Zinc, nickel and lead in copper deposited from copper(II) ammine sulphate electrolytes

Hydrometallurgy, 8 (1982) 173--183 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

173

Z I N C , N I C K E L A N D L E A D IN C O P P E R D E P O S I T E D F R O M C O P P E R ( I I ) AMMINE SULPHATE ELECTROLYTES

J. SEDZIMIR, W. GUMOWSKA and I. HARA~qCZYK Academy of Mining and Metallurgy, Cracow (Poland) (Received January 5, 1981; accepted in revised form September 30, 1981)

ABSTRACT Sedzimir, J., Gumowska, W. and Haranczyk, I., 1982. Zinc, nickel and lead in copper deposited from copper(II) ammine sulphate electrolytes. Hydrometallurgy, 8: 173--183. The influence of electrolytic variables on the content of Zn, Ni and Pb in copper deposited cathodically from ammine solution has been investigated. Solutions spiked with zinc or nickel sulphate were studied using a rotating cylindrical titanium cathode and a lead anode. The nickel and zinc contents in the cathodic deposit increase with increasing current density and their concentration in the electrolyte. A rise in the rotation speed of the cathode increases the lead and nickel contents but diminishes the zinc content in the deposit. To explain these results an attempt has been made to schematically reconstruct polarisation diagrams for each metal. Direct potential measurements are subject to considerable experimental error in the system studied. Polarisation measurements performed on a simplified system were in good agreement with the hypothetical diagram.

INTRODUCTION No d a t a are available for t h e b e h a v i o u r o f i m p u r i t y metals during t h e c a t h o d i c d e p o s i t i o n o f c o p p e r f r o m a m m i n e s u l p h a t e solution. This is o n e o f the m a i n obstacles t o using this m e t h o d for t h e r e c o v e r y o f c o p p e r f r o m solut i o n s o b t a i n e d b y leaching o f ores or scrap. In previous papers [ 1,2] t h e influence o f e l e c t r o l y t i c p a r a m e t e r 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 , q u a l i t y o f d e p o s i t a n d transfer o f lead f r o m the a n o d e have b e e n discussed. MATERIALS AND METHOD A r o t a t i n g cylindrical t i t a n i u m c a t h o d e ( d i a m e t e r 10 m m , length 60 m m ) s u r r o u n d e d b y a lead a n o d e ( d i a m e t e r 1 2 0 m m , length 70 m m ) was used. The c a t h o d e r o t a t i o n speed was c o n t r o l l e d b y m e a n s o f a s t r o b o s c o p e . All experim e n t s were p e r f o r m e d galvanostatically at a c o n s t a n t t e m p e r a t u r e o f 25 + 0.5°C. 0304-386X/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

174

A synthetic electrolyte, containing 1 mol/1 copper, 1.2 mol/1 sulphate and 6.5 mol/1 ammonia, was prepared using distilled water and a.r. reagents [ 11 The range of zinc and nickel concentrations was chosen to match their concentrations in the solutions obtained from ammonia leaching of copper ores or scrap. Solutions spiked with zinc or nickel sulphate remained clear even after several days. Furthermore, no traces of precipitate were observed after centrifuging for 10 min at 50 revolutions/s. The cathodic deposit was dissolved in concentrated nitric acid. Thioacetamide was used to precipitate most of the copper as a sulphide [3]. Zinc was determined polarographically [4] and nickel colorimetrically [3,5]. It was shown, using standard solutions, that these methods make possible the determination of zinc in copper with a precision of +- 1 p.p.m, and nickel with a precision of ± 0.5 p.p.m. The, determination of lead in copper deposits has been described previously [ 2 ] Current densities for the separate deposition of each metal were calculated from the weight gain of the cathode, the analytical data and the electrolysis time. Results presented are the mean values of two to four experiments. RESULTS AND DISCUSSION Figures 1 and 2 show the variation of the zinc, nickel and lead contents in copper, with the current density and the rotation speed of the cathode, Figure 3 shows the influence of the zinc and nickel concentrations on their contents in the cathodic deposit. The zinc, nickel and lead contents in the deposited copper are fixed by the ratio of their current densities to the total current density. Hence, electrolytic parameters which influence the trend of the cathodic polarisation lines o f the separate metals also influence the composition of the cathodic deposit. Schematic diagrams, Figs. 4 and 5, were constructed in order to analyse this problem. Figure 4 shows polarisation diagrams for the major metal A and for the impurities B and C which are, respectively, more and less noble than A. It was assumed that i B and ic, the current densities for deposition of B and C, were four orders of magnitude smaller than iA (by analogy with the practical system where it was found that iZn, iNi and iPb were three to six orders of magnitude smaller than iCu )- The shift of the curves B and C, in fig. 4, from position I to III corresponds to a change in the cathodic deposition regime from predominantly concentration- to predominantly activation-polarisation controlled. This is promoted by increases in the rotation frequency of the electrode and the concentration of the deposited metal in the bath. Fig. 1. Dependence of the zinc, nickel and lead contents in copper, cathodically deposited from ammine solution, on current density. Rotational frequency is 1 revolution/s. Fig. 2. Dependence of the zinc, nickel and lead contents in copper, cathodically deposited from ammine solution, on rotation speed of the cathode. Current density ik = 1250 A m-L

