Mineral Matter and Ash in Coal - American Chemical Society

35 Separation of Mineral Matter from Pittsburgh Coal by Wet Milling Douglas V. Keller, Jr.

Downloaded by TUFTS UNIV on November 18, 2016 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch035

Otisca Industries, Ltd., Syracuse, NY 13208

The fracture of massive Pittsburgh coal permits some of the entrained mineral matter to be released as a distinct phase. A study of the variables that affect the release of mineral matter during the wet (water) milling process of that coal indicated that the mineral matter exists as a distinct and separable phase to a content of at least 0.5 weight percent mineral matter in the product coal. The reduction of mineral matter by physical separation procedures of the free mineral matter phase from the coal phase is directly related to the log of the mode of the particle size distribution of the raw coal. That relationship also appears to be insensitive to some of the common chemical additives that are introduced to enhance the rate of milling. The term " i n h e r e n t m i n e r a l m a t t e r , o r a s h " i s a commonly used p h r a s e i n t h e c o a l l i t e r a t u r e ( 1 ) . The p h r a s e r e f e r s t o t h a t f r a c t i o n o f the m i n e r a l m a t t e r bound o r g a n i c a l l y t o t h e carbonaceous s t r u c t u r e of t h e c o a l and e s t i m a t e s o f i t s c o n t e n t suggest t h a t i t i s i n t h e range o f two w e i g h t p e r c e n t o f t h e whole m i n e r a l m a t t e r ( 1 ) . Common c l a s s i c a l p h y s i c a l s e p a r a t i o n schemes such a s d i f f e r e n t i a l 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 s or f r o t h f l o t a t i o n which a r e d i r e c t e d a t déminera l i z i n g t h e c o a l never approach t h a t lower l i m i t . F o r example, a t the o u t s e t o f t h i s i n v e s t i g a t i o n one c o u l d r e c e i v e a raw P i t t s b u r g h seam c o a l a t 30 weight p e r c e n t a s h and t h r o u g h c a r e f u l f l o a t - s i n k p r o c e s s e s r e d u c e t h a t a s h c o n t e n t i n t o t h e range o f f o u r t o f i v e weight p e r c e n t . P r a c t i c a l l y r e d u c i n g t h a t a s h c o n t e n t , however, t o below t h r e e weight p e r c e n t w i t h a r e a s o n a b l e y i e l d was q u i t e u n l i k e l y ( 2 ) . One consequence o f t h e l a r g e weight f r a c t i o n d i f f e r e n t i a l between t h e p r a c t i c a l l i m i t o f d e m i n e r a l i z a t i o n , and i f you l i k e , t h e t r u e " i n h e r e n t " l i m i t o f d e m i n e r a l i z a t i o n was a t o t a l l a c k of u n d e r s t a n d i n g a s t o whether o r n o t t h a t m i n e r a l f r a c t i o n c o u l d i n d e e d be e x t r a c t e d by p h y s i c a l means. W i t h t h i s a s a b a s i s , f u r t h e r q u e s t i o n s c o u l d be r a i s e d as t o whether o r n o t t h a t r e t a i n e d m i n e r a l m a t t e r was a t r u e d i s t i n c t m i n e r a l phase, o r i f s o , c o u l d i t be bound c h e m i c a l l y a l o n g t h e i n t e r f a c e s t o t h e c o a l s t r u c t u r e r e n d e r i n g t h o s e p a r t i c l e s i n s e p a r a b l e . Most s i m p l y , i n a l l c a s e s 0097-6156/ 86/0301 -0473S06.00/0 © 1986 American Chemical Society

Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

MINERAL MATTER AND ASH IN COAL

474

one c o u l d a s k t h e q u e s t i o n does a s e p a r a t i o n o f m i n e r a l m a t t e r from the c o a l take p l a c e i n a l l c a s e s when c o a l i s f r a c t u r e d and t o what e x t e n t , o r l i m i t , c a n t h e f r a c t u r e p r o c e s s be u t i l i z e d i n t h e demineralization of c o a l . A s e r i e s o f e x p e r i m e n t s was a s s i g n e d t o e x p l o r e t h e e x t r a c t i o n of m i n e r a l m a t t e r from c o a l i n t h e s i z e r a n g e s below 0.25 mm. The raw c o a l samples were o b t a i n e d from t h r e e d i f f e r e n t s o u r c e s i n t h e P i t t s b u r g h seam which p e r m i t t e d a degree o f comparison over a r a t h e r l a r g e geographic area. The r e s u l t s a r e i n t e r e s t i n g i n t h a t t h e y a l l o w a new p e r s p e c t i v e i n t h e d e m i n e r a l i z a t i o n o f c o a l .


