Analysis of the Inorganic Constituents in Low-Rank Coals - ACS

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Downloaded by STONY BROOK UNIV SUNY on December 14, 2016 | http://pubs.acs.org Publication Date: September 10, 1984 | doi: 10.1021/bk-1984-0264.ch010

Analysis of the Inorganic Constituents in Low-Rank Coals G. P. H U F F M A N and F. E. HUGGINS U.S. Steel Corporation, Technical Center, Monroeville, PA 15146

Computer-controlled scanning microscopy (CCSEM), Mössbauer spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used to investigate the inorganic constituents of low-rank coals. Mössbauer spectroscopy provides quantitative analysis of all iron-bearing phases, CCSEM yields chemical and size distribution data for discrete mineral phases, and EXAFS allows structural analysis at the atomic level of inorganics dispersed through the macerals. The inorganic phase distribution observed in lignites typically consists of 10-20% quartz, 20-40% kaolinite, 0-30% pyrite, 1-5% of iron sulfates, iron oxyhydroxide, barite and others, and 10-40% of dispersed calcium, with very l i t t l e calcite or i l l i t e (≤2%). The EXAFS data are consistent with a model in which Ca is bonded to six oxygen ions at an average nearest-neighbor distance of 2.4Å,has an electronic state similar to that of calcium acetate, and is randomly and molecularly dispersed throughout the coal macerals. In r e c e n t y e a r s , numerous modern a n a l y t i c a l t e c h n i q u e s have been a p p l i e d t o the a n a l y s i s o f the i n o r g a n i c c o n s t i t u e n t s i n c o a l . At t h e U. S. S t e e l T e c h n i c a l C e n t e r , Mossbauer s p e c t r o s c o p y and computerc o n t r o l l e d 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 y have been emphasized (JL~^)« With t h e advent o f v e r y i n t e n s e s y n c h r o t r o n r a d i a t i o n s o u r c e s , t h e t e c h n i q u e o f extended X-ray a b s o r p t i o n f i n e s t r u c t u r e (EXAFS) s p e c t r o s c o p y has been a p p l i e d i n many a r e a s o f m a t e r i a l s s c i e n c e , and s e v e r a l v e r y r e c e n t a r t i c l e s on EXAFS s t u d i e s o f t h e i n o r g a n i c cons t i t u e n t s o f c o a l have appeared ( 6 7 8 ) · F o r the most p a r t , p r e v i o u s l y p u b l i s h e d work has r e p o r t e d i n v e s t i g a t i o n s of bituminous c o a l s by these t h r e e t e c h n i q u e s . I n t h i s c h a p t e r , we p r e s e n t some examples o f the a n a l y s i s o f the i n o r g a n i c c o n s t i t u e n t s o f lower-rank c o a l s , p r i n c i p a l l y l i g n i t e s , by these methods. A l t h o u g h t h e s u i t e o f low-rank coals investigated i s rather limited, some d i s t i n c t d i f f e r e n c e s

0097-6156/ 84/ 0264-0159$06.00/ 0 © 1984 American Chemical Society

Schobert; The Chemistry of Low-Rank Coals ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

