Coal Science II - American Chemical Society

Chapter 5

Coal Structure The

Problem with Minerals

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: May 6, 1991 | doi: 10.1021/bk-1991-0461.ch005

G. A. Robbins Consolidation Coal Company, Research and Development, 4000 Brownsville Road, Library, PA 15129

Fourier transform infrared (FTIR) spectroscopy of coal low-temperature ashes was applied to the determination of coal mineralogy and the prediction of ash properties during coal combustion. Analytical methods commonly applied to the mineralogy of coal are c r i t i c a l l y surveyed. Conventional least-squares analysis of spectra was used to determine coal mineralogy on the basis of forty-two reference mineral spectra. The method described showed several limitations. However, partial least-squares and principal component regression calibrations with the FTIR data permitted prediction of a l l eight ASTM ash fusion temperatures to within 50 to 78°F and four major elemental oxide concentrations to within 0.74 to 1.79 wt % of the ASTM ash (standard errors of prediction). Factor analysis based methods offer considerable potential in mineralogical and ash property applications. D e s p i t e c o n s i d e r a b l e e f f o r t t o u n d e r s t a n d the c h e m i c a l n a t u r e o f the o r g a n i c a n d i n o r g a n i c p o r t i o n s o f c o a l , and s u b s t a n t i a l advances i n i n s t r u m e n t a t i o n and m e t h o d o l o g i e s , much o f c o a l ' s c h e m i c a l n a t u r e remains i n t r a c t a b l e . T h i s paper r e v i e w s methods and t h e i r l i m i t a t i o n s f o r the d e t e r m i n a t i o n o f s p e c i f i c m i n e r a l s i n c o a l , and p r e s e n t s r e s u l t s o f e f f o r t s a t C o n s o l i d a t i o n C o a l Company t o develop FTIR methods f o r r o u t i n e c o a l m i n e r a l o g y . M i n e r a l matter c h a r a c t e r i z a t i o n i n c o a l has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n . G i v e n and Y a r z a b (1) d i s c u s s e d the problems posed by m i n e r a l m a t t e r i n v a r i o u s c o a l a n a l y s e s . Furthermore, m i n e r a l m a t t e r c o m p l i c a t e s the c h e m i c a l t r e a t m e n t o f c o a l . I t a l s o has many a d v e r s e e f f e c t s on commercial c o a l u t i l i z a t i o n . C o a l consumers pay to s h i p m i n e r a l m a t t e r , t o accommodate i t s impact on c a p i t a l e q u i p ment and o p e r a t i o n s , and t o d i s p o s e o f t h e r e s u l t a n t ash. The impact o f c o a l m i n e r a l s on u t i l i z a t i o n m o t i v a t e d C o n s o l ' s initial i n t e r e s t i n m i n e r a l o g y ( 2 ) . The r e s u l t s r e p o r t e d h e r e a r e from a second phase o f t h e FTIR method development, i n w h i c h e x t e n s i v e improvements were made t o the methods. 0097-6156/91/0461-0044$06.00/0 © 1991 American Chemical Society

In Coal Science II; Schobert, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

5.

ROBBINS

Coal Structure

45

EXPERIMENTAL D e s c r i p t i o n o f Samples


Two

c o a l sample s e t s and the r e f e r e n c e m i n e r a l s a r e d e s c r i b e d below.

S m a l l Data S e t . Low-temperature (plasma) ashes (LTAs) were o b t a i n e d from t e n d i v e r s e c o a l samples ( T a b l e I ) , r a n g i n g i n rank from l i g n i t e t o l v b . I n f r a r e d s p e c t r a were o b t a i n e d o f d u p l i c a t e samples o f each LTA. A s e p a r a t e s e t o f d u p l i c a t e s g e n e r a t e d ( f o r o t h e r purp o s e s ) f o r the f i r s t f o u r LTAs l i s t e d a l s o was a n a l y z e d by FTIR. Table I .

D e s c r i p t i o n o f Coals i n Small Data Set

Coal

Rank

Seam - D e s c r i p t i o n

PI EH 11 SL 12 13 P2 P3 GE

hvAb hvAb hvBb Lig hvBb hvBb hvAb hvAb hvAb

PO

lvb

P i t t s b u r g h - Underground, C l e a n C o a l , Greene Co., PA E l k h o r n 3 - Underground, Unwashed, B r e a t h i t t Co., KY I l l i n o i s 5 ,6 - S u r f a c e , C l e a n C o a l , J a c k s o n Co., I L S c r a n t o n - S u r f a c e , M e r c e r Co., ND I l l i n o i s 6 - Underground, C l e a n C o a l , J e f f e r s o n Co., I L I l l i n o i s 6 - S u r f a c e , C l e a n C o a l , P e r r y Co., I L P i t t s b u r g h - Underground, C l e a n C o a l , M a r s h a l l Co., WV P i t t s b u r g h - Underground, C l e a n C o a l , M o n o n g a l i a Co., WV No. 2 Gas, Upper and M i d d l e E a g l e - Underground, C l e a n C o a l , R a l e i g h Co., WV Pocahontas 3 - Underground, C l e a n C o a l , Buchanan Co., VA

