Mineral Transformations during Ashing of Selected Low-Rank Coals

9 Mineral Transformations during Ashing of Selected Low-Rank Coals S. K . Falcone and H. H. Schobert

Downloaded by RUTGERS UNIV on January 2, 2018 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch009

Energy Research Center, University of North Dakota, University Station, Grand Forks, ND 58202

Inorganic species are present within low-rank coals as ion-exchangeable cations, as coordination complexes, and as discrete minerals. Variations in the inorganic associations of cations affects their relative reactivity and the processes associated with their formation of high temperature minerals. Twelve coals representing the Northern Great Plains and Gulf Coast were ashed 125°C, 750°C, and 1000°C in an oxidizing atmosphere. Each sample was then analyzed for its mineral composition by x-ray diffraction. In addition, model mixtures simulating raw coal mineralogies and organically-bound calcium and sodium were heated to 750°C and 1000°C for comparison to actual coal ashes. The processes responsible for most of the reactions identified were oxidation, dehydration, sulfur fixation, solid-state interactions and vaporization. In addition, i t was determined that organically-bound cations, specifically calcium and sodium, were more reactive than cations bound in mineral form in producing new mineral species with pre-existing minerals.

I n o r g a n i c s p e c i e s a r e i n c o r p o r a t e d i n l o w - r a n k c o a l s i n many w a y s : as i o n - e x c h a n g e a b l e c a t i o n s , a s c o o r d i n a t i o n c o m p l e x e s , and as a diverse array of discrete minerals. I n some c a s e s a n e l e m e n t w i l l be p r e s e n t i n m o r e t h a n o n e f o r m ; p o t a s s i u m , f o r e x a m p l e , o c c u r s both as an e x c h a n g e a b l e cation and i n a s s o c i a t i o n with clay minerals. The v a r i a t i o n i n a s s o c i a t i o n o f i n o r g a n i c s among t h e m u l t i p l e modes o f o c c u r r e n c e r e s u l t s i n a v e r y c o m p l e x s e r i e s o f reactions and mineral transformations when low-rank coals are ashed. In l o w - r a n k c o a l u t i l i z a t i o n p r o c e s s e s t h e b e h a v i o r o f t h e inorganic components c a n be a t l e a s t as important t o e f f e c t i v e

0097-6156/ 86/ 0301 -0114S06.00/ 0 § 1986 American Chemical Society

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


9.

FALCONE A N D SCHOBERT

Mineral

Transformations

during

115

Ashing

o p e r a t i o n as t h e b e h a v i o r o f t h e c a r b o n a c e o u s p o r t i o n o f t h e c o a l . The d e t e r m i n a t i o n o f t h e e x t e n t o f t h e c h a n g e s i n b u l k c o m p o s i t i o n and i n m i n e r a l p h a s e s d u r i n g c o n t r o l l e d l a b o r a t o r y a s h i n g i s v e r y i m p o r t a n t i n d e v e l o p i n g an u n d e r s t a n d i n g o f ash b e h a v i o r d u r i n g c o a l p r o c e s s i n g and how s u c h c h a n g e s a r e r e l a t e d t o p r o c e s s c o n d i t i o n s . In t h e p a s t , m i n e r a l o g i c a l d e t e r m i n a t i o n s u s i n g ash formed a t t h e s t a n d a r d t e m p e r a t u r e o f 750°C i d e n t i f i e d m i n e r a l s w h i c h w e r e n o t o r i g i n a l l y p r e s e n t i n t h e raw c o a l but which were a r t i f a c t s o f t h e ashing procedure. T h i s was d u e t o t h e a l t e r a t i o n o f m i n e r a l s by oxidation, d e h y d r a t i o n and o t h e r p r o c e s s e s a t h i g h t e m p e r a t u r e s . R e c e n t s t u d i e s by M i l l e r e t a l QJ, F r a z e r and B e l c h e r ( 2 J , and O'Gorman and W a l k e r (_3) h a v e c o n c e n t r a t e d on r e l a t i n g raw coal m i n e r a l o g y t o a s h m i n e r a l o g y o f ash g e n e r a t e d a t low t e m p e r a t u r e s . Low-temperature ashing (LTA) theoretically would enable one to obtain the true mineralogical composition of a coal since little mineral a l t e r a t i o n occurs at t y p i c a l LTA t e m p e r a t u r e s o f 125 C. Mitchell and G l u s k o t e r (£) e x p a n d e d t h i s c o n c e p t t o s t u d y low t o h i g h t e m p e r a t u r e m i n e r a l t r a n s f o r m a t i o n s i n ash o f s u b b i t u m i n o u s and bituminous coals. W i t h f e w e x c e p t i o n s , t h e a p p l i c a t i o n o f LTA i n ash mineralogy studies has been primarily associated with s u b b i t u m i n o u s and b i t u m i n o u s c o a l s { $ ) . In f a c t , M i l l e r e t a l (J.) a n d F r a z e r a n d B e l c h e r {2) s t a t e t h a t L T A may b e u n s u i t a b l e for obtaining the o r i g i n a l mineralogy in l i g n i t e s without appropriate pretreatment. This problem is due t o t h e h i g h o r g a n i c oxygen content with associated inorganic exchangeable cations characteristic of lignites. The presence of organically-bonded inorganics d r a s t i c a l l y increases the ashing time, thereby increasing t h e c h a n c e s o f m i n e r a l a l t e r a t i o n by o x i d a t i o n . In a d d i t i o n , t h e r e l e a s e o f o r g a n i c a l l y - b o u n d c a t i o n s and o r g a n i c s u l f u r i n c o n t a c t w i t h m i n e r a l m a t t e r can a l t e r t h e o r i g i n a l c o a l m i n e r a l o g y w i t h an extended period of low-temperature ashing. W

