Polymer Adsorption and Dispersion Stability - American Chemical

24 Adsorption of Polyacrylamides on Kaolinite and Its Flocculation and Stabilization P. SOMASUNDARAN, Y. H. CHIA, and R. GORELIK Downloaded by FUDAN UNIV on December 13, 2016 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch024

Henry Krumb School of Mines, Columbia University, New York, NY 10027 Adsorption of nonionic and anionic polyacrylamides on kaolinite clay is studied together with various flocculation properties (settling rate, sediment volume, supernatant clarity and suspension viscosity) under controlled conditions of pH, ionic strength and agitation. Adsorption and flocculation data obtained simultaneously for selected systems were correlated to obtain information on the dependence of flocculation on the surface coverage. Interestingly, optimum polymer concentration and type vary depending upon the flocculation response that is monitored. This is discussed in terms of the different properties of the flocs and the floc network that control different flocculation responses. Flocculation itself is examined as the cumulative result of many sub-processes that can depend differently on system properties. The nature of the flocculation of fines obtained in polymer solut i o n s i s dependent upon a number o f f a c t o r s i n c l u d i n g polymer c h a r a c t e r i s t i c s , s o l i d and s o l v e n t p r o p e r t i e s as w e l l as t h e hydrodynamic c o n d i t i o n s used during the f l o c c u l a t i o n and p r e f l o c c u l a t i o n stages. Understanding the r o l e o f f a c t o r s i s important f o r a c h i e v i n g the d e s i r e d f l o c c u l a t i o n s t a t e o f f i n e s , p a r t i c u l a r l y o f mineral d i s p e r s i o n s which a r e not e a s i l y processed ( 1 , 2 ) . I t i s noted here t h a t d i f f e r e n t p r o p e r t i e s o f t h e f l o c c u l a t e d systems can be the determining c r i t e r i o n i n d i f f e r e n t processes. F o r example, whereas supernatant c l a r i t y w i l l be imp o r t a n t when water i s t o be r e c y c l e d , i t can be the sediment volume o r the s e t t l i n g r a t e t h a t w i l l be important i n e f f l u e n t treatment. D i f f e r e n t polymers can have d i f f e r e n t e f f e c t s on the above-named p r o p e r t i e s o f a f l o c c u l a t i n g system, and a simultaneous study o f these e f f e c t s can provide u s e f u l information about the mechanism o f i n t e r a c t i o n between polymers and p a r t i c l e s as 0097-6156/ 84/0240-0393506.00/ 0 © 1984 American Chemical Society

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

Downloaded by FUDAN UNIV on December 13, 2016 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch024

394

P O L Y M E R A D S O R P T I O N A N D DISPERSION STABILITY

w e l l as between the polymer-coated p a r t i c l e s . In t h i s study, the e f f e c t o f polymer p r o p e r t i e s , such as charge d e n s i t y , on s e l e c t e d f l o c c u l a t i o n parameters, such as s e t t l i n g r a t e , sediment volume, and supernatant c l a r i t y , i s s t u d i e d f o r k a o l i n i t e f i n e s . The f l o c c u l a t i o n o f k a o l i n i t e has been s t u d i e d e x t e n s i v e l y i n the past (3-9). However, the r e s u l t s have been c o n t r a d i c t o r y , f o r example, i n the case of the polymer charge d e n s i t y r e q u i r e d f o r optimum f l o c c u l a t i o n . While the d i s c r e p a n c i e s might be owing p a r t l y t o the d i f f e r e n t methods used t o measure f l o c c u l a t i o n , they a l s o could have r e s u l t e d from u n c e r t a i n t i e s i n the measured p r o p e r t i e s o f k a o l i n i t e caused by aging o r i o n exchange, i n the p r o p e r t i e s o f polymer due t o the use o f d i f f e r e n t commercial samples, o r i n the p r o p e r t i e s o f the s o l u t i o n owing t o v a r i a t i o n s i n pH, i o n i c strength e t c . An accurate understanding of the f l o c c u l a t i o n and d i s p e r s i o n o f c l a y s i s , however, o f much importance s i n c e c l a y s are u n i v e r s a l l y present i n ore d e p o s i t s , o i l r e s e r v o i r s , and marine and a g r i c u l t u r a l systems and markedly i n f l u e n c e v a r i o u s processes i n v o l v i n g them. F l o c c u l a t i o n i s indeed dependent on polymer adsorption, and there are hypotheses c o r r e l a t i n g the two phenomena, but o f t e n these have been put f o r t h without d e t a i l e d measurement o f the two phenomena simultaneously (10-13). In t h i s paper, f l o c c u l a t i o n i s i n v e s t i g a t e d as a f u n c t i o n o f polymer and s o l u t i o n p r o p e r t i e s and hydrodynamic c o n d i t i o n s by measuring d i f f e r e n t p r o p e r t i e s of the system, i n c l u d i n g adsorption, u s i n g w e l l c h a r a c t e r i z e d k a o l i n i t e and polymer samples prepared s p e c i f i c a l l y f o r t h i s purpose. A l s o , the r o l e of c o n c e n t r a t i o n and charge d e n s i t y o f polyacrylamide and p o l y a e r y l a m i d e - p o l y a c r y l i e a c i d co-polymers i n determining k a o l i n i t e f l o c c u l a t i o n i s examined under c o n t r o l l e d hydrodynamic c o n d i t i o n s . Experimental M a t e r i a l s . N a - K a o l i n i t e : A homoionic sample o f k a o l i n i t e was prepared from a w e l l - c r y s t a l l i z e d sample purchased from Source C l a y s , U n i v e r s i t y o f M i s s o u r i , u s i n g a standardized technique (14) which i n v o l v e d repeated washing with d i s t i l l e d water and by treatment with NaCl s o l u t i o n s t o remove exchangeable ions such as Ca, and f r e e z e - d r y i n g o f the f i n a l product. Nitrogen s p e c i f i c surface area o f t h i s k a o l i n i t e was estimated t o be 9.4m /g and X-ray a n a l y s i s showed the c h a r a c t e r i s t i c p a t t e r n o f k a o l i n i t e . Polymers: Polyacrylamide and hydrolyzed polyacrylamide were prepared by the American Cyanamid Company s p e c i f i c a l l y f o r t h i s p r o j e c t , s t a r t i n g with l ^ C l a b e l l e d monomer. The r a d i o a c t i v i t y l e v e l o f the monomer was kept below 0.20 mC^/g i n order t o avoid s i g n i f i c a n t spontaneous p o l y m e r i z a t i o n , u t i l i z i n g a copper i n h i b i t o r . The homopolymer was synthesized by f r e e r a d i c a l s o l u t i o n p o l y m e r i z a t i o n i n water a t 40°C, u s i n g monomer r e c r y s t a l l i z e d from chloroform, an ammonium persulfate-sodium m e t a b i s u l f i t e c a t a l y s t system, and isopropanol as a chain t r a n s f e r agent. Sodium


