Effect of Caustic and Microwave Treatment on Clay ... - ACS Publications

38 Effect of Caustic and Microwave Treatment on Clay Minerals Associated with Coal C . K. Richardson, R. Markuszewski, K. S. Durham, and D . D . Bluhm Ames Laboratory and Department of Earth Sciences, Iowa State University, Ames,

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch038

IA 50011

Clay minerals typical of those occurring in coal (kaolinite, illite, and montmorillonite) were treated with 30 or 50% aqueous NaOH at room temperature for varying periods of time and then were either heated in a drying oven at 105°C or irradiated in a microwave unit for up to 3 minutes. The products were later washed with water or with 10% HCl. These experiments were performed to test and evaluate a chemical coal cleaning process based on reacting coal with aqueous NaOH, irradiating with microwave energy, and washing with acid to reduce the sulfur and ash content. X-ray diffraction analyses showed that no new reaction products were formed when these clay minerals were treated with NaOH solutions at room temperature. Heating in a drying oven at 105°C formed hydroxysodalite from kaolinite and a zeolite-like mineral from montmorillonite. Samples irradiated in the microwave unit underwent the greatest changes. Hydroxysodalite-hydroxycancrinite mixtures formed from kaolinite, nepheline formed from illite, and the montmorillonite dehydrated. The results suggest that the clay minerals selectively absorbed microwave energy and were heated to temperatures above the boiling point of the solution. The three clay minerals absorbed microwave energy to different degrees. I l l i t e appeared to reach a higher temperature than either kaolinite or montmorillonite. The mineral products became more sodium-rich and less hydrated with increased microwave exposure time, suggesting that increasingly higher temperatures were attained. Acid washing of the treated samples resulted in the removal of a l l or part of the hydroxysodalite-hydroxycancrinite mixture and part of the nepheline. Because of the significant breakdown of clay minerals during the caustic and microwave treatment, this technique is promising for the removal of ash-forming minerals from coal. 0097-6156/ 86/ 0301 -0513S06.00/ 0 © 1986 American Chemical Society

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

514

MINERAL MATTER AND ASH IN COAL


One o f the many p r o c e s s e s b e i n g c o n s i d e r e d f o r c h e m i c a l l y removing ash and s u l f u r from c o a l i n v o l v e s treatment of the c o a l w i t h concen t r a t e d (30-50%) aqueous NaOH s o l u t i o n s , i r r a d i a t i n g the c o a l w i t h microwave energy, and then washing the c o a l w i t h water and a c i d . P r e v i o u s s t u d i e s at the G e n e r a l E l e c t r i c Company by Z a v i t s a n o s et a l . (1-6) have shown t h a t such a p r o c e s s can remove up t o 80-90% of the s u l f u r and 40-50% o f the ash c o n t e n t of the c o a l . The p r e s ent study was i n i t i a t e d t o t r y t o determine the c h e m i c a l and miner a l o g i c a l changes t h a t o c c u r i n c l a y m i n e r a l s (commonly found i n c o a l ) d u r i n g the a l k a l i p r e t r e a t m e n t , d r y i n g , microwave i r r a d i a t i o n , and a c i d and water washing s t e p s of the microwave p r o c e s s f o r c l e a n ing c o a l . A s e a r c h o f the l i t e r a t u r e y i e l d e d no o t h e r work on microwave treatment o f c l a y m i n e r a l s . Numerous s t u d i e s , however, have l o o k e d i n t o the s o l u b i l i t y o f v a r i o u s c l a y m i n e r a l s i n a c i d s o l u t i o n s (7-10) and i n a l k a l i n e s o l u t i o n s (7,8,11-13), i n c l u d i n g many s t u d i e s on the Bayer p r o c e s s . The s o l u b i l i t y s t u d i e s suggest t h a t , i n gen e r a l , aqueous a c i d s o l u t i o n s remove the a l k a l i m e t a l s , a l k a l i n e e a r t h m e t a l s , and i r o n and aluminum from c l a y m i n e r a l s (7). Alka l i n e s o l u t i o n s , on the o t h e r hand, p r e f e r e n t i a l l y remove s i l i c a from c l a y m i n e r a l s ( 8 ) . A c i d s o l u t i o n s a p p a r e n t l y f i r s t remove the ex changeable i n t e r l a y e r c a t i o n s from the c l a y s , and then they a t t a c k the o c t a h e d r a l l a y e r s . A l k a l i n e s o l u t i o n s a t t a c k the t e t r a h e d r a l l a y e r s , and i f p r o l o n g e d a t t a c k o c c u r s , the s t r u c t u r e o f the c l a y i s d e s t r o y e d (8) . Fan et a l . (11,12) s t u d i e d the r e a c t i o n between c l a y m i n e r a l s and aqueous sodium c a r b o n a t e (1-3.0 M) and sodium h y d r o x i d e (1.0 M) i n s e a l e d a u t o c l a v e s at 250°C. A l t h o u g h the c o n d i t i o n s of these experiments were q u i t e d i f f e r e n t from those employed i n the p r e s e n t microwave p r o c e s s , they d i d show t h a t p r o l o n g e d high-temper a t u r e a t t a c k on c l a y m i n e r a l s by a l k a l i n e s o l u t i o n s caused them t o b r e a k down and form a s e r i e s o f hydrous sodium aluminum s i l i c a t e m i n e r a l s which c o u l d be removed by a c i d t r e a t m e n t . In the p r e s e n t work, i t was e x p e c t e d t h a t the m i n e r a l matter c o u l d absorb microwave energy and be t h e r e f o r e h e a t e d t o temperatures i n excess o f 100°C. Thus, some c l a y a l t e r a t i o n r e a c t i o n s were expected t o o c c u r at e l e v a t e d t e m p e r a t u r e s , and, i n the presence o f c a u s t i c , r e a c t i o n p r o d ucts — perhaps s i m i l a r t o those seen i n p r e v i o u s a u t o c l a v e e x p e r i ments — were e x p e c t e d t o form. Experimental The c l a y m i n e r a l s used i n t h i s study were k a o l i n i t e ( A l S i 0 ( O H ) ^ ) , illite (K Al Si ( 0 H ) ) , and m o n t m o r i l l o n i t e which was approx i m a t e l y 1/2 (Ca, N a ) ( A l , Mg, F e ^ i S i , A D Q O . Q (OH)^ · η 1^0. The c l a y s were n a t u r a l samples purchased from Ward s N a t u r a l S c i e n c e E s t a b l i s h m e n t , I n c . The k a o l i n i t e sample was o b t a i n e d from a k a o l i n d e p o s i t i n G e o r g i a . X-ray d i f f r a c t i o n (XRD) p a t t e r n s on t h i s mate r i a l showed peaks o n l y f o r k a o l i n i t e , and s c a n n i n g e l e c t r o n m i c r o scope-energy d i s p e r s i v e x-ray (SEM-EDX) a n a l y s i s y i e l d e d peaks f o r A l , S i , and minor amounts o f T i . The i l l i t e was a green s h a l e from New York which Ward's l i s t e d as 85% i l l i t e . XRD p a t t e r n s of t h i s 2

