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The reduction of the iron compounds inherent in Victorian brown coal to form ~-iron, T-iron and cementlte following heating to high temperatures under reducing ...

H y p e r f i n e Interactions 41 (1988) 795-798

795

GAMMA-IRON FORMATION BY REDUCTION OF IRON C O M P O U N D S I N H E R E N T IN BROWN COAL G.S. H A N G ' , P.S. C O O K a n d J.D. C A S H I O N

Department of Physics, Monash University, Clayton, Victoria 3168, Australia " Department of Physics, Xuzhou Teachers" College, Xuzhou, Peoples Republic of China The r e d u c t i o n o f t h e i r o n compounds i n h e r e n t in Victorian brown c o a l t o f o r m ~ - i r o n , T - i r o n and c e m e n t l t e f o l l o w i n g heating to high temperatures under reducing conditions has been observed. It is probable that the T-iron phase is responsible for the singlet reported in heat-treated coals. 1. ~ I O N S e v e r a l M~ssbauer s t u d i e s / 1 - 4 / of i r o n compounds i n c o a l s and c o a l p r o d u c t s h a v e r e v e a l e d a s i n g l e 57Fe a b s o r p t i o n p e a k a t z e r o o r s m a l l n e g a t i v e isomer shift relative to ~-iron. I n one c a s e / 1 / , t h i s species accounted for more t h a n a q u a r t e r o f t h e s p e c t r a l a r e a . Such u n s p l i t absorption peaks are c o m p a r a t i v e l y r a r e i n n o n - m e t a l l i c i r o n compounds, and most e x p l a n a t i o n s / 1 / of t h e n a t u r e o f t h e s p e c i e s r e s p o n s i b l e f o r t h i s p e a k h a v e f a v o u r e d T-Fe o r a closely allied species. I n b e d m o i s t V i c t o r i a n brown c o a l s , t h e two p r i n c i p a l i r o n s p e c i e s / 5 / are a f e r r i c o x y h y d r o x i d e and a f e r r o u s humate w h i c h a p p e a r t o be i n a pH-Eh dependent equilibrium, with the latter species essentially being in solution at room t e m p e r a t u r e . The o x y h y d r o x i d e shows a b i m o d a l p a r t i c l e size distribution with the major portion of the material in particles less t h a n a b o u t 3nm i n diameter. S i n c e t h e l e f t - h a n d p e a k s o f t h e s e two d o u b l e t s a r e a l m o s t c o i n c i d e n t at approximately zero velocity, it is very difficult to distinguish additional peaks in t h i s r e g i o n .

In a study of the alkali-digested fragments of Victorian brown coals

/4/,

a r e l a t i v e l y s t r o n g peak n e a r zero v e l o c i t y was o b s e r v e d . The t r e a t m e n t of t h e s e c o a l s was e x p e c t e d t o remove t h e two m a j o r s p e c i e s d e s c r i b e d in the p r e v i o u s p a r a g r a p h , e n a b l i n g t h e o b s e r v a t i o n o f low c o n c e n t r a t i o n s p e c i e s , with the total absorption being ~0.1~. However, a p e a k a t zero velocity in the s p e c t r a o f t h e s e s a m p l e s i s now a t t r i b u t e d t o i r o n i n t h e b e r y l l i u m window o f t h e d e t e c t o r , w i t h t h e r l g h t hand member o f t h e B e : F e d o u b l e t c o i n c i d e n t w i t h a peak f r o m i r o n i n t h e c o a l . I n an a t t e m p t t o d e t e r m i n e t h e o r i g i n o f t h e z e r o v e l o c i t y peak in r e f s / 1 - 3 / , we have e x a m i n e d brown c o a l s u b j e c t e d t o c o n d i t i o n s u n d e r w h i c h i n h e r e n t i r o n compounds may be r e d u c e d t o i r o n . This should also allow extension of the results to predict possible occurrences in nature. 2. SAMPLES The l a b o r a t o r y t e s t s were p e r f o r m e d on a s - m i n e d M a r y v a l e M2276 c o a l a s i t has t h e h i g h e s t i r o n c o n t e n t ( a p p r o x i m a t e l y 1.8 wt~) o f t h e V i c t o r i a n brown c o a l s and h a s a l r e a d y been .extensively s t u d i e d by M6ssbauer s p e c t r o s c o p y /5/. M o i s t , u n o x i d l s e d c o a l was c u t f r o m t h e c e n t r e o f a lump and s e a l e d u n d e r one a t m o s p h e r e o f N2 i n a q u a r t z t u b e f i t t e d w i t h a v a l v e s e t t o r e l e a s e a t 0.2 atmospheres overpressure. One sample was h e a t e d f o r 70 h o u r s a t 640"C and a s e c o n d sample a t 960"C f o r 1 h o u r . Both s a m p l e s were q u e n c h e d i n t o a n i c e / w a t e r mixture. They a r e h e n c e f o r t h r e f e r r e d t o a s '640~ c o a l ' and '960*C c o a l ' . A c t i v a t e d c a r b o n s a m p l e s were p r e p a r e d f r o m d r i e d Morwell bTown c o a l by f l a s h p y r o l y s i s f o l l o w e d by h e a t i n g a t 900~ The M~ssbauer s p e c t r u m o f t h i s c o a l a s - m i n e d i s s i m i l a r t o t h a t o f t h e M2276 c o a l b u t w i t h a s m a l l e r f e r r i c