SOMASUNDARAN

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ET AL.

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395

ethylene diamine t e t r a a c e t a t e was i n c l u d e d i n the r e c i p e to seques t e r any r e s i d u a l copper i n h i b i t o r i n the l a b e l l e d monomer. In o r der t o avoid broadening the molecular weight d i s t r i b u t i o n , the p o l y m e r i z a t i o n r e a c t i o n was not c a r r i e d to completion. Residual mono mer was e x t r a c t e d with methanol-water mixtures and with methanol un t i l i t was l e s s than 0.5 wt. % of the polymer. The f i n a l product was s t o r e d as the f r e e z e - d r i e d m a t e r i a l . Polyacrylamides of d i f f e r ent a n i o n i c i t i e s were prepared by h y d r o l y z i n g d i l u t e aqueous s o l u t i o n s of the homopolymer t o the d e s i r e d l e v e l with sodium hydroxide a t 43°-46°C, i n the presence of 2 wt. % i s o p r o p a n o l . These prod u c t s a l s o were s t o r e d as f r e e z e - d r i e d m a t e r i a l s . The i n t r i n s i c v i s c o s i t y of the homopolymer was found to be 1.8 d l / g i n 1 M NaCl at 30°C, u s i n g a #75 Cannon-Ubbelhode v i s c o meter. Using the Mark-Houink equation (η = KM ) with Κ = 3.73 χ 10~ and a = 0.66, the v i s c o s i t y average molecular weight i s estimated t o be 0.4 χ 1 0 daltons (15). The carboxyl l e v e l s of the hydrolyzed polyacrylamides were analyzed by i n f r a r e d a b s o r p t i o n spectroscopy, and were r e p o r t e d t o be 9 ± 2, 21 ± 2, and 33 ± 2 mole %. The polyacrylamides are designated as ΡAM or as HPAM t o i n d i c a t e the homopolymer or a hydrolyzed product. Two numbers are added a f t e r the l e t t e r s , the f i r s t i n d i c a t i n g the molecular weight i n u n i t s of a m i l l i o n , and the second the extent of h y d r o l y s i s (e^. HPAM 0.4-33 has a molecular weight of 0.4 χ 1 0 daltons and i s hydrolyzed t o the extent of 33 mole % ) .


a

4

6

;

6

Procedure. For each t e s t , a 5 gram sample of N a - k a o l i n i t e was f i r s t c o n d i t i o n e d i n 150 ml of 3 χ 10~ kmol/m NaCl s o l u t i o n i n t e f l o n b o t t l e s f o r two hours u s i n g a w r i s t - a c t i o n shaker set f o r maximum a g i t a t i o n of the suspension. I t was then s t i r r e d f o r de s i r e d i n t e r v a l s u s i n g a 1" p r o p e l l e r (three 2.54cm diameter blades a t 45° i n c l i n a t i o n ) at 1200 rpm i n a 250 ml 6.3 cm diameter beaker f i t t e d with four 0.63 cm wide b a f f l e p l a t e s (16) with 50 ml of p o l y mer s o l u t i o n added i n the beginning while the suspension was being s t i r r e d . Time of c o n d i t i o n i n g of the suspension with the polymer i s i n d i c a t e d i n each f i g u r e . Samples were then removed from t h i s f o r v a r i o u s measurements. Percent s o l i d s e t t l e d : In t e s t s where a d s o r p t i o n and f l o c c u l a t i o n were determined simultaneously, the 200 ml sample was allowed to s e t t l e f o r 30 seconds and then 100 ml of the supernatant was r e moved u s i n g a s u c t i o n device and a f t e r c e n t r i f u g a t i o n analyzed f o r r e s i d u a l polymer c o n c e n t r a t i o n . The minimum l e v e l of d e t e c t i o n was a t 0.5 t o 1 ppm and the r e p r o d u c i b i l i t y of the a d s o r p t i o n measure ments was 2-3%. The s e t t l e d 100 ml p o r t i o n was analyzed f o r s o l i d content. F l o c c u l a t i o n due t o polymer a d d i t i o n i s measured by noting % s o l i d s e t t l e d as a f u n c t i o n of polymer c o n c e n t r a t i o n . S e t t l i n g r a t e : For determing the s e t t l i n g r a t e , the 200 ml sample was t r a n s f e r r e d t o a 250 ml f l a t bottomed graduated c y l i n d e r and 4 ml of supernatant was used f o r polymer a n a l y s i s . The o r i g i n a l s o l u t i o n was made up with an equal volume of e l e c t r o l y t e s o l u t i o n and then remixed manually end over end. The s e t t l i n g r a t e was d e t e r mined by measuring the descent of the s o l i d / s o l u t i o n i n t e r f a c e as a f u n c t i o n of time and by e s t i m a t i n g the slope of the l i n e a r r e g i o n . 2

