Viscosity of Synthetic Coal Ash Slags - American Chemical Society

12 Viscosity of Synthetic Coal Ash Slags Karl S. Vorres , Sherman Greenberg , and Roger Poeppel 1

1

Downloaded by UNIV OF PITTSBURGH on May 4, 2017 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch012

2

2

2

Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 Materials Science and Technology Division, Argonne National Laboratory, Argonne, IL 60439

Twenty one compositions including SiO , Αl O , CaO, MgO and FeO were selected to represent a range of U.S. coal ashes. The viscosities of molten slags were measured over the temperature range 13001550°C. The data were plotted as log reciprocal viscosities (fluidities) versus reciprocal absolute temperatures. One or two straight line segments were observed for each composition. At higher tem peratures the activation energies are less than 100 Kcal/mole, while the values exceed 100 Kcal/mole in the lower temperature regime. The transition temperatures for the plots with two segments were about 1300-1400°C. The transition is thought to involve the appearance of a significant amount of solid material in the melt. Examination of related ternary equilibrium phase diagrams indicated that the transitions occurred in the temperature regime associated with the disappearance of the liquid phase. 2

2

3

C o a l used f o r energy c o n v e r s i o n c o n t a i n s a c o n s i d e r a b l e amount o f mineral matter. D u r i n g the c o n v e r s i o n p r o c e s s the m i n e r a l m a t t e r i s h e a t e d , and i n t h e h i g h e r temperature r e a c t o r s i s c o n v e r t e d t o a molten m a t e r i a l which f l o w s from the r e a c t o r a t a r a t e dependent on t h e v i s c o s i t y of t h e s l a g . I n s t u d i e s o f c o a l s l a g s o b t a i n e d from e l e c t r i c u t i l i t y b o i l e r s (1,2,3) t h i s b e h a v i o r has been s t u d i e d and c o r r e l a t i o n s have been determined between t h e v i s c o s i t y o f the s l a g and the c h e m i c a l c o m p o s i t i o n . These s t u d i e s have been c a r r i e d out i n a range o f gaseous environments t y p i c a l o f t h e combustion f u r nace w i t h a range of oxygen c o n c e n t r a t i o n s from almost z e r o t o 15%. The purposes of t h i s study i n c l u d e d a d e t e r m i n a t i o n of the v i s c o s i t y b e h a v i o r of s y n t h e t i c s l a g s over a range o f c o m p o s i t i o n s and temperatures c h a r a c t e r i s t i c o f s l a g g i n g g a s i f i e r operation. The c o m p o s i t i o n s were chosen t o be b r o a d l y r e p r e s e n t a t i v e of a range of c o a l s from b o t h the e a s t e r n and western U.S. The temper a t u r e s were chosen t o be i n the range of s a t i s f a c t o r y gasifier o p e r a t i o n , and w i t h i n t h e l i m i t s of t h e e x p e r i m e n t a l equipment. 0097-6156/ 86/ 0301 -0156S06.00/ 0 © 1986 American Chemical Society

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

12.

VORRES ET AL.

Viscosity of Synthetic

157

The gaseous environments were s e l e c t e d t o have the low oxygen p a r t i a l p r e s s u r e (about 1 0 ~ to 10"" atm) t y p i c a l of the s l a g g i n g gasifier. The v i s c o s i t y d a t a were to be used as i n p u t f o r a program t o i n v e s t i g a t e c o r r o s i o n of r e f r a c t o r i e s by s l a g s . A c c o r d i n g l y , the d a t a o b t a i n e d f o r the f i r s t few s l a g s were compared w i t h c o r r e l a t i o n s developed by Watt and Fereday (1,2) based on c h e m i c a l compos i t i o n and by Hoy, R o b e r t s and W i l l i a m s ( 3 ) , u s i n g a m o d i f i e d v e r s i o n of the s i l i c a r a t i o . In o r d e r to s i m p l i f y the systems f o r s t u d y , the s y n t h e t i c s l a g s were l i m i t e d t o the f i v e components: S i 0 2 , AI2O3, FeO, CaO and MgO. S i n c e they c o n t a i n e d no Na20 o r K2O, the s l a g c o m p o s i t i o n s were o u t s i d e the range of the e a r l i e r correlations. I f t h i s d i f f e r e n c e was n e g l e c t e d , then the composit i o n f o u r of the 21 s y n t h e t i c s l a g s used i n t h i s program f e l l i n s i d e the range of the c o m p o s i t i o n s f o r which the c o r r e l a t i o n s were developed. Those o u t s i d e the range had 10% AI2O3 or 0% MgO or a low silica ratio or a h i g h S i 0 2 / A l 2 0 3 ratio. The earlier c o r r e l a t i o n s , i n g e n e r a l , were f o r o n l y a p a r t of the c o m p o s i t i o n range used f o r the s y n t h e t i c s l a g s . 8


