5 Rare Earth Polishing Compounds Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 13, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch005
ROBERT V. HORRIGAN 8416 West Country Club Drive, North, Sarasota, F L 33580
There is little i n f o r m a t i o n in t h e literature r e g a r d i n g t h e first a p p l i c a t i o n s of cerium oxide o r c e r i u m - r i c h r a r e e a r t h oxide mixtures in g l a s s polishing, Duncan (1) mentions t h a t the application began in the European g l a s s i n d u s t r y about 1933, spreading to the Canadian optical i n d u s t r y about 1940. During World War II, an employee o f German-American descent working f o r the W. F. & J . Barnes Co. o f Rockford, Illinois, i n t r o d u c e d on August 31, 1943 a r a r e - e a r t h oxide (45% Ce0 ) polish c a l l e d B a r n e s i t e which enjoyed immediate success in the p o l i s h i n g of precision o p t i c s such as bombsights, range f i n d e r s , p e r i s c o p e s , and other fire c o n t r o l instruments. The Lindsay Chemical Co. o f West Chicago, Ill., e a r l y in World War II, i n t r o d u c e d a h i g h cerium oxide (90+% Ce0 ) polish c a l l e d Cerox f o r ophthalmic use. Other more s p e c i a l i z e d cerium-based products were added, a few competitors entered t h e field, and by 1960 more than 340 m e t r i c tons per year were being used f o r p o l i s h i n g m i r r o r s , p l a t e g l a s s , television tubes, opht h a l m i c lenses and precision o p t i c s . The advent o f the P i l k i n g t o n process, 1972-1973, f o r l a r g e s c a l e p l a t e g l a s s manufacture sev e r e l y reduced the market f o r cerium o x i d e , but still today over 1000 m e t r i c tons per year are s o l d in the U.S. From 1940 t o 1965, the p r i n c i p a l source of these r a r e e a r t h products was the m i n e r a l monazite (Th, RE orthophosphate) which f o r t u n a t e l y o r u n f o r t u n a t e l y , depending on one's p o i n t o f view, c o n t a i n s 4-6% thorium. Today, there i s e s s e n t i a l l y no market f o r thorium i n the U.S. The expense o f s e p a r a t i n g out thorium-free r a r e - e a r t h products from monazite i s not only e x c e s s i v e , but bound t i g h t l y i n governmental red tape because of the m i l d r a d i o a c t i v i t y of the thorium. This s i t u a t i o n does not apply i n France, B r a z i l , or I n d i a , whose governments are w i s e l y s t o c k p i l i n g a l l e x t r a c t e d thorium f o r f u t u r e atomic energy needs. L u c k i l y , the U.S. has the l a r g e s t b a s t n a s i t e (R.E. f l u o c a r bonate) mine i n the world l o c a t e d a t Mountain Pass, C a l i f o r n i a , owned and operated by Molycorp, I n c . , a s u b s i d i a r y o f Union O i l 2
2
0097-6156/81/0164-0095$05.00/0 © 1981 American Chemical Society In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 13, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch005
96
RARE EARTH ELEMENTS
Co. of C a l i f o r n i a . Proven orebody reserves at the end of December 1978 were 365,000 m e t r i c tons, w i t h i n d i c a t e d r e s e r v e s of over 3 m i l l i o n m e t r i c tons of r a r e e a r t h oxide (REO). Current mine prod u c t i o n c a p a c i t y i s 27,000 m e t r i c tons per year of b a s t n a e s i t e concentrate produced i n 3 grades: a 60% REO unleached concentrate, a 70% REO leached concentrate (SrO and CaO removed), and a 90% REO c a l c i n e d concentrate (CO2 removed). I n 1977, shipments t o t a l e d 13,521 m e t r i c tons of contained REO. P o l i s h i n g compounds consumed approximately 10% of t h i s p r o d u c t i o n . With s u i t a b l e chemical, mechanical and heat treatment, g l a s s p o l i s h i n g compounds of h i g h q u a l i t y have been produced from b a s t n a e s i t e concentrates s i n c e 1965. The S c i e n t i f i c B a s i s f o r the Uses A b r i e f d e s c r i p t i o n of g l a s s s u r f a c i n g techniques w i l l be u s e f u l p r i o r to our d i s c u s s i o n of the three p r i n c i p a l t h e o r i e s of the p o l i s h i n g mechanism. I n the ophthalmic f a c t o r y or p r e s c r i p t i o n l a b o r a t o r y , an o p t i c a l glass blank i s f i r m l y fastened to a lens chuck, which i s then pressed down on a curved g r i n d i n g t o o l and r o t a t e d at h i g h