9

J.C. Baltzer A.G., Scientific P u b l i s h i n g C o m p a n y

G.S. Hang et al., Gamma-iron formation in brown coal

796

oxyhydroxide contribution. Commercial a c t i v a t e d c a r b o n ( K i n t a l ) , w h i c h i s p r o d u c e d f r o m c o c o n u t c h a r principally for sold extraction, was a l s o e x a m i n e d . The d e t a i l s of its p r e p a r a t i o n a r e u n a v a i l a b l e b u t we may assume t h a t t h e c o c o n u t s h e l l was f i r s t c h a r r e d a n d t h e n h e a t e d , p o s s i b l y i n s t e a m , a t a t e m p e r a t u r e o f N1000*C. 3. R E ~ , ~

AND DIST~SSION

The f i t t e d p a r a m e t e r s f o r t h e H S s s b a u e r s p e c t r a t a k e n a t room t e m p e r a t u r e (RT) ( F i g . l ) a n d 4.2K ( F i g . 2 ) a r e g i v e n i n T a b l e s 1 a n d 2 r e s p e c t i v e l y . The RT s p e c t r m n o f t h e M2276 c o a l i n i t s i n i t i a l bedmoist state (Fig.la) comprises a central pair of peaks assigned to superparamagnetic ferric oxyhydroxlde. The ferrous humate species is barely visible at this temperature /5/. After heating a t 640~ (Fig.lb), the recoilless fraction increased dramatically and the spectral shape changed considerably. The two p r i n c i p a l a b s o r p t i o n d i p s c a n b e f i t t e d i n s e v e r a l ways b u t t h e p a r a m e t e r s shown i n T a b l e 1 are the most realistically interpretable set. The p a r a m e t e r s of the first doublet agree with those of microcrystalline FeO o b s e r v e d i n t h e a s h f r o m b l a c k coal combustion /6/. The s i n g l e t may b e m i c r o c r y s t a l l i n e (superparamagnetic) magnetite /7/, as it displays a similar isomer shift and iarge linewidth. Microcrystalline FeO and magnetite are certainly feasible considering that the iron is present before heating as microparticulate ferric o x y h y d r o x i d e a n d t h a t t h e r e d u c t i o n o f h e m a t i t e t o ~ - i r o n a b o v e 560~ proceeds v i a t h e s e two i n t e r m e d i a t e s . The s m a l l r e m a i n i n g d o u b l e t , with quadrupole splitting of 2.66 n~/s, represents an ionic iron(II) species. A f t e r h e a t i n g a t 960*C, a p r o m i n e n t a n d s h a r p p e a k n e a r z e r o v e l o c i t y is a p p a r e n t a t b o t h E r ( F i g . l c ) a n d 4.2K ( F i g . 2 a ) . This slnglet and the doublet of approximately zero isomer shift appear to arise from an interstitial solid s o l u t i o n o f c a r b o n i n i r o n , a n d c o r r e s p o n d t o i r o n a t o m s i n a T-Fe l a t t i c e with z e r o a n d one n e a r e s t - n e i g h b o u r carbon atoms respactively /8/. The s m a l l a r e a l r a t i o o f d o u b l e t t o s i n g l e t i m p l i e s a c a r b o n c o n t e n t l e s s t h a n 1 . 2 wt~ / 8 / . The s h a r p s e x t e t ( h y p e r f i n e f i e l d N33 T) c o r r e s p o n d s t o ~ - F e . The r e ~ i n l n g species is identified as micropartlculate ferric oxide, producing a 49.2 T sextet at

10O.0 29.9

99.0

100.0' 2a.G 29.0

100.~ I~O O

i

10OA 99.?