Experimental The P i t t s b u r g h seam c o a l s used i n t h i s i n v e s t i g a t i o n have a nominal a n a l y s i s as i l l u s t r a t e d i n T a b l e I . L o t s o f P i t t s b u r g h seam c o a l i n e x c e s s o f 1000 pounds each were r e c e i v e d from t h r e e d i f f e r e n t s o u r c e s i n Washington County: Mine (A) was l o c a t e d about 20 m i l e s west o f P i t t s b u r g h ; Mine (B) was l o c a t e d j u s t s o u t h o f P i t t s b u r g h ; Mine (C) was l o c a t e d about 40 m i l e s s o u t h o f P i t t s b u r g h . R e p r e s e n t a t i v e samples from each s o u r c e were o b t a i n e d by ASTM p r o c e d u r e s and s u b j e c t e d tc t h e f o l l o w i n g p r o c e s s i n g . A raw c o a l sample was reduced t o 250 ym χ 0 by d r y m e c h a n i c a l c r u s h i n g i n a hammermill and then ground i n a l a b o r a t o r y sample m i l l . The m e c h a n i c a l l y ground c o a l was then mixed w i t h water t o form one l i t e r o f s l u r r y w i t h 30 w e i g h t p e r c e n t s o l i d s and p l a c e d i n a s t a n d a r d 3 l i t e r l a b o r a t o r y b a l l m i l l u s i n g 3/8" a l u m i n a g r i n d i n g media. A l l o f b a l l m i l l i n g v a r i a b l e s were h e l d c o n s t a n t except f o r the d u r a t i o n o f m i l l i n g which p e r m i t t e d a v a r i a t i o n o f t h e p a r t i c l e size distribution. I n t h e event t h a t c h e m i c a l s were employed d u r i n g the b a l l m i l l i n g o p e r a t i o n , t h o s e c h e m i c a l s were i n c o r p o r a t e d i n an excess o f the amount o f t h e s t a n d a r d m i l l c o n t e n t . The c o a l water s l u r r y was removed from the m i l l , d i l u t e d w i t h water t o t e n weight p e r c e n t s o l i d s and t h e c o a l f r a c t i o n removed u t i l i z i n g t h e O t i s c a T - P r o c e s s (OTP) (2,3) . S e p a r a t i o n s by t h e T-Process a r e unique i n that agglomeration r e s u l t s i n the recovery of v i r t u a l l y a l l o f t h e carbonaceous m a t e r i a l l e a v i n g a f u l l y d i s p e r s e d m i n e r a l phase i n t h e r e s i d u e w a t e r . Many d e t a i l e d i n v e s t i g a t i o n s o f t h i s type have c o n c l u d e d t h a t m i n e r a l m a t t e r r e c o v e r e d w i t h the c o a l phase i s i n c l u d e d i n t h e c o a l ; t h a t i s t h e a s h c o n t e n t o f the p r o d u c t c o a l r e p r e s e n t s o n l y t h a t m i n e r a l m a t t e r m e c h a n i c a l l y a t t a c h e d t o o r enveloped by t h e c o a l . A n a l y t i c a l p r o c e d u r e s f o r a s h ( h i g h temperature, HT) and s u l f u r c o n t e n t s were conducted a c c o r d i n g t o ASTM p r o c e d u r e s . Low tempera t u r e a s h p r o c e d u r e s were conducted a t 550°C i n an oven w i t h an adequate s u p p l y o f oxygen. The d i f f e r e n c e i n m i n e r a l m a t t e r mor phology and c h e m i s t r y between t h i s t e c h n i q u e and t h e low temperature a s h i n g method d e s c r i b e d by G l u s k o t e r ( 3 ) , c a n be a n t i c i p a t e d from the paper by M i t c h e l l and G l u s k o t e r ( 4 ) . P r i n c i p a l l y t h e h i g h e r temperature a s h i n g p r o c e s s w i l l c o n v e r t p y r i t e t o h e m a t i t e and k a o l i n i t e t o m e t a k a o l i n i t e by t h e l o s s o f water o f c r y s t a l l i z a t i o n . Between 30% and 50% o f t h e m i n e r a l m a t t e r i n t h e P i t t s b u r g h c o a l i s c o n s i d e r e d t o be k a o l i n i t e ( 5 ) , and i n t h i s i n v e s t i g a t i o n we presumed t h a t t h e 550°C a s h i n g p r o c e d u r e d i d n o t s i g n i f i c a n t l y a l t e r the p a r t i c l e s i z e d i s t r i b u t i o n o f t h e o r i g i n a l k a o l i n i t e p a r t i c l e s . The presumption was t e s t e d by comparing t h e s i z e d i s t r i b u t i o n o f raw


