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Clays and Clay Minerals,Vol. 33, No. 3, 258-260, 1985.

N O T E S

PROPERTIES OF FLASH-CALCINED KAOLINITE

Key W ords - -D ens i t y, Dehydroxy la t i on , F l a sh ca l c ina ti on , Kao l in i te , The rma l t r ea tmen t .

F l a s h c a l c i n a t i o n i s a p r o c e s s w h e r e b y a s o l i d , u s u a l l yi n f i n e ly d i v i d e d f o r m , i s h e a t e d r a p i d l y, h e l d a t t e m -p e r a t u r e f o r a s h o r t t i m e , a n d t h e n c o o l e d r a p i d l y.H e a t i n g a n d c o o l i n g r at e s o n t h e o r d e r o f 1 0 3 - 10 s d e -g r e es p e r s e c o n d a n d t i m e s o f e x p o s u r e a t t e m p e r a t u r eo f a fe w t e n t h s o f a s e c o n d t o a b o u t t w o s e c o n d s a r et y p i c a l. A l t h o u g h t h e k i n e t ic s o f m a n y p h y s i c a l a n dc h e m i c a l t r a n s f o r m a t i o n s w i t h i n a s o l i d a t h i g h t e m -p e r a t u r e a r e q u i t e r a p i d , f l a s h - c a l c i n a t io n c o n d i t i o n sa r e su c h t h a t th e t r a n s f o r m a t i o n s m a y n o t b e c o m p l e t eb e f o r e t h e t e m p e r a t u r e i s r e d u c e d t o t h e l e v e l th a t f u r -

t h e r c h a n g e s a r e k i n e t i c a l l y f r o z e n .T h e p r o p e r t i e s o f s o l id s p r e p a r e d b y f l a sh - h e a t in g

a n d / o r - c o o l i n g p r o c e s s e s m a y b e q u i t e d i f f e re n t f r o mt h e p r o p e r t i e s o f th e m a t e r i a l t r e a t e d a t c o n d i t i o n s w h e r ec h e m i c a l a n d p h y s i c a l e q u i l i b r i u m a r e a c h i e v e d . F o re x a m p l e , t h e s p l a t c o o li n g o f m e t a ls c a n p r o d u c e a tr o o m t e m p e r a t u r e m e t a s t a b l e p h as e s n o t a t t a in a b l e b yc o n v e n t i o n a l q u e n c h t e c h n iq u e s ( L i e b e r m a n n a n dW a l t e r , 1 9 8 2) . I n c e r a m i c s , g l a ss y, w a t e r - s o l u b l e V 2 O sh a s b e e n p r o d u c e d b y s p l a t c o o li n g (L i v a ge a n d C o l -l o n g u e s , 1 9 7 6 ). T h e r e a r e n o c o m p a r a b l e r e s u l t s i n t h el i t er a t u r e o n t h e b e h a v i o r o f k a o l in i t e o r o t h e r c l a y

m a t e r i a l s s u b j e c t e d t o f l as h p r o c e s s in g .W h e n k a o l i n i t e i s h e a t e d a t n o r m a l r a t e s ( s o a k c a l -

c i n ed ) , c o m p l e t e d e h y d r o x y l a t i o n o c cu r s n e a r 6 0 0~w i t h t h e r e s u l t a n t s o l i d , k n o w n a s m e t a k a o l i n , m a i n -t a i n i n g a s e m b l a n c e o f t h e k a o l i n i t e s tr u c t u r e . F u r t h e rh e a t i n g t o n e a r 1 0 0 0 ~ r e s u l t s i n t h e t r a n s f o r m a t i o no f m e t a k a o l i n t o a n a l u m i n u m - s i l i c o n s p i n e l p lu s f r ees i li c a . A t a b o u t 1 2 0 0 ~ m u l l i t e a n d c h r i s t o b a l i t e a r ef o r m e d ( G r i m , 1 9 62 ) . T h e d e n s i t y o f t h e k a o l i n i t e s t a rt -i n g m a t e r i a l a l s o c h a n g e s d u r i n g t h e h e a t i n g p r o c e s s .R e i k e a n d M a u v e ( 19 4 2) a n d H a r m o n a n d P a r m e l e e( 1 9 4 2 ) f o u n d t h a t a s s o a k - c a l c i n a t i o n t e m p e r a t u r e i n -c r e a s e d t h e d e n s i t y i n i t i a l ly d e c r e a s e d , r e a c h e d a m i n -i m u m , a n d t h e n i n c r ea s e d , e v e n tu a l l y e x ce e d i n g t h ed e n s i t y o f t h e s t a r t i n g k a o l i n i t e . S p e c i f ic a l ly, R e i k e a n dM a u v e ( 1 9 4 2 ) , w o r k i n g w i t h a k a o l i n i t e h a v i n g a d e n -s i ty o f 2 . 6 4 g / c m 3, f o u n d a m i n i m u m d e n s i t y o f 2 . 4 7g / c m 3 a t 4 5 0 ~ B y 9 0 0 ~ t h e d e n s i t y o f t h e c a l c i n e dp r o d u c t h a d i n c r e a s e d t o t h a t o f th e r a w c l a y, a n d , a tt h e m a x i m u m c a lc i n a t io n t e m p e r a t u r e ( 1 00 0 ~ t h ed e n s i t y w a s 2 . 7 9 g / c m 3. T h i s l a t t e r v a l u e i s t h e t h e o -r e t i c a l d e n s i t y w h i c h s h o u l d e x i s t w h e n a s t o i c h i o -m e t r i c m i x t u r e o f m u l l it e a n d c h r i s to b a l i t e a r e p r o -d u c e d b y k a o l i n i te d e c o m p o s i t i o n .

