An Overview of Coal Preparation - American Chemical Society

4 A n Overview of Coal Preparation

Downloaded by INDIANA UNIV BLOOMINGTON on June 27, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch004

J. A. CAVALLARO and A. W. DEURBROUCK Coal Preparation and Analysis Laboratory, Bureau of Mines, U.S. Department of the Interior, Pittsburgh, PA 15213

Current Coal Preparation Practices 01) In the United States today coal preparation practices are directed toward maximizing Btu recovery, 3-shift-per-day operation of preparation plants, fine-size coal washing, closed water circuits, and reducing sulfur in the final product. In 1976, about 665 million tons of coal were produced, an increase of 24 million tons from 1975. Table I compares clean coal tonnage produced by mechanical cleaning and thermal drying equipment in 1975. Dense-medium processes produced 32.6% of all the clean coal, an increase of 1.6%, while the percentage of coal cleaned by jigs dropped 2.2% to 46.6%. Although only 2.5% of the coal was produced by dry cleaning, interest is growing in the application of dry separation techniques to remove impurities from western coal deposits where water is limited. The tonnage of thermally dried coal continues to decrease as increasingly stringent particulate emission regulations escalate the cost of thermal drying. Since 1970 the proportion of thermally dried bituminous coal has decreased 50%. However, the use of thermal drying will increase in the future as more of the low-sulfur western coals with high inherent moisture of about 25% are used to meet the increased demand for low-sulfur coal. Fluidized -bed dryers now dry 72.5% of all the thermally dried coal. In the United States t h e average c o a l p r e p a r a t i o n p l a n t i s o p e r a t e d o n l y a b o u t 13 h r . p e r day, 5 d a y s p e r week. A o n e - a n d two-shift-per-day operation i s being questioned as the cost of c o a l p r e p a r a t i o n i n c r e a s e s o w i n g t o i n f l a t i o n a n d a s t h e new p l a n t c i r c u i t s b e i n g d e v e l o p e d become more c o m p l e x t o meet e x a c t i n g a s h and s u l f u r l e v e l s w i t h minimum l o s s o f B t u s . Consequently, a number o f t h e new p l a n t s a r e b e i n g d e s i g n e d t o wash c o a l 24 h r . p e r day. Where e q u i p m e n t must be o u t o f s e r v i c e f o r p e r i o d i c m a i n t e n a n c e , s u c h a s pumps t h a t move a b r a s i v e m a t e r i a l , b a c k u p u n i t s are being i n s t a l l e d . T h e s e new p l a n t s n o r m a l l y w i l l h a v e c a p a c i t i e s o f 1000-1500 t o n s p e r h r . and w i l l h a v e two o r more p a r a l l e l c o a l washing c i r c u i t s . I n t h i s way, one c i r c u i t c a n b e f

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

COAL DESULFURIZATION

Table I .

M e c h a n i c a l C l e a n i n g and T h e r m a l D r y i n g o f B i t u m i n o u s C o a l and L i g n i t e , by Type o f E q u i p m e n t , 1975


Type o f

(thousand

equipment

short

tons)

Cleaners Wet methods Jigs Concentrating Classifiers Launders

124,317 28,682 6,176 2,664

tables

Dense-medium p r o c e s s e s Magnetite Sand Calcium c h l o r i d e

72,448 13,533 951

Total Flotation

86,932 11,518

T o t a l , wet methods P n e u m a t i c methods Grand

260,289 6,704

total

266,993 Dryers

Fluidized-bed Multilouver Rotary Screen Suspension or f l a s h V e r t i c a l t r a y and c a s c a d e Total a

25,866 1,969 794 2,798 4,184 70 35,681

D a t a may n o t add t o t o t a l s shown b e c a u s e o f i n d e pendent r o u n d i n g .


4.

CAVALLARO AND DEURBROUCK

Coal Preparation

37


down f o r m a i n t e n a n c e w h i l e t h e o t h e r i s p r o c e s s i n g c o a l , t h u s a v o i d i n g a c o m p l e t e shutdown of the c o a l w a s h i n g f a c i l i t y . In t h i s way, p l a n t o p e r a t i n g c o s t s c a n be c o n s i d e r a b l y r e d u c e d . S u l f u r and I t s R e m o v a l . Sulfur i n coal occurs i n three f o r m s — o r g a n i c , s u l f a t e , and p y r i t i c . Organic s u l f u r , which i s an i n t e g r a l p a r t o f t h e c o a l m a t r i x and w h i c h g e n e r a l l y c a n n o t be removed by d i r e c t p h y s i c a l s e p a r a t i o n , c o m p r i s e s 30-70% o f t h e t o t a l s u l f u r o f most c o a l s . G e n e r a l l y , the o r g a n i c s u l f u r - t o t a l s u l f u r r a t i o i s h i g h e s t f o r l o w - s u l f u r c o a l s and d e c r e a s e s as t h e t o t a l s u l f u r content increases. The s u l f a t e i s n o r m a l l y an o x i d a t i o n p r o d u c t w h i c h i s w a t e r s o l u b l e and t h e r e f o r e r e a d i l y removed d u r i n g c o a l c l e a n i n g . Sulf a t e s u l f u r c o n t e n t i s u s u a l l y l e s s t h a n 0.05%. P y r i t i c s u l f u r i n the form of m i n e r a l p y r i t e o c c u r s i n c o a l as d i s c r e t e and s o m e t i m e s m i c r o s c o p i c p a r t i c l e s . I t i s a h e a v y m i n e r a l w i t h a s p e c i f i c g r a v i t y o f a b o u t 5.0 w h e r e a s c o a l has a maximum s p e c i f i c g r a v i t y o f o n l y 1.8. The p y r i t i c s u l f u r c o n t e n t o f a c o a l g e n e r a l l y c a n be r e d u c e d s i g n i f i c a n t l y by s i z e r e d u c t i o n and s u b s e q u e n t s p e c i f i c g r a v i t y s e p a r a t i o n . F i g u r e 1 shows t h e r e s u l t s o f s t a g e c r u s h i n g f r o m 1 1 / 2 - i n c h t o p s i z e t o 200 mesh t o p s i z e and g r a v i m e t r i c a l l y s e p a r a t i n g a s a m p l e o f Lower F r e e p o r t bed c o a l f r o m O h i o and a s a m p l e o f P i t t s b u r g h bed c o a l f r o m West V i r g i n i a . F o r t h e Lower F r e e p o r t bed c o a l s a m p l e , c r u s h i n g f r o m 3/8 i n c h t o 14 mesh p r o v i d e d t h e m a x i mum change i n p y r i t i c s u l f u r r e d u c t i o n f o r t h e f l o a t - 1 . 3 0 - s p e c i f i c g r a v i t y m a t e r i a l and a l s o f o r t h e f l o a t - 1 . 6 0 - s p e c i f i c - g r a v i t y material. F o r t h e P i t t s b u r g h bed c o a l s a m p l e , maximum c h a n g e i n s u l f u r r e d u c t i o n o f a b o u t 0.8 p e r c e n t a g e p o i n t was o b t a i n e d a t e i t h e r s p e c i f i c g r a v i t y by c r u s h i n g f r o m 14 t o 48 mesh. Interesti n g l y , f u r t h e r c r u s h i n g o f t h e P i t t s b u r g h bed c o a l sample f r o m 48 t o 200 mesh showed a n a d d i t i o n a l p y r i t i c s u l f u r r e d u c t i o n o f a b o u t 0.5 p e r c e n t a g e p o i n t f o r t h e f l o a t - 1 . 6 0 - s p e c i f i c - g r a v i t y m a t e r i a l . R e g a r d l e s s o f how d i f f i c u l t and e x p e n s i v e t h e p r o c e s s i n g o f c o a l c r u s h e d t o 48 o r 200 mesh t o p s i z e i s , t h e p o t e n t i a l b e n e f i t s o f o b t a i n i n g l o w - s u l f u r p r o d u c t s may w e l l j u s t i f y t h e a d d i t i o n a l c o s t s and p r o b l e m s e n c o u n t e r e d . C o n t r o l l i n g S u l f u r Oxide Emissions. A v a i l a b l e methods f o r c o n t r o l l i n g s u l f u r oxide emissions from s t a t i o n a r y combustion sources f a l l i n t o the f o l l o w i n g major c a t e g o r i e s : (1) The p h y s i c a l r e m o v a l o f p y r i t i c s u l f u r by c o a l c l e a n i n g p r i o r to combustion. (2) The r e m o v a l o f s u l f u r o x i d e s f r o m t h e c o m b u s t i o n f l u e gas. (3) C o n v e r s i o n o f c o a l t o a c l e a n f u e l by s u c h p r o c e s s e s a s g a s i f i c a t i o n , l i q u e f a c t i o n , and chemical extraction. Of t h e s e m e t h o d s , p h y s i c a l r e m o v a l o f p y r i t i c s u l f u r i s t h e l e a s t e x p e n s i v e and most h i g h l y d e v e l o p e d . The d e g r e e o f s u l f u r


