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r

E L S E V I E R Desalinat ion 132 (2000) 299-306DESALINATION

I n v e s t ig a t io n o n p u r if ic a t io n o f h y d r o c h lo r ic a c id b y m e m b r a n em e t h o d

O leg D. Linnikov*, E.A. Anokhina, V.E. ScherbakovInstitute o f Soli d State C hemistry, U ral Branch of the Russian Academy of Sciences, 91 Pervomayskaya St.,620219 Ekaterinburg, RussiaTel. + 7 (3432) 493472; Fax + 7 (3432) 744495, e-mail: Linnikov@mail.ru

Received 27 June 20 00; accepted 11 July 2000

A b s t r a c t

A process of hydrochloric acid t ransfer through an anion exchange membrane at i ts regenerat ion from acidwas tewaters h as be en s tudied . An equat ion descr ib ing the d i ffus ion process of com ponents in the ex am ined mem branesystem has b een deriv ed and experimental ly tested. General mass t ransfer coefficients o f hydrochloric acid an d moleculesofNaC1, CdCI2, NiCI2 , and ZnC12 h rough the anion exchange membrane have b een determined.K eyw o rd s : An ion exchange me mb rane; Wastewater; Hydrochlor ic acid ; Di ffus ion process ; E lectrodialysi s ; C urrentdens i ty

1 . I n t r o d u c t i o n

P r o c e s s e s w i t h t h e u s e o f i o n e x c h a n g em e m b r a n e s a r e m o s t e c o n o m i c a l a n d p r o m i s i n gf o r c o n v e r s i o n o f i n d u s t r i a l w a s t e w a t e r s , n a t u r a lw a t e r p u r i f i c a t i o n a n d s y n t h e s i s o f v a r i o u sc h e m i c a l s [ 1 ,2 ] . T h e y a r e a l so u s e d f o r r e g e n e ra -t i o n o f m i n e r a l a c i d s f r o m p r o c e s s s o l u t i o n s .U s u a l l y a n e l e c t r o d i a l y s i s p r o c e s s c o m b i n i n gd i f f e re n t t y p e s o f t h e i on e x c h a n g e m e m b r a n e s[ 2 , 3 ] i s u s e d . B u t a s c a r c e s e l e c t i v i t y o f a n i o ne x c h a n g e m e m b r a n e s r e s u l t s i n a h y d r o g e n i o n*C or res p ond i ng au tho r .

t r a n s f e r f r o m t h e a c i d s o l u t i o n i n t o t h e s a l ts o l u t io n . T h i s d e c r e a s e s c u r r e n t e f f i c i e n c y o f t h ea c i d a n d i n c r e a se s e n e r g y c o n s u m p t i o n o f t h ep r o c e s s . T o e l i m i n a t e t h i s , t h e e l e c t r o d i a l y s i sp r o c e s s i s c a r r i e d o u t i n c a m e r a s w i t h c o m -b i n a t i o n a n d b i p o l a r m e m b r a n e s [ 3 , 4 ] . B e s i d e s ,w o r k s a i m e d a t r a i s i n g th e s e l e c t iv i t y o f t h ea n i o n e x c h a n g e m e m b r a n e s a r e e x e c u t e d [ 5 ] .M e a n w h i l e , a n a b i li ty o f h y d r o g e n i o n s to p a s sa c r o s s t h e a n i o n e x c h a n g e m e m b r a n e s m a y b eu s e d f o r p u r i f i c a t i o n o f a c i d s o l u t i o n s f r o mi m p u r i t i e s . A n i o n e x c h a n g e m e m b r a n e s a r eu s u a l l y c h a r a c t e r i z e d b y h i g h s e l e c t i v i t y i nr e l a ti o n s t o c a t i o n s o f m e t a l s . T h e r e f o r e , i f a n i o n

P r e s e n t e d a t t h e C o n f e r e n c e o n M e m b r a n e s i n D r i n k i n g an d I n d u s t r i al W a t e r P r o d u ct i o n , P a d s , F r a n c e , 3 - 6 O c t o b e r 2 0 0 0International W ater Association, European Desalination Society, American W ater Works Association, Japan W ater Wo rks Ass ociation0011-9164/00/$- See front matter 2000 E lsevier Science B.V . All fights reservedPII: S 0 0 1 1 - 9 1 6 4 ( 0 0 ) 0 0 1 6 4 - 8