1 - x

3

3 +

2

2

0 7


5


38.

RICHARDSON ET AL.

Caustic and Microwave

Treatment

515

m a t e r i a l c o n t a i n e d peaks f o r i l l i t e and q u a r t z , and SEM-EDX a n a l y s i s showed p r e s e n c e o f S i , A l , K, F e , and some Ca. The m o n t m o r i l l o n i t e sample was a powdered b e n t o n i t e from Wyoming. XRD p a t t e r n s showed peaks f o r m o n t m o r i l l o n i t e and an i l l i t e - l i k e m a t e r i a l , and SEM-EDX a n a l y s i s showed S i , A l , moderate F e , and s m a l l Ca and Κ peaks. A l l m a t e r i a l s were i n i t i a l l y ground t o a powder. Aqueous s o l u t i o n s o f 30% o r 50% NaOH were p r e p a r e d from d i s t i l l e d , d e i o n i z e d water and reagent grade NaOH. The experiments were s e t up i n such a way t h a t t h e a l k a l i / m i n e r a l r a t i o s , r e a c t i o n t i m e s , d r y i n g temperature, and a c i d c o n c e n t r a t i o n were s i m i l a r t o those t o be used l a t e r i n t h e c l e a n i n g o f c o a l w i t h microwave i r r a d i a t i o n . I n t h e GE procedure d e s c r i b e d by Z a v i t s a n o s et a l . ( 1 - 6 ) , aqueous NaOH was added t o c o a l t o make a s l u r r y w i t h an a l k a l i / c o a l r a t i o o f 0.3 - 2.0. T h e i r s l u r r y had an i n i t i a l m o i s t u r e c o n t e n t o f about 40% which was lowered t o 20% by h e a t i n g t h e s l u r r y i n a d r y i n g oven. Then the d r i e d s l u r r y was i r r a d i a t e d f o r 1 minute under i n e r t atmosphere i n a 2.45-GHz m i c r o wave apparatus at 1.0 o r 2.0 kW. The m i x t u r e was then washed w i t h water and a c i d ( e i t h e r 10% HC1 o r 10% 1^ SO^) t o remove NaOH and acid-soluble products. The p r e s e n t m i n e r a l experiments were d e s i g n e d t o l o o k at t h e m i n e r a l o g i c a l changes t h a t o c c u r as c l a y components a r e s u b j e c t e d t o each s t e p i n t h i s p r o c e s s . The microwave i r r a d i a t i o n s t e p i n our e x p e r i m e n t s , however, was not e n t i r e l y comparable t o t h a t i n the GE experiments because a s m a l l e r , 500-watt u n i t was used w h i l e t h e 2.0-kW u n i t was b e i n g c o n s t r u c t e d . F u t u r e experiments i n o u r l a b o r a t o r y w i l l m o n i t o r c l a y r e a c t i o n s at h i g h e r microwave power l e v e l s . A p p r o x i m a t e l y e q u a l amounts ( u s u a l l y 5 grams) o f the c l a y min e r a l o r c l a y m i x t u r e and 5 grams o f 30% o r 50% NaOH s o l u t i o n ( a l k a l i / c l a y r a t i o o f 0.3 and 0.5, r e s p e c t i v e l y ) were p l a c e d i n a beak er. Three d i f f e r e n t types o f experiments were then performed: 1) The c l a y m i n e r a l o r m i x t u r e was r e a c t e d w i t h t h e NaOH s o l u t i o n a t room temperature f o r v a r y i n g p e r i o d s o f time, then f i l t e r e d and water washed t o remove excess NaOH, and f i n a l l y a i r dried. 2) The c l a y m i n e r a l o r m i x t u r e was r e a c t e d w i t h NaOH f o r about 20 minutes and then p l a c e d i n a d r y i n g oven a t 105°C f o r 5 t o 30 minutes t o reduce the m o i s t u r e c o n t e n t . The sample was then removed from t h e oven, f i l t e r e d , water washed, and air dried. In a few experiments the c a u s t i c - t r e a t e d c l a y m i x t u r e was f i l t e r e d t o remove excess m o i s t u r e and then p l a c e d i n the d r y i n g oven; t h e sample was a g a i n f i l t e r e d and water washed. These two s e t s o f experiments e v a l u a t e d the e f f e c t o f the treatment s t e p s p r i o r t o microwave i r r a d i a tion. 3) The c l a y m i n e r a l o r m i x t u r e was r e a c t e d w i t h t h e aqueous NaOH s o l u t i o n f o r about 20 minutes at room temperature and t h e n was i r r a d i a t e d i n a Microwave D r y i n g / D i g e s t i o n System (Model MDS-81, CEM C o r p o r a t i o n , P. 0. Box 9, I n d i a n T r a i l , N.C. 28079) at 500 watts (2.45 GHz) f o r 30 seconds t o 3 minutes. The sample was then f i l t e r e d , water washed, and air dried.


