Study of Electrochemical Behaviour and Surface Morphology of Copper Electrodeposit from Electrorefining with Lignin-Based Biopolymer and Thiourea as Additives

Abstract

Effect of single addition of a lignin-based biopolymer additive, DP 2782, at various dosages as well as in combination with thiourea on cathode polarization during copper deposition was investigated by performing potentiodynamic and galvanostatic measurements using a potentiostat. The result of electrochemical measurements was compared with the behaviour of cathode deposition with glue and thiourea additives. Surface micro-appearance of the copper deposit after 1 h galvanostatic scan under current density of 330 A/m² and various additive type and dosage was evaluated by scanning electron microscopy (SEM) examination. It was found that the addition of 2.5 mg/l biopolymer gave sufficiently polarizing effect of copper deposition. SEM examination of the cathode surface after galvanostatic scan revealed that the combination of the biopolymer with thiourea (2.5 and 3.5 mg/l) resulted in a compact deposit which is comparable with that resulted from the test with glue and thiourea.

Introduction

In order to obtain copper deposit with a smooth surface and to control the growth of deposit, small amount of additives are commonly added in the electrolyte of copper electrorefining and electrowinning. The additives that are widely used in industrial copper electrorefining are a combination of glue or gelatine, thiourea and chloride ions. Glue has been known to have a polarizing effect on the cathode potential during copper deposition. The presence of glue in the diffusion layer increases the diffusion limiting current density, while at the electrical double layer glue hinders charge transfer process recognized by a remarkable increase of the charge transfer overpotential and exchange current density [1]. Glue is used as a levelling agent which is expected to be adsorbed on the cathode surface with highest local current density such as edge areas, butts, nodule and dendrite seeds. Glue forms an isolating film on this area and inhibits copper deposition; hence the deposition is induced to take place on the other part of the cathode surface [2]. As a result, rapid growth of the deposit and nodule formation that can lead to short circuit is avoided. Despite its effectiveness in serving as inhibition agent in copper electrorefining and electrowinning, glue is hydrolysed slowly with time (i.e. within hours) at acidic condition and high temperature of electrolyte [1, 2]. Due to the degradation of glue with time, addition of glue only at the beginning of electrolysis is not appropriate and the glue must be added continuously during electrolysis. Commercial tank houses added glue in the basis of gram per tonne of deposited copper. The dosage used varies in a wide range from 50 to 120 g/t-deposited copper due to variations of current density and other parameters used in the different tankhouses [3].

Thiourea is used as an addition agent in copper electrorefining to promote copper nucleation. In CuSO₄–H₂SO₄ solutions, thiourea (TU) is oxidized in the presence of cupric ions to produce formamidine disulphide ((NH₂)₂(NH₂)₂C₂S₂) (further abbreviated by FDS). Both TU and FDS form complexes with cuprous (Cu⁺) and cupric (Cu²⁺) cations [4–6]. All these complexes can be eventually adsorbed on the electrode surface and/or participate in redox reactions, so that the overall mechanism on how thiourea affect copper deposition is complex [5–7]. Wang and O’Keefe found that at lower TU concentrations (less than 5 ppm), the polarization curves show an initial depolarization at lower current densities (less than about 30 mA/cm²) [8]. The transition from polarizing into depolarizing effect of thiourea by the increase of current density was reported by Jin and Ghali [6]. There are critical current densities for certain TU concentrations in which polarizing effects of TU change into depolarizing effects [6]. The mechanism of polarizing and depolarizing of TU on copper cathode was believed to be related with the predominance of the three complexes (i.e. [Cu(FDS)]⁺, ([Cu(TU)]⁺ and [Cu(TU)]²⁺) in the process of ionic copper deposition.

As glue, thiourea degrades also with time in acidic copper sulphate at high temperature. However, the degradation of thiourea is reported to be slower than glue (i.e. in days) and does not make significant problem as that of glue [1]. The problem of using thiourea is associated with the susceptibility of sulphur inclusion from thiourea to the copper deposit. It is pointed out that sulphur inclusion in the copper deposit has a deleterious effect on continuous casting of copper and quality diminution of the resulted copper wire products. The dosage of glue and thiourea additions must be carefully controlled. Moreover, the proper ratio of these additives is also to be maintained. Improper individual concentration and concentration ratio of glue/thiourea has been reported to induce nodule and dendrite growth [9–11].

