Desulfurizing Coal with Alkaline Solutions Containing Dissolved

15 Desulfurizing Coal with Alkaline Solutions Containing Dissolved Oxygen

Downloaded by UNIV LAVAL on June 28, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0064.ch015

C. Y. TAI,* G. V. GRAVES, and T. D. WHEELOCK Iowa State University, Department of Chemical Engineering and Nuclear Engineering, Energy and Mineral Resources Research Institute, Ames, IA 50011 The extraction of pyritic sulfur from coal by leaching the comminuted material with hot aqueous solutions containing dissolved oxygen has been demonstrated in numerous laboratory experiments (1-6). Although acidic solutions have generally been used for such experiments, basic solutions appear to offer several important advantages. Thus Majima and Peters (7) showed that the rate of extraction of sulfur from relatively pure pyrite is much greater in basic solutions containing dissolved oxygen than in neutral solutions. Moreover it has been shown recently that basic solutions containing ammonium hydroxide and oxygen can extract a significant portion of the organic sulfur as well as most of the inorganic sulfur from coal at relatively moderate temperatures (e.g., 130°C) (4,5) whereas higher temperatures (150°-200°C) seem to be required with acidic solutions to remove organic sulfur (6). Furthermore some types of basic solutions are much less corrosive towards the common materials of construction than acidic solutions. A l t h o u g h the chemical kinetics and mechanism o f pyrite r e a c tion with an o x y g e n - b e a r i n g caustic solution have been studied, t h e y a r e n o t c o m p l e t e l y d e f i n e d . S t e n h o u s e and A r m s t r o n g ( 8 ) f o u n d s u l f a t e i o n s and i r o n o x i d e s t o be t h e f i n a l p r o d u c t s o f reaction. B o t h Fe2Û3 and Fe3Û4 were i d e n t i f i e d i n t h e r e s i d u e . I t a p p e a r e d t o t h e s e i n v e s t i g a t o r s t h a t t h e i r o n o x i d e s formed a s t a b l e l a y e r around t h e u n r e a c t e d p y r i t e . As a r e s u l t o f a l a t e r i n v e s t i g a t i o n , B u r k i n and Edward ( 9 ) c o n c l u d e d t h a t t h e f i n a l o x i d a t i o n p r o d u c t o f i r o n i s maghemite (yFe203) w h i c h i s formed t h r o u g h a s e r i e s o f t o p o t a c t i c t r a n s f o r m a t i o n s . These i n v e s t i g a t o r s o b s e r v e d t h a t t h e r a t e o f a t t a c k was more r a p i d a l o n g g r a i n b o u n d a r i e s and c r a c k s i n impure and i m p e r f e c t p y r i t e c r y s t a l s , w i t h t h e r e s u l t t h a t t h e s e p a r t i c l e s were l e a c h e d more r a p i d l y t h a n p a r t i c l e s c o n t a i n i n g few i m p e r f e c t i o n s and i m p u r i t i e s ,

*Present address: T a i w a n , R.O.C.

Chung-Shen I n s t i t u t e o f S c i e n c e and T e c h n o l o g y ,

182

In Coal Desulfurization; Wheelock, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


15.

TAi E T A L .

Alkaline

Solutions

with Dissolved

Oxygen

183

even though t h e i n s o l u b l e o x i d a t i o n p r o d u c t adhered t o t h e imper f e c t c r y s t a l s b u t n o t t o t h e p e r f e c t o n e s . As a r e s u l t o f t h e i n t e r g r a n u l a r a t t a c k by t h e l e a c h s o l u t i o n , t h e s m a l l e r c r y s t a l s w i t h i n p a r t i c l e s composed o f d i f f e r e n t - s i z e d c r y s t a l s were d e s u l f u r i z e d b e f o r e t h e l a r g e r c r y s t a l s . The r e a c t i o n r a t e i n c r e a s e d w i t h temperature a c c o r d i n g t o the A r r h e n i u s e q u a t i o n w i t h an a c t i v a t i o n energy o f 9 k c a l / m o l . A l s o t h e r e a c t i o n r a t e was c o n t o l l e d by t h e o x y g e n c o n c e n t r a t i o n a t t h e m i n e r a l - s o l u t i o n i n t e r face so t h e r a t e i n c r e a s e d w i t h the oxygen p a r t i a l p r e s s u r e . In a d d i t i o n , the r a t e increased with i n c r e a s i n g concentration o f s o d i u m h y d r o x i d e up t o 2 wt %. H i g h e r c o n c e n t r a t i o n s r e d u c e d t h e rate s l i g h t l y . I n a s o l u t i o n o f some b a s e s u c h a s s o d i u m h y d r o x i d e , t h e s t o i c h i o m e t r y o f t h e r e a c t i o n o f p y r i t e w i t h o x y g e n and t h e s u b s e q u e n t n e u t r a l i z a t i o n o f t h e a c i d p r o d u c e d c a n p r o b a b l y be r e p r e s e n t e d by t h e f o l l o w i n g e q u a t i o n s : FeS

2

+ ^| 0

2

+ 2 H 0 = y F e ^ + 2 H SC> 2

2

4

2 H S 0 . + 4 NaOH = 2 N a S 0 , + 4 H 0 2 4 2 4 2 o

o

o

(1) (2)

