AGGLOMERATION PROCESSES DURING CALCINATION OF BRUCITE

(1)

HAL Id: jpa-00225543

https://hal.archives-ouvertes.fr/jpa-00225543

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

AGGLOMERATION PROCESSES DURING CALCINATION OF BRUCITE

E. Naujokat, C. Zografou, W. Krönert

To cite this version:

E. Naujokat, C. Zografou, W. Krönert. AGGLOMERATION PROCESSES DURING CAL- CINATION OF BRUCITE. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-103-C1-108.

�10.1051/jphyscol:1986116�. �jpa-00225543�

(2)

JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au n02, Tome 47, fbvrier 1986 page cl-103

AGGLOMERATION PROCESSES DURING CALCINATION OF BRUCITE

E. NAUJOKAT, C. ZOGRAFOU and W. KRONERT

Institut fiir Gesteinshuttenkunde, Mauerstrasse 5 , D-5100 Aachen, F.R.G.

Rgsumk - Les p r o c e s s u s d T a g g l o m 6 r a t i o n d l u n e b r u c i t e d ' o r i g i n e -- c o m m e r c i a l e o n t 6 t 6 a n a l y s 6 s . Les e a r a c t 6 r i s t i q u e s d e s

a g g l o m g r a t s e t l e u r i n f l u e n c e s u r l a c o m p a c t i o n d e s p o u d r e s c a l c i n g e s s o n t g g a l e m e n t k t u d i g e s .

A b s t r a c t

-

A g g l o m e r a t i o n p r o c e s s e s a r e i n v e s t i g a t e d w i t h a c o m m e r c i a l b r u c i t e . The c h a r a c t e r i s t i c s o f t h e a g g l o m e r a t e s a s w e l l a s t h e i r i n f l u e n c e on t h e c o m p a c t i n g o f c a l c i n e d powders a r e d i s c u s s e d .

I

-

INTRODUCTION

D u r i n g t h e c a l c i n a t i o n o f compounds, i . e . b r u c i t e , v a r i o u s a g g l o m e r a - t i o n p r o c e s s e s c a n b e o b s e r v e d , which r e s u l t from complex mechanisms a n d depend on t h e n a t u r e and t h e r m a l t r e a t m e n t o f t h e s t a r t i n g m a t e r i a l .

E f f e c t s o f t h e a n i s o t r o p i c s h r i n k a g e a n d r e a r r a n g e m e n t o f p a r t i c l e s accompany t h e f o r m a t i 0 . n o f t h e a g g l o m e r a t e s and i n f l u e n c e t h e a g g l o - m e r a t e p r o p e r t i e s .

I n a d d i t i o n t o t h e t h e r m a l t r e a t m e n t , t h e a g g l o m e r a t e p r o p e r t i e s o f t h e c a l c i n e d powders a r e d e p e n d e n t on t h e c a l c i n a t i o n a t m o s p h e r e . I n t h i s work m a i n l y t h e r e s u l t s o f t h e powders which w e r e c a l c i n e d i n a i r a r e p r e s e n t e d .

I1

-

EXPERIMENTAL METHODS

I n t h e f i r s t s t a g e a c o m m e r c i a l s e a w a t e r b r u c i t e from J a p a n was c a l - c i n e d a t s e v e r a l t e v p e r a t u r e s and w i t h d i f f e r e n t s o a k i n g t i m e s and a t m o s a h e r e s . The gowders were f i r e d a t t e m p e r a t u r e s o f 400 O C , 700

"c,

1000 'C and 1300 C f o r 0 , 0 . 5 , 1 and 2 h . The c a l c i n a t i o n a t m o s p h e r e f o r e a c h s p e c i m e n was vacuum, i n e r t g a s ( a r g o n ) o r a i r .

