Kinetics of Ozonation of Cyanides

electroplating and case hardening of steel parts. The problem of the disposal of cyanide waste has become increasingly important because sewage dispos...
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Kinetics of Ozonation of Cyanides K. K. KHANDELWAL , A. J. BARDUHN, and C. S. GROVE, Jr. 1

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Syracuse University, Syracuse 10, Ν. Y.

Rates of oxidation of aqueous cyanide solutions by ozonized oxygen have been determined under a variety of controlled conditions. The reaction rate constant is more than doubled when copper(II) ions are a d d e d as a catalyst, but increasing copper(II) concentration does not markedly affect the reaction rate. The rate is not affected by copper sulfate, nitrate, acetate, or sulfide, or by temperatures from 13° to 3 0 ° C.

T h e first w a s t e c y a n i d e s o l u t i o n s of a n y significance were p r o d u c e d i n t h e g o l d a n d silver m i n i n g industries, where cyanides were used t o dissolve t h e metals f r o m t h e i r ores. S i n c e t h a t t i m e t h e use of c y a n i d e s h a s e x p a n d e d s i g n i f i c a n t l y i n t o t h e field of e l e c t r o p l a t i n g a n d case h a r d e n i n g of steel p a r t s . T h e p r o b l e m of t h e d i s p o s a l of c y a n i d e w a s t e h a s b e c o m e i n c r e a s i n g l y i m p o r t a n t because sewage d i s p o s a l r e g u l a t i o n s i n m a n y c o m m u n i t i e s r e q u i r e t h e r e m o v a l of wastes t h a t a r e t o x i c t o p l a n t a n d a n i m a l l i f e . V a r i o u s m e t h o d s f o r t r e a t i n g c y a n i d e c o n t a i n i n g i n d u s t r i a l wastes h a v e b e e n u s e d . D o d g e a n d R e a m s (3) h a v e w r i t t e n a n e x t e n s i v e l i t e r a t u r e r e v i e w o n t h i s s u b j e c t f o r t h e A m e r i c a n E l e c t r o p l a t e r s ' S o c i e t y . T h e s e m e t h o d s h a v e b e e n classified i n t o t w o general groups: p h y s i c a l methods a n d chemical methods. O n e i m p o r t a n t c h e m i c a l t r e a t m e n t of s u c h w a s t e is t r e a t m e n t w i t h gaseous ozone. T h i s m e t h o d has n o t received adequate attention. Ozone has become i m p o r t a n t i n r e c e n t y e a r s as a n i n d u s t r i a l r a w m a t e r i a l f o r t h e c h e m i c a l i n d u s t r y . I t i s n o w a v a i l a b l e i n t o n n a g e q u a n t i t i e s a t a c o m p e t i t i v e cost f o r use as a n o x i d i z i n g agent a n d as a c h e m i c a l r a w m a t e r i a l . A n excellent c o m p i l a t i o n of h i s t o r y , g e n e r a t i o n , a n d p r o p e r t i e s of ozone h a s b e e n p r e p a r e d b y H a n n a n d M a n l e y (5). S t u d i e s of ozone a n d c y a n i d e r e a c t i o n s h a v e b e e n p u b l i s h e d b y N e u w i r t h ( 7 ) , T y l e r a n d c o w o r k e r s (8), a n d W a l k e r a n d Z a b b a n (9). T h e chief r e s u l t s of these i n v e s t i g a ­ t i o n s w e r e m a t e r i a l b a l a n c e s w h i c h s i m p l y r e l a t e d t h e s i m u l t a n e o u s d i s a p p e a r a n c e of ozone a n d c y a n i d e u n d e r v a r i o u s c o n d i t i o n s (6). Theoretical

Considerations

A gaseous m i x t u r e of o x y g e n a n d ozone is b u b b l e d t h r o u g h a vessel c o n t a i n i n g a n a q u e o u s s o l u t i o n of c y a n i d e i o n . T h e ozone dissolves i n t h e l i q u i d a n d t h e n reacts w i t h t h e cyanide ions. I f the reaction i n the l i q u i d solution c a n be represented b y t h e following equation: Present address, Research and Development Division, Rice Barton Corp., Worcester, Mass. 1

78

79

KHANDELWAL, BARDUHN, A N D GROVE-KINETICS OF CYANIDE O Z O N A T I O N

n C N - + m 0 + (zs)H 0 -> n C N O " + (zs)H 0 + products 3

2

2

t h e r a t e of c y a n i d e d i s a p p e a r a n c e c a n b e expressed as : d[CNάθ