speed. To achieve the d e s i r e d l e n s c u r v a t u r e , one or two stages of diamond g r i n d i n g (generation) s u f f i c e . A l t e r n a t e l y one diamond generating step f o l l o w e d by g r i n d i n g (or f i n i n g ) w i t h l o o s e powdered a b r a s i v e s (such as corundum, emery, garnet, or s i l i c o n carbide) suspended i n a water s l u r r y may be employed. This i s rough treatment, and we may w e l l expect to f i n d some subsurface damage which h o p e f u l l y can be r e p a i r e d during the p o l i s h i n g step. The " f i n e d " l e n s , s t i l l attached to the l e n s chuck, i s r i n s e d f r e e of any adhering a b r a s i v e , and placed i n a p o l i s h e r . Here the t o o l contacted by the l e n s i s c a l l e d the " l a p " and may c o n s i s t of a wide v a r i e t y of m a t e r i a l s depending on the g o a l to be achieved. In the ophthalmic f a c t o r y , f o r example, t h i c k , hard t h e r m o p l a s t i c pads have good s u r f a c e q u a l i t y , curve c o n t r o l , long l i f e , and the a b i l i t y to operate w e l l under h i g h speeds and p r e s s u r e s . On the other hand, the p r e s c r i p t i o n l a b o r a t o r y w i l l f a v o r a p a p e r - t h i n p l a s t i c or c l o t h pad which i s used o n l y once. The p o l i s h i n g compound, u s u a l l y e e r i e o x i d e , z i r c o n i u m o x i d e , f e r r i c oxide ( j e w e l er's rouge), or s i l i c a (white rouge) i s s l u r r i e d i n water i n a c o n c e n t r a t i o n of 5-25% by weight, and i s r e c i r c u l a t e d c o n s t a n t l y over the l a p and l e n s . I n the f a c t o r y , l a r g e c e n t r a l systems c o l l e c t the p o l i s h i n g s l u r r y and pumps f u r n i s h the s l u r r y to the p o l i s h i n g bowls c o n s t a n t l y . The lens weight l o s s , w h i l e not g r e a t , i s r e a d i l y measured, and g l a s s removal r a t e i s of prime importance i n measuring the e f f i c i e n c y of a p o l i s h i n g compound. In our view, the p o l i s h i n g s l u r r y would have an i n d e f i n i t e l i f e were i t not f o r the f a c t t h a t the g l a s s products g r a d u a l l y d i l u t e and contaminate the s l u r r y . The b u i l d - u p of a l k a l i ions i s so g r e a t , t h a t d a i l y pH adjustments are necessary i n l a r g e c e n t r a l system s l u r r y tanks.
In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 13, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch005
5.
HORRIGAN
Polishing
97
Compounds
Over three hundred years ago Isaac Newton concluded t h a t p o l i s h i n g was nothing more than f i n e - s c a l e a b r a s i o n , E a r l y i n the t w e n t i e t h century, Lord R a y l e i g h found t h a t a p o l i s h e d s u r f a c e was e n t i r e l y d i f f e r e n t from a ground, or abraded s u r f a c e , and suggests ed t h a t the p o l i s h e d s u r f a c e was smooth on a molecular s c a l e , l i k e the s u r f a c e of water. L a t e r the B r i t i s h chemist, S i r George B e i l b y , a p p l y i n g chemical etchants to a p o l i s h e d s u r f a c e , found o r i g i n a l g r i n d i n g scratches to reappear. He concluded t h a t a molecular f l o w of m a t e r i a l (the " B e i l b y Layer") from h i g h to low spots took p l a c e , thus covering the s c r a t c h e s , (2, 5) Bowden and Hughes (2) at the U n i v e r s i t y of Cambridge i n the 1930 s r e a soned t h a t i f a b r a s i o n were the fundamental mechanism, then the hardness of the p o l i s h i n g m a t e r i a l should c o r r e l a t e w i t h a b i l i t y to p o l i s h ; they found t h i s not to be the case, but d i d f i n d a remarkable c o r r e l a t i o n between the m e l t i n g p o i n t of the p o l i s h i n g m a t e r i a l and the r a t e of p o l i s h . They concluded t h a t p o l i s h i n g was a m e l t i n g phenomenon, not a b r a s i o n . Let us t e s t t h e i r hypotheses a g a i n s t the m e l t i n g p o i n t s of known good p o l i s h i n g o x i d e s : ZrOo, 3000°C; Ce0 , 1950°Cj S i 0 1700°C; F e 0 , 1565°C; S n 0 , 1127*C. Except f o r s t a n n i c o x i d e , these high m e l t i n g p o i n t v a l u e s support the m e l t i n g hypothesis of Bowden and Hughes. Here the matter r e s t e d f o r twenty years or so, a b r a s i o n