A l t h o u g h these e q u a t i o n s s u g g e s t t h a t the main purpose o f t h e a l k a l i i s t o n e u t r a l i z e t h e a c i d and t o d r i v e t h e f i r s t r e a c t i o n t o c o m p l e t i o n , t h e a c t u a l r e a c t i o n mechanism i s p r o b a b l y more c o m p l e x w i t h t h e a l k a l i p l a y i n g a more s u b t l e r o l e a s w e l l a s t h e o b v i o u s one. An i n v e s t i g a t i o n by McKay a n d H a l p e r n ( 1 0 ) o f t h e r e a c t i o n o f p y r i t e w i t h o x y g e n i n a c i d i c s o l u t i o n s showed t h a t t h e p r o d u c t d i s t r i b u t i o n , and t h e r e f o r e t h e r e a c t i o n mechanism, i s q u i t e d i f f e r e n t f r o m t h a t n o t e d above f o r b a s i c s o l u t i o n s . Thus p y r i t e was c o n v e r t e d t o s o l u b l e f e r r o u s and f e r r i c s u l f a t e , s u l f u r i c a c i d , and e l e m e n t a l s u l f u r when l e a c h e d w i t h o x y g e n - b e a r i n g a c i d i c s o l u t i o n s a t 100°-130°C. The p r o d u c t d i s t r i b u t i o n depended on t e m p e r a t u r e a n d a c i d i t y , w i t h a l o w e r t e m p e r a t u r e and h i g h e r a c i d i t y f a v o r i n g t h e p r o d u c t i o n o f e l e m e n t a l s u l f u r a n d a h i g h e r tem p e r a t u r e and l o w e r a c i d i t y f a v o r i n g t h e p r o d u c t i o n o f s u l f u r i c a c i d and s o l u b l e s u l f a t e s . A l s o a t m o d e r a t e pH ( b u t n o t a t l o w ρΗ), t h e f e r r i c h y d r o x i d e h y d r o l y z e d , and f e r r i c o x i d e p r e c i p i tated. The A r r h e n i u s a c t i v a t i o n e n e r g y was d e t e r m i n e d t o be 13 k c a l / m o l , w h i c h , b e i n g s i g n i f i c a n t l y l a r g e r t h a n t h a t n o t e d above for the reaction of p y r i t e i na caustic s o l u t i o n , indicates a g r e a t e r e n e r g y b a r r i e r f o r t h e r e a c t i o n i n a n a c i d i c medium. The p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n t o e v a l u a t e t h e t e c h n i c a l f e a s i b i l i t y o f e x t r a c t i n g s u l f u r from c o a l w i t h h o t oxygen-bearing s o l u t i o n s . S i n c e i t was s o o n c o n f i r m e d t h a t a l k a l i n e o r b a s i c s o l u t i o n s r e s u l t e d i n a much g r e a t e r r a t e o f e x t r a c t i o n t h a n a c i d i c s o l u t i o n s , c o n s i d e r a b l e e m p h a s i s was p l a c e d on d e t e r m i n i n g t h e e f f e c t i v e n e s s o f d i f f e r e n t a l k a l i s a n d a l k a l i concentrations. Several h i g h - s u l f u r bituminous c o a l s , as w e l l as


184

COAL

DESULFURIZATION

p y r i t e w h i c h had been i s o l a t e d f r o m one o f t h e s e c o a l s , were l e a c h e d i n a s m a l l s t i r r e d r e a c t o r under a v a r i e t y of e x p e r i mental c o n d i t i o n s . In a d d i t i o n the o v e r - a l l e f f e c t i v e n e s s of c h e m i c a l l e a c h i n g i n c o m b i n a t i o n w i t h p h y s i c a l b e n e f i c i a t i o n was investigated.


Experimental L e a c h i n g e x p e r i m e n t s were c a r r i e d o u t i n a 1-L, s t a i n l e s s s t e e l a u t o c l a v e e q u i p p e d w i t h a r e m o v a b l e n i c k e l l i n e r and an a g i t a t o r c o n s i s t i n g o f two p i t c h e d b l a d e t u r b i n e i m p e l l e r s (7.3 cm d i a m e t e r ) mounted on a s h a f t d r i v e n by a c o m p r e s s e d a i r m o t o r . The a u t o c l a v e was a l s o e q u i p p e d w i t h a p r e s s u r e gauge, t e m p e r a t u r e i n d i c a t o r , and e l e c t r i c h e a t i n g j a c k e t . A s u s p e n s i o n o f c o a l o r p y r i t e p a r t i c l e s i n an a l k a l i n e s o l u t i o n o f a s p e c i f i c c o n c e n t r a t i o n was p l a c e d i n t h e r e a c t o r w h i c h was t h e n s e a l e d and h e a t e d t o a s p e c i f i c o p e r a t i n g t e m p e r a ture. A f t e r t h e s y s t e m a t t a i n e d t h i s t e m p e r a t u r e , o x y g e n was introduced f r o m a h i g h - p r e s s u r e c y l i n d e r e q u i p p e d w i t h a comb i n a t i o n p r e s s u r e - r e d u c i n g v a l v e and r e g u l a t o r . D u r i n g t h e s u c c e e d i n g o p e r a t i o n , t h e s y s t e m p r e s s u r e was k e p t c o n s t a n t by s u p p l y i n g o x y g e n on demand. A l s o a s m a l l amount o f gas was b l e d c o n t i n u o u s l y f r o m t h e r e a c t o r t o a v o i d any b u i l d - u p o f g a s e o u s r e a c t i o n p r o d u c t s i n the s y s t e m . R e a c t i o n c o n d i t i o n s were k e p t c o n s t a n t f o r a s p e c i f i c t i m e w h i l e t h e m a t e r i a l was l e a c h e d . At t h e end o f t h e r e a c t i o n p e r i o d , the f l o w o f o x y g e n i n t o t h e s y s t e m was s t o p p e d , and t h e s y s t e m was c o o l e d . Throughout the o p e r a t i o n t h e a g i t a t o r was k e p t r u n n i n g a t a c o n s t a n t speed w h i c h was u s u a l l y i n the r a n g e o f 250-350 rpm. A f t e r a run the l e a c h e d s o l i d s were r e c o v e r e d , d r i e d , and w e i g h e d . C o a l s a m p l e s w e r e a n a l y z e d by t h e s t a n d a r d ASTM methods o f a n a l y s i s ( 1 1 ) . When p y r i t e a l o n e was l e a c h e d , t h e amount o f s u l f u r e x t r a c t e d was d e t e r m i n e d by m e a s u r i n g t h e s u l f a t e c o n t e n t o f t h e s p e n t l e a c h a n t u s i n g t h e s u l f a t e t i t r a t i o n method o f F r i t z and Yamaraura ( 1 2 ) . C o a l s a m p l e s f r o m t h e B i g Ben, ICO, and ISU Demonstration M i n e No. 1, a l l l o c a t e d i n S o u t h e a s t e r n Iowa, were l e a c h e d . The rank of the c o a l from these mines i s h i g h v o l a t i l e C b i t u m i n o u s c o a l ; t y p i c a l c o m p o s i t i o n s a r e shown i n T a b l e I . S i n c e t h e c o a l from these mines i s v e r y heterogeneous, v a r i o u s batches used f o r s p e c i f i c s e t s o f e x p e r i m e n t s were r e - a n a l y z e d and the c o m p o s i t i o n reported along with other experimental r e s u l t s . A h a n d p i c k e d sample o f i r o n p y r i t e s was o b t a i n e d f r o m t h e c o a l d e p o s i t a t ISU D e m o n s t r a t i o n M i n e No. 1. T h i s m a t e r i a l was v e r y i m p u r e and seemed t o c o n t a i n a b o u t 74% FeS2» Nevertheless, i t was g r o u n d and s c r e e n e d i n t o v a r i o u s s i z e d f r a c t i o n s w h i c h were u s e d s u b s e q u e n t l y f o r v a r i o u s l e a c h i n g e x p e r i m e n t s . The s u l f u r c o n t e n t o f t h e d i f f e r e n t s i z e f r a c t i o n s i s shown i n T a b l e II. I n a d d i t i o n t o p y r i t i c s u l f u r , t h e m a t e r i a l c o n t a i n e d some s u l f u r i n t h e f o r m o f s u l f a t e s and some i n a f o r m w h i c h was n e i t h e r p y r i t i c nor s u l f a t e .


