Filtrability of Polymer-Flocculated Suspensions - American Chemical

27 Filtrability of Polymer-Flocculated Suspensions J. GREGORY and A. E. L. DE MOOR Downloaded via TUFTS UNIV on July 15, 2018 at 18:35:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Department of Civil Engineering, University College, London WC IE 6BT, England

The filtrability of suspensions can be a useful means of assessing the performance of polymeric flocculants. A newly-developed technique involves filtration at constant pressure, continuous monitoring of filtration rate and on-line data processing to give the specific resistance to filtration. The flocculation of kaolin suspensions by various cationic polymers has been studied using this method. Optimum polymer dosages are clearly established and show good agreement with those from other techniques. Filtrability seems not to be greatly dependent on floc size. P o l y m e r f l o c c u l a n t s a r e now u s e d i n a w i d e r a n g e o f a p p l i c a t i o n s ( 1 ) . E a r l y i n t e r e s t i n t h e s e m a t e r i a l s was l a r g e l y based on t h e i r a b i l i t y t o improve t h e d e w a t e r i n g r a t e s ( i . e . t o increase the permeability) o f suspensions. A p p l i c a t i o n s a s s o i l c o n d i t i o n e r s ( 2 ) and i n t h e d e w a t e r i n g o f p h o s p h a t e s l i m e s ( 3 ) w e r e among t h e f i r s t s u c c e s s f u l u s e s o f synthetic polymeric f l o c c u l a n t s . For t h i s reason, several test methods b a s e d on p e r m e a b i l i t y h a v e b e e n d e v e l o p e d , i n c l u d i n g t h e r e - f i l t r a t i o n r a t e method o f L a Mer ( 3 ) . Even i n a p p l i c a t i o n s o t h e r t h a n d e w a t e r i n g , p e r m e a b i l i t y methods a r e q u i t e o f t e n u s e d t o a s s e s s t h e p e r f o r m a n c e o f polymeric f l o c c u l a n t s , s i n c e , i n p r i n c i p l e , they can give a very sensitive i n d i c a t i o n of the state of aggregation of p a r t i c l e s and a r e u s e f u l i n l o c a t i n g optimum p o l y m e r c o n c e n t r a t i o n s . T r a d i t i o n a l p e r m e a b i l i t y t e s t s a r e time-consuming and s u b j e c t t o some u n c e r t a i n t i e s ( 4 ) . I n t h e p r e s e n t p a p e r , we d e s c r i b e an automated t e c h n i q u e f o r d e t e r m i n i n g t h e f i l t r a b i l i t y of f a i r l y d i l u t e suspensions, which can give u s e f u l information, on t h e b e h a v i o u r o f p o l y m e r i c f l o c c u l a n t s . 0097-6156/84/0240-0445S06.00/0 © 1984 American Chemical Society

Goddard and Vincent; Polymer Adsorption and Dispersion Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

POLYMER ADSORPTION AND DISPERSION STABILITY

446 P e r m e a b i l i t y and S p e c i f i c

Resistance

L a m i n a r f l o w t h r o u g h a p o r o u s medium i s d e s c r i b e d

by D a r c y ' s l a w s

ν = (Κ/ η)(Δ P/L)

(D 1

where ν i s t h e a p p r o a c h v e l o c i t y o f t h e f l u i d (m s " ) , Κ i s t h e p e r m e a b i l i t y o f t h e medium ( m ) , η i s t h e f l u i d v i s c o s i t y ( P a s ) , ΔΡ i s t h e p r e s s u r e d i f f e r e n c e ( P a ) and L t h e d e p t h o f t h e medium (m). The p e r m e a b i l i t y , K, i s c h a r a c t e r i s t i c o f t h e medium and c a n be r e l a t e d t o m e a s u r a b l e p r o p e r t i e s by t h e Carman-Koζeny equation: 2

1 9 9 Κ = e /5S (1-εΓ J

(2)

