Characterizing the Role of Organic Additives in Copper Electrowinning

Abstract

Additives are commonly used in copper electrowinning to improve cathode morphology. Polysaccharides such as guar are generally favoured as additives due to their compatibility with solvent extraction. Polyacrylamides are, however, being considered as possible alternatives. Understanding the influence of the structural chemistry of additives on electrowinning performance is critical to utilise them efficiently. This paper investigated polyacrylamide additives which are structurally different; their effects on copper electrodeposition were characterized and compared to that of guar. Electrochemical impedance spectroscopy (EIS) were used to characterize the effects of molecular weight and ionic charge of the polyacrylamides on electrodeposition. The findings were compared against results obtained with an industrially used guar product. Equivalent circuit modelling of EIS data indicated that an increase in polyacrylamide concentration and a decrease in additive molecular weight increased the overall system resistance.

Introduction

Copper electrowinning follows one of two major processing routes namely hydrometallurgical or pyrometallurgical processing. Oxide ores favour a hydrometallurgical processing route, where the ores characteristically undergo crushing, leaching, solvent extraction (SX), and finally electrochemical extraction of the copper in an electrowinning stage. Hydrometallurgical processing of copper has seen much growth over the past few decades with its contribution to the world’s total copper production rising from 3% in 1980 to 20% in 2005 [1]. This growth can be attributed to many factors including lower capital investments, lower energy consumption per copper cathode, and less environmental impact compared to pyrometallurgical processing [2]. The most significant success factor of hydrometallurgical processing is its ability to create high quality copper cathodes, predominantly by controlling the purity (e.g. SX) and the growth structure of the deposit on the cathode surface.

The crystal growth type and structure are predominantly determined by the real current density applied over the cathode surface. The real current density is primarily controlled by effecting proper cell design and layout, and by selecting appropriate materials of construction. However, even with the careful implementation of these control measures, current maldistribution still occurs in tankhouses resulting in system short circuiting caused by irregular crystal growth structures (dendrite growth). The solution to reducing current maldistribution has been to add an organic additive (inhibitor) to the electrowinning cell because they facilitate creation of smoother, denser, and brighter copper cathodes. Organic smoothing agents achieve this by interacting with the cathode surface to either polarize or de-polarize the copper deposition by increasing or decreasing the activity of the cuprous ions (Cu⁺) near the cathode surface, respectively [3].

Guar gum became the standard organic additive in copper electrowinning, mainly because of its compatibility with the transformative solvent extraction stage introduced in 1968 [1, 3]. Recently, a shift in the popularity of guar products has been observed on active tankhouses due to seasonality and high prices of the product [4]. Increasingly more research has been conducted on using more sustainable and economic alternative additives such as modified polysaccharides and polyacrylamides (PAMs) because these products are less expensive and are also reported to deliver a higher purity copper cathode [4–10]. Consequently, much work has been done to compare the performance of these increasingly popular organic additive products with each other and with the industry standard, guar. However, very little information is available on the relationship between adsorption mechanisms and molecular characteristics of the organic additives.

In this paper electrochemical impedance spectroscopy (EIS) was selected as a technique to extract kinetic parameters, polarization behaviour, and system impedance for copper electrodeposition in the presence of organic additives with varying molecular weights (MWs) and ionic content. This technique has recently been utilized in metal electrodeposition to describe the influence and interactions between organic additives and the substrate surface, since the electrochemical impedance structure can be affected significantly by adsorption phenomena [7, 8, 11, 12].

Experimental

Materials

A synthetic electrolyte containing 35 g/L copper (Cu²⁺), 160 g/L sulfuric acid (H₂SO₄), and 20 mg/L chloride (Cl⁻) was prepared with analytical grade reagents. De-ionized water was used for the preparation of the electrolyte. Organic additives were obtained from an industrial reagent manufacturing company (SENMIN). The organic additives were prepared before every experiment by dissolution in de-ionized water at a temperature of 40 °C for approximately 2 h. The organic additives were added to the synthetic electrolyte 30 min prior to the initiation of each experiment to allow for the organic additive to hydrolyze and unfold in the acidic solution. Two concentrations were investigated, namely 2 mg/L and 10 mg/L.