Coal Ash Slags

9

There i s an added i n t e r e s t i n d e v e l o p i n g the u n d e r s t a n d i n g of the b e h a v i o r of molten c o a l ash systems beyond somewhat e m p i r i c a l correlations of c h e m i c a l c o m p o s i t i o n s expressed i n terms of a v a r i e t y of r a t i o s . I t would be d e s i r a b l e to d e s c r i b e the f l o w b e h a v i o r i n terms of the i n t e r a c t i o n s between the i n d i v i d u a l cons t i t u e n t s , and t o u n d e r s t a n d the n a t u r e of the a c i d s and b a s e s , s u c h t h a t the r e a s o n f o r the p a r t i a l s u c c e s s of e m p i r i c a l c o r r e l a t i o n s u s i n g these c o n c e p t s can be u n d e r s t o o d .

Experimental Slag. The v i s c o s i t i e s of 21 s y n t h e t i c s l a g s , c o v e r i n g the range of c o m p o s i t i o n s e x p e c t e d i n s l a g s d e r i v e d from American c o a l s , were determined i n t h i s s t u d y . The s y n t h e t i c s l a g s were p r e p a r e d from reagent grade c h e m i c a l s . C a l c i u m c a r b o n a t e , magnesium o x i d e and m a g n e t i t e , (Fe30i+), s i l i c a and alumina were used. The components were mixed w i t h water and p r e s s e d i n t o p e l l e t s u s i n g a p r e s s u r e of 15,000 p s i g . The p e l l e t s were p a r t i a l l y d r i e d and i n s e r t e d i n AI2O3 c r u c i b l e s about 63 mm h i g h and 32 mm i n t e r n a l diameter f o r t h e v i s c o s i t y measurements. A p p r o x i m a t e l y 65 grams of s l a g were used and the melt reached a depth of about 35 mm w i t h the measuring "bob" i n s e r t e d . A l t h o u g h alumina i s not a s u i t a b l e m a t e r i a l f o r l o n g - t e r m containment of these s l a g s at e l e v a t e d t e m p e r a t u r e s , the amount of d i s s o l u t i o n was i n s i g n i f i c a n t f o r the 24 hour exposure time w i t h o u t s l a g motion w i t h r e s p e c t to the c r u c i b l e . The compos i t i o n of the s l a g s i s g i v e n i n T a b l e I . Viscometer. The a p p a r a t u s and t e c h n i q u e have been d e s c r i b e d i n d e t a i l (4_). E s s e n t i a l l y a B r o o k f i e l d Rheolog was used t o r e p l a c e the sample head i n the r o t a t i n g c y l i n d e r s l a g c o r r o s i o n a p p a r a t u s used i n slag/refractory corrosion s t u d i e s a t Argonne N a t i o n a l L a b o r a t o r y (ANL) (5). A p p r o p r i a t e s e a l s and ceramic structural components p e r m i t maintenance of the d e s i r e d low oxygen a c t i v i t y w i t h i n the measuring chamber. The v i s c o s i t y measuring bob was a


158

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

c y l i n d e r 12.8 mm d i a m e t e r and 11.1 mm h i g h . F o r measurement a t the lower oxygen p a r t i a l p r e s s u r e s d i s c u s s e d i n t h i s paper, the bob and c o n n e c t i n g s h a f t were f a b r i c a t e d of molybdenum. The s l a g was cont a i n e d i n AI2O3 c r u c i b l e s . The bob and measuring system were c a l i b r a t e d a t room temperature u s i n g a s e r i e s of NBS o i l s r a n g i n g from 10 t o 600 p o i s e . The v o l t a g e - v i s c o s i t y r e l a t i o n s h i p was l i n e a r over t h e range of r o t a t i o n a l speeds used i n t h e c a l i b r a t i o n runs (0.5-100 rpm). F o r s l a g v i s c o s i t y measurements o n l y one r o t a t i o n a l speed, 20 rpm, was used.


Table I.

S l a g Compositions

(weight % ) .