516


A l l the samples were a n a l y z e d by s t a n d a r d XRD t e c h n i q u e s . After i n i t i a l XRD a n a l y s e s o f the t r e a t e d samples were completed, many o f the samples were a c i d washed w i t h 10% HC1 and r e - a n a l y z e d t o d e t e r mine whether any a c i d - s o l u b l e p r o d u c t s were removed. Results


The r e s u l t s o f experiments on the i n d i v i d u a l c l a y m i n e r a l s a r e summarized i n Tables I - I I I . K a o l i n i t e i s u n s t a b l e i n a l k a l i n e (30% NaOH, a l k a l i / c l a y = 0.3) s o l u t i o n s (7,14) and b e g i n s t o break down even a t room temperature. The f i r s t change noted i n the XRD p a t t e r n s i n T a b l e I was t h a t the k a o l i n i t e peak i n t e n s i t i e s d e c r e a s e d , and t h e background i n c r e a s e d somewhat. T h i s may i n d i c a t e a d e c r e a s e Table I .

Treatment o f K a o l i n i t e w i t h C a u s t i c and Microwave I r r a d i a t i o n , F o l l o w e d by Water o r 10% HC1 Wash R e a c t i o n Time (min. )

No.

NaOH Room MicroConcn. Temp. 105°C wave

Type o f Products Wash

5-1 5-2

30% 30%

41 10

—

—

—

—

HgO HgO

5-3

30%

41

10

—

1^0

5-4 5-4b 10-1

30% 30% 30%

40 40 20

30 31

— —

HÔ acid R,0

10-2

30%

10-2b

—

1

20

—

0.5

HjO

30%

20

—

0.5

acid

10-3

30%

20

—

1.33

HgO

10-4 10-4b

30% 30%

20 20

—

3 3

13-1

50%

20

—

—

HjO

13-2

50%

20

22

—

HjO

13-2

50%

20

22

—

acid

13-3

50%

20

—

1

1^0

13-3

50%

20

—

1

acid

—

HjO acid

Observed by XRD

k a o l . only k a o l . (peak i n t e n s i t i e s decreased) k a o l . (peak i n t e n s i t i e s i n t e n s i t i e s decreased) kaol., hydroxysodalite kaol. kaol., sodalite-cancrinite mix. kaol., cancrinite-rich c a n c r i n i t e - s o d a l i t e mix. k a o l . , s o d a l i t e (peak i n t e n s i t i e s decreased) kaol., sodalite-rich c a n c r i n i t e - s o d a l i t e mix. kaol., sodalite k a o l . , s o d a l i t e (peak i n t e n s i t i e s decreased) k a o l . (peak i n t e n s i t i e s decreased) k a o l . (peak i n t e n s i t i e s decreased), s o d a l i t e c a n c r i n i t e mix. kaol., sodalite-cancrinite, ( a l l peak i n t e n s i t i e s decreased) kaol., sodalite-cancrinite mix. kaol., sodalite-cancrinite ( a l l peak i n t e n s i t i e s decreased)


38.


