Coal Microstructure and Pyrite Distribution - ACS Symposium Series

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Downloaded by UNIV OF BRIGHTON ALDRICH LIB on June 27, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch001

Coal Microstructure and Pyrite Distribution

RAYMOND T. GREER Department of Engineering Science and Mechanics, Engineering Research Institute, Ames Laboratory, U.S. ERDA, Iowa State University, Ames, IA 50011

The microstructure of coal is of interest in understanding coal properties. Microstructural features may affect the selection of coal beneficiation methods for removing impurities. For example, in planning sulfur removal schemes it is useful to understand the proportionment of inorganic sulfur compared with that of organic sulfur. Details of size, shape, orientation, and distribution factors for pyrite, certain pyrite and maceral groupings or fields, or other coal constituents(maceralsand other inorganic components) are of importance in supporting rational designs of sulfur removal processes. Recent scanning electron microscope (SEM) investigations help to show the interrelations between macerals and inorganic phases such as pyrite (FeS , the primary inorganic source of sulfur in coal) (1,2,3,4). This form of characterization complements optical microscope studies (5,6,7,8) and conventional transmission electron microscope (TEM) studies (9-15) which have been used to classify and to study coal constituents. SEM offers a way to visualize features hundreds and thousands of Angstroms in diameter by direct observation in three dimensions which is not available by any other technique. Many significant coal features (such as cellular components) and impurity crystals occur in this size range. Important features of coal constitution discovered by SEM are discussed below. 2

Experimental

Investigation

A p p a r a t u s . A JSM-U3 SEM w i t h a n EDAX e n e r g y d i s p e r s i v e x - r a y a n a l y s i s system i s used f o r t h e m i c r o s c a l e s t r u c t u r a l and c h e m i c a l studies. The r e s o l u t i o n c a p a b i l i t i e s o f t h e i n s t r u m e n t o n a d a y t o - d a y b a s i s a r e somewhat b e t t e r t h a n 200 Â f o r m i c r o s t r u c t u r a l i n f o r m a t i o n and s u b m i c r o m e t e r i n a c h e m i c a l a n a l y s i s mode f o r l o c a l i z e d c r y s t a l d e s c r i p t i o n . The s y s t e m p r o v i d e s s i z e , s h a p e , o r i e n t a t i o n , and d i s t r i b u t i o n i n f o r m a t i o n f o r i n o r g a n i c phases and coal constituents.

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

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COAL DESULFURIZATION


Procedure. Samples f o r SEM s t u d i e s a r e mounted i n an o r i e n t a t i o n where t h e f a c e and v e r t i c a l p o s i t i o n i n t h e c o a l seam a r e maintained. The e l e c t r o n beam a c c e l e r a t i n g p o t e n t i a l u s u a l l y u s e d i n v i e w i n g t h e s p e c i m e n s i s 25 kV. S a m p l e s r e c e i v e a vacuumd e p o s i t e d c o a t i n g o f a p p r o x i m a t e l y 200 Â g o l d ( f o r m i c r o s t r u c t u r a l s t u d i e s ; to m i n i m i z e c h a r g i n g problems) or a vacuum-deposited g r a p h i t e c o a t i n g of 20-100 Â ( f o r m i c r o c h e m i c a l s t u d i e s ; to a v o i d p o s s i b l e i n t e r p r e t a t i o n d i f f i c u l t i e s o f t h e s u l f u r and t h e g o l d x-ray fluorescence l i n e s ) . A d d i t i o n a l i n f o r m a t i o n about e x p e r i m e n t a l t e c h n i q u e a p p e a r s i n R e f . 4_. Materials. Samples o f h i g h - v o l a t i l e C b i t u m i n o u s c o a l f r o m b o t h s t r i p and s h a f t m i n e s i n Iowa f o r m t h e p r i m a r y m a t e r i a l s o f i n t e r e s t i n t h i s work. Results

