Water for Coal Beneficiation

37 Carbon Dioxide/Water for Coal Beneficiation R. Sapienza, T. Butcher, W. Slegeir, and F. Healy

Downloaded by STONY BROOK UNIV SUNY on May 20, 2018 | https://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch037

Brookhaven National Laboratory, Department of Applied Science, Upton, N Y 11973

Pressurized carbon dioxide/water solvents affect the chemical and physical properties of coal to provide a method of simultaneous cleaning and fracturing. The synergistic interaction of these two inexpensive reagents causes the selective solubilization of the alkali and alkaline earth mineral matter in the coal and the swelling of the coal matrix which weakens the coal structure. Significant improvements in coal grindability with high retention of energy content have been achieved.

The c h e m i c a l c o m p o s i t i o n , p h y s i c a l s i z e , and mode of d i s t r i b u t i o n o f the m i n e r a l matter i n c o a l g r e a t l y a f f e c t the way i n which c o a l m i n e r a l s can be s e l e c t i v e l y removed. F o r any c o a l c l e a n i n g method to be a p p l i c a b l e t o a v a r i e t y o f c o a l s , i t s h o u l d p r o v i d e b o t h p h y s i c a l l i b e r a t i o n and c h e m i c a l s e p a r a t i o n t e c h n i q u e s . Brookhaven N a t i o n a l L a b o r a t o r y (BNL) has focused on t h i s a p p r o a c h . The method i n v o l v e s the s e l e c t i v e s o l u b i l i z a t i o n o f c o a l m i n e r a l m a t t e r which a l s o causes s w e l l i n g o f t h e c o a l . This s w e l l i n g r e s u l t s i n the f r a g m e n t a t i o n o f t h e c o a l s t r u c t u r e which a f f e c t s c o a l g r i n d i n g and f a c i l i t a t e s t h e removal o f m i n e r a l m a t t e r . With t h e proper s e l e c t i o n of reagents, s e l e c t i v e s o l u b i l i z a t i o n of mineral matter occurs. T h i s work has c l a r i f i e d our u n d e r s t a n d i n g o f t h e n a t u r a l b i n d i n g f o r c e s h o l d i n g c o a l t o g e t h e r and c o u l d be of v a l u e i n i m p r o v i n g and e x t e n d i n g t h e c a p a b i l i t y o f c u r r e n t c l e a n i n g p r o c e s s e s and p r o v i d i n g a b a s i s f o r t h e development o f new c o a l c l e a n i n g methodologies. T h i s approach t o c o a l b e n e f i c a t i o n i s f l e x i b l e and i s a p p l i c a b l e t o a v a r i e t y o f c o a l s because the n a t u r e o f m a c e r a l - m a c e r a l and maceral-mineral i n t e r a c t i o n s are s i m i l a r f o r s i m i l a r coals. The c o a l i s m o d i f i e d b o t h c h e m i c a l l y and p h y s i c a l l y i n a way which a l l o w s s e l e c t e d m i n e r a l components t o be a t t a c k e d . I t i s believed t h a t improvements can be made which w i l l be more e f f e c t i v e i n terms o f degree and s p e c i f i c i t y f o r m i n e r a l m a t t e r removal, w h i l e p o t e n t i a l l y e f f e c t i n g a r o u t e t o o r g a n i c s u l f u r removal. 0097-6156/ 86/ 0301 -0500$06.00/ 0 © 1986 American Chemical Society

