36 Decomposition of Pyrite in a Coal Matrix during the Pyrolysis of Coal I. Stewart , S. G . Whiteway , P. J. Cleyle , and W. F. Caley
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1
1
2
2
1
Atlantic Research Laboratory, National Research Council, Halifax, NS B 3 H 3Z1, Canada
2
Technical University of Nova Scotia, Halifax, N S B3J 2X4, Canada
The high levels of sulphur present in Nova Scotian coals may restrict their future use in power generation. A number of beneficiation strategies, aimed at reducing the sulphur levels, are currently under investigation in the Atlantic region; among these is pyrolysis. This work provides a better understanding of the fate of pyritic sulphur from the pyrite/pyrrhotite conversion during pyrolysis. Coal from collieries in Nova Scotia were pyrolysed under argon in a tube furnace at a variety of temperatures and times. Elements of interest in the resultant char were analysed using a scanning electron microscope, equipped with energy and wavelength dispersive spectrometers. The results indicate that sulphur originating from pyrite is transferred to the nearby surrounding organic matrix during the pyrolysis process. This sulphur is in a form which may be very difficult to remove. The possibility of using magnetic separation to reduce the sulphur levels in the char was explored. The h i g h s u l p h u r c o n t e n t o f E a s t e r n C a n a d i a n c o a l s , as much as 8%, has l e d t o s t u d i e s aimed a t r e d u c i n g t h e s e s u l p h u r l e v e l s ( 1 ) . I n g e n e r a l about two t h i r d s o f t h e s u l p h u r i s p y r i t i c , o f t e n o c c u r i n g as s m a l l i n c l u s i o n s , and t h e r e f o r e d i f f i c u l t t o remove by c o n v e n t i o n a l means. One method which h o l d s promise i s p y r o l y s i s f o l l o w e d by magnetic s e p a r a t i o n . P y r o l y s i s involves the heating o f coal i n a n o n - o x i d i s i n g environment p r o d u c i n g c h a r , l i q u i d and gas. This publication deals with the r e a c t i o n s o f p y r i t e i n the c o a l , the p a r t i t i o n i n g o f the p y r i t i c s u l p h u r d u r i n g p y r o l y s i s , and t h e e f f e c t t h i s has on magnetic s e p a r a t i o n . 0097-6156/ 86/ 0301 -0485506.00/ 0 © 1986 American Chemical Society
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
MINERAL MATTER AND
486
ASH IN COAL
When m i n e r a l p y r i t e i s heated under an i n e r t atmosphere i t decomposes, i n two s t e p s , t o form f i r s t l y p y r r h o t i t e and then a t a h i g h e r temperature, iron. FeS
2
FeS
=
( 1 + x )
FeS
( 1 + x )
=
Fe
+
+
S S
( 1
(
1
.
+
x
[l ]
x )
[2]
)
Where 0 < χ < 0.22. I t i s g e n e r a l l y known t h a t , i n c o a l , r e a c t i o n s 1 and 2 a t a lower temperature, p r o b a b l y because hydrogen from the decomposing c o a l m a t r i x a i d s the r e a c t i o n :
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FeS
2
+ (1-x)H
2
->
proceed
FeS(1+x) + ( 1 - x ) H S 2
[3]
R e a c t i o n [2] was not o b s e r v e d i n t h i s study because e x p e r i m e n t a l temperatures were not h i g h enough. The f a t e o f p y r i t e i n c o a l has been the s u b j e c t o f a number o f p u b l i c a t i o n s , p a r t i c u l a r l y i n the a r e a o f l i q u e f a c t i o n , where p y r i t e or i t s p r o d u c t s a r e thought t o p l a y an important c a t a l y t i c r o l e ( 2 ) . In a p r e v i o u s p u b l i c a t i o n ( 3 ) i t was r e p o r t e d t h a t the d e c o m p o s i t i o n of p y r i t e t o p y r r h o t i t e o c c u r r e d i n the temperature range 500-550°C f o r a run-of-mine(ROM) P r i n c e c o a l . T h i s was w i t h i n the range o f 440~580°C r e p o r t e d by o t h e r workers f o r the d e c o m p o s i t i o n o f p y r i t e i n c o a l (4,5). The c u r r e n t work extends the p r e v i o u s s t u d y t o t h r e e washed c o a l s and i n c l u d e s some p r e l i m i n a r y work on magnetic separation. Magnetic s e p a r a t i o n has been a p p l i e d by o t h e r workers (6) who have shown t h a t r e j e c t i o n o f 95-99% o f p y r i t i c s u l p h u r i s p o s s i b l e , i f advantage can be taken o f the p y r i t e / p y r r h o t i t e t r a n s f o r m a t i o n . T h i s l a t t e r p r o c e s s not o n l y i n v o l v e s a l o s s o f s u l p h u r but c o n c u r r e n t 100- f o l d i n c r e a s e i n magnetic s u s c e p t i b i l i t y ( 7 ) . I t i s t h i s i n c r e a s e which makes magnetic s e p a r a t i o n a t t r a c t i v e . Experimental A l l the c o a l s s t u d i e d i n the p r e s e n t work were washed c o m m e r c i a l l y t o f r e e them o f some o f t h e i r m i n e r a l c o n t e n t . They were from t h r e e c o l l i e r i e s o f the Cape B r e t o n Development C o r p o r a t i o n : Prince, L i n g a n and No. 26. L i n g a n and No. 26 c o l l i e r i e s a r e i n d i f f e r e n t r e g i o n s o f the same c o a l seam: the P r i n c e c o l l i e r y i s i n a s e p a r a t e seam. A l l the c o a l s a r e c l a s s e d as h i g h v o l a t i l e bituminous A. T y p i c a l ash and s u l p h u r c o n t e n t s a r e g i v e n i n T a b l e I . The c o a l was c r u s h e d and s i e v e d on T y l e r s c r e e n s t o -8 +16 mesh (1-2.4 mm). A p p r o x i m a t e l y 10 g o f c o a l was p l a c e d i n an alumina boat and p y r o l y s e d under argon i n a tube f u r n a c e f o r 20 hours a t c o n s t a n t temperature i n the ranges 4 1 0 - 6 4 5 ° C A f t e r p y r o l y s i s p i e c e s o f the r e s u l t a n t char were mounted u s i n g F i s h e r Chemical Co. 'Quickmount* cold-mounting r e s i n , and p o l i s h e d in p r e p a r a t i o n f o r scanning e l e c t r o n microscopy. The o b j e c t was t o a n a l y s e f o r s u l p h u r by s c a n n i n g a c r o s s s e l e c t e d p y r i t e s e c t i o n s , and c o n t i n u i n g out i n t o the s u r r o u n d i n g c o a l m a t r i x .