EIS experiments were conducted for the following conditions: no additive labelled “Base Case”, Guar (Sendep Opt43), a low MW (anionic), a medium MW (non-ionic), a high MW (anionic), and a very high MW (non-ionic) PAM additive at the two concentrations. In this paper the additives will be referenced by their MW classification only. Each unique EIS run was performed at least twice, and the average values are presented in the figures. In this paper high and low MW are relative terms, and each PAM additive were characterized and classified according to their MW and ionic content by their manufacturer.

Equipment

All experimental work was conducted using a three-electrode system. A 150 ml glass jacketed experimental cell was used. The electrolyte inside the cell was controlled to a constant temperature of 45 °C by circulating hot water from a temperature-controlled water bath through the cell jacket. The cell was used in conjunction with a Gamry RDE710 rotating disk electrode operated at 500 rpm. A platinum tip electrode with a surface area of 0.196 cm² was used for all experimental work. The working electrode surface was polished using 3 µm diamond suspension and an appropriate polishing cloth. After at least 3 min of polishing, the electrode was rinsed with deionized water and left to air dry. A graphite counter electrode and an Ag/AgCl reference electrode (0.210 V vs. standard hydrogen electrode) in 3 M KCl were used.

Methods

A Gamry Instruments Interface 1000 potentiostat was used to measure and control the electrochemical parameters inside the cell. EIS work was conducted after depositing copper for 6 min at a potential of 0.23 V versus standard hydrogen electrode (SHE). The frequency was swept between 100 kHz and 100 MHz and the DC voltage during the experimental runs was kept at 0.23 V while an AC voltage of 5 mV rms was maintained throughout. The software utilized to capture the electrochemical measurements was Gamry Framework version 7.05, while Gamry Echem Analysist version 7.05 was used for analysis and modelling purposes.

Results and Discussion

It has been determined that the metallographic structure of the deposit shows a strong correlation with the polarization characteristics of the electrodeposit [13]. Polarization refers to a decrease in the rate of reaction (current) at a fixed potential, while de-polarization refers to an increase in the rate of reaction at a fixed potential. An example of a depolarizing agent that is widely accepted in copper electrowinning, is the chloride ion [14]. Additives that inhibit or polarize the copper electrodeposit are believed to adsorb onto the surface of the working electrode, thereby increasing the effective current density resulting in an increase of the overpotential, a phenomena which is described by the Butler–Volmer equation [3]. Previous studies conducted by Fabian [16] found that guarfloc66 de-polarized copper deposition, while a high MW PAM additive (MW 15 million Da) consistently polarized the deposit. Moats et al. [9] found that a PAM additive Cyquest N-900 polarized the cathodic deposit while a Polysaccharide (‘HydroStar’) de-polarized the cathodic deposit [9, 15]. It was expected therefore that the polyacrylamides reported in this paper would exhibit similar polarization responses to those in the literature and that the guar studied would exhibit de-polarizing behaviour. The extent to which these characteristics were affected by molecular weight and anionic content was interrogated further.

EIS Results

Nyquist plots represent the data as a function of frequency presenting the imaginary impedance on the y-axis and the real impedance on the x-axis. Figure 1a and b show the Nyquist plots for each additive at 2 mg/L and 10 mg/L respectively. Each data point represents a measurement at a specific frequency, ranging from high frequencies (left side) to low frequencies (right side). The intercept on the x-axis at the high frequency region of the curves represent the solution resistance (R_S) of each experimental condition respectively. The intercept at the end of the first semicircle is the sum of the solution resistance and the charge transfer resistance (R_CT). The semicircle diameter in the high frequency region is therefore equal to the R_CT. The double layer capacitance (C_DL) can be calculated from the frequency at the top of the semi-circle. The capacitive semi-circle or inductive loop encountered in the low frequency region is more complex and less understood in terms of numbers and parameter values. The phenomena in the low frequency region are believed to be dependent on the surface preparation of the substrate, current density, hydrodynamics, as well as the deposit growth mode and crystallographic orientation [16].

