Adsorption, Electrokinetic, and Flotation Properties of Minerals

16 Adsorption, Electrokinetic, and Flotation Properties of Minerals above the Critical Micelle Concentration Downloaded by PENNSYLVANIA STATE UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch016

Bohuslav Dobiás Department of Surface Chemistry and Flotation, Institute of Physical and Macromolecular Chemistry, University of Regensburg, 8400 Regensburg, Federal Republic of Germany

The problem of mineral flotability above the CMC with respect to the appearance of adsorption maxima has not yet been systematically studied. The aim of this paper was to correlate the adsorption maxima and the flotability of minerals with the kind of potential determining ions. The f l o t a t i o n o f m i n e r a l s i s b a s e d o n d i f f e r e n t a t t a c h ment f o r c e s o f h y d r o p h o b i z e d and h y d r o p h i l i c m i n e r a l p a r t i c l e s t o a gas b u b b l e . H y d r o p h o b i z e d m i n e r a l p a r t i c les a d h e r t o gas b u b b l e s and a r e c a r r i e d t o t h e s u r f a c e of the m i n e r a l d i s p e r s i o n where they form a f r o t h l a y e r . A mineral i s hydrophobized by the a d s o r p t i o n o f a s u i t able s u r f a c t a n t on the s u r f a c e o f the m i n e r a l component to be f l o t a t e d . The h y d r o p h o b i c i t y o f a m i n e r a l p a r t i c l e depends on the degree o f o c c u p a t i o n o f i t s s u r f a c e by s u r f a c t a n t m o l e c u l e s and t h e i r p o l a r - a p o l a r o r i e n t a t i o n in the a d s o r p t i o n l a y e r . In a number o f p a p e r s t h e r e l a t i o n s h i p was a n a l y z e d b e t w e e n t h e a d s o r p t i o n d e n s i t y of t h e s u r f a c t a n t a t t h e m i n e r a 1 - w a t e r i n t e r f a c e and the n o t a b i l i t y . However, most i n t e r p r e t a t i o n s o f a d s o r p t i o n and f l o t a t i o n measurements c o n c e r n s u r f a c t a n t c o n c e n t r a t i o n s under t h e i r CMC. A f e w p a p e r s have been p u b l i s h e d r e c e n t l y on t h e p r o b l e m o f s u r f a c t a n t a d s o r p t i o n maxima on s o l i d s i n t h e r e g i o n o f t h e CMC ( 1 - 5 ) . S c a m e h o r n e t a l . ( 1 , 2 ) and T r o g u s e t a l . ( 3 ) e x p T T i n e d t h e o r i g i n o f t h e s e maxima by v a r i o u s r a T i o s o f t h e s u r f a c t a n t s o l u t i o n t o the s o l i d , i n connection with isomeric i m p u r i t y o f t h e s u r f a c t a n t . A n a n t h a p a d m a n a b h a n and S o m a s u n d a r a n ( 4 ) e x amined c r i t i c a l l y t h e p r e s e n c e o f such maxima f r o m t h e viewpoint o f various proposed adsorption mechanisms. T h e y h a v e shown t h a t a m e c h a n i s m i n c l u d i n g m i c e l l a r e x c l u s i o n , m i x e d m i c e l l e f o r m a t i o n and p r o p e r t i e s o f s o l i d s , such as the pore s i z e , cannot e x p l a i n s a t i s f a c 0097-6156/ 86/ 0311 -0216$06.00/ 0 © 1986 American Chemical Society In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Downloaded by PENNSYLVANIA STATE UNIV on February 16, 2013 | http://pubs.acs.org Publication Date: June 5, 1986 | doi: 10.1021/bk-1986-0311.ch016

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Properties of Minerals above the Critical Micelle Concentration