To c o m p a r e t h e d e n s i t y o f a k a o l i n i te s t a rt i n g m a -t e r i a l p r o d u c e d b y f l a sh c a l c in i n g w i t h t h a t o f t h e p r o d -u c t s o f s o a k c a l c i n a t i o n , t h e f o l l o w i n g i n v e s t i g a t i o n w a s

C o p y r i g h t 9 1 9 8 5 , T h e C l a y M i n e r a l s S o c i e t y

c o n d u c t e d . T h e r e l a t io n s h i p b e t w e e n p r o d u c t d e n s i t ie sa n d d e g r ee o f d e h y d r o x y l a t i o n w a s a ls o e x a m i n e d .

E X P E R I M E N T A L

S P S g r a d e k a o l i n i te s u p p l i e d b y E C L P L t d ., S t . A u s -t e ll , C o r n w a l l , U n i t e d K i n g d o m , w a s u s e d in t h i s s t u d y.T h i s c l a y, w h i c h i s d e s i g n e d f o r t h e p a i n t i n d u s t r y, h a sa b r i g h tn e s s o f a p p r o x i m a t e l y 8 9 .0 . T h e d e n s i t y a n db o u n d w a t e r c o n t e n t w e r e m e a s u r e d a s 2 .6 0 g / c m 3 a n d1 2 . 4 % , r e s p e c t i v e l y.

F l a s h c a l c i n a t i o n w a s c a r r i e d o u t i n a r e s i s t a n c e -

h e a t e d , l a m i n a r - f l o w fu r n a c e in a h e l i u m a t m o s p h e r e .T h e r e s id e n c e t i m e o f t h e c l a y in t h e i s o t h e r m a l z o n ew a s c o n t r o l l e d b y v a r y i n g t h e f lo w r a t e o f t h e h e l i u mc a r r ie r ga s. T h e c o n s t r u c t i o n a n d o p e r a t i o n o f th e f u r -n a c e h a v e b e e n d e s c r i b e d f u l ly (D a v i e s , 1 9 8 4 ). B e -c a u s e o f i t s s ti c k y n a t u r e , d i r e c t f e e d i n g o f t h e k a o l i n i t ep o w d e r i n t o th e f u r n a ce p r o v e d i m p o s s i b l e a n d r e -q u i r e d t h e d e v e l o p m e n t o f a s p e c ia l fe e d i ng t e c h n iq u e .T h e c l a y p o w d e r w a s t h e r e f o re c o n s o l i d a t e d i n t o c h a l k -l i k e s t ic k s 1 c m i n d i a m e t e r a n d 9 c m i n l e n g t h b yf o r c i n g a t h i c k p a s t e o f c l a y a n d d i s t i l l e d w a t e r ( 2 t o 3p a r t s c l a y t o 1 p a r t o f d i s t i l l e d w a t e r ) t h r o u g h a n e x -