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

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Concept

C o a l i s s e d i m e n t a r y rock composed of m i c r o s c o p i c a l l y r e c o g n i z a b l e o r g a n i c c o n s t i t u e n t s (macérais) and i n o r g a n i c c o n s t i t u e n t s (minerals). Macérais are p r o d u c t s of c o a l i f i e d remains of p l a n t s ; they have been c l a s s i f i e d i n t o groups, i . e . v i t r i n i t e , e x i n i t e , and inertinite. C o a l m i n e r a l s a r e b e l i e v e d t o be l o c a t e d a t m a c e r a l boundaries ( 1 ) · The overwhelming m a j o r i t y of t h e s e m i n e r a l s a r e i n one of the f o l l o w i n g f o u r groups, a l u m i n o s i l i c a t e s , c a r b o n a t e s , s u l f i d e s , and s i l i c a ( q u a r t z ) . Hydrogen bonding may w e l l be an i m p o r t a n t p a r t of the f o r c e s t h a t h o l d t h e s e m i n e r a l s a t the m a c e r a l boundaries. The most abundant element i n c o a l , a f t e r c a r b o n and hydrogen, i s oxygen. Oxygen i s found combined w i t h o t h e r atoms i n c o a l to form v a r i o u s f u n c t i o n a l groups, namely, c a r b o x y l groups ( C O 2 H ) , c a r b o n y l groups ( 0 = 0 ) , and phenol (OH) groups. One of the prominent f e a t u r e s of the i n f r a r e d s p e c t r a of most c o a l s i s an i n t e n s e broad band a s s o c i a t e d w i t h 0-H v i b r a t i o n s . I t i s a l s o worth n o t i n g t h a t OH a c c o u n t s f o r between one t h i r d and two t h i r d s of t h e t o t a l oxygen i n c o a l Ç 2 ) . These bonding f e a t u r e s may c o n t r i b u t e t o h o l d the c o a l i n o r g a n i c and o r g a n i c aggregate t o g e t h e r and are p o t e n t i a l s i t e s of f u r t h e r hydrogen bonding. S i n c e the s t r u c t u r e of c o a l shows many p o s s i b l e s i t e s f o r hydrogen b o n d i n g , b r e a k i n g hydrogen bonds a l o n g the m a c e r a l b o u n d a r i e s c o u l d r e s u l t i n the s e p a r a t i o n of the m i n e r a l m a t t e r from the c o a l m a t r i x . C o n c e p t u a l l y , two r o u t e s a r e a v a i l a b l e f o r the d i s r u p t i o n of the coal matrix. One method i n v o l v e s the use of r e a g e n t s which a t t a c k the s u r f a c e of c o a l and, d u r i n g the c h e m i c a l t r a n s f o r m a t i o n , e f f e c t i v e l y " p e e l away" t h i s s u r f a c e t o expose f r e s h s u r f a c e to the reagent. T h i s method appears to be o p e r a t i v e d u r i n g c o a l g a s i f i c a tion. However, t h i s t e c h n i q u e , of n e c e s s i t y , i n v o l v e s r a d i c a l t r a n s f o r m a t i o n s of the c o a l s t r u c t u r e , r e s u l t i n g i n e x t e n s i v e c a r b o n carbon bond c l e a v a g e . The second method i n v o l v e s the p e n e t r a t i o n o f the c o a l s t r u c t u r e w i t h r e a g e n t s t h a t s w e l l the c o a l . Such methodology allows f o r v a s t l y g r e a t e r surface area, e f f e c t i v e l y longer c o a l - r e a g e n t r e a c t i o n t i m e s , and g r e a t e r p o s s i b i l i t i e s f o r s e l e c t i v e c l e a v a g e of bonds. S w e l l i n g s t r a i n s the c o a l m a t r i x , most p r o b a b l y l e a d i n g t o the r u p t u r e of hydrogen bonds. However, due t o the h i g h a v a i l a b i l i t y o f hydrogen bonds, the r e o r i e n t a t i o n of the c o a l m a t r i x a l l o w s r e o r i e n t a t i o n of the hydrogen bonding s t r u c t u r e . I t i s b e l i e v e d t h a t an a p p r o p r i a t e hydrogen bonding agent, i f a l l o w e d t o p e n e t r a t e the s w e l l e d c o a l s t r u c t u r e , would be c a p a b l e o f " t y i n g up" hydrogen bonding s i t e s i n the c o a l . The combined e f f e c t s of s w e l l i n g and bond b r e a k i n g would appear to a l l o w i n c l u d e d m i n e r a l matter to d r o p out o f the c o a l s t r u c t u r e w h i l e p h y s i c a l l y weakening the c o a l m a t r i x . U n l i k e f r a c t u r i n g , t h i s approach s h o u l d cause o n l y moderate d i s r u p t i o n of the c o a l m a t r i x and might not l e a d t o complete comminut i o n , but t h i s method c o u l d be e n v i s i o n e d as a p r e l u d e to c o a l grinding. Removal of m a t e r i a l from s t r u c t u r a l b o u n d a r i e s would y i e l d a more f r i a b l e , " s o f t " c o a l s t r u c t u r e . The l e a c h i n g of f i n e m i n e r a l p a r t i c l e s from the c o a l m a t r i x r e s u l t s i n a more porous s t r u c t u r e and s h o u l d a l s o improve the c o a l ' s g r i n d a b i l i t y . The p r i o r removal o f some m i n e r a l m a t t e r would a l s o reduce the c o a l ' s a b r a s i v e n e s s .