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t o r i l y the r e p o r t e d r e s u l t s f o r t h e a d s o r p t i o n maxima and h y s t e r e s i s . A c c o r d i n g t o t h e s e a u t h o r s , t h e m a i n reason f o r the maxima i s r e d i s s o l u t i o n o f s u r f a c t a n t m u l t i v a l e n t i o n p r e c i p i t a t e s a b o v e t h e CMC, a n d t h e b u l k p r e c i p i t a t i o n upon d i l u t i o n . The s o l u b i l i z a t i o n o f the r e a c t i o n products between s u r f a c t a n t s a n d m u l t i v a lent i n o r g a n i c c a t i o n s , o r o f weakly d i s s o c i a t e d surfactant molecules a t c > CMC w a s p r o p o s e d a l s o b y D o b i â s (5), a s a p o s s i b l e e x p l a n a t i o n o f m a x i m a o n a d s o r p t i o n - i s o t h e r m s . The main r e a s o n f o r t h e o c c u r r e n c e o f t h e s e m a x i m a i s , a c c o r d i n g t o D o b i â s a n d S t r n a d (6J , t h e f o r m a t i o n o f a new p h a s e , m i c e l l e . T h i s new p h a s e competes w i t h the monomer f o r a d s o r p t i o n s i t e s o n t h e mineral surface during the e x t r a c t i o n o f the l e t t e r from the bulk phase. In other words, the presence o f m i c e l l e s prevents the formation o f an a d s o r p t i o n l a y e r an t h e m i n e r a l i n t e r f a c e , o b v i o u s l y b e c a u s e t h e m o n o m e r is t h e r m o d y n a m i c a l l y more s t a b l e i n a t h r e e d i m e n s i o n a l m i c e l l e than i n an a s s o c i a t e d two-dimensional layer on the mineral i n t e r f a c e . T h e p r o b l e m o f m i n e r a l f l o t a b i l i t y a b o v e t h e CMC with respect t o the appearance o f a d s o r p t i o n maxima has not y e t been s y s t e m a t i c a l l y s t u d i e d . T h e a i m o f t h i s paper i s t o c o r r e l a t e t h e a d s o r p t i o n maxima a n d the f l o t a b i l i t y o fminerals with the kind o fp o t e n t i a l determining ions(PDI). Experimental The a d s o r p t i o n d e n s i t y o f s u r f a c t a n t s was d e t e r m i n e d b y c a l c u l a t i n g the d i f f e r e n c e in the s u r f a c t a n t concentrat i o n b e f o r e and a f t e r t h e a d s o r p t i o n o f a m i n e r a l , a s d e s c r i b e d e a r l i e r (5). The m e a s u r e m e n T s w i t h N a - d o d e c y l b e n z e n e sulfonate w e r e p e r f o r m e d a t p H 5.6 + 0.2 w h e r e a s t h o s e w i t h c e t y l p y r i d i n i u m c h l o r i d e a t p H 6.0 + 0.2. T h e p H v a l u e s were not a d j u s t e d . The m i n e r a l s used i n t h e experiments were o f natur a l o r i g i n , i n a c r y s t a l l i n e f o r m , a n d w e r e > 99 % p u r e Specific s u r f a c e o f the m i n e r a l s a n d t h e i r source were c h e s a m e a s g i v e n i n (5). S o l u t i o n s o l i d r a t i o s (1/g) w e r e g r e a t e r t h a n 0.5. The m i n e r a l f l o t a b i l i t y was c a l c u l a t e d f r o m k i n e t i c d a t a i n a m o d i f i e d H a l l i m o n d t u b e (7_). A l l m e a s u r e m e n t s w e r e p e r f o r m e d a t 25.0 + 0.2 ° C . R e s u l t s and

Discussion

The a d s o r p t i o n i s o t h e r m s o f v a r i o u s s u r f a c t a n t s w e r e measured on minerals with a d i f f e r e n t c h a r a c t e r o f PDIs. T h e course o f the isotherms on m i n e r a l s , with H a n d OH" o n o n e h a n d a n d t h o s e w i t h l a t i c e i o n s a s PDIs o n t h e o t h e r h a n d , i s s i m i l a r , w i t h a maximum i n a r e g i o n c l o s e t o t h e CMC. S o m e c h a r a c t e r i s t i c a d s o r p -