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300 O.D. L in n iko v e t a l. / Desa l in a t io n 1 3 2 (2 0 0 0 ) 2 9 9 -3 0 6e x c h a n g e m e m b r a n e i s u s e d f o r s e p a ra t io n o f af o u l a c i d s o l u t i o n i n o n e c e l l o f t h e e l e c tr o -d i a l y s is o u t o f c l e a n w a t e r i n o t h e r c e l l, t h e n ,b e c a u s e o f a h i g h m o b i l i t y o f h y d r o g e n i o ns , th ea c i d w i l l b e g i n t o p a s s i n t o t h e w a t e r . T h e a c i ds o l u t i o n t h a t w i l l b e c r e a t e d i n th e s e c o n d c e l l o ft h e e l e c t r o d i a l y s i s , w i l l h a v e c o n s i d e r a b l y l e s si m p u r i t i e s t h a n t h e i n i t i a l o n e ( t h e f o u l a c i ds o l u ti o n ) . T h e p r o c e s s i s n o t o n l y o f p r a ct ic a l,b u t a l s o o f t h e o r e t i c a l i n t e r e s t s in c e t h e k i n e t i c so f s u c h a c i d t r a n s f e r ( u n l i k e t h e s a l t t r a n s f e r ), a sf a r a s w e k n o w , w a s n o t y e t s t u d ie d .

2. ExperimentalA l a b o r a t o r y r i g c o n s i s t e d o f a th r e e - c el l

e l e c tr o d i a l y s is . A c e n t e r c o m p a r t m e n t o f t h ee l e c t r o d i a l y s i s w a s s e p a r a t e d f r o m t h e e l e c t r o d ec e ll s b y t w o a n i o n s e x c h a n g e m e m b r a n e s . T h e r ew a s a g r a p h i t e a n o d e a n d a n o n - c o r r o s i v e s t e e lc a t h o d e i n th e s i d e c o m p a r t m e n t s o f th e e l e c tr o -d i a ly s i s. A t f i r s t o f e v e r y r u n a d e l u t e d s o l u t i o no f h y d r o c h l o r i c a c i d ( 0 . 1 8 m o l /1 ) w a s p o u r e d i n t ot h e c e n t e r c e l l o f t h e e l e c t r o d i a l y s i s . A s o l u t i o ni m i t a t i n g i n d u s t r i a l a c i d w a s t e w a t e r w a s p o u r e di n t o t h e s i d e c o m p a r t m e n t s . T h e s o l u t i o n h a d t h ef o l l o w i n g c h e m i c a l c o m p o s i t io n , i n m g / l : N a + -1 4 0 0 , K + - 1 5 4 , C a 2 + - 6 5 0 , C u 2 + - 9 0 , F e 2 + - 1 7 0 ,N i 2 + 380 , C d 2+-- 560 , S n 2+- 126 , B i 2+- 12 , P b 2+-4 , Cr to ta l - 116 .6 , Z n 2+- - 200 , H +- 1686 , C1- -6 6 , 9 0 0 . T h e c o l o u r o f t h e s o l u t i o n w a s d e e pg r e e n . T h e s o l u t i o n i m i t a t e s e l u a t e c o m p o s i t i o na f t e r t h e r e g e n e r a t i o n h y d r o g e n f i lt e rs i n p l a t i n gi n d u s t r y . I t w a s p r e p a r e d f r o m c l e a n h y d r o -c h l o r i c a c i d , a p p r o p r i a t e s a l t s , a n d d i s t i l l e dw a t e r . T h e c i r c u l a t io n o f t h e s o l u t i o n s t h r o u g ht h e c e l ls o f th e e l e c t r o d i a l y s is w a s k e p t b y ap e r i s t a l t i c p u m p . R u s s i a n s e r i a l a n i o n e x c h a n g em e m b r a n e s H M A K - 2 a n d M A K - 2 w e r e us ed[ 6 ]. T h e t r a n s p o r t n u m b e r s o f c h l o r i d e i o n s in0 . 0 1 - 0 . 0 2 N N a C I s o l u ti o n s fo r t h e se m e m -b r a n e s a r e n o l e s s t h a n 0 . 9 5 a n d 0 . 9 4 ( f r a c t i o n s ) ,r e s p e c t i v e l y [ 6] . B e f o r e t h e r u n s, th e m e m b r a n e sw e r e k e p t i n 0. 5 M H C 1 f o r t w o d a y s T h e c o n t r o l

20

16

12

8

4

0

4

2

l

20 40 60 80 100 120 140

Fig. 1. Kinetics o f the h ydroch loricacid concentrationC nagrowth in the cen ter ce l l of the electrodialysis . Thecirculation rate of the solutions through the rig w as 7.5cm3/s, v is t ime (m in) here and in Fig. 2. C urrent density I(A/m2): 1, 70; 2, 140 ; 3,21 0; 4, 0.

