Behavior of Quartz, Kaolinite, and Pyrite during Alkaline Leaching of

34 Behavior of Quartz, Kaolinite, and Pyrite during Alkaline Leaching of Coal C.-W. Fan, R. Markuszewski, and T. D . Wheelock

Downloaded by TUFTS UNIV on November 18, 2016 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch034

Chemical Engineering Department and Energy and Mineral Resources Research Institute, Iowa State University, Ames,IA50011

Most of the mineral matter was extracted from Illinois No. 6 coal, a representative bituminous coal, by first leaching i t with a hot alkaline solution and then with a dilute mineral acid. The alkalis employed, in decreasing effectiveness, were: (1) NaOH, (2) Na CO , (3) NaHCO . The results generally agreed with those obtained in leaching quartz, kaolinite, and iron pyrite individually under similar conditions. The quartz was solubilized by the stronger alkalis. Kaolinite was largely converted to sodium hydroaluminosilicates such as analcime, hydroxycancrinite, and natrodavyne. A portion of the iron pyrite was converted to hematite, the amount depending on alkali strength and temperature. The sodium hydroaluminosilicates and hematite were subsequently extracted by acid. 2

3

3

P r e v i o u s work has shown t h a t a s h - f o r m i n g m i n e r a l m a t t e r , i n c l u d i n g i r o n p y r i t e s , c a n be removed from c o a l by l e a c h i n g t h e f i n e l y d i v i d e d m a t e r i a l w i t h a h o t c a u s t i c s o l u t i o n under p r e s s u r e f o l l o w e d by washing w i t h a d i l u t e m i n e r a l a c i d ( 1 - 6 ) . R e c e n t l y , s i m i l a r r e s u l t s have been a c h i e v e d i n l e a c h i n g f i n e - s i z e c o a l w i t h h o t sodium c a r b o n a t e s o l u t i o n s ( 7 ) . I n b o t h c a s e s , q u a r t z appears t o d i s s o l v e i n t h e h o t a l k a l i n e s o l u t i o n , w h i l e c l a y m i n e r a l s and i r o n p y r i t e a r e c o n v e r t e d i n t o a c i d - s o l u b l e compounds which a r e removed i n t h e a c i d washing s t e p . A c i d a l s o removes any c a r b o n a t e m i n e r a l s which a r e p r e s e n t i n t h e c o a l . I n t h e p r e s e n t s t u d y t h e b e h a v i o r o f some o f t h e p r i n c i p a l c o a l m i n e r a l s was t r a c e d d u r i n g a l k a l i n e l e a c h i n g . T h i s was a c c o m p l i s h e d by l e a c h i n g s e v e r a l t y p e s o f c o a l and by l e a c h i n g i n d i v i d u a l mine r a l s , which a r e commonly p r e s e n t i n c o a l , w i t h v a r i o u s a l k a l i s . D i f f e r e n t a l k a l i c o n c e n t r a t i o n s and temperatures were employed. The s o l i d r e a c t i o n p r o d u c t s were c h a r a c t e r i z e d by x - r a y d i f f r a c t i o n (XRD) and by d i s s o l u t i o n i n m i n e r a l a c i d s . Results achieved with I l l i n o i s No. 6 c o a l a r e d i s c u s s e d , s i n c e they a r e r e p r e s e n t a t i v e , and t h e r e s u l t s o f i n d i v i d u a l l y l e a c h i n g q u a r t z , k a o l i n i t e , and i r o n p y r i t e 0097-6156/ 86/ 0301 -0462$06.00/ 0 © 1986 American Chemical Society

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

34.

FAN ET AL.

Mineral

Matter

Behavior

during

463

Leaching

a r e d e s c r i b e d because t h e s e m i n e r a l s g e n e r a l l y account p o r t i o n of the m i n e r a l m a t t e r i n many c o a l s .


Experimental

f o r a major

Methods

I l l i n o i s No. 6 c o a l , o b t a i n e d from the Elm Mine i n I l l i n o i s , was d r i e d under n i t r o g e n a t 90°C f o r 24 h r . and then ground t o -74 ym size. A n a l y s i s of the d r y c o a l by ASTM methods (8) r e v e a l e d the f o l l o w i n g composition: 12.75% ash, 1.32% p y r i t i c s u l f u r , 0.24% s u l f a t e s u l f u r , 1.68% o r g a n i c s u l f u r , and 3.24% t o t a l s u l f u r . E x a m i n a t i o n by XRD showed t h e p r i n c i p a l m i n e r a l i m p u r i t i e s t o be i r o n p y r i t e , q u a r t z , k a o l i n i t e , and c a l c i t e ( T a b l e I ) . I r o n p y r i t e n o d u l e s were o b t a i n e d from r e f u s e produced i n c l e a n i n g c o a l from Mahaska County, Iowa. The n o d u l e s were c r u s h e d and then b a l l - m i l l e d to -38 ym s i z e ; p r i n c i p a l i m p u r i t i e s determined by XRD a r e i n d i c a t e d i n T a b l e I . P a r t o f t h e ground p y r i t e was t r e a t e d f o r 1 h r . w i t h e x c e s s 1.2 M h y d r o c h l o r i c a c i d a t 70°C under n i t r o g e n t o remove a c i d s o l u b l e i m p u r i t i e s ; i t was then washed and d r i e d . The a c i d - c l e a n e d m a t e r i a l c o n t a i n e d 88% i r o n p y r i t e , based on t o t a l s u l f u r c o n t e n t , and s i g n i f i c a n t amounts o f q u a r t z and k a o l i n i t e p l u s a t r a c e of titania. I n a d d i t i o n , the m a t e r i a l seemed t o c o n t a i n a nonc r y s t a l l i n e c o n s t i t u e n t which may have been c o a l . Quartz o b t a i n e d f o r t h i s s t u d y d i d not appear t o c o n t a i n s i g n i f i c a n t amounts o f any i m p u r i t i e s when examined by XRD. On the o t h e r hand, the k a o l i n which was o b t a i n e d seemed to c o n t a i n a s i g n i f i c a n t amount o f q u a r t z and t r a c e q u a n t i t i e s o f i l l i t e and t i t a n i a . For the l e a c h i n g e x p e r i m e n t s , a weighed amount o f f i n e l y d i v i d e d c o a l or m i n e r a l m a t t e r was mixed w i t h 120 ml. o f a l k a l i n e s o l u t i o n i n a 300-ml. s t a i n l e s s s t e e l (Type 316) a u t o c l a v e equipped w i t h a turbine agitator. The system was f l u s h e d w i t h n i t r o g e n and then h e a t e d t o the d e s i r e d l e a c h i n g temperature. The m i x t u r e was s t i r r e d c o n t i n u o u s l y w h i l e l e a c h i n g was conducted a t c o n s t a n t temperature and p r e s s u r e f o r a s p e c i f i e d p e r i o d . A f t e r t h i s t r e a t m e n t , the a u t o c l a v e