38



r e d u c t i o n p o s s i b l e depends on t h e w a s h a b i l i t y c h a r a c t e r i s t i c s o f t h e raw c o a l and i t s a m e n a b i l i t y t o s u l f u r r e l e a s e upon c r u s h i n g ; t h e s e c h a r a c t e r i s t i c s a r e u n i q u e and v a r y f r o m c o a l t o c o a l . S u l f u r R e d u c t i o n P o t e n t i a l S t u d i e s . I n 1965, NAPCA (EPA) f u n d e d a s t u d y t o d e t e r m i n e w a s h a b i l i t y c h a r a c t e r i s t i c s o f U.S. coals. The p u r p o s e o f t h i s c o n t i n u i n g s t u d y i s t o d e t e r m i n e t h e f o r m s o f s u l f u r i n t h e m a j o r s o u r c e s o f u t i l i t y s t e a m c o a l s and the s u l f u r r e d u c t i o n p o t e n t i a l of these c o a l s . The i n f o r m a t i o n g e n e r a t e d by t h i s s t u d y i s n e c e s s a r y t o a s s e s s t h e i m p a c t p h y s i c a l d e s u l f u r i z a t i o n o f c o a l m i g h t h a v e on t h e l e v e l o f s u l f u r o x i d e emissions from s t a t i o n a r y combustion sources. A Bureau of Mines r e p o r t e n t i t l e d " S u l f u r Reduction P o t e n t i a l o f t h e C o a l s o f t h e U n i t e d S t a t e s " (2) s u m m a r i z e s t h e w o r k p e r f o r m e d f r o m 1965 t o m i d - 1 9 7 4 . T h i s r e p o r t p r e s e n t s t h e r e s u l t s of a w a s h a b i l i t y s t u d y o f 455 raw c o a l c h a n n e l s a m p l e s w i t h s p e c i a l e m p h a s i s on s u l f u r r e d u c t i o n . The 455 U.S. c o a l s a m p l e s e v a l u a t e d c o n t a i n e d on t h e a v e r a g e 1.91% p y r i t i c s u l f u r and 3.02% t o t a l s u l f u r . Of t h e 455 s a m p l e s t e s t e d , 174 e i t h e r r e q u i r e d no b e n e f i c i a t i o n o r c o u l d be u p g r a d e d t o meet t h e EPA new s o u r c e e m i s s i o n s t a n d a r d o f 1.2 l b SO2/MM B t u . The B t u r e c o v e r y o f t h e s e s a m p l e s a v e r a g e d 88.7% and r a n g e d f r o m 5.6 t o 100%. I f o n l y t h o s e c o a l s w i t h a minimum B t u r e c o v e r y o f 50% a r e c o n s i d e r e d , t h e n 36% o f t h e 455 s a m p l e s t e s t e d w o u l d meet t h e new s t a n d a r d w i t h an a v e r a g e B t u r e c o v e r y o f 9 1 % . E f f e c t s o f B e n e f i c i a t i o n on C o a l R e s e r v e s ( 3 ) . The c o a l r e s o u r c e s i n the S t a t e s east of the M i s s i s s i p p i R i v e r c o n t a i n i n g 1.0% s u l f u r o r l e s s a r e e s t i m a t e d a t 95.3 b i l l i o n t o n s ( F i g u r e 2 ) . A l t h o u g h t h i s i s o n l y 20% o f t h e t o t a l r e m a i n i n g r e s o u r c e , i t i s s t i l l a s i g n i f i c a n t amount. A v a i l a b l e w a s h a b i l i t y d a t a i n d i c a t e t h a t much o f t h e m e d i u m - s u l f u r - c o n t e n t c o a l c a n be u p g r a d e d t o 1.0% s u l f u r o r l e s s u s i n g s t a t e - o f - t h e - a r t c o a l c l e a n i n g techniques. An e f f o r t i s now underway t o d e t e r m i n e how t h e p r e s e n t r e s e r v e s o f c o a l s e a s t o f t h e M i s s i s s i p p i R i v e r w o u l d be c a t e g o r i z e d i n terms of s u l f u r c o n t e n t i f they were s u b j e c t e d t o d i f f e r e n t l e v e l s of c o a l b e n e f i c i a t i o n . T h i s i n v o l v e s an o v e r l a y o f t h e s u l f u r r e d u c t i o n p o t e n t i a l d a t a and t h e r e s e r v e b a s e d a t a g e n e r a t e d by t h e B u r e a u o f M i n e s E a s t e r n F i e l d O p e r a t i o n s C e n t e r . This s t u d y has b e e n c o m p l e t e d f o r t h e N o r t h e r n A p p a l a c h i a n R e g i o n , where a dramatic i n c r e a s e occurs i n the p r o j e c t e d reserves of l o w - s u l f u r coal. The t o n n a g e s , by s u l f u r c a t e g o r y and e x t e n t o f p r e p a r a t i o n , for the e n t i r e Northern Appalachian Region are given i n Table I I . The p r e s e n t r e s e r v e s w i t h s u l f u r c o n t e n t l e s s t h a n 0.85% amount t o 1,727.45 m i l l i o n t o n s . C r u s h i n g t o 1 1/2 i n . t o p s i z e and w a s h i n g a t 1.60 s p e c i f i c g r a v i t y y i e l d s 4,172.32 m i l l i o n t o n s o f l o w - s u l f u r c o a l , w h i c h i s 2.4 t i m e s t h e e x i s t i n g r e s e r v e s .




Float

Coal Preparation

I 6 0 sp gr

TOP SIZE TO WHICH F E E D WAS C R U S H E D , inches ond mesh

Figure 1. Pyritic sulfur reduction potential for two Appafochian region coals as a result of stage crushing

Figure 2. Estimated remaining coal resources of all ranks, by sulfur content, in states east of the Mississippi River, Jan. 1,1965


40


C r u s h i n g t o 14-mesh t o p s i z e and c l e a n i n g a t 1.60 s p e c i f i c g r a v i t y w o u l d r e s u l t i n a t o t a l o f 8,546.31 m i l l i o n t o n s o f l o w - s u l f u r c o a l , w h i c h i s 4.9 t i m e s t h e p r e s e n t r e s e r v e s . Four percent of the Northern Appalachian Region r e s e r v e s w i t h a s u l f u r content g r e a t e r t h a n 0.85% c a n be u p g r a d e d t o l o w - s u l f u r c o a l by c r u s h i n g t o 1 1 / 2 - i n . t o p s i z e and w a s h i n g a t 1.60 s p e c i f i c g r a v i t y ; t h e p e r c e n t a g e i n c r e a s e s t o 10 when c r u s h i n g t o 14-mesh t o p s i z e and w a s h i n g a t 1.60 s p e c i f i c g r a v i t y .


Table I I .

C o a l P r e p a r a t i o n Wins L o w - S u l f u r C o a l f o r Energy Market i n Northern Appalachian Region, M i l l i o n s o f Tons

3.0% Sulfur

27,535.04

1/2- i n c h t o p s i z e

4,237.93 4,172.32

20,398.17 43,396.58

637.26 14,164.45

C r u s h e d t o 3/8-i n c h t o p s i z e C l e a n e d a t 1.30 C l e a n e d a t 1.60

6,485.79 6,224.53

22,350.99 44,595.82

651.68 9,918.26

C r u s h e d t o 14-mesh t o p s i z e C l e a n e d a t 1.30 C l e a n e d a t 1.60

7,136.62 8,546.31

21,691.77 45,873.05

395.59 5,956.40

F i g u r e s 3 and 4 show g r a p h i c a l l y t h e e f f e c t o f c r u s h i n g t o v a r i o u s t o p s i z e s on t h e t o n n a g e s o f l o w - s u l f u r c o a l p o t e n t i a l l y a v a i l a b l e i n the Northern Appalachian Region. I n examining these f i g u r e s to e s t a b l i s h the b e n e f i t s of c o a l preparation, i t i s n e c e s s a r y t o keep s e v e r a l t h i n g s i n m i n d . F i r s t o f a l l , e v e n t h o u g h t h e t o n n a g e s o f l o w - s u l f u r c o a l ( l e s s t h a n 0.85% s u l f u r ) r e s u l t i n g from c o a l p r e p a r a t i o n a r e a r e l a t i v e l y s m a l l f r a c t i o n of the t o t a l r e s e r v e base, they represent s i z a b l e q u a n t i t i e s of c o a l , e q u i v a l e n t t o many y e a r s o f p r o d u c t i o n a t c u r r e n t and f o r e s e e n r a t e s . M o r e o v e r , many p o w e r p l a n t s c o n s t r u c t e d p r i o r t o 1975 c a n be b r o u g h t i n t o c o m p l i a n c e w i t h l o c a l e m i s s i o n s t a n d a r d s by using c o a l cleaned to the appropriate s u l f u r l e v e l . I n summary, c o a l p r e p a r a t i o n has t h e p o t e n t i a l o f p r o v i d i n g s i z a b l e t o n n a g e s o f c o a l t h a t w i l l meet t h e EPA n e w - s o u r c e



Coal Preparation


10,000

TOP SIZE A F T E R CRUSHING

Figure 3. Effect of crusing and cleaning at 1.60 specific gravity on the availability of < 0.85% sulfur coal

Figure 4. Effect of crushing and cleaning at 1.30 specific gravity on the availability of < 0.85% sulfur coal


42


emission standards. I n a d d i t i o n , f o r powerplants b u i l t p r i o r to 1975, f a r g r e a t e r q u a n t i t i e s o f c o a l c a n b e b u r n e d a f t e r u p g r a d i n g by c o a l p r e p a r a t i o n .