L a r g e Data S e t . LTAs were a n a l y z e d by FTIR f o r 50 c o a l s , r a n g i n g i n rank from l i g n i t e t o l v b . These were a r e p r e s e n t a t i v e s u b s e t o f 95 unwashed and c l e a n c o m m e r c i a l c o a l s from the e a s t e r n , m i d - w e s t e r n and w e s t e r n U n i t e d S t a t e s and A l b e r t a , Canada. The 5 0 - c o a l s e t cont a i n e d no d u p l i c a t e c o a l samples, b u t d i f f e r e n t c o a l samples from the same mine were i n c l u d e d . Reference M i n e r a l s . The 42 r e f e r e n c e m i n e r a l s and the m i n e r a l c l a s s e s u s e d a r e l i s t e d i n T a b l e I I . Most o f the m i n e r a l s were o b t a i n e d from Ward's N a t u r a l S c i e n c e E s t a b l i s h m e n t , I n c . , R o c h e s t e r , New Y o r k . Many o f the s i l i c a t e m i n e r a l s were A m e r i c a n P e t r o l e u m I n s t i t u t e (API) s t a n d a r d samples o r t h e i r e q u i v a l e n t s . Numbers g i v e n i n the t a b l e ( e . g . , k a o l i n i t e 4) r e f e r t o the API s t a n d a r d designation. When a v a i l a b l e , s e v e r a l m i n e r a l s o f each t y p e were used f o r r e f e r e n c e s , to a l l o w f o r v a r i a b i l i t y i n composition, c r y s t a l l i n i t y , etc. Methods. C o a l s ground t o -60 mesh were l o w - t e m p e r a t u r e ( 0 2 plasma) ashed i n one o f t h r e e plasma a s h e r s t o c o n s t a n t w e i g h t f o r about 100-125 h o u r s f o r b i t u m i n o u s c o a l s and 200 h o u r s f o r l o w e r r a n k coals. Samples were s t i r r e d and weighed a minimum o f once each 24 hours. The c o n d i t i o n s used were 200 w a t t s f o r w a r d power and 15-20 mL 0 2 / m i n (LFE Corp. Model LTA-504 o r Denton Vacuum I n c . Model


C O A L SCIENCE II

46 Table I I . Reference M i n e r a l s


Main C l a s s e s K a o l i n - K a o l i n i t e 4, 5, 6, D i c k i t e 16, 27 Mica - B i o t i t e , Phologopite, Muscovite I l l i t e - I l l i t e 36, I l l i t e - B e a r i n g S h a l e M i x e d - L a y e r C l a y s - M e t a b e n t o n i t e 37, 42 M o n t m o r i l l o n i t e - 21, 22A, 22B, 24, 25, 26, 31 Feldspars - A l b i t e , Anorthite, Orthoclase Chlorite - Chlorite M i s c . A l u m i n o s i l i c a t e s - A t t a p u l g i t e , H a l l o y s i t e , P y r o p h y l l i t e 48 Q u a r t z - Q u a r t z , Agate Fe s u l f i d e s - M a r c a s i t e , Commercial P y r i t e , M i n e r a l P y r i t e , Pyrrotite Fe o x i d e s - G o e t h i t e , H e m a t i t e , M a g n e t i t e Fe s u l f a t e - D i f f e r e n c e spectrum weathered minus unweathered p y r i t e Siderite - Siderite C a l c i t e - C a l c i t e , Aragonite Gypsum - Gypsum, Commercial D r i e r i t e ( A n h y d r i t e ) Dolomite - Dolomite Grouped C l a s s e s (used t o s i m p l i f y t h e f i n a l r e s u l t s ) C l a y s - K a o l i n , I l l i t e , M. L. C l a y s , M o n t m o r i l l o n i t e , M i s c . Aluminosilicates Other A l u m i n o s i l i c a t e s - M i c a , F e l d s p a r , C h l o r i t e , Q u a r t z S u l f a t e - Fe S u l f a t e , Gypsum Carbonate - C a l c i t e , S i d e r i t e , D o l o m i t e Fe A l t e r e d - O x i d e s , S u l f a t e