Table I.

Material Illinois No. 6 c o a l

M a t e r i a l s Which Were C h e m i c a l l y Size, ym

Source T r i v o l i County

(111.)

-74

Impurities Indicated by XRD F e S , Si0 »

pyrite

C o a l , Mahaska County (la.)

-38

2

2

CaC0 Raw

Treated

kaolinite,

3

CaC0 , i r o n oxides, 3

Si0 , kaolinite,

Ti0

2

Ti0

2

2

Cleaned pyrite

Same as above

-38

Si0 , kaolinite,

Quartz

Ottawa Sand

-38

None

Kaolin

Old H i c k o r y No. c l a y (Ky.)

-74

Si0 ,

(111.)

2

5 ball 2

Illite,


Ti0

2

464

MINERAL MATTER AND ASH IN COAL

was c o o l e d q u i c k l y , and the c o n t e n t s o f the a u t o c l a v e were f i l t e r e d to r e c o v e r any u n d i s s o l v e d s o l i d s . The s o l i d s were washed w i t h water, d r i e d i n an oven, weighed, and d i v i d e d i n t o two p a r t s . One p a r t was a n a l y z e d by XRD o r o t h e r means, and the o t h e r p a r t was t r e a t e d w i t h a c i d t o determine the p r o p o r t i o n o f a c i d - s o l u b l e material. F o r the a c i d treatment s t e p , up t o 3 g. o f a l k a l i - l e a c h e d m a t e r i a l was mixed w i t h 300 ml. o f m i n e r a l a c i d ( a p p r o x i m a t e l y 2 M) i n a s t i r r e d , t h r e e - n e c k Pyrex r e a c t i o n f l a s k f i t t e d w i t h a r e f l u x condenser. The treatment was conducted f o r 30 min. e i t h e r a t room temperature (25°C) o r a t the b o i l i n g p o i n t (100°C). A f t e r the t r e a t ment the m i x t u r e was f i l t e r e d , and any u n d i s s o l v e d s o l i d s were washed w i t h 600 ml. o f water, d r i e d , weighed, and a n a l y z e d by XRD o r o t h e r means. XRD a n a l y s i s was performed w i t h a Siemens D500 d i f f r a c t o m e t e r u s i n g copper K radiation. The ash c o n t e n t o f the t r e a t e d c o a l was determined by ASTM Method D3174 ( 8 ) , w h i l e the s u l f u r c o n t e n t was determined w i t h a F i s h e r model 475 t o t a l s u l f u r analyzer.


a

Results with

Coal

The r e s u l t s o f l e a c h i n g I l l i n o i s No. 6 c o a l w i t h d i f f e r e n t a l k a l i n e s o l u t i o n s f o l l o w e d by a c i d treatment a r e shown i n T a b l e I I . I n each l e a c h i n g experiment 15 g. o f c o a l was l e a c h e d w i t h 120 ml. o f a l k a l i n e s o l u t i o n a t 250°C f o r 1 h r . I n some e x p e r i m e n t s the a l k a l i l e a c h e d c o a l was washed w i t h h y d r o c h l o r i c a c i d , and i n o t h e r e x p e r iments the c o a l was washed w i t h s u l f u r i c a c i d . When h y d r o c h l o r i c a c i d was employed, the a c i d washing s t e p was conducted a t the b o i l i n g p o i n t and the f i n a l water washing s t e p a t room temperature. However, when s u l f u r i c a c i d was used, the washing s t e p s were conducted a t v a r i o u s temperatures t o s t u d y the temperature e f f e c t .

T a b l e I I . R e s u l t s of T r e a t i n g I l l i n o i s No. 6 C o a l w i t h Hot A l k a l i n e S o l u t i o n F o l l o w e d by A c i d L e a c h i n g

A l k a l i Trtmt. Type

M

NaHC0 Na C0 2

3

3

NaOH N a

C

2 °3 Na C0 2

N a

2

3

C 0

3

2.0

A c i d Trtmt. Type

T.,

°C

Washing T.,

°C

Product, Yld?

