Mineral Matter and Ash in Coal - American Chemical Society

15 Estimation of Physicochemical Properties of Coal Slags and Ashes K. C. Mills

Downloaded by MIT on July 2, 2013 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch015

National Physical Laboratory, Teddington, Middlesex TW11 OLW, United Kingdom

The various methods available for estimating the melting range, viscosity, density, surface tension, heat capacity, enthalpy, thermal conductivity, absorption coefficient and emissivity of coal slags and ashes are reviewed and evaluated. New routines for estimating the density, surface tension, heat capacity and enthalpy of coal slags from their chemical composition are presented and assessed. It was concluded that the distribution of iron in the slag between free Fe, FeO and Fe O was very important as virtually every physical property of the slag was greatly influenced by this factor. 2

3

S l a g s formed d u r i n g t h e g a s i f i c a t i o n o f c o a l u s u a l l y c o n t a i n S 1 O 2 , A I 2 O 3 , i r o n o x i d e s j CaO, N a 2 0 and K 0 w i t h minor amounts o f v a r i o u s other oxides. A knowledge o f t h e p h y s i c o - c h e m i c a l p r o p e r t i e s o f t h e s e s l a g s c a n improve t h e c o n t r o l o f t h e p r o c e s s eg. t h e amount of f l u x required t o b r i n g the s l a g v i s c o s i t y t o a l e v e l s u i t a b l e f o r t a p p i n g c a n be c a l c u l a t e d from v i s c o s i t y - c o m p o s i t i o n r e l a t i o n s . P h y s i c a l p r o p e r t y d a t a f o r t h e c o a l s l a g s c a n a l s o improve p r o c e s s d e s i g n by p r o v i d i n g i n p u t v a l u e s f o r m a t h e m a t i c a l models o f t h e p r o c e s s eg. t h e r m a l p r o p e r t i e s o f t h e s l a g s a r e needed f o r h e a t b a l a n c e c a l c u l a t i o n s . There i s an a p p r e c i a b l e v a r i a t i o n i n t h e c o m p o s i t i o n o f s l a g s formed from v a r i o u s c o a l s and even from d i f f e r e n t b a t c h e s o f t h e same s t o c k on o c c a s i o n s and t h e s e composit i o n a l v a r i a t i o n s can give r i s e t o c o n s i d e r a b l e d i f f e r e n c e s i n the physical properties. As t h e c h e m i c a l a n a l y s i s i s f r e q u e n t l y a v a i l a b l e on a r o u t i n e b a s i s i t would be p a r t i c u l a r l y d e s i r a b l e t o have r e l i a b l e models f o r t h e p r e d i c t i o n o f p h y s i c o - c h e m i c a l p r o p e r t i e s from t h e i r c h e m i c a l c o m p o s i t i o n . F u r t h e r m o r e such models would have t h e f u r t h e r advantage t h a t t h e need f o r arduous i n t e r p o l a t i o n s on p s e u d o - t e r n a r y p l o t s f o r s l a g s , which a r e r e a l l y multicomponent and i n t e r a c t i v e systems, would be e l i m i n a t e d . 2

This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

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


196

MINERAL MATTER AND ASH IN COAL

The p r o p e r t i e s o f s l a g s a r e dependent n o t o n l y upon c h e m i c a l c o m p o s i t i o n , b u t upon o t h e r f a c t o r s a l s o . The most pronounced d e v i a t i o n s from a d d i t i v i t y r u l e s based on c o m p o s i t i o n a r i s e i n t h e e s t i m a t i o n o f t h o s e p r o p e r t i e s which i n v o l v e i o n i c t r a n s p o r t eg. e l e c t r i c a l c o n d u c t i v i t y . However s u r f a c e t e n s i o n v a l u e s e s t i m a t e d from a d d i t i v i t y r u l e s a r e f r e q u e n t l y i n e r r o r as b u l k thermo dynamic p r o p e r t i e s do n o t a p p l y a t s u r f a c e s . Furthermore, v i r t u a l l y a l l t h e p h y s i c a l p r o p e r t i e s o f s l a g s a r e , t o some e x t e n t , dependent upon t h e s t r u c t u r e o f t h e s l a g ( v i z . t h e l e n g t h o f s i l i c a t e c h a i n s , degree o f c r y s t a l l i n i t y e t c . ) thus e s t i m a t i o n p r o c e d u r e s have t o accommodate t h e s e s t r u c t u r a l f a c t o r s , where p o s s i b l e . There i s o n l y a l i m i t e d amount o f p h y s i c a l p r o p e r t y d a t a a v a i l a b l e f o r c o a l s l a g s and c o n s e q u e n t l y i t has been n e c e s s a r y t o examine a much b r o a d e r range o f s i l i c a t e s i n c l u d i n g magmatic l i q u i d s and t h o s e s l a g s e n c o u n t e r e d i n s t e e l m a k i n g , g l a s s m a k i n g and nonferrous processes. Thus t h e models d e s c r i b e d here s h o u l d have a much wider range o f a p p l i c a b i l i t y . A c r i t i c a l e v a l u a t i o n o f t h e e x t a n t p h y s i c a l p r o p e r t y d a t a has shown t h a t v i r t u a l l y e v e r y p h y s i c a l p r o p e r t y i s markedly dependent upon t h e d i s t r i b u t i o n o f i r o n i n t h e s l a g between FeO, Fe203 and free iron. F r e q u e n t l y t h e s e d i s t r i b u t i o n s a r e n o t r e p o r t e d and a p p r o p r i a t e v a l u e s cannot r e a d i l y be p r e d i c t e d s i n c e t h e o x i d a t i o n s t a t e depends upon ( i ) t h e p a r t i a l p r e s s u r e o f oxygen, P Q ^ (ϋ) temperature (JL) » * ( i i i ) the nature o f the other oxides present, thus CaO, Na20 and K 0 ^ ^ i n c r e a s e t h e amount o f F e 0 while S 1 O 2 i n c r e a s e s t h e amount o f FeO. Even when t h e i r o n d i s t r i b u t i o n s a r e g i v e n t h e v a l u e s a r e v u l n e r a b l e t o e r r o r owing t o d i f f i c u l t i e s i n c h e m i c a l a n a l y s i s and t h e p o s s i b i l i t y o f some r e d i s t r i b u t i o n o f the Fe/Fe / F e ^ r a t i o s d u r i n g t h e quench. N e v e r t h e l e s s i t i s s t r o n g l y recommended t h a t a l l f u t u r e p h y s i c a l p r o p e r t y d e t e r m i n a t i o n s on c o a l s l a g s s h o u l d be accompanied by c h e m i c a l a n a l y s i s f o r F e ( f r e e ) , FeO and F e 2 0 on t h e quenched specimen. 2

