Mineral Matter and Ash in Coal - American Chemical Society

32 Mineral Matter Analysis: A Technique To Improve Bituminous Coal Beneficiation

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2017 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch032

Richard B. Muter and William F. Lawrence College of Mineral and Energy Resources, West Virginia University, Morgantown, WV 26506

Mineral matter analysis can provide a more accurate and complete representation of the minerals present in bituminous coal and the effects of beneficiation processes upon them. Comparisons with conventional analytical methods are made which demonstrate some of the potential advantages of this technique in the monitoring or design of coal cleaning operations.

As a new g e n e r a t i o n o f c o a l c l e a n i n g t e c h n o l o g y becomes more commonly p r a c t i c e d , i t i s r a p i d l y becoming a p p a r e n t t h a t a new g e n e r a t i o n o f a n a l y t i c a l methods t o m o n i t o r t h e s e p a r a t i o n c h a r a c t e r i s t i c s i s a l s o r e q u i r e d . The a n a l y t i c a l methods which a r e i n p r e s e n t useage do n o t a c c u r a t e l y r e p o r t t h e m a t e r i a l s which a r e b e i n g b e n e f i c i a t e d ; r a t h e r , t h e y r e p o r t elements which a r e w i t h i n t h e m a t e r i a l s . W h i l e t h i s i n d i r e c t measurement method i swidely a c c e p t e d , b o t h because i t i s w e l l known and because i t has h i s t o r i c a l l y proven o f v a l u e t o t h e c o a l i n d u s t r y , i t may p r o v i d e a v e r y f a l s e p i c t u r e o f what t r u l y o c c u r s d u r i n g c o a l p r o c e s s i n g . C o n v e n t i o n a l c o a l c l e a n i n g o r b e n e f i c i a t i o n i s used t o remove t h o s e m a t e r i a l s w i t h i n t h e c o a l seam which a r e d e l e t e r i o u s t o c l e a n e f f i c i e n t combustion o r c o n v e r s i o n . However, a problem a r i s e s i n t h a t d i f f e r e n t m i n e r a l forms, even though t h e y may c o n t a i n t h e same e l e m e n t s , may r e a c t d i f f e r e n t l y d u r i n g c l e a n i n g . Most common a n a l y t i c a l methods d e s t r o y t h e m i n e r a l o g i c a l s t r u c t u r e s and g i v e t h e i m p r e s s i o n t h a t c o a l s a r e t o a l a r g e e x t e n t homogeneous and c o n s i s t e n t i n m i n e r a l c o n t e n t . T h i s i s a f a l s e i m p r e s s i o n as c o a l i s , i n r e a l i t y , heterogeneous i n n a t u r e and i t s m i n e r a l c o n t e n t can change markedly from sample t o sample t a k e n from t h e same s o u r c e . I t i s t h i s v a r i a b i l i t y i n m i n e r a l s c o n t e n t which r e d u c e s t h e e f f e c t i v e n e s s o f common b e n e f i c i a t i o n methods. S u l f u r a n a l y s i s p r o v i d e s a good example o f t h e f a l s e c o n c l u s i o n s which can be drawn based upon good a n a l y t i c a l t e c h n i q u e . I t i g n o r e s m i n e r a l o g i c a l c o m p o s i t i o n , a l t h o u g h i t can be improved i f an a n a l y s i s o f s u l f u r forms i s performed. However, knowing t h a t "x" p e r c e n t a g e o f p y r i t i c s u l f u r i s p r e s e n t i s s t i l l n o t s u f f i c i e n t f o r t h e d e s i g n o f t h e most e f f e c t i v e c l e a n i n g p r o c e s s . F u r t h e r i n f o r m a t i o n as t o t h e s i z e o f t h e m i n e r a l g r a i n s 0097-6156/86/0301-0443S06.00/0 © 1986 American Chemical Society