The r e a c t i v i t i e s o f t h e c a l c i n e d powders a r e d e t e r m i n e d w i t h t h e metho?

o f Haas e t a l . /1,2/. F o r t h a t p u r p o s e 0 . 2 m l H C 1 ( 0 . 1 m o l / l ) a r e added i n i n t e r v a l s o f 60 s a f t e r i n n u t o f 1 m i l l i m o l )fig0 i n 50 m l H20. pH- r a t e i s measured and p l o t t e d c n n t i n u o u s l v . I f a f t e r a d d i t i o n o f H C 1 t h e n Y - r a t e do n o t i n c r e % s e , t h e measurement c a n be s t o ~ ~ e d . depend in^ on t h e -7eight o f and c o n c e n t r a t i o n o f a c i d t h e numher o f a d d i t i o n s i s e q u i v a l e n t t o t h e p a r t o f r e a c t i v e WgO i n X. The d e t e r m i n a t i o n o f t h e l a t t i c e p a r a m e t e r s was c a r r i e d o u t by m e a s u r i n g t h e 111, 200, 220 and

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986116

(3)

30URNAL DE _PHYSIQUE

2 2 2 X-ray l i n e s o f M

B

0 f o l l o w i n g b y t h e c a l c u l a t i o n f o r a f a c e - c e n - t e r e d c u b i c s y s t e m ( ) .

F o r t h e d e s c r i p t i o n of a g g l o m e r a t e c h a r a c t e r i s t i c s , form, s i z e d i s t r i - b u t i o n , p o r o s i t y a n d e s p e c i a l l y a g g l o m e r a t e h a r d n e s s a r e i m p o r t a n t . F o r t h e o b s e r v a t i o n of t h e c h a n g e s i n a g g l o m e r a t e s form we u s e d a h e a t i n g - s t a g e s SEN. The h e a t i n g r a t e was a l w a y s 5 K/min.

The a g g l o m e r a t e h a r d n e s s was d e t e r m i n e d by f o r c e - d i s t a n c e measurements u s i n g s p e c i a l p r e s s u r e equipment / 3 / . The r e l a t i v e d e n s i t y i s deduced f r o m t h e measured w e i g h t and volume of t h e compacted powders. P l o t t i n g t h e s e r e l a t i v e d e n s i t i e s v e r s u s t h e l o g a r i t h m of t h e p r e s s u r e , a c u r v e w i t h a b r e a k p o i q t i s o b t a i n e d . The b r e a k p o i n t shows t h e p r e s s u r e which i s n e e d e d t o d e s t r o y t h e c o n t a c t p o i n t s of p o r o s a g g l o m e r a t e s / 4 , 5 / . T h i s p r e s s u r e g i v e s a n i n f o r m a t i o n on t h e b o n d i n g s t r e n g t h of t h e a g g l o m e r a t e s and a l s o i s r e l a t e d t o t h e a g g l o m e r a t e h a r d n e s s .

breakpoint

_ _ _ - -

I

log pressure

-

F i g . 1

-

T y p i c a l c u r v e of compaction of p o r o s a g g l o m e r a t e s

111

-

RESULTS AND DISCUSSTON

The magnesium o x i d e powders show d i f f e r e n t r e a c t i v i t i e s d e p e n d i n g on t h e d i f f e r e n t c a l c i n a t i o n c o n d i t i o n s . I n t h i s c a s e t h e t e c h n i c a l ex- p r e s s i o n f o r t h e r e a c t i v i t y o f MgO i s c a u s t i c i t y . F i g u r e 2 shows t h e whole r e a c t i v e amount of MgO d e p e n d i n g o n t h e c a l c i n a t i o n t e m p e r a t u r e

and t h e s o a k i n g t i m e i n a i r . I n d e v i a t i o n on an e x p e c t e d v a l u e of h i g h e r g e a c t i v i t i e s a t l o w e r t e m p e r a t u r e s t h e p l o t shows l o w e r v a l u e s a t 400 C which d u e s t o n o n d i s s o c i a t e d h y d r o x i d e . F i g u r e 3 show, t h e r e l a t i o n b e t w e e n c a l c i n a t i o n c o n d i t i o n s and c e l l - c o n s t a n t s w i t h a i r a s c a l c i n a t i o n a t m o s p h e r e . F o r i n c r e a s i n g t e m p e r a t u r e and s o a k i n g t i m e t h e c e l l - c o n s t a n t s d e c r e a s e . Powders c a l c i n e d a t l o w e r t e m p e r a t u r e s show e x t e n t e d c e l l s and more r e a c t i v e t h a n t h o s e powders w i t h i n t a c t c e l l s which were f i r e d a t h i g h e r t e m p e r a t u r e s / 6 / . F i g u r e 4 p r e s e n t s t h e c e l l - c o n s t a n t s f o r vacuum c a l c i n a t i o n s . The t e n d e n c y i s q u i t e t h e same b u t t h e c e l l s a r e s u b s t a n t i a l l y more i n t a c t and t h e r f o r e l e s s r e a c t i v e t h a n t h o s e powders which w e r e f i r e d i n a i r .

