Comminution of Asbestos: Role of the Liquids Used as Grinding Aids

449

COMMINUTION OF ASBESTOS : ROLE OF THE LIQUIDS USED AS GRINDING A I D S E. PAPIRER, J.B.

DONNET and P. ROLAND

Centre de Recherches s u r l a Physico-Chimie des Surfaces S o l i d e s (C.N.R.S.)

24, Avenue du P r e s i d e n t Kennedy 68200 Mulhouse ( F r a n c e )

ABSTRACT Asbestos ( c h r y s o t i l e s , amosite and c r o c i d o l i t e ) were s u b m i t t e d t o b a l l m i l l i n g i n t h e presence o f v a r i o u s l i q u i d s ( e s s e n t i a l l y t o l u e n e and GJater). A d r a m a t i c d i f f e r e n c e appears. Water a c t s as a p r o t e c t i v e agent whereas t o l u e n e f a c i l i t a t e s t h e f r a g m e n t a t i o n o f t h e i n d i v i d u a l m i c r o f i b r e s . F u r t h e r m o r e , t h e m a t e r i a l ground i n t o l u e n e becomes amorphous. S e v e r a l hypotheses have been made i n o r d e r t o e x p l a i n the experimental observations.

INTRODUCTION Asbestos has more t h a n 3000 p r a c t i c a l a p p l i c a t i o n s and even though i t has been r e c o g n i z e d as a p o t e n t i a l h e a l t h h a z a r d i n s e v e r a l i n s t a n c e s i t can s c a r c e l y be r e p l a c e s by any o t h e r n a t u r a l o r s y n t h e t i c f i b r e ( r e f . 1). I t s most common f o r m i s c h r y s o t i l e which i s made up o f bundles o f a l i g n e d m i c r o f i b r e s o f c o l l o i d a l dimensions. F o r many uses, such as b r a k e l i n i n g s o r i n asbestos-cement m a t e r i a l s , i t i s necessary t o s e p a r a t e t h e m c r o f i b r e s f r o m t h e bundles. T h i s "opening" procedure can be a c h i e v e d t h r o u g h a mechanical t r e a t m e n t , e.g.

grinding.

I n p r e v i o u s papers ( r e f , 2 and 3), we r e p o r t e d on t h e o p e n i n g o f c h r y s o t i l e o f Canadian o r i g i n , u s i n g b a l l m i l i n g e i t h e r i n presence o f w a t e r o r an organ c medium, G r i n d i n g i n w a t e r r e a d i l y causes t h e opening o f t h e m a c r o f i b r e s and a subsequent and i m p o r t a n t i n c r e a s e i n t h e s p e c i f i c s u r f a c e a r e a o f t h e around asbestos. F u r t h e r , on p r o l o n g e d g r i n d i n g , c h r y s o t i l e r e s i s t s a d d i t i o n a l d e g r a d a t i o n (amorphis a t i o n ) s u r p r i s i n g l y w e l l . T h i s o b s e r v a t i o n may be e x p l a i n e d by one o r t h e o t h e r o f t h e f o l l o w i n g two hypotheses. The f i r s t p o s t u l a t e s t h a t w a t e r m o l e c u l e s f o r m a c o n t i n u o u s p r o t e c t i v e c o a t i n g h a v i n g s u f f i c i e n t mechanical s t r e n g t h o r l u b r i c a t i n g a c t i o n , s t r u c t u r i n g o f water a t the mineral surface). D i f f e r e n t t e s t s support t h i s h y p o t h e s i s ( r e f . 3 ) . However, an a l t e r n a t i v e e x p l a n a t i o n i s p o s s i b l e . C h r y s o t i l e c o n s i s t s o f superimposed l a y e r s o f s i l i c a t e t r a h e d r a and magnesium h y d r o x i d e ( o r b r u c i t e ) octahedra. Due t o mismatching o f t h e s t r u c t u r a l parameters t h e s u c c e s s i v e l a y e r s become i n c u r v e d and t h e c h r y s o t i l e m i c r o f i b r e s a c q u i r e s a

450

t u b u l a r form. Mg(OH)2 always forms t h e o u t e r l a y e r . When d i s p e r s e d i n water, some o f t h e b r u c i t e d i s s o l v e s ; thus t h e pH o f t h e medium i s i n c r e a s e d and t h e e l e c t r i c s u r f a c e p o t e n t i a l ( z e t a p o t e n t i a l ) becomes n e g l i g i b l e . Now, according t o Westwood ( r e f . 4 ) when t h e f i b r e approaches i t s h i g h e s t s t a t e o f mechanical s t r e n g t h i t i s then t h a t asbestos w i l l e x h i b i t t h e h i g h e s t r e s i s t a n c e t o s t r u c t u r a l breakdown. I n an o r g a n i c s o l v e n t ( t o l u e n e , alcohols,. ..) t h e r e s u l t o f c h r y s o t i l e g r i n d i n g i s radically different.