160

THE CHEMISTRY OF LOW-RANK COALS

between the i n o r g a n i c phase d i s t r i b u t i o n s i n these c o a l s and those i n bituminous c o a l s are a p p a r e n t . Experimental Mossbauer s p e c t r o s c o p y i s a s p e c t r o s c o p y based on the resonant e m i s s i o n and a b s o r p t i o n of low-energy n u c l e a r gamma r a y s . The ^ Fe n u c l e u s e x h i b i t s the best Mossbauer p r o p e r t i e s of a l l i s o t o p e s f o r which the Mossbauer e f f e c t has been o b s e r v e d , and 57 p tfôssbauer s p e c t r o s c o p y i s perhaps the best method a v a i l a b l e f o r q u a n t i t a t i v e a n a l y s i s of the i r o n - b e a r i n g phases i n complex, m u l t i p h a s e samples. As d i s c u s s e d i n r e c e n t r e v i e w a r t i c l e s (2^ 9_ 10), e v e r y i r o n - b e a r i n g compound e x h i b i t s a c h a r a c t e r i s t i c Mossbauer a b s o r p t i o n spectrum, and the percentage of the t o t a l i r o n c o n t a i n e d i n each phase can be determined from a b s o r p t i o n peak a r e a s . D e t a i l e d d e s c r i p t i o n s of the M o s s b a u e r s p e c t r o m e t e r and d a t a a n a l y s i s p r o g r a m s used i n t h i s l a b o r a t o r y and d i s c u s s i o n of the p h y s i c a l b a s i s of the Mossbauer t e c h n i q u e are g i v e n elsewhere ( 1 0 ) . The c o m p u t e r - c o n t r o l l e d s c a n n i n g e l e c t r o n microscope (CCSEM) developed i n t h i s l a b o r a t o r y c o n s i s t s of an SEM i n t e r f a c e d by a m i n i c o m p u t e r t o a b e a m - c o n t r o l u n i t 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 s i s system. D e t a i l e d d e s c r i p t i o n s of t h i s i n s t r u m e n t and i t s use i n the d e t e r m i n a t i o n of c o a l m i n e r a l o g y and o t h e r a p p l i c a t i o n s a r e g i v e n e l s e w h e r e (3^, _5, JJ_) · B r i e f l y , t h e e l e c t r o n beam i s stepped a c r o s s the sample 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 of view. At each p o i n t , the b a c k s c a t t e r e d e l e c t r o n i n t e n s i t y i s measured by d e t e c t i n g those e l e c t r o n s t h a t a r e s c a t t e r e d from the sample at a n g l e s c l o s e t o 180°. When t h i s i n t e n s i t y i s g r e a t e r than a c e r t a i n p r e s e t l e v e l , the m i n i computer i d e n t i f i e s the f e a t u r e i n q u e s t i o n as a m i n e r a l p a r t i c l e . The g r i d d e n s i t y i s then i n c r e a s e d to 2048 χ 2048 and the p a r t i c l e a r e a i s measured. Next, the beam-control u n i t b r i n g s the beam back t o the g e o m e t r i c a l c e n t e r of the p a r t i c l e and an energy d i s p e r s i v e X-ray spectrum i s c o l l e c t e d . Each p a r t i c l e i s then p l a c e d i n t o one of up t o 30 c a t e g o r i e s ( m i n e r a l s , compounds, e t c . ) on the b a s i s of the c h e m i s t r y i n d i c a t e d by i t s X-ray e m i s s i o n spectrum, and a p p r o x i mate weight p e r c e n t a g e s of a l l c a t e g o r i e s are c a l c u l a t e d u s i n g known d e n s i t i e s and measured p a r t i c l e a r e a s . CCSEM i s c a p a b l e of measuring the s i z e and c h e m i c a l c o m p o s i t i o n of up t o 1000 p a r t i c l e s per hour f o r many k i n d s of p a r t i c u l a t e samples. EXAFS s p e c t r o s c o p y examines the o s c i l l a t o r y f i n e s t r u c t u r e above the a b s o r p t i o n edge i n the X-ray a b s o r p t i o n spectrum of a p a r t i c u l a r element. These o s c i l l a t i o n s a r i s e from i n t e r f e r e n c e between the o u t g o i n g p h o t o e l e c t r o n wave and s c a t t e r e d waves produced by i n t e r a c t i o n of the p h o t o e l e c t r o n s w i t h n e i g h b o r i n g atoms. As d i s c u s s e d elsewhere (12, 13), F o u r i e r t r a n s f o r m t e c h n i q u e s can be used t o e x t r a c t from these o s c i l l a t i o n s i n f o r m a t i o n about the bond d i s t a n c e s , c o o r d i n a t i o n numbers, and types of l i g a n d s s u r r o u n d i n g the a b s o r b i n g element. A d d i t i o n a l i n f o r m a t i o n about the v a l e n c e or e l e c t r o n i c s t a t e of the a b s o r b i n g i o n and the l i g a n d symmetry can be o b t a i n e d from examining e


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10.