15.

TAi E T A L .

Alkaline

Solutions

with Dissolved

185

Oxygen

T a b l e I . P r o x i m a t e A n a l y s i s o f Iowa C o a l s

Proximate A n a l y s i s

Source o f C o a l


Big

Ben mine

Volatile Matter

Fixed Carbon

(wt %)

Ash

Moisture

37.5

48.8

1.2

12.5

ICO mine

44.0

42.0

4.5

9.5

ISU mine

38.1

44.2

2.2

15.5

Table I I .

Composition of Iron

Pyrites

Composition Size Fraction (U.S. S e r i e s Mesh)

Iron

Pyritic Sulfur

Sulfate Sulfur

(wt %) Total Sulfur

41.7

-60/+80 39.,2

0.,5

40.9

-200/+230

36.,4

0.,7

40.3

-230/+270

36.,3

0..7

39.4

-120/+140

Other

42.5

16.6

P y r i t e Leaching Experiments 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 o u t t o d e t e r mine t h e e f f e c t o f v a r i o u s s y s t e m p a r a m e t e r s on t h e e x t r a c t i o n o f s u l f u r f r o m impure p y r i t e . F o r each experiment a s u s p e n s i o n cons i s t i n g o f 2.0 g o f p y r i t e p a r t i c l e s and 500 m l o f a n aqueous s o l u t i o n was t r e a t e d i n t h e s t i r r e d a u t o c l a v e a t 150°C w i t h o x y g e n d i s s o l v e d under a p a r t i a l p r e s s u r e o f 3.27 atm. S i n c e t h e v a p o r p r e s s u r e o f w a t e r a t t h i s t e m p e r a t u r e i s 4.56 atm, t h e t o t a l s y s t e m p r e s s u r e was 7.8 atm. U n l e s s i n d i c a t e d o t h e r w i s e , t h e l e a c h i n g o p e r a t i o n under o x y g e n p r e s s u r e a t t h e s p e c i f i e d t e m p e r a t u r e was c o n d u c t e d f o r 1 h r . The c o n v e r s i o n o f a l l forms o f s u l f u r i n t h e p a r t i c l e s t o s o l u b l e s u l f a t e was d e t e r m i n e d by a n a l y z ing the spent l e a c h a n t . The r e s u l t s p r e s e n t e d i n F i g u r e 1 i n d i c a t e t h a t among t h e v a r i o u s a l k a l i s t e s t e d , s o d i u m c a r b o n a t e was t h e most e f f e c t i v e . E s s e n t i a l l y a l l o f t h e s u l f u r was e x t r a c t e d f r o m -200/+230 mesh



186

C O A L DESULFURIZATION

p y r i t e p a r t i c l e s i n 1.0 h r by an o x y g e n a t e d s o l u t i o n c o n t a i n i n g 1.5 wt % s o d i u m c a r b o n a t e . The b e s t r e s u l t s o b t a i n e d w i t h e i t h e r s o d i u m h y d r o x i d e o r s o d i u m p h o s p h a t e were s l i g h t l y l e s s f a v o r a b l e and w i t h ammonium c a r b o n a t e much l e s s f a v o r a b l e . The ammonium c a r b o n a t e s o l u t i o n s were n o t i c e a b l y l e s s b a s i c t h a n t h e o t h e r s o l u t i o n s and t h i s f a c t o r p r o b a b l y a c c o u n t e d f o r t h e p o o r e r r e s u l t s o b t a i n e d w i t h them. However, t h e ammonium c a r b o n a t e s o l u t i o n s , as w e l l as t h e o t h e r a l k a l i n e s o l u t i o n s , a l l gave b e t t e r r e s u l t s t h a n w a t e r and o x y g e n a l o n e . Thus o n l y 27% o f the s u l f u r was e x t r a c t e d f r o m t h e p y r i t e d u r i n g a r u n made w i t h o u t any alkali. The e f f e c t i v e n e s s o f a l l o f the a l k a l i s i n c r e a s e d w i t h i n i t i a l c o n c e n t r a t i o n up t o some optimum v a l u e and t h e n d e c r e a s e d . F o r s o d i u m h y d r o x i d e t h e optimum i n i t i a l c o n c e n t r a t i o n was a b o u t 1 wt %, f o r s o d i u m c a r b o n a t e 1.5 wt %, f o r s o d i u m p h o s p h a t e 3 wt %, and f o r ammonium c a r b o n a t e 4 wt %. The s h a r p i n c r e a s e i n c o n v e r s i o n f o r i n i t i a l c o n c e n t r a t i o n s a t the low end o f the c o n c e n t r a t i o n s c a l e seems t o be r e l a t e d t o t h e amount o f a l k a l i r e q u i r e d to n e u t r a l i z e a l l of the a c i d produced i n o x i d i z i n g the pyrite. Thus i f i t i s assumed t h a t a l l o f the s u l f u r i n t h e s o l i d s was c o n v e r t e d i n t o s u l f u r i c a c i d d u r i n g t h e l e a c h i n g o p e r a t i o n , i t w o u l d have r e q u i r e d an i n i t i a l s o d i u m h y d r o x i d e c o n c e n t r a t i o n o f 0.40 wt % t o n e u t r a l i z e a l l o f t h e a c i d p r o duced. I n t e r e s t i n g l y enough, t h e d a t a p r e s e n t e d i n F i g u r e 1 show t h a t t h e c o n v e r s i o n f e l l o f f s h a r p l y when l e s s t h a n 0.40 wt % c a u s t i c was u s e d . The amount o f s o d i u m c a r b o n a t e r e q u i r e d t o n e u t r a l i z e t h e a c i d depends on t h e f i n a l p r o d u c t s o f n e u t r a l i z a t i o n . Two p o s s i b i l i t i e s a r e shown b e l o w . H S0. + N a C 0 2 4 2 3 o