o f t h e e l e c tr o d i a l y s is p r o c e s s w a s e f f e c t e d b yr e m o v i n g t h e s a m p l e s o f th e a c i d s o l u t i o n f r o mt h e c e n t e r c e ll o f t h e r ig . T h e c h a n g e s i n t h e a c i dc o n c e n t r a t i o n w i t h ti m e w e r e n o t e d . B e s i d e s , a tt h e e n d o f t h e r u n s a c h e m i c a l a n a l y s i s o f t h es o l u t i o n f r o m t h e c e n t e r c e l l o f t h e e l e c t r o -d i a l y s i s w a s c a r d e d o u t . T h e m a i n i m p u r i t i e sw e r e d e f ' m e d . T h e s e i m p u r i t i e s c o u l d c a t c h i n t ot h e c e n t er c o m p a r t m e n t o f th e r i g f r o m t h e f o u la c i d s o l u t i o n i n t h e s i d e c e l l s o f t h ee l e c t r o d i a l y s i s .

T h e f i r s t e x p e r i m e n t s w e r e c o n d u c t e d w i t ht h e a n i o n e x c h a n g e m e m b r a n e s H M A K - 2 a n dc u r r e n t d e n s i ty 7 0 - 1 2 0 A / m 2. F r o m l in e s 1 - 3 i nF i g . 1 c a n b e s e e n t h a t t h e i n c r e a s e i n t h e c u r r e n td e n s i t y o f th e e l e c t r o d i a ly s i s p r o c e s s f r o m 7 0 t o1 2 0 A / m 2 d o e s n o t r e s u l t i n t r a n s f e r r a te o fh y d r o c h l o r i c a c i d a c r o s s t h e a n i o n e x c h a n g em e m b r a n e ( t h e s l o p e o f th e l i n e s is th e s a m e ) .T h i s m a y b e i n t e r p r e t e d o n l y b y d i f f u s i o nt r an s p o r t o f h y d r o c h l o r i c a c id . T h e h y d r o g e n i o n sf o r m e d i n th e a n o d e c o m p a r t m e n t o f t h e e l e c tr o -d i a l y s i s p a s s a c r o s s t h e a n i o n e x c h a n g e m e m -b r a n e s , th e c e n t e r c e ll o f t h e r i g , a n d d i s c h a r g e i nt h e c a t h o d e . H e n c e , a c h a n g e o f t h e e le c t r o d i a l y s i sc u r r e n t d e n s i t y p r a c t i ca l l y d o e s n o t i n f l u e n c e t h e

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O.D. L inn ikov e t a l . / Desa l ina t ion 132 (2000) 299-30 6 301

Table 1Chem ical analysis results o f he solutionsfrom he centercell of he rigRu n J7o I , A / m 2 Length of nan, Ty peof Concentrat ionf ions, mg/I

min membrane Na+ Cdz+ Ni 2+ Zn 2+5 70 135 HM AK-2 11.7 2.16 140 135 HM AK -2 12.5 2.57 210 135 HM AK -2 15.8 10.518 0 420 MA K-2 50.0 9.420 0 420 MA K-2 29.2 17.1

1.2 2.01.2 4.51.8 17.36.0 30.05.6 15.3

a c i d t r a n s p o r t r a te . I t m a y b e e x p e c t e d t h a t i n t h ec a s e o f t h e p r o c e s s w i t h o u t o f m i s s in g o f c u r re n tth rough the so lu t ion , t he ac id t r anspor t r a t e wi l lb e h i g h e r , s i n c e a n e j e c t io n o f t h e h y d r o g e n i o n sf r o m t h e c e n t e r c o m p a r t m e n t o f th ee lec t rod ia lys i s due to e l ec t r i c f i e ld wi ll be absen t .T h e c o n d u c t e d e x p e r i m e n t h a s c o n f i r m e d t hi s( see F ig . 1 , l i ne 4 ) . I t can be seen tha t t he s lopeo f t h e l in e 4 i s h ig h e r , t h a n t h a t o n e o f t h e l i n e s1 - 3 .

T h e c h e m i c a l a n a l y s i s o f t h e s o l u t io n s f r o mt h e c e n t e r c e l l o f t h e r i g h a s s h o w n a f a i r l y l o wi m p u r i t y c o n c e n t r a t i o n t h e r e ( T a b l e 1 ). A v i s u a le x a m i n a t i o n a l s o c o n f i r m e d th i s . I f t h e s o l u t i o n so f t h e s i d e c o m p a r t m e n t s w e r e d e e p g r e e n, t h ea c i d s o l u t io n o f t h e c e n t e r c e ll o f t h e r i g w a sco lo r l ess .

