Optimización de Tioureación Alcalina

8/8/2019 Optimización de Tioureación Alcalina

http://slidepdf.com/reader/full/optimizacion-de-tioureacion-alcalina 1/5

Vol. 10 No. 4 Dec. 2003 J . C E N T . S O U T H U N I V . T E C H N O L .

Article ID : 1005-9784 (200 3) 04 -0292-05

O p t i m i z a t i o n o f e f f i c i e n t s t a b l e r e a g e n t o f

a l k a l i n e t h i o u r e a s o l u t i o n f o r g o l d l e a c h i n g

W A N G Y u n - y a n ( q : N,~,~, , C H A I L i - y u a n ( ~ X ' f _ ~ ; ) ,

M I N X i a o - b o ( N z j x ~ ) , H E D e - w e n ( t 3 ] ' ~ .3 ~ ) , P E N G B in g( g' ~ >-~.)

( S c h o o l o f M e t a l l u r g i c a l S ci e n c e a n d E n g i n e e r i n g , C e n t r a l S o u t h U n i v e r s i t y , C h a n g s h a 410083, C h i n a )

A b s t r a c t : Na2 SiO3 and Naz SeO~ were chosen as stable reage nts of alkaline thiourea solution substi tuting Naz SO3,

according to the st ructu re-p rope rty relationship betwe en the stabil i ty of alkaline thiourea and the struc ture of thiou-

rea and sulfi te ion, and the effect of the stable reagen ts on stabil i ty of alkaline thiourea w as investigated. Th e results

show that contra ry to Na2 SeO3, Na2 SOs and Na2 SiO3 affect the stabil i ty of alkaline thiourea solution rem arkably .

Th e stable effect of NazSiO3 on alkaline thiourea is obviously b etter than that of Na2SO3. Th e stable reag ents

Naz SO3 and NazSiO3 decrease the decomposition rate o f alkaline thiourea solution g reatly , and the decomposition

rate of alkaline thiourea reduces from 7 2 . 5 % to 33. 8% with addition of 0.3 mol • L -1 Na zSiO ~. Dissolution cu r-rents of gold in the alkaline thiourea solution containing N azSO 3 and N azSiO3 are 2.0 m A . cm z and 3.5

m A • cm z at the po tential of 0.42 V , respec tively, an d Na2 SO3 is consumed excessively due to the oxidation reac-

tion of Naz SOa occurring in the studied potential range. Na2 SiO3 is an efficient stab le reag ent of alkaline thiourea so-

lution, and gold dissolution is accelerated m uch m ore obviously by Naz SiO3 than by Naz SO3.

K ey w or ds : alkaline thiourea ; gold leaching; stable re agen t; Na2 SiO3 ; N az SO3 ; Na2 SeO3

C L C n u m b e r : TF 80 1 D o c u m e n t c o d e : A

1 I N T R O D U C T I O N

T h i o u r e a , a s a n o n t o x i c g o l d l e a c h i n g r e a g e n t

s u b s t i t u t i n g c y a n i d e , h a s a t t r a c t e d t h e in t e r e s t o f

m a n y r e s e a r c h e r s a l l o v e r t h e w o r l d s i n c e th e d i s s o -

l u t i o n o f g o l d i n t h i o u r e a s o l u t i o n w a s f i r s t r e p o r t e d

i n 1 9 4 1 , a n d a s e r i e s o f s t u d i e s o n t h e t h e o r y a n d

t e c h n o l o g y f o r l e a c h i n g g o l d b y t h i o u r e a h a v e b e e n

c o n d u c t e d [1-33. M o s t r e p o r t s o n g o l d d i s s o l u t i o n i n

t h i o u r e a s o l u t i o n w e r e c o n c e r n e d w i t h a c i di c m e d i a .

I t w a s d i f f i c u l t f o r g o l d t o d i s s o l v e i n a l k a l i n e s o l u -

t i o n b e c a u s e o f t h e p a s s i v a t i o n o f e l e m e n t s u l f u r

p r o d u c e d b y t h e i r r e v e r s i b l e d e c o m p o s i t i o n o f t h i o -

u r e a [4-63. H o w e v e r , i f t h e s t a b l e r e a g e n t o f a l k a l i n et h i o u r e a s o l u t i o n w a s a p p l i e d , g o l d c a n d i s s o l v e e f -

f i c i e n t l y i n a l k a l i n e t h i o u r e a s o l u t i o n . F r o m r e s e a r -