t r u d e r n o z z l e o f t h e p r o p e r d i a m e t e r i n t o b r a s s m o l d s .T h e m o l d s w e r e p l a c e d i n a n o v e n a n d d r i e d o v e r n i g h ta t 10 5~ T h e s t ic k s w e r e t h e n r e m o v e d f ro m t h e m o l d sa n d s t o r e d i n t h e o v e n a t 1 0 5 ~ u n t i l u s e d . T h e c l a ys t ic k s w e r e f e d i n to a g r i n d i n g c h a m b e r a b o v e t h e c a l -c i n a t i o n f u r n a c e u s i n g a s c r e w p i s t o n . F e e d r a t e w a sc o n t r o l l e d t h r o u g h a v a r i a b l e s p e ed m o t o r d r i v i n g t h ep i s to n . T h e r e - g r o u n d c l a y w a s p n e u m a t i c a l l y t ra n s -f e r r e d i n t o t h e c a l c i n a t i o n f u r n a c e u s i n g h e l i u m . T h ef e e d s y s t e m h a s b e e n d e s c r i b e d s e p a r a t e l y ( D a v i e s ,1985) .

T h e k a o l i n i t e w a s f l a s h c a l c i n e d b e t w e e n 9 0 0 ~ a n d1 25 0 *C w i t h t o t a l e x p o s u r e t i m e s , i n c l u d i n g h e a t i n ga n d q u e n c h in g , o f 0 . 2 - 2 . 0 s ec . A l t h o u g h t h e s e t e m -p e r a t u r e s a r e s u f f ic i e n t t o p r o d u c e t o t a l d e h y d r o x y l a -t i o n a t e q u i l ib r i u m , t h e e x p o s u r e t i m e s w e r e t o o s h o r tf o r e q u i l i b r i u m t o b e a t t a i n e d . T h e e x t e n t o f d e h y -d r o x y l a t i o n o f t h e p r o d u c t w a s d e t e r m i n e d f r o m t h ea d d i t i o n a l w e i g h t l o ss w h e n t h e f l a s h - c a l c i n e d s a m p l ew a s s u b s e q u e n t l y s o a k - c a l c i n e d a t 1 0 0 0 ~ f o r 2 h r . T h ed e n s i t y o f t h e p r o d u c t w a s m e a s u r e d u s i ng w a t e r d i s -p l a c e m e n t i n a p y c n o m e t e r w i t h s p e c i a l c a re t a k e n t or e m o v e a l l a i r fr o m t h e s a m p l e . S o m e v o i d s i n m a n yo f t h e p a r t i c l e s w e r e a p p a r e n t l y i n a c c e s s i b l e t o w a t e r ,t h e r e b y c a u s i n g a d e c r e a s e i n e f f e ct i v e p a r t i c l e d e n s i t y.B a s e d u p o n m u l t i p l e m e a s u r e m e n t s u s i n g t h e s a m es a m p l e , t h e a n a l y t i c a l m e t h o d s f o r t h e e x t e n t o f d e -

258

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Vol. 33, No. 3, 1985 Flash-calc ination of kaolinite 259

25

g

23~ | oo o o o~ | %% o ~

ooo o o o ~ o

o o ~ oo

oo

oI~ os o!~ oV 0'8 0'9FRACTIONAL DEHYDROXYLATION

Figure 1. Density of flash-calcined kaolin.

Figure 2. Kaol in particle flash-calcined in helium: top view.

hydroxyl ation and density were foun d to be reprodu-cible to ___2%. Brightness me asu rem ent s were con-ducted on selected samples. Electron photomicr ograph swere taken to provide visual confirmation of productproperties.

RESULTS AND DISCUSSION

Eighty-one samples of kaolinite were flash calcinedunder the above conditions. Fractional dehydroxyla-tion ranged from 0.45 to 0.99, with the more severetreatment (higher temperature and longer residencetime), as expected, prod ucing greater dehydroxy lation.

The product densities ranged from 2.19 to 2.42 g/cm 3.These values are 6.9% to 15.8% less than the densityof the raw clay, and as much as 21.5% less than thedensity of the fully soak-calcined product (Reike andMauve, 1942).

In general, product density decreased as fractionaldehydro xylation increased as shown in Figure 1. Thestraight line in Figure 1 expresses the densi ty as a func-tion of fractional dehydroxyla tion by assuming that theparticle volume does not change during calcination.Thus,

p = ,Oo(I - XY ), (I )

where

p =

P0 =

density o f the flash-calcined product,density of the original kaolinite,

X = the mass ratio of the boun d water to the de-hydroxyla ted kaolinite in the raw material, and

Y = fractional dehydroxylation .