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


444

MINERAL MATTER AND ASH IN COAL

p r e s e n t and whether t h e y a r e f i n e l y e n g r a i n e d w i t h i n t h e c o a l m a t e r i a l i t s e l f i s needed; and a d e t e r m i n a t i o n o f t h e o t h e r s u l f u r m i n e r a l s p r e s e n t , i f any, c o u l d l e a d t o f u r t h e r r e f i n e m e n t s i n t h e c l e a n i n g methods employed and t h e i r improved e f f e c t i v e n e s s . The a n a l y s i s o f s i l i c a , however, p r o v i d e s a b e t t e r example of t h e problem which e x i s t s . S i l i c o n i s t o m i n e r a l o g y what carbon i s t o o r g a n i c c h e m i s t r y and appears i n c o a l i n many forms from s i m p l e q u a r t z t o complex c l a y s ( T a b l e 1 ) . However, most a n a l y t i c a l methods d e s t r o y m i n e r a l forms t h r o u g h h i g h temperature o x i d a t i o n ( a s h i n g ) and t h e r e s u l t s a r e determined as t h e amount o f s i l i c o n p r e s e n t . Two c o a l s may have t h e same S1O2 c o n t e n t when a n a l y z e d , but t h e p h y s i c a l p r o p e r t i e s o f t h e p a r e n t m i n e r a l s may be q u i t e d i f f e r e n t . D i f f e r e n t m i n e r a l s , even o f s i m i l a r c o m p o s i t i o n s , may r e q u i r e d i f f e r e n t c l e a n i n g p r o c e s s e s and have d i f f e r e n t e f f e c t s upon p r o c e s s i n g equipment. Sand might be e a s i l y removable t h r o u g h washing o r f r o t h f l o t a t i o n ; whereas c l a y w i t h a s i m i l a r s i l i c o n c o n t e n t might adhere t o t h e c o a l m a t e r i a l o r c o l l o i d a l l y d i s p e r s e w i t h i n t h e p r o c e s s waters and be d i f f i c u l t t o remove. P h y s i c a l b e n e f i c i a t i o n i s g e n e r a l l y c o n s i d e r e d t o be a "mature" s u b j e c t i n t h a t most changes which have o c c u r r e d o v e r t h e p a s t few y e a r s have been i n terms o f equipment d e s i g n and r e f i n e m e n t o r i n t h e o r d e r i n which p a r t i c u l a r o p e r a t i o n s a r e performed. These o p e r a t i o n s a r e g e n e r a l l y based upon d i f f e r e n c e s i n p h y s i c a l c h a r a c t e r i s t i c s ( e g . s p e c i f i c g r a v i t y , hardness or b r i t t l e n e s s ) between t h e m i n e r a l s o f i n t e r e s t and t h e c o a l y m a t e r i a l s . However t h e s e p r o c e s s e s a r e s t i l l m o n i t o r e d u s i n g elemental analyses r a t h e r than s p e c i f i c m i n e r a l c o n c e n t r a t i o n measurements. F r o t h f l o t a t i o n , c o n s i d e r e d t o be a h i g h e r l e v e l o f s o p h i s t i c a t i o n i n b e n e f i c i a t i o n , i s a l s o based on d i f f e r e n c e s i n m i n e r a l o g i c a l p r o p e r t i e s . Based upon p a r t i c l e s u r f a c e c h a r a c t e r i s t i c s , t h e s e p a r a t i o n method i s more c h e m i c a l than p h y s i c a l i n n a t u r e . T e n d e n c i e s o f s p e c i f i c m i n e r a l p a r t i c l e s t o be h y d r o p h i l i c o r h y d r o p h o b i c a r e enhanced through t h e u s e o f c h e m i c a l a d d i t i v e s and t h e n a p h y s i c a l s e p a r a t i o n i s made. However, i t i s s t i l l m i n e r a l s and n o t elements which a r e b e i n g s e p a r a t e d even though c o n v e n t i o n a l a n a l y t i c a l methods would imply o t h e r w i s e . The n e x t l e v e l o f s o p h i s t i c a t i o n i n c o a l c l e a n i n g w i l l most l i k e l y be t h a t o f c h e m i c a l c o a l c l e a n i n g . I t w i l l a l s o be t h e most c o s t l y l e v e l t o d a t e , e s p e c i a l l y when t h e l a r g e tonnage amounts i n v o l v e d i n c o a l u t i l i z a t i o n a r e c o n s i d e r e d . I n o r d e r t o keep t h e s e c o s t s t o a minimum, w h i l e s t i l l a t t a i n i n g d e s i r e d r e s u l t s , p r o c e s s o p e r a t i o n s w i l l have t o be c a r e f u l l y p l a n n e d and c l o s e l y m o n i t o r e d . P r o c e s s d e s i g n e r s w i l l need t o know which m i n e r a l s a r e i n v o l v e d and whether any o f them can be removed b e f o r e employing c h e m i c a l c l e a n i n g . A c q u i r i n g a b e t t e r knowledge o f what m i n e r a l s o c c u r i n s p e c i f i c c o a l s and how t h e y a r e a f f e c t e d by l e s s e x p e n s i v e p h y s i c a l b e n e f i c i a t i o n p r o c e s s e s i s an o b v i o u s f i r s t step. As p a r t o f a much l a r g e r e f f o r t by t h e U.S. Department o f Energy, t h e C o a l R e s e a r c h Bureau o f t h e C o l l e g e o f M i n e r a l and Energy R e s o u r c e s a t West V i r g i n i a U n i v e r s i t y has been c h a r a c t e r i z i n g t h e m i n e r a l o g y and p e t r o g r a p h y o f t h r e e major b i t u m i n o u s c o a l s i n an e f f o r t t o determine whether t h e