c h e s o n g o l d l e a c h i n g i n a lk a l i n e t h i o u r e a s o l u -

t i o n E7-123 , i t w as fo u n d t h a t t h e ad d i t i v e Na2 S O3 r e -

s t r a i n e d t h e i r r e v e r s i b l e d e c o m p o s i t i o n o f t h i o u r e a

a n d i m p r o v e d i t s s t a b i l i ty i n a l k a l in e m e d i a , a n d

t h e s e l e c t iv e d i s s o l u t i o n o f g o l d f r o m s c r a p c o n t a i -

n i n g g o l d i n a l k a l i n e t h i o u r e a s o l u t i o n c a n b e c a r -

r i ed o u t . H o w e v e r t h e e f f e c t o f N a 2 SO 3 o n t h e

s t a b i li z a t io n o f a l k a l in e t h i o u r e a w a s l i m i t e d , a n d i t

w a s n e c e s s a r y t o s e l e c t ef f i c ie n t s t a b l e r e a g e n t f u r -

t h e r . I n R e f . 1 -1 31 , t h e s t r u c t u r e - p r o p e r t y r e l a t i o n -s h i p b e t w e e n t h e s t a b i l i t y o f a l k a l in e t h i o u r e a a n d

t h e s t r u c t u r e o f t h i o u r e a a n d s o d i u m s u l f it e w a s e s -

t a b l is h e d b a s e d o n t h e t h e o r y o f m o l e c u l a r m e c h a n -

i cs . I t w a s c o n c l u d e d t h a t a n e f f ic i e n t s t a b l e r e a -

g e n t s e l e c t e d f o r a l k a l i n e t h i o u r e a s h o u l d m e e t t h e

n e e d o f s o m e c o n d i t i o n s f o r t h i o u r e a e x i s t i n g i n a l -

k a l i n e m e d i a s t a b l y . T h e a i m o f t h i s s t u d y i s t o se -

l e c t m o r e e f f i c i en t s t a b l e r e a g e n t t h a n N a z S O 3

b a s e d o n t h e s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p , i n v e s -

t i g a t e t h e e f f e c t o n s t a b i l i t y o f a l k a l in e t h i o u r e a a n d

a c c e l e r a t i o n o f g o l d d i s s o l u t i o n , a n d p r o v i d e t h e o -r e t i c a l g u i d a n c e f o r l e a c h i n g g o l d f r o m g o l d o r e s

c o m m e r c i a l l y b y a l k a li n e t h i o u r e a s o l u t i o n s .

2 T H E O R E T I C A L A N A L Y S I S O N S E L E C T I O N

O F S T A B L E R E A G E N T

T h e e f f ic i e n t s t a b l e r e a g e n t s s h o u l d b e a b le t o

r e d u c e t h e e n e r g y o f t h e h i g h e s t o c c u p i e d m o l e c u l a r

o r b i ta l ( H O M O ) o f t h i ou r e a f r o m th e v i e w p o i nt of

Foundation item: Project(50004009) suppor ted by the National Na tural Science Foundation of China

Received date : 2003-10-24; Accepted date: 2003-03-18Correspondence: C H A I Li-yua n, PhD , Prof esso r; Te l: q- 86-731-8830875 ; E-m ail : Lychai@ mail . csu. edu. cn



WA NG Yun-yan , et al : Optimization of eff ici ent stable reagent of alkaline thiourea solution fo r gold leaching 293

m o l e c u l a r f ie l d b a s e d o n t h e s t r u c t u r e - p r o p e r t y r e -

l a t i o n s h i p b e t w e e n t h e s t a b i l i t y o f a l k a l in e t h i o u r e a

a n d t h e s t r u c t u r e o f t h i o u r e a a n d s u l f it e io n . I t is

t o s a y t h a t t h e r e a g e n t s h o u l d m e e t t h e n e e d s o f

t w o c o n d i t i o n s , i e , f o r m i n g h y d r o g e n - s u l f u r b o n de a s il y a n d a st a b l e o r b i c u l a r s t r u c t u r e w i t h t h i o u r e a

m o l e c u l a r i n a l k a li n e m e d i u m . S i m i l a r t o N a z S O 3 ,