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The t r e a t e d c o a l should be more amenable to c r u s h i n g and such c r u s h i n g s h o u l d o c c u r a l o n g m a c e r a l b o u n d a r i e s due t o the combined e f f e c t s of e x t e r n a l hydrogen bonding and s w e l l i n g . T h i s would r e l e a s e g r e a t e r amounts of i m p u r i t i e s of comparable p a r t i c l e - s i z e d i s t r i b u t i o n and reduce the c o s t of f o l l o w - o n p r o c e s s i n g f o r separating impurities. T h i s approach may a l s o o f f e r new ways o f f u r t h e r c o a l c l e a n - u p because the c o a l s t r u c t u r e i s most s u s c e p t i b l e to c h e m i c a l a t t a c k d u r i n g the s w e l l i n g p r o c e s s . A wide v a r i e t y of c h e m i c a l r e a g e n t s are c a p a b l e of b l o c k i n g the i n t e r n a l hydrogen bonding of the c o a l . However, f o r a c h e m i c a l c o a l b e n e f i c a t i o n system to be p r a c t i c a l t h i s hydrogen bonding system must be r e a d i l y a v a i l a b l e , i n e x p e n s i v e , be of s u f f i c i e n t l y s m a l l m o l e c u l a r s i z e , and not be expected t o i n t r o d u c e unwanted c h e m i c a l elements t h a t w i l l l e a d to c o r r o s i o n or p o l l u t i o n problems w i t h the p r o c e s s e d , comminuted c o a l . These q u a l i f i c a t i o n s s u b s t a n t i a l l y l i m i t the number of hydrogen bonding r e a g e n t s . Our i n i t i a l e x p e r i m e n t s were r e s t r i c t e d t o water combined w i t h c a r b o n d i o x i d e t o p r o v i d e a r e a g e n t system which was t a i l o r e d t o the problems. The p r o c e s s d e s c r i b e d i s r e l a t e d t o the unique e f f e c t s carbon d i o x i d e and m o i s t u r e have on c o a l . Carbon d i o x i d e r e a d i l y and e x t e n s i v e l y p e n e t r a t e s the c o a l s t r u c t u r e ( 3 ) . In f a c t , t h i s has l e d t o the advocacy of employing carbon d i o x i d e t o measure t h e i n t e r n a l a r e a of c o a l s . I t i s l i k e l y the CO2 d i f f u s e s i n t o the c o a l a l o n g the l i n e s of m i n e r a l i n c l u s i o n as has been seen f o r o t h e r gases ( 4 ) . T h i s i s i m p o r t a n t s i n c e the d i f f u s i o n of r e a g e n t s through s o l i d c o a l may be the r a t e - l i m i t i n g s t e p i n many r e a c t i o n s . T h i s d i f f u s i o n has a l s o been shown to cause a d i m e n s i o n a l e x p a n s i o n of the s t r u c t u r e . M o i s t u r e a d s o r p t i o n - d e s o r p t i o n c y c l e s of c o a l causes w e a t h e r i n g , a slow form of c h e m i c a l comminution ( 5 ) . Although a problem i n c o a l s t o r a g e , the l o s s of s t r e n g t h from the d e g r a d a t i o n of the c o a l c o u l d be p r a c t i c a l f o r f r a c t u r i n g the c o a l s t r u c t u r e i f a c c e l e r a t e d w i t h b e t t e r p e n e t r a t i o n of water. A d d i t i o n a l l y , the sorbed water can d i s s o l v e and s e r v e as a t r a n s p o r t medium f o r s o l u b l e m i n e r a l s w i t h i n the c o a l m a t r i x . In t h i s r e s p e c t , the c o u p l i n g of water w i t h carbon d i o x i d e t o g e n e r a t e a weakly a c i d i c s o l u t i o n would a l s o be b e n e f i c i a l . The a c i d l e a c h i n g of c o a l m i n e r a l matter has been employed to produce c o a l s of u n u s u a l l y low ash c o n t e n t ( 6 ) . C l e a n i n g w i t h a c i d o f f e r s an advantage over t y p i c a l g r a v i t y c l e a n i n g methods i n which c h e m i c a l c o m p o s i t i o n o f the m i n e r a l m a t t e r does not i n f l u e n c e i t s removal. Selective removal of m i n e r a l components t h a t e x e r t a d e l e t e r i o u s e f f e c t i s possible with acid cleaning. A c i d a t t a c k of the m i n e r a l o c c u r i n g i n the c l e a t s and p a r t i n g s of the c o a l i s a l s o the b a s i s of a d i s i n t e g r a t i o n p r o c e s s f o r b r e a k i n g down c o a l d u r i n g mining ( 6 ) . S e v e r a l b i t u m i n o u s c o a l s have been employed d u r i n g the c o u r s e of t h i s study. The method can be a p p l i e d t o r e l a t i v e l y unprepared c o a l s and seems to be very e f f e c t i v e f o r the removal of a l k a l i and a l k a l i n e e a r t h metals which are r e l a t e d to b o i l e r f o u l i n g . W i t h m o d i f i c a t i o n , o t h e r m i n e r a l groups c o u l d be removed. Coal t r e a t e d w i t h CO2/H2O o c c a s i o n a l l y crumbles d u r i n g p r o c e s s i n g . The system i s f l e x i b l e , may be m o d i f i e d both c h e m i c a l l y and p h y s i c a l l y , and may be i n t e g r a t e d i n t o , or used to m o d i f y , an o v e r a l l c o a l p r e p a r a t i o n


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process. The method y i e l d s a more f r i a b l e , " s o f t " c o a l s t r u c t u r e and c o u l d s i g n i f i c a n t l y reduce the energy r e q u i r e m e n t s and c o s t o f f o l l o w - o n c o a l g r i n d i n g and s e p a r a t i o n p r o c e s s .


