Analysis of Ash-Forming Mineral Matter in Raw and Supercleaned

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 30, 2015 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch033

Analysis of Ash-Forming Mineral Matter in Raw and Supercleaned Coals by Automated Image Analysis-Scanning Electron Microscopy Warren E. Straszheim, Jay G . Yousling, and R. Markuszewski Ames Laboratory and Department of Engineering Science and Mechanics, Iowa State University, Ames,IA50011

Automated image analysis (ΑΙΑ) was used in conjunction with scanning electron microscopy (SEM) and energy -dispersive x-ray (EDX) spectrometry to characterize directly the ash-forming mineral matter in raw and supercleaned samples of Illinois No. 6 and Pittsburgh No. 8 coals. The ground coals (70-80% less than 200 mesh) were cleaned to about 3% ash content by float -sink separation at 1.3 specific gravity. Samples of the raw and float material were analyzed for mineral matter phase and size distribution by the ΑΙΑ-SEM technique. For the Illinois No. 6 coal, more than 90% of the mineral matter consisted of pyrite, kaolinite, illite or quartz, which were more or less uniformly distributed among the various particle sizes. The effectiveness of cleaning was gradually increasing with increase in mineral particle size. Removals of mineral matter ranged from 75% for the smallest par ticles (less than 4 μm in diameter) to 100% for par ticles larger than 36 μm in diameter. The Pittsburgh coal was significantly different in both the character of the raw sample and the extent of cleaning observed. Compared to the Illinois No. 6 coal, the pyrite in this sample was relatively more coarse, while the other minerals were more fine-sized. The cleaning process removed more of the large-size material, while the finely grained material was relatively untouched. By u s i n g h i g h l y s o p h i s t i c a t e d and automated m i c r o s c o p e t e c h n i q u e s , t o g e t h e r w i t h p o w e r f u l m i n i - and microcomputers, i t i s now p o s s i b l e to c h a r a c t e r i z e t h e m i n e r a l components o f c o a l i n - s i t u . Combining automated image a n a l y s i s (ΑΙΑ) w i t h 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 (SEM) and e n e r g y - d i s p e r s i v e x - r a y (EDX) s p e c t r o m e t r y a l l o w s d e t a i l e d c h a r a c t e r i z a t i o n o f minerals i n coal f o r chemical composition, par t i c l e s i z e , and r e l a t i o n to t h e c o a l m a t r i x . To e v a l u a t e c o a l p a r t i c l e s produced by f i n e g r i n d i n g f o r s t u d y i n g the l i b e r a t i o n and removal e f f i c i e n c y , the AIA-SEM t e c h n i q u e p r o v i d e s i n f o r m a t i o n on the e l e m e n t a l d i s t r i b u t i o n among the v a r i o u s m i n e r a l phases. The 0097-6156/ 86/0301 -0449506.00/ 0 © 1986 American Chemical Society

In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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MINERAL MATTER AND ASH IN COAL