A t i m e r e c k o n i n g i n t h e s e r u n s w a s s t a r t e dw h e n a c i r c u l a ti o n r a t e o f t h e s o l u t io n s t h r o u g ht h e r i g h a d r e a c h e d a f i x e d l e v e l. T h i s e x p l a in s av e r t i c a l d i s p l a c e m e n t o f t h e l in e s w i t h r e s p e c t toeac h o ther in F ig . 1 .

F u r t h e r , m o r e p r o l o n g e d e x p e r i m e n t s w e r ec a r r i e d o u t w i t h o u t t h e c u r r e n t t h r o u g h t h es o l u ti o n . D u r i n g t h e r u n s i t w a s e s t a b li s h e d th a tt h e a n io n e x c h a n g e m e m b r a n e s H M A K - 2 p a rt lyp a s s e d t h e s o l u t i o n a c r o s s t h e m s e l v e s : t h em e a s u r e m e n t s o f v o l u m e s o f t h e a c i d s o l u ti o nsi n th e c o m p a r t m e n t s o f th e e l e c t ro d i a l y s i s a t th ee n d o f th e e a c h e x p e r i m e n t s h o w e d t h a t th ec o m m o n a c i d v o l u m e i n t h e s i d e c e ll s o f th e r i g

h a d d e c r e a s e d , an d t h e a c i d v o l u m e o f t h e c e n t e rc o m p a r t m e n t h a d r i s e n f o r a d e q u a t e v a l u e . I no r d e r t o c l a r if y w h e t h e r i t w a s c a u s e d b y o s m o t i co r s i m i l a r p h e n o m e n a , e x p e r i m e n t s w e r e c o n -d u c t e d i n t h e c o u r s e o f w h i c h o r d i n a r y d i s ti l le dwa te r c i r cu la t ed in a l l e l ec t rod ia lyzer ce l l s .N e v e r t h e l e s s , t h e r e s u lt o f th e s e r u n s w a s t h es a m e . T h e r e f o r e , t h e s o l u t i o n f l o w f r o m t h e s i d ec e l ls o f t h e r ig i n t o t h e c e n t e r o n e w a s c a u s e do n l y a m e c h a n i c a l p a s s i n g o f t h e s o l u t i o n b y t h ea n i o n e x c h a n g e m e m b r a n e s H M A K - 2 . S u b s e -q u e n t e x p e r i m e n t s w e r e c a r r i ed o u t w i t h t h e a n i o ne x c h a n g e m e m b r a n e s M A K - 2 ( t h e p r e l i m i n a r yr u n s w i t h d i s t i l l e d w a t e r h a d s h o w n t h a t t h e s em e m b r a n e s d i d n o t p a s s a s o l u t i o n ) . R e s u l t ss o m e o f t h e s e e x p e r i m e n t s i s s h o w n i n F i g . 2 ,a n d d a t a o f t h e i m p u r i t y c o n c e n t r a t io n s i n t h ea c i d s o l u ti o n f r o m t h e c e n t e r c o m p a r t m e n t o f t h ee lec t rod ia lys i s i s sho wn in Tab le 1 .

3 . D i s c u s s i o nT o a n a l y z e t h e e x p e r i m e n t a l r e s u lt s l e t u s

c o n s i d e r t h e k i n e t i c s o f h y d r o c h l o r i c a c i dt ra n s f e r t h ro u g h a n a n i o n e x c h a n g e m e m b r a n e .T h e f i r s t e x p e r i m e n t s h a v e s h o w n ( F i g . 1 ) t h a tt h e a c i d t r a n s f e r a c r o s s t h e m e m b r a n e i s d u e t oon ly d i f fus ion t ranspor t . He nce , t he re is a con-cen t r a t ion d is t r ibu t ion o f the d i f fus ing su bs tance( s o l u t e ) c l o s e b y t h e m e m b r a n e s u r f a c e . T h e

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30 2

C1

O.D. Linnikov et al. / Desalination 132 (2000) 299 -3 064~ , 3

16 ~ 2

' t40 8O 120 160 2O0 240 280 32O 360 40O 44O 480

Fig. 2. Kinetics of the hydrochloricacid concenlmtionCHcl(g/l) grow th in the center cell o f the rig in th e experimentswithout the curre nt throug h the s olution. 1, run 21; 2, run20; 3, run 19; 4, run 18.

s c h e m e o f t h e d i s t r ib u t i o n a t t h e s t a t i o n a r ycond i t ions i s shown in F ig . 3 . The so lu te ( in ourc a s e t h i s i s h y d r o c h l o r i c a c id ) w i t h c o n c e n t r a t i o nC 1 d i f f u s e s t h r o u g h t h e m e m b r a n e f r o m t h e c e l lo f v o l u m e V 1 i n to t h e c e ll o f v o l u m e V 2 w h e r ethe so lu te concen t r a t ion i s l ower and i s C2 .N e a r b y t h e m e m b r a n e s u r f a c e, t h e s o l u te c o n c e n -t rat ions are C11 and C2 ~, respe ct ively . Th e dist r i -b u t i o n o f t h e s o l u t e w i t h i n t h e m e m b r a n e i s n o ts h o w n i n F i g . 3.