Experimental

Procedures

A v a r i e t y of b i t u m i n o u s c o a l s have been examined i n the c o u r s e of t h i s work u s i n g CO2/H2O, i n c l u d i n g Kentucky #9 and P i t t s b u r g h Seam coal. The treatment i s c a r r i e d out i n a 2L s t a i n l e s s s t e e l a u t o c l a v e , equipped w i t h a gauge, l i q u i d sampling and gas v e n t i n g v a l v e s and a thermocouple. The thermocouple i s connected t o a p r o p o r t i o n i n g band temperature c o n t r o l l e r , which i n t u r n i s connected to a h e a t i n g mantle. T h i s system a f f o r d s p r e c i s e and r e p r o d u c i b l e temperature c o n t r o l of the a u t o c l a v e c o n t e n t s . G e n e r a l l y , a c o a l sample ( t y p i c a l l y 500g) of a p p r o p r i a t e mesh s i z e i s added t o the r e a c t o r f o l l o w e d by the a p p r o p r i a t e amount o f d i s t i l l e d water ( t y p i c a l l y 1 L ) . A f t e r c l o s u r e , the r e a c t o r i s purged and then brought to the a p p r o p r i a t e CO2 p r e s s u r e ; the weight o f CO2 I s then d e t e r m i n e d . The r e a c t o r i s heated to the d e s i r e d temperature ( t y p i c a l l y 80°C) and the p r e s s u r e i s a g a i n r e c o r d e d . Once the temperature has s t a b i l i z e d , very l i t t l e , i f any, changes i n p r e s s u r e a r e o b s e r v e d . The temperature and p r e s s u r e d a t a may be used to c a l c u l a t e the amount o f CO2 d i s s o l v e d i n the water. At the end of the d e s i r e d c o n t a c t time, the r e a c t o r i s removed from the h e a t e r , and the hot l i q u i d phase i s c a r e f u l l y t r a n s f e r r e d to a f l a s k . A f t e r c o o l i n g , the c o a l i s removed from the r e a c t o r and washed on a s i n t e r e d g l a s s f u n n e l w i t h d i s t i l l e d water. Final d r y i n g of the c o a l i s c a r r i e d out i n a vacuum oven a t 110°C. The c o o l e d l i q u i d phase, which f r e q u e n t l y c o n t a i n s a s m a l l amount o f powdered c o a l and p r e c i p i t a t e d m i n e r a l m a t t e r , i s f i l t e r e d . The water i s evaporated to d r y n e s s to determine the q u a n t i t y and n a t u r e o f m i n e r a l s l e a c h e d from the c o a l . The weight of the d r i e d c o a l i s compared w i t h t h a t o f the f e e d coal. The d r i e d c o a l i s ashed by ASTM method D-3174-73 a l o n g w i t h samples of u n t r e a t e d c o a l . The gas phase was examined f o r CO, H2, CH4, and SO2 a f t e r some t r e a t m e n t s , and i n no cases were s i g n i f i c a n t q u a n t i t i e s of t h e s e detected. W i t h i n the l i q u i d phase, a v a r i e t y of i n o r g a n i c m a t e r i a l s are found ( v i d e i n f r a ) . To a s s e s s q u a n t i t a t i v e l y the e f f e c t s of t h i s treatment on g r i n d i n g , a l a b o r a t o r y - s c a l e g r i n d i n g system was needed. U s i n g a b a t c h b a l l m i l l , the g r i n d a b i l i t y of samples has been compared u s i n g an a d a p t a t i o n o f the Bond Work Index (BWI) concept ( 7 ) . The use o f the BWI i n v o l v e s d e t e r m i n i n g the energy i n p u t r e q u i r e d t o a c h i e v e a d e s i r e d l e v e l of g r i n d i n g w i t h the Index b e i n g c a l c u l a t e d by t a k i n g i n t o account the energy i n p u t and the e x t e n t of s i z e r e d u c t i o n . T h i s approach i s c o n s i d e r e d more f l e x i b l e than o t h e r a p p r o a c h e s , such as the H a r d g r o v e , which a r e d e f i n e d o n l y f o r a g i v e n f e e d and product s i z e . In a d d i t i o n the Hardgrove t e s t r e q u i r e s o n l y a s m a l l f r a c t i o n of the c o a l to pass a g i v e n s c r e e n s i z e and so may o n l y r e f l e c t surface f r a c t u r i n g .


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504

ASH IN COAL

The a c t u a l g r i n d i n g was c a r r i e d out on a l a b o r a t o r y b a l l m i l l , u s i n g ceramic j a r s w i t h dimensions o f 5-13/16" i n t e r n a l d i a m e t e r and 5-3/8" i n t e r n a l h e i g h t . The o v e r a l l volume i s 1829cc. Initial experiments employed s t e e l b a l l s (seventeen o f 3/4" d i a m e t e r , e l e v e n o f 5/8" d i a m e t e r and f i f t y of 1" d i a m e t e r ) . The r a t e of j a r r o t a t i o n of the m i l l i s n o m i n a l l y 80 rpm. The v o l t a g e and c u r r e n t f e e d i n g of the b a l l m i l l a r e measurable and c o n s t a n t ; t h e r e f o r e , the g r i n d i n g time i s used as a measure of power i n p u t . Experiments employed 100 g of c o a l . In the BWI t e s t u n d e r s i z e d c o a l i s removed a t c a l c u l a t e d t i m e s and f r e s h feed i s added to s i m u l a t e c l o s e d c i r c u i t g r i n d i n g . This procedure i s r e p e a t e d u n t i l a c o n s t a n t mass of u n d e r s i z e d product i s produced per r e v o l u t i o n . Simpler approaches, however, i n v o l v i n g b a t c h g r i n d i n g have been shown to p r o v i d e a c c u r a t e r e s u l t s , p a r t i c u l a r l y f o r comparing the g r i n d a b i l i t y of two m a t e r i a l s ( 8 ) . T h i s l a t e r approach was used here f o r comparison of the g r i n d a b i l i t y o f t r e a t e d and u n t r e a t e d c o a l s . With the assumptions of c o n s t a n t power i n p u t to the m i l l , f i x e d feed s i z e , a s i m i l a r product s i z e d i s t r i b u t i o n c u r v e , the change i n BWI and hence g r i n d a b i l i t y can be approximated by the d i f f e r e n c e i n time r e q u i r e d to a c h i e v e a f i x e d p e r c e n t a g e of the product c o a l p a s s i n g a g i v e n s c r e e n . Condition Selection Carbon d i o x i d e p r e s s u r e s from 1 t o 75 atmospheres and temperatures from 0° t o 80°C were a s s e s s e d . Since the l i q u i d phase i s i n c o n t a c t w i t h the c o a l and i s r e s p o n s i b l e f o r m i n e r a l m a t t e r d i s s o l u t i o n , i t s c o m p o s i t i o n would be expected to have a b e a r i n g on ash r e d u c t i o n i n the c o a l . The s o l u b i l i t y of CO2 i n the l i q u i d phase i n c r e a s e s as the CO2 p r e s s u r e i n c r e a s e s and may be r e l a t e d to s w e l l i n g of c o a l s t r u c t u r e ( a l t h o u g h not l i n e a r l y ) . T a b l e I summarizes a set of experiments d i r e c t e d toward d e t e r m i n i n g the e f f e c t of aqueous phase c o n c e n t r a t i o n of CO2 on the t r e a t e d product ash c o n t e n t f o r P i t t s b u r g h Seam c o a l . At low CO2 c o n c e n t r a t i o n s , l i t t l e ash r e d u c t i o n i s o b s e r v e d , c o n s i s t e n t w i t h poor matching of the s o l u b i l i t y parameter o f water w i t h c o a l . As the CO2 c o n c e n t r a t i o n i s i n c r e a s e d , a s i g n i f i c a n t r e d u c t i o n i n ash i s observed. However, i n c r e a s i n g the c o n c e n t r a t i o n above 24 g C C ^ / l i t e r does not appear to r e s u l t i n s i g n i f i c a n t ash r e d u c t i o n s .