RICHARDSON ET AL.

i n the c r y s t a l l i n i t y o f the k a o l i n i t e , p o s s i b l y due t o the f o r m a t i o n of p a r t i a l l y s o l u b i l i z e d n o n - c r y s t a l l i n e m a t e r i a l . In the sample h e a t e d at 105°C f o r 30 minutes and i n a l l o f the microwave i r r a d i ated samples, k a o l i n i t e r e a c t e d t o form the hydrous sodium aluminum s i l i c a t e s c a l l e d h y d r o x y s o d a l i t e [Na ( A l S i 0 ) ( 0 H ) ] and h y d r o x y c a n c r i n i t e [Nag ( A l S i 0 )(0Η) ^ · (1-5)H 0]. In the microwave i r r a d i a t e d experiments, v a r y i n g m i x t u r e s o f h y d r o x y c a n c r i n i t e and h y d r o x y s o d a l i t e formed. In t h e 30-second run, the h y d r o x y c a n c r i n i t e peaks predominated, but as the exposure time was i n c r e a s e d , h y d r o x y s o d a l i t e predominated. T h i s change suggests t h a t h i g h e r tempera t u r e s were a t t a i n e d i n l o n g e r exposure runs and d e h y d r a t i o n was occurring. The experiments performed w i t h 50% NaOH ( a l k a l i / m i n e r a l = 0.5) y i e l d e d s i m i l a r r e s u l t s w i t h p o s s i b l y one e x c e p t i o n ; i n the XRD p a t t e r n f o r experiment 13-3 (44 minutes at room temperature and 1 minute i n microwave oven) t h e r e was a new peak i n a d d i t i o n t o t h e h y d r o x y s o d a l i t e - h y d r o x y c a n c r i n i t e peaks, s u g g e s t i n g t h a t a new (as yet u n i d e n t i f i e d ) m i n e r a l was f o r m i n g . A c i d washing the t r e a t e d k a o l i n i t e i n 10% HC1 removed p a r t o f the h y d r o x y s o d a l i t e - h y d r o x y c a n c r i n i t e m i x t u r e , d e m o n s t r a t i n g t h a t these m i n e r a l s a r e at l e a s t p a r t i a l l y soluble i n cold acid. Q

g


517

Treatment

g

2 1 f

χ

g

6

2 1 f

2

2

I l l i t e , a m i n e r a l t h a t forms i n a l k a l i n e s o l u t i o n s (15), i s more s t a b l e than k a o l i n i t e i n 30% NaOH. The experiments ( T a b l e I I ) i n which i l l i t e was r e a c t e d e i t h e r at room temperature o r at 105°C w i t h 30% NaOH showed t h a t no d i s c e r n i b l e r e a c t i o n took p l a c e . I n the microwave i r r a d i a t e d experiments, however, c o n s i d e r a b l e changes occurred. I l l i t e s t r o n g l y absorbed the microwave energy, c a u s i n g c o n s i d e r a b l e h e a t i n g o f t h e sample. When i l l i t e was i r r a d i a t e d f o r Table I I .

Treatment o f I l l i t e ( C o n t a i n i n g Quartz I m p u r i t i e s ) w i t h C a u s t i c and Microwave I r r a d i a t i o n , F o l l o w e d by Water or 10% HC1 Wash R e a c t i o n Time (min. )

No.

NaOH Room M i c r o Type o f Wash Concn. Temp. 105°C wave

6-1 6-2 6-3 6-4 12-1 12-lb

30% 30% 30% 30% 30% 30%

16 40 22 35 20 20

—

—

—

—

—

1 1

1^0 HgO 1^0 1^0 1^0 acid

12-2

30%

20

—

0.5

1^0

12-2b

30%

20

—

0.5

acid

12-3

30%

20

—

0.75

1^0

12-4

30%

20

—

2

1^0

a

35 15 —

a

— —

F i l t e r e d before drying in

Products

Observed by XRD

ill. (+quartz) ill. (+quartz) ill. (+quartz) ill. (+quartz) i l l . ( + q u a r t z ) , Na n e p h e l i n e i l l . (+quartz), nepheline (peak i n t e n s i t i e s decreased) i l l . (+quartz), nepheline low-Na n e p h e l i n e ) i l l . (+quartz), nepheline trace i l l . (+quartz), Na-rich nepheline i l l . (+quartz), nepheline

oven.