S l a g Number Composition Si0 CaO

2

AI2O3 FeO MgO

Si02 CaO AI2O3 FeO MgO

Si02 CaO AI2O3 FeO MgO

1

2

3

4

5

6

7

50 5 10 15 20

50 5 10 25 10

50 5 20 25 0

50 5 20 15 10

50 5 20 5 20

50 5 30 15 0

50 5 30 5 10

8

9

10

11

12

13

14

40 15 10 15 20

40 15 10 25 10

40 15 20 25 0

40 15 20 15 10

40 15 20 5 20

40 15 30 15 0

40 15 30 5 10

15

16

17

18

19

20

21

30 25 10 15 20

30 25 10 25 10

30 25 20 25 0

30 25 20 15 10

30 25 20 5 20

30 25 30 15 0

30 25 30 5 10

Procedure. V i s c o s i t y measurements were u s u a l l y made i n a d e c r e a s i n g temperature mode a t 50°C i n t e r v a l s a f t e r t h e s l a g sample had been heated t o t h e d e s i r e d maximum t e m p e r a t u r e , t y p i c a l l y about 1400-1550°C. The maximum temperature c o r r e s p o n d e d t o the maximum o p e r a t i n g temperature e x p e c t e d i n commercial c o a l g a s i f i e r s and was a l s o the temperature at which v e r y low (a few c e n t i p o i s e ) v i s c o s i t i e s were o b s e r v e d . The maximum temperatures were reached i n a c o n t r o l l e d manner t o a v o i d apparatus problems. The s l a g was h e a t e d from room temperature t o 100°C i n one hour and h e l d f o r 1/2 hour. From 100°C t o t h e maximum temperature, the h e a t i n g r a t e was 175°C/hour. F o r v i s c o s i t y measurements, the s l a g was kept a t each temperature l o n g enough t o demonstrate c o n s t a n t v i s c o s i t y (about


12.

VORRES ET AL.


Coal Ash

Slags

159


30-60 m i n u t e s ) . In t h e case of one s l a g , 12, measurements were a l s o made i n an i n c r e a s i n g temperature mode t o determine i f t h e r e were h y s t e r e s i s e f f e c t s . None were observed f o r t h i s s l a g and o t h e r work w i t h n a t u r a l s l a g s c o n f i r m e d t h i s ( 4 ) . Measurements made i n o t h e r l a b o r a t o r i e s w i t h o t h e r s l a g s have shown apparent h y s t e r e s i s ( 6 ) . The d e s i r e d oxygen p a r t i a l p r e s s u r e was m a i n t a i n e d by f l o w i n g H2-CO2-N2 ( o r argon) m i x t u r e s of t h e r e q u i r e d composit i o n through t h e i n t e r i o r of the measuring chamber throughout t h e experiment. The v a r i a t i o n of oxygen p a r t i a l p r e s s u r e w i t h gas c o m p o s i t i o n and temperature was c a l c u l a t e d u s i n g a NASA-developed code (4_). A t t h e oxygen p a r t i a l p r e s s u r e s used, t h e i r o n was maint a i n e d i n the f e r r o u s s t a t e .