F i g u r e 5 i l l u s t r a t e s t h e c h a n g e s i n geometry o f t h e whole a g g l o m e r a t e and p a r t of them i n d e p e n d e n c e on c a l c i n a t i o n t e m p e r a t u r e . S h r i n k a g e c a u s e s t h e g e o m e t r i c m o r p h o l o g i c a l c h a n g e s . Rearrangement o f p a r t s of t h e a g g l o m e r a t e s t a k e s p l a c e by b r i d g i n g and r e c r y s t a l l i z a t i o n com- b i n e d w i t h g r a i n g r o w t h . T h e s e r e a c t i o n s i n d u c e c h a n g e s - i n t h e a g g l o - m e r a t e s r a d i i a s shown i n f i g u r e 5 . Neck f o r m a t i o n t a k e s p l a c e b e c a u s e

( I ) S o f t w a r e D i f f r a c 310, S i e m e n s , K a r l s r u h e

(4)

t h e m a t e r i a l t e r d s t o l o w e r i t s f r e e e n e r g y . Concave s u r f a c e s ( r < O ) c r e a t e t e n s i l e s t r e s s e s w h e r e a s c o n v e x s u r f a c e s ( D O ) c r e a t e compres- s i v e s t r e s s /7/. D i f f e r e n t s t r e s s e s c a u s e r r a t e r i a l d i f f u s i o n t o t h e c o n t a c t p o i n t o f two p a r t i c l e s ( c o n c a v e c u r v a t u r e ) . T h e v a r i a t i o n s i n s h a p e a n d s i z e o f p a r t i c l e s i n d u c e c h a n g e o f p o r e g e o m e t r y . T h i s i s shown b y t h e a n i s o t r o p i c p o r e s h r i n k a g e r i i t h a d e c r e a s e o f p o r e volume. Powders c a l c i n e d i n a i r show a c o n s p i c u o u s c h a n g e o f t h e a g g l o -

F i g . 2 - R e l a t i o n between c a l c i n a t i o n c o n d i t i o n s and r e a c t i v i t y ; c a l - c i n a t i o n atmosphere : a i r

F i e . 3 - R e l a t i o n between c a l c i n a t i o n c o n d i t i o n s and c e l l c o n s t a n t s ; c a l c i n a t i o n atmosphere : a i r

(5)

JOURNAL DE PHYSIQUE

F i g . 4 - R e l a t i o n between c a l c i n a t i o n c o n d i t i o n s a n d c e l l c o n s t a n t s ; c a l c i n a t i o n u n d e r vacuum.

calcination atmosphere: air

calcination atmosphere: inert gas (argon)

calcination atmosphere: vacuum 5 0 pm

F i g . 5 ^,- Changp o f a g ! z l o m e r a t e g e o m e t r y i n d e p e n d e n c e o f t e m p p r a t u r e a n d c a l c i n a t i o n a t m o s p h e r e

(6)