o

o

= Na S0. + C0 + H 0 2 4 2 2 o

o

(3)

o

H S 0 . + 2 N a C 0 = N a S 0 . + 2 NaHC0 2 4 2 3 2 4 3 o

o

o

o

o

(4)

The f i r s t r e a c t i o n w o u l d have r e q u i r e d an i n i t i a l s o d i u m c a r b o n a t e c o n c e n t r a t i o n o f 0.53 wt % t o n e u t r a l i z e a l l o f t h e s u l f u r i c a c i d w h e r e a s t h e s e c o n d r e a c t i o n w o u l d have r e q u i r e d an i n i t i a l c o n c e n t r a t i o n o f 1.1 wt %. The e x p e r i m e n t a l r e s u l t s ( F i g u r e 1) i n d i c a t e t h a t t h e c o n v e r s i o n f e l l o f f s h a r p l y when l e s s t h a n a b o u t 1 wt % s o d i u m c a r b o n a t e was u s e d . I n t h e c a s e o f s o d i u m p h o s p h a t e an i n i t i a l c o n c e n t r a t i o n o f 1.65 wt % w o u l d have been r e q u i r e d t o n e u t r a l i z e a l l o f the p o s s i b l e s u l f u r i c a c i d a c c o r d i n g to the f o l l o w i n g r e a c t i o n : H S 0 . + 2 N a P 0 . = N a S 0 . + 2 Na HP0. 2 4 3 4 2 4 2 4 o

o

o

o

(5)

A l t h o u g h the e x p e r i m e n t a l r e s u l t s shown i n F i g u r e 1 i n d i c a t e t h a t the c o n v e r s i o n f e l l o f f s h a r p l y between 2 and 3 wt %, the s p a r s i t y o f t h e d a t a l e a v e c o n s i d e r a b l e d o u b t as t o the e x a c t c r i t i c a l



15.

TAI E T A L .

Alkaline

Solutions

with Dissolved

Oxygen

187

concentration. Why t h e c o n v e r s i o n d e c l i n e d a t 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 w h i c h e x c e e d e d t h e optimum l e v e l s c a n n o t be e x p l a i n e d w i t h c e r tainty. A s i m i l a r d e c l i n e was o b s e r v e d by B u r k i n and Edwards ( 9 ) when p y r i t e was l e a c h e d w i t h c a u s t i c s o l u t i o n s , a n d t h e y a t t r i b u t ed t h e d e c l i n e t o the r e d u c e d s o l u b i l i t y o f o x y g e n i n t h e s e s o l u tions. A l s o s i m i l a r b e h a v i o r was o b s e r v e d i n l e a c h i n g b o t h g a l e n a and m o l y b d e n i t e w i t h c a u s t i c s o l u t i o n s c o n t a i n i n g d i s s o l v e d o x y g e n (13,14). I n the case o f both m i n e r a l s , a d e c l i n e i n r e a c t i o n r a t e w i t h i n c r e a s i n g a l k a l i c o n c e n t r a t i o n was a t t r i b u t e d t o t h e d e c r e a s i n g s o l u b i l i t y o f oxygen. The r e s u l t s o f l e a c h i n g d i f f e r e n t s i z e d f r a c t i o n s o f p y r i t e w i t h sodium carbonate 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 a r e p r e s e n t e d i n F i g u r e 2. C l e a r l y t h e e f f e c t o f p a r t i c l e s i z e o n c o n v e r s i o n was v e r y p r o n o u n c e d w i t h t h e c o n v e r s i o n i n c r e a s i n g i n an a l m o s t e x p o n e n t i a l manner a s t h e s i z e was r e d u c e d ( F i g u r e 2 ) . P a r t i c l e s s m a l l e r t h a n 60 ym i n d i a m e t e r were c o m p l e t e l y d e s u l f u r i z e d i n 1.0 h r by s o l u t i o n s c o n t a i n i n g 2-5 wt % s o d i u m c a r b o n a t e f o r t h e c o n d i t i o n s shown i n F i g u r e 2. On t h e o t h e r h a n d , p a r t i c l e s l a r g e r t h a n 210 ym i n d i a m e t e r were o n l y a b o u t h a l f d e s u l f u r i z e d u n d e r t h e s e c o n d i t i o n s . F i g u r e 2 a l s o shows t h e advantage o f u s i n g an a l k a l i n e s o l u t i o n . The l o w e r c u r v e i n t h i s d i a g r a m r e p r e s e n t e d t h e c o n v e r s i o n o b t a i n e d when no a l k a l i was added t o t h e s y s t e m . Even w i t h t h e s m a l l e s t p a r t i c l e s o f p y r i t e , o n l y 3 5 % o f t h e s u l f u r was e x t r a c t e d i n the a b s e n c e o f a n a l k a l i . S e v e r a l r u n s were made u s i n g l o n g e r r e a c t i o n t i m e s t o s e e how the c o n v e r s i o n w o u l d change w i t h t i m e ( F i g u r e 3 ) . The r e s u l t s i n d i c a t e t h a t t h e r e a c t i o n r a t e d e c r e a s e d a s the b a t c h l e a c h i n g o p e r a t i o n p r o c e e d e d . The d e c l i n i n g r a t e c o u l d have been c a u s e d by t h e d e c r e a s i n g a v a i l a b i l i t y o f u n r e a c t e d p y r i t e , i n c r e a s i n g resistance to diffusion of reactants within p a r t i c l e s , increasing c o n c e n t r a t i o n o f some r a t e - i n h i b i t i n g r e a c t i o n p r o d u c t i n t h e l e a c h s o l u t i o n , o r a combination o f these f a c t o r s . Coal Leaching