A number of investigations have been conducted to find suitable alternative additives for copper electrorefining [12, 13]. Some alternative additives in interest are polyethylene glycol (PEG), several amine compounds, sulfonic acids and dimethyl thiourea [12]. These alternative additives are basically classified as substitutes for glue and substitutes for thiourea. Polyethylene glycols were reported to have a high thermal stability and slow chemical decomposition at higher temperatures [12]. In the present work, a fundamental study of electrochemical behaviour of copper deposition at the presence of a lignin-based biopolymer additive was made. Effect of single addition of the biopolymer additive at various dosages as well as in combination with thiourea on the cathode polarization during copper deposition was investigated by performing potentiodynamic and galvanostatic measurements using a potentiostat. The result was compared with the behaviour of glue and thiourea system. Surface micro-appearance of the copper deposit was evaluated by SEM. Moreover, the degradation behaviour of the additive was also studied.

Experimental

Potential-current and potential-time behaviour of copper electrodeposition at the presence of various types and dosages of additives was evaluated by conducting potentiodynamic and galvanostatic measurements using a potensiostat from Gamry Instrument. A pure copper plate was used as working electrode, while platinum wire and silver-silver chloride (Ag–AgCl) were used as auxiliary electrode and reference electrode, respectively. The working electrode was mounted in an acrylic resin in order to have active surface area of 1 cm × 1 cm. The back surface of the working electrode was connected with a copper wire using a solder through which connection to the potensiostat was made. The measurements were conducted in a Faraday cage to avoid electrical field disturbances from surrounding. The measurement set-up is illustrated in Fig. 1. All measurements were conducted at 65 °C in a solution containing 175 g/l of H₂SO₄, 45 g/l of Cu²⁺ and 50 mg/L of Cl⁻ (added as NaCl). The volume of solution was 250 mL and all chemicals used were of analytical grade. Cathodic polarization was made from open circuit potential to 300 mV below the open circuit potential with a scan rate of 0.167 mV/s. Galvanostatic measurements were carried out by applied current density of 330 A/m² and cathode potentials were recorded for 1 h. SEM examinations of the cathode surface after galvanostatic scans were done for several type and dosage of additives used in the galvanostatic measurements. Prior to the SEM examination, cathode surface was rinsed with distilled water and dried by an electrical dryer. Study of degradation of biopolymer additive was carried out by aging and stirring the electrolyte containing the biopolymer additive for 4, 12 and 24 h at temperature of 65 °C. Galvanostatic measurements were conducted by using the aged solution and the alteration of cathode potential at various aging time was evaluated. As a comparison, similar measurement in solution containing glue was also performed. Variations of additives concentration used in potentiodynamic and galvanostatic measurements are listed in Table 1.

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig1_HTML.gif — Fig. 1
The electrochemical mesurement set-up using a potensiostat

Table 1

Variation of additive concentrations used in potentiodynamic and galvanostatic measurements

No.	Glue (mg/l)	Thiourea (mg/l)	Biopolymer DP 2782 (mg/l)
1.	0	0	0
2.	2.5	0	0
3.	0	3.5	0
4.	2.5	3.5	0
5.	0	0	2.5
6.	0	0	25
7.	0	0	50
8.	0	3.5	2.5
9.	0	3.5	25
10.	0	3.5	50

Results and Discussion

Cathodic polarization curves at the absence and the presence of glue and thiourea with various dosages are shown in Fig. 2. It can be seen that the addition of glue with a dosage of 2.5 mg/l gave cathodic polarizing effect of copper deposition which shifted the cathode potential of copper deposition into lower level of 15–20 mV in the area of charge-transfer controlled in comparison to that at the absence of additives. Single addition of glue also promoted mass-transfer control of copper deposition indicated by the appearance of limiting current density of about 400 A/m² at cathode potential range of about 10–70 mV versus Ag/AgCl. The limiting current density is associated with the presence of glue molecules in the diffusion layer at the surface of cathode. The polarizing effect of glue is prominent and the measurement result is in accordance with the previous investigation reports [1, 11, 14]. Polarizing effect of glue is required to control the rate of deposit growth and give levelling effect of copper deposition.