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
36.
STEWART ET AL.
Decomposition
Table I .
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P r i n c e Coal
Moisture Volatiles Ash Carbon Oxygen T o t a l sulphur P y r i t e sulphur Sulphate sulphur Organic sulphur
2.24 3^.65 6.4 58.95 5.0 3.62 2.56 0.08 0.98
of Pyrite in a Coal
Matrix
487
Analyses of Coals
Lingan
Coal
1 .09 28.34 3.65 68.01 5.6 2.01 1 .42 0.09 0.56
No.
26
Coal
0.87 25.42 2.92 71.66 5.0 0.86 0.26 0.04 0.56
A J e o l - 3 5 Scanning E l e c t r o n M i c r o s c o p e equipped w i t h energyand wavelength- d i s p e r s i v e X-ray s p e c t r o m e t e r s was used f o r elemental a n a l y s i s . Spot a n a l y s e s were c a r r i e d out; t h e s e covered a p p r o x i m a t e l y 0.5 ym f o r the p y r i t e and 1 ym f o r the c o a l . The s t a n d a r d atomic number, a b s o r p t i o n and f l u o r e s c e n c e (ZAF) c o r r e c t i o n s were a p p l i e d t o a l l a n a l y s e s , u s i n g c o u n t i n g times o f 200 seconds, an a c c e l e r a t i n g v o l t a g e o f 15 kV and a p y r i t e c r y s t a l as s t a n d a r d . I n a d d i t i o n some of the samples were s t u d i e d by Fe-57 Mossbauer spectroscopy. The s p e c t r o m e t e r was an Apple H e - c o n t r o l l e d Ranger S c i e n t i f i c MS-900 o p e r a t e d i n c o n s t a n t a c c e l e r a t i o n mode, u s i n g Co-57 i n a m a t r i x o f rhodium as s o u r c e . S p e c t r a l d a t a were accumulated i n 1024 c h a n n e l s . Results The e x t e n t of p y r i t e d e c o m p o s i t i o n was f o l l o w e d by o b t a i n i n g S/Fe atomic r a t i o s a t v a r i o u s p o i n t s a c r o s s s e l e c t e d p y r i t e i n c l u s i o n s , t h e l a t t e r i n the s i z e range 10-20 ym. Figure 1 i s a p l o t of t h i s r a t i o v e r s u s " d i s t a n c e from i n t e r f a c e w i t h i n p y r i t e " f o r each o f the c o a l s at d i f f e r e n t temperatures. The r e s u l t s i n d i c a t e t h a t , f o r p y r i t e o f t h i s s i z e , d e c o m p o s i t i o n t o p y r r h o t i t e o c c u r s i n the temperature range 450-500°C, f o r a l l the c o a l s s t u d i e d . The r e a c t i o n e s s e n t i a l l y was complete a t 500°C. Mossbauer s p e c t r o s c o p y was used t o examine the c h a r s produced from the p y r o l y s i s r e a c t i o n . The r e s u l t s , which d e a l w i t h b u l k d e c o m p o s i t i o n compared t o the m i c r o s c o p i c view o b t a i n e d from SEM, tended t o c o n f i r m those o b t a i n e d above f o r the i n i t i a l temperature of decomposition. However, a number o f major d i f f e r e n c e s were observed. P y r i t e was always a component i n the s p e c t r a , even those o f c h a r s produced a t 650°C. I n a d d i t i o n the t h r e e c o a l s c o u l d be c l a s s e d i n two groups: P r i n c e and L i n g a n behaved s i m i l a r l y w i t h the f o r m a t i o n o f p y r r h o t i t e s a t around 450-500°C; on the o t h e r hand No. 26 c o a l showed l i t t l e s i g n o f any r e a c t i o n p r o d u c t even a t 650°C ( F i g s . 2-4), an anomaly c o n s i d e r i n g the f a c t t h a t s e l e c t e d g r a i n s o f s i z e 10-20 ym would have been c o m p l e t e l y r e a c t e d .
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
MINERAL MATTER AND ASH IN COAL
Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch036
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cr.
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^ Β φ• 555
D
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Δ
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Distance From Interface Within Pyrite (/*m)
F i g . 1 S/Fe atomic r a t i o v s . d i s t a n c e from i n t e r f a c e w i t h i n p y r i t e f o r P r i n c e , L i n g a n and No. 26 c o a l s .
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Decomposition
of Pyrite in a Coal
Matrix
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STEWART ET AL.
F i g . 2 Mossbauer s p e c t r a o f P r i n c e c o a l p y r o l y s e d a t temperatures.
different
In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
490
MINERAL MATTER AND ASH IN COAL
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Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch036
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