32.

MUTER AND LAWRENCE

Mineral

Matter

Analysis

445

m i n e r a l o g i c a l a s s o c i a t i o n s can be c l o s e l y f o l l o w e d t h r o u g h common p h y s i c a l b e n e f i c i a t i o n processes. A l i s t i n g of the minerals commonly p r e s e n t i n bituminous c o a l s i s p r o v i d e d i n T a b l e 1. Methodology R e p r e s e n t a t i v e samples o f t h r e e major bituminous c o a l s were o b t a i n e d f o l l o w i n g ASTM sampling p r o c e d u r e s . The s p e c i f i c c o a l s chosen i n c l u d e d a n o r t h e r n West V i r g i n i a h i g h v o l a t i l e coal, a s o u t h e r n West V i r g i n i a low v o l a t i l e c o a l , and a n o r t h e r n Illinois c o a l . Head c o a l samples were s p l i t o u t f o r each c o a l and then s c r e e n and s p e c i f i c g r a v i t y ( s i n k - f l o a t ) s e p a r a t i o n s were made. S c r e e n f r a c t i o n s produced were +1", l x l / 4 " , l/4"x8M, 8x28M, 28xl00M and -100M. G r a v i t y s e p a r a t i o n s were performed on a l l but t h e -100M f r a c t i o n a t 1.30, 1.40, 1.60, and 1.80 (SG) and a l l samples were analyzed.Déterminâtions were made o f t h e a s h , low temperature a s h , and t h e elements S i , A l , Fe, T i , Ca, Mg, Na, and K, among o t h e r s . The c o n c e n t r a t i o n s o f t h e m i n e r a l s i l l i t e , k a o l i n i t e , q u a r t z , c a l c i t e and p y r i t e were a l s o d e t e r m i n e d . Low temperature a s h i n g t e c h n i q u e s were employed t o reduce t h e o x i d a t i o n and d e c o m p o s i t i o n o f t h e m i n e r a l s which would o c c u r under normal a s h i n g c o n d i t i o n s . M i n e r a l o g i c a l a n a l y s e s were performed u s i n g X - r a y powder d i f f r a c t i o n , i n f r a - r e d s p e c t r o s c o p y , o p t i c a l p e t r o g r a p h y , and 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 . E l e m e n t a l d e t e r m i n a t i o n s were performed u s i n g atomic a b s o r p t i o n s p e c t r o s c o p y . Each o f t h e m i n e r a l i d e n t i f i c a t i o n and q u a n t i f i c a t i o n t e c h n i q u e s mentioned met w i t h v a r y i n g degrees o f s u c c e s s f o r each i n d i v i d u a l m i n e r a l . However, f o r ease o f a p p l i c a t i o n a c c u r a c y and r e p r o d u c i b i l i t y , and t h e h i g h e s t degree o f q u a n t i f i c a t i o n , X - r a y a n a l y s i s was d e t e r m i n e d t o be t h e b e s t method. A c c o r d i n g l y , t h e m i n e r a l v a l u e s shown i n t h e t a b l e s were o b t a i n e d by t h i s method. P y r i t e was determined u s i n g ASTM p r o c e d u r e s . R e s u l t s and D i s c u s s i o n In t h i s p r o j e c t , t h e f i r s t o p e r a t i o n s s t u d i e d were s i z e and s p e c i f i c g r a v i t y s e p a r a t i o n s as t h e y a r e t h e most common u n i t o p e r a t i o n s p r a c t i c e d . The m i n e r a l o g i c a l d a t a o b t a i n e d was then c o n v e r t e d i n t o w a s h a b i l i t y t a b l e s s i m i l a r t o t h o s e produced u s i n g e l e m e n t a l a n a l y s e s , e.g. a s h w a s h a b i l i t y t a b l e s . T h i s d a t a was then t r a n s f e r r e d t o g r a p h i c form t o produce a m i n e r a l s w a s h a b i l i t y diagram ( F i g u r e 1) f o r t h e head n o r t h e r n West V i r g i n i a c o a l . T h i s f i g u r e i s based on c o n v e n t i o n a l s i n k - f l o a t s p e c i f i c g r a v i t y s e p a r a t i o n s , but i t c o u l d as r e a d i l y have been produced by v a r y i n g t h e p r o c e s s i n g parameters o f a s p e c i f i c u n i t o p e r a t i o n and m o n i t o r i n g t h e m i n e r a l s e p a r a t i o n s which o c c u r e d . Such c u r v e s c o u l d be used as p r e d i c t i v e t o o l s t o e s t i m a t e t h e s e p a r a t i o n s which would o c c u r and a l s o c o u l d p r o v i d e a t h e o r e t i c a l base l i n e f o r monitoring the e f f e c t i v e n e s s of a p a r t i c u l a r cleaning operation. M i n e r a l s were a l s o f o l l o w e d t h r o u g h i n d i v i d u a l u n i t o p e r a t i o n s as p a r t o f t h e r e s e a r c h p r o j e c t . The d a t a o b t a i n e d when c o a l was c l e a n e d u s i n g a p i l o t s c a l e D e i s t e r T a b l e i s p r e s e n t e d i n T a b l e 2. An e x a m i n a t i o n o f t h e a c t u a l m i n e r a l s c o n t e n t s v e r s u s t h o s e p r e d i c t e d by t h e w a s h a b i l i t y d a t a shows a lower y i e l d o f i l l i t e and a h i g h e r c o n c e n t r a t i o n o f k a o l i n i t e and q u a r t z r e p o r t i n g i n t h e c l e a n c o a l . T h i s i m p l i e s t h a t t h e i l l i t e i s b e i n g removed t o a g r e a t e r e x t e n t than e x p e c t e d and t h a t t h e removal o f q u a r t z and k a o l i n i t e i s being retarded r e l a t i v e to p r e d i c t e d values. C o n v e n t i o n a l e l e m e n t a l a n a l y s i s , i n t h i s case f o r S i , would n o t