HC1

100

1.0

HC1

100

25

94

2.0

HC1

100

25

83

1.0

H S0

1.0 1.0

Y i e l d and

2

H

S

4

2 °4 H SO. 2 4 0

25

95

Ash 6.57

%

a

Reduction,

b

*t

Ash

s

t

3.14

48

15

2.61

2.40

80

35

1.28

1.95

90

47

25

25

94

3.40

2.72

73

27

100

25

94

3.26

3.09

74

17

100

100

94

2.93

2.55

77

31

t o t a l s u l f u r c o n t e n t on a d r y , a s h - f r e e b a s i s

'Ash c o n t e n t on a d r y b a s i s


%

34.

FAN ET AL.

Mineral

Matter

Behavior

during

465

Leaching

The r e s u l t s o f the c o a l l e a c h i n g e x p e r i m e n t s i n d i c a t e t h a t t h e ash and t o t a l s u l f u r c o n t e n t s o f the c o a l were r e d u c e d s u b s t a n t i a l l y w h i l e the p r o d u c t y i e l d was g e n e r a l l y h i g h . The r e s u l t s were a f f e c t e d by the type o f a l k a l i , and the e f f e c t i v e n e s s o f t h e a l k a l i d e c r e a s e d i n the f o l l o w i n g o r d e r : NaOH > Na C03 > NaHC03Although the g r e a t e s t r e d u c t i o n i n ash and s u l f u r c o n t e n t s was r e a l i z e d w i t h sodium h y d r o x i d e , the p r o d u c t y i e l d (84%) was the l o w e s t . The 47% r e d u c t i o n i n t o t a l s u l f u r c o n t e n t a c h i e v e d w i t h sodium h y d r o x i d e was e q u i v a l e n t t o the r e d u c t i o n w h i c h would have been r e a l i z e d i f a l l o f the i n o r g a n i c s u l f u r had been removed and none o f the o r g a n i c s u l f u r . Sodium c a r b o n a t e appeared t o be o n l y s l i g h t l y l e s s e f f e c t i v e than sodium h y d r o x i d e i n r e d u c i n g t h e s u l f u r and ash c o n t e n t s o f the c o a l , but i t p r o v i d e d a g r e a t e r y i e l d ( 9 4 % ) . Sodium b i c a r b o n a t e appeared c o n s i d e r a b l y l e s s e f f e c t i v e than the o t h e r a l k a l i s . For c o a l leached w i t h sodium c a r b o n a t e , s l i g h t l y lower ash and s u l f u r c o n t e n t s were o b t a i n e d when the m a t e r i a l was washed w i t h h y d r o c h l o r i c a c i d than when i t was washed w i t h s u l f u r i c a c i d . However, the e f f e c t i v e n e s s o f the s u l f u r i c a c i d t r e a t m e n t was improved by u s i n g h o t a c i d and by u s i n g h o t water i n s t e a d o f c o l d water t o wash the a c i d - t r e a t e d c o a l s .


2

Results with I n d i v i d u a l Minerals To e x p l a i n the p r e c e d i n g r e s u l t s w i t h c o a l , a s e r i e s o f l e a c h i n g e x p e r i m e n t s was c a r r i e d out w i t h i n d i v i d u a l m i n e r a l s . I n the f i r s t s e t o f e x p e r i m e n t s , -38 ym s i z e q u a r t z p a r t i c l e s were l e a c h e d w i t h v a r i o u s h o t a l k a l i n e s o l u t i o n s t o s t u d y the e f f e c t s o f a l k a l i t y p e , a l k a l i c o n c e n t r a t i o n , r a t i o o f a l k a l i t o q u a r t z , temperature, and l e a c h i n g time on d i s s o l u t i o n . A f t e r each l e a c h i n g , the r e a c t o r c o n t e n t s were f i l t e r e d w i t h Whatman No. 40 f i l t e r paper u s i n g s u c t i o n , and the r e s i d u e was washed w i t h c o l d w a t e r , d r i e d a t 350°C f o r 2 h r . , and weighed. The p e r c e n t a g e o f m a t e r i a l e x t r a c t e d was c a l c u l a t e d u s i n g the f o l l o w i n g e x p r e s s i o n : •m . ^, Extraction

/ o y N

(%)

Λ r\r\ = 100

w

t . of r e s i d u e τ~έ—τ~ wt. o f f e e d

ΛΛ

Λ

x 100

,

Λ N

(1)

The r e s u l t s o f l e a c h i n g q u a r t z w i t h sodium c a r b o n a t e s o l u t i o n s h a v i n g d i f f e r e n t c o n c e n t r a t i o n s a r e shown i n F i g u r e 1. F o r each p o i n t , 2.0 g. o f q u a r t z was l e a c h e d w i t h 120 ml. o f s o l u t i o n a t 250°C f o r t h e i n d i c a t e d t i m e . I t can be seen t h a t t h e amount o f m a t e r i a l e x t r a c t e d i n c r e a s e d l i n e a r l y w i t h time f o r about the f i r s t hour. D u r i n g t h i s p e r i o d the r a t e o f e x t r a c t i o n a l s o i n c r e a s e d w i t h a l k a l i c o n c e n t r a t i o n t o a p p r o x i m a t e l y the 0.5 power. F o l l o w i n g the c o n s t a n t r a t e p e r i o d , no f u r t h e r m a t e r i a l was e x t r a c t e d . Apparently the s o l u b i l i t y l i m i t was r e a c h e d , and i t seemed t o be the s o l u b i l i t y l i m i t a t room temperature r a t h e r t h a n a t the l e a c h i n g t e m p e r a t u r e . The f o l l o w i n g e v i d e n c e p o i n t e d t o the lower temperature l i m i t . The r e s i d u e r e m a i n i n g a f t e r l e a c h i n g q u a r t z f o r 2 h r . o r more was an amorphous, n o n - c r y s t a l l i n e s o l i d which was i n s o l u b l e i n h o t h y d r o chloric acid. E x a m i n a t i o n o f the r e s i d u e f o r v a r i o u s c h e m i c a l elements by energy d i s p e r s i v e x - r a y a n a l y s i s r e v e a l e d a h i g h con c e n t r a t i o n o f s i l i c o n and o n l y a t r a c e o f sodium. The r e s u l t s s u g g e s t e d t h a t q u a r t z had d i s s o l v e d i n the hot s o l u t i o n , and upon c o o l i n g some o f the s i l i c a had r e p r e c i p i t a t e d as an amorphous o r