where ε i s t h e p o r o s i t y o f t h e medium ( v o i d v o l u m e / t o t a l v o l u m e ) and S i s t h e s p e c i f i c s u r f a c e ( i . e . t h e s u r f a c e a r e a p e r u n i t volume o f p a r t i c l e s ) (m""-*-) . S t r i c t l y , t h e v a l u e 5 i n t h e Carman-Kozeny e q u a t i o n should be t r e a t e d a s an e m p i r i c a l c o n s t a n t , w h i c h h a s t o be d e t e r m i n e d experimentally. However, f o r many s y s t e m s o f i n t e r e s t , t h e v a l u e i s v e r y c l o s e t o 5. When a s u s p e n s i o n i s f i l t e r e d by a s t r a i n i n g mechanism, t h e p e r m e a b i l i t y o f t h e r e s u l t i n g f i l t e r c a k e c a n be i n t e r p r e t e d b y means o f E q u a t i o n 2. T h e r e s h o u l d be a s t r o n g dependence o f p e r m e a b i l i t y on t h e s t a t e o f a g g r e g a t i o n o f t h e p a r t i c l e s , s i n c e a g g r e g a t e s have a s m a l l e r e f f e c t i v e s u r f a c e a r e a t h a n t h e i n d i v i d u a l p a r t i c l e s and may p a c k l e s s e f f i c i e n t l y , g i v i n g a h i g h e r p o r o s i t y . Both o f t h e s e e f f e c t s should l e a d t o an i n c r e a s e d p e r m e a b i l i t y , a l t h o u g h t h e i r r e l a t i v e i m p o r t a n c e does n o t a p p e a r t o h a v e b e e n c l e a r l y e s t a b l i s h e d . One c o m p l i c a t i n g f a c t o r i s that f l o c c u l a t e d suspensions often give compressible f i l t e r c a k e s ( 5 ) , s o t h a t p o r o s i t y and s p e c i f i c s u r f a c e may change d u r i n g f i l t r a t i o n and may v a r y t h r o u g h t h e d e p t h o f t h e f i l t e r cake. The r e - f i l t r a t i o n t e c h n i q u e o f L a Mer (3) i n v o l v e s f i l t e r i n g t h e f l o c c u l a t e d s u s p e n s i o n and t h e n p a s s i n g t h e f i l t r a t e once more t h r o u g h t h i s pre-formed f i l t e r cake. The s e c o n d f i l t r a t i o n i s c a r r i e d o u t u n d e r a c o n s t a n t pressure d i f f e r e n c e ( n o r m a l l y by a p p l y i n g s u c t i o n ) and t h e t i m e t o f i l t e r a known volume i s n o t e d . T h i s r e - f i l t r a t i o n t i m e i s d i r e c t l y r e l a t e d t o t h e p e r m e a b i l i t y o f t h e f i l t e r c a k e and c a n be g r e a t l y r e d u c e d by f l o c c u l a t i o n . A l t h o u g h t h e r e - f i l t r a t i o n r a t e c a n be a s e n s i t i v e i n d i c a t o r o f f l o c c u l a t i o n , t h e m a t h e m a t i c a l t r e a t m e n t by S m e l l i e and L a Mer (6) and t h e i n t e r p r e t a t i o n o f t h e r e s u l t s h a s b e e n t h e s u b j e c t o f some c r i t i c i s m ( 4 ) . T h e r e a r e p r a c t i c a l d i f f i c u l t i e s , t o o , s u c h a s t h e d i s p r o p o r t i o n a t e e f f e c t o f a s m a l l amount o f ' f i n e s 1


27.

GREGORY AND DE MOOR

Polymer- Flocculated

447

Suspensions

( u n f l o c c u l a t e d p a r t i c l e s ) . For these reasons, the r e - f i l t r a t i o n method i s not much used i n f l o c c u l a t i o n s t u d i e s . E s s e n t i a l l y equivalent information can be obtained during the formation of the f i l t e r cake, without the need f o r a second filtration. During f i l t r a t i o n , p a r t i c l e s are deposited as a l a y e r o f i n c r e a s i n g t h i c k n e s s , so that the r e s i s t a n c e to f i l t r a t i o n i n c r e a s e s . The r e s i s t a n c e , R(m~l), i s i n v e r s e l y r e l a t e d t o p e r m e a b i l i t y and i s d e f i n e d i n terms of the volume flow r a t e : dV/dt =* FAP/nR

(3)

2 where F i s the area of the f i l t e r (m ) and V i s the volume of f i l t r a t e (m^) produced i n a time t ( s ) . The r e s i s t a n c e has two components : the r e s i s t a n c e of the support medium, R , and the f i l t e r cake r e s i s t a n c e , R . I f remains constant and a l l p a r t i c l e s are removed to give a uniform f i l t e r cake, then: m

Q

R = R + acV/F (4) m where c i s the c o n c e n t r a t i o n of the suspension (kg m-3) and α i s the s p e c i f i c r e s i s t a n c e to f i l t r a t i o n (m k g ~ l ) . The s p e c i f i c r e s i s t a n c e should depend only on the nature of the suspended p a r t i c l e s ( s i z e , shape and d e n s i t y ) and on t h e i r s t a t e of aggregation. I t i s r e l a t e d to the p e r m e a b i l i t y , K, by: α = 1/K ρ(1-ε)

(5)