W h e n t h e d i f f u s i o n p r o c e s s is s t a t io n a r y , t h et r an s f e r o f th e s o lu t e a c ro s s t h e m e m b r a n e m a yb e d e s c r i b e d b y e q u a t i o n s :

"1 dm - K , ( C , ) ( 1 )S d z1 d m _ f l ( C ; - C ~ ) ( 2 )S d z1 d m - K 2 ( C ~ - C 2 ) (3 )S d z

Fig. 3. Sch em e o f the concentration distribution of thediffusing substance solute near by the mem brane surfacewhen the diffusionprocess is stationary. I is the mem branethickness; 61, 52 are the thicknesses o f no nm ixed layers o fthe solution.

t o t h e s u r f a c e o f th e m e m b r a n e ( m / s ); 12 i s t hem a s s t r a n s f e r c o e f f i c i e n t o f t h e s o l u t e f r o m t h em e m b r a n e s u r f a c e i n t o t h e s o l u t i o n v o l u m e V 2( m / s ) .

O n e c a n s o l v e s i m u l t a n e o u s l y t h e e q u a t i o n s( 1 ) - ( 3 ) a n d t h e n o b t a i n a n e x p r e s s i o n f o r t h es o lu t e c o n c e n t ra t io n n e a r t h e m e m b r a n e s u r f ac ei n t h e c e l l o f v o l u m e V 2 :C~ = K lflC ' + K~K2C2 + K2flC2 (4 )

K l f l + K , K 2 + K 2 f l

w h e r e S i s t h e a r e a o f th e m e m b r a n e ( m 2); fl i st h e m a s s t r a n s f e r c o e f f i c i e n t o f t h e s o l u t e a c ro s sthe m em bra ne (m/s ) ; "r i s t ime ( s ) ; m i s t he m asso f t h e s o l u t e ( g ) ; /1 i s t h e m a s s t r a n s f e r c o -e f f ic i e n t o f t h e s o l u te f r o m t h e s o l u ti o n v o l u m e V 1

Subs t i tu t ion o f Eq . ( 4 ) i n to express ion (3 )g i v e s a n e q u a t i o n d e s c r i b i n g t h e d i f f u s i o nt r a n s f e r o f t h e s o l u t e a c r o s s t h e m e m b r a n e :

1 d m _ K ( C , - C 2 ) ( 5 )S d v

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O.D. L inn ikov e t aL / Desa l ina t ion 132 (2000) 299-306 303w h e r e

1K = (6)1 1 1+ - - + - -

K 1 K 2 /3When Nerns t ' s non-mixed d i f fus ion layers

concept ion is used (Fig. 3) , then KI and K2 areequa l toK I = D / r s (7 )K 2 = D / 8 (8 )wh ere t~l and ~2 are the thick nesses o f non-mix edlayers (m).Similar ly , f rom the f i rs t Fick 's diffusion lawfo l lows tha tf l = D m / l (9 )where D is the solute diffusion constant in theso lu t ion (m2/s ) ; Dm is the solute diffusionconstant in the membrane substance (m2/s) ; I isthe me mb rane th ickness (m) .

Tak ing in to cons idera t ion tha td m = V 2 d C2 (10)

C , = C - - - ~ . ( C 2 - C O (11)z ~after integrat ion of Eq. (5) , one can obtain f inal ly

I n =c O - c o

c o - c o2 V I

t v ,

w he re C~l, C~2 are the initial solute con centra tionsin the ce l l s o f vo lum e V 1 a n d V 2 , respect ively; Kis a gen eral mass t ransfer c oeff ic ient (m/s) .

The equa t ion co inc id ing wi th Eq . (5 ) anddescr ibing a dialysis process is given in wor k [7]without deduct ion. In [8] , an equat ion is der ivedanalogous to Eq. (12) , but i ts der ivat ion was m adewithout considerat ion diffusion diff icul t ies forthe solute t ransfer in the solut ion. Besides , i t wasassumed that the solubi l i ty of the diffusingsubs tance ( so lu te ) in the me mbran e mate r ia l wasa l inear funct ion of the diffusing substance(solute) concentrat ion in the solut ion. Also,integration in [8] wa s carr ied out with an error sothe de r ived equat ion w as incorrect [8 , p .60] .