m e r a t e g e o m e t r y a b o v e 200 0 C . R e a r r a n g e m e n t o f m a t e r i a l i s f o r c e d by l a t t i c e d i s t o r t i o n s a n d t h u s by- h i g h e r r e a c t i v i t i e s a r i s i n g f r o m d e - c o m p o s i t i o n o f magnesium h y d r o x i d e i n a i r ( F i g . 2,3,4). R e a c t i v i t y and l a t t i c e d i s t o r t i o n d e c r e a s e f o r p o w d e r s decomposed i n vacuum. " h e r e - f o r e t h e r e a r r a n g e m e n t o f m a t e r i a l i s much s l o w e r . Powder c a l c i n e d i n i n e r t g a s shows a n a n a l o g o u s b e h a v i o u r . A n o t h e r i m p o r t a n t f a c t f o r t h e c h a r a c t e r i z a t i o n o f a g g l o m e r a t e s i s t h e a g g l o m e r a t e s t r e n g t h . F i g u r e 6 g i v e t h e p l o t o f b r e a k p o i n t p r e s s u r e , a s d e f i n e d a b o v e , i n d e p e n d e n c e o f c a l c i n a t i o n c o n d i t i o n s f o r p o w d e r s c a l c i n e d i n a i r . R e s u l t s show t h a t a g g l o m e r a t e s t r e n g t h i n c r e a s e s w i t h c a l c i n a t i o n t e m p e r a t u r e and s o a k i n g t i m e . The s t a r t i n g m a t e r i a l i s a random a r r a n g e m e n t o f p l a t e s w i t h h e x a g o n a l l a t t i c e . I n t h e f i r s t s t a g e o f d e c o m p o s i t i o n t h i s r a n - dom a r r a n g e m e n t i s n o t d e s t r o y e d . Magnesium o x i d e e x i s t s i n t h i s f i r s t s t a g e of d e c o m p o s i t i o n i n a p s e u d o m o r p h o l o g i c a l s t r u c t u r e / 8 / . C o n t a c t p o i n t s of two p a r t i c l e s w e r e f o r m e d by b r i d g i n g b o n d s . Necks a r e b u i l t by t r a n s p o r t o f material t o t h e c o n t a c t p o i n t s a n d t h e r e f o r e t h e a g g l o m e r a t e s c o n s o l i d a t e . I n c r e a s i n g t e m p e r a t u r e i n d u c e s a s i n t e r i n g of a g g l o m e r a t e s a n d a n i n c r e a s e o f a g g l o m e r a t e h a r d n e s s .

F i g . 6

-

B r e a k p o i n t p r e s s u r e i n d e p e n d e n c e o f c a l c i n a t i o n c o n d i t i o n s f o r p o w d e r s c a l c i n e d i n a i r

I V

-

CONCLUSION

The h a n d l i n g , f o r m i n g , a n d s i n t e r i n g b e h a v i o u r s a r e p r o f o u n d l y i n - f l u e n c e d b y t h e s t a t e o f a g g l o m e r a t i o n . F o r e x a m p l e p o w d e r s w i t h h a r d a g g l o m e r a t e s h a v e p o o r c o m p a c t i o n b e h a v i o u r , s i n c e t h e a g g l o m e r a t e s a r e d i f f i c u l t t o c r u s h / a / . With h a r d n e s s o f a g g l o m e r a t e s m i c r o - s t r e s s e s of s i n t e r e d b o d i e s c a n b e e f f e c t e d / l o / .

SFFEREMCES

/ 1 / H a a s , H . , K a h l e r , P. a n d R e i s c h l , K . , Radex Rundschau ( 1 9 5 8 ) 4 7 .

/ 2 / N a u j o k a t

,

^{E . ,} v . ~ l a l l i n c k r o d t

,

^{D .} a n d Z o g r a f o u , C . , K e r a m i s c h e Z e i t s c h r i f t ( 1 9 8 3 ) 440.

(7)

c I - 1 0 8 J O U R N A L DE PHYSIQUE

/ 3 / S c h u l z , P . , D i s s . Aachen ( 1 9 8 1 ) 3 2 .

/ 4 / R e e d , J.S. a n d Runk, R . B . , T r e a t i s e o n P a t e r i a l s S c i e n c e a n d T e c h n o l o g y , V o l . 9 , Academic P r e s s .

/5/

N i e s z , D.E., B e n n e t t , R.B. a n d S n y d e r , Am. Ceram. S o c . B u l l . 5 1 ( 1 9 7 2 ) 6 7 7 .

/ 6 / Pampuch, R., L i b r a n t , Z . , P i e k a r c z g k , J . , P h y s i c s o f S i n t e r i n g

5

( 1 9 7 3 ) 2 6 3 .

/ 7 ) E x n e r , H.E., W a t e r i a l k u n d l i c h e T e c h n i s c h e R e i h e 4 ( 1 9 7 8 ) . / 8 / G o r d o n , R . S . , K i n g e r y , W.D., Am. Ceram. S o c . 49 ( 1 9 6 6 ) 6 5 h .

/9/ N i e s z , D . E . , B e n n e t t , R . B . , Cerfimic P r o c e s s i n g B e f o r e F i r i n g , d i l e y , Mew York ( 1 9 7 8 ) .

/ l o /

H a l l o r a n , J.W., t ~ l t r a s t r u c t u r e P r o c e s s i n g o f C e r a m i c s , G l a s s e s , a n d C o m p o s i t e s , W i l e g , :Jew Y o r k .