The purpose of this study was to identify mineral transformations occurring d u r i n g l o w and h i g h t e m p e r a t u r e ashing (125°, 750°, and 1 0 0 0 C ) of low-rank coals and t o examine the processes responsible f o r the mineral transformations. Twelve lowrank c o a l s were s e l e c t e d from t h e n o r t h e r n G r e a t P l a i n s and G u l f Coast. Nine North Dakota l i g n i t e s , two G u l f Coast (Texas and Alabama) lignites, and one s u b b i t u m i n o u s c o a l from Montana were studied (Table I ) . In a d d i t i o n , model m i x t u r e s o f i n o r g a n i c compounds simulating those inorganics c h a r a c t e r i s t i c a l l y found i n low-rank c o a l s were a l s o h e a t e d t o 750° a n d 1000°C. M i n e r a l s used i n t h e model m i x t u r e s t u d i e s i n c l u d e d c a l c i t e , k a o l i n i t e , q u a r t z and p y r i t e . C a l c i u m and s o d i u m a c e t a t e s w e r e u s e d t o s i m u l a t e o r g a n i c a l l y - b o u n d c a l c i u m and sodium. Sodium s u l f a t e was also used as a s o d i u m and sulfur source. Two-, threeand four-component systems were examined varying the molar r a t i o s of the c o n s t i t u e n t s . In t h i s m a n n e r t h e interaction of minerals and o r g a n i c a l l y - b o u n d inorganics can be traced during the heating process. Of p a r t i c u l a r i n t e r e s t was t h e s i m u l a t i o n of high temperature processes responsible for forming f e l d s p a t h i c a l u m i n o s i l i c a t e s c h a r a c t e r i s t i c o f m i n e r a l s f o r m e d when ashing low-rank c o a l s . The model s y s t e m s d i s c u s s e d a r e p r e s e n t e d i n Table I I . W


Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 1.06 0.86 1.09 2.86 0.71 1.11 3.58 0.56

Choctaw, Alabama McLean C o . , N o r t h D a k o t a Bowman C o . , N o r t h D a k o t a Bowman C o . , N o r t h D a k o t a

Mercer C o . , North Dakota P i k e County, Alabama A t a s c o s a C o . , Texas McHenry C o . , N o r t h D a k o t a

Al

0.49 0.66 1.06 0.33

0.49 0.50 0.73 0.87

1.61 0.54 0.29 0.44

Fe

Raw

0.68 0.36 0.47 0.26

1.87 0.48 0.25 0.39

2.58 1.02 0.52 0.89

+ + +

+ +

1.56 1.77 1.20 0.98

0.84 2.60 2.28 1.74

0.37 1.57 1.81 1.70

specified.

0.97 0.11 ND 0.11

0.56 0.91 0.38 0.59

0.08 1.18 0.67 0.94

Mg*

Ca

Coals

(Percent)+

of

C o n c e n t r a t i o n s m e a s u r e d by x - r a y f l u o r e s c e n c e ( X R F ) u n l e s s o t h e r w i s e A11 coals are l i g n i t e s except f o r Absaloka subbituminous. A b s a l o k a c o a l a n a l y s i s c o m p l e t e l y b y NAA. • C o n c e n t r a t i o n s m e a s u r e d by n e u t r o n a c t i v a t i o n a n a l y s e s ( N A A ) . NA ( N o t A v a i l a b l e ) ND ( N o t D e t e c t e d )

I n d i a n Head Pike San M i g u e l Velva

Blue Red

+ + +

Choctaw Falkirk Gascoyne Gascoyne

Si 3.5 0.90 0.35 0.66

Locality

B i g Horn C o . , Montana Mercer C o . , North Dakota Mercer C o . , North Dakota O l i v e r C o . , North Dakota

Name

and M i n o r E l e m e n t s

Inorganic Analyses

Absaloka B e u l a h Low S o d i u m B e u l a h High Sodium Center

Coal

I.

Major

Table


0.62 NA 0.60 0.09

0.09 0.01 0.27 0.13

0.33 0.14 0.46 0.40

Na*

0.44 2.28 1.88 NA

2.50 0.55 0.93 1.12

3.92 2.01 0.75 0.65

0.12 0.10 0.35 0.05

0.10 0.15 0.14 0.13

0.12 0.09 ND 0.08

Ti

0.04 0.65 0.08 0.02

0.04 0.04 0.05 0.12

0.06 0.05 0.06 0.32

Ba*

0.05 NA NA 0.03

ND 0.02 0.13 0.06

0.03 0.02 0.04 0.04

9.