In Phenomena in Mixed Surfactant Systems; Scamehorn, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


218

P H E N O M E N A IN M I X E D S U R F A C T A N T S Y S T E M S

t i o n i s o t h e r m s a r e s h o w n i n f i g u r e 1 a n d 2. T h e p l a t e a u on t h e s e i s o t h e r m s c h a r a c t e r i z e s f i l l i n g t h e f i r s t a d s o r p t i o n l a y e r t h a t s h o u l d show t h e maximum hydrophob i c i t y and t h e r e f o r e t h e optimal f l o t a b i l i t y . The value of t h e z e t a p o t e n t i a l r e a c h e s i n many c a s e s i t s maximum a t t h e CMC, t o o (5). The a d s o r p t i o n i s o t h e r m s were d i s c u s s e d e x t e n s i v e l y i n t h e p r e v i o u s p a p e r (5). T h e s u r f a c e a r e a t h a t t h e adsorbed s u r f a c t a n t m o l e c u l e s o c c u p y a t t h e r a n g e o f p l a t e a u ( Θ = 1) i s d i f f e r e n t b e c a u s e i t d e p e n d s o n t h e chemical composition o f the mineral and i t s clevage plane. The a d s o r p t i o n d e n s i t i e s ( r ) o n m i n e r a l s (C< CMC) of t h e s a l t t y p e a r e i n some c a s e s h i g h e r b e c a u s e o f p r e c i p i t a t i o n o f the ionic surfactant with multivalent c a t i o n s i nt h e bulk phase. Measurements were c a r r i e d out t odetermine t h e f r a c t i o n O f t h e p r e c i p i t a t e d sur f a c t a n t by d i v a l e n t c a t i o n s Ca and Ba leading t o a d e c r e a s e i n i t s e q u i l i b i r u m c o n c e n t r a t i o n . They showed a s h i f t o f t h e a d s o r p t i o n maximum t o w a r d s l o w e r v a l u e s of r , even a f t e r a c o r r e c t i o n o f t h e a d s o r p t i o n den s i t y d u e t o t h e p r e c i p i t a t i o n . On t h e o t h e r h a n d , a d i rect co-adsorption o f the p r e c i p i t a t e d s u r f a c t a n t on a mineral surface cannot be excluded. The d e c r e a s e o f t h e a d s o r p t i o n d e n s i t y o v e r t h e CMC a s a c o n s e q u e n c e o f t h e s o l u b i l i z a t i o n o f t h e p r e c i p i t a t e d s u r f a c t a n t leads t o an increase o f i t s equi l i b r i u m c o n c e n t r a t i o n (^5). A c c o r d i n g t o t h e s t r u c t u r e of t h e a d s o r p t i o n l a y e r o f a s u r f a c t a n t , t h e f l o t a b i l i t y should c o r r e l a t e with the surface coverage o f sur f a c t a n t m o l e c u l e s . On t h e b a s i s o f t h i s a s s u m p t i o n , t h e optimum f l o t a b i l i t y o f t h e mineral should be reached in t h e r e g i o n o f t h e f i r s t p l a t e a u on t h e a d s o r p t i o n i s o t h e r m , ( F i g . 1,2) w h e r e a s a m i n i m u m s h o u l d b e e x p e c ted i n t h e r e g i o n o f t h e maximum r v a l u e . T h e e x a m p l e s w i t h q u a r t z ( H a n d OH" a s P D I s ) a n d b a r i t e a n d f l u o r i te ( l a t t i c e ions a s PDIs) i n F i g s . 3 - 5 show a good agreement with t h i s hypothesis on t h e s t r u c t u r e o ft h e a d s o r p t i o n l a y e r . F o r t h e sake o f c l a r i t y i nthese f i g u r e s , o n l y t h e arrows show t h e c o n c e n t r a t i o n o f t h e s u r f a c t a n t a t which a p l a t e a u i s formed on t h e appro p r i a t e i s o t h e r m ( Θ = 1, f r o m F i g s . 1,2) a n d t h e C M C . A correlation o f the values f o r ( θ = 1) s h o w s t h a t a maximum f l o t a b i l i t y i s r e a c h e d i n t h e r e g i o n o f these p l a t e a u s with a r e l a t i v e l y good accuracy f o r t h e shortest flotation time. A decrease i n f l o t a b i l i t y i s characterized by an increase i n p o l a r i t y o f t h e adsorption l a y e r o f both s u r f a c t a n t s duet o the formation o f a second adsorp tion l a y e r with p o l a r groups o f t h e s u r f a c t a n t o r i e n t a t e d toward t h e bulk phase, up t o t h e CMC. T h e ana l y s i s o ft h e f l o t a t i o n measurements was c a r r i e d o u t on the basis of k i n e t i c data f o r t h e s h o r t e s t constant