New D e v e l o p m e n t s i n C o a l P r e p a r a t i o n

(4 )

Coal p r e p a r a t i o n s t a t e - o f - t h e - a r t i s changing r a p i d l y i n the United States today. The s c o p e o f t h e s e c h a n g e s and some o f t h e new c i r c u i t and e q u i p m e n t i n n o v a t i o n s w h i c h h a v e b e e n r e c e n t l y i n t r o d u c e d a r e d i s c u s s e d below. The m a j o r e m p h a s i s i s o n r e d u c i n g s u l f u r a n d m a x i m i z i n g B t u recovery. The n e e d f o r s u l f u r r e d u c t i o n stems f r o m t h e e n f o r c e ment o f SO2 e m i s s i o n s t a n d a r d s , w h i l e h i g h B t u r e c o v e r y becomes e s s e n t i a l as a r e s u l t o f t h e i n c r e a s e d v a l u e o f c o a l as an energy source. The r e s u l t i n g i r o n y s p e l l s o u t t h e c h a l l e n g e f a c i n g c o a l p r e p a r a t i o n today: How t o e c o n o m i c a l l y p r o d u c e i n c r e a s i n g amounts of c o a l that s a t i s f y t h e s t r i c t p o l l u t i o n standards. Homer C i t y P r e p a r a t i o n P l a n t . A p o s s i b l e s o l u t i o n t o t h e d u a l p r o b l e m s o f e n v i r o n m e n t a l r e s t r i c t i o n s a n d maximum r e c o v e r y i s t h e h i g h l y p u b l i c i z e d Homer C i t y m u l t i s t r e a m c o a l c l e a n i n g p l a n t ( F i g u r e 5 ) . The p l a n t i s b e i n g b u i l t f o r j o i n t o w n e r s , New Y o r k S t a t e E l e c t r i c and Gas C o r p . a n d P e n n s y l v a n i a E l e c t r i c Co., b y H e y l & P a t t e r s o n , I n c . The p l a n t w i l l p r o d u c e two streams of c l e a n c o a l . The f i r s t s t r e a m w i l l be a n e x t r e m e l y h i g h q u a l i t y p r o d u c t w h i c h w i l l meet t h e e n v i r o n m e n t a l standards f o r f i r i n g i n t h e new U n i t 3 g e n e r a t i n g s t a t i o n ( 6 5 0 MW) w i t h o u t subsequent s t a c k gas s c r u b b i n g . The s e c o n d c l e a n c o a l s t r e a m i s o f i n t e r m e d i a t e q u a l i t y and w i l l meet t h e e x i s t i n g s o u r c e r e q u i r e m e n t s f o r U n i t s 1 and 2, w h i c h a r e 600 MW e a c h . The e x p e c t e d p r e p a r a t i o n p l a n t p e r f o r m a n c e , g i v e n i n T a b l e I I I , shows t h a t t h e s u l f u r c o n t e n t i s r e d u c e d a b o u t 70% f r o m 2.8% t o 0.88% i n t h e U n i t 3 c l e a n c o a l w h i l e t h e o v e r a l l B t u r e c o v e r y i s 94.5%. Table I I I .

Homer C i t y P r e p a r a t i o n P l a n t E x p e c t e d P e r f o r m a n c e '

Recovery (weight percent) Recovery (Btu percent)* P r o d u c t Btu/pound (dry b a s i s ) Ash, percent S u l f u r , percent Pounds s u l f u r / M M B t u 5

Units 1 & 2

Unit 3

Refuse

56.2 61.6 °12,549 17.8 2.84 1.79

24.7 32.9 15,200 2.8 0.88 0.58

19.1 5.5 3,367 69.7

a

(Loss)

R e f e r e n c e : J . F. M c C o n n e l l , Homer C i t y C o a l C l e a n i n g D e m o n s t r a t i o n , 4 t h N a t i o n a l C o n f e r e n c e on E n e r g y and E n v i r o n m e n t ( E P A ) . ^ G r o s s r e c o v e r y , 94.5%; n e t r e c o v e r y i s 9 3 . 5 % w i t h l o s s e s f o r thermal dryer included. B l e n d o f m i d d l i n g and c l e a n c o a l . c





44


The p l a n t i n c l u d e s s e v e r a l i n n o v a t i v e a p p l i c a t i o n s o f e x i s t i n g t e c h n o l o g y w h i c h we a t t h e B u r e a u o f M i n e s h a v e b e e n r e s e a r c h i n g a t a p i l o t p l a n t l e v e l i n c o o p e r a t i o n w i t h t h e owners and designers. I n p r o d u c i n g t h e c l e a n c o a l f o r U n i t 3, dense-medium c y c l o n e s c l e a n a t a s p e c i f i c g r a v i t y o f s e p a r a t i o n o f 1.3; a s p e c i f i c g r a v i t y o f 1.8 i s u s e d t o r e c l e a n t h e u n d e r f l o w m a t e r i a l f o r U n i t s 1 and 2. These s e p a r a t i n g s p e c i f i c g r a v i t i e s a r e s i g n i f i c a n t l y b e y o n d t h e n o r m a l o p e r a t i n g r a n g e o f a b o u t 1.4-1.6 f o r dense-medium c y c l o n e s w a s h i n g b i t u m i n o u s c o a l . A l s o , t h e u s u a l 28-mesh b o t t o m s i z e o f t h e c o a l c l e a n e d i n t h e c y c l o n e w i l l be e x t e n d e d t o 100 mesh, o r t o a b o u t o n e - f o u r t h the u s u a l s i z e . The f e e d i s s i z e d a t 1 / 4 - i n . by 9-mesh and 9- by 100-mesh f r a c t i o n s and c l e a n e d i n two s e p a r a t e c y c l o n e c i r c u i t s , i n s t e a d o f t h e w h o l e f e e d b e i n g washed i n one c y c l o n e c i r c u i t . Because of the s m a l l p a r t i c l e s i z e of c o a l cleaned i n the 9- by 100-mesh c y c l o n e c i r c u i t , c o n v e n t i o n a l d r a i n and r i n s e s c r e e n s f o r s u b s e q u e n t m a g n e t i t e r e c o v e r y a r e r e p l a c e d by twostage magnetic s e p a r a t o r s which handle the f u l l streams of overf l o w and u n d e r f l o w p r o d u c t s f r o m t h e c y c l o n e . The c l e a n i n g o f f i n e r s i z e c o a l i n c y c l o n e s f o l l o w e d by t h e a b o v e a p p l i c a t i o n o f m a g n e t i c s e p a r a t o r s i s a l s o b e i n g c o n s i d e r e d f o r i n s t a l l a t i o n s by o t h e r companies. Another i n t e r e s t i n g f e a t u r e i s a novel c o n t r o l c i r c u i t f o r c l o s e s p e c i f i c g r a v i t y c o n t r o l of the feed s l u r r y to the densemedium c y c l o n e s . When o p e r a t i n g a t s p e c i f i c g r a v i t i e s o f s e p a r a t i o n a r o u n d 1.3, t h e ±0.10 n e a r - g r a v i t y m a t e r i a l i n t h e f e e d i s a b o u t 60%. T h e r e f o r e , t h e e n t e r i n g s l u r r y must be r i g i d l y m a i n t a i n e d a t t h e p r o p e r s p e c i f i c g r a v i t y o r m a t e r i a l w i l l be m i s placed. T h i s w i l l r e s u l t i n e i t h e r a c l e a n c o a l t h a t does not meet s p e c i f i c a t i o n s o r a d e c r e a s e i n h i g h - q u a l i t y c l e a n c o a l tonnage. F i g u r e 6 shows t h e p i l o t p l a n t c o n t r o l s y s t e m t h a t i s now b e i n g t e s t e d a t t h e B u r e a u o f M i n e s . Three s t r e a m s — f r e s h w a t e r , c o a l s l u r r y , and m a g n e t i t e s l u r r y — a r e c o m b i n e d i n t h e m i x i n g sump and r e p r e s e n t t h e f e e d t o a dense-medium c y c l o n e . Ins t r u m e n t s m e a s u r e t h e d e n s i t y and f l o w r a t e o f e a c h s t r e a m and s e n d t h e measurement s i g n a l s t o an a n a l o g c o m p u t e r . The c o m p u t e r c a l c u l a t e s t h e s e s i g n a l s , compares t h e a c t u a l d e n s i t y and f l o w f o r t h e f e e d s l u r r y w i t h t h e p r e s e t v a l u e s , and t h e n c o n t r o l s t h e w a t e r and m a g n e t i t e s l u r r y f l o w r a t e s a c c o r d i n g l y t o p r o d u c e t h e preset values. The s y s t e m n o t o n l y m a i n t a i n s s t r i c t q u a l i t y c o n t r o l a t a h i g h e f f i c i e n c y , but p r o v i d e s the f l e x i b i l i t y needed to make q u i c k c h a n g e s i n t h e s p e c i f i c g r a v i t y o f s e p a r a t i o n , s h o u l d t h e w a s h a b i l i t y c h a r a c t e r i s t i c s o f t h e raw c o a l change. B a t a c J i g . The t r e n d t o w a r d f i n e - s i z e c o a l c l e a n i n g t o remove more s u l f u r and r e c o v e r more B t u s has s p u r r e d t h e d e v e l o p ment o f c l o s e d w a t e r c i r c u i t s and many new t e c h n i q u e s and c i r c u i t s i n w a s h i n g , d e w a t e r i n g , and s i z i n g . Baum j i g s w a s h i n g c o a r s e c o a l s t i l l p r o d u c e t h e most c l e a n c o a l ; h o w e v e r , t h e B a t a c j i g , d e v e l o p e d i n Germany t o c l e a n f i n e c o a l , has b e e n i n s t a l l e d i n a t 1



Figure 6.