For the k aolin ite used in this study po = 2.60 g/cm 3and X = 0.142; hence

P = 2.60 - 0.37Y. (2)

The significance of this equati on is that flash-calcineddensities less than predicted by Eq. (2) must correspondto an increase in particle volume during flash calci-

nation; conversely, flash-calcination densities greaterthan those predicted require a decrease in particle vol-ume. All but 4 of the 81 ex perimen tal points lie belowthe line i ndicatin g that particle swelling is associatedwith flash calcination.

What appear to be voids and the bubb le- or balloon-like appearance o f some o f the flash-calcined particleswere evident by electron microscopy. Figure 2 showsa top view, and Figure 3 an edge view of the flash-calcined product. Whereas no t all of the flash-calcinedparticles exhibited this appearance, it was reasonably

comm on, pa rticularly in those samples having the low-er densities. In one test, three electron microscope ob-servers concluded that slightly more than half of theparticles contai ned voids. The raw clay was completelyfree of such voids an d exhib ited the hexagonal lamellarstructure commonly associated with kaolinite.

The bub bling may have been the result of the very

Figure 3. Kaolin particle flash-calcined in helium: edge view.

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260 Br idson, Davies , and Harr i son Clays and Clay Minera ls

r a p i d h e a t i n g r a t es w h i c h c a u s e d d e h y d r o x y l a t i o n t oo c c u r f a s t e r t h a n p r o d u c t w a t e r c o u l d d i ff u s e f r o m t h ep a r t i c l e . T h e r e s u l t w a s a b u i l d - u p i n s t e a m p r e s s u r ew i t h i n t h e p a r t i c le w h i c h , a t t h e p r o p e r t e m p e r a t u r e ,b e c a m e e l a s ti c p e r m i t t i n g b u b b l e f o r m a t i o n . T h e h i g hq u e n c h r a t e s t h e n f r o z e t h e b u b b l e s i n t o t h e p r o d u c t .

W a d a a n d W a d a ( 1 9 77 ) r e p o r t e d a n a l l o p h a n e h a v -i n g a s i m i l a r st r u c t u re . T h e a l l o p h a n e w a s d e s c r i b e da s h o l l o w s p h e r u le s o r p o l y h e d r a w i t h e x t e r n a l d i a m -e t e r 3 5 - 5 0 ~k a n d w a l l t h i c k n e s s e s 1 0 ~ o r l e ss . T h e s es a m p l e s a l so h a d l o w d e n s i ty , w i t h t h e a p p a r e n t d e n s i t yb e i n g d e p e n d e n t u p o n t h e f l u id u s e d i n th e d e n s i t ym e a s u r e m e n t s . T h i s l e d to t h e f u r t h e r s p e c u l a t i o n th a tt h e w a i ls c o n t a i n e d m o l e c u l a r - s i z e o p e n i n g s w h i c h a l -l o w e d s m a l l m o l e c u l e s ( w a t e r ) t o p e n e t r a t e b u t p r e -v e n t e d t h e e n t r y o f l a rg e m o l e c u l e s ( m e t h a n o l ) i n t o t h ev o i d s p a c e .

S u b s e q u e n t s o a k c a l c i n a t i o n ( 1 0 0 0~ f o r a p p r o x i -m a t e l y 2 h r ) o f t h e f l a s h - c a l c i n e d k a o l i n i t e s a m p l e s d i dn o t d e s t r o y t h e b u b b l e s t r u c tu r e a n d c a u s e d o n l y s m a l li n c r e a s e s in p a r t i c l e d e n s i t y. Te n f l a s h - c a l c in e d s a m -p l e s h a v i n g d e n s i ti e s o f 2 . 2 6 - 2 . 3 7 g / c m 3 e x p e r i e n c e da n a v e r a g e 2 . 7 % d e n s i t y i n c r e a se . T h e d e n s i t i e s o f t h e s ed o u b l y - c a l c i n e d s a m p l e s r a n g e d f r o m 2 . 3 5 t o 2 .4 3 g /c m 3, w e l l b e l o w t h e 2 . 79 g / c m 3 v a l u e r e p o r t e d b y R e i k ea n d M a u v e ( 1 9 4 2 ) .

T h e f l a s h - c a lc i n e d p r o d u c t w a s c l e a r l y le s s b r i g h tt h a n t h e r a w k a o l i n i t e . T h e b r i g h t n e s s o f f o u r f la s h -c a l c i n ed s a m p l e s w a s 7 9 . 3 - 8 1 . 7 % c o m p a r e d t o t h e a p -p r o x i m a t e 8 9 % b r i g h tn e s s o f th e r a w m a t e r ia l . T h e

d e c r e a s e d b r i g h tn e s s w a s p r e s u m a b l y d u e t o i m p u r i t i e si n t h e o r i g i n a l k a o l i n i te ( p e r h a p s F e ) a n d / o r t h e h e l i u mc a r r i e r g a s . H e a t t r e a t m e n t i n a i r a t a b o u t 5 0 0 ~ r e -s t o r e d t h e o r i g i n a l b r ig h t n e s s .