In work [9 ] , the der iva t ion o f the equa t ionanalogous to Eq. (5) was mad e, as wel l as in [8] ,without account of the diffusion diff icul t ies forthe solute t ransfer in the solut ion. Thereto, i t wassupposed tha t the so lu te was adsorbed by themembrane sur face in accordance wi th Henry ' slaw. I t shou ld be no ted tha t in the abovederivation of Eqs. (5),(12), the assumptions aboutthe solute adsorbt ion, or the solute dis t r ibut ionand so lub i li ty in the mem brane mate r ia l were no tmade .

In [10], the experimen tal data on the diffusionof ammonium hydrox ide ac ross an an ionexchange membrane were ca lcu la ted on theequat ion s imilar to Eq. (12) which was givenwithout deduct ion. I ts dis t inct ion from Eq. (12)i s tha t V1= V2 and K=D m/1 . There fore , theinf luence of the diffusion t ransfer of the solutef rom the so lu t ion vo lume to the membranesurface on the general kinet ics of the processagain was no t taken into account .

Com put ing o f the da ta p resen ted in F ig . 2showed tha t they were approx imated wel l byEq. (12) ( the correlat ion factor was in the rang eof 0 .998-0 .999) . The ca lcu la ted genera l masstransfer coeff ic ients K of hydrochlor ic acidth rough the examined membrane a re p resen tedin Tab le 2 . They c lea r ly demons t ra te tha tEqs . (5 ) , (12) can be used fo r descr ip t ion o f thekinet ics of the electrolyte ( in our case this ishydro chlor ic acid) transfer process across the ionexchange membrane . As in the runs 18 and 20

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3 0 4 O . D . L i n n i k o v e t a l. / D e s a l i n a t io n 1 3 2 ( 2 0 0 0 ) 2 9 9 - 3 0 6

Tab le 2Calculated results of the general mass transfer coeff icient K of the hydrochlor ic acid through the anion exch ange m em braneM A K - 2Run 3f_o Circu lation ate of he solutions V~, V2, S, K . 1 07, Sn"107, n

throu gh the rig, cm3/s rrl3 m 3 m 2 m/s m]s18 7.5 0.0011 0.00055 0.02862 4.786 0.058 1019 0.4 0.0011 0.00055 0.02862 4.105 0.089 1020 7.5 0.0006 0.00055 0.01431 4.683 0.060 1021 0.4 0.0 00 6 0 .0 0 0 5 5 0.01431 3.987 0.035 10

the c i rcu la t ion ra te o f the so lu t ions th rough ther ig was 7 .5 cm3/s , the a rea S o f the me m brane swa s d i f f e re n t i n two t ime s , s o lu t i o n v o lu m e s V1o f t h e s i d e c e l l s we r e a l s o d i f f e re n t i n two t ime s ,bu t ca lcu la ted genera l mass t rans fe r coef f ic ien tsK o f h y d r o c h lo r i c a c id i n th e s e e x p e r ime n t s we r ee q u a l. A n a lo g o u s e q u a l it y o f t h e g e n e r a l ma s st ransfe r coef f ic ien ts o f hydroch lo r ic ac id takesp l a c e i n t h e r u n s 1 9 a n d 2 1 . An d wh e n th ec i rcu la t ion ra te o f the so lu t ions i s low then thegenera l m ass t rans fe r coef f ic ien t K i s low too(Tab le 2 ) . I t shou ld be no ted tha t the exper i -me n ta l d a t a f o u n d h e r e ( i n t h i s s t u d y ) d o n o tc o n f i r m th e e q u a t io n f o r t h e d i f f u s io n t r a n sf e r o fth e e l e c t ro ly t e a c r o s s a n i o n e x c h a n g e m e mb r a n etha t was g iven in [9 ] ( the equa t ion su ppose s tha tt h e d i f f u s io n f l u x o f t h e s o lu t e mu s t b e i n d i re c tp r o p o r t i o n to d i f f e r e n c e o f two s q u a re s o f t h es o lu t e c o n c e n t r a t i o n o n t h e d i f f e r e n t me mb r a n esides) .

