Electrowinning of cobalt from a pure cobalt sulphate bath was carried out. .... 60. 80. ' I00. CONCENTRATION. OF COBALT~ 9/t. 13 o u. "6. J¢ e-. Z ~. O. 7 a. Z. Z.
Hydrometallurgy, 12 (1984) 317--333
317
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
ELECTROWINNING OF COBALT I. W I N N I N G F R O M P U R E C O B A L T S U L P H A T E B A T H
S.C. DAS and T. SUBBAIAH
Regional Research Laboratory, Bhubaneswar-13, Orissa (India) (Received October, 1982; accepted in revised form November 12, 1983)
ABSTRACT Das. S.C. and Subbaiah, T., 1984. Electrowinning of cobalt. I. Winning from pure cobalt sulphate bath. Hydrometallurgy, 12: 317--333. Electrowinning of cobalt from a pure cobalt sulphate bath was carried out. Effects of current density, cobalt concentration in the bath, temperature, etc. on current efficiency and on the nature of the deposit were studied. The results indicate that bath pH and temperature play major roles in the cobalt deposition. A high current efficiency can be achieved at relatively low temperature and relatively high bath pH (pH > 4.0). Further, at relatively low bath pH and high temperature (~60°C), a high current efficiency can also be obtained; however, in the former case the deposited cobalt is dull and brittle and malleable in nature.
INTRODUCTION C o b a l t m e t a l plays a great role in o u r industrial and e c o n o m i c developm e n t b y p r o v i d i n g various s t r u c t u r a l materials a n d o t h e r essential items. It b e c a m e a strategic m e t a l because it is used extensively in c o m m e r c i a l and m i l i t a r y aircraft engines a n d in a e r o s p a c e research. C o b a l t is recovered m a i n l y b y h y d r o m e t a l l u r g i c a l processes. Metallic c o b a l t is p r o d u c e d either by c h e m i c a l or e l e c t r o c h e m i c a l r e d u c t i o n o f c o b a l t sulphate. T h e l a t t e r r o u t e is o f m o r e i m p o r t a n c e since m o s t o f the virgin c o b a l t m e t a l n o w p r o d u c e d is o b t a i n e d b y e l e c t r o w i n n i n g f r o m a s o l u t i o n o f its salt, c o m m o n l y in t h e f o r m of s u l p h a t e [ 1 - - 5 ] . Besides a p u r e sulphate b a t h , e l e c t r o w i n n i n g o f c o b a l t f r o m a s u l p h a t e b a t h c o n t a i n i n g additives such as Na2SO4, H3BO3, (NH4)2SO4, N a F etc. has also been r e p o r t e d in the literature [ 6 - - 1 2 ] , b u t the p u r e c o b a l t s u l p h a t e b a t h is o f m o s t c o m m e r cial interest. A literature s u r v e y o n t h e e l e c t r o w i n n i n g o f c o b a l t f r o m a p u r e c o b a l t s u l p h a t e b a t h s h o w s t h e availability o f a very limited n u m ber o f p a p e r s [ 1 2 - - 1 8 ] , w h i c h are lacking in the provision o f detailed inf o r m a t i o n o n the process. In t h e p r e s e n t s t u d y , a detailed investigation was carried o u t i n t o the
0304-386X/84/$03.00
© 1984 Elsevier Science Publishers B.V.