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of Mineral

Matter from

Milling

475

m i n e r a l m a t t e r and t h a t f i r e d a t 550°C. L i t t l e change i n s i z e was noted. A M i c r o m e r i t i c s 5500L u n i t was used t o o b t a i n the p a r t i c l e s i z e d i s t r i b u t i o n data f o r t h i s i n v e s t i g a t i o n . Data from the M i c r o m e r i t i c s u n i t u s i n g a one ym mode sample was compared w i t h the d a t a developed by the m a n u f a c t u r e r from a C o u l t e r Counter on the same sample to w i t h i n t e n p e r c e n t . Even w i t h t h i s i n hand, t h e r e was no attempt t o c h a r a c t e r i z e the p a r t i c l e s or t h e i r shape on an a b s o l u t e basis. The M i c r o m e r i t i c s u n i t p r o v i d e s p a r t i c l e d i a m e t e r d a t a i n terms o f " a r e a p e r c e n t " which a r e r e a d i l y c o n v e r t e d t o "mass p e r c e n t " by a m a t h m a t i c a l format d e v e l o p e d by M i c r o m e r i t i c s , c f the i n s t r u ment handbook. The observed a r e a p e r c e n t d a t a from the M i c r o m e r i t i c s u n i t was t r a n s p o s e d t o a mass p e r c e n t base a t d a t a p o i n t s y ( i n ym) f o r a l l of the p o i n t s y 2 where χ = η + 0.5 and η = -3, -2.5, ...0..., +7, +7.5. The d a t a p o i n t a t y r e p r e s e n t s t h a t mass f r a c t i o n o f m a t e r i a l l y i n g i n t h e s i z e range χ + 0.25 ym. The d a t a g i v e n below a r e p l o t t e d w i t h the o r d i n a t e p r o v i d i n g e i t h e r the m i n e r a l m a t t e r , low temperature a s h or c o a l , c o n t e n t i n weight p e r c e n t based on the c o a l o r as (dw/d(logx)) which i s the weight p e r c e n t o f m i n e r a l m a t t e r i n t h a t p a r t i c u l a r s i z e r a n g e ( l o g x ) of a l l the m i n e r a l m a t t e r p r e s e n t i n the system. β


Coal by Wet

X

The f o l l o w i n g p a r t i c l e s i z e d i s t r i b u t i o n d a t a a r e g i v e n as a l o g d i s t r i b u t i o n i n p a r t i c l e diameter (ym) where the mass p o i n t s a r e i n t e r c o n n e c t e d f o r c o n v e n i e n c e o f comparison a t the expense o f r i g o r . T y p i c a l d i s t r i b u t i o n s a r e b e l l - s h a p e d where the mode i s d e f i n e d as the p a r t i c l e d i a m e t e r a t t h a t p o i n t of h a l f w i d t h o f the c u r v e a t the h a l f h e i g h t o f t h e maximum. The M i c r o m e r i t i c u n i t i s based on a S t o k e s Law s e t t l i n g of the p a r t i c l e s where one must choose an average s p e c i f i c g r a v i t y of p a r t i c l e s under i n v e s t i g a t i o n b e f o r e the data are recorded. In t h o s e c a s e s where low temperature ash p a r t i c l e s were i n v e s t i g a t e d , the i r o n m i n e r a l s w i t h a d e n s i t y l a r g e r than 4 gms/cc were s e p a r a t e d i n a dense f l u i d from the c l a y m i n e r a l s w i t h d e n s i t i e s l e s s than 3 gms/cc. A l l d e n s i t i e s were determined by p i c n o m e t r i c methods. The d i s t r i b u t i o n s were measured i n d i v i d u a l l y and then the s i z e d i s t r i b u t i o n s were recombined m a t h e m a t i c a l l y . The p r o d u c t c o a l s demonstrated a v e r y narrow s p e c i f i c g r a v i t y d i s t r i b u t i o n i n the range of 1.33. 1

R e s u l t s and D i s c u s s i o n F i g u r e 1 i l l u s t r a t e s the p a r t i c l e s i z e d i s t r i b u t i o n o f the m i n e r a l matter t h a t r e s u l t s from the low temperature a s h i n g o f t h r e e 5 cm cubes of b r i g h t c o a l t h a t were h a n d - p i c k e d from the v a r i o u s samples. The ash c o n t e n t s were i n the range of f i v e weight p e r c e n t . I t i s of i n t e r e s t to n o t e t h a t band of m i n e r a l matter p a r t i c l e s t h a t l i e i n the p a r t i c l e d i a m e t e r range between one and 50 ym w i t h a mode between f o u r and e i g h t ym. The p a r t i c l e s i z e d i s t r i b u t i o n d a t a from the low temperature a s h i n g o f the 5 cm cubes o f c o a l show t h a t raw c o a l from each s o u r c e has a unique " f i n g e r p r i n t " of m i n e r a l m a t t e r p a r t i c l e s d i s t r i b u t e d i n the p a r t i c l e s i z e range below 250 ym. It s h o u l d be noted t h a t the d i s t r i b u t i o n c u r v e s a r e r e p r o d u c i b l e t o w i t h i n p l u s o r minus f i v e p e r c e n t of t h e d e s i g n a t e d ash, or c o a l , c o n t e n t v a l u e s g i v e n f o r each p a r t i c l e diameter range ( y ) . I n v e s t i g a t i o n s of many o t h e r c o a l seams and c o a l s w i t h i n a p a r t i c u l a r seam i n d i c a t e t h a t indeed the m i n e r a l m a t t e r d i s t r i b u t i o n


476


Table

I.