Experiments

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 o u t t o d e t e r mine t h e e f f e c t s o f v a r i o u s a l k a l i s and a l k a l i c o n c e n t r a t i o n s on t h e d e s u l f u r i z a t i o n o f c o a l f r o m t h e I S U D e m o n s t r a t i o n M i n e No. 1. The raw c o a l was p u l v e r i z e d s o t h a t 9 0 % was f i n e r t h a n 200 mesh. F o r e a c h e x p e r i m e n t 50 g o f c o a l and 500 ml o f w a t e r o r a l k a l i n e s o l u t i o n were p l a c e d i n t h e s t i r r e d a u t o c l a v e . The c h e m i c a l t r e a t m e n t was c a r r i e d o u t f o r 2 h r a t 150°C w i t h o x y g e n d i s s o l v e d u n d e r a p a r t i a l p r e s s u r e o f 3.27 atm and u s i n g a t o t a l s y s t e m p r e s s u r e o f 7.8 atm. The t r e a t e d c o a l was r e c o v e r e d , d r i e d , w e i g h e d , and a n a l y z e d t o d e t e r m i n e t h e e f f e c t i v e n e s s o f t h e t r e a t ment . The b e s t o v e r a l l r e s u l t s were o b t a i n e d w i t h s o d i u m c a r b o n a t e among t h e f o l l o w i n g a l k a l i s w h i c h were t e s t e d i n v a r i o u s c o n c e n trations: sodium carbonate, sodium h y d r o x i d e , sodium phosphate,



188

COAL DESULFURIZATION

PYRITE -200/+23O MESH 7.8 atm. TOTAL PRESS. 150 °C 1.0 h r .

Figure 1. The effect of different alkalis and alkali concentrations on the extraction of sulfur from pyrite

Figure 2. The effect of particle size on the extraction of sulfur from pyrite

J 2

4

L

6

8

10

INITIAL CONCENTRATION, wt. %

100

150

200

PYRITE PARTICLE SIZE, microns


250


15.

TAI E T A L .

Alkaline

Solutions

with Dissolved

Oxygen

189

c a l c i u m c a r b o n a t e , c a l c i u m h y d r o x i d e , and ammonium c a r b o n a t e . The o n l y a l k a l i w h i c h a p p r o a c h e d s o d i u m c a r b o n a t e i n e f f e c t i v e n e s s was s o d i u m p h o s p h a t e . However, i t r e q u i r e d a 7 wt % i n i t i a l c o n c e n t r a t i o n o f sodium phosphate t o produce c o a l w i t h a s u l f u r c o n t e n t c o m p a r a b l e t o t h a t o b t a i n e d w i t h a 1.4 wt % i n i t i a l c o n c e n t r a t i o n of sodium carbonate. M o r e o v e r , t h e B t u r e c o v e r y was l o w e r and t h e a s h c o n t e n t o f t h e p r o d u c t h i g h e r when s o d i u m p h o s p h a t e was used to o b t a i n c o m p a r a b l e d e s u l f u r i z a t i o n . The i n i t i a l c o n c e n t r a t i o n o f s o d i u m c a r b o n a t e had a v e r y p r o nounced e f f e c t on t h e e x t e n t o f d e s u l f u r i z a t i o n and t h e r e s u l t i n g s u l f u r c o n t e n t o f t h e t r e a t e d c o a l ( T a b l e I I I , F i g u r e 4 ) . The b e s t r e s u l t s were o b t a i n e d w i t h an i n i t i a l c o n c e n t r a t i o n o f 1.4 wt % and u s i n g e i t h e r l o w e r o r h i g h e r c o n c e n t r a t i o n s r e s u l t e d i n l e s s desulfurization. The t r e a t m e n t w i t h t h e optimum c o n c e n t r a t i o n r e duced t h e p y r i t i c s u l f u r c o n t e n t o f t h e c o a l by t w o - t h i r d s . A s i m i l a r e f f e c t o f c o n c e n t r a t i o n was o b s e r v e d when s o d i u m h y d r o x i d e was u s e d w i t h the optimum i n i t i a l c o n c e n t r a t i o n b e i n g a b o u t 1 wt %. Hence, t h e e f f e c t o f a l k a l i c o n c e n t r a t i o n was l i k e t h a t e x p e r i e n c e d in leaching pyrite (Figure 1 ) . Under t h e m i l d l e a c h i n g c o n d i t i o n s used i n t h i s w o r k , m a i n l y i n o r g a n i c s u l f u r was e x t r a c t e d f r o m t h e c o a l . Essentially a l l o f t h e s u l f a t e f o r m o f s u l f u r was e x t r a c t e d a n d a good s h a r e o f the p y r i t i c s u l f u r . Unfortunately the r e s u l t s reported i n Table I I I f o r o r g a n i c s u l f u r a r e so e r r a t i c t h a t i t i s n o t p o s s i b l e t o d i s c e r n w h e t h e r o r g a n i c s u l f u r was removed o r n o t . For the e x p e r i m e n t s i n v o l v i n g sodium c a r b o n a t e , the y i e l d o f d r y c o a l was 93-94%, and t h e B t u r e c o v e r y was 88-92%. These v a l u e s c o u l d have been h i g h e r w i t h t h e e x e r c i s e o f g r e a t e r c a r e in conducting the experiments. Since the heating value o f the t r e a t e d c o a l on a m o i s t u r e and a s h f r e e (maf) b a s i s was n e a r l y t h e same as t h a t o f t h e u n t r e a t e d c o a l , i t does n o t a p p e a r t h a t t h e c o a l was s i g n i f i c a n t l y d e g r a d e d by t h e t r e a t m e n t . A l s o the oxygen c o n t e n t o f t h e c o a l , a s d e t e r m i n e d by u l t i m a t e c h e m i c a l a n a l y s i s , d i d n o t a p p e a r t o be s i g n i f i c a n t l y d i f f e r e n t f r o m t h a t o f t h e untreated coal. These r e s u l t s c o n t r a s t m a r k e d l y w i t h t h o s e r e p o r t e d b y S a r e e n e t a l . ( 3 , 4 ) f o r t r e a t m e n t s u s i n g much h i g h e r oxygen p a r t i a l p r e s s u r e s which r e s u l t e d i n n o t i c e a b l e oxygen upt a k e by I l l i n o i s No. 6 c o a l . As a r e s u l t o f t h e a l k a l i t r e a t m e n t , t h e s o d i u m and a s h c o n t e n t o f t h e c o a l i n c r e a s e d m e a s u r a b l y ( T a b l e I I I , F i g u r e 5 ) . The i n c r e a s e i n a s h c o n t e n t was d i r e c t l y p r o p o r t i o n a l t o t h e i n c r e a s e i n s o d i u m c o n t e n t and seemed c a u s e d a l m o s t e n t i r e l y by t h e a d s o r p t i o n o f s o d i u m . Most o f t h e i n c r e a s e i n s o d i u m o r a s h c o n t e n t t o o k p l a c e a t l o w e r a l k a l i c o n c e n t r a t i o n s , and more t h a n 2 wt % sodium carbonate i n t h e l e a c h a n t g e n e r a l l y produced l i t t l e a d d i tional increase. I n o t h e r w o r d s , t h e c o a l seemed t o become s a t u r a t e d when t h e l e a c h s o l u t i o n c o n t a i n e d 2 wt % s o d i u m c a r b o n a t e o r more. On t h e o t h e r hand, t h e c o a l r e t a i n e d v e r y l i t t l e s o d i u m when t h e l e a c h s o l u t i o n i n i t i a l l y c o n t a i n e d 0.8 wt % s o d ium c a r b o n a t e o r l e s s b e c a u s e t h e s o l u t i o n became a c i d i c b y t h e