-3 where ρ i s the d e n s i t y o f the p a r t i c l e s (kg m ) . S u b s t i t u t i n g the value of R from Equation 4 i n t o Equation 3 and i n t e g r a t i n g (with ΔΡ constant) gives the w e l l known p a r a b o l i c f i l t r a t i o n equation: 2

t = ( n/FAP)(R V + acV /2F) (6) m D i v i d i n g throughout by V gives the l i n e a r i z e d v e r s i o n of Equation 6: 2

t/V = nR /FAP + ( nac/2F ΔΡ)ν (7) m I f the volume of f i l t r a t e i s measured as a f u n c t i o n of time, under constant pressure, then a p l o t of t/V against V should give a s t r a i g h t l i n e , the slope of which can be used to c a l c u l a t e the s p e c i f i c r e s i s t a n c e . Equation 7 i s based on a number of assumptions and may not apply i n a l l cases, e s p e c i a l l y i f the f i l t e r cake i s compressible. A r i g o r o u s treatment of cake f i l t r a t i o n has been given by W i l l i s and Tosun ( 7 ) .


448


Experiments to determine s p e c i f i c r e s i s t a n c e , based on E q u a t i o n 7, h a v e u s u a l l y b e e n c a r r i e d o u t by some f o r m o f vacuum filtration. These methods a r e t i m e - c o n s u m i n g and s u b j e c t t o e r r o r . More r a p i d t e c h n i q u e s s u c h as t h e measurement o f c a p i l l a r y s u c t i o n t i m e (CST) c a n be u s e d ( 8 ) , a l t h o u g h t h e s e do not g i v e a b s o l u t e values of s p e c i f i c r e s i s t a n c e . N e v e r t h e l e s s , t h e CST method i s v e r y u s e f u l f o r r a p i d l y o b t a i n i n g c o m p a r a t i v e d a t a on t h e f l o c c u l a t i o n o f f a i r l y c o n c e n t r a t e d s u s p e n s i o n s by p o l y m e r s ( 9 ) . I n t h e p r e s e n t w o r k , s p e c i f i c r e s i s t a n c e has b e e n d e t e r m i n e d by an a u t o m a t e d t e c h n i q u e , w h i c h w i l l be d e s c r i b e d below. Automated F i l t r a b i l i t y

Determination

The new t e c h n i q u e (10) i s b a s e d on t h e a p p a r a t u s shown i n F i g u r e 1. A 50 m l H a m i l t o n g l a s s s y r i n g e , w i t h a T e f l o n - c o a t e d p i s t o n , i s connected v i a a three-way t a p t o a Swinnex f i l t e r h o l d e r , w h i c h i s f i t t e d w i t h a s u i t a b l e membrane f i l t e r (0.45 ym o r 0.22 ym M i l l i p o r e f i l t e r s i n t h e p r e s e n t w o r k ) . The s y r i n g e p i s t o n h a s b e e n m o d i f i e d by f i t t i n g a c i r c u l a r p l a t f o r m , on w h i c h one o r more r i n g - s h a p e d w e i g h t s c a n be p l a c e d . The t o t a l l o a d ( t y p i c a l l y 3 kg) i s chosen t o g i v e a r e a s o n a b l e f i l t r a t i o n r a t e and s e r v e s t o m a i n t a i n a c o n s t a n t p r e s s u r e d u r i n g t h e f i t r a t i o n process. R a t h e r t h a n m e a s u r i n g t h e f i l t r a t e volume d i r e c t l y , t h e movement o f t h e s y r i n g e p i s t o n i s m o n i t o r e d by a d i s p l a c e m e n t t r a n s d u c e r , which g i v e s a v o l t a g e output p r o p o r t i o n a l to the p i s t o n d i s p l a c e m e n t and h e n c e t o t h e f i l t r a t e v o l u m e . The o u t p u t o f t h e t r a n s u c e r i s d i g i t i z e d and f e d t o t h e u s e r p o r t o f a Commodore PET m i c r o c o m p u t e r , so t h a t , d u r i n g a f i l t r a t i o n e x p e r i m e n t , a s e r i e s o f t i m e and d i s p l a c e m e n t d a t a c a n be s t o r e d . A f t e r t h e s y r i n g e has b e e n f i l l e d w i t h t h e s u s p e n s i o n u n d e r t e s t and t h e l o a d a p p l i e d , t h e r e q u i r e d d a t a c a n be o b t a i n e d w i t h no f u r t h e r a t t e n t i o n b e i n g needed. F i g u r e 2 shows t h e f o r m o f t h e d i s p l a c e m e n t v s . t i m e r e s u l t s for d i f f e r e n t suspension c o n c e n t r a t i o n s . For p a r t i c l e - f r e e w a t e r (e = 0) , t h e l i n e i s s t r a i g h t , s i n c e t h e r e s i s t a n c e i s j u s t t h a t o f t h e membrane f i l t e r , w h i c h r e m a i n s c o n s t a n t . F o r s u s p e n s i o n s o f p a r t i c l e s l a r g e r t h a n t h e p o r e s i n t h e membrane, the f o r m a t i o n o f a f i l t e r cake l e a d s t o p r o g r e s s i v e l y i n c r e a s i n g r e s i s t a n c e and a d e c l i n i n g f i l t r a t i o n r a t e . The h i g h e r t h e s u s p e n s i o n c o n c e n t r a t i o n , t h e more r a p i d l y does t h e r a t e d e c l i n e . By a n a l o g y w i t h E q u a t i o n 7, t h e d i s p l a c e m e n t - t i m e d a t a c a n be l i n e a r i z e d , g i v i n g : t/D = c o n s t a n t + bD where D i s t h e d i s p l a c e m e n t o f t h e p i s t o n a t t i m e T. of t h e l i n e , b , i s g i v e n b y : b =