../images/468727_1_En_123_Chapter/468727_1_En_123_Fig1_HTML.gif — Fig. 1
Nyquist plots showing the electrochemical impedance data for copper electrodeposition. Conditions: 35 g/L copper, 160 g/L H₂SO₄ and 25 mg/L Cl at 45 °C in a RDE setup at 500 rpm, copper electrodeposited for 6 min prior to EIS, DC potential of 0.23 V versus SHE with AC rms potential of 5 mV. Additive concentration: a 2 mg/L b 10 mg/L

Equivalent Circuit Modeling Results

Copper is predominantly present as a divalent cation (Cu²⁺) in the acidic electrolyte. During copper electroplating, the cupric ions migrate towards the negative cathodes primarily via diffusion and convection. Once at the cathode surface the cupric ion undergoes a series of known and recognized reactions before it is transformed into a solid copper deposit [17]. These reaction mechanisms are less understood in the presence of organic additives. EIS is an effective tool to investigate reaction mechanisms at a solution-solids interface by quantifying the real impedance. A mathematical model, presented in Fig. 2, was implemented for further analysis of the electron transfer impedance by formulating comparable, theoretical values.

../images/468727_1_En_123_Chapter/468727_1_En_123_Fig2_HTML.gif — Fig. 2
Equivalent circuit model with reference electrode (RE), solution resistance (R_S), charge transfer resistance (R_CT), constant phase element (CPE), capacitance (C₂), Resistance (R₂), and working electrode (WE)

To model the current passing through the copper containing electrolyte during electrodeposition a series element, solution resistance (R_S) and a constant phase element (CPE), are inserted into an equivalent circuit. A constant phase element is an equivalent circuit component that models the behaviour of the double layer existing between the electrolyte and an irregular surface e.g. an imperfect capacitator [9]. The faradaic impedance components are considered to be non-ideal, and in the present study it includes the charge transfer resistance or polarization resistance (R_CT). The low frequency data is modelled to include a second constant phase element and a second resistance arranged in a parallel configuration. The low frequency components model the impedance caused by diffusion of the copper ions through the Nernst boundary layer or the interaction of copper with the organic additive. The components in the equivalent circuit, R₂ and C₂, were inserted to model the interaction of the copper with the organic additive layer; this approach was based on the model used by Fabian et al. [7] and Moats et al. [9] to describe the impedance behaviour caused by guar and polyacrylamide additives.

Table 1 contains the calculated impedance values modelled with the Echem analyst Gamry software. The content of Table 1 is discussed along with the Nyquist plots in the subsequent discussions. The CPE elements are presented in units of siemens · second^α (S·s^α). The average values of exponent α for CPE and C₂ was 0.6 ± 0.014 and 0.81 ± 0.048 respectively. The value of α did not vary significantly between concentration limits. The errors experienced with non-determinant (N.D.) values were substantially larger than the actual value.

Table 1

Summary of the parameters with their errors obtained from equivalent circuit model and model fitting