R e s u l t s and D i s c u s s i o n The d a t a o b t a i n e d were p l o t t e d as v i s c o s i t y v e r s u s temperature f o r the d i f f e r e n t m a t e r i a l s and d i s p l a y e d the e x p e c t e d exponential i n c r e a s e i n v i s c o s i t y as t h e temperature d e c r e a s e d . F o r most of the s l a g s a c h a r a c t e r i s t i c sudden i n c r e a s e i n v i s c o s i t y was n o t e d , as i n some r e l a t e d s t u d i e s ( 1 , 3 ) . Some t y p i c a l r e s u l t s a r e shown and compared w i t h t h e Watt-Fereday and m o d i f i e d silica ratio p r o j e c t i o n s , assuming o n l y a l i q u i d phase e x i s t s , i n F i g u r e s 1 and 2 ( s l a g s 1 & 12). To f u r t h e r i n v e s t i g a t e the c h a r a c t e r i s t i c s of t h e s l a g s , p l o t s of the l o g a r i t h m o f v i s c o s i t y v e r s u s temperature were made. These i n d i c a t e d s t r a i g h t l i n e s o r two l i n e segments. F o r slags with a sudden i n c r e a s e i n v i s c o s i t y at lower t e m p e r a t u r e s , two segments were o b s e r v e d . Shear r a t e s were not v a r i e d and t h e v a r i o u s types of non-Newtonian b e h a v i o r were not e x p l o r e d . A related study i n d i c a t e d p s e u d o p l a s t i c b e h a v i o r f o r t h i s type of m a t e r i a l (9_). A r r h e n i u s p l o t s were then made. These p l o t s t y p i c a l l y i n v o l v e the l o g a r i t h m of a r a t e c o n s t a n t and t h e r e c i p r o c a l o f the a b s o l u t e temperature. V i s c o s i t y i s not a r a t e parameter, but i s d e f i n e d as t h e shear s t r e s s d i v i d e d by the shear r a t e . The r e c i p r o c a l of t h e v i s c o s i t y i s the shear r a t e per u n i t shear s t r e s s and was used i n the p l o t s . A t y p i c a l example i s shown i n F i g u r e 3. S l a g 1 shows a typical high-temperature, low a c t i v a t i o n energy regime w i t h a t r a n s i t i o n t o a h i g h a c t i v a t i o n energy, low temperature regime. T h i s b e h a v i o r was noted f o r most of t h e c o m p o s i t i o n s . The o t h e r b e h a v i o r was, as i n t h e case of s l a g 12, a s i n g l e s t r a i g h t l i n e c o v e r i n g the range o f the d a t a . The s l o p e s and a c t i v a t i o n e n e r g i e s f o r s l a g s w i t h o u t the t r a n s i t i o n tended t o be i n t e r m e d i a t e i n t h e range of the v a l u e s f o r runs w i t h the t r a n s i t i o n . I n o r d e r t o compare t h e v i s c o s i t y b e h a v i o r of t h e d i f f e r e n t c o m p o s i t i o n s f o r the h i g h e r temperature regime and the s l a g s w i t h no t r a n s i t i o n s , s e p a r a t e p l o t s s u p e r i m p o s i n g t h e sample s e r i e s w i t h a c o n s t a n t wt% S i 0 2 were made and a r e shown i n F i g u r e s 4, 5 and 6. Note t h e v e r t i c a l change i n s c a l e i n F i g u r e 6. The s o l i d p o r t i o n of t h e l i n e s r e p r e s e n t s the a c t u a l range of d a t a . The dashed p a r t of t h e l i n e s was added t o f a c i l i t a t e v i s u a l comparison. Examina t i o n of the p l o t s shows two g e n e r a l t e n d e n c i e s . The r e c i p r o c a l viscosities o r f l u i d i t i e s tend t o i n c r e a s e i n a s e r i e s as t h e amounts of S i 0 2 d e c r e a s e s . A d d i t i o n a l l y , f o r a given s e r i e s with a


160

MINERAL MATTER AND ASH IN COAL

6ΟΟ1

SLAG. NO. I PRESENT WORK

500

H

C02

2

N

2

3% 3 % BAL SILICA RATIO WATT-FEREOAY

400


~ 3001

200

100

1100

1200

1300

1400

TEMPERATURE (°C)

Figure

1.

V i s c o s i t y of S l a g

1.

600. SLAG NO. 12 PRESENT WORK 500

δ

AIR H2

•

_ CO2 N

2

3% 3% BAL.H

400hi §

— SILICA RATIO WATT-FEREDAYl

3001—

200

iooh1800

1200

1300

1400

TEMPERATURE (°C)

F i g u r e 2.

V i s c o s i t y o f S l a g 12.



VORRES ET AL.


Figure 3.

Coal Ash

T y p i c a l Arrhenius

Slags

Plots.


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

Τ (°C)


1579

1513

1451

1394

T"

Figure

4.

1

1340

(I0

4

1290

1242

1198

K )

Arrhenius Plots

H

f o r Slags

1-7.


VORRES ET A L .


Coal Ash

Slags


12.


163


164

Τ CO 1513

1451

1394

1340

1290

1242

1198


1579

F i g u r e 6. Arrhenius P l o t s f o r Slags V i s c o s i t y v s . R e c i p r o c a l Temperature.

15-21, Log

of R e c i p r o c a l



12.

VORRES ET AL.


