Application of MÃ¶ssbauer Spectroscopy to Coal Characterization and

7 Application of Mössbauer Spectroscopy to

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Coal Characterization and Utilization PEDRO A. M O N T A N O Department of Physics, West Virginia University, Morgantown, W V 26506

To characterize a coal completely, a careful identification of its mineral matter is necessary.

D u e to the presence of

iron in a large percentage of the minerals appearing in coal, the

Mössbauer

effect became a very powerful tool to iden

tify iron-bearing minerals. In this chapter we review the applications of and list the

Mössbauer

Mössbauer

spectroscopy

in coal research,

parameters of the major iron-bearing

minerals in United States coals. The use of the

Mössbauer

effect as a quantitative analytical tool to determine pyritic sulfur is discussed initially, and we find the standard procedures to be as accurate as the also have used the

Mössbauer

Mössbauer

ASTM

results. W e

effect to determine the sto-

ichiometry of the pyrrhotites present i n liquefaction resi dues.

There is considerable interest in the study of the

transformation of the iron minerals in coal conversion processes, and several examples

of such applications of the

Mossbauer effect are included.

T 7 x i s t i n g a n d p r o j e c t e d shortages of n a t u r a l gas a n d p e t r o l e u m p r o d u c t s i n t h e U n i t e d States h a v e c r e a t e d a s t i m u l a t i n g e n v i r o n m e n t f o r extensive r e s e a r c h o n the u s e of c o a l as a m a j o r source o f e l e c t r i c i t y a n d s y n t h e t i c fuels.

D u e t o i t s i m p o r t a n c e as a source of e n e r g y a n d t h e

e n v i r o n m e n t a l h a z a r d s i n v o l v e d i n its u s e , c o n s i d e r a b l e

research has

b e c o m e necessary t o u n d e r s t a n d f u l l y the different c o m p o u n d s i n coal a n d h o w they transform d u r i n g processing.

appearing

T h e acceptance of a

c o a l f o r a p a r t i c u l a r process d e p e n d s c r i t i c a l l y o n b o t h t h e o r g a n i c a n d inorganic components.

A careful identification of the mineral matter is

necessary f o r a c o m p l e t e c h a r a c t e r i z a t i o n o f a c o a l . D u e t o t h e p r e s e n c e of i r o n i n a l a r g e p e r c e n t a g e o f t h e m i n e r a l s a p p e a r i n g i n c o a l , t h e M o s s -

©

0065-2393/81/0194-0135$10.25/0 1981 American Chemical Society

In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

136

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

b a u e r effect b e c a m e a u s e f u l , a n d to a c e r t a i n degree, u n i q u e a n a l y t i c a l t o o l i n t h e i d e n t i f i c a t i o n of i r o n - b e a r i n g m i n e r a l s i n c o a l . compounds,

Iron-sulfur

a l t h o u g h m a k i n g u p a r e l a t i v e l y s m a l l p o r t i o n of c o a l o r

c o a l m i n e r a l c o m p o s i t i o n , a r e significant i n t h a t t h e y m a y affect a l l phases of

the coal

industry i n c l u d i n g m i n i n g , processing,

reclamation, a n d

u t i l i z a t i o n . F o r t h e most p a r t , t h e i n f l u e n c e is deleterious a n d results i n intolerable environmental impacts, namely water a n d air pollution. H o w ever, there a r e cases s u c h as c o a l l i q u e f a c t i o n w h e r e s u l f u r c o m p o u n d s Downloaded by NORTH CAROLINA STATE UNIV on October 17, 2012 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch007

actually m a y be beneficial ( 1 , 2 , 3 ) .

T h e importance of the i r o n - s u l f u r

m i n e r a l s i n c o a l has i n c r e a s e d interest i n t h e use o f t h e M o s s b a u e r effect as a q u a n t i t a t i v e a n a l y t i c a l t o o l to d e t e r m i n e t h e a m o u n t s u l f u r (4,5,6).

of p y r i t i c

W e h a v e c a r r i e d o u t extensive studies i n the classification

of i r o n - b e a r i n g m i n e r a l s i n c o a l a n d h a v e s t u d i e d t h e t r a n s f o r m a t i o n o f the i r o n - b e a r i n g m i n e r a l s d u r i n g p r o c e s s i n g .

I n t h e f o l l o w i n g sections,

w e r e v i e w t h e subject s t a r t i n g w i t h a b r i e f d e s c r i p t i o n of c o a l a n d a fisting

of t h e m a j o r i r o n - b e a r i n g m i n e r a l s i t contains.