3


396



Sediment volume: The sample used f o r measurements of s e t t l i n g r a t e was allowed to subside overnight f o r 16 to 24 hours and the sediment volume was read on the graduated cylinder. Supernatant c l a r i t y : A f t e r subsidence, the sample was remixed manually and allowed t o s e t t l e f o r 3 minutes. Supernatant c l a r i t y was measured u s i n g a Brinkman probe c o l o r i m e t e r PC 1600 by d i p ping the probe t o j u s t below the surface of the l i q u i d . V i s c o s i t y : A 16 ml sample of the suspension removed a f t e r remixing was used f o r torque measurements u s i n g a B r o o k f i e l d viscometer (Model: LVTD) with a UL adapter. I n i t i a l torque readings were used f o r e s t i m a t i n g v i s c o s i t y i n order t o avoid problems a r i s i n g from any s e t t l i n g of the k a o l i n i t e i n the viscometer during the measurements. Results Simultaneous A d s o r p t i o n / F l o c c u l a t i o n T e s t s . R e s u l t s obtained f o r adsorption and f l o c c u l a t i o n (as measured by the amount of s o l i d s e t t l e d ) f o r the same samples are given as a f u n c t i o n of concentrat i o n of the nonionic polyacrylamide i n F i g u r e 1 and the 33% hydrol y z e d polymer i n F i g u r e 2. F l o c c u l a t i o n as measured by s e t t l i n g r a t e i s given i n F i g u r e 3 f o r the case of the nonionic p o l y acrylamide. In these t e s t s both polymers were completely depleted from s o l u t i o n up to about 100 mg/kg a d d i t i o n . As f a r as f l o c c u l a t i o n i s concerned, the nonionic polymer i s found t o i n c r e a s e both the percent s o l i d s e t t l e d and the s e t t l i n g r a t e which go through maxima a t about 25 mg/kg a d d i t i o n . While polymer a d d i t i o n had no measurable e f f e c t on percent s o l i d s e t t l e d even a t higher polymer concentrations, a t a l l l e v e l s of a d d i t i o n , the s e t t l i n g r a t e i s higher than t h a t obtained i n the absence of the polymer. I t i s c l e a r from a comparison of the polymer e f f e c t on these two f l o c c u l a t i o n responses t h a t a s i n g l e parameter cannot be used to c h a r a c t e r i z e f l o c c u l a t i o n . Even though the s e t t l i n g r a t e i s o f t e n a more s e n s i t i v e parameter, as i n the present case, i t i s o f t e n n e c e s s a r y — p a r t i c u l a r l y i n the case of s e l e c t i v e f l o c c u l a t i o n — t o a l s o measure percent s o l i d s e t t l e d t o a given l e v e l and t o analyze the s e t t l e d mass f o r m i n e r a l o g i c a l composition. I t i s noted t h a t while the nonionic polymer enhanced the amount of s o l i d s e t t l e d , the a n i o n i c polymer had no measurable e f f e c t on t h i s response even though i t s a d s o r p t i o n was comparable t o t h a t of the former. More important i s the observed l a c k of the proposed c o r r e l a t i o n s between the extent of polymer coverage and f l o c c u l a t i o n . T h i s i s f u r t h e r seen when the e f f e c t of time of s t i r r i n g of the mineral with the polymer s o l u t i o n i s examined (see F i g u r e 4 ) . While adsorption reached i t s maximum value w i t h i n 2 minutes, f l o c c u l a t i o n decreased f o r a t l e a s t 20 minutes upon continued s t i r r i n g . E v i d e n t l y the amount of polymer adsorbed cannot be considered to be the s o l e f a c t o r governing the f l o c c u l a t i o n . Polymer Concentration. The e f f e c t of polymer dosage on a d d i t i o n a l f l o c c u l a t i o n responses was measured together with adsorption


24. SOMASUNDARAN ET AL.

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397


PAM 0.4-0/Na-KAOLINITE pH =4.5 ± 0 . 2 1*3x10-* kmol/m'NaCI S/L'0.025 COND. SPEED = 1200 rpm T= 2 4 ± 2°C COND. TIME = lh

INITIAL

POLYMER CONCENTRAT ION , mg/kg

F i g u r e 1. Adsorption and f l o c c u l a t i o n obtained with Nak a o l i n i t e a t pH 4.5 as a f u n c t i o n o f dosage o f p o l y a c r y l a mide (PAM 0.4-0).

F i g u r e 2. Adsorption and f l o c c u l a t i o n obtained with Nak a o l i n i t e a t pH 4.5 as a f u n c t i o n o f dosage o f 33% hydrol y z e d polyacrylamide (HPAM 0.4-33).



POLYMER ADSORPTION AND DISPERSION STABILITY

0.2

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F i g u r e 3. S e t t l i n g r a t e o f N a - k a o l i n i t e suspension a t pH 4.5 as a f u n c t i o n o f polyacrylamide (PAM 0.4-0)dosage.