318
electrowinning of cobalt from a pure cobalt sulphate bath. A bath without any additive or buffering reagent was used since, in commercial practice, a pure COSO4 bath is usually used. Effects of several variables on current efficiency and physical appearance of the deposits were studied. EXPERIMENTAL METHODS
The synthetic electrolyte solutions were prepared from reagent grade cobalt sulphate crystals (COSO4-7H20) and deionized water. The calcium hydroxide and sulphuric acid used for adjusting the pH of the electrolyte were also of reagent grade. The cathodes used were rectangular stainless steel sheets having the following dimensions: length 95 mm, width 50 mm and thickness 2 mm. For electrical connection, strips of the same material having dimensions of length 50 mm, width 8 m m , and thickness 2 mm were welded at the centre of the top edge of these rectangular sheets. The anodes used consisted of lead--antimony containing 7% Sb and cut to the same size as the cathodes. A 250-ml Corning beaker with lid was used as the electrolysis cell. For the diaphragm cell, a 500-ml Corning beaker was used. This had a Perspex lid and a porous plastic diaphragm, fixed by Araldite (CIBA), to divide it into two compartments. The electrolysis was carried out by applying DC voltage from an electroanalyser. For measuring cell voltage and current, a precision voltmeter and ammeter were inserted in the circuit. The bath pH was measured by a digital pH meter. For stirring the electrolyte a mechanical stirrer was used. The bath temperature was maintained by having the cell in a thermostat which kept temperature fluctuations within -+ 1°C. In all experiments one anode and one cathode were taken and kept face to face 25 mm apart. After electrolysis, the cathode was removed and thoroughly washed with tap water followed by deionised water and then dried in an oven at a temperature of 100°C. The current efficiency was calculated from the weight gained by the cathode. RESULTS AND DISCUSSION
In the electrowinning, electrons are discharged from the cathode and collected at the anode. The electrode potential for the various reactions that take place at the anode or cathode are best represented by the electromotive series as :shown in Table 1. From this table, it m a y be demonstrated that the more negative the potential, the more readily the reaction takes place at the anode; the more positive the potential, the more readily the reverse .reaction takes place at the cathode. In accordance with the positions of cobalt and hydrogen, it is observed t h a t in an aqueous solution the hydrogen evolution reaction takes place more readily than the cobalt deposition. Thus it is difficult to electrowin cobalt from an
319
aqueous CoSO4--H2SO4 medium since the process is controlled by competition between cobalt and hydrogen production. So, for achieving efficiency in cobalt electrowinning, such conditions should be established that the hydrogen evolution reaction is suppressed and the cobalt deposition reaction is favoured. In the present paper, influences of various parameters, viz. cobalt concentration in the bath, current density, bath temperature and bath pH, on the cathode current efficiency and on the nature of electrodeposits were investigated. TABLE
1
Standard
electrode potential
at 25°C
Element
Anode reaction
Reversible electrode potential (V)
Zinc Iron Cobalt Nickel Lead Hydrogen Copper
Zn Fe Co Ni Pb H2 Cu
= = = = = = =
-0.76 -0.44 --0.28 - 0.25 -0.13 -0.00 +0.34
Oxygen
2H20
=O 2 +4H
Z n 2 ÷ + 2 eF e ~÷ + 2 eC o 2÷ + 2 e N i 2÷ + 2 eP b 2÷ + 2 e2 H ÷ + 2 eC u 2÷ + 2 e++4e-
+1.23
Cobalt concentration in the bath The influence of cobalt concentration was studied in the range 6--90 g/1 cobalt while carrying out the electrolysis at a current density of 250 A/m 2 for a period of 2 h at room temperature (30 -+ 1°C). The bath pH was adjusted to ~4.0 in each case. The results are presented in Fig. 1. The change in pH during electrolysis was observed to be almost the same in each case. The effect of pH was thus assumed negligible while discussing the effect of cobalt concentration on current efficiency and the morphology of the deposit. Current efficiency increased steadily from 35% to 70% as cobalt concentration was increased from 6 g/1 to 50 g/1. Beyond this any further addition of Co 2÷ to the bath had no effect. The specific power consumption with increase of Co 2÷ in the bath fell steeply from 11.7 kW h/kg cobalt to 4.8 kW h/kg cobalt at a cobalt concentration of 50 g/1 and then maintained an almost steady value. At a bath concentration of up to 12 g/1 Co 2÷, cobalt was deposited as a sheet which did not adhere properly to the cathode and started peeling off in the electrolysis time of 2 h. Bright s m o o t h deposits of an almost grey colour started depositing bey o n d a cobalt concentration of 25 g/1.
320
13
,°°t
uo "6
80 J¢
0
e-
>" U Z hi
60 Z ~ O
G U,.
40
7
a. Z
pZ
Z O
5
20
u
-"1 U O
0 0
I 20
I 40
CONCENTRATION
I 60
I 80 OF COBALT~
I ' I00
3
=-
120
9/t
Fig. 1. E f f e c t o f c o b a l t c o n c e n t r a t i o n on current e f f i c i e n c y . Current density, 2 5 0 A / m 2 ; bath temperature, 30°C, duration o f electrolysis, 2 h.
Current density Using a bath containing 40 g/1 of cobalt and having a pH of ~ 4 . 0 the influence of current density was studied (in the range of 50 A / m 2 to 1000 A/m 2) on the cell voltage, power consumption, current efficiency and deposit morphology.