PE-250 plasma a s h e r ) o r 280 w a t t s f o r w a r d power and 90 mL 0 2 / m i n (Branson/IPC plasma a s h e r ) . The l a t t e r a s h e r and c o n d i t i o n s were u s e d o n l y f o r a s h i n g s m a l l b a t c h e s o f c o a l (up t o e i g h t d i f f e r e n t samples o f 0.5 t o 1 g e a c h ) . When l a r g e r b a t c h e s were ashed, e i g h t d i s h e s c o n t a i n i n g 2 g each o f t h e same c o a l were p l a c e d i n t h e a s h e r . A s h i n g c o n t a i n e r s were made o f P y r e x o r c e r a m i c . M i n e r a l s t a n d a r d s were hand c r u s h e d t o -1/4 i n c h , t h e n ground to a f i n e powder i n a b a l l m i l l ( a l u m i n a e l e m e n t s ) o r B l e u l e r Model 526/LFS678 puck m i l l . The r e s u l t a n t powder was a e r o d y n a m i c a l l y c l a s s i f i e d i n a Bahco Model 6000 m i c r o p a r t i c l e c l a s s i f i e r and t h e f i n e s t f r a c t i o n (#18 t h r o t t l e ) was c o l l e c t e d . A size criterion of 90% o r more b y w e i g h t o f p a r t i c l e s 5 m i c r o n and s m a l l e r i n d i a m e t e r was u s e d f o r t h e m i n e r a l s t a n d a r d s . S i z e s were v e r i f i e d b y C o u l t e r C o u n t e r . D u p l i c a t e 13 mm KBr p e l l e t s were p r e p a r e d and t h e s p e c t r a were w e i g h t - s c a l e d b y t e c h n i q u e s s i m i l a r t o t h o s e r e p o r t e d b y P a i n t e r (3) and E l l i o t ( 4 ) . W i t h one e x c e p t i o n , a l l t h e m i n e r a l s t a n d a r d s p e c t r a were averages o f s p e c t r a from d u p l i c a t e p e l l e t s . The one e x c e p t i o n was t h e i r o n s u l f a t e spectrum, w h i c h was o b t a i n e d as t h e d i f f e r e n c e spectrum by s u b t r a c t i n g t h e spectrum o f HC1-washed w e a t h e r e d p y r i t e from t h a t o f t h e weathered p y r i t e . A weight corr e c t i o n was a p p l i e d t o t h e d i f f e r e n c e spectrum. LTA samples were ground f o r 30 minutes i n a Wig-L-Bug (15mg LTA, 50 mg KBr and 500 mg acetone i n an a g a t e v i a l ) , d r i e d on a h o t



5.

ROBBINS

Coal Structure

47

p l a t e a t 40°C and t h e n s t o r e d i n t h e d e s s i c a t o r . A n a l i q u o t o f each sample m i x t u r e was d i l u t e d w i t h KBr, 13 mm p e l l e t s were made and t h e r e s u l t i n g s p e c t r a were w e i g h t - s c a l e d b y t e c h n i q u e s s i m i l a r t o those r e p o r t e d b y o t h e r s ( 3 . 4 ) . LTAs were n o t s i z e - c l a s s i f i e d because o f l i m i t e d q u a n t i t y . However, t h e g r i n d i n g c o n d i t i o n s were chosen t o give the smallest p a r t i c l e s achievable f o r various reference minerals. A l l s p e c t r a were r u n o n a N i c o l e t 7199 FTIR spectrometer e q u i p p e d t o o p e r a t e i n t h e m i d - i n f r a r e d (wide-band MCT d e t e c t o r ) . A l e a s t s q u a r e s a n a l y s i s program p r o v i d e d b y N i c o l e t (MCOMP) was e x t e n s i v e l y m o d i f i e d f o r e f f i c i e n t r o u t i n e u s e w i t h a l a r g e number o f reference minerals. The r e f e r e n c e m i n e r a l w i t h t h e l o w e s t n e g a t i v e c o n c e n t r a t i o n i n each l e a s t - s q u a r e s d e c o m p o s i t i o n o f t h e LTA s p e c t r a was o m i t t e d upon each i t e r a t i o n , u n t i l o n l y n o n - n e g a t i v e c o n c e n t r a t i o n s were o b t a i n e d . G e n e r a l l y 12 t o 18 m i n e r a l s remained i n t h e final calculation. In a d d i t i o n t o spectra o f the reference minerals l i s t e d i n T a b l e I I , t h e l e a s t - s q u a r e s components i n each i t e r a t i o n i n c l u d e d 3 " s p e c t r a " r e p r e s e n t i n g 1) m o i s t u r e i n K B r b l a n k ( o b t a i n e d b y subt r a c t i o n o f 2 K B r b l a n k s p e c t r a ) , 2) a c o n s t a n t b a s e l i n e o f f s e t ( 1 abs from 4000 t o 400 c m ' ) , and 3) a s l o p i n g l i n e a r b a s e l i n e ( l i n e from 1 abs a t 4000 cm" t o 0 abs a t 400 cm" ). The f i n a l m i n e r a l component c o n c e n t r a t i o n s were n o r m a l i z e d t o 100%, d i s r e g a r d i n g t h e c o n t r i b u t i o n s o f t h e t h r e e a r t i f i c i a l components. The n o r m a l i z e d l e a s t - s q u a r e s r e s u l t s f o r each sample were combined w i t h t h e a s h e l e m e n t a l c o m p o s i t i o n o f each r e f e r e n c e m i n e r a l t o c a l c u l a t e t h e e l e m e n t a l c o m p o s i t i o n o f t h e ASTM o x i d i z e d a s h c o r r e s p o n d i n g t o each LTA. T h i s was done b y m u l t i p l y i n g t h e c o n c e n t r a t i o n o f each r e f e r e n c e m i n e r a l i n a sample b y t h e c o n c e n t r a t i o n o f each e l e m e n t a l o x i d e i n t h e r e f e r e n c e m i n e r a l , t h e n summing o v e r each o x i d e . For p a r t i a l l e a s t - s q u a r e s (PLS) o r p r i n c i p a l component r e g r e s s i o n (PCR), t h e i n f r a r e d s p e c t r a were t r a n s f e r r e d t o a DEC VAX 11/750 computer v i a t h e NIC-COM s o f t w a r e package from N i c o l e t . T h i s package a l s o p r o v i d e d u t i l i t y r o u t i n e s used t o p u t t h e s p e c t r a i n t o f i l e s c o m p a t i b l e w i t h t h e PLS and PCR s o f t w a r e . The PLS and PCR program w i t h c r o s s - v a l i d a t i o n was p r o v i d e d b y D a v i d H a a l a n d o f Sandia N a t i o n a l Laboratory. A d e t a i l e d d e s c r i p t i o n o f t h e program and t h e p r o c e d u r e s used i n i t has been g i v e n ( 5 ) . 1