and

Discussion

Microstructural Features. To see t h e i n t e r r e l a t i o n o f i n o r g a n i c p h a s e s s u c h as p y r i t e t o t h e c o a l c o n s t i t u e n t s (macérais), m i c r o g r a p h s have been chosen f i r s t to p r e s e n t the r e l a t i v e o r g a n i z a t i o n o f t h e m i c r o s t r u c t u r a l components. T h e s e w i l l be f o l l o w e d by a d d i t i o n a l m i c r o g r a p h s r e p r e s e n t a t i v e o f t h e common ways w h i c h p y r i t e c r y s t a l s occur i n c o a l . F i g u r e 1 p r o v i d e s d e t a i l s of c e l l u l a r compression. The c o a l s p e c i m e n f r o m t h e L o v i l i a ( s h a f t ) m i n e i s e x a m i n e d i n two o r i e n t a t i o n s : t h e top o f a f r e s h f r a c t u r e s u r f a c e c o r r e s p o n d i n g t o v a r i o u s m a g n i f i c a t i o n v i e w s o f the top o f t h e c o a l seam ( F i g u r e s l a - c ) ; and t h e s i d e , end-on v i e w o f t h e s t r a t i f i e d material (Figures l d - f ) . F r a g m e n t a t i o n and c o m p r e s s i o n f e a t u r e s of c e l l u l a r m a t e r i a l are seen f o r other c o a l s i n F i g u r e s 2, 3 , and 4 and i n R e f s . 1^ and 4_. I n F i g u r e s l a - c , the c o a l i f i e d p l a n t f i b e r s run from l e f t to r i g h t i n t h e i m a g e s . M o s t o f t h e f i b e r s a r e a p p r o x i m a t e l y 25 ym t h i c k , as s e e n f o r e x a m p l e i n a h i g h m a g n i f i c a t i o n v i e w i n F i g u r e le. M i c r o s t r u c t u r a l d e t a i l o f p i t s and o t h e r i n d i v i d u a l c e l l u l a r f i b e r d e t a i l s can be d i s c r i m i n a t e d i n t h i s v i e w as w e l l . The o p e n i n g s and r e g i o n s f o r m i n g p a t h w a y s o f i n t e r c o m m u n i c a t i o n f o r f l u i d s and gas b e t w e e n and w i t h i n c e l l s can be s e e n i n s i d e v i e w s ( F i g u r e s l d - f ) of the stacked f i b e r s . As t h e r e s o l u t i o n o f t h e s c a n n i n g e l e c t r o n m i c r o s c o p e i s o f t h e o r d e r o f 150 t o 200 A n g s t r o m s , m i c r o p o r e s o c c u r r i n g b e l o w t h i s s i z e w i l l n o t be o b s e r v e d , even a t the h i g h e s t m a g n i f i c a t i o n s . F i g u r e I f shows t h e c o a l o p e n i n g s i n t h i s c a s e t o be o f t h e o r d e r o f 20 ym o r l e s s , as i n f l u e n c e d by t h e d e g r e e o f c o m p r e s s i o n o f t h e f i b e r s s e e n i n t h e p a r t i c u l a r f i e l d o f view.* C o n s i d e r a b l e c o m p r e s s i o n o f t h e p l a n t s o u r c e m a t e r i a l i n t h e c o a l can o c c u r , and i n c e r t a i n e x a m p l e s t h e c e l l u l a r d e t a i l may be c o m p l e t e l y a b s e n t down t o t h e r e s o l u t i o n l i m i t of t h i s e l e c t r o n m i c r o s c o p y t e c h n i q u e . The c e l l w a l l s a r e o f t h e o r d e r o f 2 ym t h i c k i n t h e s e s i d e v i e w s ( F i g u r e I f , f o r e x ample) . F i g u r e 2 r e p r e s e n t s an e x a m p l e o f b o t h c o m p r e s s i o n and d i s -


GRÉER


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1.

Figure 1. Coal from the Lovilia mine (Iowa) showing detailed cellular features of the coalified plant material (a) (top left) Top view; scale bar, 200 \xm. (b) (top right) Top view; scale bar, 100 pm. (c) (middle left) Top view; scale bar, 20 ^m. (d) (middle right) Side view; scale bar, 200 fxm. (e) (bottom left) Side view; scale bar, 100 \xm. (f) (bottom right) Side view; scale bar, 20 pm.