F A L C O N E A N D SCHOBERT

Mineral

Transformations

during

Ashing

117

Experimental


T h e m i n e r a l m a t t e r c o m p o s i t i o n o f e a c h c o a l s a m p l e was d e t e r m i n e d d i r e c t l y b y X - r a y d i f f r a c t i o n (XRD) o f l o w t e m p e r a t u r e a s h ( L T A ) . A L F E M o d e l 5 0 4 f o u r - c h a m b e r o x y g e n p l a s m a l o w t e m p e r a t u r e a s h e r was used. The a s h i n g p r o c e d u r e s u s e d w e r e m o d i f i e d f r o m M i l l e r a n d G i v e n s * t e c h n i q u e {§) f o r l o w t e m p e r a t u r e a s h i n g o f s u b b i t u m i n o u s and b i t u m i n o u s c o a l s . One s e t o f s a m p l e s was i o n - e x c h a n g e d i n 1M ammonium a c e t a t e a t 70°C f o r 2 4 h o u r s a n d f r e e z e - d r i e d p r i o r t o low temperature ashing. T h i s p r o c e d u r e was r e p e a t e d t w i c e t o e n s u r e removal o f i o n - e x c h a n g e a b l e c a t i o n s . A n o t h e r , b u t u n t r e a t e d , sample s e t was a l s o a s h e d . P r e l i m i n a r y c o m p a r i s o n o f sample s e t s showed the exchanged samples t o have lower ashing time and identical mineralogy except f o r t h e presence of b a s s a n i t e i n non-exchanged samples. T h i s d i f f e r e n c e w i l l be d i s c u s s e d l a t e r . The L T A o p e r a t i n g p r o c e d u r e s u s e d w e r e a s f o l l o w s : a radio frequency power of approximately 150W a n d a n o x y g e n flow of 1 0 0 c c / m i n a t 2 p s i were m a i n t a i n e d a l o n g w i t h a chamber p r e s s u r e o f 1mm H g . Samples were s t i r r e d once e v e r y two hours d u r i n g t h e f i r s t e i g h t hours and e v e r y e i g h t hours d u r i n g t h e r e m a i n i n g a s h i n g t i m e . Samples were also ashed at 750 C i n accordance with ASTM p r o c e d u r e D3174-73 ( 2 ) and w i l l be r e f e r r e d t o a s ASTM s a m p l e s . S a m p l e s w e r e t h e n a s h e d a t 1000°C f o l l o w i n g t h e same p r o c e d u r e f o r 750°C c o a l a s h i n g a n d w i l l be r e f e r r e d t o a s ΗΤΑ ( h i g h t e m p e r a t u r e ash) samples. M i n e r a l s and o t h e r i n o r g a n i c s used i n model s y s t e m s A t h r o u g h J w e r e g r o u n d t o - 6 0 mesh t o m a t c h t h e p a r t i c l e s i z e o f t h e c o a l s u s e d for ashing. Samples were r a p i d l y heated t o 7 5 0 C and h e l d f o r two hours at temperature. Half o f t h e s a m p l e was r e m o v e d a n d a i r quenched. T h e r e m a i n i n g p o r t i o n o f t h e s a m p l e was r e t u r n e d t o t h e f u r n a c e a n d h e a t e d t o 1000°C, h e l d f o r t w o h o u r s , a n d t h e n a i r quenched. M i n e r a l o g i c a l c o m p o s i t i o n o f a s h s a m p l e s a n d m o d e l s y s t e m s was d e t e r m i n e d b y XRD. X - r a y f l u o r e s c e n c e ( X R F ) a n a l y s i s was a l s o u s e d f o r bulk elemental analysis of the ash. Raw c o a l a n a l y s i s was performed b y XRF a n d n e u t r o n activation (NAA). XRF e l e m e n t a l a n a l y s e s o f raw c o a l samples a r e l i s t e d i n T a b l e I . The n e u t r o n activation analyses were performed at North Carolina State University. e

W

R e s u l t s and D i s c u s s i o n M i n e r a l o g i c a l phases formed a t d i f f e r e n t temperatures f o r each coal sample a r e summarized i n T a b l e III. The m a j o r m i n e r a l phases detected b y XRD i n L T A s a m p l e s are quartz, pyrite, bassanite, k a o l i n i t e and p l a g i o c l a s e . The p r o c e s s e s r e s p o n s i b l e f o r s u b s e q u e n t mineral transformations include oxidation, vaporization, sulfur fixation, dehydration and solid-state interactions. The t e m p e r a t u r e s a t w h i c h s p e c i f i c t r a n s f o r m a t i o n s o c c u r a r e a s s i g n e d on t h e b a s i s o f p r e v i o u s e x p e r i m e n t a l work by M i t c h e l l a n d G l u s k o t e r ( 4 ) a n d p u b l i s h e d c h e m i c a l d a t a i n t h e Handbook o f C h e m i s t r y and P h y s i c s (S). In a d d i t i o n t o m i n e r a I - m i n e r a I interactions it is b e l i e v e d t h a t r e a c t i o n s b e t w e e n m i n e r a l s and e x c h a n g e a b l e cations occur (j)).



2

2

2

3

2

5

5

2

2

+

4

4

4

2

3

2

C a ( C H 0 ) + NaC H 0 + K a o l i n i t e (Al Si 0 (0H) )

2

3

5

2

E.

2

2

2

3

NaC H 0 + K a o l i n i t e (Al Si 0 (0H) )

2

2

D.

2

Ca ( C H 0 ) 2 Kaolinite (Al Si 0 (0H) )

3

2

P y r i t e (FeS )

C.

2

C a l c i t e (CaC0 ) + P y r i t e (FeS )

3

B.

2

Ca ( C H 0 ) 2

A.

+

Compound Mixtures

Table I I .

1:1:1

1:1

1:1

1:2

1000

750

1000

750

1000

750

1000

750 3

2

4

7

4

2

3

3

4

2

2

7

4

Nepheline (NaAlSi0 ) + Gehlenite (Ca Al Si0 )

Carnegieite (NaAlSi0 )

4

Nepheline (NaAISi0 )

Amorphous + Carnegieite (NaAlSi0 )

2

3

4

4

3

Gehlenite ( C a A l S i 0 J

Amorphous + CaO

2

4

3

4

Anhydrite (CaS0 ) + C a l c i t e (CaC0 ) + Hematite ( F e 0 ) Anhydrite (CaS0 ) + Hematite ( F e 0 ) + Magnetite ( F e 0 ) + CaO

2

3

Anhydrite (CaS0 ) + Magnetite ( F e 0 J + Hematite ( F e 0 )

2

1000

4

Anhydrite (CaS0 ) + Magnetite ( F e 0 )

750

2:1

Analyzed Minerals

Temp.