N a 2 S e O 3 a n d N a 2 S i O 3 c a n f o r m h y d r o g e n b o n d

w i t h t h e h y d r o g e n a t o m o f t h i o u r e a m o l e c u l a r

t h r o u g h o x y g e n a t o m a n d s t ab l e o r b i c u la r s t r u c t u r e

w i t h t h e s u l fu r a t o m o f t h i o u r e a m o l e c u l a r t h r o u g h

i n t e r l i n k o f c o u p l e i s o la t e d e l e c t r o n s o f s e l e n i u m

a t o m o r s il i co n a to m . M o r e o v e r a q u e o u s N a z S iO s

p r e s e n t s t h e p r o p e r t y o f c o l l o i d , t h e o r b i c u l a r

s t r u c t u r e w i t h t h i o u r e a m o l e c u l a r c a n d i s p e rs e i n

s o l u t i o n e q u a b l y a n d m a k e t h i o u r e a m o l e c u l a r m o r e

s t a b l e . T h e r e f o r e , N a e S e O 3 a n d N a zS i O 3 a r e s e -

l e c t e d a s s ta b l e r e a g e n t s f o r a lk a l i n e t h i o u r e a , a n d

t h e i r st a b l e e f f e c t s a r e c o m p a r e d w i t h t h a t o f

N a 2 S O 3 t o o p t i m i z e a n e f f i c i e n t s t a b l e r e a g e n t .

W h e t h e r i n ac i di c o r a lk a l i n e m e d i a , t h i o u r e a

b e i n g o x i d i z e d e a s il y is t h e m a i n c a u s e o f i t s i n s t a -

b i l i t y , s o t h e s t a b l e r e a g e n t w i t h r e d u c i n g p r o p e r -

t i es sh o u l d b e s e l e c t ed . T h e s t r u c t u r e o f N a z Se O 3

a n d N a z S O 3 w i t h r e d u c i n g p r o p e r t y a r e s i m i l a r ,

f u r t h e r m o r e t h e v a l e n cy o f s e l e n i u m i n NazSe03 is4 , b u t i t is a n o x i d a n t w i t h m e d i u m s t r e n g t h , s o i t

i s d i f f i c u l t t o i m p r o v e t h e s t a b i l i t y o f a l k a l i n e t h i o -

u r e a. O n t h e c o n t r a r y , i t c a n p r o m o t e t h e d e c o m -

p o s i t i o n o f t h i o u r e a .

T h e s t r u c t u r e o f C S ( N H 2 ) z , S O ~ - a n d S iO 3 -

b e l o n g s t o A B3 ty p e i n m i c r o s t r u c t u r e , t h e r e e x i s t

b i g a w a y - f i e l d rt b o n d s i n m o l e c u l e , i e , ~ i n th i o u -

r e a a n d 7 t~ i n S O3 - a n d S i O ~ - . B e c a u s e t h e b o n d

e n e r g y o f C - S ( 2 5 9 . 2 k J • m o l - ~ ) i s s m a l l e r t h a n

t h a t o f C - N ( 2 9 1 . 3 k J • m o l - ~ ) i n t h i o u r e a m o l e -

c u l e , s o t h e C = S b o n d b r e a k s e a s i l y , a n d t h e n s u l -

f u r a t o m b e c o m e s S 2 - i o n , t w o H + i o n s a r e e a s y t o

d i ss o c ia t e i n C = N b o n d a n d f o r m H 2 0 w i t h O H - .

I n a l k a l i n e m e d i a , t h i o u r e a i s i n s t a b l e a n d a p t t o

d e c o m p o s e t o f o r m s u l f i d e a n d c y a n a m i d b e c a u s e o f

p l e n t y o f O H - i o n s. H o w e v e r , t h e s t a b i l i ty o f a l -

k a l in e t h i o u r e a c a n b e i m p r o v e d b y S O ~ - o r S i O z - ,

b e c a u s e a m o l e c u l a r s t a b i l i t y f ie l d w i t h t h e h i g h e s t

e n e r g y c a n b e c o n s ti t u t e d t h r o u g h t h e o x y g e n a t o m

i n S O ~ - o r S iO ~ - s t r u c t u r e a n d h y d r o g e n a t o m i n

t h i o u r e a b a se d o n t h e s a m e s t r u c t u r e . T h u s i t c a n

b e s e e n t h a t N a 2 S i O 3 i s m o s t l i k e l y t o b e t h e o p t i -

m i z e d s t a b l e r e a g e n t f o r a l k a l in e t h i o u r e a , w h i l e

N a 2 S eO 3 i s n o t a s u i t a b l e o n e w i t h i n N a 2 S O 3 ,

N a z S e O 3 a n d N a z S i O 3 .