Coal Ash Slags

165

f i x e d wt% S i 0 2, the f l u i d i t i e s are g r e a t e r f o r the lower amounts of A l 20 3. E a r l i e r work (7,10,11) d e s c r i b e d the c h e m i c a l b e h a v i o r of the s l a g c o n s t i t u e n t s i n terms of a c i d s and bases. V o r r e s (_7) used the concept of i o n i c p o t e n t i a l ( r a t i o of i o n i c charge t o c r y s t a l l o g r a p h i c r a d i u s ) to d i f f e r e n t i a t e s l a g c o n s t i t u e n t s on the b a s i s of their ability to a t t r a c t a common a n i o n ( o x i d e i o n s i n these systems). The s t r o n g e s t a c i d s a t t r a c t anions most s t r o n g l y or a r e most a b l e to e f f e c t i v e l y compete f o r anions to complete a r e g u l a r c l o s e packed c o o r d i n a t i o n . The bases are not a b l e t o compete f o r a n i o n s , and s e r v e p r i m a r i l y as oxide i o n donors. T h i s type of system i n v o l v e s the f o r m a t i o n of l a r g e polymers by the a c i d s , and polymer b r e a k i n g by the bases. I n the presence of a l a r g e amount of a v e r y s t r o n g a c i d , Si02> some a c i d i c cons t i t u e n t s such as AI2O3 and Fe203, behave w i t h an amphoteric c h a r acter. I r o n i s q u i t e i n t e r e s t i n g s i n c e i t i s such a common c o n s t i t u e n t i n c o a l ash, e s p e c i a l l y i n e a s t e r n U.S. c o a l s , and does have two common v a l e n c e s t a t e s . F e r r r i c i r o n behaves as a weak a c i d w h i l e f e r r o u s i s c l a s s i f i e d as a weak base. Thus i n the more o x i d i z i n g environment of a b o i l e r , c o a l s l a g s have a more a c i d i c o r h i g h l y p o l y m e r i z e d c h a r a c t e r than i n the l e s s o x i d i z i n g e n v i r o n ment of the s l a g g i n g g a s i f i e r or c y c l o n e combustor. For the measurements d i s c u s s e d i n t h i s c h a p t e r , a l l i r o n s p e c i e s are b e l i e v e d to e x i s t i n the f e r r o u s s t a t e . I n a g e n e r a l way, the l a r g e r the amount of S i 0 2 and t o t a l a c i d c o n s t i t u e n t s , the l a r g e r the s i z e of h y p o t h e t i c a l average silica polymer s p e c i e s i n the m e l t . G a s k e l l has d e s c r i b e d a r e l a t i o n s h i p between s i l i c a s p e c i e s and the b a s i c c o m p o s i t i o n ( 8 ) . The c o r r e l a t i o n s developed by Watt and Fereday were i n t e n d e d f o r a p p l i c a t i o n t o a l i q u i d system. As the temperature v a r i e s and increases, the composition of a dynamic system i s s u b j e c t t o changes due to p r e f e r e n t i a l v o l a t i l i z a t i o n of s p e c i e s w i t h h i g h e r vapor p r e s s u r e s . The a l k a l i e s t y p i c a l l y have the h i g h e r vapor pressures i n these systems. The acidity can be expected to s l i g h t l y i n c r e a s e as a r e s u l t of t h i s v o l a t i l i z a t i o n . As the temperatures are reduced, s o l i d phases w i l l c r y s t a l l i z e from the melt. The Watt and Fereday c o r r e l a t i o n s do not apply t o this region. The s t u d i e s r e p o r t e d here i n c l u d e the temperature r e g i o n i n which c r y s t a l s can be expected to form. The authors d i d not f i n d l i t e r a t u r e r e f e r e n c e s to phase e q u i l i b r i a s t u d i e s f o r the S i 0 2-Al203-Ca0-Mg0-Fe0 or Si0 -Al203"Ca0-Mg0 systems i n the comp o s i t i o n r e g i o n of i n t e r e s t . As a r e s u l t some s p e c u l a t i o n on b e h a v i o r i s i n v o l v e d i n any e x t e n s i v e i n t e r p r e t a t i o n of the d a t a . A c t i v a t i o n e n e r g i e s and the temperature ranges of d a t a are given i n Table I I . I n i t i a l s t a t i s t i c a l a n a l y s e s have not shown a s t r o n g c o r r e l a t i o n of a c t i v a t i o n e n e r g i e s w i t h any of the s l a g constituents. A wide range of a c t i v a t i o n e n e r g i e s w i t h v e r y h i g h v a l u e s was o b t a i n e d f o r the lower temperature regime. In a number of cases both the temperature range and the number of p o i n t s were s m a l l , such t h a t the r e l a t i v e e r r o r i n these v a l u e s can be l a r g e . When the d a t a cover the range from 1300-1400°C the t r a n s i t i o n from the lower to the h i g h e r a c t i v a t i o n e n e r g i e s g e n e r a l l y o c c u r r e d i n t h i s range. One reason for the observed results is that the 2