Molar Ratios

3

Dehydration; i n t e r s t i t i a l i n f i l l i n g i n reordered k a o l i n i t e s t r u c t u r e I n t e r s t i t i t a l i n f i l l i n g of Ca i n reordered c l a y s t r u c t u r e ; s t r u c t u r a l transformation of sodium a l u m i n o s i l i c a t e

Dehydration; I n t e r s t i t i a l i n f i l l i n g i n reordered k a o l i n i t e s t r u c t u r e

I n t e r s t i t i a l i n f i l l i n g i n reordered Kaolinite structure

Dehydration, o x i d a t i o n

Oxidation; Decomposition fo CaC0

Oxidation; P y r i t i c s u l f u r f i x a t i o n

Oxidation

Oxidation; P y r i t i c s u l f u r f i x a t i o n

Processes

Synthetic Compound Mixtures and Transformations That Occur During Heating


ζ ο

Χ

>

73

H m

25

r

>

73

ζ

m


4

2

2

2

2

3

4

2

2

2

2

2

2

2

3

2

2

2

3

2

2

3

5

2

5

2

2

2

4

4

4

2

3

2

2

K a o l i n i t e ( A l S i 0 5 ( 0 H ) ) + Quartz ( S i 0 ) + Na C H 0 + Ca ( C H 0 ) + P y r i t e (FeS )

5

J.

2

Kaolinite ( A l S i 0 ( 0 H ) ) + NaC H 0 + C a ( C H 0 \ + P y r i t e (FeS )

2

I.

3

Kaolinite (Al Si 0 (0H) ) + Na S0 + C a ( C H 0 )

4

5

H.

2

2

Na S0 + K a o l i n i t e ( A l S i 0 ( 0 H ) )

2

C a l c i t e (CaC0 ) + K a o l i n i t e (Al Si 0 (0H)4)

G.

F.

1:2:1:1:1

1:1:1:1

1:1:1

1:1

2.5:1

1000

750

1000

750

1000

750 4

7

6

8

2

4

4

2

2

2

3

6

3

2

8

3

7

4

4

8

4

4

4

4

4

1 - 2

3

1 - 2

4

Quartz ( S i 0 ) + Anhydrite (CaS0 ) + Hauyne ( N a , C a ) _ ( A l S i 0 ) ( S 0 ) + Hematite ( F e 0 ) + Magnetite ( F e 0 )

2

2

2

Quartz ( S i 0 ) + Anhydrite (CaS0 ) + Hematite ( F e 0 )

3

6

3

1

2

Hauyne ( N a , C a ) _ ( A l S i 0 ) ( S 0 ) + Magnetite ( F e 0 ) + Magnetite ( F e 0 ) + Anhydrite (CaS0 ) + Gehlenite ( C a A l S i 0 )

2

4

2

Magnetite ( F e 0 ) + Anhydrite (CaS0 )

4

4

Gehlenite ( C a A l S i 0 ) + Nepheline (NaAlSi0 ) + Hauyne ( N a , C a ) _ (A1S10 ) ( S 0 j .

2

Na S0 + Amorphous

4

Nepheline (NaAlSi0 )

1000

4

NaS0 + Amorphous

7

750

2

Gehlenite ( C a A I S i 0 ) + CaO + M u l l i t e 2

3

C a l c i t e (CaC0 ) + Amorphous Phase

1000

750

infilling

infilling

Oxidation

Interstitial

substitution

Dehydration; o x i d a t i o n , s u l f u r f i x a t i o n

I n t e r s t i t i a l i n f i l l i n g i n reordered k a o l i n i t e s t r u c t u r e ; Oxidation

Dehydration; O x i d a t i o n , P y r i t i c sulfur fixation

Interstitial

Dehydration

Interstitial

Dehydration

Dehydration and c o l l a p s e of k a o l i n i t e structure I n t e r s t i t i a l i n f i l l i n g in reordered k a o l i n i t e structure


120

M I N E R A L M A T T E R A N D A S H IN C O A L


Table

III.

Mineralogical

Composition

Samp!e Absal oka

LTA ( 125°C)+ Quartz Pyrite Kaolinite Plagioclase Bassanite

ASTM (750°C) ΗΤΑ (1000°C) Anhydrite Quartz Magnetite Anhydrite Hematite Hematite Quartz Melilite Plagioclase Nepheline

Beulah-Low

Quartz Pyrite Kaolinite Bassanite

Quartz Hematite Magnetite Anhydrite

Anhydrite Pyroxene Magnetite Hauyne Hematite Quartz

Quartz Bassanite Kaolinite Pyrite

Anhydrite Hematite Magnetite Quartz Melilite Hauyne

Anhydrite Melilite Magnetite Hematite Hauyne Quartz Corundum

Center

Quartz Bassanite Pyrite Kaolinite

Anhydrite Hematite Quartz

Anhydrite Hauyne Pyroxene Melilite Hematite Quartz

Choctaw

Quartz Pyrite Kaolinite Bassanite PI a g i o c l a s e

Anhydrite Quartz Hematite Magnetite Plagioclase Pyroxene

Anhydrite Hematite Quartz Magnetite PI a g i o c l a s

Falkirk

Quartz Kaolinite Pyrite

Anhydrite Quartz Hematite Magnetite Melilite (trace)

Anhydrite Quartz Melilite Hematite Magnetite Hauyne

Gascoyne B l u e High Sodium

Quartz Kaolinite Pyrite Calcite Sodium S u l f a t e (trace)

Anhydrite Quartz Hematite Magnetite Nosean Melilite

Anhydrite Melilite Hauyne Quartz

Beulah-High

•Minerals

Sodium

Sodium

listed

in decreasing order

o f peak

intensities

and


9.

of

F A L C O N E A N D SCHOBERT

Mineral

Ash Samples Determined

b y XRD*


Sample

LTA

Transformations

( 125°C)

during

Ashing

121

ΗΤΑ (1000°C)