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T h e c o n t e n t s o f B and C in t h e c a t h o d i c d e p o s i t can be c a l c u l a t e d b y m e a n s o f eqn. (1) (X here r e p r e s e n t s B or C.) f r o m

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t h e c o r r e s p o n d i n g v a l u e s o f t h e current densities iA, i B and i C , w h i c h can be e x p r e s s e d in t h e s a m e units as in Fig. 4, at d i f f e r e n t p o i n t s o n its abscissa Fig. 3. D e p e n d e n c e of the zinc and nickel contents in copper, cathodically deposited from a m m i n e solution, on their concentrations in the electrolyte. Current density: 1 2 5 0 A m'L Fig. 4. Schematic polarisation diagrams for cathodic deposition of the major metal A and of the impurities B (more noble) and C (less noble). It is assumed that B and C are present in the electrolyte at m u c h smaller concentrations than A. Factors w h i c h decrease concentration polarisation will shift lines from I to III.

178

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179 The majority of t he results shown in Figs. 1--3 c o m p l y with this h y p o t h e sis. However, t h e sharp dr op in zinc c o n t e n t with increasing r o t a t i o n speed does n o t fit into t he scheme. To obtain a b e t t e r insight into the mechanism of the process Figs. 6 and 7 (Am -2)

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were constructed. These represent the dependence of the partial current density (determined coulometrically) on the total current density ik (determined amperometrically) and on t he r o t a t i o n of the cathode. The results in Fig. 6 indicate t hat the partial current densities of copper, zinc and nickel decrease with an increase in t he r o t a t i o n frequency, c o n t r a r y to expectation. Figure 7 shows t hat the sum iCu + ix is smaller t han ik and n ot -- as was assumed in the discussion of Figs. 4 and 5 -- equal to i k . This shows th at there is a cathodic reaction which has n o t been considered. In a

180 p r e v i o u s p a p e r [1] this was s h o w n t o be t h e r e d u c t i o n o f c o p p e r ( I I ) to c o p p e r ( I ) c o m p l e x e s . It s e e m s t h a t this r e a c t i o n has a k e y p o s i t i o n in t h e e x p l a n a t i o n o f t h e m e c h a n i s m o f t h e processes investigated. Figure 8, which r e p r e s e n t s a h y p o t h e t i c a l p o l a r i s a t i o n d i a g r a m , has b e e n c o n s t r u c t e d t a k i n g it i n t o a c c o u n t . D(~'posi[ K)f \

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In t h e a b s e n c e o f a n e x t e r n a l c u r r e n t t h e velocities o f t h e c a t h o d i c (Cu~Tn + e -~ C u ~ n ) a n d a n o d i c (Cu -~ C u ~ n + e} r e a c t i o n s [6,7] are equal. H e n c e t h e c o r r o s i o n r a t e a n d p o t e n t i a l are d e t e r m i n e d b y t h e p o i n t o f i n t e r s e c t i o n o f t h e p o l a r i s a t i o n lines. T h e c a t h o d i c d e p o s i t i o n o f c o p p e r begins at t h e p o t e n tial e °. T h e r a t e o f this p r o c e s s a n d h e n c e t h e c u r r e n t d e n s i t y icu can be calc u l a t e d f r o m t h e c h a n g e in t h e mass o f t h e e l e c t r o d e a n d f r o m t h e electrolysis time. F i g u r e 8 e x p l a i n s w h y t h e c u r r e n t e f f i c i e n c y o f t h e investigated p r o c e s s ( e x p r e s s e d b y t h e r a t i o icu :ik ) is less t h a n u n i t y . I t also e x p l a i n s t h e d e c r e a s e in t h e r a t e s o f c o p p e r , zinc a n d nickel d e p o s i t i o n , a n d rise in t h e v e l o c i t y o f lead d e p o s i t i o n w i t h t h e increasing r o t a t i o n f r e q u e n c y o f t h e c a t h o d e (Fig. 6). T h e increase in t h e c a t h o d e r o t a t i o n s p e e d d i m i n i s h e s t h e c o n c e n t r a t i o n p o l a r i s a t i o n o f r e a c t i o n [1] (Fig. 8). As a result, t h e p o l a r i s a t i o n line, which