Table

I.

Effect

Concentration 8 10 24 37 43

o f CO2 C o n c e n t r a t i o n P i t t s b u r g h Seam C o a l

CO2 D i s s o l v e d

g/L

on Ash

Reduction

Removal-

i n Ash,

5 5 16 15 15


%


37.

SAPIENZA ET AL.

Carbon

Dioxide/

Water for

Coal

Beneficiation

505

I n i t i a l t e s t s were conducted w i t h P i t t s b u r g h Seam c o a l (NA1361 - A r k w r i t e Mine) s i z e d t o 1 3/8 i n c h e s χ 3/8 i n c h e s . Reaction c o n d i t i o n s s t u d i e d f o r t h i s sample i n c l u d e d 75 - 80 atmospheres CO2 at 80°C. These c o n d i t i o n s were chosen because they a r e above t h e c r i t i c a l p o i n t of CO2 y e t below the b o i l i n g p o i n t of water. Super c r i t i c a l p r e s s u r e was chosen t o maximize the amount of CO2 p r e s e n t i n the water. F i n a l l y , we have found t h a t s i m p l y t r e a t i n g the c o a l i n water w i t h o u t CO2 o f f e r s l i t t l e change i n m i n e r a l c o n t e n t or ease of grinding. T e s t s analogous t o the CO2/H2O t r e a t m e n t s , u s i n g e i t h e r n i t r o g e n or h e l i u m i n p l a c e of the CO2, r e s u l t e d i n o n l y s m a l l changes i n the c o a l . These t e s t s i n d i c a t e t h a t p r e s s u r i z e d water a l o n e i s not i m p o r t a n t t o the p r o c e s s . Rapid decompression tests u s i n g CO2 i n the absence o f water were c a r r i e d out t o d e t e r m i n e whether f r a c t u r i n g i s due t o pore-entrapped CO2 c a u s i n g s t r e s s on the c o a l s t r u c t u r e d u r i n g p r e s s u r e r e l e a s e . L i t t l e change i n t h e c o a l s i z e o r g r i n d i n g time was observed f o r r a p i d CO2 decompression. We b e l i e v e t h e s e t e s t s j o i n t l y p o i n t t o pronounced synergy o f water w i t h carbon d i o x i d e . Results The c o m p o s i t i o n a l d i f f e r e n c e s i n t r e a t e d and u n t r e a t e d samples of P i t t s b u r g h Seam c o a l are p r e s e n t e d i n T a b l e I I . The 80°C, 1200 p s i CO2 treatment a f f o r d e d a d e c r e a s e i n ash c o n t e n t and v o l a t i l e matter. A s i g n i f i c a n t d e c r e a s e i n s u l f u r was a l s o o b s e r v e d . The s m a l l change i n h e a t i n g v a l u e s u p p o r t s the b e l i e f t h a t t h i s p r o c e s s does not d r a s t i c a l l y a l t e r the c o a l s t r u c t u r e .

T a b l e I I . PROXIMATE ANALYSIS OF TREATED UNTREATED PITTSBURGH SEAM COAL Untreated Ash V o l a t i l e Matter F i x e d Carbon Sulfur

6.48% 38.30% 55.06% 2.49%

Treated 5.93% 37.67% 56.01% 1.96%

AND

Change -8.5% -1.6% +1.7% -21.0%

U l t i m a t e ash a n a l y s e s f o r the t r e a t e d and u n t r e a t e d P i t t s b u r g h Seam c o a l are shown i n T a b l e I I I . In accordance w i t h the r e s u l t s d e s c r i b e d , a l k a l i and a l k a l i n e e a r t h elements g e n e r a l l y appear to be removed e f f i c i e n t l y , a l t h o u g h the r e s u l t s f o r sodium appear anomalous. S i g n i f i c a n t q u a n t i t i e s of i r o n and t i t a n i u m a r e a l s o removed. The s i l i c o n c o n c e n t r a t i o n appears t o remain c o n s t a n t w h i l e the aluminum c o n c e n t r a t i o n i n c r e a s e s . The d a t a p r e s e n t e d i n T a b l e IV can be used t o e v a l u a t e t h e e f f e c t o f treatment on the c o n c e n t r a t i o n s o f s e v e r a l elements f r e q u e n t l y encountered i n c o a l m i n e r a l m a t t e r . Optical emission s p e c t o s c o p y was used t o o b t a i n approximate measurements. The f i r s t two columns l i s t c o n c e n t r a t i o n s found i n the ash from u n t r e a t e d and t r e a t e d samples, r e s p e c t i v e l y , and the t h i r d column l i s t s r e l a t i v e c o n c e n t r a t i o n s i n the s o l i d formed from e v a p o r a t i n g the l i q u i d


MINERAL MATTER AND ASH IN COAL

T a b l e I I I . U l t i m a t e A n a l y s i s o f Ash From T r e a t e d and U n t r e a t e d Samples o f P i t t s b u r g h Seam C o a l Component Si02 A1 0 Fe 0 Ti0 CaO MgO Na 0 K 0 2

2

Untreated 40.45% 23.08% 13.88% 1.11% 8.27% 1.30% 1.60% 1.37%

3

3


2

2

2

Table IV.