466

ο —

//


-

—

//

Δ / /

/

• 1.0 M 0 2.0 M Δ

-

// /

Ϊ 0.0

1 0.5

3.0 M

250°C 1 1.0

1 1.5

I 2.0

1 2.5

I 3.0

LEACHING TIME, h r . F i g u r e 1. D i s s o l u t i o n o f q u a r t z (2.0 g.) by h o t sodium s o l u t i o n s (120 ml.) h a v i n g d i f f e r e n t c o n c e n t r a t i o n s .

carbonate



34.

FAN ET AL.

Mineral

Matter

Behavior

during

Leaching

467

g l a s s y m a t e r i a l . On the o t h e r hand, the r e s i d u e r e m a i n i n g a f t e r q u a r t z had been l e a c h e d f o r l e s s than 1 h r . appeared t o be l a r g e l y unreacted quartz. Furthermore, when q u a r t z was l e a c h e d f o r 1 h r . a t the h i g h e r temperature o f 300°C w i t h a 1.0 M sodium c a r b o n a t e s o l u t i o n , the amount o f m a t e r i a l e x t r a c t e d was o n l y 69% which was no g r e a t e r than the maximum amount e x t r a c t e d a t 250°C. From p u b l i s h e d s o l u b i l i t y d a t a (9) f o r s i l i c a i n v a r i o u s a l k a l i n e s o l u t i o n s , one would expect the s o l u b i l i t y o f q u a r t z i n a sodium c a r b o n a t e s o l u t i o n to i n c r e a s e markedly w i t h temperature. Even though the amount o f s i l i c a e x t r a c t e d seemed t o be l i m i t e d by the room temperature s o l u b i l i t y l i m i t , t h i s l i m i t was r a i s e d by i n c r e a s i n g e i t h e r the a l k a l i c o n c e n t r a t i o n or the r a t i o o f a l k a l i t o quartz. Thus by i n c r e a s i n g the sodium c a r b o n a t e c o n c e n t r a t i o n from 1.0 M t o 2.0 M, the maximum amount e x t r a c t e d was r a i s e d from 70% t o 98.5%. Moreover, when the q u a n t i t y o f q u a r t z l e a c h e d f o r 1 h r . w i t h a 1.0 M s o l u t i o n a t 250°C was reduced from 2 g. t o 1 g., the p e r c e n t age e x t r a c t e d i n c r e a s e d from 58% t o 95%. Quartz was a l s o l e a c h e d f o r 1 h r . a t 250°C w i t h 2.0 M s o l u t i o n s of e i t h e r sodium h y d r o x i d e o r sodium b i c a r b o n a t e . I n each case 2.0 g. o f q u a r t z was l e a c h e d w i t h 120 ml. o f s o l u t i o n . Most o f the q u a r t z (99.5%) was c o n v e r t e d t o s o l u b l e sodium s i l i c a t e s and e x t r a c t ed by the hot sodium h y d r o x i d e s o l u t i o n . On the o t h e r hand, o n l y 10% of the q u a r t z was e x t r a c t e d by the sodium b i c a r b o n a t e s o l u t i o n , and the r e s i d u e appeared to be e n t i r e l y q u a r t z . These v a l u e s were n o t i c e a b l y d i f f e r e n t from the 58% e x t r a c t e d by 1.0 M sodium c a r b o n a t e under s i m i l a r c o n d i t i o n s . T h e r e f o r e , t h e s e s o l u t i o n s were not e q u i v a l e n t i n l e a c h i n g a b i l i t y even though each p r o v i d e d the same number o f moles o f sodium. None of the r e s i d u e s r e m a i n i n g a f t e r l e a c h i n g q u a r t z w i t h any o f the a l k a l i s were a c i d - s o l u b l e . In a second s e t o f e x p e r i m e n t s , -74 ym s i z e k a o l i n p a r t i c l e s were l e a c h e d w i t h h o t a l k a l i n e s o l u t i o n s t o s t u d y the c o n v e r s i o n o f k a o l i n i t e t o v a r i o u s sodium h y d r o a l u m i n o s i l i c a t e compounds ( T a b l e III) under d i f f e r e n t l e a c h i n g c o n d i t i o n s . I n each experiment, 15 g. of k a o l i n was l e a c h e d w i t h 120 ml. o f a l k a l i n e s o l u t i o n . The s o l i d r e a c t i o n p r o d u c t was r e c o v e r e d by f i l t r a t i o n , washed w i t h water, d r i e d i n an oven a t 95°C, and a n a l y z e d by XRD. A l t h o u g h t h i s method of a n a l y s i s i d e n t i f i e d the m i n e r a l s p r e s e n t , i t c o u l d p r o v i d e o n l y an approximate i n d i c a t i o n o f the r e l a t i v e p r o p o r t i o n s o f the v a r i o u s minerals present. The amount o f q u a r t z was p a r t i c u l a r l y d i f f i c u l t to a s s e s s because the method o f d e t e c t i o n was v e r y s e n s i t i v e t o t h i s mineral. T h e r e f o r e , the r e s u l t s a r e r e p o r t e d o n l y i n terms of major, minor, and t r a c e q u a n t i t i e s p r e s e n t i n the p r o d u c t as i n d i c a t e d by XRD ( T a b l e I V ) . Because the s m a l l amount o f t i t a n i a i n the k a o l i n was a p p a r e n t l y not a f f e c t e d by even the most r i g o r o u s l e a c h i n g c o n d i t i o n s , the p r o d u c t always c o n t a i n e d a t r a c e o f t h i s m a t e r i a l and no f u r t h e r mention seems n e c e s s a r y . When k a o l i n was l e a c h e d w i t h 1.0 M sodium c a r b o n a t e a t 200°C f o r 1 h r . , most o f the k a o l i n i t e was c o n v e r t e d t o the s o d a l i t e - t y p e natrodavyne (NS) w h i l e the q u a r t z and i l l i t e i m p u r i t i e s were not a f f e c t e d n o t i c e a b l y ( T a b l e I V ) . I n c r e a s i n g the l e a c h i n g temperature to 250°C r e s u l t e d i n the c o n v e r i o n o f the k a o l i n i t e t o a m i x t u r e o f mixed-type n a t r o d a v y n e (NCS) and a n a l c i m e (A) and complete d i s s o l u t i o n o f the q u a r t z i m p u r i t y . The i l l i t e i m p u r i t y was not a f f e c t e d . L e a c h i n g a t 300°C and above r e s u l t e d i n the c o n v e r s i o n o f the