T

a n c

2

2 +

2

3

+

3

Models f o r E s t i m a t i n g P h y s i c o - C h e m i c a l P r o p e r t i e s . M e l t i n g Range E m p i r i c a l r u l e s have been f o r m u l a t e d f o r t h e e s t i m a t i o n o f m e l t i n g range on t h e b a s i s o f t h e b a s i c i t y o f t h e s l a g o r ash. How e v e r t h e v a r i o u s c o n s t a n t s used i n t h e c a l c u l a t i o n s a r e a p p l i c a b l e t o v e r y narrow c o m p o s i t i o n a l ranges and thus a l a r g e number o f c o n s t a n t s a r e r e q u i r e d t o r e p r e s e n t t h e s l a g s formed from d i f f e r e n t coals. A more u n i v e r s a l approach has been adopted by Gaye^Z^^) who e x p r e s s e d , t h e dependency o f l i q u i d u s temperature, T u q , upon c h e m i c a l c o m p o s i t i o n as p o l y n o m i a l s f o r each o f t h e phases ( o r compounds) formed by t h e s l a g . The maximum c a l c u l a t e d v a l u e o f ^ l i q f ° "the v a r i o u s phases i s t h e T ^ value. Good agreement was o b t a i n e d between t h e c a l c u l a t e d and e x p e r i m e n t a l v a l u e s f o r T ^ q f o r the systems, S i 0 + A I 2 O 3 + MgO + FeO and S 1 O 2 + A 1 0 + MgO+CaQ. However development o f t h i s model t o c o v e r t h e multicomponent c o a l s l a g s c o u l d prove d i f f i c u l t . r

q

2

2

3


15.

Physicochemical

MILLS

Properties

197

of Slags and Ashes

A second model due t o Gaye (9.) would appear t o o f f e r more promise; i t i s based on the Kapoor-Frohberg model f o r the e s t i m a t i o n o f a c t i v i t i e s and assumes t h a t b o t h a c i d i c and b a s i c o x i d e s are made up o f s y m m e t r i c a l c e l l s and t h e s e i n t e r a c t t o form assymmetrical c e l l s . The a c t i v i t i e s o f the v a r i o u s o x i d e s c a l c u l a t e d f o r q u a t e r n a r y s l a g s w i t h t h i s model a r e i n e x c e l l e n t agreement w i t h t h o s e determined e x p e r i m e n t a l l y . V a l u e s o f T ^ q f o r a g i v e n com p o s i t i o n can be d e r i v e d by d e t e r m i n i n g the temperature a t which the s o l i d phase formed has an a c t i v i t y o f one. The model has been developed f o r systems based on seven components, S 1 O 2 1 A I 2 O 3 , CaO, MgO, FeO, Fe203 and MnO. The model w i l l have t o be e n l a r g e d t o i n c l u d e N a 0 e t c b e f o r e i t can be used f o r the r e l i a b l e e s t i m a t i o n o f T ^ i q o f c o a l s l a g s b u t i t would seem t o have c o n s i d e r a b l e promise and has the d e c i d e d advantage t h a t a c t i v i t y d a t a f o r the v a r i o u s component o x i d e s a r e a l s o p r o d u c e d .


2

V i s c o s i t y ( η ) . S e v e r a l models have been r e p o r t e d f o r the e s t i m a t i o n o f v i s c o s i t i e s (η) o f s i l i c a t e m e l t s t o c o v e r t h e c o m p o s i t i o n a l ranges o f g l a s s e s ( i 2 > i i ) s t e e l m a k i n g s l a g s (12*14), magmas^~~IS) and c o a l s l a g s and a s h e s ( 1 2 ~ 2 4 ) . The temperature (T) dependence o f the v i s c o s i t y i s e x p r e s s e d i n the form o f the A r r h e n i u s r e l a t i o n s h i p ( e q u a t i o n 1) o r the F r e n k e l r e l a t i o n s h i p ( e q u a t i o n 2 which i s sometimes known as the Weymann e q u a t i o n ) where A and Β a r e con s t a n t s , Ε i s the a c t i v a t i o n energy and R i s the Gas C o n s t a n t . η= η=

A exp AT exp

(E/RT) (B/RT)

(1) (2)