3 E X P E R I M E N T A L

T h e c u r v e s o f s t e a d y p o l a r i z a t i o n o f s p e c i m e n s

w e r e m e a s u r e d w i t h CHI660A e l e c t r o c h e m i c a l

w o r k s t a t i o n c o n t r o l le d b y a c o m p u t e r . T h r e e -

e l e c t r o d e s y s t e m w a s a p p l i e d , t h e H s t y l e ce l l w i t h

s a n d c o r e g l a s s s e p t u m s e r v e d a s e l e e t r o b a t h , a

p l a t i n u m f i l a m e n t ( t h e e x p o s e d a r e a i s 0 . 4 4 2 m m z )

a n d g o l d ( t h e e x p o s e d a r e a i s 1 c m z ) s e r v e d a s

w o r k e l e c t r o d e s , a p l a t i n u m p i e c e w i t h g r e a t a r e a

c o m p a r e d w i t h w o r k i n g e l e c t ro d e s e r v e d as c o u n t e r

e l e c t r o d e , a n d a H g / H g O e l e c tr o d e d i p p e d i n 1

t o o l * L -1 N a O H s o l u t i o n a s r e f e r e n c e e l e c t r o d e .

S a l t b r i d g e w i t h L u g g i n c a p i l la r y w a s u s e d t o e l im -

i n a t e b o r d e r p o t e n t i a l b e t w e e n d i f f e r e n t s o l u t i o n s

a n d d e c r ea s e O h m i c r e s i s t a n c e o f s o l u t i o n . T h e

s u r f a c e o f s p e c i m e n s w a s p o l i s h e d b y 1 5 /~ m a b r a -

s i ve p a p e r , d i p p e d s u c c e s s i v e l y in a c e t o n e a n d

w a s h e d b y d o u b l y d i s t il l e d w a t e r b e f o r e i m m e r s i o n

i n t h e t e s t s o l u t i o n . B e f o r e p o l a ri z a t i o n m e a s u r e -

m e n t s c o m m e n c e d , t h e s o l u ti o n w a s p u r g e d b y a

s t r e a m o f p u r i f i e d N2 f o r a t l e a s t 1 5 ra i n i n o r d e r t o

r e m o v e t h e d i s s ol v e d o x y g e n . T h e v o l u m e o f so l u -

t i o n u s e d i n th e e x p e r i m e n t s w a s 2 0 0 m L a n d t h e

c e ll w a s m a i n t a i n e d w i t h t h e r m o s t a t w a t e r b a t h t o

k e e p c o n s t a n t te m p e r a t u r e . T h e b e h a v i o r s w a ss t u d i e d b y l i n e a r s w e e p v o l t a m m e t r y a t a s ca n r a t e

o f 1 0 m V • s -1 f o r g o l d e l e c t r o d e a n d 1 m V • s - 1

f o r p l a t i n u m f i la m e n t . A l l p o t e n t i a l s w a s q u o t e d v s

t h e st a n d a r d h y d r o g e n e l e c t ro d e ( S H E ) , t h e f r e sh -

l y p r e p a r e d s o l u t i o n s w e r e u s e d a s e l e c t r o l y t e s fo r

a n o d i c p o l a r i z a t i o n .

T h e e l e c t r o l y t e w a s c o m p o s e d o f 0 . 1

m o l • L - 1 C S ( N H z ) 2 , 0 . 2 5 m o l • L - 1 s t a b l e r e a -

g e n t . T h e c o n d i t i o n s w e r e c o n t r o l l e d a t p H 1 2. 5

a n d 2 5 * C .

T h e d e c o m p o s i t i o n r a t e o f t h i o u r e a i n a l k a l i n e

s o l u t i o n w a s c a l cu l a t e d b y m e a s u r i n g t h e c o n c e n -t r a t i o n o f t h e d i s s o c ia t e d t h i o u r e a w i t h t h e c a p a c i t y

m e t h o d . T h e v a l u e - a d d e d c u r r e n t d e n s i t y o f g o l d

d i s s o l u t i o n i s d e f i n e d a s t h e ' d i f f e r e n c e b e t w e e n t h e

c u r r e n t d e n s i t y o f g o l d d i s s o l u t io n i n a l k al i n e t h i o -

u r e a s o l u t i o n w i t h o r w i t h o u t s t a b l e r e a g e n t a n d

t h a t i n s o l u t i o n o n l y w i t h s t a b l e r e a g e n t a t c e r t a i n

p o t e n t i a l .