ASTM (750°C)

Gascoyne RedLow S o d i u m

Quartz Kaolinite Pyrite

Quartz Anhydrite Hematite Magnetite

Quartz Anhydrite Pyroxene Hematite Hauyne

I n d i a n HeadHigh Sodium

Quartz Pyrite Kaolinite Bassanite

Anhydrite Quartz Hematite Nosean Melilite Hauyne Sodium S u l f a t e

Melilite Hematite Anhydrite Hauyne Magnetite Pyroxene

Λ

Pike

Quartz Pyrite Kaolinite

Anhydrite Quartz Pyrite

Anhydrite Hematite Melilite Anorthite Quartz

San M i g u e l

Zeolite (Heulandite) Quartz Kaolinite Pyrite Bassanite PIagioclase

Zeolite Anhydrite Hematite Quartz Plagioclase (Anorthite) Melilite

Plagioclase (Anorthite) Hematite Quartz Magnetite Anhydrite

Velva

Quartz Kaolinite Pyrite Bassanite

Anhydrite Quartz CaO

Anhydrite Gehlenite Quartz Hauyne

occurrence.

Peak

identification

not

conclusive.


122


P y r i t e ( F e S ) i s p r e s e n t i n a l l LTA s a m p l e s . While M i l l e r et al Q) stated that pyrite may be oxidized with increased low t e m p e r a t u r e a s h i n g t i m e i n l i g n i t e s no e v i d e n c e o f o x i d i z e d f o r m s o f i r o n was s e e n b y X R D . T h i s may b e a t t r i b u t e d t o t h e p r e t r e a t m e n t o f s a m p l e s w i t h ammonium a c e t a t e , t h e r e b y r e d u c i n g a s h i n g t i m e s a s much a s 50%. I n ASTM s a m p l e s p y r i t e i s o x i d i z e d t o h e m a t i t e ^ 2 0 3 ) a n d magnetite (FeoO^. A c c o r d i n g t o M i l l e r and G l u s k o t e r ( 4 ) , p y r i t e oxidizes a t 500°C. With t h e o x i d a t i o n of pyrite to iron oxide r a t h e r than iron s u l f a t e , p y r i t i c s u l f u r i s released. The f o r m a t i o n of sodium and c a l c i u m sulfates detected in ASTM ash can be associated with the release of t h i s p y r i t i c sulfur or organic sulfur and t h e i r i n t e r a c t i o n w i t h c a r b o n a t e s a s w e l l as w i t h organicallybound c a l c i u m and s o d i u m .


2

I n t h e m o d e l m i x t u r e s t u d i e s c a l c i u m a c e t a t e was o b s e r v e d t o r e a c t w i t h p y r i t e a t 750° a n d 1 0 0 0 ° C t o g e n e r a t e a n h y d r i t e XRD p e a k s of g r e a t e r i n t e n s i t y than those observed f o r the system of c a l c i t e r e a c t i n g w i t h p y r i t e ( T a b l e I I , Systems A and B ) . Sulfur fixation by c a l c i u m released from c a l c i u m acetate by p y r i t i c s u l f u r was greater than in the c a l c i t e case probably due t o t h e similar t e n p e r a t u r e s a t w h i c h c a l c i u m a c e t a t e decomposes and p y r i t e o x i d i z e s (400° t o 5 0 0 ° C ) . C a l c i t e , on t h e o t h e r h a n d , does n o t decompose until h i g h e r t e m p e r a t u r e s (900°C); t h e r e f o r e t h e c a l c i u m necessary t o form a n h y d r i t e i s not as r e a d i l y a v a i l a b l e . Most o f t h e S O 2 f o r m e d b y o x i d a t i o n o f p y r i t e w o u l d be l o s t t o t h e a t m o s p h e r e b y t h e time c a l c i t e decomposes. Bassanite (CaS04.1/2H20) is present in some of the sample LTAs. While bassanite may f o r m f r o m t h e d e h y d r a t i o n of gypsum (CaS04.2H20) during t h e LTA p r o c e d u r e no s i g n i f i c a n t gypsm was detected in the orignal coal m i n e r a l o g y o f t h e samples studied. Therefore, hemihydrated calcium s u l f a t e (bassanite) formed a t low temperature may b e d u e t o the fixation of organic sulfur by o r g a n i c a l l y - b o u n d c a l c i u m c a t i o n s n o t c o m p l e t e l y r e m o v e d by t h e i o n e x c h a n g e p r o c e d u r e (JLL_10). In t h i s c a s e , b a s s a n i t e i s s i m p l y an a r t i f a c t of the low temperature ashing procedure. This phenomenon i s t y p i c a l o f c o a l s h a v i n g abundant a l k a l i c a t i o n s a s s o c i a t e d w i t h c a r b o x y l g r o u p s (_6). At such low temperatures i t i s u n l i k e l y t h a t calcite would react with organic sulfur to form C a S U 4 l / 2 H 0 . Continued increases in ashing temperature results in complete d e h y d r a t i o n o f b a s s a n i t e t o a n h y d r i t e (CaSO/i) a t 400°C. Anhydrite i s a m a j o r m i n e r a l p h a s e i n ASTM a n d ΗΤΑ s a m p l e s . e