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F i g u r e 1. A d s o r p t i o n i s o t h e r m s o f N a - d o d e c y 1 b e n z e n e sulfonate on MgF (1), CaF (2), S r F ( 3 ) , B a F ( 4 - r /15) and b a r i t e (5) r .10" - number o f m o l e c u l e s adsorbed per c m 9

ά

9

ά

9

ά

9

ά

2




F i g u r e 2. Adsorption isotherms o f c e t y l p y r i d i n i u r a c h l o r i d e on Τ 1 Ο 2 ( 1 - Γ 2 ) and S 1 O 2 (2 - ) Γι,Γ2 -number o f m o l e c u l e s adsorbed p e r cm^ (Γ · 1 0 - 1 3 ) . Γ ι

F i g u r e 3. F l o t a t i o n r e c o v e r y R ( i n %) o f f l u o r i t e as dependent on t h e c o n c e n t r a t i o n c ( i n mol/L) o f Na-dodecylbenzenesulfonate.




DOBIÂS

F i g u r e 4. F l o t a t i o n r e c o v e r y R ( i n %) o f q u a r t z as dependent on t h e c o n c e n t r a t i o n c ( i n mol/L) o f c e t y l pyridinium chloride.

R

[%]

F i g u r e 5 . F l o t a t i o n r e c o v e r y R ( i n %) o f b a r i t e as dependent on t h e c o n c e n t r a t i o n c ( i n mol/L) o f Na-do decylbenzenesulfonate.


222


time. In t h i s case the f l o t a b i l i t y i s maximal a t only one c o n c e n t r a t i o n o f t h e s u r f a c t a n t ( 8 ) . F o r a c o n s t a n t f l o t a t i o n t i m e o f 10 min, t h e maximum f l o t a b i l i t y c a n not be determined reliably. Scamehorn e t a l (1,2) have s t u d i e d the i n f l u e n c e o f a d s o r p t i o n o f i s o m e r i c a l l y p u r e s u r f a c t a n t s and t h e i r m i x t u r e s o n t h e o c c u r r e n c e o f a d s o r p t i o n m a x i m a . We have i n v e s t i g a t e d , t h e r e f o r e , the f l o t a b i l i t y o f b a r i t e in s o l u t i o n s o f pure N a - a l k y l s u l f a t e s ( C ^ , C > a n d C ,) a n d t h e i r m i x t u r e s a b o v e t h e CMC. T n e d e c r e a s e in f l o t a b i l i t y begins a t a c o n c e n t r a t i o n corresponding t o t h e a d s o r p t i o n d e n s i t y Θ = 1, a n d r e a c h e s a m i n i m u m a t t h e s u r f a c t a n t c o n c e n t r a t i o n * CMC ( F i g . 6 ) . S i m i l a r l y a s i n F i g s . 3 a n d 4, o n l y a r r o w s f o r e = 1 a n d C M C , r e s p . , were u s e d . The minimum f l o t a b i l i t y o f b a r i t e i n the mixtures o f pure s u r f a c t a n t s C + C a n d C*q + C.. i s a l s o r e a c h e d i n t h e r e g i o n o f c o n c e n t r a t i o n 5 CMC ( F i g . 7 , 8 ) . N o a d s o r p t i o n i s o t h e r m s w e r e d e t e r m i n e d for the f l o t a t i o n experiments s h o w n s i n c e we g a t h e r e d from t h e d a t a p u b l i s h e d b y Scamehorn e t a l ( 2 ) , who studied the change o fthe adsorption density as a func tion o f the composition o f binary isomer mixtures o f Na-alkylbenzene sulfonates on alumina and k a o l i n i t e . The l o c a t i o n o f f l o t a t i o n maxima d e p e n d s o n t h e r a t i o of the isomers used. 1 2