4 I N F L U E N C E O F S T A B L E R E A G E N T S O N

S T A B I L IT Y O F A L K A L I N E T H I O U R E A

4 . 1 I n f l u e n c e o f s t a b l e r e a g e n t s o n o p e n c i r c u i t

p o t e n t i a l o f a l k a l i n e t h i o u r e a

I n f l u e n c e s o f N a z S O 3 , N a 2 Si O 3 a n d N a 2 Se O 3



294 Journal CSO T Vol. 10 No. 4 2003

on the open circuit potential vs time of alkaline thi-

ourea solution are depicted in Fig. 1.

The open circuit potential of alkaline thiourea

solution is shown with curve 1. Curves 2, 3 and 4

represent the open circuit potential vs time of alka-

line thi ourea solution containing Naz SO3, Naz SiO3

and Na2SeO3, respectively. The original open cir-

cuit potential and the stable potential are listed in

Table 1. It can be concluded tha t the original open

circuit potential of the solution decreases from

--0.20 to --0.25 and --0.29 V, respectively, due

to addition of Na2SO3 and NazSiO3 into the solu-

tion. The solution, furthermore, becomes stable

only after 500 s, and both stable pot ential s are

--0. 27 V. When NazSeO3 is added into the solu-

tio n, the original and the stable open circuit poten-

tials move positively from - -0. 20 to -- 0. 18 V,

and from --0.26 to --0.25 V, respectively. More-

over, the solution similar to alkaline thiourea solu-

tion without stable reagent does not reach a stable

state until more than 25 min later. The res ults

show tha t contrary to NazSeO3, NazSO3 and

NazSiO3 remarkably affect the stability of alkaline

thiourea solution.

-0.16 t 1 '-CS(NH2)2/

4.~, 3--CS(NH2)2+Na2SeO3-0.181 2--CS(NH2)2+Na2SO3

-0.201 lla-~ 4---CS(NI-I2)2+Na2SiO3

-0.26[ ,,,,,~

-0.28[- 3-0.301

-500 560 1 500 2 500 3 500t/ s

Fig. 1 Curves of open circuit potentia l vs

time of alkaline thiourea solutions

containing different stable re agents

T a b l e 1 Original open circuit potenti als andstable potentials of alkaline thiourea

solutions containing different stable re agents

Solution Original potential/V Stable potential/V

CS(NHz)z --0. 20 --0.26

CS(NHz)z +Na2SO3 --0. 25 --0.27

CS(NHz)z +NazSiO3 --0 .29 --0.27

CS(NH2)z +NazSeO3 --0. 18 --0. 25

4 . 2 I n f l u e n c e o f s t a b l e r e a g e n t s o n d e c o m p o s i t i o n

r a t e o f a l k a l i n e t h i o u r e a

Fig. 2 illustrates the influence of the concen-

tration of NazSO3 and Na2SiO3 on the stability ofalkaline thiourea solution. The decomposition rate

(r) of thiourea is up to 72.5% in alkaline media

without stable reagent. Stable reagents Na2SO3

and Na2 SiO3 decrease the decomposition rate of al-

kaline thiourea solution greatly, and the decompo-

sition rate decreases gradually with increasing the

concentration of the stable reagents. Tha t is, the

addition of stable reagent improves the stability of

alkaline thiourea prominently. And the stable

effect of NazSiO3 on alkaline thiourea is obviously

bett er t han that of Naz SO3, and the decomposition

rate of alkaline thiourea reduce from 72.5% to

51.3% and 33.8 % with adding 0. 15 mol • L -1 and

0.30 mol • L -1 NazSiO3, respectively. Theref ore ,

NazSiO3 can be considered as the optimized effi-

cient stable reagent.

70

4030 II--Na2SiO3

20 , J , ~ , , ,0 0.05 0.10 0.15 0.20 0.25 0.30

c/(mol • L t)

Fig. 2 Influence of stable reagent s on

decomposition rate of alkaline thiourea

I N F L U E N C E O F S T A B L E R E A G E N T S O N

G O L D D I SS O L U T I O N I N A L K A L I N E T H I O -

U R E A S O L U T I O N

5 . 1 I n f l u e n c e o f s t a b l e r e a g e n t s o n g o l d d i s s o l u t io n

i n a l k a l i n e t h i o u r e a s o l u t i o n

In order to find out whether the efficiency of

gold leaching can be improved in alkaline thiourea

solution by stable reagent, the electrochemical be-

haviors of gold in alkaline thiourea solution contai-

ning NazSO3, NazSeO3 and NazSiO3 are reviewed

by the method of anodic polarization, and the re-suits are shown in Fig. 3. The addition of NazSO3

and NazSiO3 accelerates anodic current densi ty

largely. The anodic curre nt apex occurs at the po-

tentia l of 0. 44 and 0.42 V, the according current

dens ity is 15 and 4 mA • cm -z , respectively. The

anode is passivated because of the reaction between

the products of thiourea decomposition and the

fresh gold surface. Naz SeO3 hard ly affects the gold

dissolution, and NazSO3 and NazSiO3 can improve

gold dissolution obviously.