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig2_HTML.gif — Fig. 2
Cathodic polarization curves of copper deposition at the absence and the presences of glue and thiourea with various dosages

Single addition of 3.5 mg/l thiourea gave also polarizing effect of copper deposition. Different to glue, no limiting current density was observed at the range of potential-current recorded at the presence of 3.5 mg/l thiourea which indicates that thiourea did not promote mass-transfer control of copper deposition. As has been pointed out in the previous section, thiourea tends to be adsorbed at the cathode surface and forms complexes with cuprous (Cu⁺) and cupric (Cu²⁺) cations that leads to initiation of copper nucleation. In combination with glue, the addition of thiourea of 3.5 mg/l (i.e. 1.4× glue) diminished the polarizing effect of glue. The presence of thiourea is needed to synergistically act with glue at an optimum dosage of both additives.

Effect of single addition of DP2782 biopolymer additive at various dosages (i.e. 1×, 10× and 20× of glue) on the cathode polarization during copper deposition is shown in Fig. 3. It was found that additions of DP 2782 biopolymer additive at concentrations of 2.5, 25 and 50 mg/l gave polarizing effect of copper deposition in comparison to that without additive. At lower current densities (i.e. less than about 250 A/m²), the polarizing effect of the biopolymer is lower than that of glue-thiourea system with a dosage of 2.5 mg/l and 3.5 mg/l, respectively. At higher current density, the biopolymer with a dosage of 2.5 mg/l exhibited higher polarizing effect than glue-thiourea system with the previous mentioned dosage. In order to know the effect of the DP 2782 biopolymer additive in combination with thiourea, similar polarization measurements were also carried out at the presences of the biopolymer and thiourea and the results are presented in Fig. 4. The addition of thiourea reduced the polarizing effect of DP 2782. As the previous measurement results at the absence of thiourea, the biopolymer dosage of 2.5 mg/l tend to give the highest polarizing effect among three dosages tested. This measurement results indicates that the biopolymer has a potential to be used in combination with thiourea. The polarizing effect of the biopolymer tends to decrease by the increase of the biopolymer dosage from 2.5 mg/l to 25 mg/l and 50 mg/l. The know-how of the biopolymer role and mechanism in affecting copper deposition is to be further clarified.

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig3_HTML.gif — Fig. 3
Cathodic polarization curves of copper deposition at the presence of various dosages of biopolymer of DP 2782 (i.e. 1×, 10× and 20× of glue) compared with glue-thiourea and the solution without additives

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig4_HTML.gif — Fig. 4
Cathodic polarization curves of copper deposition at the presence of various dosages of DP 2782 biopolymer (i.e. 1×, 10× and 20× of glue) compared with glue-thiourea and the solution without additives

To evaluate the profiles of cathode potential versus time at a constant current density, galvanostatic measurement by using potensiostat was also conducted. The applied current density used was 330 A/m², refers to that used in the electrorefining plant of PT. Smelting Gresik in Indonesia. The alteration of cathode potential reported versus Ag/AgCl reference electrode for 1 h was recorded. As a baseline, galvanostatic measurements of copper deposition at the absence of additive, at the presences of 2.5 mg/l glue, 3.5 mg/l thiourea, and 2.5 mg/l glue + 3.5 mg/l thiourea were also performed and the results are depicted in Fig. 5. The results obtained were similar with the potentiodynamic measurement results, in which glue polarizes the cathode, while thiourea tends to diminish the polarizing effect of glue. The presence of both glue and thiourea with a dosage of 2.5 mg/l and 3.5 mg/l, respectively shifted the cathode potential to 30–70 mV lower than that without additive during 1 h of copper deposition.

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig5_HTML.gif — Fig. 5
Profile of cathode potential versus time during 1 h galvanostatic measurements at the absence and the presences of glue and thiourea at various dosages