H U F F M A N A N D HUGGINS

Analysis of Inorganic Constituents

161


the X-ray a b s o r p t i o n near-edge s p e c t r a , o r XANKS, In the energy r e g i o n v e r y c l o s e to the a b s o r p t i o n edge ( w i t h i n a p p r o x i m a t e l y ±20-30 eV). Such XANES s p e c t r a f r e q u e n t l y p r o v i d e c h a r a c t e r i s t i c f i n g e r p r i n t s f o r d i f f e r e n t t y p e s of l i g a n d bonding to an a b s o r b i n g i o n (6^, The X-ray a b s o r p t i o n s p e c t r a of c a l c i u m - c o n t a i n i n g c o a l s and r e f e r e n c e compounds d i s c u s s e d i n t h i s paper were r e c o r d e d at the S t a n f o r d S y n c h r o t r o n R a d i a t i o n L a b o r a t o r y d u r i n g a d e d i c a t e d run of the Stanford Positron-Electron Acceleration R i n g at an electron energy of 3.0 GeV. The c a l c i u m K-edge o c c u r s at 4038 eV and d a t a were c o l l e c t e d f r o m 3800 t o 5000 eV, u s i n g a d o u b l e S i ( 1 1 1 ) monochromator and a f l u o r e s c e n c e d e t e c t o r s i m i l a r to t h a t of S t e r n and Heald ( 1 6 ) . A more d e t a i l e d d i s c u s s i o n of t h i s work i s g i v e n elsewhere ( 1 7 ) . The samples examined were p r e d o m i n a n t l y l i g n i t e s from the Pust seam i n Montana. However, d a t a f o r two North Dakota l i g n i t e s , f o r s l a g g i n g and f o u l i n g d e p o s i t s produced by those l i g n i t e s , and f o r s e v e r a l subbituminous c o a l s a r e a l s o i n c l u d e d . R e s u l t s and

Discussion

Mossbauer s p e c t r o s c o p y r e s u l t s f o r a l l samples i n v e s t i g a t e d a r e summarized i n T a b l e I . The p e r c e n t a g e s of the t o t a l i r o n c o n t a i n e d i n the sample a s s i g n e d t o each of the i r o n - b e a r i n g m i n e r a l s i d e n t i f i e d a r e g i v e n i n columns 1 t o 4, and the weight p e r c e n t a g e s of p y r i t i c s u l f u r , determined from the p y r i t e a b s o r p t i o n peak a r e a s as d i s c u s s e d elsewhere ( I), a r e g i v e n i n column 5. I t i s seen t h a t p y r i t e and m i n e r a l s ( i r o n s u l f a t e s and i r o n o x y h y d r o x i d e ) t h a t are p r o b a b l y d e r i v e d from p y r i t e by w e a t h e r i n g a r e the o n l y i r o n - b e a r i n g s p e c i e s i n these low-rank c o a l s . N o t a b l y absent are c o n t r i b u t i o n s from i r o n - b e a r i n g c l a y s and s i d e r i t e which are common c o n s t i t u e n t s of b i t u m i n o u s c o a l s ( 1 , 2). P y r i t e and i r o n o x y h y d r o x i d e a r e d i f f i c u l t to s e p a r a t e w i t h room-temperature Mossbauer s p e c t r o s c o p y ( 1 8 ) . For example, i n F i g u r e 1 a r e shown the room temperature and 77 Κ s p e c t r a o b t a i n e d from the Pust seam, C, l i g n i t e , which had been s t o r e d f o r s e v e r a l y e a r s p r i o r to measurement. Although i t i s quite d i f f i c u l t t o d e t e r m i n e the r e l a t i v e amounts of p y r i t e and o x y h y d r o x i d e from the room-temperature Mossbauer spectrum, the s p e c t r a l c o n t r i b u t i o n s of the two phases a r e r e a d i l y r e s o l v e d a t 77 K. CCSEM r e s u l t s f o r the approximate weight p e r c e n t a g e s of a l l i n o r g a n i c phases a r e g i v e n i n T a b l e I I . Perhaps the most i n t e r e s t i n g a s p e c t of the CCSEM r e s u l t s f o r t h e s e low-rank c o a l s as compared w i t h s i m i l a r d a t a f o r b i t u m i n o u s c o a l s i s the abundance of C a - r i c h phases. In most c a s e s , t h e s e phases a r e not c a l c i t e , but a r e C a - e n r i c h e d macérais i n which the Ca i s u n i f o r m l y d i s p e r s e d throughout the c o a l , as i l l u s t r a t e d by F i g u r e 2. The C a - e n r i c h e d macérais appear l i g h t g r a y i n the b a c k s c a t t e r e d e l e c t r o n image shown i n F i g u r e 2a; the dark gray m a t e r i a l i s the epoxy mounting. The r e l a t i v e l y u n i f o r m d i s p e r s i o n of the Ca i s i l l u s t r a t e d by the Ca X - r a y map o f F i g u r e 2b, and an energy d i s p e r s i v e X-ray spectrum o b t a i n e d from an i n d i v i d u a l