PAM 0.4-0/Na-KAOLINITE pH =4.5 ± 0 . 2 Ci = 25 mg/kg S / L » 0.025 I =3x10-* kmol/m NaCI T=24±2 C COND. SPEED* 1200 rpm 3

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CONDITIONING TIME , min.

F i g u r e 4. Diagram i l l u s t r a t i n g k i n e t i c s o f adsorption and f l o c c u l a t i o n f o r N a - k a o l i n i t e / p o l y a c r y l a m i d e (PAM 0.4-0) system.



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f o r both the nonionic and the a n i o n i c polymer. C o n d i t i o n i n g was done i n these t e s t s a t 1200 rpm f o r 10 minutes. On the b a s i s o f the r e s u l t s given i n F i g u r e 4, adsorption can be considered t o have a t t a i n e d e q u i l i b r i u m during t h i s time f o r these systems. Results obtained f o r v a r i o u s f l o c c u l a t i o n responses are given i n F i g u r e s 5 and 6 f o r the nonionic and the a n i o n i c p o l y mer r e s p e c t i v e l y . I t i s seen from F i g u r e 5 t h a t while the s e t t l i n g r a t e goes through a maximum as a f u n c t i o n o f PAM conc e n t r a t i o n , the transmittance o f the supernatant i n c r e a s e s from 38% a t 0 mg/kg t o 80% a t 25 mg/kg and then remains a t t h a t l e v e l t i l l a t l e a s t 50 mg/kg. The sediment volume does not vary t o any s i g n i f i c a n t extent. Adsorption o f the polymer on the other hand i s found t o i n c r e a s e continuously i n the complete concent r a t i o n range o f 0 t o 100 mg/kg. In c o n t r a s t , with the 33% hydrolyzed polyacrylamide, while the a d s o r p t i o n reaches i t s maximum value a t about 50 mg/kg, both the s e t t l i n g r a t e and the supernatant transmittance are found t o go through a maximum i n the 10-25 mg/kg ranges. A l s o i n t h i s case the sediment volume i s a f f e c t e d by the polymer a d d i t i o n . I t i s t o be noted t h a t the maximum e f f e c t i s seen a t d i f f e r e n t polymer c o n c e n t r a t i o n s f o r d i f f e r e n t responses. V i s c o s i t y o f the suspension a l s o responds to the PAMA a d d i t i o n , but again the polymer c o n c e n t r a t i o n r e q u i r e d f o r t h i s e f f e c t i s d i f f e r e n t from those f o r changes i n other responses. These d i f f e r e n c e s i n the e f f e c t o f polymers on v a r i o u s f l o c c u l a t i o n responses have important t h e o r e t i c a l and p r a c t i c a l i m p l i c a t i o n s and can be e x p l a i n e d i n terms o f v a r i o u s c h a r a c t e r i s t i c s o f f l o e s and f l o c - a g g r e g a t e s . Polymer a d s o r p t i o n o r a t t a c h ment o f p a r t i c l e s t o polymer can occur i n any number of c o n f i g u r a t i o n s , and as a r e s u l t the aggregation o f p a r t i c l e s a l s o can take p l a c e i n many ways, l e a d i n g t o d i f f e r e n t f l o e and suspension s t r u c t u r e s which w i l l respond d i f f e r e n t l y t o d i f f e r e n t t e s t s . Thus, a wide s i z e d i s t r i b u t i o n o f f l o e can y i e l d both a t u r b i d supernatant and a r e l a t i v e l y high s e t t l i n g r a t e . V i s c o s i t y and sediment volume w i l l a l s o depend on f l o e s t r u c t u r e , but more importantly on p o s s i b l e i n t e r f l o c aggregation t o form network s t r u c t u r e s (17-19). They a l s o can be d i r e c t l y a f f e c t e d i f the polymer adsorbs with long loops and t a i l s and by a l t e r a t i o n s i n the s t r u c t u r e o f the f l u i d medium i n the v i c i n i t y o f the adsorbed s p e c i e s . Indeed, such a l t e r a t i o n s i n the f l u i d s t r u c t u r e and attachments o f u l t r a f i n e s and c o l l o i d s t o the polymer can be expected t o depend on the charges o f both the p a r t i c l e and the polymer. Thus, a t 50 mg/kg polymer a d d i t i o n , even though both types of polymer adsorb t o the same extent, the sample c o n t a i n i n g the nonionic polymer i s i n a f l o c c u l a t e d s t a t e with a c l e a r supernatant whereas the one c o n t a i n i n g the a n i o n i c polymer i s d i s p e r s e d and the supernatant i s t o t a l l y t u r b i d . The a n i o n i c polymer has adsorbed t o s a t u r a t i o n a t t h i s l e v e l o f a d d i t i o n , but under these c o n d i t i o n s aggregation between the p a r t i c l e s can be prevented by the e l e c t r o s t a t i c r e p u l s i o n between them. However,



PAM 0.4-0/No-KAOLINITE pH = 4 . 5 ± 0 . 2 1 = 3x10"* kmol/m»NoCI S/L»0.025 COND. SPEED = 1200rpm T= 2 4 ± 2 C COND. TIME = lOmln


e

INITIAL POLYMER CONCENTRATION, mg/kg

F i g u r e 5. Various f l o c c u l a t i o n responses together with ads o r p t i o n obtained with N a - k a o l i n i t e a t pH 4.5 as a f u n c t i o n of polyacrylamide (PAM 0.4-0) dosage.


Polyacrylamides


SOMASUNDARAN ET AL.