Element Si Al Ca Mg Fe Ti Na Mn Ba V Ni

Treated 42.52% 28.18% 11.83% 1.02% 2.01% .59% 1.57% .43%

A n a l y s i s of Leached M i n e r a l M a t t e r and Ash In Ash % Untreated Treated major major major 1 1 0.5 0.1 0.1 0.1 0.08 0.03

major major 0.03 0.05 1 0.08 0.01 0.005 0.01 0.01 0.005

In Leached Mineral Matter,% major maj o r major major 1 1 major 0.1 0.01 0.05 0.1


37.

SAPIENZA ET AL.

Carbon

Dioxide/

Water for

Coal

Beneficiation

507


phase. The l e a c h e d m i n e r a l matter c o n t a i n e d very s i g n i f i c a n t q u a n t i t i e s of s i l i c o n , aluminum, c a l c i u m , magnesium, and sodium. T h i s removal was a l s o r e f l e c t e d i n the r e t a i n e d ash. Although s i l i c o n and aluminum were removed from the c o a l , t h e i r p e r c e n t a g e c o n c e n t r a t i o n s i n the t r e a t e d ash d i d not d i f f e r s i g n i f i c a n t l y from t h a t of the u n t r e a t e d c o a l . I t i s important to n o t e t h a t the more c o r r o s i v e elements, the a l k a l i s and a l k a l i n e e a r t h s , appear t o be p a r t i c u l a r l y r e s p o n s i v e to t h i s t r e a t m e n t . These r e s u l t s g e n e r a l l y i n d i c a t e a t r e n d toward a reduced f r a c t i o n of b a s i c components i n the ash. F i g u r e 1 i l l u s t r a t e s the e f f e c t which t h i s might have on the f u s i o n temperature of the ash. The curve i n t h i s f i g u r e (9) r e p r e s e n t s a c o r r e l a t i o n of d a t a from an e x t e n s i v e number of c o a l f r a c t i o n samples. F o r the case of the P i t t s b u r g h Seam c o a l , treatment w i t h C02/water reduced the p e r c e n tage of b a s i c components from 29.4% t o 18.4% as shown on the figure. The curve i n F i g u r e 1 i n d i c a t e s t h a t t h i s r e d u c t i o n i n p e r c e n t a g e b a s i c components s h o u l d i n c r e a s e ash f u s i o n t e m p e r a t u r e s . In very r e c e n t r e s u l t s (10) i n which f u s i o n temperatures have been measured, an i n c r e a s e has been c o n f i r m e d . The e f f e c t of r e a c t i o n time on ash removal was a l s o e v a l u a t e d i n a l i m i t e d s e t of t e s t s . A f t e r one hour 14.5% o f the ash was removed and a f t e r 20 hours 18.8% was removed. T h i s i n d i c a t e s t h a t r e a c t i o n times as s h o r t as one hour may be s u f f i c i e n t t o a d e q u a t e l y beneficiate coal. F i g u r e 2 compares dry g r i n d i n g times r e q u i r e d f o r P i t t s b u r g h Seam c o a l ( f r o m A r k w r i t e Mine, NA1361) i n i t i a l l y s i z e d t o 1-3/8" t o 3/8". One curve d e p i c t s the weight of u n t r e a t e d c o a l r e m a i n i n g on a 18 mesh (US) s c r e e n a f t e r s p e c i f i e d g r i n d i n g t i m e s . The second c u r v e i s f o r the same c o a l t r e a t e d w i t h CO2/H2O. For t h e s e samples, the g r i n d i n g and s i e v i n g p r o c e s s was r e p e a t e d to a f f o r d an i n d i c a t i o n of d e v i a t i o n i n t h e s e p r o c e s s e s . The f i g u r e i n d i c a t e s a pronounced improvement i n g r i n d i n g . Two-thirds of the t r e a t e d c o a l was below 18 mesh a f t e r about 5 m i n u t e s , w h i l e f o r the u n t r e a t e d c o a l about 70 minutes was r e q u i r e d . Seven-eighths passage r e q u i r e d 10 minutes f o r the t r e a t e d c o a l and w e l l i n excess of 150 minutes f o r the u n t r e a t e d c o a l . For f i n e r f i n a l g r i n d s i z e s , the t r e a t e d c o a l a l s o appears t o r e q u i r e much l e s s g r i n d i n g power. F i g u r e 3 d e p i c t s the amount o f c o a l r e m a i n i n g on a 200 mesh s i e v e as a f u n c t i o n of g r i n d i n g time. Again an o r d e r of magnitude r e d u c t i o n i n g r i n d i n g time i s a f f o r d e d f o r the t r e a t e d c o a l , w i t h 45% o f the c o a l p a s s i n g through the 200 mesh s i e v e a f t e r 35 m i n u t e s . The u n t r e a t e d c o a l reached t h a t l e v e l o n l y a t about 400 m i n u t e s . The product s i z e d i s t r i b u t i o n a f t e r s h o r t g r i n d i n g times i s a l s o very r e v e a l i n g . T h i s a l l o w s b e t t e r e v a l u a t i o n of the e f f e c t of feed c o a l s i z e . The r e s u l t s f o r the 1" χ 3/4" feed are shown i n T a b l e V. Comparison of the 10 minute g r i n d i n g time r e s u l t s f o r t r e a t e d and u n t r e a t e d r e s u l t s show t h a t , as i n our p r e v i o u s l y r e p o r t e d r e s u l t s , t h e r e i s a s i g n i f i c a n t improvement i n g r i n d a b i l ity. T h i s i s p a r t i c u l a r l y prominent at the minus-1/2" s i z e which e s s e n t i a l l y means t h a t the feed c o a l has been weakened. The d i f f e r e n c e i s l e s s s i g n i f i c a n t , however, w i t h a 200 mesh p r o d u c t . U s i n g t h i s s h o r t g r i n d i n g time approach comparisons of wet and dry g r i n d i n g were a l s o s t u d i e d . T a b l e VI shows t h a t the d i m i n i s h i n g improvement from CO2/water treatment w i t h f i n e r c o a l s i z e s o c c u r s f o r both wet and d r y g r i n d i n g .