518


2 minutes i n the beaker, a r e a c t i o n r i n g formed which was hot and glowing when the microwave u n i t was opened. XRD p a t t e r n s showed t h a t t h e anhydrous s i l i c a t e n e p h e l i n e [(Na,K). Al^SiÔjç] has formed. The p o s i t i o n o f the n e p h e l i n e peaks s h i f t e d s l i g h t l y w i t h i n c r e a s i n g exposure time, i n d i c a t i n g t h a t the n e p h e l i n e was more p o t a s s i u m - r i c h ( a p p r o x i m a t e l y K N a A l S i 0 ) i n i t i a l l y and became more s o d i u m - r i c h i n 45-second and 1-minute r u n s . In the 2-minute microwave r u n , changes i n peak i n t e n s i t i e s and a d d i t i o n a l peak s h i f t s ( i n XRD) suggest t h a t n e p h e l i n e was b e g i n n i n g t o c o n v e r t t o another m i n e r a l . Washing the microwave i r r a d i a t e d i l l i t e s w i t h c o l d HC1 r e s u l t e d i n removal o f o n l y s m a l l amounts o f the n e p h e l i n e . M o n t m o r i l l o n i t e began t o break down i n 30% NaOH s o l u t i o n s a t room temperature ( T a b l e I I I ) . The f i r s t change noted, as w i t h k a o l i n i t e , was t h a t the peak i n t e n s i t i e s d e c r e a s e d i n the XRD p a t t e r n . A d e c r e a s e i n peak i n t e n s i t i e s was a l s o seen i n the XRD p a t t e r n s f o r samples h e a t e d t o 105°C. Microwave i r r a d i a t i o n o f m o n t m o r i l l o n i t e caused h e a t i n g o f the sample and some changes i n the XRD p a t t e r n . The t y p i c a l 13.6-Â peak d i s a p p e a r e d and the 9.6-Â peak t y p i c a l o f i l l i t e s and micas became s t r o n g e r , i n d i c a t i n g t h a t the montmorill o n i t e was undergoing d e h y d r a t i o n and the l a y e r s were c o l l a p s i n g . Another m o n t m o r i l l o n i t e peak s p l i t and formed two peaks, s u g g e s t i n g t h a t o t h e r s t r u c t u r a l m o d i f i c a t i o n s were o c c u r r i n g . A l t h o u g h some s t r u c t u r a l m o d i f i c a t i o n s o f the m o n t m o r i l l o n i t e have o c c u r r e d , the XRD p a t t e r n d i d not r e v e a l the p r e s e n c e o f a new m i n e r a l . A c i d washing o f the t r e a t e d samples d i d not change the XRD p a t t e r n s i g n i f i c a n t l y , s u g g e s t i n g t h a t any a l t e r a t i o n p r o d u c t formed was not acid-soluble.


2

Table I I I .

3

3

1 2

Treatment o f M o n t m o r i l l o n i t e w i t h C a u s t i c and Microwave I r r a d i a t i o n , F o l l o w e d by Water o r A c i d Wash R e a c t i o n Time (min. )

No.

NaOH Room MicroConcn . Temp.. 105°C wave

9-1

30%

40

—

9-2

30%

41

16

9-3 11-1 11-lb 11-2 11-3 ll-3b

30% 30% 30% 30% 30% 30%

40 20 20 20 20 20

a

a

16

— — — — —

F i l t e r e d before drying

Type o f Wash

—

ILO

—

ILO

— 0.5 0.5 1 1.33 1.33 in

HjO \0 acid HgO HgO acid

Products

Observed by XRD

mont, (peak i n t e n s i t i e s decreased) mont, (peak i n t e n s i t i e s decreased) mont. mont., dehydrated mont. mont., dehydrated mont. mont., dehydrated mont. mont., dehydrated mont. mont., dehydrated mont.

oven •

The r e s u l t s o f experiments i n which b i n a r y and t e r n a r y m i x t u r e s of the t h r e e c l a y m i n e r a l s were r e a c t e d w i t h 30% NaOH and i r r a d i a t e d i n the microwave oven a r e summarized i n T a b l e s IV and V, r e s p e c t i v e ly. R e a c t i n g m i x t u r e s o f these c l a y m i n e r a l s d i d not r e s u l t i n the


38.


RICHARDSON ET AL.

Table

IV.

Treatment

519

Treatment o f B i n a r y C l a y M i x t u r e s o f K a o l i n i t e , and M o n t m o r i l l o n i t e w i t h 30% NaOH and Microwave I r r a d i a t i o n , F o l l o w e d by Water or 10% HC1 Wash

Illite,


R e a c t i o n time (min. )

No.

Initial Sample

14-1

kaol.-ill.

26

1

14-lb k a o l . - i l l .

26

1

14-2

kaol.-ill.

22

2

14-3

kaol.-ill.

7

0.5

14-4

kaol.-ill.

24

0.5

15-1

ill.-mont.

39

0.5

15-2

ill.-mont.

35

1

15-3

ill.-mont.

36

2

16-1

kaol,-mont. 35

0.5

16-2

kaol,-raont. 31

1

16-3

kaol.-mont. 25

2

Room M i c r o - Type .of Temp. wave Wash

Products

Observed by XRD

1^0

i l l . , k a o l . , neph., s o d a l i t e cancrinite a c i d i l l . , k a o l . , neph., s o d a l i t e cancrinite ( l i t t l e change from above samp. HjO i l l . , kaol., sodalite-cancrin i t e , neph. (peak s h i f t i n g ) HjO i l l . , k a o l . , neph., s o d a l i t e cancrinite 1^0 i l l . , k a o l . (peaks d e c r e a s e d ) , neph., s o d a l i t e - c a n c r i n i t e HgO i l l . , mont., t r a c e o f neph., dehydrated mont. 1^0 i l l . , mont., neph., d e h y d r a t e d mont. \0 i l l . , mont., neph., d e h y d r a t e d mont. HgO kaol., sodalite-cancrinite mix., mont. HgO k a o l . , raont., s o d a l i t e c a n c r i n i t e , d e y d r a t e d mont. HjO k a o l . , mont., s o d a l i t e c a n c r i n i t e , dehydrated mont.