468


Table I I I .

V a r i o u s Sodium H y d r o a l u m i n o s i l i c a t e s Produced K a o l i n i t e w i t h Hot A l k a l i n e S o l u t i o n s

Mineral Analcime


Hydroxycancrinite

i n Leaching

Symbol

Chemical formula N a 0 - A l i0 - 4 ( S i 0 ) •2(HO) 2 N a 0 - A l i0 - 2 ( S i O ) • (NaOH)-n(H 0) 2

2

3

2

2

3

2

2

3

2

A HC

Hydroxysodalite

(same as above)

HS

Natrodavyne (sodalite-type)

N a 0 - A l i0 - 2 ( S i 0 ) • ^ ( N a C 0 ) - n ( H 0 )

NS

Natrodavyne (cancrinite-type)

(same as above)

NC

Natrodavyne (mixed type)

(same as above)

NCS

2

2

3

2

2

3

2

k a o l i n i t e t o t h e c a n c r i n i t e - t y p e n a t r o d a v y n e (NC). Only a t 350°C d i d t h e i l l i t e i m p u r i t y appear t o be a f f e c t e d . A s i m i l a r t r e n d was o b s e r v e d when k a o l i n was l e a c h e d f o r 1 h r . at 250°C w i t h sodium c a r b o n a t e s o l u t i o n s o f d i f f e r e n t c o n c e n t r a t i o n s ( T a b l e I V ) . A t t h e l o w e s t c o n c e n t r a t i o n (0.2 Μ ) , p a r t o f t h e k a o l i n i t e was c o n v e r t e d t o t h e s o d a l i t e - t y p e n a t r o d a v y n e w h i l e t h e i m p u r i t i e s were untouched. A t t h e h i g h e s t c o n c e n t r a t i o n o f sodium c a r b o n a t e (2.0 Μ ) , t h e k a o l i n i t e was c o n v e r t e d t o t h e c a n c r i n i t e type n a t r o d a v y n e , and a l t h o u g h t h e q u a r t z was e x t r a c t e d , t h e i l l i t e remained. Somewhat s i m i l a r changes were o b s e r v e d when t h e l e a c h i n g time was v a r i e d w h i l e h o l d i n g t h e c o n c e n t r a t i o n o f sodium c a r b o n a t e a t 1.0 M and t h e temperature a t 250°C. W i t h a l e a c h i n g time o f 0.5 h r . the k a o l i n i t e was c o n v e r t e d t o a m i x t u r e o f a n a l c i m e and s o d a l i t e type n a t r o d a v y n e , whereas w i t h a l e a c h i n g time o f 2.0 h r . t h e k a o l i n i t e was l a r g e l y c o n v e r t e d t o t h e c a n c r i n i t e - t y p e n a t r o d a v y n e . Not a l l o f t h e q u a r t z was e x t r a c t e d when l e a c h i n g was conducted f o r 0.5 h r . , b u t a l l o f t h e q u a r t z appeared t o be removed when l e a c h i n g was conducted f o r l o n g e r p e r i o d s . However, t h e i l l i t e remained even after 2 h r . of leaching. When k a o l i n was l e a c h e d w i t h 1.0 M sodium h y d r o x i d e a t 250°C f o r 1 h r . , most o f t h e k a o l i n i t e appeared t o be c o n v e r t e d t o a n a l c i m e ( T a b l e I V ) . I n c r e a s i n g t h e c o n c e n t r a t i o n t o 2.0 M r e s u l t e d i n c o n v e r t i n g most o f t h e k a o l i n i t e t o h y d r o x y c a n c r i n i t e (HC). I n e i t h e r case t h e q u a r t z i m p u r i t y was e x t r a c t e d b u t t h e i l l i t e i m p u r i t y remained. L e a c h i n g k a o l i n w i t h 2.0 M sodium b i c a r b o n a t e a t 250°C for 1 h r . converted the k a o l i n i t e to the s o d a l i t e - t y p e natrodavyne, but i t had no apparent e f f e c t on t h e i m p u r i t i e s . To i n v e s t i g a t e t h e d i s s o l u t i o n o f t h e sodium h y d r o a l u m i n o s i l i c a t e s produced by a l k a l i n e l e a c h i n g , 3.0 g. p o r t i o n s o f t h e l e a c h e d p r o d u c t were t r e a t e d w i t h 300 m l . o f a c i d i n a s t i r r e d f l a s k f o r 30 min. E i t h e r 2.0 M h y d r o c h l o r i c a c i d o r 1.8 M s u l f u r i c a c i d was u t i l i z e d . W h i l e h y d r o c h l o r i c a c i d was always used a t t h e b o i l i n g p o i n t , s u l f u r i c a c i d was sometimes used a t t h e b o i l i n g p o i n t and sometimes a t room t e m p e r a t u r e . From XRD a n a l y s i s o f t h e s o l i d