" ~ H.O

100.0 I;9.8

-12

|

,

I ,

|

|

-8

-4

0

,4

8

~ 12

Velocity (turn/see wrt a-Fe) F i g . 1 . 57Fe MBssbauer s p e c t r a a t RT of (a) initial m o i s t c o a l , ( b ) 6400C c o a l , ( c ) 960"C c o a l , (d) a c t i v a t e d carbon from coconut char, and (e) activated carbon from coal char.

-12

-8 -4 0 4 8 12 Velocity (mm/sec wrt a-Fe)

Fig.2. 57Fe M ~ s s b a u e r s p e c t r a a t 4.2K o f ( a ) 960~ c o a l , (b} a c t i v a t e d c a r b o n from coconut char, and (c) activated carbon from coal char.

G.S. Hang et al., Gamma-iron formation in brown coal

797

4.2K w h i c h c o l l a p s e s s u p e r p a r a m a g n e t i c a l l y t o t h e s e c o n d d o u b l e t a t RT. The large linewidths of this ferric oxide spectrum at both temperatures reflect a site distribution, indicative of poor crystallinity. The RT ( F i g . l d } and 4.2K ( F i g . 2 b ) s p e c t r a o f a c t i v a t e d c a r b o n f r o m c o c o n u t char resemble those of the 960~ coal, showing the ]-Fe(C) slnglet, a sharp ~-Fe s e x t e t and a b r o a d f e r r i c o x i d e s e x t e t . The f e r r i c o x i d e e v i d e n t l y occurs as l a r g e r p a r t i c l e s t h a n i n t h e 960~ c o a l s i n c e i t r e m a i n s m a g n e t i c a l l y split at RT. The l a r g e u n c e r t a i n t y i n t h e f i t o f t h e d o u b l e t s p r e v e n t s an unambiguous interpretation, e x c e p t t h a t a l a r g e component s p l i t s o u t a t 4.2K i n t o a p a t t e r n characteristic of a poorly crystalline ferric o x i d e and an i o n i c iron(II) species appears to be present. The spectra of activated carbon prepared from coal display the same general features, but with several important distinctions. The ~ - F e s e x t e t is less intense than in the other samples at 4.2K (Fig.2c), and indiscernable at RT (Fig.le). The ferric oxide magnetic spectrum at 4.2K is more intense, and two s e x t e t s were r e q u i r e d f o r an a c c e p t a b l e f i t t o what i s p r o b a b l y a c o n t i n u o u s distribution of hyperfine fields. F i n a l l y , and most s i g n i f i c a n t l y , a prominent s e x t e t a p p e a r s ( 2 0 . 6 T a t RT, 2 5 . 4 T a t 4.2K) w h i c h was n o t s e e n i n t h e o t h e r samples ; its p a r a m e t e r s a g r e e w i t h t h o s e o f FeRC / 8 / . The s l m u l t a n e o u s p r e s e n c e o f = - F e , T-Fe(C) and Fe3C i n t h i s sample i l I u s t r a t e s the progressive carburation of iron. V i c t o r i a n brown c o a l c o n t a i n s a p p r o x i m a t e l y 67 wt~ w a t e r , so t h a t a water-gas reaction C

+

HzO

..... >

CO

+

H2

occurs when the coal is heated in an inert atmosphere. The resultant Ha c c o u n t s f o r t h e o b s e r v e d r e d u c t i o n o f F e ( I I I ) t o F e ( I I ) and F e ( 0 ) i n t h e 640~ and 960~ c o a l s . With t h e a b u n d a n c e o f c a r b o n a v a i l a b l e , i t is not surprising that the reduced iron is extensively carbided. The f o r m a t i o n o f i r o n c a r b i d e s has been documented p r e v i o u s l y for coal coking /9/ and l i q u e f a c t i o n /10/, and i n c a t a l y s t s u s e d i n F i s c h e r - T r o p s c h s y n t h e s i s o f h y d r o c a r b o n s f r o m CO and H2 / 1 1 / .