1

1

RESULTS AND DISCUSSION A C r i t i q u e o f Methods f o r C o a l

Mineralogy

T h i s s u r v e y o f f i v e major methods f o r c o a l m i n e r a l o g y a n d t h e i r l i m i t a t i o n s i n c l u d e s o n l y methods w h i c h c a n p r o v i d e a "complete" mineral analysis. Methods o f l i m i t e d applicability, such as Mossbauer, a r e o m i t t e d . X - r a y D i f f r a c t i o n . XRD i s t h e most common method used f o r c o a l mine r a l o g y (6.7.8). I t s major advantage i s t h e a b i l i t y t o unequivoc a l l y i d e n t i f y many m i n e r a l s . The main d i s a d v a n t a g e s a r e : 1) r e l i ance o n r e f e r e n c e m i n e r a l s , 2) r e q u i r e s c a r e f u l a t t e n t i o n t o sample p r e p a r a t i o n , and 3) l o w s e n s i t i v i t y t o c e r t a i n m i n e r a l s ( e s p e c i a l l y many c l a y s ) due t o poor c r y s t a l l i n i t y and t o p a r t i c l e o r i e n t a t i o n effects. Many l a b o r a t o r i e s a n a l y z e a s e p a r a t e c o n c e n t r a t e d c l a y

American Chemical Society Library 1155Science 16th II; St., N.W. H., et al.; In Coal Schobert, ACS Symposium Series; American Chemical Society: Washington, DC, 1991. Washington, O.C. 20Q36