f o r m a t i o n o f any m i n e r a l s not a l r e a d y found i n experiments w i t h i n d i v i d u a l minerals. R e a c t i o n s w i t h k a o l i n i t e and i l l i t e r e s u l t e d i n the f o r m a t i o n o f both h y d r o x y s o d a l i t e - h y d r o x y c a n c r i n i t e and nepheline. R e a c t i o n s w i t h k a o l i n i t e and m o n t m o r i l l o n i t e r e s u l t e d i n the f o r m a t i o n o f h y d r o x y s o d a l i t e - h y d r o x y c a n c r i n i t e and dehydrated montm o r i l l o n i t e , and r e a c t i o n s w i t h i l l i t e and m o n t m o r i l l o n i t e r e s u l t e d i n the f o r m a t i o n o f n e p h e l i n e and dehydrated m o n t m o r i l l o n i t e . When a t e r n a r y mixture o f the c l a y s ( e q u a l amounts o f each m i n e r a l ) was r e a c t e d w i t h 30% NaOH s o l u t i o n and i r r a d i a t e d i n the microwave oven, h y d r o x y s o d a l i t e - h y d r o x y c a n c r i n i t e and t r a c e s o f n e p h e l i n e were found ( T a b l e V ) . The temperature d i d not appear t o have r i s e n as h i g h i n the t e r n a r y m i x t u r e s . Thus, n e p h e l i n e was not found i n 30-seconds e x p e r i m e n t s , and o n l y s m a l l amounts were found i n 1- and 2-minute experiments. The 13.6-Â m o n t m o r i l l o n i t e peak remained i n a l l p a t t e r n s , s u g g e s t i n g t h a t the m o n t m o r i l l o n i t e d i d not dehydrate completely. A c i d washing removed p a r t o f the r e a c t i o n p r o d u c t s from these m i x t u r e s . S e v e r a l t e s t s were performed t o i n c r e a s e s o l u b i l i z a t i o n by u s i n g warm (60°C) o r hot (80°C) a c i d r a t h e r than c o l d a c i d . These


520


T a b l e V.

Treatment of T e r n a r y C l a y M i x t u r e s of K a o l i n i t e , and M o n t m o r i l l o n i t e w i t h 30% NaOH and Microwave I r r a d i a t i o n , F o l l o w e d by Water or 10% HC1 Wash

Illite,

R e a c t i o n time (min. )


No.

Room M i c r o - Type o f Temp. wave Wash

17-1

23

0.5

HjO

17-lb

23

0.5

acid

17-2

22

1

HgO

17-3

22

2

HgO

17-3b

22

2

acid

Products

Observed by

XRD

k a o l . , i l l . , mont., t r a c e s o d a l i t e cancrinite k a o l . , i l l . , mont, (peak i n t e n s i t i e s d e c r e a s e d ) , s o d a l i t e - c a n c r i n i t e mix. (peak i n t e n s i t i e s decreased) k a o l . , i l l . , mont., s o d a l i t e - c a n c r i n i t e , t r a c e neph., dehydrated mont. k a o l . , i l l . , mont., s o d a l i t e - c a n c r i n i t e , neph., dehydrated mont. k a o l . , i l l . , mont., s o d a l i t e - c a n c r i n i t e , neph., dehydrated mont, (peak i n t e n s i t i e s decreased)

runs i n d i c a t e t h a t c o n s i d e r a b l y more p r o d u c t , a l t h o u g h not a l l of i t , was removed at 80°C. Fan et a l . (11,12) were a b l e to remove s i m i l a r p r o d u c t s c o m p l e t e l y by u s i n g b o i l i n g HC1 o r H S 0 . 2

lt

Discussion These experiments have a l l o w e d an e v a l u a t i o n of the e x t e n t o f r e a c t i o n between c l a y m i n e r a l s and a l k a l i s o l u t i o n d u r i n g the p r e treatment p a r t o f the microwave c o a l c l e a n i n g p r o c e s s . It i s e v i dent from these r e s u l t s t h a t some r e a c t i o n s began t o o c c u r between the a l k a l i s o l u t i o n and the c l a y m i n e r a l s , p a r t i c u l a r l y w i t h k a o l i n i t e , b e f o r e the microwave i r r a d i a t i o n . The new product peaks, howe v e r , were s m a l l on the XRD p a t t e r n s compared t o the s t a r t i n g mater i a l s , s u g g e s t i n g t h a t the amount of m a t e r i a l t h a t has r e a c t e d may be q u i t e s m a l l . The v i s u a l appearance o f the m a t e r i a l a l s o sugg e s t e d t h a t o n l y a s m a l l r e a c t i o n r i m was forming i n the c o n t a i n e r . S i n c e o v e r a l l the changes i n the c l a y m i n e r a l s d u r i n g a l k a l i p r e treatment at room temperature were r a t h e r minor, v e r y l i t t l e ash removal can be expected d u r i n g o n l y the p r e t r e a t m e n t p a r t o f the coal cleaning process. On the o t h e r hand, s u b s t a n t i a l changes d i d o c c u r under m i c r o wave i r r a d i a t i o n . The c l a y m i n e r a l s appeared t o absorb microwave energy e f f i c i e n t l y enough f o r c o n s i d e r a b l e h e a t i n g t o o c c u r . Comp l e t e d e h y d r a t i o n o f the c l a y - a l k a l i s l u r r y o c c u r e d w i t h i n the f i r s t 30 seconds. No measurement o f the temperatures a t t a i n e d was p o s s i b l e because temperature probes c o u l d not be i n s e r t e d i n the m a t e r i a l d u r i n g microwave h e a t i n g w i t h o u t p e r t u r b i n g the microwave f i e l d . A probe was not i n s e r t e d immediately a f t e r i r r a d i a t i o n because c o n s i d e r a b l e c o o l i n g took p l a c e as soon as the oven door was opened and because the r e a c t i o n p r o d u c t s formed a h a r d c r u s t which was d i f f i c u l t t o p e n e t r a t e w i t h a probe. These experiments a l s o p o i n t e d out


38.