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t o r t i o n of the c o a l i f i e d p l a n t c e l l u l a r f e a t u r e s . The s p e c i m e n was t i l t e d so t h a t b o t h t h e t o p and s i d e o f t h e s p e c i m e n c o u l d be seen i n t h i s v i e w . Across the c e n t r a l r e g i o n of the micrograph this i s particularly clear. An a n a l o g y w o u l d be t o t a k e a s t a c k of s o l i d r e c t a n g u l a r b o x e s ( e a c h open a t t h e e n d s ) , e a c h a p p r o x i m a t e l y 25 ym χ 25 ym on t h e end by s e v e r a l h u n d r e d ym i n l e n g t h , to r e p r e s e n t a f i b e r , and c r u s h and bend t h e s t a c k by c o m p r e s s i o n p e r p e n d i c u l a r to the long dimension of the boxes. A r e s u l t some what s i m i l a r i n a p p e a r a n c e t o F i g u r e 2 c o u l d be o b t a i n e d . V a r i a t i o n s o f c e l l u l a r f o r m s and f r a g m e n t s a r e s e e n i n F i g u r e s 3 and 4. These r e p r e s e n t f r e s h f r a c t u r e s u r f a c e s of the c o a l t h a t r e v e a l t h e m a t e r i a l was a l r e a d y f r a g m e n t e d and c r u s h e d p r i o r to t h e s c a n n i n g m i c r o s c o p e i n v e s t i g a t i o n . T h i s i s a way t h e c o a l c o n s t i t u e n t s c a n o c c u r and s t a c k w i t h i n t h e seam i t s e l f , e x c l u s i v e of s a m p l i n g and p r e p a r a t i o n f o r e x a m i n a t i o n on a m i c r o s c a l e . T h u s , a w i d e r a n g e o f d e t a i l may be p r e s e n t , f r o m e x a c t s u b c e l l u l a r f e a t u r e s w h e r e l i t t l e o r no p e r t u r b a t i o n c a n be d e t e c t e d by m i c r o s t r u c t u r a l e x a m i n a t i o n , t o t h a t where t h e r e i s a complete absence of d e t a i l o f t h e s o u r c e m a t e r i a l . T h e s e c e l l u l a r o p e n i n g s c a n be f i l l e d w i t h p y r i t e , gypsum, c a l c i t e o r o t h e r m i n e r a l i n c l u s i o n s which were d e p o s i t e d e i t h e r a t t h e t i m e o f e a r l y s t a g e s o f c o a l i f i c a t i o n o r a f t e r t h e c o a l had formed. I n c e r t a i n p r o c e s s i n g schemes t o remove c r y s t a l l i n e p y r i t e , t h e p r e s e n c e o f t h i s t y p e o f open m i c r o m e t e r s i z e n e t w o r k m i g h t be u s e d t o a d v a n t a g e d e p e n d i n g i n p a r t on t h e o c c u r r e n c e , d i s t r i b u t i o n and s u r f a c e c h a r a c t e r i s t i c s o f t h e t y p e o f c o a l c o n stituent. M i c r o c h e m i c a l F e a t u r e s . M i n e r a l i n c l u s i o n s s u c h as p y r i t e a r e r e v e a l e d o v e r a v a r i e t y o f s c a l e s o f m i c r o s c o p i c and m a c r o scopic investigations. The s u l f u r - b e a r i n g p h a s e s a r e o f p a r t i c u l a r i n t e r e s t i n v i e w o f a t t e m p t s t o remove s u l f u r p r i o r t o combus tion. The p r i m a r y f o r m s o f s u l f u r i n c o a l i n c l u d e : (1) P y r i t i c s u l f u r : FeS (2) Sulfate sulfur: CaSOi *2H 0 (3) O r g a n i c s u l f u r : a. m e r c a p t a n o r t h i o l RSH (R and R b e i n g a l k y l b. s u l f i d e or thio-ether RSR or a r y l groups) c. disulfide RSSR d. a r o m a t i c s y s t e m s c o n t a i n i n g HC — CH the thiophene r i n g || || HC CH 2

+

2

f

1

1

V A s i g n i f i c a n t p o r t i o n o f t h e s u l f u r i n c o a l may o c c u r as p y r i t e i n t h e f o r m o f i n d i v i d u a l c r y s t a l s , o r as a s s e m b l i e s o f c r y s t a l s f o r m i n g f r a m b o i d s (a w o r d c o i n e d by R u s t (16) i n 1935 m e a n i n g b e r r y - l i k e ) , o r a s a s s e m b l i e s o f f r a m b o i d s i n t e r c o n n e c t e d by a d d i t i o n a l p y r i t e o c c u r r i n g o v e r a l a r g e - s i z e r a n g e up t o c e n t i m e t e r s i n w i d t h o r g r e a t e r . P y r i t e ( F e S , c u b i c ) and m a r c a s i t e ( F e S , 2


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GRÉER


Figure 2. Compressed coalified plant cellular material showing both the top and the side of the fibers. Number 6, Midland, III. coal. Scale bar, 15 μm.

Figure 3. Phnt debris. Mich mine (Iowa). Scale bar, 10 μm.

Figure 4. Plant debris. Both fragmenta tion and distortion are seen. Horton, KY. Scale bar, 10 μm.



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orthorhombic) are observed i n c o a l samples, the c u b i c form b e i n g t h e most a b u n d a n t o f t h e s u l f i d e m i n e r a l s . O t h e r i n o r g a n i c f o r m s o f s u l f u r s u c h a s gypsum a r e r e l a t i v e l y l o w i n abundance (

Coal Microstructure and Pyrite Distribution - ACS Symposium Series

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