Coal and Coal Products Analytical ... - ACS Publications

12 Scanning Electron Miscroscope-Based Automated Image Analysis (SEM-AIA) and Mössbauer Spectroscopy Quantitative Characterization of Coal Minerals Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 9, 2016 | http://pubs.acs.org Publication Date: November 12, 1982 | doi: 10.1021/bk-1982-0205.ch012

F. E . HUGGINS, G. P. H U F F M A N , and R. J. L E E U.S. Steel Research, Monroeville, PA 15146

Many coal-utilization problems derive from the mineral matter in coal and require for their solution quantitative determinations of this component. For such determinations, we use automated image analysis (ΑΙΑ) in the scanning electron microscope (SEM-AIA) and Fe Mössbauer spectroscopy. SEM-AIA involves measurement of the areas of typically 1024 particles in a polished coal section and classification of each particle into one of 29 categories on the basis of its energy-dispersive x-ray spectrum. From such data, the relative weight percentages of the different minerals are derived. Mössbauer spectroscopy is used for classifying certain iron-bearing categories that could arise from two or more distinct minerals with identical energy-dispersive spectra. Because these techniques are used directly on the coal, provide information on mineral particle size, and are more sensitive to minor constituents than other techniques, they compare well with other techniques such as Fourier transform infrared spectroscopy and x-ray diffraction commonly used for quantitative coal mineralogies. 57

T h e r e i s c u r r e n t l y much i n t e r e s t i n 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 and t h e b e h a v i o r and p r o p e r t i e s o f such m a t t e r during c o a l - u t i l i z a t i o n technologies. F o r t h e most p a r t , t h e mineral matter has d e l e t e r i o u s e f f e c t s d u r i n g u t i l i z a t i o n , and c o n s i d e r a b l e a t t e n t i o n must o f t e n be g i v e n t o m i n i m i z i n g p r o b lems a r i s i n g f r o m t h i s i n o r g a n i c , n o n c o m b u s t i b l e component i n coal. On t h e p o s i t i v e s i d e , h o w e v e r , t h e m i n e r a l m a t t e r appears t o have d e s i r a b l e c a t a l y t i c p r o p e r t i e s i n c e r t a i n c o a l c o n v e r s i o n t e c h n o l o g i e s a n d c a n be u s e d b y g e o l o g i s t s i n exploration t o delineate c o a l - q u a l i t y trends within a c o a l deposit. A l t h o u g h some m i n e r a l o g i c a l i n f o r m a t i o n c a n be i n f e r r e d from standard c h e m i c a l and other a n a l y s e s o f t h e c o a l and t h e a s h r e m a i n i n g a f t e r c o n t r o l l e d c o m b u s t i o n , a f u l l

0097-6156/82/0205-0239$06.00/0 © 1982 American Chemical Society

Fuller; Coal and Coal Products: Analytical Characterization Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

240

COAL

AND

COAL

PRODUCTS

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 9, 2016 | http://pubs.acs.org Publication Date: November 12, 1982 | doi: 10.1021/bk-1982-0205.ch012