m i n e r a l s are c l a s s i f i e d by u s i n g a c h e m i s t r y d e f i n i t i o n f i l e based on the r e l a t i v e amounts o f elements p r e s e n t as determined by EDX spectrometry. F o r a s t a t i s t i c a l l y s i g n i f i c a n t number o f p a r t i c l e s , both s i z e d i s t r i b u t i o n and volume f r a c t i o n can be o b t a i n e d and used to c h a r a c t e r i z e the m i n e r a l matter c o n t e n t . The c o n v e n t i o n a l chemi c a l a n a l y s e s f o r the b u l k c o a l samples can be r e l a t e d to the AIA-SEM r e s u l t s f o r comparison o f the d a t a . Other i n s t r u m e n t a l t e c h n i q u e s , such as x - r a y d i f f r a c t i o n (XRD) and F o u r i e r t r a n s f o r m i n f r a r e d s p e c t r o s c o p y ( F T I R ) , can be used t o i d e n t i f y the m i n e r a l phases p r e s e n t i n c o a l . Sometimes they can be used to p r o v i d e q u a n t i t a t i v e e s t i m a t i o n , but t h e i r u t i l i t y i s l i m i t e d to samples w i t h l a r g e m i n e r a l c o n t e n t s . Furthermore, such t e c h n i q u e s use b u l k samples and p r o v i d e o n l y average v a l u e s . Thus, when a p p l i e d i n c o a l p r e p a r a t i o n , they are l i m i t e d to c a l c u l a t i n g o n l y an "average" c l e a n i n g e f f e c t i v e n e s s and do not o f f e r i n f o r m a t i o n on 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 phases i d e n t i f i e d as removed o r r e t a i n e d . On the o t h e r hand, AIA-SEM p e r m i t s c l e a n i n g e f f e c t i v e n e s s to be e v a l u a t e d w i t h r e s p e c t to both m i n e r a l phase and particle size distribution. Such i n f o r m a t i o n i s important f o r any c o a l p r e p a r a t i o n p r o c e s s , e s p e c i a l l y s i n c e i t can r e l a t e g r i n d i n g and l i b e r a t i o n o f m i n e r a l p a r t i c l e s to the e f f e c t i v e n e s s o f a c l e a n i n g p r o c e s s ( 1). In p r a c t i c e , t h e r e f o r e , problems a s s o c i a t e d w i t h removing a p a r t i c l e s i z e o r c h e m i c a l c l a s s o f p a r t i c l e s c o u l d be d e t e c t e d and remedied f o r more e f f e c t i v e c l e a n i n g . The AIA-SEM t e c h n i q u e has been used i n Ames L a b o r a t o r y to study the e f f e c t o f g r i n d i n g on the l i b e r a t i o n o f m i n e r a l matter from f i n e c o a l and i t s subsequent removal d u r i n g c l e a n i n g . S e v e r a l s e r i e s o f c o a l s have been c h a r a c t e r i z e d by t h i s t e c h n i q u e i n r e c e n t y e a r s , thus d e m o n s t r a t i n g i t s u s e f u l n e s s . In t h i s work, the AIA-SEM t e c h nique was a p p l i e d to determine the c o a l m i n e r a l c h a r a c t e r b e f o r e and a f t e r c l e a n i n g by a f l o a t - s i n k t e c h n i q u e . Experimental Sample D e s c r i p t i o n and P r e p a r a t i o n . The two b i t u m i n o u s c o a l s b e i n g t e s t e d f o r c l e a n a b i l i t y were from the I l l i n o i s No. 6 seam, Randolph County, I l l i n o i s , and from the P i t t s b u r g h No. 8 seam, Lewis County, West V i r g i n i a . The c o a l s were ground to a t y p i c a l power p l a n t g r i n d ( i . e . , 70-80% l e s s than 200 mesh o r -75 ym). The c o a l s were then c l e a n e d by f l o a t - s i n k s e p a r a t i o n ( u s i n g h a l o g e n a t e d hydrocarbons) at 1.3 s p e c i f i c g r a v i t y i n a c e n t r i f u g e to produce a v e r y low-ash, c l e a n c o a l f r a c t i o n (ash c o n t e n t sed as wt. % on a d r y b a s i sÏ , except f o r moisture. C a l c u l a t e d u s i n g the m o d i f i e d P a r r f o r m u l a : M i n e r a l Matter = 1.13 \ (ash) + 0.47 ( p y r i t i c s u l f u r ) , as defined i n r e f . (2).