T h e t r a n s f o r m a t i o n of t h e i r o n - b e a r i n g m i n e r a l s , e s p e c i a l l y p y r i t e , is d i s c u s s e d i n t h e last sections of this c h a p t e r . A c r i t i c a l e v a l u a t i o n of t h e M o s s b a u e r effect as a p o s s i b l e q u a n t i t a t i v e a n a l y t i c a l t o o l is d i s c u s s e d , a n d f a v o r a b l e a n d u n f a v o r a b l e aspects of this t e c h n i q u e a r e c o n s i d e r e d .

Coal: Organic and Inorganic Components C o a l has a v e g e t a b l e o r i g i n (7,8).

T h e m a t e r i a l f r o m w h i c h i t is

c r e a t e d a c c u m u l a t e d i n marshes f r o m t h e r e m a i n s of p l a n t s , i n lakes f r o m algae a n d t h e r e m a i n s of a n i m a l p l a n k t o n , o r i n lagoons f r o m s h a l l o w water organic muds.

Three major periods c a n be distinguished i n the

f o r m a t i o n of c o a l : T h e first is t h e peat p e r i o d , t h a t is, w h e n p l a n t r e m a i n s are d e c o m p o s e d a n d a l t e r e d , m a i n l y b y b i o c h e m i c a l processes t a k i n g p l a c e i n t h e v e g e t a b l e mass w i t h t h e h e l p o f a n a e r o b i c b a c t e r i a . I n t h e s e c o n d p e r i o d , after t h e b e d is c o v e r e d , a p h y s i c o c h e m i c a l a l t e r a t i o n of the p l a n t substance takes p l a c e d u r i n g t h e diagenesis process; there is a n increase

i n c a r b o n content,

a l o w e r i n g of oxygen,

dehydration, a n d

t r a n s f o r m a t i o n o f the p e a t to l i g n i t e . I n t h e last p e r i o d l i g n i t e is c o n v e r t e d i n t o h i g h e r - r a n k c o a l a n d a n t h r a c i t e as t h e r e s u l t of m e t a m o r p h i s m . T h e m i n e r a l s t h a t w e r e p r e s e n t i n t h e p e a t b o g c a n a c t as catalysts, or c a n react c h e m i c a l l y w i t h t h e o r g a n i c m a t e r i a l , a n d t h e i r p r e s e n c e is reflected i n t h e p r o p e r t i e s o f t h e c o a l

(7,8).

C o a l is c o n s e q u e n t l y

a

sedimentary rock consisting of a n organic part w i t h a d d e d minerals, d i a g e n e t i c o r syngenetic i n o r i g i n . T h e c o m p o s i t i o n a n d p r o p e r t i e s of a c o a l a r e c o n t r o l l e d b y t h e o r i g i n a l m a t e r i a l , t h e c o n d i t i o n s of a c c u m u l a tion, a n d the method b y w h i c h t h e material w a s converted into coal.


7.

MONTANO

Coal Characterization

and

137

Utilization

T h e c o a l is classified o n the basis of fixed c a r b o n a n d calorific v a l u e s c a l c u l a t e d o n a m i n e r a l m a t t e r - f r e e basis. T h e h i g h e r - r a n k coals, w i t h a h i g h d e g r e e of m e t a m o r p h i s m , are classified a c c o r d i n g to

fixed

carbon

o n the d r y basis; t h e l o w e r - r a n k coals a c c o r d i n g to calorific v a l u e o n t h e m o i s t basis ( 9 ) . between

T h e a g g l o m e r a t i n g c h a r a c t e r is u s e d also to differentiate

groups.

Table I

gives

a g e n e r a l classification of

the

coals

( A S T M D388-66). T h e r e are f o u r major l i t h o l o g i c c o m p o n e n t s

of c o a l — v i t r a i n , f u s a i n ,


a n d a t t r i t a l c o a l ( c l a r a i n a n d d u r a i n ) ( J O , 11)—and

t h e y are not e q u i v a

lent i n t h e i r genetic a n d p r a c t i c a l r e l a t i o n s h i p s . V i t r a i n a n d f u s a i n a p p e a r i n the c o a l as lenses a n d i n c l u s i o n s of l i m i t e d size; b o t h seem to h o m o g e n e o u s substances.