=

fc'[0.]«[CN-]»

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I t w a s a s s u m e d i n c o r r e l a t i n g t h e d a t a t h a t t h e ozone c o n c e n t r a t i o n i n t h e l i q u i d r e m a i n s s u b s t a n t i a l l y c o n s t a n t a f t e r t h e first m i n u t e o r t w o of t h e s t a r t of t h e r e a c ­ t i o n , because : T h e c o n c e n t r a t i o n of ozone i n t h e gas phase w a s m a i n t a i n e d f a i r l y c o n s t a n t a n d r a n g e d f r o m 70 t o 9 0 m g . of ozone p e r l i t e r of o x y g e n . I n m o s t of t h e d a t a r e p o r t e d here, t h e r a t e of ozone a p p l i c a t i o n w a s 2 0 t o 2 5 m g . per minute. T h e s o l u b i l i t y of ozone, i n w a t e r a t 2 0 ° C , is l o w (25 m g . p e r l i t e r of w a t e r ) . T h e r a t e c o n s t a n t s p r e s e n t e d a r e f a i r l y consistent w h e n b a s e d o n t h i s a s s u m p t i o n , w h i c h seems p l a u s i b l e . H o w e v e r , t h e o b s e r v e d r e a c t i o n r a t e m a y d e v i a t e s o m e w h a t f r o m a c a l c u l a t e d r a t e d u r i n g t h e first m i n u t e o r t w o of t h e r e a c t i o n t i m e . C o n s e ­ q u e n t l y , o n t h e a s s u m p t i o n t h a t t h e ozone c o n c e n t r a t i o n i n t h e s o l u t i o n is c o n s t a n t , the r a t e e q u a t i o n c a n b e s i m p l i f i e d b y c o m b i n i n g t h e c o n s t a n t s , Κ = Jfe'[0 ]8

T h u s , the n e w rate equation becomes: d[CNάθ

X[CN-]»

Apparatus A Welsbach T - 2 3 M o d e l laboratory ozonator was utilized to produce ozonized oxygen f r o m cylinder oxygen w h i c h h a d been scrubbed to remove carbon dioxide. F i g u r e 1 s h o w s t h e g e n e r a l features of t h e e x p e r i m e n t a l e q u i p m e n t , w i t h t h e 1 5 0 0 - m l . WELSBACH OZONATOR MODEL T-23 ,

ί

PRESSURE GAGE PRESSURE REGULATOR \ SHUT-OFF VALVE AIR j 100 psia ^ CYLINDER OXYGEN

Ά

MERCURY TRAP

-PRESSURE GAGE THERMOMETER

VOLTMETER' GAS ROTAMETER

I-

REGULATOR AND GAGE

7

PYREX REACTOR 1500 πα

A'

NaOH SOLUTION TO DESTROY OZONE

CONSTANT COLD WATER FEED DRAIN

ο (} < » < ι < >| DRY GAS FEED TO OZONATOR

J_JJ

L

20 C CONSTANT TEMP­ ERATURE BATH CON­ TROLLED BY THERMO­ STAT AND RELAY e

"CARBOXATE" U-TUBE EXIT WATER

SILICA GEL GAS DRYER

26" MERCURY MANOMETER (SAFETY VALVE)

EXHAUST GAS - ABS0R3ER 1+00 c c

SAMPLE GAS ABSORBER ,\ 1*00 c e . I

c LINE PRESSURE 10 psig

.NaOH SOLUTION "-TO DESTROY

WATER ROTAMETER

^ COLD -®~TAP WATER

^1 WET TEST METER NO.

Figure 1.

Flow diagram of apparatus for ozonation of cyanides

b o r o s i l i c a t e glass r e a c t o r used f o r b a t c h r e a c t i o n s . T h e 1500-ml. absorption tower p a c k e d w i t h b o r o s i l i c a t e glass r i n g s a n d e q u i p p e d w i t h a p u m p t o r e c y c l e t h e s o l u t i o n f r o m t h e b o t t o m t o t h e t o p of t h e t o w e r is n o t s h o w n .