38.8 44.6 51.0 37.9 59.3 106 70.1 83.4 14.0 52.0 39.0 101 35.5 50.8 107 86.0 40.3 66.8 20.1 56.5 111

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 1340-1268 1315-1244 1335-1299 1312-1273 1520-1500

1295-1170 1330-1314 1300-1265 1460-1420 1400-1380

1315-1246 1351-1328

374 148

960 106 363

85.6 288 261

90.0 469

240

297

9

Low Temperature Ea Range

9

Sum Base - sum o f mole f r a c t i o n s o f CaO, MgO, FeO. B/A = sum base/mole f r a c t i o n s of S i 0 + Α 1 0 ο ·

1440-1340 1460-1315 1456-1344 1513-1335 1462-1312 1550-1520 1515-1415 1353-1192 1455-1295 1454-1330 1439-1300 1436-1262 1535-1460 1484-1400 1434-1339 1390-1231 1414-1315 1448-1202 1459-1351 1402-1294 1532-1415

High Temperature Ea Range

Slag # 2

.483 .515 .568 .529 .495 .584 .543 .384 .409 .451 .420 .393 .464 .431 .286 .272 .335 .313 .293 .345 .321

Si0

.057 .061 .134 .125 .117 .209 .192 .056 .060 .133 .124 .116 .205 .190 .056 .060 .132 .123 .115 .203 .189

2

3

.052 .055 .061 .057 .053 .063 .058 .154 .164 .181 .169 .158 .186 .173 .255 .272 .299 .279 .261 .308 .286

Mole F r a c t i o n s A1 0 CaO

.121 .215 .237 .133 .041 .147 .045 .120 .214 .235 .132 .041 .145 .045 .119 .212 .234 .131 .041 .144 .045

FeO

T a b l e I I . A c t i v a t i o n E n e r g i e s , Range o f Temperatures and Mole


.288 .154 .000 .158 .295 .000 .162 .286 .152 .000 .156 .293 .000 .161 .284 .151 .000 .155 .291 .000 .159

MgO

Fractions.

.461 .424 .298 .348 .389 .210 .265 .560 .530 .416 .457 .492 .331 .378 .658 .635 .533 .565 .593 .452 .490

Sum Base

0.85 0.74 0.42 0.53 0.64 0.26 0.36 1.27 1.11 0.71 0.84 0.97 0.49 0.61 1.92 1.74 1.14 1.30 1.46 0.82 0.96

Β /A

12.

VORRES ET A L .