AIA-SEM A n a l y s i s . The AIA-SEM system c o n s i s t s o f a JEOL (Japan E l e c t r o n O p t i c s L a b o r a t o r y ) model JSM-U3 s c a n n i n g e l e c t r o n m i c r o scope, a LeMont S c i e n t i f i c B-10 image a n a l y z e r , and a T r a c o r N o r t h e r n TN-2000 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 z e r . The image a n a l y z e r i s a software-based system w i t h a s s o c i a t e d e l e c t r o n i c s f o r SEM beam c o n t r o l , image a m p l i f i c a t i o n , and t h r e s h o l d i n g . The software base f o r image a n a l y s i s a l l o w s the a p p r o p r i a t e a n a l y s i s a l g o r i t h m t o be s e l e c t e d f o r the p a r t i c u l a r sample and image c o n d i t i o n s e n c o u n t e r e d . The p a r t i c l e b o u n d a r i e s , or the s o - c a l l e d p a r t i c l e e x t e n t s , are determined from the p o i n t s at which h o r i z o n t a l scans c r o s s a f e a t u r e , and p a r t i c l e o u t l i n e i s r e c o n s t r u c t e d from these a d j a c e n t chords o f a p a r t i c l e . Once the o u t l i n e has been d e t e r m i n e d , the x-ray d a t a are c o l l e c t e d from the c e n t e r o f the p a r t i c l e s . Samples were a n a l y z e d i n the SEM u s i n g 25-kV beam v o l t a g e , 1-2 nA sample c u r r e n t s , 300x m a g n i f i c a t i o n , and b a c k s c a t t e r e d e l e c t r o n imaging. A p o i n t d e n s i t y o f 1024 p i x e l s a c r o s s the s c r e e n was used to p r o v i d e ± 1 0 % a c c u r a c y on measurements as s m a l l as 1% o f the f i e l d o f view. EDX data were c o l l e c t e d f o r 4 s e c . p e r p a r t i c l e at a t y p i c a l c o u n t i n g r a t e o f 1000 counts per s e c . A p p r o x i m a t e l y 4000 p a r t i c l e s were a n a l y z e d per sample at a r a t e o f 200 p a r t i c l e s per hour. Regions o f i n t e r e s t were s e t t o m o n i t o r the i n t e n s i t i e s o f 30 elements. However, the o n l y elements o c c u r r i n g w i t h s i g n i f i c a n t f r e q u e n c y were: Na, Mg, A l , S i , P, S, C l , K, Ca, T i , and F e . Data Handling. The p a r t i c l e s were c l a s s i f i e d i n t o one o f n i n e mine r a l c a t e g o r i e s based on the r e l a t i v e amounts o f the elements p r e s ent and a c c o r d i n g to the d e f i n i t i o n s g i v e n i n Table I I . The c a t e g o r i e s were d e r i v e d from p r e v i o u s l y e s t a b l i s h e d g u i d e l i n e s (3,4) and i n c l u d e d the common c o a l m i n e r a l s p y r i t e , q u a r t z , c a l c i t e , s i d e r i t e , k a o l i n i t e , and i l l i t e . S e v e r a l o t h e r m i n e r a l s were i d e n t i f i e d , but they o c c u r r e d i n such s m a l l amounts t h a t they were c l a s s i f i e d t o g e t h e r i n t o one common c a t e g o r y t i t l e d "MINORS". T h i s c a t e g o r y i n c l u d e d the m i n e r a l s gypsum, d o l o m i t e , r u t i l e , alumina, and



452

MINERAL MATTER AND ASH IN COAL

apatite. In a d d i t i o n , s e v e r a l o t h e r c a t e g o r i e s were d e f i n e d to accommodate p a r t i c l e s not c o r r e s p o n d i n g to any o f the above d e f i n i tions. For example, a "SILICATES" c a t e g o r y was d e f i n e d to i n c l u d e p a r t i c l e s w i t h s i g n i f i c a n t s i l i c o n c o n t e n t , yet w i t h the b a l a n c e o f the elements i n such p r o p o r t i o n s t h a t the p a r t i c l e would not f i t i n t o e i t h e r the q u a r t z , k a o l i n i t e , or i l l i t e c a t e g o r i e s . A "MIS CELLANEOUS" c a t e g o r y was p r o v i d e d to i n c l u d e those p a r t i c l e s whose c o m p o s i t i o n d i d not a l l o w them to f a l l i n t o any o f the above-men t i o n e d c a t e g o r i e s . F u r t h e r d e s c r i p t i o n s o f the i n s t r u m e n t a l , s t a t i s t i c a l , c l a s s i f y i n g , and p r o c e s s i n g t e c h n i q u e s are g i v e n elsewhere (5). Table

II.

Chemical D e f i n i t i o n s

Mineral Phase

f o r M i n e r a l Phases

D e f i n i t i o n by Chemical Composition ( i n % R a n g e )

3

Specific Gravity

PYRITE KAOLINITE ILLITE

S 10-80; Fe 10-70 A l 15-80; S i 15-85; A l / S i 0.33-3.0 A l 10-50; S i 20-85; Mg 0-15; Ca 0-35; Fe 0-40; T i 0-15 QUARTZ S i 65-100 CALCITE Ca 70-100 SIDERITE Fe 70-100; Mn 0-30; Ni 0-30 MINORS ( i n c l u d e s the : f o l l o w i n g c a t e g o r i e s ) GYPSUM S 10-80; Ca 10-70 DOLOMITE Mg 10-60; Ca 60-100 RUTILE T i 70-100 ALUMINA A l 65-100 APATITE Ρ 15-40; Ca 30-100 SILICATES S i 20-80 MISCELLANEOUS (no • r e s t r i c t i o n s , a l l p a r t i c l e s accepted) a