E s t i m a t e d v i s c o s i t i e s have been c a l c u l a t e d u s i n g t h e s e v a r i o u s models and the c l o s e s t agreement w i t h e x p e r i m e n t a l v a l u e s was o b t a i n e d w i t h the models due t o Riboud e t a l and U r b a i n e t a l (Γ7)(18). These e s t i m a t i o n p r o c e d u r e s use the F r e n k e l r e l a t i o n s h i p and thus t h e i r s u p e r i o r i t y may be l a r g e l y due t o the use o f e q u a t i o n (2). The model due t o S c h o b e r t which i n v o l v e s pétrographie c l a s s i f i c a t i o n has n o t been a s s e s s e d and t h i s p r o c e d u r e may p r o v i d e r e l i a b l e e s t i m a t e s o f v i s c o s i t y f o r c o a l s l a g s but c o u l d not be a p p l i e d t o s l a g s c o v e r i n g a wide range o f c o m p o s i t i o n . Thus e f f o r t i n the p r e s e n t s t u d y was f o c u s s e d p r e d o m i n a n t l y on the Riboud and U r b a i n models. Model due t o Riboud e t a l . The s l a g c o n s t i t u e n t s a r e c l a s s i f i e d i n f i v e d i f f e r e n t c a t e g o r i e s i n t h i s model. The mole f r a c t i o n s (x) f o r those c a t e g o r i e s b e i n g g i v e n by (i) x("Si0 ) = x(Si0 ) + X(P0 ) + x(Ti0 ) + x(Zr0 ) (ii) x("CaO") = x(CaO) + x(MgO) + x(FeO) + x ( F e 0 ) (iii) x(Al 0 ) (iv) x ( C a F ) and (v) x("Na 0") = x(Na 0) + x ( K 0 ) . The parameters A and Β o f e q u a t i o n (2) are c a l c u l a t e d from the mole f r a c t i o n o f the f i v e c a t e g o r i e s by u s i n g e q u a t i o n s (3) and (4) and the M

2

2

2 i 5

2

2

l e 5

2

3

2

2

2

2

A = exp(-19.81+1.73x("CaO")+5.82x(CaF )+7.02x(Na 0)-33.76x(A1 0 )(3) Β =+31140-23896x("Ca0")-46356x(CaF )-39159x("Na 0")+68833x(Al 0 )(4) v i s c o s i t y f o r the temperatures i n q u e s t i o n by use o f e q u a t i o n 2. 2

2

2

2


2

3

2

3

198


Model due t o U r b a i n e t a l ^ Q ) . In t h i s model the parameters A and Β are c a l c u l a t e d by d i v i d i n g the s l a g c o n s t i t u e n t s i n t o t h r e e c a t e g o r i e s ( i ) " g l a s s f o r m e r s " , X Q = x ( S i 0 ) + x(P205) ( i i ) " m o d i f i e r s " , x = x(CaO) + x(MgO) + x ( N a 0 ) + x(K20) + 3x(CaF2) + x(FeO) + x(MnO) + 2 x ( T i 0 ) + 2 x ( Z r 0 2 ) and ( i i i ) "amphoterics", x = x(Al 0 ) + x(Fe 0 ) + x(B 0 ). 2

M

2

2

A

2

3

2

3

2

3

However we c o n s i d e r t h a t F e 0 behaves more l i k e a m o d i f i e r than an amphoteric and i n our r e v i s e d programme 1.5 x ( F e 0 ^ 5 ) has been added t o xjyj and x ( F e 0 ) removed from x^. "Normalized" values X Q * and xjyj* and x^* a r e o b t a i n e d by d i v i d i n g the mole f r a c t i o n s , X Q , X and x by the term (1 + 2 x ( C a F ) + 0.5 x ( F e 0 i . 5 ) + x(Ti0 ) + x(Zr0 )). U r b a i n proposed t h a t the parameter Β was i n f l u e n c e d b o t h by the r a t i o , β = xjJj/Ujyj + x j ) and by xg. The parameter Β can be e x p r e s s e d i n the form o f e q u a t i o n (5) where B-p B and B can be o b t a i n e d by e q u a t i o n ( 6 ) . 2

3

e

2

3

M

A

2

2

2


2

Β = B

B.

o

Β

+

= a.

χ

χ

£

b.p

+

+

B (x*)

+

c.p

2

2

+

B (x*)

3

3

(5)

3

2

(6)

B , B^ B and B can be c a l c u l a t e d from the e q u a t i o n s l i s t e d i n T a b l e I and t h e s e parameters are then i n t r o d u c e d i n t o e q u a t i o n (5) t o c a l c u l a t e B. The parameter A can be c a l c u l a t e d from Β by e q u a t i o n (7) and the v i s c o s i t y o f the s l a g ( i n PaS) can then be determined u s i n g e q u a t i o n ( 8 ) . 0

2

3

- InA = 0.2693 Β + 11.6725 η

= 0.1

Table I. Β

The =

AT exp

(10

3

B/T)

relationship of B

13.8

(7)

+ 39.9355 β

Q >

(8) Βχ, B

2

- 44.049

ο Β

= 30.481

ι

B

2

B

3

-

-

3

w i t h the f u n c t i o n β

2 pι 2

117.1505 (J + 129.9978

=-40.9429 + 234.0486 ρ\ - 300.04 = 60.7619

and B

β

ι

ρ

153.9276 {J\ + 211.1616

2

2 β

M o d i f i c a t i o n s t o the U r b a i n Model. U r b a i n — . has r e c e n t l y m o d i f i e d the model t o c a l c u l a t e s e p a r a t e Β v a l u e s f o r d i f f e r e n t i n d i v i d u a l m o d i f i e r s , CaO, MgO and MnO. The g l o b a l Β v a l u e f o r a s l a g c o n t a i n i n g a l l t h r e e o x i d e s can be d e r i v e d u s i n g e q u a t i o n (9)

x(Ca0)B(Ca0) + x(Mg0)B(Mg0) + x(Mn0)B(Mn0) Β(global)=

(9) x(Ca0) + x(Mg0) + x(Mn0)


15.