80

A D V A N C E S IN

Analytical

CHEMISTRY SERIES

Procedures

T h e i o d o m e t r i c m e t h o d f o r t h e d e t e r m i n a t i o n of ozone c o n c e n t r a t i o n i n o x y g e n consists of a b s o r b i n g t h e s a m p l e c o n t a i n i n g ozone i n a 2 % s o l u t i o n of p o t a s s i u m i o d i d e . T h e i o d i n e p r o d u c e d is t i t r a t e d w i t h s t a n d a r d p o t a s s i u m t h i o s u l f a t e s o l u t i o n , u s i n g s t a r c h as a n i n d i c a t o r .

OZONE CHEMISTRY AND TECHNOLOGY Downloaded from pubs.acs.org by UNIV LAVAL on 07/15/16. For personal use only.

T h e pyridine-benzidine colorimetric method the c y a n i d e i n s o l u t i o n .

(1, 2) was u s e d f o r d e t e r m i n a t i o n of

T h e 1 5 0 0 - m l . s a m p l e of 100 p . p . m . of p o t a s s i u m c y a n i d e as C N ~ ( c y a n i d e i o n s ) a q u e o u s s o l u t i o n , w i t h p o t a s s i u m h y d r o x i d e a d d e d t o raise t h e p H , was i n t r o d u c e d i n t o the reactor (bottle t y p e or a b s o r p t i o n c o l u m n ) . T h e d e s i r e d c a t a l y t i c s o l u t i o n was a d d e d b y m e a n s of a p i p e t . T h e o z o n i z e d o x y g e n was passed i n t o t h e r e a c t o r a n d t h e e x h a u s t gas was m e a s u r e d b y a w e t - t e s t m e t e r , a f t e r t h e excess ozone h a d b e e n s c r u b b e d o u t b y a p o t a s s i u m i o d i d e s o l u t i o n . A t t h e s a m e t i m e a s e p a r a t e s a m p l e of e x i t gas was a n a l y z e d b y u s i n g p o t a s s i u m i o d i d e s o l u t i o n a n d m e a s u r i n g t h e gas w i t h a gas m e t e r . T h e gas s t r e a m t o t h e r e a c t o r was l e d i n t o a n o t h e r gas a b s o r p t i o n b o t t l e w i t h p o t a s s i u m i o d i d e s o l u t i o n a t definite i n t e r v a l s , so t h a t s a m p l e s of t h e r e s i d u a l c y a n i d e s o l u t i o n c o u l d be w i t h d r a w n . T h e c o n c e n t r a t i o n of ozone i n t h e o x y g e n w a s c a l c u l a t e d as m i l l i g r a m s of ozone p e r l i t e r of o x y g e n . T h e a m o u n t of ozone t o be a p p l i e d t o t h e c y a n i d e s o l u t i o n was d e t e r m i n e d f r o m t h i s . T h e c y a n i d e c o n c e n t r a t i o n as a f u n c t i o n of t i m e was c a l c u l a t e d f r o m the periodic cyanide analyses. V a r i a b l e s S t u d i e d . E f f e c t of c o p p e r ( I I ) c o n c e n t r a t i o n f r o m c o p p e r s u l f a t e o n the reaction rate. E f f e c t o n t h e r e a c t i o n of c o p p e r ( I I ) f r o m s a l t s — c u p r i c c h l o r i d e , n i t r a t e , acetate, a n d sulfate. E f f e c t o n t h e r e a c t i o n r a t e due t o t h e p h y s i c a l m a n n e r i n w h i c h ozone a n d c y a n i d e interact, i n a reactor bottle or a p a c k e d c o l u m n . E f f e c t o n t h e r e a c t i o n r a t e due t o c a t i o n s of c o p p e r ( I I ) , n i c k e l ( I I ) , m a n g a n e s e ( I I ) , a n d c a d m i u m ( I I ) sulfate. E f f e c t of s o l u t i o n t e m p e r a t u r e o n r e a c t i o n r a t e o v e r t h e r a n g e 13° t o 30° C . D i s c u s s i o n of

Results

A series of r u n s was m a d e t o s t u d y t h e o z o n a t i o n o f c y a n i d e u n d e r a v a r i e t y of c o n d i t i o n s . F i g u r e s 2 t o 5 are g r a p h i c a l r e p r e s e n t a t i o n s o f r e s u l t s f r o m r u n s 55 t o 8 3 .

Curve No.

κ.