Nominal A n a l y s i s o f P i t t s b u r g h Seam C o a l


Weight (Dry V o l a t i l e Matter F i x e d Carbon BTU/lb Carbon Hydrogen Nitrogen Chlorine Sulfur Oxygen ( d i f f ) Ash

^

Percent Basis)

35 58 14,200 77.3 5.2 1.5 0.1 1.5 7.6 6.8

1.0 -

*

Ζ

0.8 -

P a r t i c l e Diameter

(microns)

F i g u r e 1. P a r t i c l e s i z e d i s t r i b u t i o n s o f t h e low temperature a s h p r o d u c t s from t h r e e d i f f e r e n t 5 cm b l o c k s o f P i t t s b u r g h seam coal: (O) C o a l Α; ( Δ ) C o a l B; and (O) C o a l C.



35.

KELLER

Separation

of Mineral

Matter from

Coal by Wet

Milling

All

v a r i e s w i d e l y b o t h i n shape and magnitude and as such cannot be a n t i c i p a t e d from o t h e r c o a l p r o p e r t i e s . The m i n e r a l matter d i s t r i b u t i o n i s a fundamental p r o p e r t y o f t h a t c o a l which i s an uncon t r o l l e d n a t u r a l v a r i a b l e i n t h e e x t r a c t i o n of m i n e r a l m a t t e r . In o r d e r t o demonstrate t h a t the 5 cm cube was c o n s t i t u t e d of an a c c u m u l a t i o n of much s m a l l e r u n i t volumes, each of which r e p r e s e n t e d the whole c o a l i n m i n e r a l matter p a r t i c l e s i z e d i s t r i b u t i o n , a s t u d y of a s e r i e s of s i z e c l a s s i f i e d p a r t i c l e s was u n d e r t a k e n . F o r example, i f raw c o a l (C) were ground t o 250 ym χ 0, and then s e p a r a t e d w i t h s t a n d a r d s i e v e s i n t o the s i z e f r a c t i o n 53 χ 44 ym, we would be a f f o r d e d a d r y m i x t u r e of raw c o a l p a r t i c l e s and m i n e r a l p a r t i c l e s w i t h an average s i z e o f 4 8 + 4 ym. Figure 2 i l l u s t r a t e s the p a r t i c l e s i z e d i s t r i b u t i o n of the low temperature ash p r o d u c t o f the p r o d u c t c o a l a f t e r the f r e e m i n e r a l m a t t e r p a r t i c l e s were removed. A g a i n we have the c h a r a c t e r i s t i c c u r v e v e r y s i m i l a r t o t h e c u r v e shown i n F i g u r e 1 ( C ) . In f a c t , t h a t p o r t i o n o f t h e c u r v e t h a t l i e s below 3 ym can most u s u a l l y be superimposed on o t h e r c u r v e s o b t a i n e d i n a s i m i l a r manner from the o t h e r s i z e f r a c t i o n s o f c o a l (C). P r o v i d i n g , t h a t i s , t h a t the o r i g i n a l c o a l p a r t i c l e d i a m e t e r s a r e l a r g e r t h a n 10 ym. The c o n c l u s i o n of t h a t s t u d y i n d i c a t e d t h a t p r o d u c t c o a l p a r t i c l e s l a r g e r i n diameter t h a n the band of m i n e r a l m a t t e r m i c r o p a r t i c l e s seemed to c o n t a i n the whole p a r t i c l e d i s t r i b u t i o n o f the s m a l l micropartides. That i s , t h e r e appeared t o be a r e l a t i v e l y homoge neous d i s t r i b u t i o n of p a r t i c l e s t h r o u g h o u t , l i m i t e d i n top s i z e by the l a r g e s t p a r t i c l e i n the t e s t . To e x p l o r e t h a t a s p e c t i n more depth, a raw c o a l was wet m i l l e d to s m a l l e r s i z e s . When a raw c o a l i s wet b a l l - m i l l e d f o r a s u f f i c i e n t t i m e t o produce a s l u r r y w i t h a p a r t i c l e diameter mode i n the range of 4 ym t h e r e r e s u l t s two forms o f m i n e r a l m a t t e r : That f r a c t u r e d away from the c o a l and t h a t which i s s t i l l enveloped i n the c o a l p a r t i c l e s . F i g u r e 3 i l l u s t r a t e s a t y p i c a l p a r t i c l e s i z e d i s t r i b u t i o n f o r the s e p a r a t e d p r o d u c t c o a l as compared to the s e p a r a t e d f r e e m i n e r a l m a t t e r (90 weight p e r c e n t ash) from one m i l l i n g t e s t . The s e p a r a t e d m i n e r a l m a t t e r i s c l e a r l y s m a l l e r i n d i a m e t e r t h a n t h e c o a l which i s p r o b a b l y due t o i t s more b r i t t l e p r o p e r t i e s . Note t h a t i n F i g u r e s 3 and 4 an i n t e g r a t i o n o f the c u r v e s w i l l y i e l d 100% o f the m i n e r a l m a t t e r (or ash) under c o n s i d e r a t i o n r a t h e r t h a n t h e a s h c o n t e n t o f the c o a l as was t h e c a s e i n F i g u r e s 1 and 2. When the p r o d u c t c o a l shown i n F i g u r e 3 was s u b j e c t e d to low temperature a s h i n g as d e s c r i b e d above and t h a t a s h p r o d u c t sub j e c t e d to p a r t i c l e s i z e a n a l y s i s , a c u r v e as i s i l l u s t r a t e d i n Figure 4 results. C l e a r l y the enveloped m i n e r a l m a t t e r i n the p r o duct c o a l p a r t i c l e s i s c o n s i d e r a b l y s m a l l e r i n diameter than the c o a l p a r t i c l e s from which they came and as such a r e n o t a v a i l a b l e f o r s e p a r a t i o n by the T - P r o c e s s . The T - P r o c e s s s e p a r a t i o n r e j e c t s a l l p a r t i c l e s of pure m i n e r a l m a t t e r and a g g l o m e r a t e s as p r o d u c t c o a l a l l p a r t i c l e s t h a t have any f r a c t i o n of c o a l exposed t o the l i q u i d system. Two v e r y i m p o r t a n t p o i n t s a r e i l l u s t r a t e d i n F i g u r e s 1-4: F i r s t l y , i t appears t h a t as c o a l f r a c t u r e takes p l a c e i n t h i s system, m i n e r a l m a t t e r p a r t i c l e s a r e e j e c t e d from t h e f r a c t u r e d c o a l system and most w i t h o u t a t t a c h e d c o a l . The i m p l i c a t i o n i s t h a t the c o a l m i n e r a l m a t t e r i n t e r f a c e s a r e not c h e m i c a l l y bound. I f those i n t e r f a c e s were c h e m i c a l l y bound, one would o b s e r v e m i n e r a l m a t t e r