0

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INITIAL POLYMER

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Adsorption

75

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CONCENTRATION, mg/kg

Figure 6. Various f l o c c u l a t i o n responses together with ads o r p t i o n obtained with N a - k a o l i n i t e a t pH 4.5 as a f u n c t i o n of 33% hydrolyzed polyacrylamide (HPAM 0.4-33) dosage.


402



while p a r t i c l e / p o l y m e r - p o l y m e r / p a r t i c l e aggregation w i l l be prevented by such r e p u l s i o n , p a r t i c l e / p o l y m e r - p a r t i c l e aggregat i o n conceivably c o u l d occur s i n c e the p a r t i c l e s are not completel y covered by the a n i o n i c polymer under such c o n d i t i o n s (the nonionic polymer adsorbs t o a g r e a t e r extent than the a n i o n i c polymer a t polymer a d d i t i o n s higher than 50 mg/kg, and there i s no apparent reason f o r a l a r g e r surface coverage p e r molecule by the a n i o n i c polymer). E v i d e n t l y the e l e c t r o s t a t i c r e p u l s i o n i s s u f f i c i e n t t o prevent even p a r t i c l e / p o l y m e r - p a r t i c l e aggregat i o n . Whether o r not the entropy changes i n t h i s system are s u f f i c i e n t t o produce such s t a b i l i z a t i o n o f the suspension i s not clear at present. Polymer Charge D e n s i t y . The e f f e c t o f polymer charge d e n s i t y i s i l l u s t r a t e d i n d e t a i l i n F i g u r e 7. F o r reference purposes, values obtained i n the absence o f the polymer are 0.4 mm/s, 38%, 25 cm and 1.56 cp f o r s e t t l i n g r a t e , supernatant transmittance, sediment volume and v i s c o s i t y , r e s p e c t i v e l y . While a l l the responses are s e n s i t i v e t o the polymer charge d e n s i t y , i t i s c l e a r l y seen t h a t a maximum e f f e c t i s obtained a t d i f f e r e n t l e v e l s of a n i o n i c i t y f o r d i f f e r e n t responses. S e t t l i n g r a t e and sediment volume are s e n s i t i v e i n the 0 t o 20% h y d r o l y s i s range, whereas v i s c o s i t y and supernatant transmittance become s e n s i t i v e a t higher a n i o n i c i t i e s . D i s c r e p a n c i e s i n the l i t e r a t u r e regarding the optimum charge d e n s i t y f o r f l o c c u l a t i o n might p a r t l y be the r e s u l t o f use o f d i f f e r e n t techniques t o monitor f l o c c u l a t i o n . I t i s t o be noted t h a t the v a l u e s obtained f o r the optimum charge d e n s i t y can be dependent on the polymer dosage, s o l u t i o n propert i e s — s u c h as pH and i o n i c s t r e n g t h — a n d hydrodynamic c o n d i t i o n s . 3

Discussion The r e s u l t s o f t h i s study c l e a r l y show the complex dependence of the f l o c c u l a t i o n process on polymer dosage and charge d e n s i t y . I t i s seen t h a t the form o f dependence v a r i e s markedly among the responses monitored. In a d d i t i o n t o the f a c t o r s s t u d i e d here, i t can a l s o be expected t o depend upon s e v e r a l other p h y s i c o chemical c o n d i t i o n s o f the system, i n c l u d i n g the type o f mixing. The f i n a l s t a t e o f f l o c c u l a t i o n achieved by a mineral/polymer system w i l l depend upon many i n t e r a c t i o n s i n the system as determined by v a r i o u s chemical and hydrodynamic p r o p e r t i e s o f the p a r t i c l e s , polymer, d i s s o l v e d o r g a n i c s and the f l u i d s . These i n c l u d e e l e c t r o s t a t i c i n t e r a c t i o n between the p a r t i c l e s and i n t e r a c t i o n o f p a r t i c l e s with t h e f l u i d governed by t h e i r wett a b i l i t y , morphology and d e n s i t y (17-19); the extent o f adsorption of the polymer and i t s i n f l u e n c e on the i n t e r a c t i o n o f p a r t i c l e s , the o r i e n t a t i o n or c o n f i g u r a t i o n o f the adsorbed polymers (and s u r f a c t a n t when i t i s present) and r e s u l t a n t i n t e r a c t i o n o f adsorbed l a y e r s ; the hydrodynamic s t a t e o f the system and i t s i n fluence on the i n t e r a c t i o n o f f l o e s themselves.


Polyacrylamides

SOMASUNDARAN ET AL.

Adsorption

on

Kaolinite

POLYACRYLAMIDE (MW~0.4xlO )/No-KAOLINITE Cl = 10 mg/kg 1= 3x10"* kmol/m NoCI pH= 4.5±0.2 COND. SPEED = 1200 rpm S/L* 0.025 COND.TIME = 10min T= 24±2°C e


3

I

I

1

1

1

1

1

0

10

20

30

40

Uo^ I

POLYMER CHARGE DENSITY, mole % F i g u r e 7. Diagram i l l u s t r a t i n g the e f f e c t o f polymer charge d e n s i t y on v a r i o u s f l o c c u l a t i o n responses o f Nak a o l i n i t e a t pH 4.5.