u n d e r s t a n d i n g o f m i n e r a l - r e l a t e d phenomena i n c o a l u t i l i z a t i o n r e q u i r e s a d e t a i l e d knowledge o f the m i n e r a l o g y o f t h e c o a l . A number o f t e c h n i q u e s h a v e b e e n u s e d t o d e t e r m i n e m i n e r a l o g i e s of c o a l . As d i s c u s s e d i n a r e c e n t r e v i e w (J_), t h e m o s t common t e c h n i q u e s a r e x - r a y d i f f r a c t i o n , i n f r a r e d s p e c t r o s c o p y , o p t i c a l m i c r o s c o p y , and e l e c t r o n m i c r o s c o p y . X-ray d i f f r a c t i o n a n d i n f r a r e d s p e c t r o s c o p y c a n be c o n s i d e r e d " b u l k " methods b e c a u s e t h e y a r e g e n e r a l l y b e s t p e r f o r m e d on t h e m i n e r a l - m a t t e r c o n c e n t r a t e o b t a i n e d by r e m o v a l o f t h e m a c é r a i s by l o w - t e m p e r a t u r e a s h i n g (2). The m i c r o s c o p e methods c a n be c o n s i d e r e d " p a r t i c u l a t e " methods b e c a u s e m i n e r a l g r a i n s i n t h e c o a l a r e s i z e d and c l a s s i f i e d i n d i v i d u a l l y . T h e s e m i c r o s c o p e methods a r e u s u a l l y u s e d w i t h o u t s e p a r a t i o n o f m i n e r a l s and m a c é r a i s b e c a u s e the c o a l macérais can s e r v e as a b a c k g r o u n d m a t r i x t o s e p a r a t e m i n e r a l p a r t i c l e s and t o p r o v i d e c o n t r a s t f o r t h e d i m e n s i o n a l measurement o f t h e p a r t i c l e . A l t h o u g h p a r t i c u l a t e methods a r e p o t e n t i a l l y more i n f o r m a t i v e t h a n b u l k methods b e c a u s e o f t h e e x t r a i n f o r m a t i o n o b t a i n e d on p a r t i c l e s i z e , t h e i r a p p l i c a t i o n i n c o a l m i n e r a l o g y has been r e l a t i v e l y l i m i t e d . The m a i n r e a s o n f o r t h i s h a s b e e n t h e l a c k o f a u t o m a t i o n , as s u c h m i c r o s c o p i c m e a s u r e m e n t s a r e t i m e - c o n s u m i n g and t e d i o u s i f s u f f i c i e n t d a t a h a v e t o be generated manually. R e c e n t l y , however, t h e a p p l i c a t i o n o f m i c r o c o m p u t e r s and i m a g e - a n a l y s i s s y s t e m s t o m i c r o s c o p y has p r o g r e s s e d t o t h e e x t e n t t h a t most o f t h e o p e r a t i o n , d a t a c o l l e c t i o n , and a n a l y s i s c a n now be done a u t o m a t i c a l l y * A number o f r e p o r t s o f s u c h a u t o m a t e d m i c r o s c o p e t e c h n i q u e s u s e d i n c o a l r e s e a r c h have appeared r e c e n t l y i n the l i t e r a t u r e

(2-8). I n t h i s p a p e r we d e s c r i b e t h e u s e o f a 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 (SEM) e q u i p p e d w i t h a u t o m a t i c - i m a g e - a n a l y s i s (ΑΙΑ) c a p a b i l i t y f o r the determination of c o a l m i n e r a l o g i e s . Because one o f t h e m a i n l i m i t a t i o n s o f t h e SEM-AIA t e c h n i q u e i s i t s i n a b i l i t y to d i s t i n g u i s h the v a r i o u s i r o n - b e a r i n g minerals, Fe MiJssbauer s p e c t r o s c o p y i s u s e d t o s u p p l e m e n t d a t a f r o m SEM-AIA with respect to iron-bearing minerals. The c o m b i n a t i o n o f t h e s e two t e c h n i q u e s u s u a l l y p r o v i d e s a d e t a i l e d , q u a n t i t a t i v e c h a r a c t e r i z a t i o n of the m i n e r a l s i n c o a l . 5 7

Experimental

Methods

Sample P r e p a r a t i o n . Our s a m p l e - p r é p a r â t i o n p r o c e d u r e s f o r SEM-AIA on c o a l h a v e b e e n m o d i f i e d f r o m t h o s e d e s c r i b e d e a r l i e r {8). A p p r o x i m a t e l y 7 g o f -60 mesh c o a l i s m i x e d i n t o a s l u r r y w i t h a b o u t 2 g o f an epoxy p l a s t i c and p r e s s e d i n a s t a i n l e s s s t e e l d i e t o f o r m a c y l i n d r i c a l s a m p l e p e l l e t 1.6 cm i n d i a m e t e r and 3 t o 4 cm i n l e n g t h . A s l i c e , a p p r o x i m a t e l y 1 cm t h i c k , i s c u t f r o m t h e p e l l e t w i t h a d i a m o n d w a f e r i n g saw a t a n



12.