2

Kaolinite ( A l 2 ">2°5(° )4) P t only in LTA samples. K a o l i n i t e d e h y d r a t i o n o c c u r s a p p r o x i m a t e l y f r o m 400° t o 525°C ( 4 J . With removal of water by d e h y d r a t i o n , the k a o l i n i t e structure collapses, retaining some d e g r e e o f o r d e r a n d f o r m i n g m e t a k a o l i n . No m e t a k a o l i n was d e t e c t e d b y XRD i n ASTM s a m p l e s , p e r h a p s d u e t o i t s poorly defined c r y s t a l l i n e structure. However, i t i s b e l i e v e d t h a t t h e b a s i c k a o l i n i t e components a r e p r e s e n t i n an amorphous f o r m i n ASTM a s h . With i n c r e a s i n g temperature t h e c o l l a p s e d kaolinite s t r u c t u r e forms corundum ( Ύ - Α Ι 2 Ο 3 ) . While m u l l i t e (3Al?03.2Si02) and c r i s t o b a l i t e ( S i 0 ) have been r e p o r t e d t o f o r m f r o m w e l l - o r d e r e d k a o l i n i t e s i n b i t u m i n o u s c o a l s a t 1 0 0 0 ° C ( 4 J n e i t h e r were o b s e r v e d i n ΗΤΑ s a m p l e s . According t o Grim Q _ l ) , t h e absence of m u l l i t e suggests that the original kaolinitic structure was poorly defined. I t h a s a l s o b e e n s u g g e s t e d b y G r i m (11) t h a t t h e p r e s e n c e S

H

i

s

r

e

s

e

n

2


9.

Mineral

FALCONE A N D SCHOBERT

during

123

Ashing

of impurities i n the form of a l k a l i ions, such as i n lignites, r e t a r d s t h e development o f m u l l i t e s and C r i s t o b a l i t e . The m e c h a n i s m f o r t h i s i s not f u l l y u n d e r s t o o d . The c o l l a p s e d k a o l i n i t i c s t r u c t u r e a c t s as a s o u r c e o r framework for several different a l u m i n o s i l i c a t e c o m p l e x e s f o r m e d i n ΗΤΑ a s h samples. Common minerals found are as follows: anorthite (CaAl Si 0 ), pyroxenes (Ca,Na)(Mg,Fe,Al)(Si,A1) 0 ), melitites (Na,Ca)2(Mg,Fe,Al)(Si,Al) 07, hauyne (Na,Ca) o(AISi0 J(S0 )ι , nosean (NasAl Si 0 4S04) and n e p h e l i n e ( ( N a , K ) A I S i 0 ) . A t IOOCrC aluminosilicate minerals form from solid-state reactions of k a o l i n i t i c m a t e r i a l with c a t i o n s d e r i v e d from c a r b o n a t e s , o x i d e s , o r sulfates. Interstitial i n - f i l l i n g of a l k a l i cations occurs within t h e d e h y d r a t e d k a o l i n i t e s t r u c t u r e w i t h i n c r e a s i n g t e m p e r a t u r e due to thermal expansion and reordering of the collapsed clay structure. I n some c o a l s , p a r t i c u l a r l y t h o s e h i g h i n s o d i u m , t h e s e a l u m i n o - s i l i c a t e s a r e a l s o s e e n i n ASTM s a m p l e s . In t h e model s y s t e m s C t h r o u g h I ( T a b l e I I ) d i f f e r e n t s o u r c e s o f c a l c i u m and sodium were m i x e d w i t h c l a y ( k a o l i n i t e ) and h e a t e d t o observe t h e i r r o l e i n forming the a l u m i n o s i l i c a t e s t y p i c a l of ash fouling deposits. In s y s t e m s C a n d D c a l c i u m a c e t a t e and s o d i u m a c e t a t e were mixed w i t h k a o l i n i t e i n equal molar r a t i o s . X-ray d i f f r a c t o m e t e r p a t t e r n s showed t h a t b o t h s y s t e m s w e r e , f o r t h e most p a r t , a m o r p h o u s a t 750°C. However, i n s y s t e m Ε where sodium and c a l c i u m a c e t a t e w e r e p r e s e n t i n e q u a l m o l a r r a t i o s c a r n e g i e t i t e was f o r m e d a t 750°C. Carnegietite i s a polymorph of n e p h e l i n e . In c a r n e g i e t i t e t h e sodium c a t i o n i s t e t r a h e d r a l l y c o o r d i n a t e d whereas i n n e p h e l i n e t h e sodium c a t i o n i s o c t a h e d r a l l y c o o r d i n a t e d . 2

2

8

2

2

6


Transformations

6

2

6

6

4

4

2

4

At temperatures greater than 900°C the formation of new aluminosilicates would be anticipated with the reordering of c o l l a p s e d c l a y s t r u c t u r e s and i n f i l l i n g o f i n t e r s t i t i a l v o i d s p a c e s by c a t i o n s . In s y s t e m s C a n d D n e p h e l i n e and g e h l e n i t e were formed r e s p e c t i v e l y a t 1000°C. In s y s t e m Ε b o t h n e p h e l i n e and g e h l e n i t e w e r e f o r m e d a t 1000°C s h o w i n g no d e t e c t a b l e m u t u a l i n t e r s t i t i a l void filling of calcium and sodium within t h e same aluminosilicate structures. To e s t a b l i s h t h a t t h e r e s u l t s o b s e r v e d w i t h t h e 1:1 c a l c i urn:sodium ratios were not artifacts of a stoichiometric l i m i t a t i o n o f r e a c t a n t s t h e m o l a r r a t i o s o f c a l c i u m t o sodium were v a r i e d f r o m 1:4 t o 4 : 1 i n a n a t t e m p t t o s a t u r a t e t h e s y s t e m w i t h respect to calcium or sodium. X-ray d i f f r a c t o m e t e r patterns of v a r i o u s r a t i o c o m b i n a t i o n s showed t h a t s o d i u m and c a l c i u m c o n t i n u e d t o f i l l v o i d s i n t h e same m a n n e r a s w i t h a 1:1 r a t i o . The f a c t t h a t s o d i u m and c a l c i u m do n o t m u t u a l l y i n f i l l v o i d o f s p e c i f i c m i n e r a l s reflects the preferred oxide coordination behavior of t h e two cations, which in turn i s determined by t h e i r respective ionic charges and r a d i i . Calcium prefers tetrahedral coordination, so t h a t i n general the r e a c t i o n of calcium ions with k a o l i n i t e would result in a gehlenite structure, in which the calcium is tetrahedral ly coordinated. On t h e o t h e r hand, sodium favors octahedral coordination, thus forming nepheline structures with sodium i n octahedral site. In system F c a l c i t e was u s e d as a c a l c i u m source f o r the f o r m a t i o n o f g e h l e n i t e . The r e a c t i o n d i d n o t p r o c e e d a s c o m p l e t e l y as w i t h c a l c i u m a c e t a t e , as i n f e r r e d from two o b s e r v a t i o n s : 1) some of the calcium remained as c a l c i u m o x i d e upon d e c o m p o s i t i o n of