2100

1

1

10

" 30

20

1 40

1 50

1 60

1 70

1 80

PERCENT BASIC COMPONENTS F i g u r e 1.

Effect

of Ash C o m p o s i t i o n on F u s i o n

Temperatures

Run-of-mine Pittsburgh Seam coal

I

0

I

I

20

I

I

40

I

I

60

I

I

80

I

I

100

I

I

120

I

I

140

Grinding Time, min. F i g u r e 2.

I n i t i a l G r i n d i n g R e s u l t s - 18 Mesh P r o d u c t



SAPIENZA ETAL.

Carbon

Dioxide/

Water for Coal

Beneficiation

509

T a b l e V. Product S i z e D i s t r i b u t i o n 1" χ 3/4" Feed - S h o r t G r i n d i n g Times % o f Product Less Than Screen S i z e - Dry G r i n d i n g Treated Coal Run--of-Mine C o a l Screen S i z e 10 M i n . 6 M i n . 2 Min. 10 Min. G r i n d Grind Grind Grind 1/2" 1/4" 6 mesh 40 mesh 120 mesh 400 mesh

18.4 14.9 14.7 14.1 12.8 11.4

71.7 68.3 61.3 57 45.3 19.4

74 63.1 57.8 48 32 21.1

41.4 32.1 25.4 14.3 7.6 4.7


510


T a b l e V I . Comparison o f Wet and Dry G r i n d i n g Summary 1/2" χ 1/4" Feed C o a l - 30 Second G r i n d i n g Time


Weight % f P r o d u c t Les>s Than S i e v e S i z e Shown 4 1 2 3 Run-of-Mine Run-of-Mine Treated Coal Treated Coal Coal Coal Dry G r i n d i n g Dry G r i n d i n g Wet G r i n d i n g Wet G r i n d i n g 6 mesh

17.1

32.9

24.0

14.1

40 mesh

4.9

8.2

5.8

7.9

80 mesh

2.9

4.3

4.0

4.1

120

mesh

2.2

3.2

3.3

2.9

200

mesh

1.6

2.1

2.6

3.2

E x p e r i m e n t s were a l s o performed t o determine t h e i n i t i a l breakage r a t e o f u n t r e a t e d c o a l (no p r e s s u r e ) , p r e v i o u s l y t r e a t e d c o a l (no p r e s s u r e w h i l e g r i n d i n g ) , and under p r o c e s s c o n d i t i o n s u s i n g an i n t e g r a t e d r e a c t o r / g r i n d e r system. ( A f t e r the exposure t i m e , the m i l l was run as i n t h e o t h e r t e s t s w h i l e s t i l l under pressure.) The product s i z e d i s t r i b u t i o n s a r e g i v e n i n T a b l e V I I . The i n i t i a l breakage r a t e f o r the i n t e g r a t e d t r e a t m e n t / g r i n d i n g systems was 3.7 t i m e s g r e a t e r than t h a t f o r t h e u n p r e s s u r i z e d wet g r i n d i n g and 1.8 times g r e a t e r than u n p r e s s u r i z e d wet g r i n d i n g o f previously treated coal. T h i s demonstrates t h a t t h e c o a l a g g r e g a t e i s weaker (more g r i n d a b l e ) w h i l e i n t h e C02/water s o l v e n t s y s t e m t h a n i t i s when removed.

Table VII. Integrated Reactor Grinder Test 1/2" χ 1/4" Feed - 30 Second G r i n d

Results

Weight ?» o f Products• L e s s Than S i e v e S i z e Shown Sieve

Size

Untreated

-1/2 -1/4 -6 mesh -40 mesh

100.00 15.80 1.70 0.67

-80 mesh -120 mesh -200 mesh

0.38 0.28 0.19

Treated 100.00 31.60 5.40 2.15 1.40 0.98 0.59

Integrated

System

100.00 58.20 12.90 3.50 1.60 1.00 0.69


37.

SAPIENZA ET AL.