(a) Influence of current density on cell voltage The total voltage across the cell may be divided into the following components: (i) decomposition potential of the electrolyte, (ii) polarisation potential, and (iii) potential drop due to ohmic resistance of the electrolyte and electrical connections. Each of the voltage components is affected by the various process variables (current density, electrolyte composition and temperature, nature of the cathode and organic additives), resulting in a total cell voltage which varies depending on the operating conditions. In this work the variation of total cell voltage with increasing current densities was established, but no attempt was made to investigate the effect of current densities on the individual components of the total cell voltage.
321
o
S.O
10,0 °
/(.0
8.0,, 0
o 3.0
6,0 )--
I--
Z
O
~ W 0
::3)
].,.O z
2.0
°
u
o
~ I
1.0
_j2.0 ._o I
I
100
200
i
~
I
I
300 400 500 600 CURRENT DENSITY~ A / m 2
1
I
800
900
~
700
0
1000
Fig. 2. E f f e c t of current density on cell voltage and p o w e r c o n s u m p t i o n . Co concentration, 40 g/l; bath t e m p e r a t u r e , 30°C; duration of electrolysis, 2 h.
Figure 2, line A shows the variation of cell voltage with current density. The line, which is calculated by applying a least squares m e t h o d , is plotted together with the values actually recorded. This shows that current density and the cell voltage o b e y a linear relationship in the range of current densities studied; this relationship may be represented by eqn.
(I): V = a + mi
(1)
where V is the total cell (V) and i is the current density (A/m2). The values of a and m from Fig. 2, line A are 2.69 and 0.0019, respectively. (b) Influence of current density on power consumption
Figure 2, line B shows the effect of current density on power consumption. The line was calculated by applying a least squares method. Thus, as expected from the voltage/current relationship previously established, the p o w e r consumption is directly proportional to the current density. This relationship is expressed by an equation of the following general form: P = al + m l i
(2)
where P is the specific power consumption (kW h/kg of Co), i is the current density (A/m2), and al and ml are intercept and slope, respectively. The values of a~ and m~, calculated from Fig. 2, line B, are 3.115 and 0.006, respectively.
322
(c) I n f l u e n c e o f current d e n s i t y o n current efficiency
It has been shown [24] that the current efficiency in cobalt electrowinning is given by the equation: 90.969 V -
P
(%)
(3)
where V is the total cell voltage (V), and P is the specific power consumption (kW h/kg of Co). Since V = a + mi and P = al + m l i from eqns. (1) and (2), we find 7 =
90.969 (a + mi)
(4)
a I + mid
Therefore, if the values of total cell voltage and power consumption at a given current density are known, the current efficiency may be calculated from eqn. (4). The values calculated from this equation, together with the values actually recorded, are plotted in Fig. 3.
I00
10
80
8
60
6
o
,>.. 0
3;
z
z o
I1.
e~
I-
4
40:
Z W
o ~.~
OC :D
o
2
2O
re W
3~ (
I
:200 400 CURRENT
......
I
I
I
..
600 600 1000 DENSITY ,t ~li/m 2
Fig. 3. E f f e c t of c u r r e n t d e n s i t y o n c u r r e n t e f f i c i e n c y a n d p o w e r c o n s u m p t i o n , Co c o n c e n t r a t i o n , 40 g/l; b a t h t e m p e r a t u r e , 30°C; d u r a t i o n o f electrolysis, 2 h.
The current efficiency increased as the current density was raised from 50 A/m 2 to 100 A/m 2. Beyond this it regularly decreased. This was probably due to a decrease o f the effective metal ion concentration (Co 2÷) in the vicinity of the cathode, as a result of which hydrogen evolution
323
increases. Thus, the condition b e y o n d a current density of 100 A/m: favours the hydrogen evolution reaction and suppresses the cobalt deposition reaction.
(d) Influence of current density on deposit morphology At a current density of 50 A/m 2, the deposit was dull grey, coarse and not properly adhering to the cathode. When the current density was increased to 100 A/m 2 a bright, greyish and adherent deposit was obtained. Beyond this current density comparatively brighter sheet deposits were obtained, which were adherent and smooth.