2

(8) The

2

nacA /2F AP


slope (9)

27.

GREGORY AND DE M O O R

Polymer-Flocculated

Suspensions

OUTPUT

DISPLACEMENT TRANSDUCER

LOAD

TEFLON PISTON

SAMPLE INLET

3 WAY TAP

FILTER MEMBRANE

Figure 1 .

Filtrability

apparatus.


449

450


where A i s the c r o s s - s e c t i o n a l area of the s y r i n g e . In the present method, t/D values are computed and d i s p l a y e d during a f i l t r a t i o n experiment. At the end of a run a p l o t of t/D vs. D i s drawn on a Bryans 50000 d i g i t a l p l o t t e r . Such a p l o t i s shown i n F i g u r e 3 f o r a suspension of k a o l i n p a r t i c l e s at a concentation of 35 mg/1. A f t e r a b r i e f i n i t i a l phase, when the pores of the membrane f i l t e r are being blocked and the f i r s t l a y e r s of p a r t i c l e s deposited, the p l o t shows good l i n e a r behaviour, i n d i c a t i n g a uniform f i l t e r cake and the absence of s i g n i f i c a n t c o m p r e s s i b i l i t y e f f e c t s . The slope of the l i n e i s determined a u t o m a t i c a l l y by a l i n e a r r e g r e s s i o n r o u t i n e and the s p e c i f i c r e s i s t a n c e i s c a l c u l a t e d from the slope (and other necessary data, which are entered i n i t i a l l y ) . The e n t i r e sequence of o p e r a t i o n s , from the s t a r t of the f i l t r a t i o n to the computation of s p e c i f i c r e s i s t a n c e i s c a r r i e d out a u t o m a t i c a l l y under microcomputer c o n t r o l . T h i s technique has been thoroughly evaluated f o r a range of d i l u t e suspensions and shown to give c o n s i s t e n t r e s u l t s (10). The s p e c i f i c r e s i s t a n c e s obtained are independent of a p p l i e d l o a d , suspension c o n c e n t r a t i o n and membrane type, as expected f o r noncompressible f i l t e r cakes. Tests with uniform l a t e x p a r t i c l e s have given p e r m e a b i l i t i e s i n very good agreement with Equation 2, using a value of 5 f o r the Carman-Koζeny constant. The method i s b e t t e r s u i t e d to d i l u t e , r a t h e r than concentrated suspensions, s i n c e the data can be obtained i n a short time (a few minutes at most) and only t h i n f i l t e r cakes are formed. With higher c o n c e n t r a t i o n s , much longer f i l t r a t i o n times are needed and the t h i c k e r f i l t e r cakes are more l i k e l y to show c o m p r e s s i b i l i t y e f f e c t s and n o n - l i n e a r behaviour. E f f e c t of F l o c c u l a t i o n on F i l t r a b i l i t y Some p r e l i m i n a r y experiments have been conducted using the new technique, with k a o l i n suspensions and c a t i o n i c polymers. The k a o l i n was from BDH L t d . and was dispersed by high-speed s t i r r i n g at around n e u t r a l pH. The r e s u l t i n g suspension was allowed to stand overnight and the sedimented m a t e r i a l was r e j e c t e d . The remaining suspension contained p a r t i c l e s up to about 2 ym i n s i z e . F i n a l suspensions f o r the f l o c c u l a t i o n experiments were made up i n 10~3M NaCl, to c o n t r o l the i o n i c strength. The c a t i o n i c polymers used were as f o l l o w s : A. Poly(dimethylaminoethyl methacrylate), with a molecular weight of about 5000, f u l l y quaternized with dimethyl sulfate. B. As A, but with a molecular weight of about 150,000.


GREGORY AND DE MOOR


Suspensions

F i g u r e 2. Form o f d i s p l a c e m e n t v s . t i m e c u r v e s f o r d i f f e r e n t suspension concentrations.