RICHARDSON ET AL.

t h a t not a l l c l a y m i n e r a l s absorbed the microwave energy to the same e x t e n t . The i l l i t e samples were the o n l y ones i n which the p r o d u c t rim was glowing a f t e r opening the microwave u n i t , s u g g e s t i n g t h a t i l l i t e absorbed microwave energy more s t r o n g l y than the o t h e r c l a y m i n e r a l s and reached h i g h e r temperatures. A l s o , anhydrous r e a c t i o n p r o d u c t s were formed i n these e x p e r i m e n t s . The r e a c t i o n s between c l a y m i n e r a l s , NaOH s o l u t i o n s , and m i c r o wave i r r a d i a t i o n showed t h a t the c l a y m i n e r a l s t r u c t u r e s began t o b r e a k down, and the r e l e a s e d A l and S i (and some K) combined w i t h Na from the s o l u t i o n t o form new m i n e r a l s . The exact r e a c t i o n path c o u l d not be determined from the p r e s e n t e x p e r i m e n t s . S i n c e the new m i n e r a l s formed had a p p r o x i m a t e l y the same A l : S i r a t i o as the c l a y m i n e r a l s , no excess A l (as A 1 0 or A 1 ( 0 H ) ) or q u a r t z was e x p e c t e d , nor was any found i n the XRD p a t t e r n . The f o l l o w i n g e q u a t i o n s , a r r a n g e d w i t h i n c r e a s i n g time, seem most r e a s o n a b l e t o d e s c r i b e the r e a c t i o n s observed f o r k a o l i n i t e and i l l i t e : 2


521

Treatment

3

3

kaolinite: SAlgSiÔçiOH)^ (kaolinite)

+ 8NaOH = Na Alg S i g O ^ ( 0 Η ) · (1-5)1^0 + (hydroxycancrinite) 8

2

(3-8)H 0 2

S A l g S i ^ O ç i O H ^ + 8NaOH - Na Alg S i 0 (0H)« + 9H 0 (kaolinite) (hydroxysodalite; 8

6

2 l |

2

illite: 4/3KAl (Si A10 )(OH) (ideal i l l i t e ; 2

3

1 Q

4/3KAl (Si AlO )(OH) (ideal i l l i t e ) 2

3

1 0

2

2

+ 2NaOH -

KN^ A l S i j O ^ + 2H 0 (low-Na n e p h e l i n e )

+ 3NaOH = N a K A l 3

3

S^0 + 1/3 (nepheline)

l#

χ 6

2

KOH

+ 8/3H 0 2

A l l o f these r e a c t i o n s consume a l k a l i and r e l e a s e water, r e i n f o r c i n g the o b s e r v a t i o n t h a t a l k a l i (Na) a d d i t i o n and d e h y d r a t i o n r e a c t i o n s were o c c u r r i n g , p a r t i c u l a r l y w i t h i n c r e a s e d microwave exposure time. These p r o d u c t s were s i m i l a r to those observed i n a u t o c l a v e exper iments run at 250-300°C f o r 1 hour ( 1 2 ) . In our t r e a t e d samples, however, XRD p a t t e r n s f o r the s t a r t i n g c l a y m a t e r i a l s were c l e a r l y v i s i b l e , i n d i c a t i n g t h a t a l t h o u g h h i g h temperature r e a c t i o n s d i d o c c u r , they d i d not go to c o m p l e t i o n . T h i s i m p l i e s t h a t at the microwave power l e v e l s and i r r a d i a t i o n times used i n these e x p e r i ments, not a l l o f the c l a y m i n e r a l s i n a c o a l sample would be con v e r t e d t o s o l u b l e p r o d u c t s . T h i s c o n v e r s i o n p r o c e s s , however, may be much more complete at the power l e v e l s to be used i n the m i c r o wave c o a l c l e a n i n g p r o c e s s . Furthermore, the p r e t r e a t r a e n t , i r r a d i a t i o n , and washing c y c l e s can be r e p e a t e d s e v e r a l times t o improve the c o n v e r s i o n . S t i l l the r e s u l t s o f these experiments are e n c o u r a g i n g and suggest t h a t most or a l l of the k a o l i n i t e and i l l i t e ( t h e two most abundant c l a y m i n e r a l s ) i n c o a l c o u l d be c o n v e r t e d t o s o l u ble p r o d u c t s and removed, r e d u c i n g c o n s i d e r a b l y the ash content of the c o a l . These experiments are the f i r s t p a r t of a program d e s i g n e d t o e v a l u a t e each s t e p o f the microwave c o a l c l e a n i n g p r o c e s s . A d d i t i o n a l experiments of the type r e p o r t e d here w i l l be performed w i t h