THE CHEMISTRY OF LOW-RANK

162

COALS

TABLE I Mossbauer R e s u l t s f o r Low-Rank C o a l s


P e r c e n t of T o t a l Sample

Pyrite

J a r o s i te

Iron Contained i n F e r r o u s I r o n OxyS u l f a t e hydroxide

Wt. A of Pyritic Sulfur 0.30

Pust seam, A-3

100

Pust seam, A-4

92

7

Pust seam, A-6

91

9

Pust seam, A-7

100+

0.03+

Pust seam, B-3

100

0.06

Pust seam, B-5

100

0.05

Pust seam, B-7

100

0.09

0.15

Pust seam, C

43

26

-

N.Dakota l i g n i t e , heavy f o u l i n g

91

6

2

N.Dakota l i g n i t e , light fouling

95

Rosebud subbituminous

65

16

81

19

Colstrip subbitumnous

2.26

1

31

0.29 0.50

5

0.38

0.15 19 -

+Very weak spectrum; sample c o n t a i n e d o n l y 0.07% i r o n .


0.27


10.


Analysis of inorganic Constituents

163

VELOCITY IN MM/S Figure

1.

Pyrite

( Ρ ) , jarosite

indicated.

Mossbauer Key:

top,

spectra (J), room

and

of

the

iron

Pust

seam,

oxyhydroxide

temperature;

C, (0)

and b o t t o m ,

lignite. are 77

K.


Schobert; The Chemistry of Low-Rank Coals ACS Symposium Series; American Chemical Society: Washington, DC, 1984. 6

16

36

28

23

18 22 56

E v e l e t h , Sbb. C o l s t r i p , Sbb. A b s a l o k a , Sbb. 22 6 8

20

-

12 3

2

2

3

-1

--1

-

4 8 3

6

10

19

26 1 1 1 1 5 4

1

-

3 3

--

3

2

2

1

14

--

7

6

1

5

--

4

-

7 29 5

8

23

14

15 51 49 22 38 27 13

26

* M i n e r a l s denoted by C a - S r - A l - P a r e p r o b a b l y c r a n d a l l i t e ( C a , S r ) A l ~ ( ( P 0 , ) ( 0 H ) H 0. 9

-

1

1

-

-

-

-3

2 3

-

quantitative.