220,

î

176

132

-

8 8 L

Q \

44

0

0

2 4 5 DISPLACEMENT (CM)

8

F i g u r e 3. L i n e a r i z e d p l o t f o r a n u n f l o c c u l a t e d 35 mg/1 k a o l i n suspension.


452 C.

P O L Y M E R ADSORPTION A N D DISPERSION STABILITY

P o l y ( 1 - e t h y l 2 methyl 5 v i n y l p r i d i n i u m bromide) w i t h m o l e c u l a r weight of about 1 m i l l i o n . These m a t e r i a l s h a v e b e e n u s e d p r e v i o u s l y ( 1 1 ) .

a

Procedure. I n i t i a l l y , t h e f l o c c u l a t i o n p r o c e d u r e a d o p t e d was t o add p o l y m e r t o t h e c l a y s u s p e n s i o n , s t i r r a p i d l y f o r 15 s e c o n d s t o e n s u r e good m i x i n g and t h e n t o f l o w t h e t r e a t e d s u s p e n s i o n t h r o u g h a 3 m l e n g t h o f c o i l e d 1 mm d i a m e t e r t u b i n g . Tube f l o w i s known t o be an e f f e c t i v e method o f a p p l y i n g s h e a r t o a s u s p e n s i o n and h e n c e p r o m o t i n g o r t h o k i n e t i c f l o c c u l a t i o n ( 1 2 ) . The f l o c c u l a t e d s u s p e n s i o n was t h e n t r a n s f e r r e d t o t h e s y r i n g e and t h e s p e c i f i c r e s i s t a n c e was d e t e r m i n e d as d e s c r i b e d previously. R e s u l t s a r e shown i n F i g u r e 4 f o r t h r e e d i f f e r e n t k a o l i n c o n c e n t r a t i o n s , f l o c c u l a t e d w i t h p o l y m e r A. The p o l y m e r c o n c e n t r a t i o n i s shown as a p e r c e n t a g e o f t h e c l a y c o n c e n t r a t i o n . I n a l l c a s e s , p l o t s o f t/D v s . D w e r e l i n e a r . The s p e c i f i c r e s i s t a n c e o f t h e u n f l o c c u l a t e d k a o l i n i s a b o u t 12 χ 1012 m/kg and f a l l s t o a b o u t 1 χ 1012 m/kg a t t h e optimum flocculant concentration. I t i s c l e a r t h a t t h e amount o f p o l y m e r r e q u i r e d t o g i v e t h e minimum s p e c i f i c r e s i s t a n c e i s a b o u t 0.5% o f t h e c l a y c o n c e n t r a t i o n i n a l l c a s e s . This p r o p o r t i o n a l i t y i n d i c a t e s t h a t the polymer i s s t r o n g l y adsorbed and p r o b a b l y a c t s by a c h a r g e n e u t r a l i z a t i o n , r a t h e r t h a n a b r i d g i n g mechanism. The amount o f c a t i o n i c p o l y m e r r e q u i r e d t o n e u t r a l i z e t h e n e g a t i v e c h a r g e o f c l a y p a r t i c l e s must be p r o p o r t i o n a l to the c l a y c o n c e n t r a t i o n . R e s t a b i l i z a t i o n of the k a o l i n occurs at excess polymer doses because the p a r t i c l e s become p o s i t i v e l y c h a r g e d . The more p r o n o u n c e d r e s t a b i l i z a t i o n o b s e r v e d f o r t h e h i g h e s t c l a y c o n c e n t r a t i o n i n F i g u r e 4 may be a r e s u l t o f t h e more r a p i d p o l y m e r a d s o r p t i o n u n d e r t h e s e conditions. The p r o c e d u r e u s e d t o o b t a i n t h e r e s u l t s i n F i g u r e 3 i s n o t e n t i r e l y s a t i s f a c t o r y , s i n c e t r a n s f e r of the f l o c c u l a t e d s u s p e n s i o n t o t h e s y r i n g e i n e v i t a b l y c a u s e s some b r e a k - u p o f f l o e s w h i c h may a f f e c t t h e f i l t r a t i o n b e h a v i o u r . In order to i n v e s t i g a t e t h i s p o i n t , two d i f f e r e n t p r o c e d u r e s were a d o p t e d : a) The s u s p e n s i o n was t r a n s f e r r e d t o t h e s y r i n g e a f t e r a d d i n g t h e p o l y m e r and s t i r r i n g r a p i d l y f o r 15 s e c o n d s , b u t w i t h o u t tube flow. I n t h i s c a s e , no v i s i b l e f l o e s had f o r m e d so t h a t no s i g n i f i c a n t b r e a k - u p w o u l d be e x p e c t e d . The s u s p e n s i o n was f i l t e r e d immediately a f t e r t r a n s f e r . Since the f i l t r a t i o n of the samples i s q u i t e r a p i d , t h e r e i s v e r y l i t t l e o p p o r t u n i t y f o r f l o c c u l a t i o n i n the s y r i n g e . b) A f t e r p o l y m e r a d d i t i o n and 15 s e c o n d s o f r a p i d m i x i n g , t h e s u s p e n s i o n was t r a n s f e r r e d t o t h e s y r i n g e and t h e n s u b j e c t e d t o 10 m i n u t e s o f s l o w s i t r r i n g i n s i t u , u s i n g a s m a l l m a g n e t i c b a r . T h i s r e s u l t e d i n q u i t e l a r g e f l o e s , w h i c h c o u l d be f i l t e r e d without disturbance.