Dense-medium specific gravity control circuit


46



l e a s t t h r e e U.S. p l a n t s t o p r o c e s s 1 / 2 - i n . by 0 raw c o a l . It is g e n e r a l l y i n s t a l l e d a f t e r a Baum j i g w h i c h c l e a n s t h e p l u s 1/2i n c h raw c o a l . In the Batac j i g , a i r p u l s a t i o n s are produced i n m u l t i p l e chambers d i r e c t l y b e n e a t h t h e bed s c r e e n , a s shown i n F i g u r e 7. The p u l s e s i n e a c h chamber a r e c o n t r o l l e d i n d e p e n d e n t l y and p r e c i s e l y . The j i g p r o v i d e s s i g n i f i c a n t l y h i g h e r t h r o u g h p u t i n t h e same s p a c e a s a c o n v e n t i o n a l Baum j i g b e c a u s e o f i t s i m p r o v e d e f f i c i e n c y and e l i m i n a t i o n o f t h e a d j a c e n t a i r chamber. The B a t a c j i g c l e a n s w i t h a s h a r p n e s s s i m i l a r t o t h a t o f c o n c e n t r a t i n g t a b l e s f o r t h e same s i z e r a n g e . Novel F i n e - S i z e Cleaning C i r c u i t . An i n t e r e s t i n g new f i n e s i z e c o a l c l e a n i n g c i r c u i t has b e e n i n s t a l l e d i n a P e n n s y l v a n i a p r e p a r a t i o n p l a n t by R o b e r t s & S c h a e f e r Co. The p l a n t was o r i g i n a l l y s t r i c t l y an a i r t a b l e p l a n t , b u t a dense-medium c y c l o n e has b e e n i n c o r p o r a t e d t o r e c l e a n t h e s e c o n d and t h i r d r e f u s e d r a w s from the a i r t a b l e . The f i r s t d r a w i s e s s e n t i a l l y p u r e r e f u s e w i t h no m i s p l a c e d f l o a t m a t e r i a l . W i t h t h e s e c o n d and t h i r d d r a w s w i d e o p e n , t h e amount o f s i n k m a t e r i a l r e p o r t i n g t o t h e c l e a n c o a l i s reduced. A t t h e s e t a b l e s e t t i n g s , a b o u t 20% o f t h e f e e d i s sent to the cyclone to maximize Btu recovery w h i l e upgrading the f i n a l product. The c y c l o n e p r o d u c e s a 5.6%-ash c l e a n c o a l , w h i c h i s combined w i t h t h e c l e a n c o a l from the a i r t a b l e . The n e t r e s u l t s a r e a 5% i n c r e a s e i n y i e l d and a 20% d e c r e a s e i n a s h c o n tent. The a i r t a b l e i s r e c e i v i n g a t t e n t i o n f r o m w e s t e r n c o a l p r o d u c e r s who a r e l o o k i n g f o r a d r y method o f c l e a n i n g c o a l i n the water-scarce Western S t a t e s . D e w a t e r i n g and S i z i n g . One o f t h e b i g g e s t p r o b l e m s i n p r o c e s s i n g f i n e - s i z e c o a l i s d e w a t e r i n g and s i z i n g . The s e v e r e b l i n d i n g t e n d e n c y o f m i n u s 28-mesh c o a l s l u r r i e s on c o n v e n t i o n a l s c r e e n s makes f o r e x t r e m e l y i n e f f i c i e n t and u n r e l i a b l e s e p a r a tions. A new s c r e e n i n g d e v i c e has b e e n d e v e l o p e d by D e r r i c k M a n u f a c t u r i n g Corp. w h i c h p u r p o r t s t o have s o l v e d t h e b l i n d i n g problem. I t c o n s i s t s of a "sandwich s c r e e n " concept, which binds two f i n e s c r e e n s u r f a c e s one on t o p o f t h e o t h e r . The a c t i o n o f t h e two s c r e e n s a g a i n s t e a c h o t h e r , f r o m a s p e c i a l h i g h f r e q u e n c y low a m p l i t u d e v i b r a t i n g motor, p r e v e n t s n e a r - s i z e p a r t i c l e s from becoming lodged i n the top s c r e e n . The o p e n i n g s i n t h e b o t t o m screen are s l i g h t l y l a r g e r than those i n the top to ensure t h a t p a r t i c l e s a r e not t r a p p e d between s c r e e n s . D e r r i c k claims these s c r e e n s c a n make s h a r p s e p a r a t i o n s down t o 400 mesh. The f i n a l s e c t i o n o f t h e s c r e e n may be a s l o t t e d n a t u r a l r u b b e r s u r f a c e f i t t e d w i t h a p a t e n t e d vacuum a r r a n g e m e n t t o h e l p remove a d d i t i o n a l surface moisture. C l o s e d Water C i r c u i t s . I n a d d i t i o n to the enforcement of a i r e m i s s i o n s t a n d a r d s , s t r e a m p o l l u t i o n and r e f u s e d i s p o s a l l a w s h a v e become more s t r i n g e n t r e c e n t l y . T h u s , new p r e p a r a t i o n p l a n t s u s e some t y p e o f c l o s e d w a t e r c i r c u i t w i t h a g r a v i t a t i o n a l


Coal Preparation



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


48


c l a r i f i e r as t h e p r i m a r y p i e c e o f e q u i p m e n t i n t h e s y s t e m . Two new t h i c k e n e r d e s i g n s — t h e L a m e l l a and t h e E n v i r o - C l e a r — a r e o f i n t e r e s t f o r p o s s i b l e a p p l i c a t i o n a t p l a n t s i t e s where l a n d a r e a i s a t a premium. T h e s e u n i t s h a v e b e e n i n s t a l l e d a t o l d e r p l a n t s w h i c h had s p a c e l i m i t a t i o n s and w e r e e x p a n d i n g t h e i r p l a n t c a p a c i t y o r e l i m i n a t i n g waste ponds. B o t h compact t h i c k e n e r s may r e q u i r e as l i t t l e as o n e - t e n t h o f t h e s p a c e o c c u p i e d by a c o n v e n t i o n a l c i r c u l a r t a n k t h i c k e n e r . The c l a r i t y of t h e i r o v e r f l o w i s c o m p e t i t i v e w i t h t h a t of c o n v e n t i o n a l t h i c k e n e r s w i t h o v e r f l o w c o n c e n t r a t i o n s o f 500 t o 100 ppm o r l e s s of s o l i d s , while underflow concentrations are s l i g h t l y higher at 30-40% s o l i d s . E a c h t h i c k e n e r o p e r a t e s on a d i f f e r e n t d e s i g n p r i n c i p l e . The L a m e l l a ( F i g u r e 8) has t h e same s e t t l i n g a r e a f o r a g i v e n f l o w r a t e as a c o n v e n t i o n a l t h i c k e n e r by v i r t u e o f a s e r i e s o f p l a t e s . The s e t t l i n g o u t o f s o l i d s on t h e s e p l a t e s i s much l i k e t h a t i n a s t a c k of c o n v e n t i o n a l t h i c k e n e r s , but the p l a t e s a r e i n c l i n e d to a l l o w continuous removal of the s o l i d s . L a m e l l a s have f e e d l o a d i n g r a t e s o f 0.5 t o 0.6 g p m / f t ^ , and u s e f l o c c u l a n t d o s a g e s o f a b o u t 2 ppm. These v a l u e s a r e i n l i n e w i t h those of the l a r g e c i r c u l a r tank t h i c k e n e r s . At present, the l a r g e s t Lamella w i l l h a n d l e 1,500 gpm. The E n v i r o - C l e a r ( F i g u r e 9) has a much h i g h e r f e e d l o a d i n g r a t e , 3 t o 4 gpm/ft^, because i t does not have the s e t t l i n g a r e a of the L a m e l l a t h i c k e n e r . I t i s a bottom-fed u n i t which a c t u a l l y u s e s t h e s e t t l e d s o l i d s a s a f i l t e r i n g medium f o r t h e w a t e r as i t f l o w s upward t o t h e o v e r f l o w . T h e r e a r e o n l y two z o n e s i n t h i s t h i c k e n e r , c l a r i f i e d l i q u i d and c o m p r e s s e d s o l i d s . Feed f l u c u a t i o n s c a n be a b s o r b e d by a l l o w i n g t h e s l u d g e bed l e v e l t o r i s e o r f a l l w i t h no d e t r i m e n t t o t h e o v e r f l o w c l a r i t y . Both u n i t s a r e more s e n s i t i v e t o f e e d v a r i a t i o n s , and t h e y l a c k t h e huge advantage of a l a r g e storage c a p a c i t y which c o n v e n t i o n a l t h i c k eners have. The e x a m p l e s d i s c u s s e d show t h a t t h e r e a r e i n n o v a t i o n s i n a l l areas of c o a l p r e p a r a t i o n . The new e q u i p m e n t , n o v e l techniques, c o m p l e x c i r c u i t r y , and h i g h e r l e v e l s o f c o n t r o l a r e a l l needed i n o r d e r t o i n s u r e t h a t c o a l m a r k e t demands c a n be met economically. Summary o f U.S.