-

1

-

7 5 3 3 5 5 4

1

Ca-Sr-Al-P* Smectite Rutile Ca-Sr-Al-P* Gypsum Rutile

Smectite

Rutile Ca-Fe-S Ca-Fe Ca-Sr-Al-P* Jarosite Smectite

-S m e c t i t e

6 1 1 8 4 3

1

4 1 1 ό 4 2 1

1 1 3

Phases

Smectite Rutile Fe-Ca-S

Fe Fe- CaI l l i t e P y r i t e S u l f a t e s r i c h r i c h + B a r i t e A p a t i t e Other

percentages)

+As d i c u s s e d i n the t e x t , percentages g i v e n f o r t h e C a - r i c h c a t e g o r y a r e not

23 25 22

13

13

24

N.Dakota l i g . , heavy f o u l . N.Dakota l i g . , light foul. Rosebud, Sbb.

13 3 2 5

-3

3

8

20 27 35 43 38 38 29

45

12 7 7 10 14 16 9

seam, seam, seam, seam, seam, seam, seam,

7

Mixed Quartz K a o l i n i t e S i l i c a t e s

A-4 A-6 A-7 B-3 8-5 B-7 C

Pust Pust Pust Pust Pust Pust Pust

Pust seam, A-3

Sample

CCSEM R e s u l t s f o r Low-Rank C o a l s (approximate weight

TABLE I I


Analysis of inorganic Constituents



Figure

2.

Ca X - r a y X-ray

Backscattered electron map o f

spectrum

the from

same an

area

(b),

individual

image

of

and an maceral

Pust

lignite

(a),

energy-dispersive (c).



166

THE CHEMISTRY OF LOW-RANK COALS

m a c é r a i is shown in Figure 2c. As discussed below, EXAFS data indicate that this dispersed calcium is present as salts of carboxylic acids. The backscattered electron intensity of the Ca-enriched macérais is significantly smaller than that of calcite, and CCSEM can make a distinction, albeit somewhat imprecisely, between Ca-enriched macé r a i s and calcite or other Ca-rich minerals on this basis. However, the CCSEM programs have not been properly calibrated to deal with the case of macérais enriched in an inorganic component such as Ca at this point. Consequently, the percentages indicated in Table II for the Ca-rich category are only a qualitative indication of the relative amounts of this species in the various low-rank coals examined. On the basis of the backscattered electron intensity, it appears that calcium is dispersed throughout the macérais of the lignites that have been examined, and is present partially in dispersed form and partially as calcite in the subbituminous coals. In fresh bituminous coals, calcium is present almost exclusively as calcite (3^5)· For comparison with Tables I and II, Table III gives the range and typical values of the mineral distributions observed in bituminous coals by the CCSEM and Mossbauer techniques, derived from studies of perhaps a hundred different bituminous coal samples in this laboratory. Some obvious differences in mineralogy are apparent. In addition to the difference in calcium dispersion and abundance a l ready noted, it is seen that certain minerals common in bituminous coals, such as Fe-bearing clays ( i l l i t e and chlorite) and siderite, are virtually absent in the low-rank samples of Tables I and II. Conversely, minerals such as barite (BaSO^), apatite (Ca^(PO^)3OH), and other Ca, Sr phosphates are rather uncommon in bituminous coals. EXAFS and XANES data for a Ca-rich sample of the Pust seam, A, lignite can be briefly summarized by reference to Figures 3 and 4. Figure 3 shows the XANES of the lignite, calcium acetate, and a fresh bituminous coal from the Pittsburgh seam rich in calcite. The strong similarity between the lignite and calcium acetate spectra is apparent. Similarly, a close similarity is also observed for the XANES of the fresh bituminous coal and that of a calcite standard. Examination of the XANES of several other standard compounds (CaO, Ca(OH) , and CaS0^.2H 0) showed that none of these phases were present in detectable amounts in either coal. The strong similarity of the XANES of calcium acetate and that of the lignite is direct evidence that the calcium in this coal is associated with carboxyl groups in the macérais and is not contained in very fine (

Analysis of the Inorganic Constituents in Low-Rank Coals - ACS

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