404


Among the p r o p e r t i e s measured here, the s e t t l i n g r a t e i s mainly a measure o f the s i z e o f the f l o e s and i n l a t e r stages the c o m p r e s s i b i l i t y o f f l o e s and f l o e networks, and the supernatant c l a r i t y i s a measure o f the s i z e d i s t r i b u t i o n of f l o e s and s i z e dependent capture o f the p a r t i c l e s and f l o e s by the polymer. The sediment volume and the p u l p v i s c o s i t y on the other hand, are d i r e c t measures, not only o f f l o e s i z e and s t r u c t u r e but a l s o of adsorbed polymer l a y e r s . I t i s t o be noted i n t h i s regard t h a t i t i s t h i s l a t t e r aspect which makes i t p o s s i b l e t o estimate the t h i c k n e s s o f adsorbed polymer l a y e r s by measuring the v i s c o s i t y o f the medium and the suspension i n the presence of polymers (20,21). T h i s combination of e f f e c t s i s another reason one cannot always expect c o r r e l a t i o n between v a r i o u s f l o c c u l a t i o n responses. In order t o generate information on the mechanism of f l o c c u l a t i o n by polymers i t i s , however, necessary t o c o r r e l a t e f l o c c u l a t i o n with v a r i o u s system p r o p e r t i e s , p a r t i c u l a r l y a d s o r p t i o n . Thus, i f p a r t i c l e / p o l y m e r - p o l y m e r / p a r t i c l e contact i s the aggregat i o n mechanism, the f l o c c u l a t i o n responses should be expected t o continuously increase with surface coverage. On the other hand, i f p a r t i c l e / p o l y m e r - p a r t i c l e contact i s predominant and i f the polymer adsorption i s e s s e n t i a l l y i r r e v e r s i b l e , maximum f l o c c u l a t i o n might be expected under submonolayer c o n d i t i o n s . In order to determine the nature o f t h i s r e l a t i o n s h i p f o r the present systems, s e l e c t e d f l o c c u l a t i o n responses are p l o t t e d i n F i g u r e s 8 and 9 as a f u n c t i o n of surface coverage f o r the nonionic and the a n i o n i c polymer r e s p e c t i v e l y . The assumptions i n v o l v e d i n t h e computation o f the surface coverage are t o be noted a t t h i s p o i n t : 1. S a t u r a t i o n adsorption corresponds t o monolayer formation; 2. C o n f i g u r a t i o n o f the adsorbed species remains u n a l t e r e d from low t o high c o n c e n t r a t i o n s . Indeed, these assumptions cannot be considered v a l i d f o r polymer adsorption p a r t i c u l a r l y i f i t i s p o s s i b l e f o r the adsorbed polymer t o move from a f l a t c o n f i g u r a t i o n t o one with loops and t r a i n s (22,23). In s p i t e o f these d i f f i c u l t i e s , i t can be s a f e l y s t a t e d t h a t the f l o c c u l a t i o n response e x h i b i t s a maximum with respect t o surface coverage, the optimum surface coverage being about 0.1 f o r the nonionic polymer and 0.1 t o 0.2 f o r the a n i o n i c , depending on the f l o c c u l a t i o n response measured. I t i s seen from F i g u r e 9 t h a t f u l l surface coverage leads t o d i s p e r s i o n , suggesting t h a t p a r t i c l e / polymer-polymer/particle aggregation i s n o t the predominant mechanism i n t h i s case. A l s o , the 0.5 coverage c r i t e r i o n proposed i n the p a s t (13) appears not t o be v a l i d f o r the present system under the t e s t e d c o n d i t i o n s . E v i d e n t l y , i n t h i s case, f l o c c u l a t i o n i s favored when a major f r a c t i o n o f the surface remains bare and a v a i l a b l e f o r aggregation by b r i d g i n g . Indeed, such b r i d g i n g should be i n f l u e n c e d both by the e l e c t r o s t a t i c p r o p e r t i e s of the polymer and the mineral and by the polymer c o n f i g u r a t i o n , which i n t u r n w i l l depend upon molecular weight, charge d e n s i t y , pH and i o n i c s t r e n g t h . While the nonionic polyacrylamide can be


Polyacrylamides

SOMASUNDARAN ET AL.

Adsorption

on

Kaolinite


1.4

0

2

0

I

I = 3x10'* kmol/m'NoCI COND. SPEED= 1200rpm COND. TIME* lOmin

I 0

ι 0

'

'

1 2 ADSORPTION

ι

ι

1

«

3 4 DENSITY, mg/g

ι

ι

0.1 0.2 0.3 0.4 SURFACE COVERAGE

ι 0.5

F i g u r e 8. F l o c c u l a t i o n of N a - k a o l i n i t e a t pH 4.5 as meas ured by s e t t l i n g r a t e and supernatant c l a r i t y as a f u n c t i o n of p a r t i c l e surface coverage by polyacrylamide (PAM 0.4-0).


405


HPAM 0.4-33/No-KAOLINITE


Ο

pH = 4.5

±0.2

ADSORPTION DENSITY, mg/g i

0

l

l

0.2

i

0.4

0.6

l

0.8

l

1.0

SURFACE COVERAGE

F i g u r e 9. F l o c c u l a t i o n o f N a - k a o l i n i t e a t pH 4.5 as measured by the s e t t l i n g r a t e and the supernatant c l a r i t y as a func t i o n o f p a r t i c l e surface coverage by 33% hydrolyzed p o l y acrylamide (HPAM 0.4-33).



24.

SOMASUNDARAN ET AL.