Fusain

resembles

char

coal

be

a n d retains a

d i s t i n c t i v e p l a n t structure. V i t r a i n appears i n s h i n y b l a c k b a n d s ; i t has a v i t r e o u s a p p e a r a n c e a n d its p l a n t o r i g i n is m o r e c o n c e a l e d .

T h e attrital

coals, c l a r a i n a n d d u r a i n , are c o m p l e x aggregates c o n s i s t i n g of a g r o u n d mass a n d p r e s e r v e d p o r t i o n s of p l a n t s i n a n y p r o p o r t i o n . T h e c o a l m a c e r a l s are the o r g a n i c c o m p o n e n t s distinguishable by microscopic

i n s p e c t i o n (10,11).

i n three groups r e l a t e d to the a f o r e m e n t i o n e d

of

coal that

are

T h e y are classified

lithologic

components,

n a m e l y , v i t r i n i t e , e x i n i t e , a n d i n e r t i n i t e . E a c h of these groups

include

f u r t h e r s u b d i v i s i o n s . T h e necessity for this p e t r o g r a p h i c classification is r e l a t e d to the heterogeneous c h a r a c t e r of c o a l (see F i g u r e s 1 a n d 2 ) . F r o m the p o i n t of v i e w of a solid-state c h e m i s t or p h y s i c i s t , c o a l is a c o m p o s i t e m a t e r i a l w i t h o r g a n i c a n d i n o r g a n i c constituents. T h e c a r b o n structure of coals c a n b e v i e w e d as c o n s i s t i n g of h y d r o a r o m a t i c structures w i t h a r o m a t i c i t y i n c r e a s i n g f r o m l o w - r a n k to h i g h - r a n k coals

(12,13).

T h e hetero atoms o x y g e n , n i t r o g e n , a n d s u l f u r are associated w i t h the c o a l i n v a r y i n g amounts. T h e o r g a n i c s u l f u r is d i s t r i b u t e d t h r o u g h o u t the entire c o a l mass a n d c a n n o t be s e p a r a t e d b y c o n c e n t r a t i o n ; s u l f u r i n r i n g s is the most difficult to r e m o v e .

N i t r o g e n i n t h e c o a l is f o u n d to be m a i n l y

i n r i n g positions, a n d c o n s e q u e n t l y , i t is difficult to r e m o v e for c l e a n i n g processes. O x y g e n is present i n c o a l i n p h e n o l i c h y d r o x y l , o p e n ethers, a n d r i n g ethers.

I n g e n e r a l , l o w e r - r a n k coals are r i c h i n o x y g e n .

The

trace element content i n c o a l is c o m p l i c a t e d , a n d m a n y coals c a n h a v e m o r e t h a n 60 trace elements i n v a r y i n g a m o u n t s . F r o m the p o i n t of v i e w of t h e M o s s b a u e r spectroscopist, the i n o r g a n i c constituents of c o a l are of c e n t r a l i m p o r t a n c e .

N o e v i d e n c e has

been

f o u n d of a n y detectable a m o u n t of i r o n associated w i t h t h e o r g a n i c p a r t of c o a l . C o n s e q u e n t l y , i n a n y M o s s b a u e r s t u d y of c o a l o n l y t h e m i n e r a l m a t t e r is a n a l y z e d . A c e r t a i n a m o u n t of m i n e r a l grains a n d c l a y m a t e r i a l is a l w a y s present i n c o a l (7,8,14,15).

T h e m i n e r a l s are u s u a l l y c l a y s

( k a o l i n i t e , i l l i t e , m i x e d l a y e r c l a y s , e t c . ) , sulfides

(pyrite, marcasite,


138


Table I.

Classification

Fixed Carbon Limits (% DMf)


Equal or Greater Than Anthracitic Meta-Anthracite Anthracite Semianthracite Bituminous Low-volatile bituminous coal M e d i u m - v o l a t i l e bituminous coal H i g h - v o l a t i l e A bituminous coal H i g h - v o l a t i l e B bituminous coal H i g h - v o l a t i l e C bituminous coal

98 92 86 78 69

Less

Than

98 92 86 78 69

Subbituminous Subbituminous A coal Subbituminous B coal Subbituminous C coal Lignite Lignite A Lignite B

Figure I . Thin section of a coal The majority of the sample is vitrinite; light spots are sphoronite and dark spots are attrital (courtesy of W. C. Grady Coal Research Bureau, West Virginia University).


7.