478


^

1.0 •

3

~


ο

0.8 -

Particle

Diameter

(microns)

F i g u r e 2. P a r t i c l e s i z e d i s t r i b u t i o n o f t h e low temperature a s h p r o d u c t o f a 48 + 4 ym p r o d u c t c o a l a f t e r t h e f r e e m i n e r a l m a t t e r was removed.

25h

I I 10"

. 1

.

. 10 P a r t i c l e Diameter

10

0

1

(microns)

F i g u r e 3. When t h e p r o d u c t s o f wet m i l l i n g 250 ym χ 0 P i t t s b u r g h c o a l (C) a r e s e p a r a t e d , two p r o d u c t s e v o l v e : P r o d u c t c o a l and separated mineral matter. The p a r t i c l e s i z e d i s t r i b u t i o n s f o r each a r e i l l u s t r a t e d .



35.

KELLER

Separation

of Mineral

Matter from

Coal by Wet

Milling

479

r e j e c t i o n o n l y when t h e p a r t i c l e s were broken i n t o s m a l l e r p i e c e s . The r e s u l t would be a l a r g e i n c r e a s e i n the sub-micron p a r t i c l e p o p u l a t i o n i n the r e l e a s e d m i n e r a l m a t t e r , c f . F i g u r e 3, as w e l l as t h a t i n the low temperature a s h of the p r o d u c t c o a l . The l a t t e r i s not observed as i s i l l u s t r a t e d below. Secondly, g i v e n the low temperature a s h s i z e d i s t r i b u t i o n of the raw c o a l and a knowledge o f the raw c o a l p a r t i c l e s i z e d i s t r i b u t i o n a f t e r wet m i l l i n g , we a r e i n a p o s i t i o n to p r e d i c t the ash c o n t e n t of the p r o d u c t c o a l . C o n s i d e r F i g u r e 5 where a h y p o t h e t i c a l low temperature a s h p a r t i c l e s i z e d i s t r i b u t i o n i s superimposed on a T-Process product c o a l s i z e d i s t r i b u t i o n . The p r o d u c t c o a l p a r t i c l e s w i t h d i a m e t e r s l y i n g between χ and dx c o n t a i n no m i n e r a l matter p a r t i c l e s l a r g e r t h a n dx as t h o s e p a r t i c l e s were removed d u r i n g the s e p a r a t i o n p r o c e s s . The m i n e r a l m a t t e r r e t a i n e d i n t h e p r o d u c t c o a l p a r t i c l e s i s the c u m u l a t i v e m i n e r a l m a t t e r c o n t e n t r e p r e s e n t e d by the low temperature a s h c u r v e . S i n c e the m i n e r a l c o n t e n t g i v e n on the o r d i n a t e i n F i g u r e 5 i s based on 100%, the a s h c o n t e n t i n each s i z e range can be e s t i m a t e d by m u l t i p l y i n g the mass f r a c t i o n o f t h a t p o i n t by the t o t a l ash c o n t e n t i n the raw c o a l sample, i . e . 250 ym χ 0,