The above current density in alkaline thiourea

solution represents the total current density

including current density of gold dissolution in al-

kaline thiourea and in stable reagent solution, the



WA NG Yun-yan , et al : Optimization of eff ici ent stable reagent of alkaline thiourea solution f or gold leaching 295

a n o d i c d e c o m p o s i t i o n c u r r e n t d e n s i t y o f t h i o u r e a

a n d t h e s ta b l e r e a g e n t . T h e r e f o r e , t h e p a rt i a l c u r-

r e n t d e n s i t y a n d v a l u e -a d d e d c u r r e n t d e n s i t y s h o u l d

b e in v e s t i g a t e d i n o r d e r t o s t u d y t h e a c c e l e r a t i o n

e f f ec t s o f g o l d d i s s o l u t i o n i n a l k a l i n e t h i o u r ea s o l u -

t ion .

201--CS(NH2)2

16 2---CS(NH2)2+Na2SO33---CS(NH2)2+Na2SeO ....4---CS(NH2)2+Na2SiO 3 "" "...2. . .

~ 1 2 "'""

8 ."

4 ." 4- "

- 0 . 2 6 0 ' .2 0 1 4 0 1 6VN

F i g . 3 A n o d i c l in e a r s w e e p v o l t a m m o g r a m s

o f A u e l ee t r o d e i n d i f f e r en t s o l u t i o n s

5 . 2 A n o d i c b e h a v i o r o f Na2SO3 a n d N a 2 S i O 3 i n

a l k a l i n e m e d i a

T h e an o d i c p o l a r i za t io n o f N azS O 3 an d

N a 2 S i O 3 a r e s t u d i e d w i t h w o r k i n g e l e c t r o d e ( P t ) i n

a l k a l i n e s o l u t i o n i n o r d e r t o f i n d o u t w h e t h e r t h e

s t ab l e r eag en t i t s e l f i s o x i d i zed , an d t h e r e s u l t i s

s h o w n i n F i g . 4.

12 1 - - N a E S O3

10 2 - -Na 2SiO 3 ~ 1

2

0 ~ ~ . . . . . . . . . ~".."'""2

-0.: 0 012 014 016

V N

F i g . 4 A n o d i c p o l a r i za t i o n b eh av i o r o f N az S O 3

an d N az S i O 3 i n a l k a l i n e m ed i a

F r o m F i g . 4 , i t c an b e s een t h a t t h e an o d i c

c u r r e n t d e n s i t y b e g i n s t o r is e s l o w l y a t 0 . 1 V a n d

i n c r eas e s r ap i d l y a f t e r 0 . 3 5 V w i t h t h e s c an n i n g

p o t en t i a l m o v i n g p o s i t i v e l y , w h i ch i n d i ca t e s t h a t

t h e an o d i c r eac t i o n o f N a2S O 3 o ccu r s i n P t e l ec -

t r o d e , r e s u l t in g i n e x c e s s i v e c o n s u m p t i o n o f

N a2S O 3 i t s e lf . W h i l e t h e an o d ic cu r r en t d en s i t y o f

N azS i O 3 i s v e r y l i t t l e i n t h e s t u d i ed p o t en t i a lr an g e , an d t h e o x i d a t i o n r eac t i o n h a r d l y o ccu r s . S o

N azS i O 3 i s f u r t h e r d em o n s t r a t ed t o b e t h e o p t i m a l

s t a b | e r e a g e n t f o r i m p r o v i n g t h e s t a b i l i t y o f a l k a -

l i n e t h i o u r ea s o l u t i o n .