446

MINERAL MATTER AND ASH IN COAL

Table 1

Q

QUARTZ

Q

FELDSPARS

S

Q

Scanning Electron Microscopy

KAOLINITE

Optical Petrography

S

Normative Calculations

ILLITE

Infrared Spectroscopy

X-ray Powder Diffraction


M i n e r a l s p r e s e n t i n t h e n o r t h e r n West V i r g i n i a b i t u m i n o u s c o a l . The s y m b o l s i n d i c a t e t h e a n a l y t i c a l p r o c e d u r e s a v a i l a b l e f o r e a c h m i n e r a l a n d w h e t h e r t h e p r o c e d u r e c a n be u s e d f o r quantitative (Q),semi-quantitative (S), o r i d e n t i f i c a t i o n ( I ) analyses only.

Q

S

I

Variable

Q

S

I

Al (Si 0 û) (0H)

Q

S

I

Si0

I

Variable

I

KA1

S

MUSCOVITE CARBONATES CALCITE

Q

DOLOMITE

Q

BASSANITE

S

DISULFIDES

Q

4

(AlSi 0

4

3

I

I

CaC0

S

I

I

CaMg(C0 )

I

CaS0 .l/2H 0

S

4

I

FeS

2

2

MARCASITE

s

FeS

2

I

s

RUTILE

s

2

4

FeS

HEMATITE

) (0H)