C O N C L U S I O N S

F l a s h p r o c e s si n g o f m e t a l s a n d c e r a m i c s i s k n o w nt o p r o d u c e a l l o y s a n d g l a s s e s n o t p r e v i o u s l y o b s e r v e da t r o o m t e m p e r a t u r e . T h e p r e l i m i n a r y r e s u lt s o f t h ep r e s e n t s t u d y i n d i c a t e t h a t c l a y p r o d u c t s h a v i n g u n i q u ep r o p e r t i e s m a y a l so b e p r o d u c e d b y f l as h c a l c in a t i o n .T h e d e g r ee o f d e h y d r o x y l a t i o n ca n b e c o n t r o l l e d o v e r

a w i d e r a n g e o f v a l u e s . D e n s i t i e s o f fl a s h - c a l c i n e d p r o d -

u c t s a s l o w a s 2 . 1 9g / c m 3 c a n b e p r o d u c e d . A c o r r e -l a t io n b e t w e e n d e g r e e o f d e h y d r o x y l a t i o n a n d p r o d u c td e n s i t y w a s s h o w n , a n d p r o d u c t s a p p e a r e d t o c o n t a i nn u m e r o u s i n t e r n a l v o id s . S o m e h a d a b a l l o o n - l i k e a p -p e a r a n c e .

A C K N O W L E D G M E N T S

S u p p o r t p r o v i d e d t o D P H d u r in g t h e p e r i o d o f t h isr e s e a r c h b y t h e S c i e n c e a n d E n g i n e e r i n g R e s e a r c hC o u n c i l i s g r a te f u l ly a c k n o w l e d g e d . A p p r e c i a t i o n i s a l s oe x p r e s se d t o E C L P, L t d . f o r p r o v i d i n g c o n s u l t a t i o n o nt h e p r o j e c t a n d s u p p l y i n g k a o l i n i te s a m p l e s .

Depa r tmen t o f Chemica l Eng inee r ing D. BRIDSONUniversi ty o f Ex ete r T. W. D Av m sExeter, U ni ted Kingdom

Depa r tmen t o f Chemica l Eng inee ri ng D.P. HARRISONLouisiana State Universi tyBaton Rouge , Louis iana 70803

R E F E R E N C E S

Davies , T. W. (1984) Equ ipm ent for the s tudy of the f lashheat ing of par t ic le suspens ions :High Temp. Tech .2 , 141 -147.

D a v i es , T. W . ( 19 85 ) T h e p r o d u c t i o n o f c o n c e n t r a t e d p o w -der suspens ions a t low f low ra tes :Powder Tech. 42,1-5 .

Gr im , R.E . (1962)Appl ied C lay Minera logy:M c G r a w - H i l l ,New York , 98 -101 .

Harm on, G. G. and Parm elee , C. W. (1942) Tes t ing andclass i fica t ion of ba l l c lays : thermal h is tory :Bull . Amer. Ce-ram. Soc. 2 1 , 2 8 3 - 2 8 6 .

L iebe rmann , H . and Wai t e r, J . ( 1982) Rap id so l i d i f ic a t i ono f m e ta s tab l e m a te r ia l s : i nRapidly Sol id i f ied Amorphousand Crystal l ine Alloys,B. H. Kear, B. C. Geissen , and M.Cohen, eds . , Elsevier, New Y ork , 111-122.

L ivage , J . andCo l longues , R . ( 1976) Sem iconduc t ingp rop -e r t ie s o f amorp hous V205 p repa red by sp l a t coo ling :M a t .Sci. Engr. 23, 297-299.

Reike , R. and Mau ve, L . (1942) Indica t ions as to the minera lcons t i tuents of kaol in :Ber. Deut. Keram. Ges.23, 119-151(c i ted in Gr im, R. E . (1968)Clay Mineralogy: 2nd ed . ,McG raw-Hi l l , New York , p . 308) .

W ada, S . I . and W ada, K. (1977) Den si ty and s t ruc ture ofa l lophane:Clay Miner. 12, 289-298.

(Received 27 June 1984; accepted 25 October 1984; Ms.1390)