MONTANO

of Coals b y


Matter (%)

Than

2 8 14 22 31

Figure 2. collapsed.

139

Utilization

Rank

Volatile

Greater

and

Limits Equal or Less Than

Caloric Value Limits BTU Per Pound (Moist Mineral-MatterFree Basis) Equal or Greater Than

Less

Than

2 8 14 22 31 14 13 11 10

000 000 500 500

14 000 13 000 11 500

10 500 9 500 8 300

11 500 10 500 9 500

6 300

8 300 6 300

Photomicrograph of fusinite. Most cell walls are broken and (Courtesy of W. C. Grady Coal Hesearch Bureau, West Virginia University.)


140


sphalerite, g a l e n a , e t c . ) , carbonates ( l i k e c a l c i t e , a n k e r i t e , siderite, d o l o m i t e ) , q u a r t z , a n d other m i n e r a l s i n lesser a m o u n t s l i k e r u t i l e , h e m a t i t e f e l d s p a r , etc.

T h e a m o u n t , character, a n d d i s t r i b u t i o n of the m i n e r a l

m a t t e r i n the c o a l g r e a t l y i n f l u e n c e the p h y s i c a l p r o p e r t i e s .

T h e clay

m i n e r a l s , p y r i t e , a n d c a l c i t e are the m a i n m i n e r a l substances (15).

They

c a m e i n t o t h e c o a l seam b y i n f i l t r a t i o n i n t h e course of a c c u m u l a t i o n of the peat.

T h e c l a y w a s b r o u g h t to the s w a m p b y r u n n i n g w a t e r , a n d

the sulfides a n d c a l c i t e w e r e f o r m e d i n the c o a l joints a n d c a v i t i e s . Downloaded by NORTH CAROLINA STATE UNIV on October 17, 2012 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch007

o r i g i n of

finely

The

d i s s e m i n a t e d p y r i t e p r o b a b l y c a n b e a t t r i b u t e d to t h e

a c t i v i t y of s u l f u r - f o r m i n g b a c t e r i a . L a r g e p y r i t e lenses i n t h e c o a l are d u e to the d e p o s i t i o n of i r o n sulfides w h e n t h e h y d r o g e n sulfide o b t a i n e d f r o m the d e c o m p o s i t i o n of p l a n t m a t e r i a l interacts w i t h i r o n dissolved i n the swamp water.

compounds

C o a l l y i n g n e a r t h e surface c a n c o n t a i n

c l a y , h y d r a t e d f e r r i c o x i d e , a n d ferrous carbonates

or sulfates b r o u g h t

b y p e r c o l a t i n g surface w a t e r s a n d d e p o s i t e d i n cracks of t h e c o a l seam. T h e presence of sulfates i n t h e c o a l is almost a definite i n d i c a t i o n of weathering.

Iron-Bearing Minerals in Coal I r o n Sulfides.

T h e m a j o r g r o u p of i r o n - s u l f u r c o m p o u n d s

is the d i s u l f i d e g r o u p c o n s i s t i n g of p y r i t e a n d m a r c a s i t e . p y r i t e is the most a b u n d a n t .

i n coal

O f the t w o ,

P y r i t e a n d marcasite can be

identified

r e a d i l y b y x - r a y d i f f r a c t i o n ( X R D ) , b u t b e c a u s e p y r i t e is u s u a l l y d o m i n a n t i n a n y p a r t i c u l a r c o a l , the t w o

d i m o r p h s u s u a l l y are

considered

c o l l e c t i v e l y as p y r i t e . A significant c h a r a c t e r i s t i c of p y r i t e i n c o a l is the f a c t t h a t i t occurs i n v a r i o u s m o r p h o l o g i c a l forms

(16).

G e n e r a l l y p y r i t e falls i n t o

two

m a j o r classes. T h e first class consists of f r a m b o i d s , i n d e p e n d e n t e u h e d r a l crystals a n d aggregates

of e u h e d r a l crystals ( s y n g e n e t i c

pyrite).

The

s e c o n d class consists of the m a s s i v e occurrences, d e n d r i t i c , i r r e g u l a r , a n d cleat fillings, u s u a l l y greater t h a n 100

i n mean diameter.

O f a l l the m i n e r a l s i n c o a l , p y r i t e is p r o b a b l y t h e most

deleterious

i n the c o a l i n d u s t r y . I t is the source of a c i d m i n e d r a i n a g e ( 1 7 ) , a n d t h e m a j o r source of S 0

2

p o l l u t i o n i n the c o m b u s t i o n process.