Polyacrylamides

Adsorption

on Kaolinite

407

expected to be i n a c o i l e d form, the 33% hydrolyzed polymer w i l l be more extended a t pH 4.5 s i n c e about h a l f o f the carboxylate groups w i l l be i n d i s s o c i a t e d a n i o n i c carboxylate form. The higher v i s c o s i t y obtained f o r the a n i o n i c polymers supports the above c o n s i d e r a t i o n . Such expansion of the polymer due t o charge r e p u l s i o n can f a c i l i t a t e b r i d g i n g due to i n c r e a s e d polymer length but can a l s o r e t a r d i t v i a e l e c t r o s t a t i c r e p u l s i o n between the polymer and the s i m i l a r l y charged f a c e s o f the k a o l i n i t e p a r t i c l e . The net e f f e c t of these competing phenomena w i l l a c t u a l l y determine the f l o c c u l a t i o n power o f the hydrolyzed p o l y acrylamide. In the present study, reduced f l o c c u l a t i o n was obtained upon i n c r e a s i n g the charge d e n s i t y o f the polyacrylamide, suggesting the predominating c o n t r i b u t i o n of the e l e c t r o s t a t i c r e p u l s i o n . The higher sediment volume obtained with hydrolyzed polyacrylamides can a l s o be a t t r i b u t e d to such changes i n conf i g u r a t i o n o f the polymer upon h y d r o l y s i s : The charged polymer can produce voluminous f l o e s due to both i t s expanded c o n f i g u r a t i o n and to longer loops owing to the fewer contact p o i n t s expected between the s i m i l a r l y charged polymer and mineral p a r t i c l e s . Indeed some a d d i t i o n a l contacts can occur between the p o l y mer and the edges o f the k a o l i n i t e p a r t i c l e s . The presence of excess s a l t , p a r t i c u l a r l y of b i v a l e n t i n o r ganics can reduce the e l e c t r o s t a t i c r e p u l s i o n between the a n i o n i c polymer and the k a o l i n i t e p a r t i c l e s and enhance f l o c c u l a t i o n (24,25). The optimum f l o c c u l a t i o n of f i n e c o a l and c o a l r e f u s e (which contained 13 to 65% c l a y s ) obtained by Lewellyn and Wang(24) with hydrolyzed polyacrylamide c o n t a i n i n g 70% a c r y l a t e i n f a c t might have r e s u l t e d from the use o f r e c y c l e d water which cont a i n e d 55 ppm Ca and 30 ppm Mg. Other works i n l i t e r a t u r e have reported maximum f l o c c u l a t i o n of k a o l i n i t e to occur with 30% hydrolyzed polyacrylamide (5,6,10,26,27). Such d i f f e r e n c e s i n r e p o r t e d r e s u l t s could e a s i l y r e s u l t from v a r i a t i o n s i n water chemistry, o r i g i n a t i n g e i t h e r with the water used or with d i s s o l v e d o r g a n i c s r e l e a s e d by the mineral i t s e l f . Summary F l o c c u l a t i o n u s i n g polyacrylamide and hydrolyzed polyacrylamides was i n v e s t i g a t e d by studying a number of f l o c c u l a t i o n responses of N a - k a o l i n i t e under c o n t r o l l e d chemical and hydrodynamic c o n d i tions . The f l o c c u l a t i o n responses s t u d i e d are s e t t l i n g r a t e , percent s o l i d s e t t l e d , supernatant c l a r i t y , sediment volume and s l u r r y v i s c o s i t y . The polymer c o n c e n t r a t i o n and polymer a n i o n i c i t y r e q u i r e d f o r maximum f l o c c u l a t i o n were seen c l e a r l y t o depend on the response s t u d i e d . Both the s e t t l i n g r a t e and the supernat a n t c l a r i t y with the nonionic polyacrylamide f l o c c u l e n t showed a t pH 4.5 a marked i n c r e a s e to a maximum a t about 25 mg/kg, whereas with the a n i o n i c polymer s e t t l i n g r a t e and supernatant c l a r i t y showed maxima a t 10-25 mg/kg, but the system was t o t a l l y




408

d i s p e r s e d a t higher concentrations. Sediment volume d i d not show any measurable change over the e n t i r e concentration range of nonionic polymer. In c o n t r a s t , with the a n i o n i c polyacrylamide (33% hydrolyzed) both the sediment volume and the s l u r r y v i s c o s i t y showed i n c r e a s e s with polymer concentration i n the 0 t o 50 and 25 t o 50 mg/kg ranges r e s p e c t i v e l y . V i s c o s i t y and sediment volume, however, remained a t the h i g h e s t l e v e l s up t o the h i g h e s t t e s t e d concentrations o f a n i o n i c polymer. The d i f f e r e n t e f f e c t s o f polymers on the above responses a r e d i s c u s s e d i n terms o f v a r i o u s c h a r a c t e r i s t i c s o f f l o e s and f l o c - a g g r e g a t e s . While s e t t l i n g r a t e and percent s o l i d s e t t l e d can be expected t o be governed e s s e n t i a l l y by the coarser r e g i o n o f the p a r t i c l e s i z e d i s t r i b u t i o n , b y the d e n s i t y and compressive strength o f the f l o e s , and by the s t r u c t u r e o f any three dimensional networks t h a t can t r a p the f l u i d , the supernatant c l a r i t y w i l l depend on any s h i f t i n the f i n e region o f the s i z e d i s t r i b u t i o n and on the a b i l i t y o f the polymers and the network t o capture the u l t r a f i n e s and c o l l o i d s . Sediment volume and s l u r r y v i s c o s i t y on the other hand can depend on, i n a d d i t i o n t o the f l o e s i z e d i s t r i b u t i o n and the compressive strength o f the f l o e s , a l s o the t h i c k ness o f the adsorbed l a y e r s o f the polymer on the p a r t i c l e s . A l l the measured responses were s e n s i t i v e t o the a n i o n i c i t y of the polymer used as the f l o c c u l e n t . S e t t l i n g r a t e showed a maximum with 0% h y d r o l y s i s and the supernatant c l a r i t y exh i b i t e d a maximum i n the 0 - 20% h y d r o l y s i s range. While the a n i o n i c i t y d i d not have a s i g n i f i c a n t e f f e c t on the v i s c o s i t y i n the 0-20% range, f u r t h e r i n c r e a s e s i n a n i o n i c i t y produced a measurable decrease i n v i s c o s i t y . These e f f e c t s a r e accounted f o r i n terms o f e l e c t r o s t a t i c i n t e r a c t i o n s between the polymer l a y e r s on the p a r t i c l e s and a l t e r a t i o n s i n the c o n f i g u r a t i o n o f the adsorbed polymers due t o the presence o f the charged groups. F l o c c u l a t i o n was c o r r e l a t e d with both adsorption d e n s i t y and estimated surface coverage f o r the nonionic and 33% hydrolyzed polyacrylamides. Maximum s e t t l i n g r a t e was obtained with the nonionic f l o c c u l e n t a t 0.1 and with the hydrolyzed sample a t 0.2 surface coverage. Supernatant c l a r i t y showed a maximum at a surface coverage o f N a - k a o l i n i t e by the hydrolyzed polyacrylamide o f 0.1. A t higher surface coverages (such as 0.5) considered i n the past t o be optimum f o r f l o c c u l a t i o n , complete d i s p e r s i o n was obtained with both the nonionic and the a n i o n i c polymer. Acknowledgments The authors wish t o acknowledge the p a r t i c u l a t e and multiphase p r o c e s s i n g program o f the N a t i o n a l Science Foundation (CPE-8011013) f o r support o f t h i s work and Dr. F. Halverson o f American Cyanamid f o r polymer syntheses and h e l p f u l d i s c u s s i o n s .