Bath temperature Temperature is an important factor during the electrodeposition of cobalt. The current yields and energy requirements are dependent on bath temperature [12]. Feneau et al. [18] suggested a bath temperature of 65°C to be the best operating condition. In the present study the influence of temperature was studied in the range 30--75°C. The results are reported in Figs. 4 and 5. At all the temperatures studied the bath pH was the same (~4.0). Very little variation in pH was observed to occur during the electrolysis. The effect of pH was thus assumed to be negligible in the discussion of the effect of temperature on current efficiency and the nature of the deposit. At a bath temperature of 30°C (room temperature} 76% current efficiency was achieved. As the temperature was raised, the current efficiency increased slowly until a bath temperature of 60°C was reached (Fig. 4).
o_~ >..
u
IOC
Z L~ u.
"
90: 80
~_ 7 o (lc
~
60
u
50
1
0
J
20
40
TEMPERATURE
I
I
60
80
I00
°C
Fig. 4. E f f e c t o f b a t h t e m p e r a t u r e on c a t h o d e c u r r e n t efficiency. Co c o n c e n t r a t i o n , 40 g/l; c u r r e n t d e n s i t y , 100 A / m 2 ; d u r a t i o n o f electrolysis, 2 h.
324
2.7
S.O
o (..i o
I.J 0
Z.6
4.0
~
2.5
3.0
z
0 pn
z
-I, 2 . 4
2.0 ~ Z 0 er
1.0 ..,
2.3
0
2.2~ 0
I 20
I L 40 60 TEMPERATURE j°C
I 80
Io0
Fig. 5. Effect of bath temperature on cell voltage and power consumption. Co concentration, 40 g/l; current density, 100 A/m 2 ; duration of electrolysis, 2 h. B e y o n d 60°C t h e gain in c u r r e n t e f f i c i e n c y was negligible. T h e b e h a v i o u r o f b a t h voltage and specific p o w e r c o n s u m p t i o n with variation o f t e m p e r a t u r e are r e p o r t e d in Fig. 5. Bath voltage c o n t i n u o u s l y decreased as the b a t h t e m p e r a t u r e was raised. A similar b e h a v i o u r was also observed in t h e case o f specific p o w e r c o n s u m p t i o n . C o b a l t was d e p o s i t e d as a greyish sheet at r o o m t e m p e r a t u r e (30°C). As the t e m p e r a t u r e o f t h e b a t h increased t o ~ 4 5 ° C the brightness of the d e p o s i t i m p r o v e d ; at higher t e m p e r a t u r e (60--75°C), the deposits o b t a i n e d were m u c h brighter. T h e c o b a l t sheets d e p o s i t e d in the l o w e r t e m p e r a t u r e range (up t o 45°C) were s m o o t h and brittle in n a t u r e while in the higher t e m p e r a t u r e range the deposits were n o n - u n i f o r m , s o m e w h a t r o u g h and malleable in nature.
Duration of electrolysis The i n f l u e n c e o f electrolysis t i m e o n c o b a l t e l e c t r o w i n n i n g at various b a t h t e m p e r a t u r e s (30--75°C) was studied. T h e results are r e p o r t e d in Fig. 6. T h e results s h o w e d t h a t for each t e m p e r a t u r e studied, the c u r r e n t e f f i c i e n c y fell with d u r a t i o n o f electrolysis. A c o m p a r i s o n o f the n a t u r e o f the fall o f c u r r e n t efficiencies at various t e m p e r a t u r e s i n d i c a t e d t h a t in t h e l o w e r t e m p e r a t u r e range t h e c u r r e n t efficiencies fell rapidly (Fig. 6), i.e., f r o m 86% t o 76% at 30°C and f r o m 88% to 78.5% at 45°C, respectively, in 2 h o f electrolysis time. At higher t e m p e r a t u r e s c u r r e n t efficiencies
325
remained steady for up to 2 h of electrolysis tikne, but b e y o n d that period they fell slowly (Fig. 6). As the bath temperature was increased the steepness in the fall of current efficiency decreased. Beyond a bath temperature of 60°C, the behaviour of current efficiency with time was similar. The fall of current efficiency with time may be due to unavailability of Co 2+ at the interface. At lower temperatures (up to 45°C) the deposit changed with time of electrolysis from blackish grey to shiny grey and the deposit was smooth. At room temperature, however, b e y o n d 6 h of electrolysis time, the deposit started cracking. At higher temperatures the brightness increased with the passage of time b u t roughness also increased. Decrease of concentration of Co 2÷ ion in solution and of the bath pH with time m a y perhaps contribute towards the morphology of the deposit. I00
9O
(J z w
BO
u tl. ti. t~J FZ
u
7O
6c
5C 0
I I
] 2 DURATION
I 5 OF
i 4 ELECTROLYSIS
I 5
I 6
I 7
I 8
I 9
I0
HOUR~
Fig. 6. E f f e c t o f d u r a t i o n of electrolysis on current efficiency. Co concentration, 40 g/l; current density, 100 A/rn 2.