Not o n l y does d e f i b e r i z a t i o n occur, b u t t h e i s o l a t e d

m i c r o f i b r e s a l s o break down i n t o s h o r t e r segments : a phenomenon accompanied by a much sharper i n c r e a s e i n t h e s p e c i f i c s u r f a c e area ( S ) ,

o r a higher i n i t i a l grinding

r a t e than t h e one encountered when g r i n d i n g i n water. However, immediately a f t e r t h i s i n i t i a l stage, S decreases, as i s usual, due t o agglomeration and amorphisation o f t h e ground fragments. The amorphisation i s caused by t h e i n c r e a s i n g number o f h i g h l y r e a c t i v e s i t e s , which a r e c o n t i n u o u s l y formed by c r i s t a l f r a g m e n t a t i o n . The purpose o f t h e p r e s e n t study i s t o v e r i f y t h e r e s u l t s o b t a i n e d w i t h t h e Canadian c h r y s o t i l e ( s e r p e n t i n e ) ; f i r s t l y , by i n v e s t i g a t i n g c h r y s o t i l e s of d i f f e r e n l o r i g i n s and secondly by examining asbestos o f t o t a l l y d i f f e r e n t s t r u c t u r e s (amphiboles).

EXPERIMENTAL Asbestos c h a r a c t e r i s t i c s The main p r o p e r t i e s o f t h e asbestos samples a r e gathered i n Table I. TABLE 1

1 isio2

C h a r a c t e r i s t i c s o f t h e asbestos samples Sample

CHRYSOTILE

r

MOSITE

C R O C I OO-1TE

Origin

Canada SouthAfrica

Formula

I

SouthAfrica

FeO

Composi t i on % CaO MgO

2.02

0.89

lInit i a ' Surfacf Na20 H20 Area n2/g 59.78 0.10 12.82 15.2

139.70/ 0.70

1.08

10.30 0.04 12.81 18.3

( F e + b 17 Si8022 49.70 39.70

1.04

6.44 0.09

1.92

0.7

1.45

1.06 6 . 2 0

2.59

1.1

Mg3 (Si 0 )

38.75

(OH12 NaFe3+ SouthA f r i c a (Fe2+Mg)3 50.9

I

si8022 (OH)2

I I

20.50 16.85

-

451

Grinding c o n d i t i o n s

I n a t y p i c a l run, 30 g o f asbestos a r e mixed w i t h 300 cm3 o f l i q u i d i n a 600 cm3

b a l l m i l l , c o n t a i n i n g 1.8 kg o f s t a i n l e s s s t e e l b a l l s (1.2 cm i n diameter). The b a l l m i l l i s v i b r a t e d a t a frequency o f 25 Hz, w i t h a peak amplitude o f 2 cm. Analysis o f the ground samples A f t e r g r i n d i n g , t h e samples are recovered by c e n t r i f u g a t i o n . They are then oven d r i e d a t 150°C. F i n a l l y , several examinations are undertaken : X-ray a n a l y s i s (powder d i f f r a c t i o n ) e l e c t r o n microscopy (JEOL l O O B ) ,

i n t h e transmission and i n scanning modes

a f t e r gold coating measurement o f t h e s p e c i f i c surface area, w i t h N2 a t

-

195°C using "Micromeretics"

semi-automati c equipment measurement o f the degree o f c h r y s o t i l e amorphisation according t o a p r e v i o u s l y described method ( r e f . 5) I n some cases, I . R .

spectroscopy, as w e l l as D.T.A.

and D.T.G.

were a l s o applied.

RESULTS AND D I S C U S S I O N Asbestos f i b r e s a r e i n t e r e s t i n g s t a r t i n g m a t e r i a l s f o r a g r i n d i n g experiment. They have an i n i t i a l c h a r a c t e r i s t i c shape (micrographs A l , A2, A3) and t h e i r g r i n d i n g leads t o m i c r o f i b r e s which a l s o show a marked morphology. Hence i t becomes very easy t o f o l l o w any change i n morphology and s t r u c t u r e as a r e s u l t o f t h e mechanical treatment. G r i n d i n g i n water The v a r i a t i o n s i n t h e s p e c i f i c s u r f a c e areas o f t h e t h r e e samples ( c h r y s o t i l e , amosite, c r o c i d o l i t e ) , as a f u n c t i o n o f t h e g r i n d i n g d u r a t i o n , i s i l l u s t r a t e d i n f i g u r e 1. C l e a r l y , t h e s p e c i f i c surface areas o f a l l t h e samples are p r o g r e s s i v e l y augmented.