27.

GREGORY AND DE MOOR


Suspensions

4

F i g u r e 4. S p e c i f i c r e s i s t a n c e o f k a o l i n suspensions t r e a t e d w i t h d i f f e r e n t amounts o f p o l y m e r A. K a o l i n concentrations: 1) 70 mg/1, 2 ) 140 mg/1, 3) 280 mg/1. Polymer c o n c e n t r a t i o n expressed as a percentage o f k a o l i n concentration.


453

454

POLYMER ADSORPTION AND

DISPERSION STABILITY

The r e s u l t s i n F i g u r e 5 compare t h e s e p r o c e d u r e s and the p r e v i o u s one ( r a p i d mix and t u b e f l o w ) f o r a k a o l i n c o n c e n t r a t i o n o f 140 mg/1. I n t h i s and s u b s e q u e n t F i g u r e s , s p e c i f i c r e s i s t a n c e s a r e p l o t t e d as p e r c e n t a g e s o f t h e o r i g i n a l v a l u e ( i . e . f o r the u n f l o c c u l a t e d k a o l i n ) . E v i d e n t l y , the procedure makes l i t t l e d i f f e r e n c e t o t h e r e s u l t s and e s s e n t i a l l y t h e same 'optimum p o l y m e r c o n c e n t r a t i o n w o u l d be c h o s e n i n e a c h c a s e . The r e s u l t s a t h i g h p o l y m e r c o n c e n t r a t i o n s show d i f f e r e n t b e h a v i o u r , d e p e n d i n g on t h e p r o c e d u r e e m p l o y e d , b u t no s a t i s f a c t o r y e x p l a n a t i o n seems a p p a r e n t . I n v i e w o f t h e n a t u r e o f t h e r e s u l t s i n F i g u r e 5, c o m p a r i s o n o f t h e e f f e c t s o f d i f f e r e n t p o l y m e r s was c a r r i e d out u s i n g t h e simplest technique, i . e . f i l t r a t i o n immediately a f t e r rapid mixing. 1

B e h a v i o u r o f D i f f e r e n t P o l y m e r s . The e f f e c t s o f p o l y m e r s A, Β and C, on t h e s p e c i f i c r e s i s t a n c e o f a 140 mg/1 kaolin s u s p e n s i o n a r e shown i n F i g u r e 6. The r e s u l t s a r e r e m a r k a b l y s i m i l a r , b e a r i n g i n mind t h e v e r y d i f f e r e n t m o l e c u l a r w e i g h t s o f these m a t e r i a l s . Again, there i s a strong i n d i c a t i o n that c h a r g e n e u t r a l i z a t i o n i s t h e p r e d o m i n a n t e f f e c t and t h a t p o l y m e r b r i d g i n g p l a y s a m i n o r r o l e , i f any. The l e s s p r o n o u n c e d r e s t a b i l i z a t i o n w i t h t h e h i g h e r m o l e c u l a r w e i g h t p o l y m e r s may be a r e s u l t of a s l o w e r a d s o r p t i o n or of a non-uniform charge d i s t r i b u t i o n (11). I t i s worth n o t i n g that the s p e c i f i c r e s i s t a n c e s o b t a i n e d by f l o c c u l a t i o n w i t h c a t i o n i c p o l y m e r s a r e much l e s s t h a n t h o s e a c h i e v e d by s i m p l e s a l t s . F o r i n s t a n c e , when t h e k a o l i n s u s p e n s i o n i s c o m p l e t e l y d e s t a b i l i z e d by t h e a d d i t i o n o f a c a l c i u m s a l t , t h e s p e c i f i c r e s i s t a n c e i s o n l y r e d u c e d by a f a c t o r o f a b o u t two. A t e n - f o l d r e d u c t i o n can e a s i l y be a c h i e v e d by c a t i o n i c p o l y m e r s . The f l o c c u l a t i o n and r e s t a b i l i z a t i o n b e h a v i o u r i n d i c a t e d by t h e f i l t r a b i l i t y r e s u l t s i s w e l l m a t c h e d by o t h e r t e s t m e t h o d s , i n c l u d i n g s i m p l e s e t t l i n g t e s t s and a n e w l y - d e v e l o p e d o p t i c a l m o n i t o r i n g technique (13). A l l o f t h e s e methods g i v e e s s e n t i a l l y t h e same optimum p o l y m e r c o n c e n t r a t i o n . E f f e c t of S t i r r i n g . The r a t h e r s m a l l e f f e c t o f t u b e f l o w and s l o w s t i r r i n g on t h e s p e c i f i c r e s i s t a n c e r e s u l t s i n F i g u r e 5 was u n e x p e c t e d and has b e e n c h e c k e d by a s e r i e s o f t r i a l s i n w h i c h a k a o l i n s u s p e n s i o n was s u b j e c t e d t o v a r i o u s p e r i o d s o f s l o w s t i r r i n g , f o l l o w i n g p o l y m e r a d d i t i o n and r a p i d m i x i n g . F o r t h i s p u r p o s e , 500 ml o f a 140 mg/1 k a o l i n s u s p e n s i o n was t r e a t e d w i t h an amount o f p o l y m e r A c o r r e s p o n d i n g t o 0.5% of t h e c l a y c o n c e n t r a t i o n ( i . e . t h e optimum d o s e ) . A f t e r 15 s e c o n d s o f r a p i d m i x i n g , t h e s u s p e n s i o n was s u b j e c t e d t o s l o w (30 r.p.m.) s t i r r i n g , u s i n g a p a d d l e s t i r r e r . Samples w e r e w i t h d r a w n a t d i f f e r e n t i n t e r v a l s d i r e c t l y i n t o t h e s y r i n g e and a f i l t r a b i l i t y d e t e r m i n a t i o n was c a r r i e d o u t .