MILLS

Physicochemical

Properties

199

of Slags and Ashes \ -L J , Ι Ο )

Assessment o f t h e V i s c o s i t y Models. These two models have"— —- been used t o c a l c u l a t e t h e v i s c o s i t i e s o f s l a g s w i t h w i d e l y - v a r y i n g c o m p o s i t i o n s and i t has been found t h a t b o t h g i v e v a l u e s which agree w e l l w i t h experiment. The model o f U r b a i n g i v e s a s l i g h t l y b e t t e r f i t than t h e Riboud model. The d i s c r e p a n c i e s between t h e e x p e r i mental v a l u e s and t h e p r e d i c t e d v a l u e s a r e o f t h e o r d e r o f + 30% which a r e o f s i m i l a r magnitude t o t h e e x p e r i m e n t a l u n c e r t a i n t i e s f o r v i s c o s i t y measurements. Density ( p ) (25) R e c e n t l y Keene — has r e p o r t e d t h a t t h e d e n s i t y a t 1673K o f molten s l a g s c a n be o b t a i n e d w i t h i n + 5% u s i n g t h e e q u a t i o n ( 1 0 ) p = 2.49 + 0.012 (% FeO + % F e ^ + % MnO + % NiO)

(10)


gem An a d d i t i v e method f o r t h e e s t i m a t i o n o f d e n s i t i e s (p) i n s l a g s has been w i d e l y used f o r some time (26,27). i t h i s method, t h e molar volume, V, c a n be o b t a i n e d from e q u a t i o n s (11) and (12) below where Μ, χ and 7 a r e t h e m o l e c u l a r weight, mole f r a c t i o n and t h e p a r t i a l molar volume, r e s p e c t i v e l y , and 1, 2 and 3 denote t h e v a r i o u s oxide c o n s t i t u e n t s o f the s l a g . n

V = Μ χ χ

V = x

+ M x

χ

l V l

2

x V

+

2

2

+ M x / ρ

2

3

+

x V 3

(11)

3

(12)

3

The p a r t i a l molar volume i s u s u a l l y assumed t o be e q u a l t o t h e molar volume o f t h e pure component ( V ° ) . B o t t i n g a and W e i l l (.£§) produced a s e r i e s o f v a l u e s o f V f o r v a r i o u s o x i d e s assuming a c o n s t a n t v a l u e f o r V ( S i 0 2 ) and i t was c l a i m e d t h a t good e s t i m a t i o n s o f t h e d e n s i t y c o u l d be o b t a i n e d f o r c o m p o s i t i o n s c o n t a i n i n g between 40 and 80% S i 0 « Two more r e c e n t s t u d i e s ( J L ? ) ( 3 0 ) a l s o c o n c l u d e d t h a t V ( S 1 O 2 ) was independent o f c o m p o s i t i o n and have r e v i s e d the V v a l u e s f o r t h e v a r i o u s o x i d e s . However i t has been p o i n t e d o u t (31f32) t h a t t h e d e n s i t y o f t h e s l a g i s a l s o r e l a t e d t o i t s structure. S i l i c a t e s l a g s c o n t a i n a m i x t u r e o f c h a i n s , r i n g s and b a s i c s i l i c a t e u n i t s , which a r e dependent upon t h e s i l i c a con c e n t r a t i o n and upon t h e n a t u r e o f t h e c a t i o n s p r e s e n t . Thus t h e d e n s i t i e s o f s i l i c a t e slags estimated using a constant value f o r V ( S i 0 ) w i l l be s u b j e c t t o e r r o r as t h e arrangement o f t h e s e s i l i c a t e chains v a r i e s with s i l i c a concentration. F u r t h e r m o r e Grau and M a s s o n ( ^ J ) p o i n t e d o u t t h a t f o r t h e s e r i e s , M0, M S i 0 , M S i 0 7 , t h e p a r t i a l molar volume o f S i 0 i s not constant. They c a l c u l a t e d a Δ ν term f o r t h e d i f f e r e n c e s between any two members o f t h e s e r i e s and i n t h i s way, c a l c u l a t e d v a l u e s were d e r i v e d f o r t h e systems FeO + S i 0 , PbO + S i 0 , FeO + MnO + S i 0 and FeO + CaO + S i 0 , f o r c o m p o s i t i o n s i n t h e range Si02 1·0· However t h i s method i s n o t s u i t a b l e f o r c a l c u l a t i n g d e n s i t i e s o f multicomponent systems. Very r e c e n t l y , B o t t i n g a e t al(33.) have p r e s e n t e d a model i n which t h e p a r t i a l molar volumes o f a l u m i n a - s i l i c a t e l i q u i d s were c o n s i d e r e d t o be composition-dependent. 2