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and Môssbauer Spectroscopy

241

a n g l e o f 54.7° t o t h e a x i s o f t h e c y l i n d e r . T h i s angle of cut h a s b e e n f o u n d (9) t o m i n i m i z e b i a s t h a t m i g h t a r i s e f r o m p o s s i b l e p r e f e r r e d o r i e n t a t i o n o f c l a y m i n e r a l s due t o c o m p r e s s i o n of the coal/epoxy mixture. The r e s u l t i n g o v a l - s h a p e d p e l l e t i s p l a c e d f a c e down i n a o n e - i n c h b a k é l i t e r i n g f o r m a n d s u r r o u n d e d by more e p o x y t o f o r m a c y l i n d r i c a l p e l l e t t h a t f i t s c o n v e n i e n t l y i n t o an a u t o m a t i c p o l i s h i n g m a c h i n e . The c o a l / e p o x y p e l l e t i s then ground, p a r a l l e l t o the c u t f a c e , w i t h g r i d s impregnated w i t h c u b i c boron n i t r i d e and f i n a l l y p o l i s h e d w i t h 3 um and 1 um d i a m o n d p a s t e s . The u s e o f c u b i c b o r o n n i t r i d e a n d d i a m o n d p a s t e f o r g r i n d i n g and p o l i s h i n g m i n i m i z e s c o n t a m i n a t i o n by m a t e r i a l s t h a t c a n be m i s t a k e n f o r m i n e r a l s i n t h e SEM a n a l y s i s . The sample p e l l e t i s t h e n c u t p a r a l l e l t o t h e p o l i s h e d f a c e t o r e d u c e i t s h e i g h t t o a b o u t 0.6 cm, mounted on an a l u m i n u m s t u b , a n d c a r b o n - c o a t e d f o r u s e i n t h e SEM. S a m p l e - p r e p a r a t i o n p r o c e d u r e s f o r Mô'ssbauer s p e c t r o s c o p y are r e l a t i v e l y simple. D e p e n d i n g on t h e i r o n c o n t e n t o f t h e c o a l , 0.3 t o 1.0 g o f v e r y f i n e l y c r u s h e d (-400 mesh) c o a l i s p l a c e d i n a p l e x i g l a s s c o m p r e s s i o n h o l d e r o f 1.3-cm-diameter. The f i l l e d h o l d e r i s t h e n p l a c e d i n a m e t a l l i c s h i e l d w i t h a 1.3 cm d i a m e t e r o p e n i n g t h a t e n a b l e s t h e sample t o be e x p o s e d t o t h e γ-ray beam f r o m t h e Mo'ssbauer s o u r c e . Identical proce dures w i t h a d i f f e r e n t m e t a l l i c s h i e l d arrangement i n s i d e a vacuum s h r o u d a r e u s e d f o r c r y o g e n i c e x p e r i m e n t s . SEM A u t o m a t e d Image A n a l y s i s . our l a b o r a t o r y f o r a b r o a d range o f t e r i z a t i o n o f a i r - p o l l u t i o n samples inclusions i n steel. In t h i s paper, m i n e r a l s i n c o a l w i l l be d e s c r i b e d , t i o n s o f t h e r a n g e and p o t e n t i a l o f elsewhere (10,11).

SEM-AIA h a s b e e n u s e d i n a p p l i c a t i o n s , from c h a r a c t o measurement o f only i t s application to as more g e n e r a l d e s c r i p SEM-AIA h a v e b e e n g i v e n