3

3

S

S

1 0

2

CaS0 .2H 0

I Q

8

2

PYRITE

APATITE

1

S

I

GYPSUM IRON

Q

4

Variable

S

Q

Formulae

Ca (F,Cl,0H) 5

Fe 0 2

I

2

Ti0

3

2


(P0 ) 4

3

2

Mineral

MUTER AND LAWRENCE


Ο J

I

I

I

5 I

I

I

Matter

ι

CUMULATIVE ASH, FLOAT 10 15 ι ι I f ι ι ι I < ι

ι

C U M U L A T I V E C A L C I T E , LBS.

i l l

I

I

0

20

Analysis

1

i ι

ι

ι ,

I

40

I 100

ι

20 I

1

1

1

1

25 I

I

I

1

60

C U M U L A T I V E I L L I T E , KAOLINITE . Q U A R T Z , A N D

[ 0

ι

I

I

I 200

»

80 PYRITE,

1

100 LBS.

— I 300

C U M U L A T I V E TOTAL MINERAL M A T T E R , L B S .

F i g u r e 1. W a s h a b i l i t y Curves f o r t h e Most Common M i n e r a l s i n N o r t h e r n West V i r g i n i a H i g h V o l a t i l e B i t u m i n o u s C o a l .


448

MINERAL MATTER AND

ASH IN COAL

show any changes o c c u r r i n g as t h e i l l i t e r e d u c t i o n would be o f f s e t by t h e g r e a t e r t h a n p r e d i c t e d amounts o f the o t h e r two m i n e r a l s . F u r t h e r c l e a n i n g o f t h i s c o a l based upon removing q u a r t z o r k a o l i n i t e would be more e f f e c t i v e t h a n i f a p r o c e s s t o remove i l l i t e were used, but c o n v e n t i o n a l a n a l y s i s would n o t p r o v i d e t h i s type of planning information.


T a b l e 2. A c t u a l v s . p r e d i c t e d r e c o v e r y (pounds per t o n o f f e e d c o a l ) o f p r o d u c t s from a N o r t h e r n West V i r g i n i a h i g h v o l a t i l e bituminous c o a l u s i n g a p i l o t s c a l e D e i s t e r t a b l e .

Clean

Coal

Ash

Actual 1705

Predicted 1650

118

120

126

120

Illite

16

29

Kaolinite

34

30

Quartz

21

20

Calcite

13

4

Pyrite

30

23

Mineral

Matter

Future

Impact S a y i n g t h a t m i n e r a l o g i c a l a n a l y s i s may become as common as elemental a n a l y s i s i n the c o a l i n d u s t r y i s not too strong a s t a t e m e n t . F u t u r e p r e p a r a t i o n o p e r a t i o n s w i l l be q u i t e s i m i l a r i n c o m p l e x i t y t o c u r r e n t o r e p r o c e s s i n g p l a n t s w i t h advanced p h y s i c a l and c h e m i c a l b e n e f i c i a t i o n methods b e i n g employed. The p l a n t o f the f u t u r e w i l l be d e s i g n e d based upon t h e p r o p e r t i e s o f t h e s p e c i f i c c o a l s b e i n g p r o c e s s e d and w i l l p r o b a b l y be a m u l t i p l e u n i t o p e r a t i o n , m u l t i p l e end p r o d u c t , end use r e l a t e d p l a n t . The economic s u c c e s s o f such p l a n t s w i l l depend upon t h e a b i l i t y t o m o n i t o r and c l o s e l y c o n t r o l t h e m a t e r i a l s which e n t e r t h e i n d i v i d u a l processes w h i l e having the f l e x i b i l i t y to handle c o a l s from d i f f e r e n t seams. Such c o n t r o l w i l l be d i f f i c u l t w i t h o u t an a c c u r a t e i d e n t i f i c a t i o n o f t h e m i n e r a l s p r e s e n t and a knowledge o f t h e changes w h i c h t h e y undergo i n s p e c i f i c c l e a n i n g o p e r a t i o n s .

REFERENCES Coal Preparation, 4th edition, AIME, New York, N.Y. 1979 ASTM Standards, Section 5, Volume 05.05, 1982 Personal Communications: Dr. John Renton, William Grady, W. Va. Univ. RECEIVED November 4, 1985


Mineral Matter and Ash in Coal - American Chemical Society

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