34.

FAN ET AL.

Mineral

T a b l e IV.

Matter Behavior

Leaching

469

R e s u l t s o f L e a c h i n g 15 g. K a o l i n w i t h 120 m l . A l k a l i n e S o l u t i o n

Leaching conditions M Alkali °C hr.


during

M i n e r a l p r o d u c t s e x c l u d i n g TiO, type, (amount) a

Na C0 2

3

1.0

200

1.0

NS(maj), S i O ( m a j ) , k a o l i n i t e ( m i n ) , illite(tr)

Na C0 2

3

1.0

250

1.0

NCS(maj), A ( m a j ) ,

Na C0 2

3

1.0

300

1.0

NC(maj), A ( m i n ) ,

Na C0 2

3

1.0

350

1.0

NC(maj)

Na C0 2

3

0.2

250

1.0

S i O ( m a j ) , k a o l i n i t e ( m i n ) , NS(min) illite(tr)

Na C0 2

3

0.5

250

1.0

S i 0 ( m a j ) , NS(maj),

Na C0

?

illite(tr) illite(tr)

?

illite(tr)

2

3

1.0

250

1.0


Na C0 2

3

2.0

250

1.0

NC(maj),

Na C0 2

3

1.0

250

0.5

A ( m a j ) , NS(maj), S i 0 ( m i n ) ,

Na C0 2

3

1.0

250

1.0


Na C0

3

1.0

250

2.0

NC(maj), A ( m i n ) ,

illite(tr)

NaOH

1.0

250

1.0

A ( m a j ) , HS(min),

illite(tr)

NaOH

2.0

250

1.0

HC(maj), A ( m i n ) ,

illite(tr)

2.0

250

1.0

NS(maj), S i 0 ( m a j ) ,

2

2

NaHC0

3

Amount:

illite(tr)

illite(tr)

2

2

illite(tr)

illite(tr)

illite(tr)

maj = major q u a n t i t y , min = minor q u a n t i t y , t r = t r a c e quantity



470


r e s i d u e r e m a i n i n g a f t e r t h e a c i d t r e a t m e n t , i t was found t h a t a l l o f the s o d a l i t e - t y p e and c a n c r i n i t e - t y p e compounds were d i s s o l v e d by the a c i d s whether h o t o r c o l d . On t h e o t h e r hand, a n a l c i m e was c o m p l e t e l y d i s s o l v e d o n l y by t h e b o i l i n g a c i d s . I n a t h i r d s e t o f e x p e r i m e n t s , -38 ym p y r i t e p a r t i c l e s were l e a c h e d w i t h h o t a l k a l i n e s o l u t i o n s t o study t h e c o n v e r s i o n o f i r o n p y r i t e t o i r o n o x i d e and s o l u b l e s u l f u r s p e c i e s . I n each experiment, 5 g. o f a c i d - c l e a n e d p y r i t e was l e a c h e d w i t h 120 m l . o f a l k a l i n e s o l u t i o n f o r 1 h r . I n a d d i t i o n t o a n a l y z i n g t h e s o l i d r e s i d u e by XRD, t h e t o t a l s u l f u r c o n t e n t o f t h e l e a c h a t e was d e t e r m i n e d i n o r d e r to e s t i m a t e p y r i t e c o n v e r s i o n . When p y r i t e was l e a c h e d f o r 1 h r . w i t h 1.0 M sodium c a r b o n a t e a t 250°C, o n l y 12.7% o f t h e p y r i t e was c o n v e r t e d t o i r o n o x i d e and s o l u b l e s u l f u r s p e c i e s ( T a b l e V ) . I n c r e a s i n g t h e l e a c h i n g temp e r a t u r e t o 300°C r a i s e d t h e c o n v e r s i o n t o 26.4%, and i n c r e a s i n g t h e temperature t o 350°C r a i s e d t h e c o n v e r s i o n t o 44.8%. I n each case the s o l i d r e s i d u e c o n s i s t e d p r i n c i p a l l y o f h e m a t i t e and u n r e a c t e d pyrite. I n c r e a s i n g t h e sodium c a r b o n a t e c o n c e n t r a t i o n , w h i l e m a i n t a i n i n g the l e a c h i n g time a t 1 h r . and temperature a t 250°C, i n c r e a s e d t h e p y r i t e c o n v e r s i o n o n l y s l i g h t l y ( T a b l e V ) . On t h e o t h e r hand, when p y r i t e was l e a c h e d w i t h 2.0 M sodium h y d r o x i d e a t 300°C f o r 1 h r . , a c o n v e r s i o n o f 62% was a c h i e v e d . A g a i n , h e m a t i t e appeared t o be t h e p r i n c i p a l s o l i d r e a c t i o n product. When t h e p y r i t e r e s i d u e from t h e a l k a l i n e l e a c h i n g s t e p was t r e a t e d with a c i d , a l l of the hematite d i s s o l v e d i n e i t h e r hot hydroc h l o r i c a c i d or hot s u l f u r i c a c i d . However, the h e m a t i t e appeared t o d i s s o l v e incompletely or very slowly i n c o l d s u l f u r i c a c i d . U n r e a c t e d p y r i t e was n o t touched by t h e a c i d s whether h o t o r c o l d . Behavior