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COAL SCIENCE


f r a c t i o n ( l e s s t h a n 2 /xm o r 5 /im). However, s e n s i t i v i t y i s s t i l l low and o t h e r l i m i t a t i o n s may a r i s e : 1) the s e p a r a t e d c l a y f r a c t i o n may n o t be r e p r e s e n t a t i v e , and 2) the s e p a r a t i o n p r o c e d u r e can a l t e r the sample. The o r i g i n a l c o a l , i n s t e a d o f the LTA, can be a n a l y z e d by XRD. However, t h i s i s not s a t i s f a c t o r y , s i n c e s e n s i t i v i t y i s even l o w e r . An e x t e n s i v e i n t e r l a b o r a t o r y c o m p a r i s o n o f XRD r e s u l t s w i t h I l l i n o i s 6 c o a l showed h i g h l y v a r i a b l e r e s u l t s ( 7 ) . That s t u d y a l s o i n c l u d e d r e s u l t s from FTIR, SEM and o t h e r methods. I n f r a r e d Spectroscopy. The use o f IR (9.10.11.12) and FTIR (3.4) f o r c o a l m i n e r a l o g y has been r e p o r t e d . P a i n t e r and coworkers (3) d e m o n s t r a t e d t h a t FTIR can p r o v i d e a v i r t u a l l y complete a n a l y s i s . P a i n t e r , Brown and E l l i o t t ( 4 ) , and o t h e r s (9.10.11) d i s c u s s sample p r e p a r a t i o n , r e f e r e n c e m i n e r a l s , and d a t a a n a l y s i s . The advantages o f IR a r e : 1) h i g h s e n s i t i v i t y t o m o l e c u l a r s t r u c t u r e , 2) unequiv o c a l i d e n t i f i c a t i o n o f a number o f m i n e r a l s , 3) s m a l l sample s i z e (a few m i l l i g r a m s ) , and 4) r a p i d a n a l y s i s time (once LTA i s a v a i l a b l e ) . Disadvantages include: 1) reliance on reference minerals, 2) r e q u i r e s c a r e f u l a t t e n t i o n t o sample p r e p a r a t i o n , and 3) l i m i t e d s e l e c t i v i t y ( d i s c r i m i n a t i o n among s i m i l a r m i n e r a l s ) . The p r o b l e m w i t h l i m i t e d s e l e c t i v i t y i n c l u d e s some o f the m i n e r a l s w h i c h are problems f o r XRD: i l l i t e , muscovite, smectites and m i x e d - l a y e r c l a y s . Poor c r y s t a l l i n i t y c r e a t e s problems w i t h b o t h XRD and FTIR. The IR spectrum o f an amorphous m a t e r i a l l a c k s sharp d i s t i n g u i s h i n g f e a t u r e s b u t r e t a i n s s p e c t r a l i n t e n s i t y i n the regions t y p i c a l of i t s composition. The X - r a y d i f f r a c t i o n p a t t e r n shows low i n t e n s i t y r e l a t i v e t o w e l l - d e f i n e d c r y s t a l l i n e s t r u c t u r e s . The major problem f o r IR i s s e l e c t i v i t y : f o r XRD i t i s s e n s i t i v i t y . I n an i n t e r l a b o r a t o r y FTIR comparison ( 7 ) , two l a b o r a t o r i e s gave s i m i l a r r e s u l t s f o r k a o l i n i t e , c a l c i t e , and i l l i t e , b u t substan t i a l l y d i f f e r e n t r e s u l t s f o r m o n t m o r i l l o n i t e and q u a r t z . E l e c t r o n Beam/X-ray S p e c t r o s c o p y . S e v e r a l methods b a s e d on p o i n t c o u n t o r automated image a n a l y s i s (ΑΙΑ) i n scanning electron m i c r o s c o p y - e n e r g y d i s p e r s i v e X - r a y have been r e p o r t e d (13-18). P o i n t c o u n t a n a l y s i s can determine m i n e r a l o g y ; ΑΙΑ can a l s o d e t e r mine the s i z e d i s t r i b u t i o n o f the m i n e r a l s . These methods o b t a i n a point-by-point or particle-by-particle elemental analysis. A m i n e r a l d i s t r i b u t i o n and a n a l y s i s i s o b t a i n e d by c l a s s i f y i n g each e l e m e n t a l c o m p o s i t i o n i n t o one o f the m i n e r a l c a t e g o r i e s . Such methods have s e v e r a l advantages: 1) they can be automated, 2) the c o m p o s i t e e l e m e n t a l c o m p o s i t i o n can be checked a g a i n s t t h a t o f the b u l k sample, 3) i t i s p o s s i b l e t o r u n c o a l (not n e c e s s a r y t o o b t a i n LTA), and 4) they p r o v i d e some i n f o r m a t i o n on s t a t i s t i c a l and perhaps s p a t i a l , size, or morphological distribution of the minerals. For these r e a s o n s , such methods have become more p o p u l a r in recent years. The major d i s a d v a n t a g e s are: 1) the chemical i n f o r m a t i o n and thus the s e l e c t i v i t y i s l i m i t e d , s i n c e i t uses o n l y e l e m e n t a l c o m p o s i t i o n , 2) i t r e l i e s on a s u i t a b l e c l a s s i f i c a t i o n scheme f o r m i n e r a l c a t e g o r i e s , and 3) d a t a c o l l e c t i o n can be time-consuming ( e s p e c i a l l y f o r ΑΙΑ), r e q u i r i n g many h o u r s per sample.