Bureau of Mines C o a l P r e p a r a t i o n Program

(5)

P r i o r t o 1965, t h e m a i n t h r u s t o f t h e B u r e a u o f M i n e s c o a l p r e p a r a t i o n r e s e a r c h e f f o r t was d i r e c t e d t o w a r d a s h r e d u c t i o n w i t h l i t t l e e f f o r t aimed a t the e n v i r o n m e n t a l a s p e c t s of c o a l u t i l i z a tion. The r e s e a r c h i m p e t u s c l e a r l y mandated by t h e C l e a n A i r A c t o f 1970 i s t h a t S 0 e m i s s i o n s must be c o n t r o l l e d , t r a c e e l e m e n t s p r e s e n t i n t h e raw c o a l must be i d e n t i f i e d and t r a c k e d t o f i n a l d i s p o s a l , and t h e e n v i r o n m e n t a l e f f e c t s o f t h e c o a l p r e p a r a t i o n p r o c e s s i t s e l f must meet s t r i n g e n t r e q u i r e m e n t s . W i t h t h i s l e g i s l a t e d mandate, an e n t i r e l y new d i m e n s i o n o f c o a l p r e p a r a t i o n r e s e a r c h has emerged. The B u r e a u o f M i n e s has X



Coal Preparation


4.

49

Figure 8. Lamelfo thickener

Figure 9.

Enviro-Clear thickener


50

coal

desulfurization


i n i t i a t e d a c o a l p r e p a r a t i o n p r o g r a m commensurate w i t h t h e v i t a l e n e r g y demands o f t h i s N a t i o n . The f o r m e r s p e c t r u m o f c o a l p r e p a r a t i o n r e s e a r c h has n o t b e e n e l i m i n a t e d b u t r a t h e r b r o a d e n e d t o meet t h e s e new demands. The p r i m a r y o b j e c t i v e o f t h e B u r e a u o f M i n e s C o a l P r e p a r a t i o n Program i s to i n c r e a s e the a v a i l a b i l i t y of e n v i r o n m e n t a l l y a c c e p t a b l e f u e l s f o r combustion use through the removal of p o l l u t i n g constituents. T h i s i s t o be a c c o m p l i s h e d by a d v a n c i n g t h e s t a t e o f - t h e - a r t o f c o a l c l e a n i n g t o a l e v e l where l o w - p o l l u t a n t f u e l s c a n be p r o d u c e d i n a c o s t - e f f e c t i v e manner i n c o m m e r c i a l - s c a l e q u a n t i t i e s w i t h o u t a l t e r i n g t h e b a s i c f o r m of t h e p r o c e s s e d f u e l . Two-Stage C o a l - P y r i t e F l o t a t i o n . F r o t h f l o t a t i o n of c o a l s u f f e r s as a d e s u l f u r i z a t i o n method b e c a u s e of t h e t e n d e n c y f o r the f i n e - s i z e p y r i t e p a r t i c l e s to f l o a t w i t h the c o a l . As a c o n s e q u e n c e , t h e B u r e a u o f M i n e s has b e e n c o n d u c t i n g r e s e a r c h t o i m p r o v e f l o t a t i o n p r o c e d u r e s so t h a t b e t t e r p y r i t e r e j e c t i o n i s possible. E a r l y l a b o r a t o r y f l o t a t i o n w o r k showed t h a t t h e q u a n t i t y of p y r i t e r e p o r t i n g w i t h the c l e a n c o a l f r o t h product i n c r e a s e d w i t h i n c r e a s i n g a d d i t i o n s o f t h e f r o t h i n g a g e n t and w i t h lengthy f r o t h c o l l e c t i o n times. T h e r e f o r e , some d e s u l f u r i z a t i o n was made p o s s i b l e by u s i n g s t a r v a t i o n amounts o f r e a g e n t and v e r y b r i e f s l u r r y residence times. S u b s t a n t i a l s u l f u r r e d u c t i o n s w e r e a l s o a c h i e v e d w i t h some c o a l s by t w o - s t a g e r o u g h e r - c l e a n e r f l o t a t i o n i n w h i c h t h e c l e a n c o a l f r o t h c o n c e n t r a t e f r o m t h e f i r s t s t a g e was r e f l o a t e d . I n a l l o f t h i s e a r l i e r w o r k , i t was r e c o g n i z e d t h a t a p o r t i o n o f t h e p y r i t e r e p o r t e d t o t h e c l e a n c o a l f r o t h p r o d u c t b e c a u s e i t was a t t a c h e d t o f l o a t a b l e c o a l o r b e c a u s e i t was t o o f i n e and became m e c h a n i c a l l y entrapped i n the f r o t h . To combat t h i s p r o b l e m , a u n i q u e t w o - s t a g e f l o t a t i o n p r o c e s s was d e v e l o p e d and p a t e n t e d (6) by t h e B u r e a u . The p r o c e s s c o n s i s t s o f a f i r s t - s t a g e c o a l f l o t a t i o n s t e p i n w h i c h h i g h - a s h r e f u s e and t h e c o a r s e o r s h a l e a s s o c i a t e d p y r i t e a r e removed as t a i l i n g s . The f i r s t - s t a g e c o a l f r o t h c o n c e n t r a t e i s t h e n r e p u l p e d i n f r e s h w a t e r , and a c o a l d e p r e s s a n t , a p y r i t e c o l l e c t o r , and a f r o t h e r a r e added. A h i g h s u l f u r - c o n t e n t p r o d u c t i s t h e n f l o a t e d , and t h e s e c o n d - s t a g e u n d e r f l o w i s c o l l e c t e d as t h e f i n a l c l e a n c o a l . I n l a b o r a t o r y and p i l o t p l a n t f l o t a t i o n t e s t s w i t h c o a l s from v a r i o u s coalbeds throughout the E a s t e r n U n i t e d S t a t e s , p y r i t i c s u l f u r r e d u c t i o n s o f up t o 90% w e r e a c h i e v e d u s i n g t h i s t e c h n i q u e ( 7 ) . The f i r s t c o m m e r c i a l i n s t a l l a t i o n o f t h e c o a l - p y r i t e f l o t a t i o n p r o c e s s w i l l go on l i n e a t t h e L a n c a s h i r e No. 25 p r e p a r a t i o n p l a n t o f t h e B a r n e s and T u c k e r Co. i n A u g u s t 1977. This i s a c o o p e r a t i v e e f f o r t w i t h the Bureau of Mines. I f i t works succ e s s f u l l y , i t w i l l mean t h e r e c o v e r y o f an a d d i t i o n a l 8 t o n s p e r h o u r o f f i n e - s i z e c o a l t h a t i s now b e i n g d i s c a r d e d as w a s t e because of i t s h i g h s u l f u r content. R e s e a r c h i s b e i n g c o n d u c t e d as t h e U n i v e r s i t y o f U t a h t o i n v e s t i g a t e t h e d e p r e s s a n t and c o l l e c t o r a d s o r p t i o n r e a c t i o n s