124


calcite a t 900°C, a n d 2 ) m u l l i t e , a p o o r l y c r y s t a l l i n e f o r m o f d e h y d r a t e d r e o r d e r e d k a o l i n i t e , was s t i l l p r e s e n t a t 1000°C i n t h e presence of calcium oxide. I n s y s t e m s G a n d H , s o d i u m s u l f a t e was u s e d a s a s o d i u m s o u r c e mixed w i t h k a o l i n i t e and c a l c i u m a c e t a t e . Sodium s u l f a t e m e l t s a t a p p r o x i m a t e l y 884°C. Therefore, a t 750°C n o i n t e r a c t i o n between k a o l i n i t e a n d s o d i u m s u l f a t e was s e e n ( s y s t e m s G a n d H ) . In system G n e p h e l i n e was f o r m e d a t 1000°C, w i t h n o n e o f t h e s u l f u r r e l e a s e d f r o m t h e s o d i u m s u l f a t e i n v o l v e d i n t h e f o r m a t i o n o f a n y new h i g h temperature minerals. However, i n system H a d d i t i o n a l feldspathoids ( i . e . , g e h l e n i t e and hauyne) o t h e r t h a n n e p h e l i n e were f o r m e d . In this case, s u l f u r was i n v o l v e d i n forming new h i g h temperature minerals. System I was comprised of calcium and sodium acetates, k a o l i n i t e , and p y r i t e t o see i f s u l f u r - c o n t a i n i n g aluminosilicates w o u l d be f o r m e d . A s e x p e c t e d , a n h y d r i t e ( C a S O ^ w a s f o r m e d by p y r i t i c s u l f u r f i x a t i o n by c a l c i u m . A t 1000°C n o t o n l y d i d t h e a n h y d r i t e p e r s i s t but a s u l f u r c o n t a i n i n g a l u m i n o s i l i c a t e , hauyne, was f o r m e d i n a d d i t i o n t o g e h l e n i t e . In systems A through I the major aluminosilicate minerals produced were s i l i c a - d e f i c i e n t . However, i n s y s t e m s J where s i l i c a was p r o v i d e d i n excess i n t h e form of q u a r t z t h e system still produced only feldspathoids containing only two-thirds a s much s i l i c a as t h e i r s i l i c a - r i c h c o u n t e r p a r t s ( i . e . a l k a l i feldspars). In a d d i t i o n , t h e q u a r t z p e a k s i n s y s t e m J were n o t s u b s t a n t i a l l y reduced a t higher temperatures. These two f a c t s s u g g e s t t h a t S i 0 i n t h e f o r m o f q u a r t z i s i n a c t i v e u p t o a n d a t 1000°C. This idea i s s u p p o r t e d by t h e f a c t t h a t q u a r t z peaks a r e a l s o q u i t e e v i d e n t i n d i f f r a c t o m e t e r p a t t e r n s o f a s h s a m p l e s g e n e r a t e d b e t w e e n 750°C a n d 1000°C. It appears that temperatures i n excess o f 1000°C a r e required f o r quartz to contribute to the formation of s i l i c a - r i c h minerals. XRD f a i l e d t o d e t e c t c a l c i t e i n L T A s a m p l e s p o s s i b l y d u e t o i t s e x t r a c t i o n by ammonium a c e t a t e s o l u t i o n o r b e c a u s e t h e a m o u n t s o f c a l c i t e were below d e t e c t i o n l i m i t s ( ^ 5 % ) . F o r t h e most part, c a l c i u m i s s u p p l i e d t o t h e s y s t e m b y gypsum a n d o r g a n i c a l l y - b o u n d calcium. C a l c i u m , whether i n t h e form o f b a s s a n i t e , c a l c i t e , o r c a t i o n s i n LTA s a m p l e s , f o r m s a n h y d r i t e i n ASTM s a m p l e s . I n ΗΤΑ samples c a l c i u m r e a c t s p r i m a r i l y with dehydrated k a o l i n i t e forming aluminosilicates. Figure 1 displays a typical X-ray diffractometer pattern s e q u e n c e f r o m L T A , A S T M , a n d ΗΤΑ s a m p l e s o f t h e B e u l a h H i g h S o d i u m lignite. Major peaks a r e i d e n t i f i e d a c c o r d i n g t o t h e m i n e r a l phases present. Mineral transformations at higher temperatures are c h a r a c t e r i z e d by t h e f o r m a t i o n o f numerous f e l d s p a t h o i d s . When comparing several of these d i f f r a c t o m e t e r patterns t h e r e i s little difference among L T A s a m p l e s f r o m d i f f e r e n t c o a l s ; on t h e o t h e r h a n d , t h e v a r i o u s ASTM a n d ΗΤΑ a s h s a m p l e s a r e q u i t e d i f f e r e n t . By c o m p a r i n g t h e m i n e r a l o g i c a l d i f f e r e n c e s i n t h e a s h e s t o t h e raw c o a l elemental compositions, it c a n be seen t h a t samples containing h i g h e r amounts o f sodium t e n d t o form a l u m i n o - s i 1 i c a t e s a t lower temperatures (750°C) t h a n s a m p l e s h i g h in calcium. High-sodium c o a l s such as B e u l a h H i g h Sodium and G a s c o y n e B l u e d e v e l o p complex silicates i n ASTM s a m p l e s a n d a r e p r o n e t o f o r m i n g a s h f o u l i n g 2


9.