2

2

4

3

2

2

2

2

x

=

t

2

2

o


200


New Model f o r C a l c u l a t i n g the D e n s i t i e s o f S l a g s . S l a g s c o n t a i n i n g S i 0 , A l ° 3 and P 2 O 5 c o n s i s t o f c h a i n s , r i n g s and complexes which are dependent upon the amount and n a t u r e o f the c a t i o n s p r e s e n t . Thus i t i s n e c e s s a r y t o make the p a r t i a l molar volumes dependent upon c o m p o s i t i o n f o r o x i d e s o f t h i s t y p e . I f i n a b i n a r y system, e q u a t i o n (12) were a p p l i c a b l e and i f = V± i e . V i s independent o f c o m p o s i t i o n , t h e n the c u r v e o f V as a f u n c t i o n o f c o m p o s i t i o n w i l l be t h a t shown by the s o l i d l i n e i n F i g u r e l a and the two x^V^ c o n t r i b u t i o n s by the d o t t e d l i n e s . I f we now c o n s i d e r a b i n a r y s i l i c a t e system the molar volume (V) would have the form shown as a s o l i d l i n e i n F i g u r e l b . I t i s r e a s o n a b l e t o assume t h a t V(MyO) i s independent o f c o m p o s i t i o n and would have the form o f x i V ^ i n F i g u r e l b . The parameter x can be d e r i v e d f o r S i 0 by use o f e q u a t i o n (13) below. 2

2

v

2

x V


2

Thus X 2

2

2

2

= V - x V

2

w i l l have the form o f the c u r v e shown i n F i g u r e l b .

V

1

(13)

1

I t i s p o s s i b l e t o d e r i v e xV f o r S i 0 i n t e r n a r y and q u a t e r n a r y s l a g s by u s i n g e q u a t i o n ( 1 4 ) . V a l u e s f o r xV ( S i 0 ) have been d e r i v e d u s i n g e x p e r i m e n t a l d e n s i t y d a t a f o r the systems, FeO + S i 0 , CaO + S i 0 , MnO + S i 0 , N a 0 + S i 0 , K 0 + S i 0 and CaO + FeO + S i 0 and a r e p l o t t e d a g a i n s t x ( S i 0 ) i n F i g u r e 2. I t can be seen from t h i s f i g u r e t h a t t h e r e i s e x c e l l e n t agreement between the x V ( S i 0 ) c a l c u l a t e d from d i f f e r e n t s o u r c e s , w i t h the e x c e p t i o n o f t h a t f o r the MnO + S i 0 system. However the r e l i a b i l i t y o f the e x p e r i m e n t a l d a t a f o r t h i s system, have been q u e s t i o n e d p r e v i o u s l y . From t h i s c u r v e f o r x V ( S i 0 ) we can d e r i v e the r e l a t i o n s h i p , V ( S i 0 ) = 19.55 + 7.966.x(Si0 )· The recommended v a l u e s f o r V f o r the v a r i o u s o x i d e s a t 1500 °C a r e g i v e n i n T a b l e I I . 2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

xV(Si0 ) 2

= V - x V 1

1

- x V 2

2

- x V 3

3

(

1

4

)

V a l u e s f o r xV ( A l 0 3 ) were determined i n a s i m i l a r manner by u s i n g e x p e r i m e n t a l d e n s i t y d a t a f o r the systems, CaO + A l 0 3 , C a F + A 1 0 , S i 0 + A 1 0 , MgO + CaO + A 1 0 and MnO + S i 0 + A 1 0 The xV ( A 1 0 ) r e s u l t s are p l o t t e d i n F i g u r e 3 and the r e l a t i o n s h i p V ( A 1 0 ) = 28.31 + 32 x ( A l 0 ) - 31.45 χ 2 ( Α 1 0 ) was d e r i v e d from t h i s curve. There a r e few e x p e r i m e n t a l d a t a f o r the d e n s i t y o f phosphate s l a g s but x V ( P 0 ) v a l u e s were d e r i v e d from d a t a f o r the systems CaO + FeO + P 2 O 5 and N a 0 + P 2 O 5 . A c o n s t a n t v a l u e o f V = 65.7 cm3 m o l o b t a i n e d from the s e l e c t e d l i n e a r r e l a t i o n s h i p . 2

2

2

3

2

2

2

2

3

2

3

2

2

2

3

3

3

2

2

3

2

3

5

- 1

2

In o r d e r t o p r o v i d e a temperature c o e f f i c i e n t , the temperature dependencies o f the molar volumes (dV/dT) o f many s l a g systems were examined and a mean v a l u e o f 0.01% K was adopted. - 1


was

Physicochemical

Properties

of Slags and Ashes


MILLS


202

MINERAL MATTER AND

ASH IN COAL

T a b l e I I . Recommended v a l u e s f o r the p a r t i a l molar volumes, o f v a r i o u s s l a g c o n s t i t u e n t s a t 1500 °C Si0 A1 0 CaO MgO 2

2

3

19.55 28.31 20.7 16.1

+ 7.966 x ( S i 0 2 ) + 32 χ ( Α 1 0 ) - 3 1 . 4 5 χ 2

2

(A1 0 ) 2

3

FeO Fe 0 2

3

MnO Na 0 2

Units


3

o f V = cm

mol

=10

m

15.8 38.4 15.6 33

CaF P 2 Ti0 2

2

V,

31.3 O 5 65.7 24

mol

Assessment o f d e n s i t y models. An a n a l y s i s o f the u n c e r t a i n t i e s a s s o c i a t e d w i t h the e s t i m a t i o n o f d e n s i t i e s w i t h t h i s model has not y e t been completed. However on the b a s i s o f t h o s e v a l u e s o b t a i n e d so f a r the s t a n d a r d d e v i a t i o n o f the f a c t o r ( p e s t " pexpt/ pexpt) i s between 1 and 2% and l e s s than t h a t r e c o r d e d u s i n g the method due to B o t t i n g a et a l (J£). The e x p e r i m e n t a l u n c e r t a i n t i e s a s s o c i a t e d w i t h d e n s i t y measurements f o r s l a g s a r e c a . + 2%. Surface