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i n v o l v e d i n the C o a l - P y r i t e F l o t a t i o n Process. The o b j e c t i v e s o f t h i s p r o g r a m w i l l be t o — (1) I d e n t i f y and c h a r a c t e r i z e the h y d r o p h i l i c polymeric coal depressants. (2) Determine the important o p e r a t i n g v a r i a b l e s which c o n t r o l the a d s o r p t i o n - d e s o r p t i o n r e a c t i o n s and r e l a t e these r e s u l t s t o the f l o t a t i o n response o f t h e mineral constituents. (3) E s t a b l i s h procedures, i f necessary, t o allow f o r subsequent c o a l f l o t a t i o n and p l a n t water r e c y c l e without d e l e t e r i o u s e f f e c t s from r e s i d u a l reagent concentrations. High-Gradient Magnetic Separation. High-gradient magnetic s e p a r a t i o n (HGMS) u s e s l a r g e - c a p a c i t y d e v i c e s a s a p r a c t i c a l means t o s e p a r a t e s m a l l , w e a k l y m a g n e t i c p a r t i c l e s o n a l a r g e scale. This technology, which i s being u t i l i z e d commercially to p u r i f y k a o l i n c l a y , i s being i n v e s t i g a t e d through s e v e r a l Bureau o f Mines c o n t r a c t s t o determine i t s p o t e n t i a l f o r a p p l i c a t i o n i n the c o a l p r e p a r a t i o n i n d u s t r y . G e n e r a l E l e c t r i c Co., i n c o n j u n c t i o n w i t h t h e M a s s a c h u s e t t s I n s t i t u t e o f Technology and E a s t e r n A s s o c i a t e d C o a l Corp., i s a t t e m p t i n g t o e s t a b l i s h the t e c h n i c a l f e a s i b i l i t y o f removing a s u b s t a n t i a l f r a c t i o n o f t h e i n o r g a n i c s u l f u r f r o m d r y c o a l powders by HGMS. In a d d i t i o n t o sponsoring these c o n t r a c t s , the Bureau w i l l e s t a b l i s h i n h o u s e c a p a b i l i t y f o r HGMS i n t h e n e a r f u t u r e . A highg r a d i e n t m a g n e t i c s e p a r a t o r o f t h e c a n i s t e r t y p e h a s been o r d e r e d , and a r e s e a r c h p r o g r a m i n v o l v i n g a w i d e v a r i e t y o f c o a l s w i l l b e i n i t i a t e d w i t h i n s e v e r a l months. C o a l Prep/FGD C o m b i n a t i o n S t u d y . I n a s t u d y j u s t c o m p l e t e d f o r the Bureau o f Mines by the Hoffman-Muntner Corp. ( 8 ) , t h e e c o n o m i c p o t e n t i a l o f c o a l p r e p a r a t i o n combined w i t h s t a c k g a s s c r u b b i n g was e v a l u a t e d . The c o n c e p t o f p h y s i c a l c o a l c l e a n i n g combined w i t h f l u e gas d e s u l f u r i z a t i o n i s n o t new. I n t h e H o f f m a n - M u n t n e r s t u d y , a c t u a l c o a l u s e a r e a s , c o a l s o u r c e a r e a s , a n d t h e most p r o b a b l e c o a l b e d s o u r c e a r e d e f i n e d a n d u s e d t o make a n e c o n o m i c e v a l u a tion of physical coal cleaning. Then t h e c o s t o f a new u t i l i t y p l a n t r e m o v i n g SO2 ( e x c l u s i v e l y ) b y s t a c k g a s s c r u b b i n g t o meet t h e new s o u r c e e m i s s i o n s t a n d a r d was e v a l u a t e d . T h i s was f o l l o w e d b y a s i m i l a r e v a l u a t i o n o f t h e combined u s e o f p h y s i c a l c o a l c l e a n i n g p l u s s t a c k g a s s c r u b b i n g t o a t t a i n t h e same SO2 emission l e v e l . T h e s e c a s e s t u d i e s i n d i c a t e t h a t f o r many s i t u a t i o n s t h e e c o n o m i c a d v a n t a g e o f a combined a p p r o a c h i s q u i t e s i g n i f i c a n t . T h i s a d v a n t a g e depends o n :


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(1) The a v a i l a b i l i t y o f c o a l s c a p a b l e o f s i g n i f i c a n t r e d u c t i o n s i n a s h a n d s u l f u r by p h y s i c a l u p g r a d i n g w i t h minimal Btu l o s s . (2) A c l e a n c o a l s u l f u r l e v e l t h a t i s c o m p a t i b l e w i t h s i g n i f i c a n t l y l e s s than f u l l - s c a l e scrubbing requirements. Even so, t h e range o f v a r i a b l e s i s such t h a t each c o a l s o u r c e - u s e r c o m b i n a t i o n must b e i n d i v i d u a l l y a s s e s s e d . Chemical D e s u l f u r i z a t i o n . P r e s e n t methods o f p h y s i c a l c o a l preparation i n r e l a t i o n t o s u l f u r reduction apply only to the p a r t i a l removal o f p y r i t i c s u l f u r . A s t h e s e p h y s i c a l methods do n o t remove o r g a n i c a l l y bound s u l f u r , t h e y c a n be a p p l i e d e f f e c t i v e l y only to selected coals. T h e r e f o r e , t h e Bureau of Mines p r o g r a m h a s b e e n expanded t o i n c l u d e r e s e a r c h o n e x p e r i m e n t a l and developmental techniques using chemical, i n a d d i t i o n t o p h y s i c a l , methods f o r c o a l d e s u l f u r i z a t i o n s o t h a t more c o a l s w i l l be made a v a i l a b l e w i t h i n t h e framework o f e n v i r o n m e n t a l r e s t r a i n t s . P r e s e n t l y , t h e B u r e a u i s engaged i n a n i n h o u s e r e s e a r c h program t o develop i n n o v a t i v e c h e m i c a l treatments s p e c i f i c a l l y aimed a t o r g a n i c s u l f u r r e m o v a l . The c h e m i c a l o x i d a t i o n o f t h e o r g a n i c s u l f u r t o a f o r m w h i c h c o u l d b e removed more r e a d i l y a p p e a r s t o have p o t e n t i a l and i s b e i n g i n v e s t i g a t e d a c t i v e l y . We e n v i s i o n t h a t t h e optimum d e s u l f u r i z a t i o n o f c o a l w i l l i n c l u d e b o t h p h y s i c a l and c h e m i c a l t r e a t m e n t . The r a w c o a l w i l l be c r u s h e d f i n e enough t o m a x i m i z e p y r i t e r e l e a s e . I t w i l l t h e n be p h y s i c a l l y t r e a t e d u s i n g wet p r o c e s s e s t o remove t h e l i b e r a t e d pyrite. The f i n e - s i z e wet c o a l p r o d u c t w i l l t h e n b e t r e a t e d b y a c h e m i c a l p r o c e s s t h a t w i l l remove s u f f i c i e n t o r g a n i c and r e maining p y r i t i c s u l f u r t o p r o v i d e an e n v i r o n m e n t a l l y a c c e p t a b l e f i n a l product. Dewatering. Hot S u r f a c t a n t S o l u t i o n A s D e w a t e r i n g A i d ( 9 ) . A p r i n c i p a l area of c o a l preparation research i n v o l v e s dewatering. M o i s t u r e r e m o v a l c a n b e a c c o m p l i s h e d by m e c h a n i c a l m e t h o d s , c h e m i c a l t r e a t m e n t , o r t h e r m a l d r y i n g . C o a l f i n e r t h a n 28 mesh i s t h e most d i f f i c u l t s i z e t o d e w a t e r a n d i s n o r m a l l y f i l t e r e d and t h e n , when n e c e s s a r y , t h e r m a l l y d r i e d . Some e a r l y r e s e a r c h w o r k i n o u r l a b o r a t o r y h a s s u g g e s t e d t h a t t h e u s e o f s u r f a c t a n t s d u r i n g t h e vacuum f i l t r a t i o n c y c l e w i l l i n c r e a s e t h e amount o f w a t e r removed. The t e c h n i q u e c o n s i s t s o f p o u r i n g a c o a l s l u r r y o n t o a m o d i f i e d f i l t e r l e a f a p p a r a t u s and washing t h e f i l t e r cake w i t h a s u r f a c t a n t s o l u t i o n d u r i n g t h e a i r drying cycle. The e f f i c i e n c y o f t h e p r o c e s s depends o n t h e amount of s u r f a c t a n t i n t h e wash w a t e r and o n t h e t e m p e r a t u r e o f t h e wash water. I t i s p o s s i b l e t o i m p r o v e on t h e r e s u l t s b y h e a t i n g t h e washed f i l t e r c a k e s w i t h s t e a m . U s i n g t h i s t e c h n i q u e , t h e m o i s t u r e c o n t e n t o f s l u r r i e s made w i t h m i n u s 35-mesh c o a l c o n t a i n i n g a b o u t 2 5 % minus 325-mesh m a t e r i a l was r e d u c e d t o a b o u t 1 0 % . I n c o n t r a s t , u n t r e a t e d f i l t e r cakes c o n t a i n e d about 19% m o i s t u r e .