92

91

93

93

94

94

94

2.0

5.0

8.0

10.0

15.0

chemical

chemical

chemical

chemical

89

90

88

11,010

10,940

10,950

10,590

10,880

10,580

10,780

18.5 19.4 19.7 18.2

— — —

17.8

17.7

13,800

13,520

13,520

16.4

chemical

91

89

—

93

1.4

93

1.0

chemical

10,740

16.3

chemical

90

—

94

0.8

16.2

13,430

chemical

10,950

94

0.8

chemical

92

17.5

—

10,440

88

0

chemical

95

15.3

15.5

13,970

13,660

maf

1.52

1.90

—

1.29

1.14

0.80

0.33

0.10

0.10

—

0.03

0.05

Ash Na ( w t . % ) (wt %)

11,577

as r e e v d .

Heat Value ( B t u / l b )

wash

Btu Recov. (%) 11,240

Wt Yield (%)

none

Treatment

Na2CC>3 Cone. (wt %)

0.17

3.,21

2.,34

1.,63

1.,76

1.,75

1.,23

1.,11

1.,46

1.,89

1.,62

0.18

0.11

0.09

0.11

0.07

0.06

0.05

0.03

0.05

0.11

0.60

2.,28

(lb/10

6

Btu)

3.66 1.80

2.65

2.28

5.17

4.02

4.24

3.15

1.85

2.39

2.99

3.90 1.81

2.39

4.23

3.47

1.80 2.32

5.29

5.63

5.63

2.89

2.25

1.81

S u l f a t e Organic Total

3.,22

Pyritic

Sulfur Distrib.

T a b l e I I I . R e s u l t s o f L e a c h i n g ISU C o a l w i t h D i f f e r e n t C o n c e n t r a t i o n s o f A l k a l i



AL.

Alkaline Solutions tvith Dissolved Oxygen

191

PYRITE -120/+Ί40

7.8

MESH

atm.

150 °C

2

3

Figure 3. The effect of reaction time on the extraction of sulfur from pyrite

REACTION T I M E , h r .

-200 7.8

4 INITIAL

Figure 4.

6

8

SODIUM CARBONATE

10

MESH I S U COAL a t m . TOTAL P R E S S .

