J., unpublished data (1977). 9. Miller, F. G., "The Effect of Froth Sprinkling on Coal Flota tion Efficiency," Trans. AIME (1969), 244, 158. 10. Gaudin, Α. Μ., Flotation, 2nd ed., McGraw-Hill, New York (1957) 11. Yancey, H. F., Taylor, J. Α., "Froth Flotation of Coal; Sulfur and Ash Reduction," U.S. Bur. Mines Rep. Invest. (1935), 3263. 12. Miller, F. G., "Reduction of Sulfur in Minus 28 Mesh Bitumin ous Coal," Trans. AIME (1964), 228, 7. 13. Zimmerman, R. Ε., "Flotation of Bituminous Coal," Trans. AIME (1948), 117, 338. 14. Miller, K. J., "Coal - Pyrite Flotation," Trans. AIME (1975), 258, 30. 15. Hirt, W. C., Aplan, F. F., unpublished data (1977). 16. Rastogi, R. C., Aplan, F. F., "Coal Flotation as a Rate Process," accepted for publication in Trans. AIME.



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COAL DESULFURIZATION

17. Bushell, C. H. G., "Kinetics of Flotation," Trans. AIME (1962), 223, 266-278. 18. Arbiter, Ν., Harris, C. C., "Flotation Kinetics," in Froth Flotation - 50th Anniversary Volume, D. W. Fuerstenau, Ed., AIME, New York (1962). 19. Rastogi, R. C., Hirt, W. C., Aplan, F. F., "An Evaluation of Several Variables Influencing the Rate of Coal Flotation," submitted to Trans. AIME. 20. Rastogi, R. C., Hirt, W. C., Aplan, F. F., unpublished data (1977). 21. Bonner, C. Μ., Im, C. J., Choudhry, V., Aplan, F. F., "The Influence of Oil on Pyritic Sulfur Rejection During Coal Flotation," submitted to Trans. AIME. 22. Bonner, C. Μ., Aplan, F. F., "Frother Comparisons in the Flotation of Coal," accepted for publication in Trans. AIME. 23. Bonner, C. Μ., Hirt, W. C., Aplan, F. F., unpublished data (1977). 24. Aplan, F. F., "Mineral Processing - Evaluation to Indicate Processing Approach," Section 27.3 in SME Mining Engineering Handbook, A. B. Cummins and I. A. Given, Eds., pp. 27, 15-28, 86-88, New York (1973). 25. Bonner, C. Μ., Hirt, W. C., Im, C. J., Aplan, F. F., unpublished data (1977).


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