5 . 3 I n f l u e n c e o f Na2SO3 a n d Na2SiO3 on g o l d d i s -

s o l u t i o n i n a l k a l i n e t h i o u r e a s o l u t i o n

T h e an o d i c b eh av i o r s o f g o l d i n a l k a l i n e t h i o u -

r ea s o l u t i o n , i n N a2 S O 3 s o l u t i o n an d i n a l k a l i n e

t h i o u r ea s o l u t i o n co n t a i n i n g N a2 S O a a r e s h o w n i n

F i g . 5 . T h e d i s s o l u t i o n cu r r en t d en s i t y o f g o l d is

v e r y l it t l e in a l k a l i n e t h i o u r ea s o l u t i o n , an d eo n t r a -

r y t o t h a t i n a l k a l i n e t h i o u r ea s o l u t i o n w i t h

Na2 SO3. I t s eem s as i f Na2 SO3 can largely accele r -

a t e g o l d d i s s o l v in g , b u t t h e m a j o r i t y o f d i s s o lv i n g

c u r r e n t s h o w s t h e c u r r e n t d e n s i t y o f t h e s t a b l e r e a -

g e n t . F o r e x a m p l e , t h e a n o d ic a p e x c u r r e n t d e n s i t y

i s 1 5 .5 m A • cm - z i n a l k a l i n e t h i o u r ea s o l u t i o n

w i t h N azS O 3 a t 0 . 4 5 V , w h i l e t h e an o d i c cu r r e n td e n s i t y r e a c h e s 1 3 . 0 m A • c m - 2 in N a 2 SO 3 s o l u -

t ion .

F i g . 5

20

, - ,15

1--CS(NH2)2- 2__Na2SO3

3__CS(NH2)2+Na2SO3 / 2. . . - . . ,

. " • •

• / ""-3j " j

- J /

. . ~ , ¢

- - 1

-0.2 0 0:2 0:4 0'.6VN

I n f l u en ce o f N a2 S O 3 o n an o d i c l i n ea r s w e ep

v o l t a m m o g r a m s o f A u e l e c tr o d e

H o w ev e r t h e i n f l u en ce o f N az S iO 3 o n g o l d d i s -

s o l u t i o n i n a l k a l i n e t h i o u r e a s o l u t i o n i s n o t t h e

s a m e a s t h a t o f N a z S O 3 , a n d t h e r e s u l t s a r e s h o w n

in Fig . 6 .

4

,---,3

1

-0.2

1 - CS( NI - I 2 )2__Na2SiO3 ...-"'"'-..3--CS(NI-I2)2+Na2SiO /"

?/

' 3. . . . - ' "

I I

0.2 0.4 016V /v

F i g . 6 I n f l u en ce o f N a2 S iO 3 o n an o d i c li n ea r s w ee p

v o l t a m m o g r a m s o f A u e l e c tr o d e



296 J o u r n a l C S U T V o l . 1 0 N o . 4 2 0 0 3

The anodic dissolution of gold electrode in

NazSiO3 solution does not occur, and NazSiO3 it-

self does not change. How ever the gold dissolving

current de nsity is improved evidently. The anodic

current density rises slowly with potential moving

positively, the maximum anodic current density is

4 mA • cm -2 at the potential of 0.42 V, and then

gold is passivat ed, the current density begins to

descent. It is concl uded that Na2SiO3 accelera tes

gold dissolving greatly before 0.42 V.

5 . 4 A c c e l e r a t i o n e f f e c t s N a 2 S O 3 a n d N a 2S iO 3 o n

g o l d d i s s o l u t i o n i n a l k a l i n e t h i o u r e a s o l u t i o n

The above results indicate that both NazSO3

and Na2SiO3 can improve gold dissolution. The

value-added current densities of Na2SO3 and

Na2SiO3 on gold dissolution at different potentialsare show n in Fig. 7. The height of column in Fig. 7

indicates that the acceleration effect of Na2SiO3 on

alkaline thiourea is obviously better than that of

Na2 SO3. The curren t den sity of gold dissolution in

the alkaline thiourea solution containing Na2SO3

andNa2Si O3 is 2.0 and 3. 5 mA • cm -2, respec-

tively at the potential of 0.42 V. The reb y Na2SiO3

is also optimal for promoting gold dissolution in al-

kaline thiourea solution.

3 . 5

3 . 0

¢ '2 .5

.o 2.0

~.~ 1.5

< 1.0

0.5

0

[" "] NaeSO3

~:~ Na,2SiO3

0.2 0.3V/V

0.4 0.5 0.6

Fig. 7 Value-added current density vs

potential of Au electrode in different solutions

6 C O N C L U S I O N S

1) Co nt ra ry to Na2 SeO3, Na2 SO3 and Naz SiO3

affect the stability of alkaline thiourea solution re-

markably . The stable effect of Na2SiO3 on alkaline

thiourea is more obvious than that of NazSO3, and

the decomposition rate of alkaline thiourea decrea-

ses from 72.5% to 33. 8~ with addition of 0. 3

m ol . L -1 Na2SiO3.