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S u r f a c e Phenomena i n C o a l D e w a t e r i n g . An i n v e s t i g a t i o n t o i m p r o v e m e c h a n i c a l methods f o r d e w a t e r i n g f i n e c o a l t o m i n i m i z e t h e need f o r t h e r m a l d r y i n g i s b e i n g c a r r i e d o u t u n d e r a g r a n t w i t h Syracuse U n i v e r s i t y . B l a c k Water S t u d i e s . Treatment of b l a c k waters from c o a l p r e p a r a t i o n p l a n t s i s c o m p l i c a t e d by t h e h e t e r o g e n e o u s n a t u r e o f t h e w a t e r s and by t h e l a c k o f i n f o r m a t i o n on t h e b e h a v i o r o f r e l a t i v e l y s i m p l e systems. T h i s water c o n s i s t s of m i x t u r e s of f i n e c o a l , c l a y m i n e r a l s , q u a r t z , c a l c i t e , p y r i t e , and o t h e r m i n e r a l p a r t i c l e s d i s p e r s e d i n water. E f f e c t i v e treatment of the e f f l u e n t s must be c a r r i e d o u t r e g a r d l e s s o f w h e t h e r t h e w a t e r i s t o be r e u s e d o r d i s c h a r g e d . Under a g r a n t , t h e 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 i s i n v e s t i g a t i n g the treatment of b l a c k water from c o a l p r e p a r a t i o n p l a n t s by a f l o c c u l a t i o n p r o c e s s . R e f u s e Pond S u r v e y . In coal preparation plants u t i l i z i n g wet c l e a n i n g m e t h o d s , t h e f i n e c o a l r e f u s e m a t e r i a l c o n s t i t u t e s a b o u t 10% o f t h e t o t a l r e f u s e m a t e r i a l and i s d i s c h a r g e d i n s u s p e n s i o n e i t h e r as t a i l i n g s from the f r o t h f l o t a t i o n p r o c e s s or a s wet f i n e s t a k e n d i r e c t l y f r o m t h e c o a r s e c o a l w a s h e r y c i r c u i t s . The U n i v e r s i t y o f A l a b a m a c u r r e n t l y has a g r a n t f r o m t h e B u r e a u t o s u r v e y e x i s t i n g c o a l w a s t e ponds i n A l a b a m a t o d e t e r m i n e the l o c a t i o n of t h e wastes, the approximate q u a n t i t y of c o a l i n e a c h w a s t e p o n d , and t h e p o t e n t i a l r e c o v e r y o f t h e c o a l b a s e d on w a s h a b i l i t y t e s t s of r e p r e s e n t a t i v e samples of the waste. R e f u s e Pond S t a b i l i z a t i o n S t u d y . A l t h o u g h r e f u s e ponds c o n t i n u e t o be u s e d f o r t a i l i n g s d i s p o s a l , o t h e r c o n s i d e r a t i o n s s u c h as g r o u n d and s u r f a c e w a t e r p o l l u t i o n , l a c k o f s u i t a b l e t e r r a i n , and e n v i r o n m e n t a l r e s t r i c t i o n s may r e n d e r t h e p o n d i n g method t o t a l l y u n a c c e p t a b l e o r p r o h i b i t i v e l y e x p e n s i v e i n t h e future. Under a c o n t r a c t j u s t s i g n e d w i t h t h e Dravo L i m e Co., t h e c r i t i c a l p a r a m e t e r s f o r l a n d f i l l - t y p e d i s p o s a l w i l l be e s t a b l i s h e d by d e t e r m i n i n g t h e e n g i n e e r i n g p r o p e r t i e s and t h e p h y s i c a l and chemical c h a r a c t e r i s t i c s a f f e c t i n g s t a b i l i z a t i o n f o r t y p i c a l samples o f p r e p a r a t i o n p l a n t wastes from t h e t r e a t m e n t of e a s t e r n bituminous coals. C a l c i l o x a d d i t i v e , a Dravo-developed product, w i l l be one o f t h e p r i n c i p a l s t a b i l i z i n g a g e n t s u s e d i n t h i s study. L i g n i t e Upgrading. L i g n i t e d e p o s i t s i n the North DakotaM o n t a n a r e g i o n r e p r e s e n t one o f t h e l a r g e s t f o s s i l f u e l r e s e r v e s i n the United States. However, t h e u t i l i z a t i o n o f t h i s f u e l has b e e n l i m i t e d by i t s h i g h i n h e r e n t m o i s t u r e and i t s o f t e n h i g h s o d i u m c o n t e n t . L i g n i t e c o n t a i n s a b o u t 38% m o i s t u r e , and b e c a u s e of t h i s h i g h m o i s t u r e c o n t e n t , the market a r e a f o r l i g n i t e i s l i m i t e d t o a b o u t a 3 0 0 - m i l e r a n g e . Beyond t h i s d i s t a n c e , t h e c o s t of d e l i v e r e d B t u s exceeds t h a t of h i g h e r rank c o a l s . 1



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R e d u c i n g m o i s t u r e c o n t e n t t o 10% o r l e s s a t r e a s o n a b l e c o s t s w o u l d make t h i s l o w - s u l f u r c o a l c o m p e t i t i v e o v e r a much w i d e r market a r e a . Other problems a s s o c i a t e d w i t h t h e h i g h m o i s t u r e content of l i g n i t e a r e f r e e z i n g i n r a i l r o a d c a r s d u r i n g c o l d w e a t h e r and s l a c k i n g upon e x p o s u r e t o a i r , w h i c h c a u s e s d u s t p o l l u t i o n p r o b lems. L i g n i t e i s a l s o e x t r e m e l y r e a c t i v e t o w a r d o x y g e n and i s thus s u s c e p t i b l e t o spontaneous combustion. The s o d i u m c o n t e n t o f l i g n i t e v a r i e s w i d e l y and may be a s h i g h a s 1.2%. L i g n i t e c o n t a i n i n g l e s s t h a n 0.3% s o d i u m c a n be f i r e d w i t h o u t d i f f i c u l t y w h i l e h i g h e r percentages o f sodium c a u s e b o i l e r t u b e f o u l i n g , r e d u c e d e f f i c i e n c i e s , and e x c e s s i v e downtime t o remove a s h d e p o s i t s . The s o d i u m c o n t e n t o f l i g n i t e c a n be r e d u c e d b y i o n e x c h a n g e u s i n g m e t a l s a l t s w h i c h s u p p l y c a t i o n s t h a t a r e exchangeable w i t h sodium; t h e i r e f f e c t i v e n e s s depends upon t h e s i z e o f t h e l i g n i t e p a r t i c l e and t h e c o n tact time. The g o a l o f o u r l i g n i t e - u p g r a d i n g p r o g r a m i s t o p r o d u c e a f i n a l p r o d u c t c o n t a i n i n g l e s s t h a n 1 0 % m o i s t u r e and 0.3% s o d i u m . C r u s h e d r a w l i g n i t e w i l l be p r o c e s s e d i n a n i o n - e x c h a n g e v e s s e l where t h e s o d i u m c o n t e n t w i l l be r e d u c e d t o a n a c c e p t a b l e l e v e l . The i o n - e x c h a n g e p r o d u c t w i l l be d e w a t e r e d , p e l l e t i z e d , and h e a t d r i e d t o the d e s i r e d moisture content. This f i n a l product w i l l be a w e l l compacted w a t e r - r e s i s t a n t p e l l e t w h i c h w i l l n o t combust s p o n t a n e o u s l y and w i l l h a v e s u f f i c i e n t m e c h a n i c a l s t r e n g t h t o r e s i s t b r e a k a g e d u r i n g h a n d l i n g and s t o r a g e . Equipment Performance S t u d i e s . A s i g n i f i c a n t p a r t o f t h e B u r e a u ' s p r o g r a m f o r many y e a r s h a s b e e n equipment p e r f o r m a n c e evaluations. These s t u d i e s p r o v i d e t h e n e c e s s a r y i n f o r m a t i o n f o r c o a l p r e p a r a t i o n p l a n t o p e r a t o r s and equipment d e s i g n e r s t o s e l e c t and recommend e x i s t i n g c o a l w a s h e r s . I n t h i s study, o n l y two t y p e s o f c o a l - w a s h i n g e q u i p m e n t r e m a i n t o be e v a l u a t e d : j i g w a s h e r s , t o be c o m p l e t e d by t h e end o f 1978, and f r o t h f l o t a t i o n c e l l s , t o be c o n c l u d e d i n 1981. At t h i s time, an e v a l u a t i o n o f t h e performance of a Batac j i g i s underway. E a r l y i n 1978, a s t u d y w i l l b e g i n c o v e r i n g t h e p r i n c i p a l p i e c e s o f c o a l d e w a t e r i n g and d r y i n g e q u i p m e n t . A l l o f t h e s e s t u d i e s w i l l b e done by c o n t r a c t . Each i s s c h e d u l e d t o t a k e a p p r o x i m a t e l y 3 y e a r s . S c r e e n s , c o n t r i f u g e s , vacuum and p r e s s u r e f i l t e r s , and f i n a l l y t h e r m a l d r y e r s w i l l be e v a l u a t e d . Trace Elements. There i s an a c u t e awareness t h a t t r a c e elements i n c o a l might c o n t r i b u t e s u b s t a n t i a l q u a n t i t i e s o f p o t e n t i a l l y hazardous m a t e r i a l s t o t h e environment. Since c e r t a i n t r a c e elements c o n c e n t r a t e s e l e c t i v e l y i n p a r t i c u l a r s p e c i f i c g r a v i t y f r a c t i o n s o f raw c o a l , r e d u c t i o n o f t h e m i n e r a l m a t t e r may c a r r y t h e c o n c u r r e n t b e n e f i t o f r e d u c i n g t h e t r a c e e l e m e n t concentration i n the clean coal.