5.00 2.65 2.75 2.65 2.80 5.00 2.30 2.90 4.50 4.00 3.20 2.70 2.00

S p e c i f i c a t i o n s may be g i v e n f o r the amount o f o t h e r elements t h a t a r e allowed to be p r e s e n t . Such s p e c i f i c a t i o n s a l l o w minor amounts o f elements not s p e c i f i c a l l y l i s t e d i n the c l a s s d e f i n i t i o n to be p r e s e n t , but they p l a c e an upper l i m i t on the a l l o w a b l e amount.

Using the ΑΙΑ p r o c e d u r e s , m i n e r a l p a r t i c l e s were c l a s s i f i e d i n t o s i z e s and m i n e r a l phases. A r e a - e q u i v a l e n t d i a m e t e r , i . e . the d i a m e t e r o f a c i r c l e w i t h the same a r e a as t h a t measured f o r the m i n e r a l p a r t i c l e , was used as the s i z e parameter f o r d a t a p r e s e n t a tion. Using a v a i l a b l e l i t e r a t u r e v a l u e s f o r the s p e c i f i c g r a v i t y o f the i n d i v i d u a l m i n e r a l s , the d a t a were then expressed as the weight f r a c t i o n o f the m i n e r a l matter w i t h i n a g i v e n m i n e r a l / s i z e c a t e g o r y . The weight f r a c t i o n d a t a were then n o r m a l i z e d u s i n g the m i n e r a l m a t t e r c o n t e n t to p r e s e n t the m i n e r a l o g i c a l e s t i m a t e s on a d r y c o a l b a s i s i n o r d e r to p r o v i d e a common base f o r comparing the c o a l s b e f o r e and a f t e r the c l e a n i n g p r o c e s s . R e s u l t s and D i s c u s s i o n ASTM a n a l y t i c a l d a t a p r e s e n t e d i n Table I i n d i c a t e t h a t t h e f l o a t - s i n k s e p a r a t i o n a c h i e v e d an 84% and a 56% removal o f ash from



33.

STRASZHEIM ET AL.

Ash-Forming

Mineral

Matter

Analysis

453

the I l l i n o i s and P i t t s b u r g h c o a l s , r e s p e c t i v e l y , w i t h c o r r e s p o n d i n g d e c r e a s e s i n t o t a l and p y r i t i c s u l f u r o f 50 and 91% and o f 43 and 97%, r e s p e c t i v e l y . I f e x p r e s s e d as a r e d u c t i o n i n the t o t a l m i n e r a l m a t t e r c o n t e n t , based on u s i n g a m o d i f i e d Parr formula (2), t h e c o r r e s p o n d i n g v a l u e s are 84% f o r the I l l i n o i s c o a l and 59% f o r the Pittsburgh coal. The ΑΙΑ d a t a f o r the I l l i n o i s c o a l and f o r the P i t t s b u r g h c o a l are much more i n t e r e s t i n g . R e s u l t s f o r the I l l i n o i s c o a l ( T a b l e s I I I a - c ) show t h a t p y r i t e , q u a r t z , and two c l a y s ( k a o l i n i t e and i l l i t e i n a p p r o x i m a t e l y equal p r o p o r t i o n s ) make up t h e b u l k o f t h e m i n e r a l m a t t e r , 72% i n the raw and 80% i n the c l e a n c o a l . Further more, a l l t h e m i n e r a l phases except p y r i t e are r a t h e r u n i f o r m l y d i s t r i b u t e d i n the raw c o a l over the e n t i r e range o f p a r t i c l e s i z e from l e s s than 4 ym to more than 36 ym i n d i a m e t e r . Table

Ilia.

ΑΙΑ R e s u l t s f o r I l l i n o i s No. 6 Raw C o a l (200 χ 0 Mesh), E x p r e s s e d as Weight Percent o f Dry Coal Particle

Mineral Phase

Analysis of Ash-Forming Mineral Matter in Raw and Supercleaned

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