24. SOMASUNDARAN ET AL.

Polyacrylamides

Adsorption

on

Kaolinite

409

Literature Cited 1. Somasundaran, P., in "Research Needs in Mineral Processing"; Somasundaran, P.; Fuerstenau, D.W., eds.; NSF Workshops Rep. 1976. 2. Read, A.D.; Hollick, C.T., Minerals Sci. Eng. 1976, 8, 202. 3. Gardner, W.M.; Montemayer, L.W., B.S. Thesis, M.I.T. 1953. 4. Michaels, A.S., Ind. Eng. Chem. 1954, 46, 1485. 5. Michaels, A.S.; Morelos, O., Ind. Eng. Chem. 1955, 47, 1801. 6. Dollimore, D.; Harridge, T.A., Trans. J. Brit. Ceram. Soc. 1971, 70, 191 (b) Water Res. 1972 (c) J. Colloid Interf. Sci. 1973, 42, 581. 7. Roberts, K.; Kowalewska, J.; Friberg, S., J. Colloid Interf. Sci. 1974, 48, 361. 8. Halverson, F., Proc. 10th Annual Meeting, Canadian Mineral Processors Ottawa, Canada 1978. 9. Solomchenko, Ν.Y.; Kochetkova, Ε.I.; Sokolova, Ν.Ρ., Colloid J. USSR 1981, 43, 310. 10. Smellie, R.H., Jr.; La Mer, V.K., J. Colloid Sci. 1958, 13, 589. 11. Linke, W.F.; Booth, R.B., Trans. AIME 1960, 217, 364. 12. Kuzkin, S.F.; Nebera, W.P.; Zolin, S.N., Proc. 7th Int. Miner. Process Congress New York, 1964. 13. La Mer, V.K.; Healy, T.W., J. Phys. Chem. 1963, 67, 2417. 14. Hollander, A.F.; Somasundaran, P.; Gryte, C.C., Adsorption from Aqueous Solutions"; Tewari, P.H., ed.; Plenum, 1981. 15. American Cyanamid Technical Bulletin, "Cyanamer™ Polyacryl amides", 1967. 16. Wang, Y.C., Carrier Flotation of Anatase from Clay and its Physicochemical Mechanisms, D.E.Sc. Thesis, Columbia Univer sity, New York, 1980. 17. Somasundaran, P.; Smith, E.L., Jr.; Harris, C.C., "Proceed ings of the First International Conference in Particle Technology", IITRI, Chicago, 1973, 144-150. 18. Harris, C.C.; Somasundaran, P.; Jensen, R.R., Powder Tech nology, 1975, 11, 75-84. 19. Nagaraj, D.R.; McAllister, L.; Somasundaran, P., Int. J. Miner. Process. 1977, 4, 111-129. 20. Doroszkowski, A.; Lambourne, R., J. Colloid Interf. Sci. 1968, 26, 214. 21. Sato, T.; Ruch, R., Stabilization of Colloidal Dispersion by Polymer Adsorption, Marcel Dekker, Inc., NY, 1980. 22. Theng, B.K.G., Formation and Properties of Clay-Polymer Com plexes, Elsevier Scientific Publishing Company, N.Y., 1979. 23. Strombert, R.R., in "Treatise on Adhesion and Adhesives"; Patrick, R.L., ed.; Marcel Dekker, NY, 1967; Vol. 1. 24. Lewellyn, M.E.; Wang, S.S., presented at 183rd ACS National Meeting, New York, 1981. 25. O'Gorman, J.V.; Kitchener, J.A., Int. J. Miner. Process. 1973, 1, 33.


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ADSORPTION

AND

DISPERSION

STABILITY

26. Werneke, M.F., presented at 108th AIME Meeting, New Orleans, LA, 1979. 27. Pearse, M.J.; Barnett, J., Filtr. Sep. 1980. 17, 460.


R E C E I V E D October 7, 1983


Polymer Adsorption and Dispersion Stability - American Chemical

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