181

represents the dependence of ik (i k equal to the sum of il and i2) on the potential, shifts from I toward II. This means that, at a constant total current density (i k ), the potential will increase and the deposition rate of copper will decrease from iTCu to iIICu. A similar effect takes place in the case of zinc and nickel. The increase in lead current density with cathode rotation speed (Fig. 6) can be explained by deposition within the range of dominant concentration polarisation (Fig. 8). Figure 8 cannot be considered to be a result proved by our experiments; it is only a hypothesis which makes all experimental observations consistent. The verification of this hypothesis would require direct determination of the polarisation lines. In the present system the proper determination of the polarisation lines is difficult. At the high applied current densities even small changes in the position of a Luggin capillary significantly influence the measured potential. The geometry of the system, with its rotating cylindrical (A m - 2 )

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182 electrode, increases this effect. Therefore an a t t e m p t was made to determine the polarisation lines indirectly. Advantage was taken of the fact thaL the rotation frequency of the electrode, within the range 0--1 revolutions/s, does not influence the rate of copper deposition [1 ]. This can be explained by the dominant influence of natural convection due to nitrogen evolution at the anode [ 1 ]. These facts lead to the conclusion that the polarisation diagram determined for a motionless flat cathode would be helpful in discussing the case of a cylindrical electrode rotating at frequencies not greater than ] revolution/s. Two capillaries were used simultaneously for potential measurements. These were situated at a distance of several mm and several tens of mm (measured with a precision of -+ 0.2 mm) from the electrode surface. The results so obtained enabled calculation of the value of the potential at the electrode surface This m e t h o d also eliminated errors connected with the screening of the electrode by the Luggin capillary placed in the traditional position. The m e t h o d presented permitted determination of the dependence of the copper cathode potential on the current density it~. These data, combined with the independently established dependencies of icu, izn, iNi, ipb on itz, enabled the authors to construct the polarisation lines shown in Fig. 9. These are in good agreement with the trend of the hypothetical curves in Fig.8 and seem to confirm the assumptions involved in its construction and, which is more important, the conclusions drawn from it. CONCLUSIONS (1) The influence of electrolytic variables on the zinc and nickel content of copper, deposited cathodically from ammine solution spiked with these elements, was investigated. The influence was f o u n d to increase with increasing cathodic current density and increasing zinc and nickel concentrations in the electrolyte. A sharp decline in the zinc content is observed with the increasing rotation speed of the cathode. The content of lead (from the anode) in the copper does not depend practically on the current density but increases significantly with the rotation frequency of the cathode. (2) Electrolysis time and the analytically determined masses of the deposited metals enabled the authors to calculate the separate current densities for the cathodic deposition of each metal. These increased with the applied current density. Increase in the rotation frequency of the cathode reduced the current densities of copper, zinc and nickel but increased that for lead deposition. (3) An a t t e m p t was made to analyse the influence of the electrolytic variables on the composition of the cathodic deposit obtained from the solutions containing traces of the more and less noble metals besides the salt of the major metal. This analysis leads to the conclusion that in the present system

183 another cathodic reaction exists apart from the deposition of metals. Previous publications have shown t h a t this is due to partial reduction of Cu 2÷ to Cu ÷ complexes. Taking all these processes into account, hypothetical polarisation diagrams were constructed which make all the observed facts in the system consistent. (4) Precise experimental determination of the polarisation lines, in a traditional way, is not possible in this system. The high current densities applied and the geometry of the system are sources of significant experimental error in measurements of the electrode potential. An indirect m e t h o d of measurement, for a simplified system, enabled the authors to eliminate the error connected with the potential drop in the layer of electrolyte between the end of a Luggin capillary and the electrode surface. It also eliminated the error caused by the screening of the electrode by the capillary; this is important at high current densities. The dependence of ik on the electrode potential, together with the previous. ly determined dependence of icu, izn, iNi, ipb on ik, enabled the construction of polarisation diagrams for the separate deposition of each metal. The results were in good agreement with this qualitative reconstruction of the polarisation diagram and seemed to confirm the assumption s about the process mechanism which have been presented in the text. REFERENCES 1 Sedzimir, J. and Kustowska, B., 1980. Hydrometatlurgy, 6: 171--183. 2 Sedzimir, J. and Jasinska, B., 1981. Archiwum Hutnictwa (in English), 26: 199--211. 3 Jackwerth, E. and Willmer, P., 1976. Fresenius Zeitschrift filr Analytische Chemie, 279(1) 23--27. 4 Milner, G., 1962. Polarografia. PWN Warszawa. 5 Christopherson, M. and Sandell, E., 1954. Analytica Chimica Acta, 10: 1--9. 6 Sedzimir, J. and Bujanska, M., 1978. Hydrometallurgy, 3: 233--248. 7 Sedzimir, J. and Bujanska, M., 1980. Corrosion Science, 20: 1029--1040.