T h e g e n e r a l ma s s t r a n sf e r c o e f f ic i e n t s o f t h eimpur i ty molecu les in runs 18 , 20 ca lcu la ted byEq . (12) (us in g the da ta o f Tab le 1 ) a re p resen tedin Tab le 3 . As can be seen , in two cases (CdCI2and NiCI2) va lues K, ca lcu la ted on thee x p e r ime n ta l d a t a o f th e r u n s 1 8 an d 2 0 , we r eno t equa l . I t m ay be caused by a che mica l ana lysise r ro r . At la rge , the genera l mass t rans fe rc o e f f i ci e n t s o f t h e imp u r i t y mo le c u l e s t h r o u g ht h e a n i o n e x c h a n g e m e m b r a n e w e r e i n o rd e r o fm a g n i t u d e b e l o w t h a n t h a t o n e o f h y d r o c h lo r i c

Table 3Calculated results o f the general ma ss transfer coefficientsof the impurity molecules through the anion excha ngem e m b r a n e M A K - 2R u n N _ o K . 1 0 8 ,m /s

NaC1 CdCI2 NiCI2 ZnC1218 2.80 1.30 1.22 12.9620 3.20 4.80 2.28 12.62Av erag e 3.00 3.05 1.75 12.79

ac id . Th is a l lows us ing the p rocess fo r regene-r a t i o n o f h y d r o c h lo r i c a c id f r o m a c id wa s t e -wate rs . O n the bas is o f the da ta in Tab le 3 , thee x a n ima te mo le c u l e s o f imp u r i t i e s ma y b ea r r a n g ed in o r d e r o f m a g n i tu d e i n a c c o r d a n c ewi th the ab i l i ty to t rans fe r th rough the an ione x c h a n g e me mb r a n e ( t h e ma s s t r a n s f e r r a t e o fth e imp u r i t y mo le c u l e s a c r o s s t h e a n io ne x c h a n g e m e m b r a n e i n c r e a s e s f r o m l e f t t o r i g h t) :NiCI2 < N aC I _< CdC12 < ZnC12

C o m p a r i s o n o f t h e r e s u l ts o b t a in e d w i th t h eda ta o f o ther au thors [8 ,10-14] i s d i f f icu l tbecause the i r s s tud ies we re ca r r ied ou t wi th o therty p e s o f i o n e x c h a n g e m e mb r a n e s . B e s id e s , i t i sn o t a lwa y s c l e a r wh a t r e g ime , o u t e r d i f f u s io nprocess ( the genera l ra te o f the p ro cess i sd i r e c t e d b y t r a n s f e r o f s o lu t e f r o m th e b u lk o f

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O . D . L i n n i k o v e t a l. ~ D e s a l i n a t i o n 1 3 2 ( 2 0 0 0) 2 9 9 - 3 0 6 3 0 5s o l u t i o n t o t h e s u r f a c e o f th e m e m b r a n e ) , o ri n s i d e d i f f u s i o n p r o c e s s ( t h e g e n e r a l r a t e o f t h ep r o c e s s i s d i re c t e d b y d i f f u s i o n o f th e s o l u t ea c r o s s t h e m e m b r a n e ) , t h e m e a s u r e m e n t s w e r em a d e .

N e v e r t h e l e s s , t h e r e s u l t s o b t a i n e d a l l o wm a k i n g a p p r o x i m a t e d e s i g n o f a p p ar a tu s a n dp a r a m e t e r s o f t h e p u r i f i c at i o n p r o c e s s o fh y d r o c h l o r i c a c i d a t i t s r e g e n e r a t i o n f r o m a c i dw a s t e w a t e r s .

c l , -

CH( Z - -D

DmIK

4 . C o n c l u s i o n s1 . I n t h e e x p e r i m e n t a l c o n d i t i o n s e x a m i n e d ,

h y d r o c h l o r i c a c i d t r a n s f e r t h r o u g h t h e a n i o ne x c h a n g e m e m b r a n e i s r e a l i z e d b y d i f f u s i o n ,a n d i s in d e p e n d e n t o f t h e c u r r e n t d e n s i t y att h e e l e c t r o d i a l y s i s p r o c e s s .

2 . T h e g e n e r a l m a s s t r a n s f e r c o e f f i c i e n t o fh y d r o c h l o r i c a c i d a c r o s s t h e a n i o n e x c h a n g em e m b r a n e i s i n o r d e r o f m a g n i t u d e a b o v et h a n t h o s e o n e s o f m o l e c u l e s o f N a C 1 , C d C I2 ,N i C l 2 , a n d Z n C l 2 a t t h e s a m e e x p e r i m e n t a lc o n d i t i o n s . T h i s a l l o w s u s i n g t h e p r o c e s s f o rr e g e n e r a t i o n o f h y d r o c h l o r i c a c i d f r o m a c i dw a s t e w a t e r s .