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55

The B u r e a u o f M i n e s r e c e n t l y c o m p l e t e d a w a s h a b i l i t y s t u d y (10) s h o w i n g t h e t r a c e e l e m e n t c o n t e n t s o f v a r i o u s s p e c i f i c g r a v i t y f r a c t i o n s f o r 10 c o a l s a m p l e s c o l l e c t e d f r o m v a r i o u s U.S. c o a l producing regions. R e l i a b l e a n a l y t i c a l methods w e r e d e v e l o p e d t o d e t e r m i n e cadmium, chromium, c o p p e r , f l u o r i n e , m e r c u r y , manganese, n i c k e l , and l e a d i n t h e w h o l e c o a l s and t h e v a r i o u s s p e c i f i c g r a v i t y f r a c t i o n s of the c o a l s . Most o f t h e t r a c e e l e m e n t s of i n t e r e s t c o n c e n t r a t e d i n t h e h e a v i e r s p e c i f i c g r a v i t y f r a c t i o n s of the c o a l , i n d i c a t i n g t h a t they are a s s o c i a t e d w i t h m i n e r a l matter; removal of t h i s m a t e r i a l w o u l d s i g n i f i c a n t l y r e d u c e t r a c e e l e m e n t s up t o 88%. I n a r e l a t e d , but g r e a t l y expanded, e f f o r t , the Bureau, t h r o u g h EPA, i s a b o u t t o f u n d a p r o g r a m w i t h B i t u m i n o u s C o a l R e s e a r c h . The o b j e c t i v e s o f t h i s p r o j e c t a r e — (1) To p r e p a r e a c o m p r e h e n s i v e s t a t e - o f - t h e - a r t assessment of the f a t e of t r a c e elements i n c o a l m i n i n g , p r e p a r a t i o n , t r a n s p o r t a t i o n , and u t i l i z a t i o n ; and (2) To d e t e r m i n e t h e e f f e c t o f c o a l c l e a n i n g on t r a c e elements. Computer S i m u l a t i o n . To a s s i s t i n t h e p r e d i c t i o n o f f u l l s c a l e p r e p a r a t i o n p l a n t o p e r a t i o n , a computer s i m u l a t i o n program i s b e i n g developed under c o n t r a c t w i t h the U n i v e r s i t y of P i t t s burgh (11). The s t a n d a r d p r e d i c t i o n o f p r o d u c t q u a l i t y ( s u c h as a s h , s u l f u r , B t u , and y i e l d ) i s c a l c u l a t e d b a s e d on t h e e x t e n s i v e equipment performance d i s t r i b u t i o n d a t a o b t a i n e d from p r e v i o u s U.S. B u r e a u o f M i n e s i n v e s t i g a t i o n s . S e v e r a l o t h e r f e a t u r e s , t h o u g h , a r e more n o v e l and i n c l u d e t h e m a t h e m a t i c a l m o d e l i n g o f t h e r o t a r y b r e a k e r , o t h e r c r u s h e r s , and b o t h wet and d r y s c r e e n s . The o v e r a l l c a p a b i l i t y o f t h e computer p r o g r a m i s s u c h t h a t a b u i l d - y o u r - o w n p r e p a r a t i o n p l a n t c a n be s i m u l a t e d . A l l i e d t o t h i s work i s a study b e i n g conducted f o r the Bureau by t h e H o f f m a n - M u n t n e r C o r p o r a t i o n . The i n f o r m a t i o n g e n e r a t e d under t h i s c o n t r a c t w i l l c o n s t i t u t e the economic s u b r o u t i n e f o r the t o t a l computer s i m u l a t i o n program. M

n

New F a c i l i t y . I n t h e U n i t e d S t a t e s t h e r e i s no a v a i l a b l e f u l l y i n t e g r a t e d d e m o n s t r a t i o n - t y p e f a c i l i t y f o r t e s t i n g a new c o a l washing t e c h n i q u e , flowscheme, or p i e c e of equipment. Cons e q u e n t l y , a s i g n i f i c a n t t i m e l a p s e e x i s t s between development of new e q u i p m e n t and t e c h n i q u e s and t h e i r a c c e p t a n c e by t h e c o a l industry. To a l l e v i a t e many o f t h e p a s t p r o b l e m s o f i n t r o d u c i n g new technology to the i n d u s t r y , a c e n t r a l c o a l p r e p a r a t i o n process d e v e l o p m e n t f a c i l i t y has b e e n d e s i g n e d . T h i s w i l l p e r m i t u n b i a s e d e n g i n e e r i n g d a t a t o be s c a l e d up r e a d i l y t o f u l l - s i z e c o m m e r c i a l coal preparation plant operation. Moreover, the expense of e v a l u a t i n g p r o c e s s e s t h a t p r o v e t o be o f l i m i t e d v a l u e t o t h e i n d u s t r y w i l l be r e d u c e d g r e a t l y .



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The p l a n t w i l l have a n o m i n a l c a p a c i t y o f 10-25 t o n s p e r h o u r o f r a w c o a l , d e p e n d i n g on t h e f l o w s c h e m e u s e d . P r o c e s s f l e x i b i l i t y i s a prime design r e q u i s i t e . New c r u s h i n g , s c r e e n i n g , d e w a t e r i n g , and c l e a n i n g e q u i p m e n t and c o a l w a s h i n g c i r c u i t s w i l l be e v a l u a t e d . T h i s p l a n t w i l l be used t o p r o c e s s c o a l s from d i f f e r e n t a r e a s a n d w i l l p r o v i d e optimum o p e r a t i n g c o n d i t i o n s needed t o p r o d u c e a c o a l o f a c c e p t a b l e s u l f u r , a s h , and t r a c e element c o n t e n t . An a d d i t i o n a l f u n c t i o n o f t h e d e m o n s t r a t i o n p l a n t w i l l b e t o p r o d u c e t o n l o t s o f s p e c i f i c a t i o n c o a l f o r com b u s t i o n t e s t i n g , s t a c k gas s c r u b b i n g work, o r f e e d s t o c k t o c o a l conversion processes. This b r i e f overview of c o a l preparation research a t the F e d e r a l B u r e a u o f M i n e s shows a w i d e s p e c t r u m o f i n t e r e s t i n p h y s i c a l , a s w e l l a s c h e m i c a l , c o a l p r e p a r a t i o n . As t h e N a t i o n g i v e s i n c r e a s i n g a t t e n t i o n t o e n e r g y - r e l a t e d problems, t h e pace of these c o a l p r e p a r a t i o n r e s e a r c h e f f o r t s i s l i k e l y t o accelerate.

Literature Cited

1. Deurbrouck, A. W., Jacobsen, P. S., "Coal Cleaning--State-of the-Art," Coal and Environmental Technical Conference and Equipment Exposition, Louisville, Ky., Oct. 22-24, 1974. 2. Cavallaro, J. Α., Johnston, M. T., Deurbrouck, A. W., "Sulfur Reduction Potential of the Coals of the United States," BuMines RI 8118, 1976, 323 pages. 3. Hucko, R. E., Cavallaro, J. Α., "Effect of Coal Preparation on Northern Appalachian Reserves," 4th National Conference on Energy and the Environment, American Institute of Chemical Engineers, Cincinnati, Ohio, Oct. 3-7, 1976, pp. 307-316. 4. Killmeyer, R. P., Jr., "State-of-the-Art of Coal Cleaning and Recent Developments in Equipment and Circuits," Third Kentucky Coal Refuse Disposal and Utilization Seminar, Pineville, Ky., May 11-12, 1977. 5. Hucko, R. E., Deurbrouck, A. W., "Overview of the U.S. Bureau of Mines Coal Cleaning Program," Coal Contaminant/Removal Technology Conference, Research Triangle Park, N.C., Apr. 27-28, 1977. 6. Miller, K. J., "Flotation of Pyrite From Coal," U.S. Patent No. 3807557, Apr. 30, 1974. 7. Baker, A. F., Miller, K. J., Deurbrouck, A. W., "Desulfuriza tion of Coal by Froth Flotation," Int. Coal Prep. Cong., Paris, France, March 1973, Paper 27E, 13 pp.; Available for consultation at the Bureau of Mines Coal Preparation and Analysis Laboratory, Bruceton, Pa. 8. Hoffman, L., Aresco, S. J., Holt, E. C., Jr., "Engineering/ Economic Analyses of Coal Preparation WithSO Cleanup Processes for Keeping Higher Sulfur Coals in the Energy Market," NCA/BCR Coal Conference and Expo III, Louisville, Ky., Oct. 19-21, 1976. 2


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9. Baker, A. F., Deurbrouck, A. W., "Hot Surfactant Solution as Dewatering Aid During Filtration," 7th Int. Coal Prep. Cong., Sydney, Australia, May 23-28, 1976. 10. Cavallaro, J. Α., Gibbon, G. Α., Hattman, Ε. Α., Schultz, H., Deurbrouck, A. W., "A Washability and Analytical Evaluation of Potential Pollution From Trace Elements in Coal," Joint EPA/Bureau report, not yet published. 11. Gottfried, B. S., Jacobsen, P. S., "A General Distribution Curve for Characterizing the Performance of Coal Cleaning Equipment," BuMines RI 8238, to be published in 1977.


An Overview of Coal Preparation - American Chemical Society

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