Tension

(Y)

Methods f o r e s t i m a t i n g the s u r f a c e t e n s i o n o f ^ s l a g s based on the a d d i t i o n o f the p a r t i a l molar c o n t r i b u t i o n s (Y) o f the i n d i v i d u a l c o n s t i t u e n t s have been r e p o r t e d by Appen (34) by B o n i and Derge (35) and by P o p e l ( 3 6 ) . A l l t h e s e methods make use o f e q u a t i o n (15) where 1, 2, 3 e t c denote the v a r i o u s s l a g c o n s t i t u e n t s . Υ

= χ

1

Ϋ

+

1

χ

Ϋ

2

2

+

χ

3

Ϋ

3

+

(15)

V a l u e s o f Y i a r e o f t e n t a k e n t o be the s u r f a c e t e n s i o n o f the pure components, γ ° , and have a l s o been o b t a i n e d by i t e r a t i v e p r o c e d u r e s . F i g u r e 4a shows a t y p i c a l c l o t o f Y as a f u n c t i o n o f χ f o r a b i n a r y s l a g and the i n d i v i d u a l X i Y i c o n t r i b u t i o n s have a l s o been i n c l u d e d . These methods work w e l l f o r c e r t a i n s l a g m i x t u r e s but break down when s u r f a c e - a c t i v e c o n s t i t u e n t s , such as Ρ 2 Ο 5 » are p r e s e n t . These components m i g r a t e p r e f e r e n t i a l l y t o the s u r f a c e and cause a s h a r p d e c r e a s e i n the s u r f a c e t e n s i o n and c o n s e q u e n t l y o n l y v e r y s m a l l c o n c e n t r a t i o n s a r e r e q u i r e d t o cause an a p p r e c i a b l e d e c r e a s e i n Y. Thus some u n r e p o r t e d o r u n d e t e c t e d i m p u r i t y c o u l d have a marked e f f e c t on the s u r f a c e t e n s i o n o f the s l a g and t h e r e b y produce an a p p a r e n t e r r o r i n the v a l u e e s t i m a t e d by the model. In t h i s r e s p e c t s u r f a c e t e n s i o n d i f f e r s from a l l the o t h e r p h y s i c a l p r o p e r t i e s which are e s s e n t i a l l y bulk p r o p e r t i e s . New Model f o r C a l c u l a t i n g the S u r f a c e T e n s i o n o f S l a g s . F i g u r e 4a shows the s u r f a c e t e n s i o n o f two s l a g c o n s t i t u e n t s which a r e n o t s u r f a c e a c t i v e . F o r a b i n a r y m i x t u r e w i t h one s u r f a c e a c t i v e component the s u r f a c e t e n s i o n - c o m p o s i t i o n r e l a t i o n s h i p w i l l have the form o f t h a t shown i n F i g u r e 4b where 2 denotes the s u r f a c e - a c t i v e c o n s t i t u e n t . J^f we assume t h a t χ Y f o r component 1 i s u n a f f e c t e d , t h e n the term Y x , p a r t i a l molar c o n t r i b u t i o n o f the s u r f a c e t

2

n

e

2


Physicochemical

Properties

of Slags and Ashes


MILLS

2 X

a n

Figure 4 The c o m p o s i t i o n dependence of Ύ, /Y^> d 2^2 ^ y ^- 8 systems w i t h (a) n o n - s u r f a c e - a c t i v e c o n s t i t u e n t s and (b) one s u r f a c e - a c t i v e c o n s t i t u e n t .

X

i

n

a

r

s

a


204

MINERAL MATTER AND

ASH IN COAL

a c t i v e m a t e r i a l . can be c a l c u l a t e d by e q u a t i o n (16) below. The term ( X 2 Y 2 ) can s i m i l a r l y be c a l c u l a t e d f o r t e r n a r y and q u a t e r n a r y systems p r o v i d i n g t h e r e i s o n l y one s u r f a c e a c t i v e component.

xY = Υ -Χ Υ 2

2

χ

(16)

λ

The c o m p o s i t i o n a l dependence o f the ( x Y ) term is^shown i n F i g u r e 4b and i t s h o u l d be n o t e d t h a t as x —+ 1 then χ Υ — * γ . V a l u e s o f (xY) f o r v a r i o u s s u r f a c e a c t i v e m a t e r i a l s d e r i v e d from e x p e r i m e n t a l s u r f a c e t e n s i o n d a t a are shown i n F i g u r e 5. i t i s p o s s i b l e to deal w i t h the c o m p o s i t i o n a l dependence o f t h e s e (xY). V a l u e s by c o n s i d e r i n g two c u r v e s v i z . one o p e r a t i n g up t o the p o i n t Ν and the o t h e r r e p r e s e n t i n g v a l u e s o f χ where χ > N . The p a r t i a l s u r f a c e t e n s i o n f o r n o n - s u r f a c e a c t i v e c o n s t i t u e n t s i s shown i n T a b l e I I I and the e q u a t i o n s f o r c a l c u l a t i n g Y f o r s u r f a c e a c t i v e components and v a l u e s o f Ν a r e g i v e n i n T a b l e IV. Thexe i s a c o n s i d e r a b l e d i s c r e p a n c y i n the r e l a t i o n s h i p s between x Y * 2 2°3 o b t a i n e d from e x p e r i m e n t a l d a t a on two d i f f e r e n t systems, and a mean v a l u e has been adopted u n t i l f u r t h e r d a t a become a v a i l a b l e . 2