or m e l t i n g , take your p i c k , and each s i d e had i t s adherents. But now a t h i r d hypothesis was proposed and g r a d u a l l y took precedence over the f i r s t two. I n 1931 Grebenschikov (3) noted t h a t the presence or absence of water i n f l u e n c e d the p o l i s h i n g of g l a s s , and suggested that a l a y e r of s i l i c i c a c i d would b u i l d up on the s u r f a c e of the g l a s s being p o l i s h e d . This l a y e r would p r o t e c t the g l a s s from f u r t h e r e r o s i o n were i t not f o r the f a c t t h a t the p o l i s h i n g agent was at work to sweep away t h i s l a y e r and expose a f r e s h surface. Further work by C o r n i s h and Watt (4) and S i l v e r n a i l and Goetz i n g e r (5) e s t a b l i s h e d the a c t i v e r o l e played by the presence of water, and these authors concluded t h a t a chemical-mechanical hypothesis would f i t the observed data. I n the case of e e r i e oxide p o l i s h i n g of g l a s s , Cornish and Watt suggest the formation of a "CeO-Si" a c t i v a t e d complex which permits the r u p t u r e of the O-Si-0 bonds by h y d r o l y s i s . The complex "CeO-Si" then breaks a p a r t , the hydrated s i l i c a i s swept away along w i t h a l k a l i s r e leased from the g l a s s s u r f a c e , and the process r e p e a t s . The above i s a good example of Ce0 a c t i n g l i k e a c a t a l y s t . The author would l i k e to suggest another p o s s i b i l i t y which may add to the chemical theory. The e f f i c i e n t p o l i s h i n g compounds p r e v i o u s l y mentioned have not only high m e l t i n g p o i n t s , but a l s o have l a r g e u n s a t i s f i e d c o o r d i n a t e v a l e n c i e s . T y p i c a l l y , f o r example, the z i r c o n i u m atom w i l l a t t r a c t a cloud of h y d r o x y l r a d i c a l s to s a t i s f y i t s c o o r d i n a t e v a l e n c e s . Thus, a h i g h concentrat i o n of h y d r o x y l r a d i c a l s are r e a d i l y a v a i l a b l e at the g l a s s s u r f a c e to speed the h y d r o l y s i s r e a c t i o n . f
2
2
3
2
2
In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
2 >
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 13, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch005
98
RARE EARTH ELEMENTS
The f r e e world market f o r cerium-oxide based p o l i s h i n g compounds i s not l a r g e — a p p r o x i m a t e l y 4400 m e t r i c tons per y e a r — , and we do not see a s u b s t a n t i a l growth p o t e n t i a l d e s p i t e the 11% annual growth i n s a l e s of s p e c t a c l e l e n s e s . The reason i s twof o l d : f i r s t , f a s t e r more e f f i c i e n t p o l i s h i n g compounds a r e a v a i l a b l e which can be used i n s l u r r y c o n c e n t r a t i o n s one-half t h a t of a few years ago; second, f u l l y h a l f the market f o r ophthalmic g l a s s lenses has been captured by p l a s t i c lenses of CR-39 polymer. Cerium oxide i s i n e f f e c t i v e i n p o l i s h i n g t h i s m a t e r i a l ; s p e c i a l l y t r e a t e d alumina or s t a n n i c o x i d e a r e used. Table I shows our estimate of 1979 cerium-oxide based p o l i s h ing product consumption. TABLE I . CONSUMPTION OF CERIUM-OXIDE BASED POLISHING COMPOUNDS (1979) a COUNTRY
CONSUMPTION (METRIC TONS/YR.)
United S t a t e s Canada South America Far East Western Europe
a.
Ce0
2
1,600 200 350 850 1,400 4,400
content v a r i e s from 45-90%.
The estimated end-use p a t t e r n f o r cerium oxide based p o l i s h ing compounds i n the U.S. (1979) i s i l l u s t r a t e d i n Table I I . TABLE I I . ESTIMATED END USE PATTERN IN THE UNITED STATES FOR CERIUM-OXIDE BASED POLISHING COMPOUNDS (1979) a END USE, U.S.
CONSUMPTION PERCENT METRIC TONS
Glass l e n s e s , ophthalmic Glass lenses, p r e c i s i o n Mirrors TV tube f a c e p l a t e s M i s c . - photomasks, gem stones
a.
Ce0
o
720 192 320 240 128 1,600
45 12 20 15 8 100
content v a r i e s from 45-90%.
Competitive Advantage o f the Rare Earths - Competition from Other M a t e r i a l s As our s a l e s f o r c e never t i r e s o f r e p e a t i n g , " I t i s n ' t the cost per pound of cerium o x i d e t h a t m a t t e r s ; what matters i s your
In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 13, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch005
5.