of M i n e r a l Matter

During

Coal

Leaching

F o l l o w i n g the preceding experiments with i n d i v i d u a l m i n e r a l s , f u r t h e r c o n s i d e r a t i o n was g i v e n t o t h e b e h a v i o r o f t h e m i n e r a l m a t t e r i n c o a l

T a b l e V.

Alkali

R e s u l t s o f L e a c h i n g 5 g. A c i d - c l e a n e d P y r i t e w i t h Alkaline Solution for 1 hr.

Leaching conditions Cone., M Temp., °C

Conv., %

120 m l .

Residue

Na C0 2

3

1.0

250

12.7

FeS ,

Fe 0

3

Na C0 2

3

1.0

300

26.4

FeS ,

Fe 0

3

Na C0 2

3

1.0

350

44.8

FeS ,

Fe 0

3

Na C0 2

3

1.0

250

12.7

FeS ,

Fe 0

3

Na C0

3

2.0

250

14.5

3.0

250

15.8

2.0

300

62.0

FeS ,

Fe 0

3

2

N a

2

C 0

NaOH

3

2

2

2

2

2


2

2

2

2

2


34.

Mineral

FAN ET AL.

Matter

Behavior

during

Leaching

471

i t s e l f d u r i n g l e a c h i n g . The c o a l was examined by XRD a f t e r b o t h t h e a l k a l i n e l e a c h i n g s t e p and t h e a c i d / w a t e r washing s t e p . Leaching I l l i n o i s No. 6 c o a l w i t h 2 M sodium b i c a r b o n a t e a t 250°C f o r 1 h r . appeared t o c o n v e r t e s s e n t i a l l y a l l o f t h e k a o l i n i t e to natrodavyne and a n a l c i m e and a s m a l l p o r t i o n o f t h e i r o n p y r i t e t o h e m a t i t e w h i l e not g r e a t l y a f f e c t i n g t h e q u a r t z and c a l c i t e . However, i t appeared t h a t some o f t h e q u a r t z was c o n v e r t e d t o a n a l c i m e s i n c e t h i s m i n e r a l has a h i g h e r r a t i o o f s i l i c a t o alumina than i s p r e s e n t i n k a o l i n i t e . F o l l o w i n g t h e subsequent a c i d washing s t e p , t h e r e m a i n i n g m i n e r a l m a t t e r c o n s i s t e d m o s t l y o f q u a r t z and p y r i t e . L e a c h i n g I l l i n o i s No. 6 c o a l w i t h 1 M sodium c a r b o n a t e under s i m i l a r c o n d i t i o n s a l s o c o n v e r t e d t h e k a o l i n i t e t o natrodavyne and a n a l c i m e and p a r t o f t h e p y r i t e t o h e m a t i t e ; t h e q u a r t z was e i t h e r c o n v e r t e d t o s o l u b l e sodium s i l i c a t e s or analcime. A f t e r washing t h e a l k a l i - t r e a t e d c o a l w i t h a c i d , p y r i t e seemed t o c o n s t i t u t e most o f t h e r e m a i n i n g m i n e r a l matter. L e a c h i n g I l l i n o i s No, 6 c o a l w i t h 2 M sodium h y d r o x i d e a t 250°C f o r 1 h r . seemed t o c o n v e r t a l l o f t h e k a o l i n i t e t o h y d r o x y c a n c r i n i t e and a l a r g e p o r t i o n o f t h e p y r i t e t o h e m a t i t e ; most o f t h e q u a r t z was e x t r a c t e d o r c o n v e r t e d . A f t e r washing t h e c o a l w i t h a c i d , o n l y a s m a l l amount o f p y r i t e remained. These r e s u l t s were g e n e r a l l y i n good agreement w i t h those observed i n t h e l e a c h i n g o f i n d i v i d u a l minerals. Conclusions Most o f t h e m i n e r a l m a t t e r was removed from a r e p r e s e n t a t i v e c o a l ( I l l i n o i s No. 6) by f i r s t l e a c h i n g t h e f i n e l y ground m a t e r i a l w i t h a h o t a l k a l i n e s o l u t i o n and then washing t h e p r o d u c t w i t h a m i n e r a l a c i d and w a t e r . The e f f e c t i v e n e s s o f v a r i o u s a l k a l i s f o r c o n v e r t i n g or e x t r a c t i n g m i n e r a l m a t t e r d e c r e a s e d i n t h e f o l l o w i n g o r d e r : NaOH > Na C03 NaHC03. C o a l r e c o v e r y , on t h e o t h e r hand, i n c r e a s e d i n t h e same o r d e r . E x a m i n a t i o n o f t h e c o a l by XRD a f t e r each s t e p and a study o f t h e b e h a v i o r o f i n d i v i d u a l m i n e r a l s when s u b j e c t e d t o t h e same t r e a t m e n t i n d i c a t e d good agreement i n r e s u l t s . In other words, t h e r e s u l t s were g e n e r a l l y s i m i l a r r e g a r d l e s s o f whether t h e m i n e r a l s were l e a c h e d i n d i v i d u a l l y o r i n a m i x t u r e w i t h c o a l . The r e s u l t s o f l e a c h i n g s e v e r a l o f t h e most p r e v a l e n t m i n e r a l s i n c o a l on an i n d i v i d u a l b a s i s a r e summarized below. I t was shown t h a t f i n e - s i z e q u a r t z p a r t i c l e s r e a d i l y d i s s o l v e i n 1-2 M sodium c a r b o n a t e o r sodium h y d r o x i d e a t 250°C. However, when q u a r t z i s e x t r a c t e d by a h o t s o l u t i o n which i s then c o o l e d , an amorphous, a c i d - i n s o l u b l e m a t e r i a l i s produced under some c o n d i t i o n s . A l i k e l y p o s s i b i l i t y i s t h a t amorphous s i l i c a p r e c i p i t a t e s when t h e room temperature s o l u b i l i t y l i m i t o f t h e m a t e r i a l i s exceeded. F o r m a t i o n o f t h e amorphous m a t e r i a l i s p r e v e n t e d by u s i n g h i g h e r concentrations of a l k a l i or a higher r a t i o of a l k a l i to s i l i c a . I t was a l s o shown t h a t k a o l i n i t e r e a c t s w i t h h o t a l k a l i n e s o l u t i o n s t o form v a r i o u s sodium h y d r o a l u m i n o s i l i c a t e s which a r e a c i d soluble. The p a r t i c u l a r sodium h y d r o a l u m i n o s i l i c a t e formed depends on t h e type o f a l k a l i employed, t h e a l k a l i c o n c e n t r a t i o n , t h e t r e a t ment temperature, and l e n g t h o f t r e a t m e n t . When sodium c a r b o n a t e i s employed, k a o l i n i t e i s l a r g e l y c o n v e r t e d t o t h e s o d a l i t e - t y p e natrodavyne under l e s s r i g o r o u s c o n d i t i o n s and t o t h e c a n c r i n i t e - t y p e natrodavyne under more r i g o r o u s c o n d i t i o n s . Under i n t e r m e d i a t e >