However, pyrite

m a y h a v e a b e n e f i c i a l effect as a p o t e n t i a l c a t a l y s t i n c o a l l i q u e f a c t i o n processes ( 1 , 2 , 3 , 1 8 , 1 9 ) . P y r i t e , F e S , is a c u b i c c r y s t a l t h a t c a n b e c o n s i d e r e d as a n N a C l - l i k e 2

g r o u p i n g of i r o n atoms a n d S p a i r s . I t has f o u r m o l e c u l e s i n a c e l l 2

(20),

w i t h a l a t t i c e constant e q u a l to 5.40667 A a n d space g r o u p s y m m e t r y T . h

The iron i n F e S

2

experiences a s l i g h t l y d i s t o r t e d o c t a h e d r a l s y m m e t r y .

I n p y r i t e the i r o n i o n is i n t h e l o w - s p i n c o n f i g u r a t i o n i r o n ( I I ) . d-electrons are o c c u p y i n g the T

2g

T h e six

g r o u n d state a n d n o m a g n e t i c m o m e n t


7.

MONTANO


and

141

Utilization


DETECTOR

36t—i

Figure

3.

i -1

i

i i 0 VELOCITY ( m m / s )

L

i 1

Scattering Mossbauer spectrum of a single crystal of (Courtesy of Guettinger and Williamson.)

is p r e s e n t at the i r o n site (21).

pyrite.

I n F i g u r e 3, the M o s s b a u e r s p e c t r u m of

a single c r y s t a l of p y r i t e is s h o w n . G u e t t i n g e r a n d W i l l i a m s o n (28)

have

f o u n d t h a t the r e l a t i v e intensities of the t w o M o s s b a u e r transitions i n p y r i t e a r e e q u a l a n d i n d e p e n d e n t of t h e s i n g l e - c r y s t a l o r i e n t a t i o n ( F i g u r e 3 ) . T h e v a l u e s of t h e M o s s b a u e r p a r a m e t e r s f o r F e S are g i v e n i n T a b l e 2

II.

T h e t e m p e r a t u r e d e p e n d e n c e of t h e c e n t e r shift is p r o b a b l y

due

c o m p l e t e l y to the second-order D o p p l e r shift. T h e m a g n e t i c a n d e l e c t r i c p r o p e r t i e s of p y r i t e d e p e n d s t r o n g l y o n t h e presence of i m p u r i t i e s i n the crystals. M a g n e t i c s u s c e p t i b i l i t y m e a s u r e m e n t s are sensitive to t h e presence

of m a g n e t i c i m p u r i t i e s o n t h e

s a m p l e ; v e r y s m a l l a m o u n t s , f o r e x a m p l e , of C o S a p p r e c i a b l y t h e v a l u e of the s u s c e p t i b i l i t y (23). d u c t o r w i t h a z e r o - t e m p e r a t u r e b a n d gap (24)

2

or N i S

2

will

P y r i t e is a

of a b o u t 0.84 e V .


change semicon

142


Table II.

Iron Sulfides IS

Pyrite (FeS ) Marcasite (FeS ) Greigite (Fe S ) t

t

s


Amorphous

t

Fe S 4

4

Sphalerite (ZnFe)S (FeS) synthetic Troilite (FeS) natural Fe S (pyrrhotite monoclinic) 7

8

0.31 0.25 0.70 0.40 0.45 0.35 0.51 0.66

(Isomer Shifts with Respect to a-Iron)

(mm/s) ± ±

QS

0.01 0.01

0.61 0.56 0.3 ( r = 4.2K) 0 0.4 ± 0.06 0.82 ± 0.12 0.88 0.80

0.81 0.86

(mm/s) ± ±

0.01 0.01

± ±

0.06 0.12

0 —

322 486 465 253 (4.2 K )

(28)

(102)

315 ( R T ) 310 ( R T )

-0.32 -0.28

0.69 ±

0.08

0.18 ±

0.64 ± 0.64 ±

0.10 0.10

0.31 0.30 0.31 0.43 0.31 0.23 0.00 0.30 0

0.15

307

0.15 0.15 0.03 0.04 0.04 0.0 0.1 0.1

255 225 305 224 253 302 274 256 0

(32)

± 8 (32)

Fe .88iS 0

F e».»

Application of MÃ¶ssbauer Spectroscopy to Coal Characterization and

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