\

V

Ν

Ν

Ν V

ν

4

]ppm Cu "*

Rate Constant Κ

Symbol

1

0

0.90

Ο

2

1.33

2.00

β

3

2.67

2.20

θ

Data From Run No's. 61 to 70 55 t o 60 71 and

72

oN

1

c;

s c

Ό V

Ή

1

J

«

Reaction Time θ Minutes :

igure 2.

Effect of

copper(il) sulfate on ozonation rate of cyanide in water solution

potassium

KHANDELWAL, BARDUHN, A N D GROVE-KINETICS OF CYANIDE

ve •

Type of Reactor Bottle

2.67

2.20

β

Data From Run No's. 75 to 77*

2.67

1.90

Ο

73 and

ppm C u

44-

Rate Constant

J*Q

Packed Tower

80

81

OZONATION

Symbol

-

Ik

•Note: Curve No. 1 i s the calculated curve f o r K=2.20. The experimental points shown are f o r the runs i n which S0£ ions were substituted by the following ions and the Rate Constant remained unchanged: ( C l " ) , (NO"^^ > and ( A c " ) · 2

2

OZONE CHEMISTRY AND TECHNOLOGY Downloaded from pubs.acs.org by UNIV LAVAL on 07/15/16. For personal use only.

60

16

2k

20

28

Reaction Time θ Minutes

Figure 3. Effect of type of reactor used a n d different anions of copper salts on ozonation rate of potassium cyanide in water solution

Curve No.

2.67 ppm cations

1

Cu

2

Ni *

3

Μη "

1+

Cd

Rate Constant Κ

Symbol

Data From Run No's.

2.20

β

71 and 72

1.50

e

78

1.33 0.95

®

80

0

79

4 4

4

44

16

4 4

20

2k

Reaction Time 0 Minutes

Figure 4.

Metal sulfates used as catalysts on ozonation rate of potas­ sium cyanide in water solution

A l l r u n s w e r e c o n d u c t e d a t 2 0 ° C , w i t h t h e e x c e p t i o n of r u n s 81, 8 2 , a n d 8 3 . T h e p H was m a i n t a i n e d a t 11.3 ± 0.5 i n o r d e r t o h o l d t h i s v a r i a b l e c o n s t a n t . T h e s e r u n s w e r e a l l c o n d u c t e d i n t h e b o t t l e - t y p e r e a c t o r , w i t h t h e e x c e p t i o n of 73 a n d 74, w h i c h w e r e conducted i n the packed absorption tower. I n F i g u r e 6 t y p i c a l d a t a a r e p l o t t e d as r e s i d u a l c y a n i d e i n s o l u t i o n vs. r e a c t i o n time. Rate of K C N disappearance = r = —

= — du

du

w h e r e r = slope of c u r v e i n F i g u r e 6. Now, r = Z [ C N - ] w h e r e η is t h e o r d e r of r e a c t i o n w i t h respect t o t h e c y a n i d e c o n c e n t r a t i o n . M

ADVANCES IN CHEMISTRY SERIES

82 ! Curve No.

:

,

1

!

1

1

Reaction Temp. Rate Constant deg. Centigrade Κ

1

I

!

Symbol

1

i

Data From Run No's.

1

13

2.15

2

20

2.20

Not shown

J

Ο an α

d

on

0

81

d 03 ar

^1 2

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Ο

^3

16 20 2k Reaction Time θ Minutes

Figure 5.

Effect of temperature on ozonation rate of potassium anide in water solution

cy­

2.67 p.p.m. Cu(SOi) used as catalyst T o find n, t h e f o l l o w i n g s i m p l i f i c a t i o n is c o n d u c t e d : Log r = η log [ C N " ] + log Κ F i g u r e 7 s h o w s t h e p l o t of r e s i d u a l p o t a s s i u m c y a n i d e as p a r t s p e r m i l l i o n of c y a n i d e vs. r e a c t i o n r a t e r. T h e slope of these lines is c o n s t a n t a n d is expressed a s : Slope = η =

A (log r) A(log C N - )

C u r v e 4 i n F i g u r e 7 i s t h e c a l c u l a t e d r e s u l t of d a t a s h o w n i n F i g u r e 6. C u r v e s s i m i l a r t o F i g u r e 6 w e r e o b t a i n e d f o r a l l t h e d a t a a n d c a l c u l a t i o n s of slope r a t v a r i o u s

90 Symbo1 8o

0 θ

g I

7

Data Froiη Run No. 73 7^

0

10 ο

1» a

i " 1

3 0