t h a t was used to g e n e r a t e the low temperature a s h c u r v e . A s p e c i f i c c a s e i s examined i n T a b l e I I where the p a r t i c l e s i z e d i s t r i b u t i o n d a t a from the low temperature a s h i n g of a 5 cm cube of c o a l and 44-53 ym c o a l was r e l a t e d t o two p r o d u c t c o a l samples m i l l e d under d i f f e r e n t c o n d i t i o n s a l l o f which o r i g i n a t e d from the same s o u r c e c o a l ( C ) . The f i r s t column i n T a b l e I I p r o v i d e s the average p a r t i c l e d i a m e t e r p o i n t s (y) a t which the d a t a were o b s e r v e d 1, e. the p a r t i c l e s w i t h d i a m e t e r s l y i n g between χ and χ + dx. A comparison of t h e low temperature a s h d a t a f o r the 5 cm cube, column 2, and f o r the 44-53 ym c o a l , column 3, i l l u s t r a t e s t h a t the f r a c t u r e of c o a l from a 5 cm cube t o 48 ym does not s i g n i f i c a n t l y d i s t u r b the m i n e r a l matter p a r t i c l e s l y i n g i n the range o f d i a m e t e r s below 6 ym. S i n c e a 4 ym p a r t i c l e of p r o d u c t c o a l ought t o have the complete m i n e r a l m a t t e r p a r t i c l e d i s t r i b u t i o n s m a l l e r than 4 ym enveloped i n t h a t p a r t i c l e , a c u m u l a t i v e a s h f r a c t i o n of the l e s s e r p a r t i c l e d i a m e t e r ash s h o u l d be e q u i v a l e n t t o the ash c o n t e n t w i t h i n the 4 ym p a r t i c l e s , t h a t i s , 2.67 weight p e r c e n t ash, c f . column 4 a t 4 ym. A t e s t o f t h i s r e l a t i o n s h i p i s a f f o r d e d i n column 5 where we o b s e r v e the mass f r a c t i o n of p a r t i c l e s a t v a r i o u s d i a m e t e r s of a p r o d u c t c o a l from a s t a n d a r d m i l l r u n w i t h no c h e m i c a l s added. The p r e d i c t e d a s h c o n t e n t o f the p r o d u c t c o a l i s determined by a summation, o v e r a l l p a r t i c l e d i a m e t e r s o f the p r o d u c t o f the a s h c o n t e n t o f each d i a m e t e r cumulated by i n c r e a s i n g d i a m e t e r , ( X ) , times the mass f r a c t i o n o f the p r o d u c t c o a l a t the p a r t i c u l a r d i a m e t e r , ( y ) . The p r e d i c t e d ash c o n t e n t f o r t h i s c a s e i s 1.23 weight p e r c e n t ash which can be compared t o the o b s e r v e d v a l u e u s i n g ASTM p r o c e d u r e s of 1.12 weight p e r c e n t a s h . Following i d e n t i c a l m i l l i n g p r o c e d u r e s , except f o r the a d d i t i o n of 20 pounds per t o n l i g n i n s u l f o n a t e , a d i s p e r s a n t , we o b t a i n a much f i n e r s i z e d i s t r i b u t i o n as i s i l l u s t r a t e d i n column 6, T a b l e I I . The p r e d i c t e d a s h c o n t e n t i n t h a t c a s e i s 0.77 weight p e r c e n t ash w h i l e the o b s e r v e d v a l u e was 0.91 weight p e r c e n t a s h . The l a r g e d i s c r e p a n c y i n the f i n e r c o a l c a s e was p r o b a b l y due to i m p e r f e c t s e p a r a t i o n p r o c e d u r e s t h a t were caused by the p r e s e n c e o f t h e d i s p e r s a n t . The a b i l i t y to p r e d i c t ash c o n t e n t s u s i n g t h i s p r o c e d u r e has been a p p l i e d t o


480


25r


0

I

10"

• 1

.

.—

10

0


.

1

10

1

(microns)

F i g u r e 4. P a r t i c l e s i z e d i s t r i b u t i o n o f t h e low temperature a s h p r o d u c t from t h e same p r o d u c t c o a l sample shown i n F i g u r e 3.

F i g u r e 5. A h y p o t h e t i c a l p a r t i c l e s i z e d i s t r i b u t i o n c u r v e o f low temperature a s h and p r o d u c t c o a l .



35.