GREGORY AND DE MOOR


%

POLYMER

Suspensions

A

F i g u r e 5. E f f e c t o f d i f f e r e n t f l o c c u l a t i o n procedures, u s i n g p o l y m e r A and a k a o l i n c o n c e n t r a t i o n o f 140 mg/1. Procedures: 1) R a p i d m i x i n g o n l y , 2) R a p i d m i x i n g a n d t u b e f l o w ( a s i n F i g u r e 1); 3) R a p i d m i x i n g a n d s l o w s t i r r i n g i n s y r i n g e . S p e c i f i c r e s i s t a n c e shown a s percentage o f o r i g i n a l value.


455

456


In t h i s procedure, v i s i b l e f l o e s became apparent a f t e r about 5 minutes of slow s t i r r i n g and the f l o e s i z e continued to increase up to about 30 minutes. A f t e r long s t i r r i n g times, f l o e s appeared to be q u i t e strong and survived t r a n s f e r to the syringe without much break-up. Figure 7 shows the s p e c i f i c r e s i s t a n c e of the f l o c c u l a t e d samples immediately a f t e r r a p i d mixing and a f t e r i n c r e a s i n g times of slow s t i r r i n g . The major r e d u c t i o n i n s p e c i f i c r e s i s t a n c e (about 85%) has already occurred during the r a p i d mixing phase. Further prolonged slow s t i r r i n g , during which the f l o e s grow considerably i n s i z e , produces only a r e l a t i v e l y minor r e d u c t i o n . P r a c t i c a l l y a l l of the r e d u c t i o n i n s p e c i f i c r e s i s t a n c e has occurred a f t e r 15 minutes. One p o s s i b l e explanation of t h i s behaviour i s i n terms of a model f o r f l o e s t r u c t u r e postulated by Michaels and Bolger (14) and elaborated by van de Ven and Hunter ( 1 5 ) . Floes are regarded as aggregates, not of i n d i v i d u a l p a r t i c l e s , but of 'micro-fIocs or f l o c c u l i . M i c r o - f I o c s would be formed during the r a p i d mixing period and t h e i r s i z e would depend on the shear c o n d i t i o n s . During the slow s t i r r i n g period f l o e growth i s by c o l l i s i o n and aggregation of the m i c r o - f I o c s . The i m p l i c a t i o n of the present r e s u l t s i s that the formation of micro-fIocs gives the major improvement i n f i l t r a b i l i t y , most l i k e l y by an e f f e c t i v e r e d u c t i o n i n the surface area of the p a r t i c l e s . Aggregates of micro-fIocs might s t i l l be q u i t e permeable, so that most of the remaining p a r t i c l e surface would be a c c e s s i b l e to the flowing l i q u i d and the r e s i s t a n c e to flow would not be much l e s s than that of the m i c r o - f I o c s . However, during the formation of a f i l t e r cake, f l o e s must be subject to considerable d i s r u p t i v e forces and l a r g e aggregates may not s u r v i v e . Scanning e l e c t r o n micrographs of f i l t e r cakes formed a f t e r v a r i o u s periods of slow s t i r r i n g have shown no obvious d i f f e r e n c e s . T