Mineral

FALCONE AND SCHOBERT

Transformations

during

125

Ashing

LTA 1-125 C)

υ


—1

I—

ASTM I750°CI I

Me

Ha

10 ΗΤΑ I1000°C1

86-

**** 30

10

50

40

°2Θ KEY:

Q

Quartz

Β

Bassanite

Ρ

Η

Hematite

M

Magnetite

Me

Pyrite

A

Melilite

Ha Hauyne

Anhydrite

Κ

Kaolinite

F i g u r e 1. X - r a y d i f f r a c t o g r a m s o f L T A , ASTM a n d ΗΤΑ s a m p l e s o f Beulah high sodium c o a l .


126

M I N E R A L MATTER A N D A S H IN C O A L

deposits. M e l i l i t e s , hauyne, n e p h e l i n e , nosean and pyroxenes a r e t y p i c a l o f s u c h a l u m i n o s i l i c a t e s i n t h e ASTM a n d ΗΤΑ a s h s a m p l e s . T h e s e a r e m i n e r a l s commonly f o u n d i n t h e f o u l i n g d e p o s i t s o f most lignites.


C o n c l u d i n g Remarks The o b s e r v a t i o n s presented i n t h i s study r e f l e c t t h e p r e l i m i n a r y r e s u l t s o f an i n v e s t i g a t i o n o f t h e m i n e r a l t r a n s f o r m a t i o n s seen i n the high temperature combustion o f low-rank c o a l s . The m i n e r a l o g i e s of t h e raw c o a l s studied do n o t v a r y a g r e a t deal. Quartz, k a o l i n i t e , p y r i t e , and b a s s a n i t e a r e found i n abundance i n each LTA sample. G r e a t e r d i f f e r e n c e s between samples a r e apparent a t h i g h e r t e m p e r a t u r e s , where complex a l u m i n o s i l i c a t e s p r e d o m i n a t e . This i s a r e f l e c t i o n o f d i f f e r e n c e s n o t s o much i n o r i g i n a l m i n e r a l m a t t e r b u t in the total inorganic composition of the coal. Specifically, the presence o f exchangeable a l k a l i c a t i o n s accounts f o r d i f f e r e n c e s i n a s h i n g b e h a v i o r between c o a l samples (9J. Both t h e type of t h e c a t i o n s ( s p e c i f i c a l l y , sodium o r c a l c i u m ) and t h e amounts o f e a c h type incorporated i n the coal are important i n determining the m i n e r a l p h a s e s f o r m e d i n t h e ΗΤΑ a s h . The p r o c e s s e s r e s p o n s i b l e f o r most o f t h e r e a c t i o n s identified are oxidation, dehydration, sulfur fixation, solid-state interactions and v a p o r i z a t i o n . Isolating specific reactions o c c u r r i n g i n a m u l t i - c o m p o n e n t s y s t e m i s d i f f i c u l t ; t h e u s e o f model s y s t e m s was h e l p f u l i n t r a c i n g m i n e r a l t r a n s f o r m a t i o n s . Additional work planned in this area includes research on n o n - e q u i l i b r i u m systems and e x p e r i m e n t s i n r e d u c i n g atmospheres. Acknowledgments The authors wish t o thank Diane Rindt f o r her help i n x-ray d i f f r a c t i o n a n a l y s i s and Steve Braun f o r h i s a s s i s t a n c e i n p r e p a r i n g ΗΤΑ a s h s a m p l e s . T h i s work was p e r f o r m e d u n d e r a U . S . D e p a r t m e n t o f E n e r g y C o o p e r a t i v e DOE s u p p o r t A g r e e m e n t N o . D E - F C 2 1 - 8 3 F E 6 0 1 8 1 .

Literature Cited 1. 2. 3. 4. 5. 6.

7. 8.

Miller, R.N.; Yarzab, R.F.: and Given, Peter. Fuel 1979, 58, 4. Frazer, F.W.; and Belcher, C.B. Fuel 1973, 52,41. O'Gorman, J . V . : and Walker, P.L. Fuel 1973, 52,71. Mitchell, R.S.; and Gluskoter, H.J. Fuel 1976, 55,90. Gluskoter, J . J . Fuel 1965, 44, 285. Miller, R.N.; and Given, Peter. 'A Geochemical Study of the Inorganic Constituents in Some Low-Rank Coals' Technical Report, Pennsylvania State University to the U.S. Department of Energy, Rep. FE-2494-TR-1, 1978. Annual Book of American Society of Testing Materials Standards 1979, Part 26: Gaseous Fuels; Coal and Coke; Atmospheric Analysis. Handbook of Chemistry and Physics 54th edition, Chemical Rubber Company, 1973.


9.

FALCONE AND SCHOBERT

9. 10. 11.

Mineral Transformations during Ashing

127

Morgan, M.E.; Jenkins, R.G.; and Walker, P.L. J r . Fuel 1981, 60, 189. Painter, P.C.; Coleman, M.M.: Jenkins, R.G.; and Walker, P.L. Jr. Fuel 1978, 57, 125. Grim, R.E. Clay Mineralogy, McGraw-Hill, New York, 1968, Chapter 9.


RECEIVED June 13, 1985


Mineral Transformations during Ashing of Selected Low-Rank Coals

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