150° C 2.0 h r . I 12

CONCENTRATION,

I

WT. %

Sulfur content of IS U coal after leaching



192

COAL

DESULFURIZATION

end o f a r u n . W a s h i n g w i t h w a t e r d i d n o t remove s o d i u m f r o m the t r e a t e d c o a l , b u t w a s h i n g w i t h d i l u t e a c i d (0.1N hydrochloric a c i d ) d i d remove i t . A n o t h e r s e r i e s o f e x p e r i m e n t s was c o n d u c t e d t o see what e f f e c t r e d u c i n g the s i z e o f t h e c o a l w o u l d have on d e s u l f u r i z a t i o n by c h e m i c a l l e a c h i n g . ISU c o a l was f i r s t p u l v e r i z e d to -35 mesh ( a b o u t 50% b e l o w 200 mesh) w i t h a M i k r o - S a m p l m i l l ( P u l v e r i z i n g Machinery D i v i s i o n , A m e r i c a n - M a r i e t t a Co.). The c o a l was t h e n wet-ground i n a ceramic j a r m i l l f o r d i f f e r e n t time p e r i o d s . The p a r t i c l e s i z e d i s t r i b u t i o n o f the c o a l a f t e r g r i n d i n g f o r 2, 4, 8, and 20 h r i s shown i n F i g u r e 6. A f t e r g r i n d i n g f o r 2 h r , 97% o f t h e c o a l was f i n e r t h a n 400 mesh (38 ym) s i z e . The p u l v e r i z e d o r g r o u n d c o a l was t h e n l e a c h e d i n t h e s t i r r e d a u t o c l a v e f o r 2 h r a t 140°C w i t h a 2.0 wt % s o l u t i o n o f s o d i u m c a r b o n a t e c o n t a i n i n g o x y g e n d i s s o l v e d u n d e r a p a r t i a l p r e s s u r e o f 2.4 atm and w i t h a t o t a l s y s t e m p r e s s u r e o f 5.8 atm. F o r e a c h l e a c h i n g r u n 25 g o f c o a l was m i x e d w i t h 500 ml o f s o l u t i o n . The r e s u l t s p r e s e n t e d i n T a b l e IV show t h a t more s u l f u r was e x t r a c t e d f r o m c o a l w h i c h had been ground i n t h e b a l l m i l l t h a n f r o m c o a l w h i c h had o n l y been p u l v e r i z e d w i t h the M i k r o - S a m p l m i l l . Thus t h e p y r i t i c s u l f u r c o n t e n t o f t h e b a l l - m i l l e d c o a l was r e d u c e d 87% w h e r e a s t h e p y r i t i c s u l f u r c o n t e n t o f t h e p u l v e r i z e d c o a l was r e d u c e d o n l y 71%. However, g r i n d i n g t h e c o a l f o r more t h a n 2 h r d i d n o t p r o duce any a d d i t i o n a l b e n e f i t . Combined P h y s i c a l and

Chemical

Cleaning

A f i n a l s e t o f e x p e r i m e n t s i n v o l v e d combined p h y s i c a l and c h e m i c a l c l e a n i n g o f s e v e r a l Iowa c o a l s . Each c o a l was c r u s h e d t o 6 mm χ 0 s i z e w i t h a d o u b l e r o l l c r u s h e r and t h e n c l e a n e d by g r a v i t y s e p a r a t i o n i n an o r g a n i c l i q u i d h a v i n g a s p e c i f i c g r a v i t y o f 1.60. The c l e a n e d f l o a t f r a c t i o n was r e c o v e r e d , d r i e d , and p u l v e r i z e d w i t h the M i k r o - S a m p l m i l l . The p u l v e r i z e d c o a l was wetg r o u n d n e x t i n t h e c e r a m i c j a r m i l l f o r 20 h r and f i n a l l y l e a c h e d i n the s t i r r e d a u t o c l a v e f o r 2.0 h r w i t h a 2.0 wt % s o d i u m c a r b o n a t e s o l u t i o n . The e f f e c t s o f d i f f e r e n t l e a c h i n g t e m p e r a t u r e s and o x y g e n p a r t i a l p r e s s u r e s were s t u d i e d . The r e s u l t s p r e s e n t e d i n T a b l e V show t h a t g r a v i t y s e p a r a t i o n removed a s u b s t a n t i a l p a r t o f the p y r i t i c s u l f u r f r o m e a c h c o a l and t h a t c h e m i c a l l e a c h i n g removed most o f the r e m a i n i n g i n o r g a n i c s u l f u r , b o t h p y r i t i c and s u l f a t e . Thus t h r o u g h t h e combined t r e a t m e n t , the i n o r g a n i c s u l f u r c o n t e n t of each of the t h r e e c o a l s was r e d u c e d 93-95%. The t r e a t e d ICO c o a l came c l o s e to m e e t i n g the f e d e r a l new s o u r c e p e r f o r m a n c e s t a n d a r d o f 0.6 l b S/10 Btu. F o r the p h y s i c a l l y c l e a n e d and f i n e l y g r o u n d c o a l , i t d i d n o t seem to make any d i f f e r e n c e what l e a c h i n g t e m p e r a t u r e i n t h e r a n g e o f 120°-150°C o r what o x y g e n p a r t i a l p r e s s u r e i n t h e r a n g e o f 2.4-5.2 atm was u s e d b e c a u s e e s s e n t i a l l y a l l o f the i n o r g a n i c s u l f u r was removed even u n d e r the m i l d e s t t r e a t m e n t c o n d i t i o n s . 6


ET

AL.

Alkaline Solutions with Dissolved Oxygen


TAI


193


chemical

3-•108-8 b.mill (8hr) 11,237

11,070

b.mill

chemical

3-•108-6

(4 h r )

11,500

b.mill (2hr)

chemical

3-•108-7

11,360

11,040

H.V. (Btu/lb)

pulverized

pulverized

Grind

chemical

none

Treatment

16.2

16.6

14.7

15.8

12.0

Ash (wt %)

0.34

0.30

0.29

0.67

2.31

Pyritic

0.05

0.05

0.04

2.06

2.17

2.10

2.32

2.19

0.54 0.05

Organic

Sulfate

Sulfur Distribution (lb/10

R e s u l t s o f T r e a t i n g D i f f e r e n t G r i n d s o f ISU C o a l

3-•108-5

Run No.

Table IV.


b

2.46

2.51

2.43

3.05

5.04

Total

Btu


B i g Ben B i g Ben B i g Ben B i g Ben

ICO ICO ICO ICO ICO ICO ICO

ISU ISU ISU ISU ISU

Source

25 25

—

40 40 40 25 25

—

40 40 20

—

Amt.(g)

Coal

none float/sink chemical chemical

none float/sink chemical chemical chemical chemical chemical

none float/sink chemical chemical chemical

Treatment

T a b l e V.

— 2.9 3.2

132 150

2.7 3.7 5.2 3.2 3.2

120 120 120 150 150

—

—

5.8 7.8

—

4.7 5.8 7.3 7.8 7.8

—

4.4 5.8 6.9

2.4 2.7 2.4

Total

—

2

(atm.)