2


2

2

a n (

2

Table

Oxide Y /mNm-1 Table

2

2

2

5

2

3

2

3

-2 -0.8

2

A1 0 655 2

a t 1500

x-jYi f o r x
N

χ χ χ χ χ χ χ χ

The r e p o r t e d v a l u e s o f ( d Y / d T ) f o r v a r i o u s s l a g systems were examined and a mean v a l u e o f -0.15 mN m K was a p p l i e d as a temperature c o e f f i c i e n t . -1

- 1

Assessment o f the Model. The s t a n d a r d d e v i a t i o n o f the f a c t o r ( ( Y e s t - Y e x p t ) / y e x p t ) was ca.+ 10%. Undoubtedly* much o f the u n c e r t a i n t y a r i s e s from e x p e r i m e n t a l e r r o r s , where the e f f e c t o f u n r e p o r t e d s u r f a c e a c t i v e i m p u r i t i e s and the n a t u r e o f the gaseous atmosphere c o u l d have a marked e f f e c t on the v a l u e o f s u r f a c e tension. Another major s o u r c e o f e r r o r i s the amount o f F e 0 3 p r e s e n t i n the s l a g , and t h i s i n v e s t i g a t i o n has shown c l e a r l y t h a t Fe 0 i s v e r y s u r f a c e a c t i v e . Few i n v e s t i g a t o r s r e p o r t the (Fe3+/Fe +) r a t i o which i s dependent upon, ( ΐ ) Ρ θ 2 , ( ϋ ) Τ and ( i i i ) the 2

2

3

2



Figure 5 The c o m p o s i t i o n a l dependence o f x i ? i , various surface active constituents.


for


206

n a t u r e and amount o f o t h e r o x i d e s p r e s e n t so even, when quoted, the r a t i o may be i n e r r o r . Thus, i f a d e c r e a s e i n Y i s r e c o r d e d when N a 0 i s added t o the s l a g , i t i s q u e s t i o n a b l e whether t h i s d e c r e a s e i s due t o an i n c r e a s e i n F e 0 3 c o n t e n t o r t o the s u r f a c e a c t i v i t y o f the N a 0 . In t h i s i n v e s t i g a t i o n , attempts were made t o a d j u s t Y f o r an i n c r e a s e i n F e 0 3 c o n t e n t b u t the e f f e c t o f b a s i c o x i d e s on the r a t i o i s n o t w e l l documented and some e r r o r may r e s u l t . A major u n r e s o l v e d problem c o n c e r n s the s i t u a t i o n where the s l a g c o n t a i n s more than one s u r f a c e a c t i v e component. It i s possible t h a t t h e r e i s some c o m p e t i t i o n f o r s i t e s on the s u r f a c e and hence the d e c r e a s e i n ^ would n o t be as sharp as t h a t c a l c u l a t e d from the summation o f ( X A Y A + B ^ B ) where A and Β denote s u r f a c e a c t i v e constituents. In t h i s c a s e , t h e model may o v e r e s t i m a t e the d e c r e a s e i n Y . There a r e no e x t a n t d a t a t o c o n f i r m t h i s p o s s i b i l i t y , and ( N a 0 + K 0 ) has been i n c l u d e d as a s i n g l e c o n t r i b u t i o n i n the model. 2

2

2

2

X


2

2

Thermal P r o p e r t i e s The computations of the t h e r m a l l o s s e s i n the c o n v e r t e r by c o n d u c t i v e and r a d i a t i v e p r o c e s s e s r e q u i r e knowledge o f the f o l l o w i n g thermal p r o p e r t i e s , heat c a p a c i t y , enthalpy, thermal c o n d u c t i v i t y , a b s o r p t i o n c o e f f i c i e n t and e m i s s i v i t y . Heat C a p a c i t y C^ and E n t h a l p y

(H^ -

H 2

98^

When a s i l i c a t e l i q u i d i s c o o l e d the s t r u c t u r e o f the s o l i d formed i s dependent upon the c o o l i n g r a t e and the t h e r m a l h i s t o r y o f the sample. C o n s i d e r a l i q u i d a t a temperature c o r r e s p o n d i n g t o the p o i n t C i n F i g u r e 6, a r a p i d quench w i l l produce a g l a s s and the e n t h a l p y e v o l v e d w i l l f o l l o w the p a t h CLGA. By c o n t r a s t , a v e r y slow c o o l i n g r a t e w i l l r e s u l t i n the f o r m a t i o n o f a c r y s t a l l i n e s l a g , t h e e n t h a l p y e v o l u t i o n f o l l o w i n g the p a t h CL DB. I t w i l l be n o t e d from F i g u r e 6 t h a t ( H - H g ) c r y s t = ( H - H g ) g l a s s + A , where ^ e n t h a l p y o f the endothermic t r a n s f o r m a t i o n o f crystal—"glass). The C v a l u e s f o r the v a r i o u s phases can be summarized a s : Cp(crystal) = C ( g l a s s ) < C (supercooled l i q u i d ) = C (liquid) H