SEM-AIA o f c o a l u s e s two d i s t i n c t s i g n a l s e m i t t e d f r o m t h e s a m p l e i n r e s p o n s e t o i r r a d i a t i o n by t h e 20-kV e l e c t r o n beam. One o f t h e s e s i g n a l s i s t h e b a c k - s c a t t e r e d - e l e c t r o n (BSE) r a d i a t i o n , which p r o v i d e s e x c e l l e n t c o n t r a s t between m i n e r a l p a r t i c l e s and t h e b a c k g r o u n d m a t r i x o f c o a l m a c é r a i s and e p o x y ( F i g u r e 1); i t i s u s e d t o l o c a t e t h e m i n e r a l p a r t i c l e s a n d measure t h e i r a r e a . A f o u r - c h a n n e l BSE d e t e c t i o n s y s t e m i s used t o c o l l e c t t h i s r a d i a t i o n because i t i n c r e a s e s t h e c o n t r a s t and s i g n a l - t o - n o i s e r a t i o a n d m i n i m i z e s g e o m e t r i c v a r i a t i o n i n the s i g n a l . The o t h e r s i g n a l u s e d i s t h e c h a r a c t e r i s t i c x-ray spectrum a r i s i n g from the elements i n the mineral p a r t i c l e . T h e s e x - r a y s a r e c o l l e c t e d by means o f a n e n e r g y - d i s p e r s i v e x - r a y s p e c t r o m e t e r and a r e u s e d t o t y p e t h e mineral p a r t i c l e .


242

COAL AND COAL PRODUCTS


The SEM-AIA s y s t e m c o n s i s t s o f a n SEM (ETEC A u t o s c a n ) i n t e r f a c e d by means o f two m i n i c o m p u t e r s t o a b e a m - c o n t r o l unit (LeMont S c i e n t i f i c ) and an 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 z e r unit (Tracor Northern). A d i a g r a m o f t h e s y s t e m i s shown s c h e m a t i c a l l y i n F i g u r e 2. T h e b e a m - c o n t r o l u n i t c o n s i s t s o f a m i n i c o m p u t e r - c o n t r o l l e d d i g i t a l scan g e n e r a t o r and an a n a l o g comparator. T h e d i g i t a l s c a n g e n e r a t o r moves t h e e l e c t r o n beam i n a s p e c i f i c p a t t e r n through t h e f i e l d o f view t o l o c a t e p a r t i c l e s a n d t o measure t h e i r a r e a . To l o c a t e p a r t i c l e s , t h e beam i s s t e p p e d a c r o s s t h e s a m p l e i n a c o a r s e g r i d p a t t e r n , w i t h t y p i c a l l y 300 χ 300 g r i d p o i n t s c o v e r i n g the f i e l d o f view. A t each p o i n t , t h e i n t e n s i t y o f t h e BSE r a d i a t i o n i s s a m p l e d , a n d t h e m i n i c o m p u t e r d e c i d e s w h e t h e r t h e beam i s on o r o f f a m i n e r a l p a r t i c l e . The i n t e n s i t y l e v e l o f t h e BSE r a d i a t i o n t h a t d i s c r i m i n a t e s b e t w e e n m i n e r a l and background i s s e t manually a t t h e b e g i n n i n g o f each f i e l d o f view u s i n g t h e a n a l o g comparator. Once a p a r t i c l e i s l o c a t e d , t h e g r i d d e n s i t y i s i n c r e a s e d t o 2048 χ 2048 g r i d p o i n t s t o i n c r e a s e t h e p r e c i s i o n o f t h e a r e a measurement o f t h e particle. The s i z e a n d s h a p e o f t h e p a r t i c l e a r e d e t e r m i n e d from t h e c o n t i g u o u s group o f p o i n t s above t h e a n a l o g comparator l e v e l (11). The b e a m - c o n t r o l u n i t t h e n p o s i t i o n s t h e e l e c t r o n beam a t t h e c e n t e r o f t h e p a r t i c l e a n d c o n t r o l o f t h e o p e r a t i o n i s t r a n s f e r r e d t o the minicomputer i n t h e e n e r g y - d i s p e r s i v e x-ray u n i t . The e n e r g y - d i s p e r s i v e x - r a y s p e c t r u m o f t h e p a r t i c l e i s c o l l e c t e d f o r two t o t h r e e s e c o n d s i n a m u l t i c h a n n e l a n a l y z e r , a n d t h e p e r c e n t a g e s o f x - r a y s f r o m 11 p r i n c i p a l e l e m e n t s i n c o a l a s h (Na, Mg, A l , S i , P, S, C l , K, C a , T i , F e ) a r e s t o r e d i n t h e m i n i c o m p u t e r memory a l o n g w i t h d i m e n s i o n a l a n d l o c a t i o n a l d a t a a b o u t t h e p a r t i c l e t r a n s f e r r e d f r o m t h e beamc o n t r o l minicomputer while t h e x-rays a r e being recorded. The x - r a y s p e c t r u m i s c l a s s i f i e d by means o f a s o r t i n g r o u t i n e i n t o one o f 29 c o m p o s i t i o n a l c a t e g o r i e s . C o n t r o l i s t h e n r e t u r n e d to t h e beam-control minicomputer and t h e next p a r t i c l e i s s o u g h t by t h e b e a m - c o n t r o l unit. The a n a l y s i s i s t e r m i n a t e d o n c e 1023 p a r t i c l e s h a v e b e e n l o c a t e d , measured, and c l a s s i f i e d . However, t o g e t a good r e p r e s e n t a t i o n o f p a r t i c l e s a c r o s s a broad s i z e range, a l i m i t i s p u t on t h e number o f p a r t i c l e s m e a s u r e d i n v a r i o u s s i z e r a n g e s ( T a b l e I ) . The a r e a o f t h e s a m p l e s c a n n e d t o c o l l e c t t h e r e q u i r e d number o f p a r t i c l e s i n e a c h s i z e r a n g e i s recorded. The r a t i o (Mj) o f t h e a r e a s c a n n e d i n t h e t o p s i z e range t o t h a t i n t h e j t h s i z e range i s used as a w e i g h t i n g f a c t o r i n the c a l c u l a t i o n of the weight percentages o f the different minerals. The f o r m u l a f o r t h i s c a l c u l a t i o n i s based on t h e a r e a o f t h e p a r t i c l e s o f a g i v e n t y p e , m u l t i p l i e d by t h e d e n s i t y o f t h e m i n e r a l and t h e f a c t o r , Mj:



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Figure 1. Secondary electron image (left) and inverted backscattered electron (BSE, right) image of a polished sample of a bituminous coal (D seam, Colorado). The minerals are black and the maceral epoxy background is light gray in the inverted BSE image.

DUAL DISC UNIT TTY

τ τ Y

DUAL DISC UNIT

MINI COMPUTER

MINI COMPUTER

DIGITAL SCAN GENERATOR

ANALOG COMPARATOR

ELECTRON DETECTION

MULTI CHANNEL ANALYZER

X-RAY DETECTION

HIGH SPEED PRINTER

Figure 2. Schematic diagram of the SEM-AIA system.


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Table

I.

Size-Range C l a s s e s

Lower


Class

Limit

(um)

m

SEM-AIA o f C o a l

Maximum Number o f Particles

Upper L i m i t ( un)

1

10.0

50.0

300

2

5.0

10.0

200

3

2.5

5.0

175

4

1.0

2.5

175

5

0.2

1.0

173

Table

Major*

II.

SEM-AIA M i n e r a l

Accessory**

Categories

Mixed"*"

Quartz Kaolinite IIlite Pyrite Fe-rich Calcite

Pyrite/Jarosite Silicate/Sulfur Silicate/Pyrite Mixed s u l f a t e / s u l f i d e Mixed Carbonate Mixed C h l o r i d e

Mixed

Trace

Montmorillonite Chlorite Gypsum Jarosite Fe S u l f a t e Halite Sylvite S i l i c a t e s Dolomite Ankerite Rutile Apatite Sulfur

element

Unknown 2

spare

* U s u a l l y e a c h >3 wt% o f m i n e r a l m a t t e r a n d sum o f m a j o r categories

>90 wt % .

** U s u a l l y e a c h categories

Coal and Coal Products Analytical ... - ACS Publications

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