1 4

1

Q

1

2


DOBIÂS



R [%]

-U

-3

-2

Igc

F i g u r e 7. F l o t a t i o n r e c o v e r y R ( i n %) o f b a r i t e as dependent on the c o n c e n t r a t i o n c ( i n mol/L) o f C C\2 a l k y l s u l f a t e - N a mixtures. 1 0

R [%]

TÎ

Γ3

-2

Igc

F i g u r e 8. F l o t a t i o n r e c o v e r y R ( i n %) o f b a r i t e as dependent on t h e c o n c e n t r a t i o n c ( i n mol/L) o f C ^ ~ Ci4-alkylsulfate-Na mixtures. 0


224



Conclusions 1. A t c o n c e n t r a t i o n > C M C , m i n e r a l f l o t a b i l i t y r a p i d l y decreases regardless o f t h e character o f PDIs. This d e crease i nf l o t a b i l i t y i s i n a good agreement with t h e occurrence o f a d s o r p t i o n maxima a n d gives evidence o f the change i n t h e character a n d structure o f t h e a d sorption layer. 2. T h e a p p e a r e a n c e o f m a x i m a o n t h e a d s o r p t i o n isotherms and d e c r e a s e i n f l o t a b i l i t y c a n b e e x p l a i n e d b y t h e h y pothesis that i n the presence o fmicelles no adsorption layer o f the surfactant can be formed, the character of which corresponds t ot h e e q u i l i b r i u m s t a t e only with monomers ( s u f f i c i e n t l y hydrophobic adsorption layer). Due t o a h e t e r o g e n e i t y o f f o r c e s a c t i n g a t t h e s u r f a c tant ion mineral i n t e r f a c e i t c a nbe assumed that a t c o n c e n t r a t i o n s S CMC some o f t h e m o l e c u l e s w i l l b e b o u n d much m o r e f i r m l y i na t h r e e - d i m e n s i o n a l m i c e l l e t h a n i n a two-dimensional adsorption layer. Acknowledgment T h a n k s a r e d u e t o M i s s . U. for her technical assistanc ments. The f i n a n c i a l s u p p o r t gemeinschaft i s gratefully

Literature 1. 2. 3. 4. 5. 6. 7. 8.

Wamsler from t h i s department e with the f l o t a t i o n measureo f the Deutsche acknowledged.

Forschungs-

Cited

Scamehorn, J.F.; S c h e c h t e r , R.S.; Wade, W . H . ; J. Coll. I n t e r f a c e Sci. 1982, 85, 463. Scamehorn, J.F.; S c h e c h t e r , R . S . ; Wade, W . H . ; J. Coll. I n t e r f a c e Sci. 1982, 85, 479. Trogus, F.J.; S c h e c h t e r , R . S . ; Wade, W . H . ; J. Coll. I n t e r f a c e Sci. 1979, 70, 293. Ananthapadmanabhan, K.P.; Somasundaran, P . ; Colloids and Surfaces 1983, 77, 105. Dobias, B.; Colloid & Polymer Sci. 1977, 255, 682; ibid 1978, 256, 465. Dobias, B.; S t r n a d , J.; 8th Scandinavian Symposium on Surface Chemistry, Lund 4. - 6. June 1984. Dobias, B.; T r a n s . I n s t n . M i n . Metall. S e c t . C , Mineral Process E x t r . Metall. 1983, 92, C 164. Dobias, B.; Bergakademie 1964, 16, 624.

RECEIVED February 10, 1986


Adsorption, Electrokinetic, and Flotation Properties of Minerals

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