3 . I n a c c o r d a n c e w i t h t h e a b i l i t y t o t r a n s f e rt h r o u g h t h e a n i o n e x c h a n g e m e m b r a n e t h ee x a n i m a t e m o l e c u l e s o f th e i m p u r i t ie s m a yb e a r r a n g e d i n o r d e r o f m a g n i t u d e (t h e m a s st r a n s f e r r a te o f th e i m p u r i t y m o l e c u l e s a c r o sst h e a n i o n e x c h a n g e m e m b r a n e i n c r e as e s fr o mle f t t o r i gh t ) :N iCI 2 < N a CI < CdC12 < Z nC12

g l

5 . S y m b o l sC1, C2 - - S ol u t e c o n c e n t r a t i o n s i n t h e c e l l s o f

v o l u m e I"1 a n d V :, r e s p e c t i v e l yC ~l, C~2 - - I n i t i a l s o l u t e c o n c e n t r a t i o n s i n t h e

c e l l s o f v o l u m e / /'1 a n d V 2, r e s p e c -t i v e l y

K2

LMNSs.Vl,V2

S o l u te c o n c e n t r a ti o n s n e a r t h e m e m -b r a n e s u r f a c e i n t h e c e l ls o f v o l u m eV1 a nd V 2, r e sp e c t i v e lyC o n c e n t r a t i o n o f h y d r o c h l o r i c a c i d ,g / 1S o l u t e d i f f u s i o n c o n s t a n t i n s o l u t io n ,m 2 / sS o l u t e d i f f u s i o n c o n s t a n t i n t h em e m b r a n e s u b s ta n c e , m 2 /sC u r r e n t d e n s i ty , A / m 2G e n e r a l m a s s t r a n s f e r c o e f f i c i e n t ,rn/sM a s s t r a n s f e r c o e f f i c ie n t o f t h e s o l u t ef r o m t h e s o l u t i o n v o l u m e V1 t o t h es u rf a ce o f t h e m e m b r a n e , m / sM a s s t r a n s f e r c o e f f i c ie n t o f t h e s o l u t ef r o m t h e m e m b r a n e s u r f a c e i n t o t h es o l u t i o n v o l u m e V 2, m / sM e m b r a n e t h i ck n e s s , mM a s s o f t h e s o l u te , g 'N u m b e r o f m e a s u re m e n t s o f KA r e a o f th e m e m b r a n e , m 2S t a n d a r d d e v i a t i o n o f K , m / sS o l u t i o n v o l u m e s o f th e s i d e c e l l s o f

the r i g , m 3G r e e k

~2

- - M a s s t r a n s f e r c o e f f i c ie n t o f t h e s o l u t ea c ro s s t h e m e m b r a n e , m / s

- - T h i c k n e s s e s o f n o n m i x e d l ay e rs , m- - T i m e , s

R e f e r e n c e s[1] A.F.Mazanko, G.D. Kamarjan and O .D. Romashin,

Indust rial m em br an e e lea t rolys is . Chem is t ry ,M oscow , 1989 (in Russian).

[2] Indu str ial processes with the use o f mem branes.Nauka, M oscow, 1976 ( in R ussian).

[3] V .P . Greben, N.J. Pivovarov, I .G. Rodzik and N.J.Kovarskii, Zh PK h, 64 (4) (1991) 794 (in Russian).

[4] V .P . Greben, N. J . P ivovarov and I .G . Rodz ik,Proc. , Russian conference on Solid State Chem istry

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306 O . D . L i n n i k o v e t a l. / D e s a l i n a t i o n 1 3 2 ( 2 0 0 0 ) 2 9 9 - 3 0 6and New Mater ia ls , Ekater inburg, 1 (1996) 64 ( inRussian) .

[5] R. Simons, Desalination, 78 (1990) 297.[6] M emb ranes and mem brane techniques : Sup pl . to

cata logue, NIITACH IM, Cherkassi , 1 99 0 ( inRussian) .[7] S . -T . Hw ang and K. Kam merm eyer , Mem branes inSepa ra t ion . John W iley & Sons , New Y ork , 1975 .

[8] N.I. Nikolaev , Diffusion in M em branes. Chem istry,Moscow , 1980 (in Russian) .

[9] Ju .G . Fro lov , Course o f Col lo id Chemis try .Chem islry, M oscow, 1982 ( in Russian).

[10] S. Sethu, J . Appl. Polym . Sei. , 8(5) (1964) 2249.[11] C. M . Peterson and E.M. Livingston, J . Appl. Polym.

Sei., 8(3) (1964) 1429.[12] M.A . Peterson and H.P. G regor , J . Eleelrochem. Soe.,106 (12) (1959) 1051.[13] N. L akshminarayanaiah, Chem . Rev., 65 (5) (1965)

491.[14] S.A . Meehkovskii , J . Phys. Chem ., 52(4) (1978) 1084

(in Russian).