Bath pH The electrowinning of cobalt in aqueous C O S O 4 - - H 2 S 0 4 medium is controlled by competition between cobalt and hydrogen production and determined by the acidity. The acidity plays a most harmful part with respect to the efficiency of cobalt deposition. Since the rate of hydrogen evolution decreases with increasing pH, the current efficiency for the deposition of cobalt would be expected to rise with increasing pH. However, the pH value should not exceed the value above which precipitation of cobalt hydroxide occurs. At pH < 2.5 hydrogen discharge [8] is the first electrochemical reaction that occurs after current is switched on. When H ÷ is exhausted in the double layer, Co(OH)2 is precipitated on the cathode, causing passivation. At pH 3.5--4.9 the potential j u m p e d to 250 mV at ~ 1 0 A/m 2 and Co 2÷ was deposited. In the present study, the results of the effect of bath pH on the electrowinning of cobalt are
326
reported in Figs. 7--9 and Tables 2--4. Figure 7 reports the effect of electrolysis time on bath pH at room temperature. Here a comparison is shown between two baths, i.e., (i) a bath having a low starting pH (4.0) and (ii) a bath having a high starting pH (7.0). In the former case, the pH fell slowly for about 30 rain and then remained almost constant (i.e. at pH 2.5--3.0).
7°t 6.0
5.0 '1O.
-r
I,¢ 4.0 3.0
_
~
2.0
1.0
0
l
20
I
I
I
I
f
40 60 60 I00 120 DURATION OF ELECTROLYSISs MIN.
140
Fig. 7. E f f e c t o f d u r a t i o n o f electrolysis o n b a t h pH. Co c o n c e n t r a t i o n , 40 g/l; c u r r e n t d e n s i t y , 100 A / m s , b a t h t e m p e r a t u r e , 30°C.
In the latter case, the pH fell abruptly to ~4.0 during the first 10 min electrolysis time; during the rest of the period, the fall in the bath pH was similar to that observed in the former case. The results indicate that the more alkaline the bath becomes, the less is the rate of fall in the pH during electrolysis. The effect on bath pH of temperature with time of electrolysis is reported in Fig. 8. Here, for both temperatures reported (30°C and 60°C) the nature of the fall in pH was almost the same, but at the higher temperature the rate of decrease in bath pH was slower. Above 60°C, there was practically no change in the behaviour. Figure 9 shows the effect of bath pH on current efficiency at different temperatures. Since it was difficult to maintain a constant bath pH with regular addition of alkali (i.e., Ca(OH) 2 slurry), electrolysis was started at about pH 4.0; during electrolysis the bath pH was measured at different times, and current efficiency was calculated. As observed from Fig. 9, the nature of the curves at 30°C and 45°G is similar. In both cases the current efficiency fell dras-
327 5.0
4.5
4.0
60°C
5.0 ,lb. :1: b-
T E tdP. ~O°C 2.5
2.0
1.5
1.0
I
0
20
I
I
I
I
I
40 60 80 I00 120 DURATION OF ELECTROLYSIS~ MIN.
140
Fig. 8. E f f e c t o f d u r a t i o n o f e l e c t r o l y s i s a t d i f f e r e n t b a t h t e m p e r a t u r e s . t r a t i o n , 4 0 g/l; c u r r e n t d e n s i t y , 1 0 0 A / m s .