KELLER

Separation

of Mineral

Matter from

Coal by Wet

Milling

481

s e v e r a l d i f f e r e n t c o a l s from d i f f e r e n t seams and t h e same seam w i t h r e s u l t s usually w i t h i n ten percent. A c a r e f u l examination of these d a t a l e n d s c r e d e n c e t o t h e o b s e r v a t i o n t h a t f o r the most p a r t t h e m i n e r a l m a t t e r i n c l u d e d i n c o a l to the m i c r o n p a r t i c l e s i z e range i s indeed a d i s t i n c t s e p a r a b l e phase c a p a b l e of p h y s i c a l s e p a r a t i o n by fracture. The e x t e n t to which m i l l i n g can be c a r r i e d out and s t i l l a t t a i n e f f e c t i v e d e m i n e r a l i z a t i o n i s t o a degree l i m i t e d by our knowledge of m i l l i n g . C o a l Β was reduced i n ash i n t o the range o f 0.5 weight p e r c e n t , however, t h a t l i m i t appeared t o be a f u n c t i o n o f m i l l i n g phenomena as f u r t h e r m i l l i n g d i d not a l l o w f u r t h e r ash r e d u c t i o n . C l e a r l y , a c c o r d i n g t o the a n a l y s i s shown i n column 5 o f T a b l e I I and the subsequent d i s c u s s i o n , one might expect a monotonie d e c r e a s e i n p r o d u c t c o a l ash c o n t e n t w i t h p a r t i c l e s i z e d i s t r i b u t i o n mode to z e r o m i n e r a l m a t t e r which has not been o b s e r v e d . What was not a n t i c i p a t e d was the o b s e r v a t i o n t h a t the r e l a t i o n s h i p appeared to be i n s e n s i t i v e to what might be c o n s i d e r e d as r a t h e r s e v e r e changes i n the c h e m i c a l environment d u r i n g the m i l l i n g p r o c e s s even though some o f the c h e m i c a l a d d i t i v e s made s i g n i f i c a n t changes i n the m i l l i n g e f f i c i e n c y , i . e . , s p e c i f i c a r e a i n c r e a s e per u n i t i n p u t energy. C o n s i d e r the e f f e c t of t h r e e c h e m i c a l a d d i t i v e s : calcium h y d r o x i d e , sodium l i g n i n s u l f o n a t e , and sodium s u l f o s u c c i n a t e , as compared to the case of no a d d i t i v e s on the p a r t i c l e s i z e d i s t r i b u t i o n o f the p r o d u c t c o a l from a s t a n d a r d m i l l r u n shown i n F i g u r e 6 where a l l c o n d i t i o n s were i d e n t i c a l . C l e a r l y , t h e r e were no o b v i o u s effects. In the next s e r i e s , shown i n F i g u r e 7, we i n v e s t i g a t e d the a d d i t i v e s ammonium h y d r o x i d e , sodium h y d r o x i d e , and a h i g h e r c o n c e n t r a t i o n of sodium l i g n i n s u l f o n a t e . C l e a r l y , a dynamic d i f f e r e n c e i n s p e c i f i c s u r f a c e a r e a per u n i t i n p u t energy was o b s e r v e d . The e x p l a n a t i o n o f the d i f f e r e n c e s i s beyond the scope o f t h i s paper, but what was i n t e r e s t i n g was t h e e f f e c t t h a t the c h e m i c a l s had on the f r a c t u r e mechanism t h a t a f f e c t s the r e l e a s e o f m i n e r a l m a t t e r . F i g u r e 8 i l l u s t r a t e s a p l o t o f t h e ash c o n t e n t i n the p r o d u c t c o a l v e r s u s t h e mode of the p a r t i c l e s i z e d i s t r i b u t i o n o f the p r o d u c t coal. A l l of t h e t e s t s u t i l i z i n g c h e m i c a l a d d i t i v e s were m i l l e d under i d e n t i c a l c o n d i t i o n s u t i l i z e d i n t h e t e s t w h i c h produced the "no a d d i t i v e " d a t a w i t h a 2 ym mode. The t h r e e o t h e r "no a d d i t i v e " t e s t s were m i l l e d f o r extended t i m e s t o a c h i e v e s m a l l e r p a r t i c l e s i z e d i s t r i b u t i o n and t h e s e d a t a a r e shown i n F i g u r e 8. Some o f the c h e m i c a l a d d i t i v e s do a l t e r the s p e c i f i c m i l l i n g r a t e d u r i n g the p r o c e s s , but t h a t change does not a l t e r the r e l a t i o n s h i p between p a r t i c l e f r a c t u r e and m i n e r a l m a t t e r r e l e a s e d . Such l e n d s s u p p o r t to t h e presumed model t h a t the u l t r a f i n e m i n e r a l i s t o some degree homogeneously d i s t r i b u t e d and i s o n l y r e l e a s e d w i t h the f r a c t u r e of the c o a l p a r t i c l e . Conclusion High ranked b i t u m i n o u s c o a l s l i k e those o f the P i t t s b u r g h seam c o n t a i n a d i s t r i b u t i o n o f d i s c r e t e m i n e r a l m a t t e r p a r t i c l e s i n the s i z e range from 50 t o 1 ym which can be r e l e a s e d and p h y s i c a l l y s e p a r a t e d from t h e c o a l by normal f r a c t u r e mechanisms e x p e r i e n c e d i n wet b a l l m i l l i n g . S e p a r a t i o n of the p r o d u c t c o a l from the m i n e r a l m a t t e r d i s p e r s e d i n water was a c h i e v e d by a g g l o m e r a t i o n methods.


482


Table I I .