T a b l e 1. P a r a m e t e r s a) f o r 67Fe M~ssbauer s p e c t r a t a k e n a t room t e m p e r a t u r e . Sample 8

Singlets r Area

M o i s t c o a l b lI 640~

6 .36

Doublets A F .67

Area

.7

>90

coal

.35

.80

44

1.02 1.27

.71 2,66

.66 .23

52 4

960"C c o a l

-.08

.85

27

-.02 .37

.66 .82

.22 .78

6 31

Activ. carbon (coconut char)

-.08

.42

22

.66 1.13

.79 1.72

.92 1.07

II 80

Activ. carbon (coal char)

-.08

.40

I0

.82 1.06

.73 1.60

.64 .89

47 8

6

E

.00

.00

Sextets B F

.28

36

-.03 .00 ' 3 3 . 1 .30 .40 - . 0 1 4 9 . 6 1.28

18 22

.18 .02 .89 - . 0 1

33.1

Area

20.6 .60 47.5 1.36

a) 6 = i s o m e r s h i f t w r t ~ - F e a t RT, A = q u a d r u p o l e s p l i t t i n g , e = eQVzz/4, r = F~]M, a l l e x p r e s s e d i n " v - / s . B = h y p e r f i n e f i e l d i n T e s l a , b) A r e a = p e r c e n t a g e s p e c t r a l a r e a . From r e f / 8 / .

30 5

G.S. Hang et al., Gamma-iron formation in brown coal

798

T a b l e 2. P a r a m e t e r s a) f o r Sample 6 960~

coal

Activ. carbon (coconut char} Activ. carbon (coal char)

57Fe MSssbauer s p e c t r a

Singlets F Area

6

Doublets A r

t a k e n a t 4.2K.

Area

~

s

Sextets B F

Area

.04

.45

26

.16

.64

.39

7

.11 .50

.00 34.2 .30 -.01 49.2 1.19

31 36

.04

.54

18

.55 1.38

.86 2.28

1.20 1.93

12 22

.II .45

-.01 33.8 .36 -.02 49.7 1.50

18 30

-.05

.36

13

.38 1.36

.64 2.38

.89 .35

15 3

.31 .12 .54 .45

-.01 -.01 -.07 -.04

22 5 26 16

25.4 .42 34.3 .28 4 5 . 9 1.21 50.6 .72

a) See footnote (a) of Table I.

4. CONCLUSIONS This work has demonstrated that iron carbides form readily in Victorian brown coal at high temperatures in reducing conditions. By extension, it appears probable'that the singlet species observed in refs /1-3/ can all be assigned to T-Fe(C). However, considering the conditions prevailing in these experiments, we consider it unlikely that iron carbides could be formed in coal deposits by any natural process such as the heat from a nearby firehole. A

~

We would l i k e t o t h a n k J o h n H a r r i s a n d C o l i n McAllan f o r . p r o v i d i n g the coal-based activated carbon. We a r e g r a t e f u l t o Roman L i e b a c h a n d D a v i d Cookson for their technical assistance, and are pleased to acknowledge the financial s u p p o r t o f t h e A u s t r a l i a n R e s e a r c h G r a n t s Scheme a n d t h e A u s t r a l i a n I n s t i t u t e of Nuclear Science and Engineering. References /1/ G.P. Huffman and F.E. Huggins, Fuel 87(1978)592. /2/ P.A. Montano~ Private co---unlcatlon. / 3 / J . D . C a s h l o n , D . J . Cookson, L . J . Brown a n d D.G. Howard, i n I n d u s t r i a l A p p l i c a t i o n s o f t h e M o s s b a u e r E f f e c t , e d s G . J . Long a n d J . G . S t e v e n s ( P l e n u m , New York, 1987) p . 5 9 5 . / 4 / J . D . C a s h l o n , P. HcNamara a n d J . R . F r a s e r , P r o c . I n t . C o n f . Coal S c i e n c e (Pergamon, Sydney, 1985) p . 8 4 9 . /8/ P.S. Cook and J.V. Cashion, Geochim. Cosmochim. Acts, 51(1987)1467. /6/ C.P. Huffman, F.E. Huggins and G.R. Dunmyre, Fuel, 60(1981)585. / 7 / T.K. McNab, R.A. Fox a n d A . J . F . B o y l e , J . A p p l . P h y s . , 3 9 ( 1 9 6 8 ) 5 7 0 3 . / 8 / D.L. W i l l i a m s o n , K. Nakazawa a n d G. K r a u s s , M e t a l l . T r a n s . A , 10(1979)1351. / 9 / P.K. M i t r a , H . K . S h a r m a , D.E.As, D.N.Tewary a n d K. R a j a , Current Sci. 50(1981)130. / 1 0 / P . S . Cook a n d J . D . C a s h i o n , F u e l , 6 6 ( 1 9 8 7 ) 6 6 9 . / 1 1 / G. LeCaer, J.M. D u b o l s , M. P i j o l a t , V. P e r r i c h o n a n d P. B u s s i e r e , J.Phys.Chem. 86(1982)4799.

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