HORRIGAN
Polishing
99
Compounds
TABLE I I I . REPRESENTATIVE SPHERE AND TORIC POLISHING MACHINE CHARACTERISTICS, 1930 - 1980 NORMAL SPINDLE SPEED
TYPICAL SPINDLE FORCE
TYPICAL SPINDLE TIME
MODEL
APPROX. ERA OF ORIGIN
Hand Pan
1930's
300 RPM V a r i a b l e
15 Min.
Robinson- Greyhound Houchin #113
1950's
450 RPM
30 Lbs.
8 Min.
CMV
1CM-10
1970's
Upper=l,200 RPM Lower=l,800 RPM
90 Lbs.
1 Min.
Coburn
608
1970's
2,400 RPM
MANUFACTURER SPHERES Bausch & Lomb
100 Lbs. 3/4 Min.
TORICS Bausch & Lomb (used t o "rock" a t o r r i c ) Hand Pan
1930*s
American O p t i c a l 427
1940 s
Optek
400
1960's
American Optical
Super Twin 1970 s
f
f
Upper= Lower =
300 RPM V a r i a b l e
30 Min.
400 RPM 30 RPM
20 Lbs.
15 Min.
400 CPM*
30 Lbs.
5 Min.
550 CPM*
40 Lbs.
k\ Min.
*Cycles Per Minute.
In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 13, 2015 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch005
100
RARE EARTH ELEMENTS
cost per thousand p o l i s h e d s u r f a c e s ! " A h i g h q u a l i t y cerium oxide p o l i s h may be p r i c e d at $3.50 per pound; a good q u a l i t y z i r c o n i a based p o l i s h may be $1.50 per pound; red rouge ( f e r r i c oxide) may be p r i c e d at $0.40 per pound; and w h i t e rouge ( p r e c i p i t a t e d s i l i ca) a t perhaps $0.20 per pound. Without g e t t i n g i n t o d e t a i l s , a recent example may c l a r i f y the c o m p e t i t i v e advantage of cerium oxide. One of the l a r g e s t l e n s manufacturers i n America was persuaded to s w i t c h from z i r c o n i a to a h i g h q u a l i t y cerium o x i d e . I n the f i r s t n i n e months a savings of $400,000 was r e a l i z e d by more than h a l v i n g the amount of powder used, s h o r t e n i n g the time r e q u i r e d f o r p o l i s h i n g each l e n s , and i n c r e a s i n g the y i e l d of f i n ished l e n s e s i n the b a r g a i n . Other f a c t o r s are important, too. Z i r c o n i a has a nasty t e n dency to s e t t l e out rock-hard i n tanks and p i p e s , and besides the m a t e r i a l l o s s , clean-up c o s t s are severe. C e r i a w i l l s e t t l e event u a l l y , but always i s s o f t and easy to re-suspend. F e r r i c oxide (red rouge) i s an e x c e l l e n t , but slow p o l i s h , and a bad p o l l u t a n t due to i t s i r r e v e r s i b l e s t a i n i n g q u a l i t y . White rouge i s a very slow p o l i s h , and i s r a r e l y seen i n use today. High p o l i s h i n g speeds are e s s e n t i a l i n todays economy, and the l a t e s t equipment employs much h i g h e r s p i n d l e speeds and p r e s sures than those used j u s t a few years ago. Cerium oxide i s i d e a l under these more modern c o n d i t i o n s . A s p h e r i c a l l e n s that r e q u i r e d 8 minutes to p o l i s h 15 years ago i s now p o l i s h e d i n l e s s than one minute. A t o r i c ( c y l i n d e r ) l e n s that p r e v i o u s l y took 15 minutes to p o l i s h , now r e q u i r e s 4-1/2 minutes. Table I I I i l l u s t r a t e s the progress which has been made i n p o l i s h i n g machines over the l a s t f i f t y y e a r s . BIBLIOGRAPHY 1.
Duncan, L. K. "Cerium Oxide f o r Glass (1970), 41 ( 7 ) , 387-393.
2.
C o r n i s h , D. C. "The Mechanism of Glass Polishing" B.S.I.R.A. Research Report R267, British Scientific Instrument Research A s s o c i a t i o n , South Hill, C h i s l e h u r s t , Kent (1961).
3.
Grebenschikov, I . V. Keram. i S t e k l o , (1931), 7,
4.
C o r n i s h , D. C.; Watt, J. M. "The Mechanism of Glass P o l i s h i n g " a r e p o r t presented at the Symposium on the Surface Chemistry of G l a s s , Am. Ceramic Soc. Meeting, Wash. D.C. (May 11, 1966).
try,
5. Polishing"
Silvernail,
Polishing"
Glass
Indus-
36.
W. L.; Goetzinger, N. J . "The Mechanism of Glass Glass I n d u s t r y , (1971), 52 ( 4 ) , 130-152, 52 ( 5 ) ,
172-175. RECEIVED February 18,
1981.
In Industrial Applications of Rare Earth Elements; Gschneidner, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