Data P o i n t

ym

P a r t i c l e S i z e Data F o r Low Temperature Ash P r o d u c t s and P r o d u c t C o a l

Low Temperature Ash - Wt.%

5 cm Cube

44-53ym


(y) 0.35 0.50 0.71 1.00 1.41 2.0 2.83 4.0 5.66

0.03 0.06 0.19 0.35 0.49 0.67 0.63 0.66 0.66

0.06 0.1 0.18 0.28 0.38 0.52 0.60 0.55 0.55

Product Coal Standard No Additives

44-53ym (Cumula t i v e X) 0.06 0.16 0.34 0.62 1.0 1.52 2.12 2.67 3.22

00 0.08 0.05 0.09 0.13 0.21 0.19 0.15 0.06 0.04

- Wt.%

Standard 20 pounds/ton Ligninsulfonate (Z) 0.16 0.09 0.15 0.23 0.16 0.11 0.16 0.03 0.01

y = 5.66 No A d d i t i v e s

έζ y = 0.35

Ligninsulfonate

< y < y y

1.12 Wt.% Ash Observed

y

0.77 Wt.% Ash P r e d i c t e d

y = 5.66

ΣΖ y = 0.35

1.23 Wt.% A s h P r e d i c t e d

( y ( y y

0.91 Wt.% Ash Observed

y


(microns)

F i g u r e 6. The e f f e c t s o f t h r e e c h e m i c a l a d d i t i v e s on p r o d u c t c o a l p a r t i c l e s i z e d i s t r i b u t i o n while m i l l i n g a t constant c o n d i t i o n s , ( O ) No A d d i t i v e s ; ( A ) 2 l b s / t o n C a l c i u m H y d r o x i d e ; ( D ) 2 l b s / t o n Sodium L i g n i n s u l f o n a t e ; and ( Ο ) 20 l b s / t o n Sodium Sulfosuccinate.


35.

KELLER

Separation

of Mineral

Matter from

Coal by Wet Milling

483


^ 0.5 "


(microns)

F i g u r e 7. The e f f e c t s o f t h r e e c h e m i c a l a d d i t i v e s on p r o d u c t c o a l p a r t i c l e s i z e d i s t r i b u t i o n while m i l l i n g a t constant conditions, ( O ) No A d d i t i v e s ; ( A ) 36 l b s / t o n Ammonium H y d r o x i d e ; ( D ) 7.9 l b s / t o n Sodium H y d r o x i d e ; and ( O ) 20 l b s / t o n Sodium Ligninsulfonate.

-

of

I7

ι 1 οm Β /•

-I

I

I- I 1 1 1 1

1

1

1

1 1 1 1 1 1

Mode o f PSD (microns) F i g u r e 8. Weight p e r c e n t a s h i n p r o d u c t c o a l v e r s u s t h e mode o f the p a r t i c l e s i z e d i s t r i b u t i o n of the product c o a l , ( O ) No a d d i t i v e s ; ( O 0 7.9 l b s / t o n Sodium H y d r o x i d e ; ( O ) 36 l b s / t o n Ammonium H y d r o x i d e ; ( ® ) 20 l b s / t o n Sodium L i g n i n s u l f o n a t e ; 20 l b s / t o n C a l c i u m H y d r o x i d e , ( X ) 2 l b s / t o n C a l c i u m H y d r o x i d e and ( Δ ) 2 l b s / t o n Sodium L i g n i n s u l f o n a t e .


484


D e m i n e r a l i z a t i o n of c o a l by t h i s mechanism appears t o be a p r e d i c t a b l e p r o c e s s w i t h an e r r o r i n the range o f 10 p e r c e n t . The d e m i n e r a l i z a t i o n of P i t t s b u r g h seam c o a l has been a c h i e v e d to the range o f 0.5 weight p e r c e n t a s h , a l i m i t w h i c h appears to be c o n t r o l l e d by the mechanics o f the b a l l m i l l . A p p l i c a t i o n of t h e p r o c e d u r e s used i n t h i s i n v e s t i g a t i o n t o c o a l s from o t h e r seams have r e s u l t e d i n p r o d u c t c o a l s w i t h a s h c o n t e n t s below 0.3 wt. %. F u r t h e r i n v e s t i g a t i o n s a r e underway which f o c u s on the mechanism of f r a c t u r e d u r i n g wet m i l l i n g . Acknowledgments


The a u t h o r extends h i s s i n c e r e a p p r e c i a t i o n to W. B u r r y and D. S. K e l l e r f o r t h e i r e f f o r t s i n the development of the e x p e r i m e n t a l p r o c e d u r e s and d a t a a n a l y s i s .

Literature Cited 1. 2.

3.

4. 5. 6.

Leonard, J. W., "Coal Preparation", Amer. Inst. Mining Met. Pet. Eng., Inc., New York (1979). Keller, Jr., D. V., "Otisca T-Process, A New Coal Beneficiation Approach for the Preparation of Coal Slurries", Coal Gasification, Liquifaction, and Conversion to Electricity Conference, University of Pittsburgh, August 1982. Keller, Jr., D. V., "Coal Refining by Physical Methods for the Preparation of Coal Slurries With Less Than 1 wt. % Ash", Fifth International Symposium on Coal Slurry for Combustion and Technology, Tampa, Florida, April 1983, U.S. DOE, Pittsburgh, p. 269. Gluskoter, H. L., Fuel, 44, 285 (1965). Mitchel, R. S.; Gluskoter, H. L., Fuel, 55, 90 (1976). O'Gorman, J. V.; Walker, P. L., "Mineral Matter and Trace Elements in U.S. Coals", U.S. Office of Coal Research, R & D Report No. 61, (1972).

RECEIVED July 23, 1985


Mineral Matter and Ash in Coal - American Chemical Society

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