T

H

v

i

t

i

s

t

n

2

8

T

2

v

i

t

8

e

p

p

p

p

I t can be seen from F i g u r e 6 t h a t a t the g l a s s temperature ( T ) t h e r e i s a sudden i n c r e a s e i n C (ACpS ) as the g l a s s t r a n s forms i n t o a s u p e r c o o l e d l i q u i d . Drop c a l o r i m e t r y s t u d i e s on the g l a s s phase a t temperatures between T ^ and T ^ w i l l produce p r o g r e s s i v e l y more c r y s t a l l i z a t i o n as i s approached and c o n s e q u e n t l y the (H-p - H g ) - Τ r e l a t i o n s h i p w i l l be s i m i l a r t o t h a t d e p i c t e d by the d o t s i n F i g u r e 6 and n o t the p a t h , AGLC; the magni tude o f the apparent e n t h a l p y o f f u s i o n (^H^ ) w i l l be dependent upon the f r a c t i o n o f the sample c r y s t a l l i z e d d u r i n g a n n e a l i n g a t temperatures between T and T . The C v a l u e s f o r t h e l i q u i d and s u p e r c o o l e d l i q u i d s have been r e p o r t e d t o be c o n s t a n t and i n d e p e n dent o f temperature(.22). I t f o l l o w s from the t r i a n g l e GEL i n F i g u r e 6 that C (T - T ) = AH . Thus, i t i s p o s s i b l e t o e s t i m a t e the e n t h a l p y o f a s l a g w i t h a g l a s s y s t r u c t u r e from e s t i m a t e s o f p ( g l a s s ) , C ( l i q ) and T However the e s t i m a t i o n o f T is 1

g l

p

g

2

q

8

us

g l

l i q

g l

p

f u s

p

l i q

g l

c

p

g l

g l


15.

Physicochemical

MILLS

Properties

of Slags and Ashes

207

d i f f i c u l t as i t c a n v a r y between 700 and 1100 Κ and the v a r i o u s e s t i m a t i o n r u l e s s u g g e s t e d a r e known t o be prone t o a p p r e c i a b l e errors (38). I n s p e c t i o n o f l i t e r a t u r e d a t a (,2§» 2.9) i n d i c a t e s t h a t the g l a s s t r a n s f o r m a t i o n o c c u r s when Cp a t t a i n s a v a l u e o f c a . 1.1 J K ~ g " ; t h i s r u l e has been used i n t h e development o f t h e f o l l o w i n g model f o r t h e e s t i m a t i o n o f (H
3.5 and values of k can be c a l c u l a t e d u s i n g e q u a t i o n (21), where G i s the S t e f a n - B o l t z m a n n c o n s t a n t . In r e c e n t y e a r s f o r m u l a e have been


a

n

(

a

p

7

s

7

2

1

1

R

R

R

R

R

2

k

3

_ 16 η Τ σ " 3a

R

u

i

;

p r o p o s e d f o r the c a l c u l a t i o n o f kR f o r o p t i c a l l y t h i n conditions. The a b s o r p t i o n c o e f f i c i e n t {oc) i s markedly dependent upon the amounts o f F e + and M n p r e s e n t i n the s l a g s a t l e v e l s o f FeO .< 5% the f o l l o w i n g r e l a t i o n s h i p can be s u p p l i e d , aicm" ) = ll(%Fe0). Temperature appears t o have l i t t l e e f f e c t on the a b s o r p t i o n c o e f f i c i e n t o f g l a s s e s but the a b s o r p t i o n o f magmas have been r e p o r t e d t o i n c r e a s e w i t h i n c r e a s i n g t e m p e r a t u r e » Ë§) C r y s t a l l i z a t i o n o f the s l a g w i l l r e s u l t i n a l a r g e i n c r e a s e i n the a b s o r p t i o n (or e x t i n c t i o n ) c o e f f i c i e n t which c o u l d reduce k to v i r t u a l l y zero. 2

2 +

1

β

R


15.

MILLS

Physicochemical

Properties

211

of Slags and Ashes

E m i s s i v i t y (ε) The e m i s s i v i t y (ε) o f a s e m i - t r a n s p a r e n t medium i s a b u l k p r o p e r t y i n c o n t r a s t t o ε (metal) which i s s o l e l y dependent upon t h e s u r f a c e . On t h e b a s i s o f t h e s p e c t r a l and t o t a l normal e m i s s i v i t y d a t a r e p o r t e d f o r c o a l and m e t a l l u r g i c a l s l a g s , t h e v a l u e ε = 0.8 + 0.1 can be adopted f o r c o a l s l a g s i n t h e range (1100 - 1900 K ) . CONCLUSIONS


1) E s t i m a t i o n p r o c e d u r e s based on t h e c h e m i c a l c o m p o s i t i o n o f t h e s l a g have now been d e v e l o p e d f o r t h e p r e d i c t i o n o f v i s c o s i t y , s u r f a c e t e n s i o n , d e n s i t y and h e a t c a p a c i t y . 2) The a c c u r a c y o f t h e s e e s t i m a t i o n r o u t i n e s f o r some p h y s i c a l p r o p e r t i e s ( e g - v i s c o s i t y , s u r f a c e t e n s i o n ) c a n be improved when more r e l i a b l e e x p e r i m e n t a l d a t a become a v a i l a b l e . 3) has and the of

The d i s t r i b u t i o n o f i r o n i n t h e s l a g a pronounced e f f e c t on v i r t u a l l y a l l i t i s recommended t h a t e x p e r i m e n t a l s l a g s h o u l d always be accompanied by iron.

between F e , FeO and F e 0 3 the p h y s i c a l p r o p e r t i e s data f o r the p r o p e r t i e s o f values f o r the d i s t r i b u t i o n 2

4) The development o f models f o r t h e p r e d i c t i o n o f some p h y s i c a l p r o p e r t i e s (thermal c o n d u c t i v i t i e s , a b s o r p t i o n c o e f f i c i e n t ) i s r e s t r i c t e d by t h e l i m i t e d amount o f e x p e r i m e n t a l d a t a a v a i l a b l e .

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RECEIVED June 17, 1985


Mineral Matter and Ash in Coal - American Chemical Society

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