Profiles of cathode potential versus time during 1 h galvanostatic measurements at the absence of additives and at the presences of glue-thiourea and DP 2782 with various dosages are shown in Fig. 6. It can be seen that DP 2782 tend to give polarizing effect at all dosages except at 2.5 mg/l after 2800 s. This results confirms the potentiodynamic measurement result that indicated the polarizing effect of DP 2782. Different to the potentiodynamic measurement result, the polarizing effect of 2.5 mg/l DP 2782 (single addition and in combination with thiourea) is notably lower than that of glue with the similar dosage. At the initial period of electrolysis, the cathodic polarization level at the presence of DP 2782 of 2.5 and 25 mg/l is about 20 mV. The correlation between the galvanostatic profiles and copper deposit formation was investigated by conducting SEM examination of the deposit surface after the galvanostatic test. Surface examinations by SEM were conducted on the deposit from the galvanostatic run without additive, with glue-thiourea 2.5–3.5 mg/l, DP 2782 2.5 mg/l (single addition), DP 2782-thiourea 2.5–3.5 mg/l and DP 2782-glue 2.5–2.5 mg/l. SEM micrographs of the cathode surface obtained from 1 h galvanostatic run with current density of 330 A/m² under the previous mentioned additives dosages are presented in Fig. 7.

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig6_HTML.gif — Fig. 6
Profile of cathode potential versus time during 1 h galvanostatic measurements at the absence and the presences of glue, thiourea and DP 2782 biopolymer additives at various dosages

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig7_HTML.gif — Fig. 7
Micro-appearance of cathode surface after 1 h galvanostatic test a without additives magnification 500× b without additives magnification 2000× c with glue/thiourea 2.5/3.5 mg/l d with DP 2782 2.5 mg/l e with DP 2872-thiourea 2.5/3.5 mg/l and f with with DP 2872-glue 2.5/3.5 mg/l

The initial stage of copper nucleation and growth is a crucial step that determined the quality of copper cathode products. Deposition of a homogenous of initial layer on the entire surface of the cathode is required to obtain low roughness of the cathode surface. SEM examination revealed that without the use of additives, copper deposits tends to grow into columnar crystals and eventually lead to dendrite growth (Fig. 7a and b). At the presence of glue and thiourea (i.e. 2.5 and 3.5 mg/l), flat surface was obtained although micro voids exists between the crystals (Fig. 7c). Deposit with a tendency to form columnar crystals surface was also obtained from the test with single addition of DP 2782 (Fig. 4d). The crystal size of the copper deposit obtained by this condition was apparently coarser than that obtained from the condition with glue and thiourea. Meanwhile, compact and smooth surface of the deposit was obtained from the galvanostatic test with DP 2782 in combination with thiourea as can be seen in Fig. 7e. A combination of DP 2782 and glue resulted in a porous surface with voids between the crystals (Fig. 7f). Among all conditions, galvanostatic tests with glue and thiourea (i.e. 2.5 and 3.5 mg/l) and DP 2782 combined with thiourea (i.e. 2.5 and 3.5 mg/l) resulted in qualitatively the most smooth surface.

One of the main objective of the investigation with the biopolymer additive is to look for the additive which has more stability in high acidic solution and temperature. Obtaining an additive which effectively work at high acidic solution and temperature is important to be compatible with actual condition of industrial tank house which has variations in temperature as well as acid concentration from cell to cell (due to cropping activities). Degradation of glue and DP 2782 were compared by measuring cathode potentials after the solutions containing these additives were exposed at 65 °C for 4, 12 and 24 h. Profiles of cathode potential during 1 h obtained from galvanostatic tests of in solutions containing glue and DP 2872 at various aging times are depicted in Figs. 8 and 9, respectively. As has been predicted, degradation of glue has significantly occurred after 4 h which was indicated by a loss of the polarizing effect of glue. After 24 h, the polarizing effect of glue was completely removed and the cathode has even higher potential in comparison to that without additive. In contrast to glue, profile of cathode potential at the presence of DP 2782 in solution aged for 4, 12 and 24 are relatively unchanged. Thermal stability of the biopolymer would offer the possibility to dope the additive with the basis of gram per liter of electrolyte rather than in gram per tonne of deposited copper. Further long-term electrolysis test is to be done to evaluate the effectiveness of the biopolymer additive in producing good cathode quality. An active and stable additive is required to cope with tank house cell condition for producing a good quality of copper deposit.