Coal Ash

Slags

167


c o n c e n t r a t i o n of c r y s t a l l i t e s i n c r e a s e d s u f f i c i e n t l y t o i n t e r a c t w i t h one a n o t h e r , h i n d e r i n g r o t a t i o n of the bob. I n o r d e r t o i n t e r p r e t the d a t a the t e r n a r y e q u i l i b r i u m phase diagrams f o r the systems Si02-Al2Û3"MO were examined where MO i s e i t h e r CaO, FeO, or MgO. The mole f r a c t i o n s of each of the cons t i t u e n t s were c a l c u l a t e d as a l s o shown i n T a b l e I I , and the t e r n a r y diagram c o r r e s p o n d i n g to the major base i n the group CaO, FeO or MgO was s e l e c t e d . U s u a l l y a t e r n a r y e u t e c t i c was found i n the temperature r e g i o n which would be expected f o r a system most c l o s e l y c o r r e s p o n d i n g t o the sample c o m p o s i t i o n . This e u t e c t i c temperature was c l o s e t o the observed t r a n s i t i o n temperatures in the v i s c o s i t y d a t a . Many of the h i g h e s t temperatures used were s i g n i f i c a n t l y below those a s s o c i a t e d w i t h the appearance of one or more s o l i d phases from the melt. These s o l i d phases c o u l d have been p r e s e n t through the e n t i r e s e r i e s of measurements on the s l a g . The s i l i c a r i c h systems are known t o be slow t o i n i t i a t e f o r m a t i o n of solid phases. K a l m a n o v i t c h has shown t h a t the observed phases are not always those which are expected on the b a s i s of the c o r r e s p o n d i n g n o r m a l i z e d phase e q u i l i b r i u m diagrams ( 1 2 ) . I t i s thought t h a t the t r a n s i t i o n i s due to a change from a system which has a s i g n i f i c a n t amount of l i q u i d phase t o one i n which the system i s p r e d o m i n a n t l y s o l i d phases, and the measured apparent viscosity reflects the l a r g e energy requirement f o r two s o l i d s u r f a c e s or p a r t i c l e s t o move past one another through some s h e a r i n g a c t i o n . The t r a n s i t i o n s have a l s o been observed i n s t u d i e s of n a t u r a l c o a l ashes which have had i n c r e a s i n g amounts of c a l c i u m o x i d e added. The d a t a were i n t e r p r e t e d i n terms of a s a t u r a t i o n and the a u t h o r s f e l t t h a t the base then s e r v e d as an f u r t h e r p o l y m e r i z e r i n the network ( 1 3 ) . There i s a c o n t i n u i n g i n t e r e s t i n e s t a b l i s h i n g a r e l a t i o n s h i p between the c o m p o s i t i o n of a molten o x i d e system c o n t a i n i n g s i l i c a and the v i s c o s i t y of the system over a range of temperatures. Systems of t h i s type are of i n t e r e s t , f o r example, i n the study o f magmas ( 9 ) , f l u x e s f o r c a s t i n g s t e e l (14) and i n metallurgy. C o r r e l a t i o n s have been developed by B o t t i n g a and W e i l f o r magma systems (15). U r b a i n has described correlations for silicate materials (16). The i n t e r p r e t a t i o n of the v i s c o s i t y - t e m p e r a t u r e b e h a v i o r of these complex systems i s d i f f i c u l t s i n c e many a s p e c t s of the melt c o n d i t i o n s must be s i m u l t a n e o u s l y c o n s i d e r e d . These i n c l u d e : the c h e m i c a l c o m p o s i t i o n of the melt t o e s t a b l i s h the n a t u r e of the p o l y m e r i c network i n c l u d i n g the amphoteric b e h a v i o r of s p e c i e s l i k e AI2O3 and F e 0 3 , as w e l l as the a c i d / b a s e b e h a v i o r of mixed v a l e n c e c o n s t i t u e n t s such as i r o n o x i d e s , and the f o r m a t i o n of i m m i s c i b l e l i q u i d phases sometimes a s s o c i a t e d w i t h the e x i s t e n c e of s e v e r a l t y p e s of s t a b l e anions of s i g n i f i c a n t l y d i f f e r e n t s i z e or charge i n the system; the n a t u r e of the c o n t a i n e r s i n c e some of i t may diss o l v e and a f f e c t the c o m p o s i t i o n of the melt; the e x i s t e n c e of a solid phase t o e s t a b l i s h the e f f e c t on the c o m p o s i t i o n of the r e s i d u a l l i q u i d phase (the s o l i d phase may not be the one expected from r e l a t e d phase e q u i l i b r i u m s t u d i e s ) ; the r e l a t i v e amount of the l i q u i d and s o l i d phases t o e s t a b l i s h the c o m p o s i t i o n of the l i q u i d phase ( t h i s c o m p o s i t i o n changes as the s o l i d c r y s t a l l i z e s out o f 2


168


the m e l t ) ; the thermal h i s t o r y of the sample s i n c e the r e a c t i o n s i n v i s c o u s media tend t o be s l u g g i s h , such t h a t c r y s t a l l i n e phases may be slow t o form and changes i n t h e p o l y m e r i c e q u i l i b r i u m s i z e may not have been a c h i e v e d . I n a d d i t i o n the l o s s o f v o l a t i l e s p e c i e s and the r e s u l t a n t change i n the c o m p o s i t i o n o f t h e r e s i d u a l l i q u i d must be c o n s i d e r e d . I n p r a c t i c e , many i n v e s t i g a t o r s c o n s i d e r a number o f t h e s e f a c t o r s . F u t u r e work w i l l be improved t o the e x t e n t t h a t more o f them a r e i n c l u d e d i n the r e p o r t s o f r e s e a r c h i n this field.


Conclusions A s e r i e s o f 21 s y n t h e t i c c o a l observed t h a t :

ash slags

were

studied.

I t was

(1)

P l o t s o f the l o g a r i t h m of v i s c o s i t y v e r s u s temperature showed one o r two s t r a i g h t line segments, c o n s i s t e n t w i t h o t h e r observations on s i m i l a r systems i n t h e temperature range studied.