A simple e m p i r i c a l k i n e t i c equation can be formulated. An i n t e r e s t i n g

p o i n t which should be u n d e r l i n e d i s t h a t , i r r e s p e c t i v e o f the treatment time, of e i t h e r s e r p e n t i n e o r amphiboles, no agglomeration o f the ground m a t e r i a l i s observed. A l l asbestos samples r e s i s t remarkably w e l l t o amorphisation, a phenomenon which has

n o t been r e p o r t e d f o r o t h e r minerals. Possibly, the same hypotheses, as g i v e n p r e v i o u s l y , w i l l e x p l a i n t h e unexpected behaviour o f asbestos submitted t o m i l l i n g i n pure water. The s i m i l i t u d e i n the behaviours o f a l l asbestos types a r e f u r t h e r confirmed by the examination o f the e l e c t r o n micrographs (81, 82, 83). We do indeed observe the opening o f t h e f i b r e bundles, b u t even f o r t h e long treatment d u r a t i o n s , m i c r o f i b r e s are s t i l l present. However, due t o a f a t i g u e mechanism ( p r o g r e s s i v e accumulation of s t r u c t u r a l d e f e c t s ) , some f i b r e s are segmented even i n water. F u r t h e r , the remarkable

452

r e s i s t a n c e towards amorphisation can be convincingly demonstrated i n t h e case o f the c h r y s o t i l e , of South African o r i g i n , by following the quantity of amorphous Mg(OH)2 which appears during grinding. Table 2 gathers the experimental r e s u l t s . I

r Fig. 1. Grinding i n water

N

G IN

I

50

WATER

I

h

100

7-

Table 2 Resistance t o amorphisation o f c h r y s o t i l e (South Africa) Mg(OH)2 amorphous

1.40

1.43 1.94

2.50 2.57 4.11

1 ,

1.03 0.98 1.09 1.04 1.03

As seen, no accumulation of amorphous Mg(OH)2 i s recorded. The amount i s t r u l y r e l a t e d t o t h e e x t e n t of developed surface a r e a . X-ray diffractometry a s well a s IR spectroscopy e n t i r e l y support the conclusions of the chemical a n a l y s i s .

453

Grinding i n toluene Figure 2 shows the v a r i a t i o n s i n t h e s p e c i f i c surface areas o f t h e 3 asbestos samples ground i n pure toluene.

I

G R l NDlNG IN TOLUENE 1

chrysot ile ramosite

I

\

\

\

\

\

\

\

\

0

20

50

h

F i g . 2. G r i n d i n g i n toluene A f t e r a s i g n i f i c a n t r i s e i n S (common i n g r i n d i n g experiments), t h e phenomenon o f agglomeration i n t e r v e n e s . The i n i t i a l r i s e i n S w i t h time may be described by a simple e m p i r i c a l r e l a t i o n s h i p . The p o i n t t o note i s t h a t t h i s l a t t e r equation i s t o t a l l y d i f f e r e n t from t h e one p r e v i o u s l y observed i n water, thus p o i n t i n g t o a q u i t e d i f f e r e n t f r a c t u r e mechanism. E l e c t r o n microscopy ( C l ,

C2, C3) demonstrates

t h i s d i f f e r e n c e and confirms our e a r l i e r r e s u l t s ( r e f . 2 and 3 ) obtained w i t h the c h r y s o t i l e o f Canadian o r i g i n . Segmentation o f t h e m i c r o f i b r e s does indeed occur. I n t h e previous study, segmentation was a t t r i b u t e d t o a phenomenon due t o environmental s t r e s s c r a c k i n g : t h e i n i t i a l r a t e o f g r i n d i n g c o u l d be d i r e c t l y r e l a t e d t o the cohesive energy ( 6 parameter) o f the l i q u i d s used as g r i n d i n g a i d s . P r e l i m i nary r e s u l t s obtained w i t h o t h e r types o f asbestos p o i n t t o a very s i m i l a r trend. For t h e l o n g g r i n d i n g d u r a t i o n s , e l e c t r o n microscopy demonstrates the amorphisation s t e p (Dl, 02, D"). which i s n o t observed o r g r e a t l y delayed when g r i n d i n g i n water

( E l , E2, E3). When m i l l i n g i n toluene, c l e a r l y , t h e g r i n d i n g residues become associated. Amorphisation i s the main process when t h e g r i n d i n g d u r a t i o n i s extended. This means, i n t h e case o f c h r y s o t i l e , t h a t more and more amorphous Mg(OH)2 should be formed.