Carbon

Dioxide/

Water for

Coal

Beneficiation

511


Conclusions The p r o c e s s of t r e a t i n g c o a l w i t h CO2/water has been shown t o remove as much as 15% o f the ash. Ash removal a p p a r e n t l y i n c r e a s e s w i t h i n c r e a s i n g CO2 c o n c e n t r a t i o n i n the system up to a p p r o x i m a t e l y 24g/L and i s i n v a r i a n t w i t h h i g h e r c o n c e n t r a t i o n s . I t has been shown t h a t , i n a d d i t i o n to ash, s u l f u r i s removed w i t h treatment and t h a t the v o l a t i l e s c o n t e n t and h e a t i n g v a l u e are unchanged. S p e c i f i c ash elements which have been shown t o be removed i n c l u d e s i l i c o n , aluminum, c a l c i u m , magnesium, sodium, and i r o n . The o v e r a l l e f f e c t i s a r e d u c t i o n i n the b a s i c f r a c t i o n i n the ash which c o u l d be v e r y b e n e f i c i a l i n r a i s i n g the ash f u s i o n temperature. From the r e s u l t s of the g r i n d i n g t e s t s w i t h c o a l which was t r e a t e d w i t h CC^/water i t i s c l e a r t h a t the c o a l i s more g r i n d a b l e . The r e s u l t s of the g r i n d a b i l i t y t e s t s , however, a r e a p p a r e n t l y dependent upon m i l l f e e d c o a l s i z e , and product c o a l s i z e . With t h e 1-3/8" χ 3/8" f e e d c o a l , exposure t o the CC^/water environment reduced the time r e q u i r e d f o r g r i n d i n g t h r o u g h 18 mesh by a f a c t o r o f 10. With a s m a l l e r feed c o a l and a f i n e r product s i z e the b e n e f i t s g a i n e d by C02/water exposure were s i g n i f i c a n t l y reduced i n the i n i t i a l s e r i e s of t e s t s . F u r t h e r t e s t i n g , however, showed a c o n s i s t e n t r e d u c t i o n i n b o t h breakage r a t e improvement w i t h t r e a t ment and improvement i n the p r o d u c t i o n r a t e of f i n e product w i t h d e c r e a s i n g feed c o a l s i z e . The p r o d u c t i o n r a t e o f f i n e (minus-200 mesh) c o a l which i s most important s i n c e r e q u i r e d power consumption i s very high i n t h i s i n d u s t r i a l l y s i g n i f i c a n t s i z e r e g i o n . F i n a l l y , r e s u l t s from the i n t e g r a t e d - r e a c t o r g r i n d e r system showed a s i g n i f i c a n t improvement i n g r i n d a b i l i t y even w i t h t h e 1/2" χ 1/4" feed c o a l . T h i s c o n f i r m s the s u g g e s t i o n t h a t the c o a l i s weakest w h i l e s t i l l i n the presence of the C02/water s o l v e n t system. Acknowledgments T h i s r e s e a r c h was p a r t i a l l y supported by a g r a n t through t h e Morgantown Energy Technology C e n t e r from the U.S. Department of Energy under C o n t r a c t No. DE-AC02-76CH00016 ( 1 1 ) . Current support i s b e i n g r e c e i v e d from the R e s e a r c h Department of the F l o r i d a Power and L i g h t Company. F i n a l l y , the authors w i s h to thank Dr. Arden W a l t e r s of FPL f o r s u g g e s t i n g the w r i t i n g of t h i s paper and we a l s o express a p p r e c i a t i o n f o r h i s p r e s e n t a t i o n of our r e s u l t s as p a r t of t h i s Symposium at the ACS N a t i o n a l Meeting i n P h i l a d e l p h i a . A d d i t i o n a l Note S i n c e the c o m p l e t i o n of t h i s p r o j e c t o t h e r p r o c e s s e s u t i l i z i n g carbon d i o x i d e and water b e n e f i c a t i o n have been developed ( 1 2 ) . S p e c i f i c a l l y , i n t h i s work l i q u i d carbon d i o x i d e near the c r i t i c a l p o i n t i s used t o c o a l e s c e the c o a l and l e a v e behind ash components. The c o a l feed i s a water s l u r r y i n t h i s p r o j e c t . The c o a l i s c l e a n e d by the c h a r a c t e r i s t i c s of two i m m i s c i b l e phases i n sharp c o n t r a s t to the m i s c i b l e carbon d i o x i d e water t r e a t m e n t s d e s c r i b e d i n t h i s paper.


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Literature Cited


1.

Berkowitz, N. "An Introduction to Coal Technology"; Academic Press: New York, 1979. 2. Tschamler, R.; DeRuiter, E. "Chemistry of Coal Utilization"; Lowry, H., Ed.; John Wiley & Sons, New York, 1963; Chap. 2. 3. Fuller, E. L. "Coal Structure"; Gorbaty, M.; Ed., Am. Chem. Soc.: Washington, 1981; pp. 293. 4. Schmitt, R. J. of Inst. of Energy. 1981, 54,63. 5. Dryden, I. C. G.; op. c i t . Ref. 3, Chap. 6. 6. Sustmann, H.; Lehnert, R. Brennstoff-Chemie. 1937, 18, 353. 7. Bond, F. C. AIME Trans. Mining Engineering, May 1952, pp. 484, 493. 8. Lowrison, G. C. "Crushing and Grinding"; Butterworths Mono graph in Chemistry and Chemical Engineering, 1974, pp. 75-76. 9. Bryers, R. W.; Taylor, T. E. ASME Winter Annual Meeting, Houston, Nov. 30-Dec. 5, 1975, ASME 75-WA/CD-3. 10. Trinidad, R.; Sapienza, R.; and Butcher, T. "Effects of Carbon Dioxide on Low Rank Coal Grindability and Slurry Preparation," Final Report to Electric Power Research Institute, unpublished data. 11. Sapienza, R.; Slegeir, W.; Butcher, T.; Healy, F. "Coal Fracturing and Heteroatom Removal," Annual Report, Sept. 1983, Brookhaven National Laboratory, BNL-51749. 12. Morsi, Β. I.; Klinzing, G. E. "Licado Process for Super-Clean Coal," Quarterly Report, December 1983, U.S. Department of Energy, DOE/PC-63048-TI. RECEIVED August 30, 1985


Water for Coal Beneficiation

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