—

0

Pressure

—

120 135 150

—

iemp. (°C)

11,800 12,790 10,950 11,680

12,040 12,690 11,630 11,480 11,640 11,220 11,740

10,990 12,390 11,650 11,490 10,770

η. ν . (Btu/lb)

10.6 10.0

— —

11.6 11.6 10.8 14.1 13.2

— —

Ι.37 Ο,.42 0,.09 0.16

0.,37 0.,28 0..03 0.,04

0.,25 0.,20 0.,02 0.,02 0.,02 0.,03 0.,04

0.,33 0.,23 0.,06 0.,07 0.,04

3.,96 1,,38 0,,13 0.,18 0..21 1,,66 0.,70 0,.10 0,.15 0,.09 0,.14 0.11

Sulfate

%) P y r i t i c

12.2 12.1 24.0

—

(wt

Sulfur Distrib.

R e s u l t s o f Combined P h y s i c a l and C h e m i c a l T r e a t m e n t


,53 ,41 ,34 ,32 ,04

0..61 0,.81 0,.89 0.81

0.,62 0.,64 0.,58 0.,64 0.,60 0.,56

0. ,61

2. 2. 2. 2. 2.

2.36 1.47 1.00 1.01

2.52 1.53 0.76 0.75 0.74 0.77 0.71

6.81 4.02 2.53 2.57 2.29

Organic Total

(lb/10° B t u )

196

COAL DESULFURIZATION

T h e r e f o r e , i t s h o u l d have b e e n p o s s i b l e t o o b t a i n a s good r e s u l t s even w i t h m i l d e r treatment c o n d i t i o n s o r a s h o r t e r r e a c t i o n time.


Conclusions D i l u t e a l k a l i n e s o l u t i o n s c o n t a i n i n g oxygen d i s s o l v e d u n d e r p r e s s u r e were v e r y e f f e c t i v e a t e l e v a t e d t e m p e r a t u r e s f o r e x t r a c t i n g t h e i n o r g a n i c s u l f u r f r o m c o a l . Of v a r i o u s a l k a l i s t e s t e d , s o d i u m c a r b o n a t e gave t h e b e s t r e s u l t s . T h i s m a t e r i a l a l s o has the advantages o f b e i n g r e a d i l y a v a i l a b l e , low i n c o s t , and r e l a t i v e l y n o n c o r r o s i v e i n aqueous s o l u t i o n t o w a r d s s t e e l and o t h e r common m a t e r i a l s o f c o n s t r u c t i o n . Although s u f f i c i e n t a l k a l i s h o u l d be u s e d t o n e u t r a l i z e a l l o f t h e a c i d w h i c h i s p r o d u c e d t h r o u g h o x i d a t i o n o f t h e c o a l s u l f u r , an e x c e s s i v e c o n c e n t r a t i o n o f a l k a l i seems t o s l o w t h e r a t e o f d e s u l f u r i z a t i o n . On t h e o t h e r h e a d , t h e r a t e o f d e s u l f u r i z a t i o n c a n be i n c r e a s e d by r e d u c i n g t h e s i z e o f t h e c o a l a n d / o r p y r i t e p a r t i c l e s . Leach i n g c o a l f i n e s w i t h d i l u t e sodium carbonate s o l u t i o n s a t tempera t u r e s up t o 150°C and w i t h o x y g e n p a r t i a l p r e s s u r e s up t o 5 atm f o r up t o 2 h r does n o t seem t o d e g r a d e h i g h v o l a t i l e b i t u m i n o u s coal. However, t h e c o a l does a d s o r b s o d i u m f r o m t h e l e a c h s o l u t i o n w h i c h c a n be removed s u b s e q u e n t l y by w a s h i n g w i t h d i l u t e acid. C h e m i c a l l e a c h i n g c a n be combined a d v a n t a g e o u s l y w i t h p h y s i c a l c l e a n i n g s i n c e t h e l a t t e r i s more a d e p t a t r e m o v i n g c o a r s e r p a r t i c l e s o f p y r i t e w h i l e t h e f o r m e r i s more a d e p t a t removing the microscopic p a r t i c l e s . Ac k n o w l e d gemen t T h i s work was s p o n s o r e d by t h e Iowa C o a l P r o j e c t and c o n d u c t e d i n t h e E n e r g y and M i n e r a l R e s o u r c e s R e s e a r c h I n s t i t u t e a t Iowa S t a t e U n i v e r s i t y .

Literature Cited

1. Nelson, H. W., Snow, R. D., Keyes, D. B., Ind. Eng. Chem. (1933) 25, 1355. 2. Agarwal, J. C., Giberti, R. Α., Irminger, P. F., Petrovic, L. J., Sareen, S. S., Min. Congr. J. (1975) 61 (3), 40. 3. Sareen, S. S., Giberti, R. Α., Irminger, P. F., Petrovic, L. J., Preprint 50c, 80th National Meeting AIChE, (Sep. 7-10, 1975), Boston. 4. Agarwal, J. C., Giberti, R. Α., Petrovic, L. J., U.S. Patent No. 3,960,513, 1976. 5. Sareen, S. S., This volume, in press. 6. Friedman, S., LaCount, R. B., Warzinski, R. P., This volume, in press. 7. Majima, Η., Peters, Ε., Trans. Metall. Soc. AIME (1966) 1409. 8. Stenhouse, J. E., Armstrong, W. M., Can. Min. Metall. Bull. (1952) 45, 49.


15.

TAI

ET

AL.

Alkaline

Solutions

with

Dissolved

Oxygen

197


9. Burkin, A. R., Edwards, Α. Μ., in "Mineral Processing," (A. Roberts, Ed.), pp. 159-169, Pergamon, Oxford, 1965. (Procedding of the Sixth International Congress held at Cannes, May 26-June 2, 1963.) 10. McKay, D. R., Halpern, J., Trans. Metall. Soc. AIME (1958) 301. 11. Book ASTM Stand., Part 26, "Gaseous Fuels; Coal and Coke; Atmospheric Analysis," Methods D2015-66, D2492-68, D3174-73, D3177-75, 1976. 12. Fritz, J. S., Yamamura, S. S., Anal. Chem. (1955) 27, 1461. 13. Anderson, J. E., Halpern, J., Samis, C. S., J. Met. (1953) 5, 554. 14. Dresher, W. H., Wadsworth, M. E., Fassell, W., Jr., J. Met. (1956) 8, 794.


Desulfurizing Coal with Alkaline Solutions Containing Dissolved

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