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