2) Oxidation reaction to Na2SO3 occurs in the

studied potential range, resulting in excessive con-

sumption of NazSO3, while Na2SiO3 almost does

not change, which illustrates that Na2SiO3 is the

optimal efficient stable reagent of alkaline thiourea

solution.

3) Both Na2SO3 and NazSiO3 can accelerate gold

dissolution, but Na2 SiO3 is optimization for pr omotin g

gold dissolution in alkaline thiourea solution.

R E F E R E N C E S

[11 Gaspar V, Mejerovich A S, Meretukov M A, et al.

Practical application of potential-pH diagrams for Au-

CS ( N Hz ) 2 Hz O and Ag-CS ( N Hz ) z- Hz O systems for

leaching gold and silver with acidic thiourea solution

[J]. Hydrometallurgy, 1994, 34(3) : 369-381.

[2] WANG Qing-jiang, CHENG Gui-feng, ZONG Wei. Study

on the improvement process of leaching gold from sulfide

ore with thiourea [J]. Journal of East China Normal Uni-

versity, 1998,(1): 61-65. (in Chinese)

[-31 LI Jin-shan. Study on the factors influencing thiourea

consumption in the gold-leaching process with acidic

thiourea solution[J] . Engineering Chemistry and Met-

allurgy, 1994, 15( 3): 229-234. (in Chinese)

[4] SA Ben-jia, ZHANG Fang-yu, CHENG Hua. Study

on gold dissolving in alkaline thiourea solution [J ].

Gold, 1984, 5 (3) .. 45-47. (in Chinese)

[5 ] Farinha P A, Correia M J N, Carvalho J R. Leaching

of gold from a Portuguese concentrate with thiourea

[J ]. Minerals Engineering, 1992, 5 (8) .. 953-959.

[6 ] FANG Z H. Leaching kinetics of a Au-Cu sulfide con-

centrate with thiourea [J] . Engineering Chemistry and

Metallurgy, 1993, 14(4)..319-326.

[-7] CHAI Li-yuan, Okido M. Dissolution theory of gold

in alkaline thiourea solution(I) anodic behavior on gold

in alkaline thiourea solution containing NazSO3 [J].Transactions of Nonferrous Metals Society of China,

1999,9(1) : 145-151.

[81 CHAI Li-yuan, Okido M. Dissolution theory of gold

in alkaline thiourea solution(II) electrochemical study

on Au-Fe mix anode in alkaline thiourea solution con-

taining NazSO3 [J ]. Transactions of Nonferrous Met-

als Society of China, 1999,9 (2) : 393-398.

[91 CHAI Li-yuan, Okido M. Dissolution theory of gold

in alkaline thiourea solution(III) thermodynamics on

dissolution of gold in alkaline thiourea solution contai-

ning Na2SO3 [Jl. Transactions of Nonferrous Metals

Society of China, 1999,9 (3) : 646-650.

[10] OkidoM, Ishikawa M, CHAI Li-yuan. Anodic dissolu-tion of gold in alkaline solutions containing thiourea, thi-

osulfate and sulfite ions[J1. Transactions of Nonferrous

Metals Society of China, 2002,12(3):519-563.

[111 ZHANG Chuar~fu, CHAI Li-yuan, ZHONG Hai-yun,

et al. Selective dissolution of gold in an alkaline thiourea

solution by electrolysis[J]. Journal of Central South Uni-

versity of Technology, 1997, 4(2) : 73-78.

[12] WEI Da-wei, CHAI Li-yuan, Ryoichi I, et al. Gold

leaching in an alkaline thiourea solution[J]. Journal of

the Electrochemical Society, 1999,146(2) :559-563.

[131 CHAI Li-yuan, MIN Xiao-bo. Structure-property re-

lationship between the stability of alkaline thiourea

and the structure of thiourea and sulfite ion [J ]. Jour-

nal of Central South University of Technology, 2002,

33(5) :473-476. (in Chinese)

( E d i t e d b y L I Y a n - h o n g )

Optimización de Tioureación Alcalina

Documents

Transcript of Optimización de Tioureación Alcalina