Co concen-
tically below a bath pH of about 2.0. At 30°C it was possible to achieve more than 90% current efficiency only at a bath pH greater than 2.7, whereas at 45°C greater than 90% current efficiency could be achieved b e y o n d a pH of only 2.0. As the pH tended towards a lower value at both temperatures the current efficiency fell rapidly (it is around 55% and 65% at a pH of 1.0 at 30°C and 45°C, respectively). At 60°C or above the nature of the curve was completely different from the curves at lower temperatures. (At 75°C the points almost coincided with those at 60°C so they are not shown in Fig. 9). Here the current efficiency decreased rapidly at first until a bath pH of ~ 2 . 4 was reached. It then remained constant until a pH of 1.7, bey o n d which it steadily decreased. At higher temperatures, close to 100% current efficiency could be achieved at a pH of 3.0 or above and greater than 90% current efficiency could be achieved at a bath pH of over 1.75. The results also indicated that at these temperatures an 80--90% current efficiency could also be possible in an even more acidic pH range, i.e.,
328 pH 1.25 to 1.75. Further, f r om this study it is seen t h a t higher current efficiencies can be obtained with higher bath temperatures (even in a m o r e acidic pH range). The nature of c a t hode deposit changed with the bath pH. The deposit was comparatively dull in the higher pH range, and brighter in the more acidic pH range, at all t e m per a t ur es studied. In the same pH range the deposit became brighter as the bath t e m p e r a t u r e increased.
90 U Z Itl
80
X - 60°C 0 - 45°C
u. iu
,-
I'Z
:30°C
70
nr Og U
60
5
I. O
0
2. O
~
BATH
3.0
4.0
pH
Fig. 9. Effect of bath pH on current efficiency. Co concentration, 40 g/l; current density, 100 A/m s. F r o m the above study it is observed that, when cobalt is electrowon f r o m a pure COSO4 bath having any value of starting pH, t he bath pH falls during the course of electrolysis c o n t i n u o u s l y to a m ore acidic value, n o t only at r o o m t e m p e r a t u r e b u t also at higher temperatures. This results in a decrease in current efficiency. The electrochemical reaction taking place during electrowinning m a y be written as follows: ca t h o d e :
2 Co 2+ + 4 e- -~ 2 Co
(5)
anode:
2 H20 -~ 02 + 4 I-F + 4 e-
(6)
329 Alternatively, the t o t a l r e a c t i o n m a y be w r i t t e n as follows:
(7)
2 COSO4 + 2 H~O = 2 Co + 2 H2SO4 + 02
Thus Co 2÷ is d e p o s i t e d at the c a t h o d e s i m u l t a n e o u s l y with the p r o d u c t i o n o f HzSO4 at the a n o d e . The acid g e n e r a t e d (eqns. 7) c o n t i n u o u s l y lowers the b a t h pH. If, h o w e v e r , the g e n e r a t e d acid can be neutralised, the b a t h p H can be k e p t high and this s h o u l d increase the c u r r e n t efficiency. An a t t e m p t was m a d e in this regard b y s t u d y i n g the e f f e c t o f electrolysis t i m e on b a t h pH, c u r r e n t e f f i c i e n c y and n a t u r e o f c a t h o d e d e p o s i t for t h r e e baths at d i f f e r e n t c o n d i t i o n s : (i) Bath p H raised t o ~ 7 . 0 b y adding Ca(OH)2 slurry. (ii) Bath p H raised t o ~ 7 . 0 by adding Ca(OH)2 slurry and t h e n adding 3.0% excess Co(OH)2 slurry. (iii) Bath p H raised t o ~ 7 . 0 b y adding Ca(OH)2 slurry, t h e n adding 3.0% excess Co(OH)2 slurry t o it and keeping this Co(OH)2 slurry in suspension. TABLE2 Study of electrolysis time on bath pH and current efficiency in electrowinning of cobalt w
Time (h)
10 40 60 90 120
Bath without excess Co(OH)~
Bath with excess Co(OH)2
pH
With stirring (-150 min -1) Without stirring
3.89 2.60 2.45 -2.12
Current efficiency
(%)
99.9 98.1 96.8 -88.4
pH
Current efficiency (%)
pH
Current efficiency (%)
6.90 6.88 6.84 6.80 6.79
99.9 99.9 98.5 98.2 98.2
3.72 3.06 2.56 -2.12
99.9 98.9 95.6 -87.04