(2)

P l o t s o f the l o g a r i t h m o f the r e c i p r o c a l o f v i s c o s i t y versus r e c i p r o c a l o f a b s o l u t e temperature a l s o showed one o r two s t r a i g h t l i n e segments, i n d i c a t i n g one or two mechanisms were o p e r a t i v e over the temperature range.

(3)

F o r t h r e e s e r i e s , v a r y i n g i n S i 0 2 c o n t e n t , those g r e a t e s t S i 0 2 c o n t e n t had the h i g h e s t v i s c o s i t i e s .

(4)

W i t h i n a s e r i e s of g i v e n Si02 c o n t e n t , those members w i t h the h i g h e s t AI2O3 c o n t e n t had the h i g h e s t v i s c o s i t y .

(5)

F o r slags e x h i b i t i n g a t r a n s i t i o n i n behavior, the t r a n s i t i o n temperature could usually be a s s o c i a t e d with a ternary e u t e c t i c temperature i n t h e phase e q u i l i b r i u m diagram f o r t h e most c l o s e l y r e l a t e d t e r n a r y system.

(6)

Many o f the s l a g s p r o b a b l y had a s o l i d phase p r e c i p i t a t i n g from the l i q u i d phase d u r i n g the c o o l i n g p e r i o d b e f o r e t h e t r a n s i t i o n temperature.

w i t h the

Acknowledgment s The a u t h o r s g r a t e f u l l y acknowledge the support from the Chemical S c i e n c e s D i v i s i o n of the O f f i c e o f B a s i c Energy S c i e n c e s and from the S u r f a c e G a s i f i c a t i o n M a t e r i a l s Program of t h e U.S. Department of Energy, under c o n t r a c t number W-31-109-ENG-38.

Literature Cited 1. 2.

Watt, J . D.; Fereday, F. J . Inst. Fuel 1969, 42, 101. Watt, J . D. J . Inst. Fuel 1969, 42, 131.


12. VORRES ET AL.

Viscosity of Synthetic Coal Ash Slags


3.

A,

169

Hoy, H. R.; Roberts, A. G.; Williams, D. M. IGE Journal 1965, 5, 444. 4. Chen, J.; Greenberg, S.; Poeppel, R. "The Viscosity of Coal Slags as a Function of Composition, Temperature and Oxygen Partial Pressure", U. S. Department of Energy ANL/FE-83-30 (March, 1984). 5. Greenberg, S.; R. Poeppel, R.; et al., "The Corrosion of Ceramic Refractories Exposed to Synthetic Coal Slags By Means of the Rotating-Cylinder Technique: An Interim Report", U. S. Department of Energy ANL/FE-83-31, (September, 1984). 6. Streeter, R. C.; Diehl, Ε. K.; Schobert, H. H. Preprints Am. Chem. Soc. Div. Fuel Chem. 1983, 28(4), 174. 7. Vorres, K. S. Preprints Am. Chem. Soc. Div. Fuel Chem. 1977, 22(4), 118. 8. Gaskell, D. R. Metallurgical Treatises, Metallurgical Society of AIME, 1982, 59. 9. Weed, H. C.; Ryerson, F. J.; Piwinskii, A. J . Preprints Am. Chem. Soc. Div. Fuel Chem. 1984, 29(4), 157 and subsequent chapter in this volume. 10. Sage, W. L.; McIlroy, J . B. Trans. ASME, J . Eng. Power, Ser. 1960, 82, 145. 11. Reid, W. T. in "Chemistry of Coal Utilization", Second Supplementary V o l . , E l l i o t t , Μ. Α., Ed.; Wiley-Interscience, New York, 1981; p. 1389. 12. Kalmanovitch, D. P.; and J . Williamson, J . Preprints, Am. Chem. Soc Fuel Chem. Div. 1984, 29(4), 162 and subsequent chapter in this volume. 13. Wang, C-s; Wang, C-y "The Relationship between Viscosity and Mole Composition of Coal Ash with Addition of Calcia", (Chinese) J . Coal Science, 1984, 1, 39-45. 14. McCauley, W. L . ; Apelian, D. Preprints Am. Chem. Soc. Div. Fuel Chem. 1984, 29(4), 151 and this volume. 15. Bottinga, Y.; Weil, D. F. Am. J. Science 1972, 272, 438-75. 16. Urbain, G.; Cambier, F . ; Deletter, M.; Anseau, M. R. Trans. Brit. Ceram. Soc. 1981, 80, 139-41. RECEIVED October 24, 1985


Viscosity of Synthetic Coal Ash Slags - American Chemical Society

Recommend Documents