T

1

Conclusions The method described here provides a convenient means of determining the s p e c i f i c f i l t r a t i o n r e s i s t a n c e of f a i r l y d i l u t e suspensions. Results f o r c l a y suspensions f l o c c u l a t e d by c a t i o n i c polymers show that the s p e c i f i c r e s i s t a n c e gives a s e n s i t i v e i n d i c a t i o n of f l o c c u l a t i o n and i s a u s e f u l guide i n the s e l e c t i o n of optimum f l o c c u l a n t concentrations. In a s e r i e s of t r i a l s not reported here, i t has been shown that the s p e c i f i c r e s i s t a n c e r e s u l t s are very w e l l matched by r e - f i l t r a t i o n r a t e data, as expected. The r e s u l t s a l s o agree w e l l with other, unrelated techniques. For more concentrated suspensions, some d i s c r e p a n c i e s have been found between p e r m e a b i l i t y methods and other measures of f l o c c u l a t i o n (4).


GREGORY AND DE MOOR

Polymer-Flocculated

Suspensions

F i g u r e 7. E f f e c t o f slow s t i r r i n g p e r i o d on s p e c i f i c r e s i s t a n c e o f f l o c c u l a t e d k a o l i n (0.5% polymer A ) .


457

458


There i s a problem with the present technique, which has not so f a r been mentioned. Attempts to study f l o c c u l a t i o n by high molecular weight a n i o n i c polymers have proved l a r g e l y unsuccessful and the reason was found to be blockage of pores i n the membrane f i l t e r by the polymer. S i m i l a r e f f e c t s were found f o r the highest molecular weight c a t i o n i c polymer (C), but only a t concentrations r a t h e r higher than those used i n t h i s work. This problem could be overcome by using a support medium with l a r g e r pores, but then primary ( u n f l o c c u l a t e d ) p a r t i c l e s would not be e f f i c i e n t l y r e t a i n e d . Membrane f i l t r a t i o n of polymer s o l u t i o n s u s i n g a method l i k e that described here might provide a simple means o f e s t i m a t i n g molecular weights. Even though the new technique i s l a r g e l y automated, i t i s s t i l l a r a t h e r lengthy procedure, compared to some other methods of f l o c c u l a t i o n t e s t i n g (13), and would probably not be used simply t o f i n d optimum f l o c c u l a n t c o n c e n t r a t i o n s . However, as a means o f studying the f i l t r a b i l i t y of suspensions and the e f f e c t o f p a r t i c l e aggregation, e i t h e r i n fundamental work or i n p r a c t i c a l a p p l i c a t i o n s , i t could prove to be u s e f u l . Acknowledgment This work was supported by a grant from the Science and Engineering Research C o u n c i l .

Literature Cited 1. Halverson, F.; Panzer, H.P. in Kirk-Othmer:Enclclopedia of Chemical Technology; John Wiley: New York, 1980; Vol. 10, pp. 489-523. 2. Ruehrwein, R.A.; Ward, A. Soil Sci. 1952, 73, 485-492. 3. La Mer, V.K. Disc. Faraday Soc. 1966, 42, 248-254. 4. Slater, R.W.; Kitchener, J.A. Disc. Faraday Soc. 1966, 42, 267-275. 5. Grace, H.P. Chem. Eng. Prog. 1953, 49, 303-318. 6. Smellie, R.H.; La Mer, V.K. J. Coll. Sci. 1958, 23. 589-599. 7. Willis, M.S.; Tosun, I. Chem. Eng. Sci. 1980, 35, 2427-2438. 8. Gale, R.S.; Baskerville, R.C. Water Pollution Control 1968, 67, 233-241. 9. O'Gorman, J.V.; Kitchener, J.A. Intl. J. Miner Proc. 1974, 1, 33-49. 10. de Moor, A.E.L.; Gregory, J. Proc. Water Filtration Symposium, KVIV Antwerp, 1982. 11. Gregory, J.J. Coll. Interface. Sci. 1976, 55, 35-44. 12. Gregory, J. Chem. Eng. Sci. 1981, 36, 1789-1794. 13. Gregory, J.; Nelson, D.W. in "Advances in Solid-Liquid Separation"; Gregory, J., Ed; Ellis-Horwood: Chichester, 1984. 14. Michaels, A.S.; Bolger, J.C. Ind. Eng. Chem. Fundam. 1962, 1. 153-162. 15. van de Ven, T.G.M.; Hunter, R.J. Rheol. Acta 1977, 16, 534-543. RECEIVED

October 7, 1983 Goddard and Vincent; Polymer Adsorption and Dispersion Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

Filtrability of Polymer-Flocculated Suspensions - American Chemical

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