Π

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ROBBINS

Coal Structure

49


O p t i c a l Microscopy. O p t i c a l microscopy i s the t r a d i t i o n a l t o o l o f g e o l o g i s t s and p e t r o g r a p h e r s f o r m i n e r a l i d e n t i f i c a t i o n and c h a r a c terization ( 1 9 ) . I t has two main advantages: 1) p o s i t i v e i d e n t i f i c a t i o n o f m i n e r a l s c a n be a c h i e v e d , and 2) i n f o r m a t i o n i s o b t a i n e d on m i n e r a l d i s t r i b u t i o n and morphology. However, q u a n t i t a t i o n i s d i f f i c u l t , and t h e a n a l y s i s i s time-consuming, r e q u i r e s h i g h l y t r a i n e d t e c h n i c i a n s , and i s n o t amenable t o a u t o m a t i o n . A l t h o u g h i n common use i n pétrographie s t u d i e s , such methods have been d i s p l a c e d by XRD, IR, and SEM-EDS methods f o r m i n e r a l o g i c a l studies. Thermal T e c h n i q u e s . Thermal t e c h n i q u e s , e s p e c i a l l y d i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) have been u s e d f o r m i n e r a l i d e n t i f i c a t i o n (19) and f o r c o a l m i n e r a l o g y ( 2 0 ) . The advantages o f t h e r m a l t e c h n i q u e s a r e : 1) s m a l l sample s i z e , 2) l i t t l e sample p r e p a r a t i o n ( a p p l i c a b l e t o whole c o a l s ) , 3) p o t e n t i a l l y r a p i d a n a l y s i s , and 4) i n f o r m a t i o n r e l e v a n t t o combustion b e h a v i o r may be p r o v i d e d . The disadvantages are: 1) t h e c h e m i c a l i n f o r m a t i o n i s l i m i t e d , r e s u l t i n g i n a l a c k o f s e l e c t i v i t y due t o o v e r l a p p i n g c u r v e s f o r i n d i vidual minerals, 2) i d e n t i f i c a t i o n / q u a n t i f i c a t i o n depends on r e f e r e n c e m i n e r a l s (though perhaps l e s s s e n s i t i v e t o such problems t h a n XRD o r I R ) , and 3) i t i s n o t d e v e l o p e d f o r q u a n t i t a t i v e u s e . I n t e r p r e t a t i o n o f t h e r m a l d a t a i s d i f f i c u l t , b u t c o u l d be improved by a p p r o p r i a t e s o f t w a r e . V a r i a t i o n s , such as u s i n g d i f f e r e n t gases t o h i g h l i g h t o r suppress f e a t u r e s , have been u s e d ( 2 0 ) . Detection l i m i t s o f l e s s t h a n 1 wt % t o about 30 wt % were r e p o r t e d f o r d i f f e r e n t minerals. G e n e r a l Comments on M i n e r a l o g i c a l Methods. The l a c k o f a measure o f q u a n t i t a t i v e accuracy i s a general problem w i t h mineralogy. A major l i m i t a t i o n f o r XRD and IR methods i s t h e use o f r e f e r e n c e m i n e r a l s , and t h e s t a n d a r d s used a r e from g e o l o g i c a l s o u r c e s n o t r e l a t e d t o the c o a l a n a l y z e d . T h i s l i m i t a t i o n w i l l be a v o i d e d by o n l y p h y s i c a l s e p a r a t i o n o f t h e c o a l m i n e r a l s o r by d a t a a n a l y s i s t e c h n i q u e s (such as f a c t o r a n a l y s i s ) w h i c h do n o t r e q u i r e r e f e r e n c e m i n e r a l s . Many s t u d i e s have r e p o r t e d s u c c e s s f u l q u a n t i t a t i o n o f m i n e r a l m i x t u r e s . T h i s i s a n e c e s s a r y , b u t n o t s u f f i c i e n t , c r i t e r i o n f o r a good minera l o g i c a l a n a l y s i s . The d i f f e r e n c e s between m i n e r a l m i x t u r e s and a u t h e n t i c samples a r e c o n s i d e r a b l e , and good performance on r e a l samples i s n o t guaranteed. The c r u c i a l a s p e c t s o f SEM-EDS and r e l a t e d techniques are appropriate c l a s s i f i c a t i o n o f the elemental d a t a , and o b t a i n i n g a s t a t i s t i c a l l y s u f f i c i e n t number o f d a t a points. For c l a s s i f i c a t i o n o f data, factor analysis, cluster a n a l y s i s , o r r e l a t e d m u l t i v a r i a t e t e c h n i q u e s appear t o be s u i t able (21). The IR methods have p r o g r e s s e d from hand-drawn b a s e l i n e s and peak h e i g h t o r a r e a f o r q u a n t i t a t i o n , t o s p e c t r a l s u b t r a c t i o n , t o leastsquares methods. Least-squares analysis e l i m i n a t e s the r e l i a n c e on s i n g l e peaks f o r q u a n t i t a t i o n and t h e s u b j e c t i v i t y o f s p e c t r a l s u b t r a c t i o n . However, n e g a t i v e c o n c e n t r a t i o n c o e f f i c i e n t s a r e a problem w i t h l e a s t - s q u a r e s a n a l y s i s , s i n c e they have no p h y s i c a l meaning. N e g a t i v e components c a n be o m i t t e d a c c o r d i n g t o some c r i t e r i o n and t h e l e a s t - s q u a r e s p r o c e s s i t e r a t e d u n t i l o n l y