Cobalt concentration = 40 g/l, bath pH = 6.90, current density = 100 A/m 2, and bath temperature --- 30°C. The results are r e p o r t e d in Table 2. As o b s e r v e d f r o m the results, baths (i) and (ii) b e h a v e d in a similar m a n n e r . In the case o f b a t h (iii), h o w e v e r , the b a t h p H r e m a i n e d m o r e or less c o n s t a n t t h r o u g h o u t the 120 rain electrolysis time, and the c u r r e n t e f f i c i e n c y was close t o 100%. In b o t h baths (i) and (ii), bright, greyish deposits were o b t a i n e d , which c o u l d be stripped easily f r o m the base. In b a t h (iii) a bluish bright sheet was o b t a i n e d t h r o u g h out. At the end o f this electrolysis, it was o b s e r v e d t h a t the deposit a d h e r e d t o t h e base and t h a t , w h e n stripping was a t t e m p t e d , the deposit w o u l d c o m e o f f o n l y in pieces. T h e s e pieces were hard and very brittle. At t h e a n o d e a t h i c k layer o f c o b a l t o x i d e (black c o l o u r e d ) was also d e p o s i t e d . T h e bright, bluish c o l o u r , and t h e h a r d and brittle n a t u r e o f t h e d e p o s i t e d
330
cobalt are probably due to the inclusion of CoOOH and Co(OH)2 in the deposit. Nakahara and Mahajan [ 1 9 ] , from their study on the influence of solution pH on the microstructure of electrodeposited cobalt, reported that at pH 1.6 the deposition was accompanied by the evolution of a large a m o u n t of hydrogen and that the cobalt deposit contained an appreciable a m o u n t of hydrogen as cobalt hydride. At pH 5.7 the deposit incorporated cobalt hydroxide. Leidheiser et al. [20] studied the electrodeposition of cobalt at a pH of 4.9. From MSssbauer emission studies they supported the observations made by Okinaka and Nakahara [21] in that the elect r o w o n cobalt from a solution at pH 4.9 occludes CoOOH. According to them [ 2 0 ] , CoOOH is formed at the anode by oxidation of Co(II) and is carried to the cathode and occluded during deposition. Apparently, reduction of CoOOH to metallic cobalt is incomplete and a small amount of this o x y h y d r o x i d e retains its chemical integrity during and after inclusion in the deposit. Diaphragm cells have been used to protect the catholyte from acid generated at the anode in the case of the electrowinning of nickel [23] and also of cobalt [14]. In the present study an a t t e m p t was made also in this regard by using a diaphragm cell. Table 3 reports a comparative study of the electrowinning of cobalt in a diaphragm cell (i) and in a cell without diaphragm (ii). In the case of cell (ii) the bath pH decreased rapidly with increase of time of electrolysis and thus with decreasing current efficiency. In case of cell (i) the catholyte pH slowly decreased whereas the fall in anolyte pH was rapid, and more than 90% current efficiency was achieved throughout the period of electrolysis studied. Table 4 shows the result of the electrowinning of cobalt in a diaphragm cell where the bath pH was raised to ~ 7 . 0 and excess Co(OH)2 , added to the anode compartment, was kept in suspension by stirring throughout the period of electrolysis. It was observed that the catholyte pH remained constant during the whole range of the electrolysis period studied, but the anolyte TABLE3 C o m p a r a t i v e s t u d y of e l e c t r o w i n n i n g of c o b a l t in cell w i t h a n d w i t h o u t d i a p h r a g m Time
Cell w i t h d i a p h r a g m
Cell w i t h o u t d i a p h r a g m
(h) Anolyte pH
Catholyte pH
Current efficiency
Bath pH
Current efficiency (%)
2.67 2.13 1.83 1.66
90.3 86.0 80.1 75.9
(%) 10 40 60 120
2.48 1.90 1.74 1.43
4.16 3.54 3.13 2.54
99.8 91.0 91.0 90.8
C o b a l t c o n c e n t r a t i o n = 4 0 g/l, c u r r e n t d e n s i t y = 100 A / m 2, b a t h t e m p e r a t u r e = 30°C, a n d e l e c t r o l y t e p H = 4.20.