454

TABLE 3 Amorphisation o f chrysotile (South Africa)

Duration ( h )

Mg(OH)2 amorphous

Total

moles/g

mg/m2

0

1.04

1.32

4

1.27

1.06

8

1.36

1.03

24

1.75

0.86

48

1.80

0.79

72

2.07

0.79

360

10.28

30.48

Between 72 and 360 h of treatment severe degradation of chrysotile, freeing the Mg(OH)2 from the asbestos structure, o r in other words crystalline amorphisation occurs. CONCLUSION This study highlights the predominant influence o f the environment in grinding experiments.On the one hand, liquids can prevent fracture. This i s seen w i t h water, which becomes structured a t the mineral surface and thus acquires unusual protective o r lubricating properties. On the other hand liquids can f a c i l i t a t e grinding by acting as ordinary s t r e s s cracking agents or by changing the e l e c t r i c surface charge of the solids. More work i s however needed before a c l e a r understanding of the rupture of the asbestos microfibres will be reached. Especially the pure mechanical aspect and the time f a c t o r have t o be analysed in greater depth. ACKNOWLEDGMENTS The contributions of Dr. A . Eckhardt (Electron microscopy) and Mr. F. Muller (Photographs) are greatly appreciated. REFERENCES 1 B.J. Feder, The New York, Times (1981) July 6th, 01-D2 2 E. Papirer, P . Roland and 3.8. Donnet, International Conference on Asbestos, Torino ( I t a l y ) May 26-30, 1980, Vol. 1, 249-263 3 E. Papirer and P. Roland, Clays and Clay Minerals, 29(1981)161-170 4 A.R.C. Westwood, J.S. Ahearn and J . J . Mills, Colloids and Surfaces, 2(1981)1-35 5 E . Papirer, G. Dovergne and P. Leroy, Bull. SOC. Chim. F 1976(5)651-653

455

ASBESTOS

A1

Chrysotile

A2

Amosite

A3

Croci do1 ite

456

I N WATER

B3

GRINDING ( s h o r t d u r a t i o n s )

Crocidol it e ( 2 4 h r s )

I N TOLUENE

c3

GRINDING ( l o n g durations)

D1

(15 days)

Chrysoti l e

D2

(15 days)

Amos ite

E2

Croci do1 i te

E3

(32 h r s )

El

458 DISCUSSION

-

Bruijne Is there a reason why the final specific surface area of the asbestos samples, a f t e r grinding is the same ?

-

Papirer The final specific surface area is constant when chrysotiles, of different origins are submitted t o grinding in water. This is indeed a pertinent observation. So far, the authors do not have a satisfactory explanation in the case of asbestos. The situation probably corresponds t o the formation of microcracks which are unable t o grow in size so as t o meet Griffith's criterion. When grinding in organic solvents, the l i m i t i n g value of the specific surface area is significantly lower. Apparently, the r a t e of amorphization becomes higher than the comminution r a t e and hence chrysotile becomes amorphous before it reaches the same stage than in water.

-

Westwood You have conducted any grinding experiments in which the chemistry of the working environment was changed in some progressive fashion, e.g. so as t o gradually increase or decrease the surface potential o f the solid ?

-

Papirer The results o f a l l the grinding experiments reported in the paper can be explained by the chernomechanical theory, even those obtained when grinding inwater which can nevertheless be considered as a particular system. To check Westwood's theory, grinding o f chrysotile asbestos (of Canadian origin) was performed during 6 h in a DMF-DMSO mixture, keeping constant the experimental conditions outlined earlier. So far, the course o f the grinding was only assessed by the specific surface areas ( S ) determination. The figure below illustrates the variation of S w i t h the composition of the DMF-DMSO mixture. Strikingly, the same trends (discontinuities) as those recorded by Westwood, are observed in the case of the comminution of asbestos. Possibly also similar explanations can account for the experimental data. More work is however needed t o reach a better insight into the fracture mechanisms which cause the fragmentation of the individual microfibres.

1 0 1 .

DMSO

I

20

I

.

.

40

VOL%

. .

60

.

80

I

DMF

DMF

Variation of the specific surface area of chrysotile samples (of Canadian origin) submitted t o grinding (6 h) in mixtures of DMF and DMSO.