C O A L SCIENCE Π


50

p o s i t i v e c o n c e n t r a t i o n c o e f f i c i e n t s remain. However, t h i s does n o t ensure t h a t the l e a s t - s q u a r e s s o l u t i o n i s a g l o b a l minimum. H a a l a n d and coworkers (5) d i s c u s s e d o t h e r problems w i t h c l a s s i c a l l e a s t - s q u a r e s (CLS) and i t s performance r e l a t i v e t o p a r t i a l l e a s t - s q u a r e s (PLS) and f a c t o r a n a l y s i s ( i n the form o f p r i n c i p a l component r e g r e s s i o n ) . One o f the d i s a d v a n t a g e s o f CLS i s t h a t i n t e r f e r e n c e s from o v e r l a p p i n g s p e c t r a are n o t h a n d l e d w e l l , and a l l the components i n a sample must be i n c l u d e d f o r a good a n a l y s i s . For a m a t e r i a l such as c o a l LTA, t h i s i s a s i g n i f i c a n t l i m i t a t i o n . F a c t o r a n a l y s i s e x t r a c t s i n f o r m a t i o n from the sample d a t a s e t ( e . g . , IR s p e c t r a ) and does n o t r e l y on r e f e r e n c e m i n e r a l s . How e v e r , because a b s t r a c t f a c t o r s have no p h y s i c a l meaning, r e f e r e n c e m i n e r a l s may be needed i n t a r g e t t r a n s f o r m a t i o n s o r o t h e r p r o c e d u r e s to e x t r a c t m i n e r a l o g i c a l i n f o r m a t i o n . One v a l u a b l e p i e c e o f i n f o r m a t i o n o b t a i n a b l e w i t h o u t the use o f e x t r a n e o u s d a t a i s the number o f components r e q u i r e d t o r e p r e s e n t the d a t a w i t h i n e x p e r i m e n t a l error. Reported a p p l i c a t i o n s of f a c t o r a n a l y s i s to mineralogy by FTIR a r e few ( 1 2 ) . However, one commercial l a b o r a t o r y i s o f f e r i n g r o u t i n e FTIR m i n e r a l a n a l y s e s t o the p e t r o l e u m i n d u s t r y , b a s e d on r e l a t e d methods ( 2 2 ) . The n e x t s e c t i o n o f t h i s paper d e s c r i b e s the use o f c l a s s i c a l l e a s t - s q u a r e s a n a l y s i s o f FTIR d a t a t o determine c o a l m i n e r a l o g y . T h i s i s f o l l o w e d by p r o m i s i n g p r e l i m i n a r y r e s u l t s o b t a i n e d u s i n g f a c t o r a n a l y s i s techniques. R e s u l t s Using C l a s s i c a l Least-Squares R e s u l t s o f c l a s s i c a l l e a s t - s q u a r e s a n a l y s i s o f FTIR s p e c t r a o f t e n c o a l s u s i n g f o r t y - t w o r e f e r e n c e m i n e r a l s were e v a l u a t e d w i t h r e g a r d to r e p r o d u c i b i l i t y and a c c u r a c y as d e s c r i b e d below. Rep roduc i b i 1 i tν. M i n e r a l o g i c a l r e s u l t s from the t e n - c o a l s e t ( T a b l e I I I ) a r e p r e s e n t e d as ranges o f v a l u e s . I n most c a s e s , the r e p r o d u c i b i l i t y i s good. Q u a r t z , f o r example, shows c o n s i s t e n t l y h i g h r e p r o d u c i b i l i t y . However, i l l i t e , m i x e d - l a y e r c l a y s and m o n t m o r i l l o n i t e i n the f i r s t f o u r c o a l s show h i g h v a r i a b i l i t y . These m i n e r a l s are s i m i l a r i n c o m p o s i t i o n and s p e c t r a l f e a t u r e s . The v a r i a b i l i t y i n the FTIR r e s u l t s f o r these samples i s r e l a t e d t o v a r i a b i l i t y i n the s p e c t r a l d a t a . The s e t o f t e n d u p l i c a t e s p e c t r a gave a p o o l e d SD o f 0.03 abs, w h i l e the s e p a r a t e s e t o f f o u r d u p l i c a t e s p e c t r a gave a p o o l e d SD o f 0.08 abs. R e s u l t s from the f i r s t f o u r samples i n the t a b l e i n c l u d e d the p o o r e r s p e c t r a l d a t a , w h i l e the r e s u l t s from the l a s t s i x samples were o b t a i n e d from the b e t t e r s p e c t r a . No s p e c i f i c r e a s o n i s apparent f o r the h i g h e r e r r o r i n the f o u r - c o a l s e t ; however, s a m p l i n g , i n s t r u m e n t a l , and environmental f a c t o r s may have c o n t r i b u t e d . The r e p r o d u c i b i l i t y f o r t o t a l c l a y c o n t e n t i s good, even when v a r i a b i l i t y i n i n d i v i d u a l c l a y concen t r a t i o n s i s h i g h . I r o n s u l f i d e s , o x i d e s and s u l f a t e s a l s o show some v a r i a b i l i t y , p a r t i c u l a r l y i n the f i r s t and f o u r t h samples. A c c u r a c y . A l t h o u g h t h e r e i s no way t o measure the a c c u r a c y o f the FTIR m i n e r a l o g i c a l r e s u l t s , t h e r e are t h r e e a r e a s i n w h i c h i t can be addressed (2.4). The f i r s t method i s t o compare p y r i t e r e s u l t s o b t a i n e d by FTIR w i t h the c o n v e n t i o n a l ASTM d e t e r m i n a t i o n . The