331 TABLE 4 Results of electrowinning of cobalt in diaphragm cell with excess Co(OH)2 added to the anode compartment Time Anolyte (min) pH
Catholyte pH
Current efficiency (%)
10 40 60 90 120 150 180 240
6.85 6.85 6.85 6.84 6.84 6.84 6.83 6.85
99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.9
6.80 6.75 6.66 6.66 6.55 6.50 6.24 4.02
Cobalt concentration = 40 g/l, bath pH = 6.92, cobalt hydroxide suspension in the anode compartment = ~ 3% by weight, stirring of anolyte = 100--150 min -1, current density = 100 A/m 2, and bath temperature -- 30°C. p H r e m a i n e d s t e a d y u p t o 1 8 0 m i n , a f t e r w h i c h it s u d d e n l y fell t o 4.02. This i n d i c a t e s t h a t t h e excess Co(OH)2 a d d e d was e x h a u s t e d d u r i n g t h e c o u r s e o f n e u t r a l i z a t i o n o f the acid g e n e r a t e d at t h e a n o d e . Close t o 100% curr e n t e f f i c i e n c y was a c h i e v e d t h r o u g h o u t t h e p e r i o d o f electrolysis studied. C o b a l t was d e p o s i t e d as bright, bluish s h e e t having s o m e b l u e s p o t s on t h e surface. T h e s h e e t s t a r t e d t o swell a n d c r a c k a f t e r a p e r i o d o f electrolysis o f 240 m i n . T h e d e p o s i t e d c o b a l t was brittle in n a t u r e . CONCLUSIONS In t h e e l e c t r o w i n n i n g o f c o b a l t f r o m a p u r e c o b a l t s u l p h a t e b a t h , it was o b s e r v e d t h a t c u r r e n t d e n s i t y , b a t h p H a n d b a t h t e m p e r a t u r e h a d significant e f f e c t s o n c u r r e n t e f f i c i e n c y , e n e r g y c o n s u m p t i o n a n d d e p o s i t m o r p h o l o g y . C u r r e n t densities u p t o 100 A / m 2 gave u n a c c e p t a b l e d e p o s i t s . F o r c u r r e n t densities higher t h a n 100 A / m 2 , c u r r e n t e f f i c i e n c y and e n e r g y consumption were poor, although deposits of better morphology were o b t a i n e d . L o w e r b a t h t e m p e r a t u r e s w e r e n o t suitable. I n c r e a s e of b a t h temperature increased current efficiency, decreased energy consumption and improved deposit morphology. Lower bath temperature and lower b a t h p H w e r e u n s u i t a b l e so far as c u r r e n t e f f i c i e n c y is c o n c e r n e d . At l o w e r b a t h t e m p e r a t u r e a n d higher b a t h p H ( > 4.0), higher c u r r e n t e f f i c i e n c y was achieved. F o r m a i n t a i n i n g higher b a t h p H , a d i a p h r a g m cell was used. A l t h o u g h a bright u n i f o r m d e p o s i t w i t h close t o 1 0 0 % c u r r e n t e f f i c i e n c y was achieved, t h e d e p o s i t e d m e t a l s a d h e r e d t o t h e s u b s t r a t e a n d w e r e h a r d a n d brittle. A c o b a l t d e p o s i t o f b e t t e r m o r p h o l o g y w i t h high c u r r e n t eff i c i e n c y ( ~ 90%) was a c h i e v e d at higher b a t h t e m p e r a t u r e (~60°C} and
332 at a b a t h p H r a n g e o f p H 2 . 0 - - 4 . 0 . T h u s , t h e p r e s e n t s t u d y o n t h e electrowinning of cobalt from a pure cobalt sulphate bath demonstrates the f o l l o w i n g c o n d i t i o n s f o r g e t t i n g s m o o t h , u n i f o r m , c o m p a r a t i v e l y bright a n d m a l l e a b l e d e p o s i t s w h i c h c a n be easily s t r i p p e d f r o m t h e s u b s t r a t e , w i t h a c u r r e n t e f f i c i e n c y o f m o r e t h a n 90% a n d w i t h m i n i m u m e n e r g y consumption: (a) C o b a l t c o n c e n t r a t i o n o f n o t less t h a n a b o u t 50 g/1. C o b a l t d r o p is ~ 1 0 g/1 (based o n 50 g/l) (b) C u r r e n t d e n s i t y o f 100 A / m 2 (c) B a t h t e m p e r a t u r e o f 60°C (d) p H range 2 . 0 - - 4 . 0 . ACKNOWLEDGEMENTS T h e a u t h o r s are i n d e b t e d t o Prof. P.K. Jena, D i r e c t o r a n d Dr. R.P. Das, Assistant D i r e c t o r , R e g i o n a l R e s e a r c h L a b o r a t o r y , B h u b a n e s w a r , Orissa, I n d i a f o r t h e i r i n t e r e s t in this w o r k . T h e y are also g r a t e f u l t o Prof. J e n a f o r his k i n d p e r m i s s i o n t o p u b l i s h this p a p e r .
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