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig8_HTML.gif — Fig. 8
Profile of cathode potential versus time in solution containing 2.5 mg/l glue at various aging times

../images/468727_1_En_124_Chapter/468727_1_En_124_Fig9_HTML.gif — Fig. 9
Profile of cathode potential versus time in solution containing 50 mg/l DP 2782 at various aging times

Conclusion

A fundamental electrochemical study using three electrode system coupled with surface analysis by scanning electron microscopy has been performed to study the possibility of using a biopolymer additive in copper electrolytic refining. The investigation results demonstrated that the biopolymer dosed at 2.5 mg/l was sufficient to give polarizing effect of copper deposition measured by potentiodynamic and galvanostatic scan and, hence has a potential to be used as glue-substitute. SEM examination of the cathode surface after galvanostatic scan revealed that the combination of the biopolymer with thiourea (i.e. 2.5 and 3.5 mg/l) resulted in a compact deposit which is comparable with the cathode quality resulted from the test with glue and thiourea (i.e. 2.5 and 3.5 mg/l). Study of the additive degradation by aging time for 24 h shows that the DP2782 biopolymer has a good thermal and chemical stability at high temperature and low pH which is indicated by a little alteration of cathode potential measured by galvanostatic scan at various aging time.

Acknowledgements

The authors thank PT. Smelting Gresik for providing glue and thiourea sample used in this research.

References

1.
Winand R (1984) Industrial electrodeposition of copper. Problems connected to the behaviour of organic additions. Process metallurgy 3, application of polarization measurements in the control of metal deposition. Elsevier, pp 133–145
2.
Stantke P (2002) Using CollaMat to measure glue in copper electrolyte. JOM 19–22
3.
Moats M, Robinson T, Davenport WG, Demetrio, KS (2007) Electrolytic copper refining—2007 world tank house operating data. In: Proceedings of copper—cobre 2007 international conference, vol. 5, pp 195–242
4.
Suarez DF, Olson FA (1992) Nodulation of electrodeposited copper in the presence of thiourea. J Appl Electrochem 22:1002–1010Crossref
5.
Jin S, Ghali E (2001) Effects of thiourea on the copper cathode polarization behaviour in acidic copper sulphate at 65 ℃. Metall Mater Trans B 32B:887–893Crossref
6.
Tarallo A, Heerman L (1999) Influence of thiourea on the nucleation of copper on polycrystalline platinum. J Appl Electrochem 29:585–591Crossref
7.
Krzewska S et al (1984) Electrochemical determination of thiourea and glue in the industrial copper electrolyte. Metall Trans B 15B
8.
Wang CT, O’Keefe TJ (1984) The influence of additives and their interactions on copper electrorefining. In: Richardson PE, Srinivasan S, Woods R (eds) International symposium on electrochemistry in mineral and metal processing, vol 84–10. The Electrochemical Society, Pennington, NJ, USA, p 655e670
9.
Veilleux B, Lafront AM, Ghali E (2001) Computerized scaled cells to study the effect of additive ratios and concentrations on nodulation during copper electrorefining. J Appl Electrochem 31(9):1017–1024Crossref
10.
Veilleux B, Lafront AM, Ghali E (2001) Effect of thiourea on nodulation during copper electrorefining using scaled industrial cells. Can Metall Q Can Inst Mining Metall Pet 40(3):343–354
11.
Mubarok Z, Filzwieser I, Paschen P (2005) Electrochemical and metallographic characterization of inhibitor variation in copper refining electrolysis. In: Proceedings of European metallurgical conference, Dresden, Germany
12.
Stelter M, Bombach H, Nesterov N (2002) Using polyethylene glycols as alternative inhibitors in copper electrorefining. JOM 32–36
13.
Tantavichet N, Priztker M (2006) Copper electrodeposition in sulphate solutions in the presence of benzotriazole. J Appl Electrochem 36
14.
Jin S, Ghali E (2001) Effects of gelatine, thiourea and chloride ion on the copper cathode polarization behaviour in acidic copper sulphate at 65 ℃. Can Metall Q 40(4):433–440Crossref

No.	Glue (mg/l)	Thiourea (mg/l)	Biopolymer DP 2782 (mg/l)
1.	0	0	0
2.	2.5	0	0
3.	0	3.5	0
4.	2.5	3.5	0
5.	0	0	2.5
6.	0	0	25
7.	0	0	50
8.	0	3.5	2.5
9.	0	3.5	25
10.	0	3.5	50

No.	Glue (mg/l)	Thiourea (mg/l)	Biopolymer DP 2782 (mg/l)
1.	0	0	0
2.	2.5	0	0
3.	0	3.5	0
4.	2.5	3.5	0
5.	0	0	2.5
6.	0	0	25
7.	0	0	50
8.	0	3.5	2.5
9.	0	3.5	25
10.	0	3.5	50