MINERAL MATTER AND

522

ASH IN COAL

q u a r t z and p y r i t e to complete our t e s t s on the most abundant miner als i n coal. A l l the m i n e r a l s w i l l be i r r a d i a t e d at h i g h e r power l e v e l s f o r v a r y i n g p e r i o d s of time i n the l a r g e r microwave u n i t . These f u t u r e experiments w i l l h e l p determine the optimum power l e v e l s and times n e c e s s a r y to get complete c o n v e r s i o n of the m i n e r a l matter to s o l u b l e products. The r e s u l t s of these experiments w i l l be used to guide our c o a l c l e a n i n g program and h e l p us a t t a i n the h i g h e s t l e v e l s of ash and s u l f u r removal.


Acknowledgment s Ames L a b o r a t o r y i s o p e r a t e d f o r the U. S. Department o f Energy by Iowa S t a t e U n i v e r s i t y under C o n t r a c t No. W-7405-Eng-82. T h i s work was supported by the A s s i s t a n t S e c r e t a r y f o r F o s s i l Energy, O f f i c e o f C o a l U t i l i z a t i o n , through the P i t t s b u r g h Energy Technology C e n t e r . The u n l i m i t e d use of an MDS-81 microwave u n i t on extended l o a n from the CEM C o r p o r a t i o n i s g r a t e f u l l y acknowledged. V a l u a b l e d i s c u s s i o n s were h e l d w i t h P r o f . G. Fanslow of the E l e c t r i c a l E n g i n e e r i n g Department.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Zavitsanos, P.D.; Golden, J.A; Bleiler, K.W. "Coal Desulfuri zation by a Microwave Process," Tech. Prog. Report, Feb. - May 1981, General Electric Co., Philadelphia, PA, 1981. Zavitsanos, P.D.; Golden, J.A; Bleiler, K.W. "Coal Desulfurization by a Microwave Process," Tech. Prog. Report, Jan. 1982, General Electric Co., Philadelphia, PA, 1982. Zavitsanos, P.D.; Golden, J.A; Bleiler, K.W. "Coal Desulfur ization by a Microwave Process," Tech. Prog. Report, May 1982, General Electric Co., Philadelphia, PA, 1982. Zavitsanos, P.D.; Golden, J.A.; Bleiler, K.W. "Coal Desulfurization by a Microwave Process," Tech. Prog. Report, Sept. 1982, General Electric Co., Philadelphia, PA, 1982. Zavitsanos, P.D.; Golden, J.Α.; Bleiler, K.W. "Coal Desulfur ization by a Microwave Process," Tech. Prog. Report, Dec. 1982, General Electric Co., Philadelphia, PA, 1982. Zavitsanos, P.D.; Golden, J.A.; Bleiler, K.S.; Jain, K. "Coal Desulfurization by a Microwave Process," Tech. Prog. Report, March 1983, General Electric Co., Philadelphia, PA, 1983. Carroll, D.; and Starkey, H.C. "Reactivity of Clay Minerals with Acids and Alkalies," J. Clays and Clay Minerals 1971, 19, 321-333, Nutting, P.G. "The Action of Some Aqueous Solutions on Clays of Montmorillonite Group," U.S. Geological Survey, Prof. Paper, 197F, 1943, 219-235. Gastuche, M.C. "Study of the Alteration of Kaolinite by Various Chemical Reagents," S i l i c . Ind. 1959, 24, 237-244. Gastuche, M.C.; Delmon, B.; Vielvoye, L. "Kinetics of Hetero geneous Reactions. Attack on the Silicon-Aluminum Network of Kaolinite by Hydrochloric Acid," Bull. Soc. Chim. Franc. 1960, 1, 60-70.


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RICHARDSON ET AL.

523

11.

Fan, C.-W.; Markuszewski, R.; Wheelock, T.D. "Process for Producing Low-Ash, Low-Sulfur Coal," Am. Chem. Soc. Div. Fuel

12.

Fan, C.-W.; Markuszewski, R.; Wheelock, T.D. "Behavior of Mineral Matter During Alkaline Leaching of Coal," Am. Chem. Soc. Div. of Fuel Chem. Preprints 1984, 29(4), 319-325. Eremin, N.I.; Tkacheva, L.V.; Makarenko, V.N. "Investigation of the Kinetics of the Decomposition of Kaolinite in Alkaline and Aluminate Solutions," Soviet Non-ferrous Metals Research 1978,

13.


Caustic and Microwave Treatment

Chem. Preprints 1984, 29(1), 114-119.

6(5),

197-199.

14.

Montoya, J.W.; Hemley, J.J. "Activity Relations and Stabilities in Alkali Feldspar and Mica Alteration Reactions," Econ. Geol.

15.

Deer, W.A.; Howie, R.A.; Zussman, J. "An Introduction to the Rock-Forming Minerals"; Longman:London, 1975.

1975,

70, 577-582.

RECEIVED August 30, 1985


Effect of Caustic and Microwave Treatment on Clay ... - ACS Publications

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