5.

ROBBINS

Coal Structure


Table I I I .

51

FTIR M i n e r a l o g i c a l R e s u l t s U s i n g C l a s s i c a l L e a s t - S q u a r e s , wt % o f LTA

Mineral Class Kaolin Mica Illite Mixed Layer Clays Montmorillonite Feldspar Chlorite Misc. Clays Quartz Fe S u l f i d e Fe Oxide Fe S u l f a t e Siderite Calcite Gypsum Dolomite Grovped Classes Clays Other Alumino silicates Sulfates Carbonates Fe-Altered

PI (a) Min. Max. 25. 8 33. 9 2. 6 6. 4 7.,1 13. 6 0.,0 11. 3* 4.,2 13. 4 0..0 3. 8 0.,6 2. 5 8 8 9. 1. 7.,7 13. 8 9.,2 13. 2 4..7 8.,7 4.,4 11.,2 0..0 0. 0 0,.0 0.,0 0..0 0.,0 0..0 0.,0

EH (a) Min. Max. 11. 8 16. 6 9. 3 2. 9 20. 2 33. 8 6. 2 16. 6 3.,4 5. 9 0.,0 1. 3 4. 9 3.,4 0.,2 2. 3 12.,5 13. 6 2.,0 5. 6 5.,3 9. 2 2.,9 3. 8 1.,5 2.,4 0.,0 0.,0 0..0 0.,5* 0..0 0..0

11 (a) Min. Max. 15.,1 21. 8 0. 0 3. 3 3.,7 47.,4 0.,0 15. 8 0.,0 26. 0 0. 0 0..0 0.,0 2. 3 4. 0.,0 3** 11.,0 14. 6 0.,0 5.,3 6. 8 8.,5 4.,5 0..0 0.,0 0..0 0,.0 2..8* 0..0 2. 0..0 0,.0

54,.0

60,.0

55..3

59..7

67,.7

71..5

19,.6

14,.6 4,.4 0,.0 9,.3

22..1 11..2 0,.0 20,.0

20..5 3,.2 1..5 8,.2

27..1 3..8 2..4 12..4

13,.3 2,.3 0,.0 7,.5

17, 2 5..0 2,.8* 13,.0

Mineral Class Kaolin Mica Illite Mixed Layer Clays Montmorillonite Feldspar Chlorite Misc. Clays Quartz Fe S u l f i d e Fe Oxide Fe S u l f a t e Siderite Calcite Gypsum Dolomite

13 Min. Max. 13,.3 14,.0 6,.1 6..7 9,.4 10,,8 0,.0 0,.0 13,.5 13, 8 3,.2 3, 8 0,.0 0,,0 0 .0 0,.0 14,.2 14,.2 19,.4 19..6 4,.9 5,.6 9 .0 9..3 4,.0 3 .7 0 .0 1,.6 0,.0 0 .0 0 .0 0,.0

P2 Min. Max. 17,.6 18..6 5,.3 5..5 5,.3 5..5 0,.0 0..0 11,.7 11..8 1,.6 2..0 0,.0 0..0 2,.8 3..3 8,.1 8..1 28,.0 29..0 2,.9 3..0 14,.8 14..9 0,.0 0,.0 0 .0 0,.0 0,.0 0,.0 0 .0 0,.0

P3 Min. Max. 20,.7 21,.2 4,.3 3,.6 10,.0 10..4 0 .0 0,.0 4,.9 5. 9 9,.7 10,.1 2,.1 2..5 2 .8 3,.8 4 .9 6..0 20,.8 22,.3 4 .8 5,.7 10 .8 11,.1 0 .6 1,.1 0,.0 0 .0 0,.0 0 .0 0 .0 0,.1

36 .9

37,.9

37,.7

38,.9

39 .4

24 .0 9 .0 4 .0 ,2

24,.1 9,.3 5,.3 I*.

Coal Science II - American Chemical Society

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