2


472

MINERAL MATTER AND

ASH IN COAL

t r e a t m e n t c o n d i t i o n s , the mixed-type n a t r o d a v y n e and a n a l c i m e a r e p r o d u c e d . When sodium h y d r o x i d e i s employed, k a o l i n i t e i s l a r g e l y c o n v e r t e d t o a n a l c i m e and h y d r o x y c a n c r i n i t e w i t h t h e l a t t e r b e i n g f a v o r e d by h i g h e r a l k a l i c o n c e n t r a t i o n s . I t was f u r t h e r shown t h a t i r o n p y r i t e r e a c t s w i t h hot a l k a l i n e s o l u t i o n s to form h e m a t i t e and s o l u b l e s u l f u r s p e c i e s . Sodium h y d r o x i d e i s c o n s i d e r a b l y more e f f e c t i v e than sodium c a r b o n a t e f o r t h i s r e a c t i o n . However, the e f f e c t i v e n e s s o f sodium c a r b o n a t e s o l u t i o n s can be i n c r e a s e d by i n c r e a s i n g the t r e a t m e n t t e m p e r a t u r e and to a l e s s e r e x t e n t by i n c r e a s i n g the a l k a l i c o n c e n t r a t i o n . The h e m a t i t e produced i s r e a d i l y d i s s o l v e d by h o t m i n e r a l a c i d s .

Literature Cited


1.

2. 3. 4.

5. 6. 7. 8. 9.

Crawford, A. The de-ashing of coal by combined j i g washing, froth flotation, and extraction with caustic soda, BIOS Final Report No. 522, Item No. 30, Feb. 19, 1946, British Intelligence Objectives Sub-Committee, London, (A.T.I.-118668, Central Air Documents Office, Wright-Patterson Air Force Base, Dayton, Ohio). Reggel, L.; Raymond, R.; Wender, I.; Blaustein, B. D. Am. Chem. Soc. Div. of Fuel Chem. Preprints 1972, 17(1), 44. Reggel, L.; Raymond, R.; Blaustein, B. D. U.S. Patent 3 993 455, 1976. Stambaugh, E. P. In "Coal Desulfurization: Chemical and Physical Methods"; Wheelock, T. D., Ed.; ACS Symposium Series No. 64, American Chemical Society: Washington, D.C., 1977; pp. 198-205. Stambaugh, E. P.; Sachsel, G. F. U.S. Patent 4 055 400, 1977. Yang, R. T.; Das, S. K.; Tsai, B. M. C. Fuel 1985, 64, 735-742. Fan, C.-W.; Markuszewski, R.; Wheelock, T. D. Am. Chem. Soc. Div. of Fuel Chem. Preprints 1984, 29(1), 114. American Society for Testing and Materials. "Annual Book of ASTM Standards"; Part 26 (Methods D 3174 and D 3177), Philadelphia, PA, 1975. Iler, R. K. "The Chemistry of Silica"; John Wiley and Sons: New York, 1979.

RECEIVED August 12, 1985


Behavior of Quartz, Kaolinite, and Pyrite during Alkaline Leaching of

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