Additive	mg/L	R_S (Ω cm²)	R_CT (Ω cm²)	CPE (mS s^a/cm²)	R₂ (Ω cm²)	C₂ (S s^a/cm²)	Chi-square goodness of fit
Base case	–	0.682 ± 0.272	1.952 ± 0.069	1.47 ± 1.24	0.271 ± 0.088	0.334 ± 0.325	0.0007
Guar	2	0.664 ± 0.022	2.070 ± 0.069	2.24 ± 0.54	0.078 ± 0.064	0.323 ± 0.743	0.0004
Guar	10	0.916 ± 0.030	1.957 ± 0.145	2.29 ± 0.62	0.457 ± 0.207	0.600 ± 0.782	0.0007
Low MW PAM	2	0.525 ± 0.017	N.D.	0.91 ± 0.21	N.D.	N.D.	0.0019
Low MW PAM	10	0.595 ± 0.016	8.987 ± 0.163	1.32 ± 0.13	1.705 ± 0.803	1.169 ± 0.727	0.0015
Medium MW PAM	2	0.321 ± 0.017	3.841 ± 0.164	2.13 ± 0.23	1.315 ± 0.576	0.973 ± 0.293	0.0013
Medium MW PAM	10	0.450 ± 0.020	5.077 ± 0.118	2.59 ± 0.27	0.641 ± 0.241	0.377 ± 0.142	0.0014
High MW PAM	2	0.614 ± 0.031	N.D.	1.69 ± 0.34	N.D.	0.270 ± 0.115	0.0005
High MW PAM	10	0.580 ± 0.019	6.610 ± 7.332	1.35 ± 0.24	N.D.	0.262 ± 0.595	0.0013
Very high MW PAM	2	0.426 ± 0.020	3.359 ± 0.075	2.86 ± 0.34	0.511 ± 0.217	1.255 ± 0.423	0.0015
Very high MW PAM	10	0.314 ± 0.020	4.836 ± 0.142	2.64 ± 0.26	1.673 ± 0.506	0.602 ± 0.145	0.0030

Discussion

Figure 1 and Table 1 indicate a charge transfer resistance, R_CT, value of 1.95 Ω cm² at 45 °C and 500 rpm in the high frequency loop for the base case scenario. The R_CT value for the high frequency loop was also determined in previous EIS studies in similar electrolytic solutions, as shown in Table 2. These R_CT values are compared with the present study to confirm the reliability of results and experimental accuracy.

Table 2

Collection of charge transfer values and conditions obtained from similar previous studies

Study reference	R_CT value (Ω cm²)	Conditions
Kelly et al. [18]	1.97	RDE, 400 rpm, 36 mA/cm²
Kelly et al. [18]	1.47	RDE, 2500 rpm, 43 mA/cm²
Gabrielli et al. [11]	1.70	RDE, 100 rpm, 25 mA/cm²
Fabian et al. [7]	1.45	RCE, 25 rpm, 30 mA/cm²
Current study	1.95	RDE, 500 rpm, 30 mA/cm²

The R_CT value obtained in the present study correlates best with the R_CT value obtained in the study of Kelly et al. [18]. The slightly lower reported R_CT value in this work compared to that of Kelly et al. [18] can be due to the higher disk rotation setting, since a higher disk rotation setting is associated with a lower R_CT in the system. Comparing the R_CT value of the current study with that in the work of Gabrielli et al. [11] and Fabian et al. [7], the higher R_CT value in the current study can be due to the lower chloride content in the electrolyte (25 mg/L) compared to the reported chloride content (35 mg/L) in the electrolyte used by Gabrielli et al. [11]. Fabian et al. [7] utilized a rotating cylinder electrode (RCE) as opposed to a RDE, reporting that a more uniform current distribution may be achieved in a RCE and hence the lower reported R_CT value. In Fig. 1 the system resistance decreased when guar was added to the system, indicating that guar de-polarized the copper deposition. The de-polarization of the copper deposition in the presence of guar aligns with the findings of Fabian et al. [7]. It is therefore concluded that the present study produced realistic impedance values that can be corroborated with those reported in previous studies.

The Nyquist plots indicate an increasing trend of overall system resistance with a decrease in MW of the PAM organic additives at both levels of additive concentrations. Lower degrees of polarization/R_CT are possibly related to low surface coverage. In a previous study Grchev et al. [19] concluded that the adsorption of non-ionic PAM in 0.5 M sulfuric acid solution on gold and mild steel is strongly dependant on the MW of the polymer, electrode potential, and temperature. Major highlights drawn from that work were:

i.
The maximum surface coverage in the temperature range of 20–80 °C for a concentration of 20 mg/L polyacrylamide remains unaffected by the MW of the polymer.
ii.
The surface coverage of dissolved polyacrylamide at concentrations ranging between 2–3 mg/L decreased from about 0.52 to 0.02 as the MW of the polyacrylamide increased from 5 × 10³ Da to 1.5 x 10⁶ Da.

The EIS results of the present study are aligned with the findings of Grchev et al. [19]. It is concluded that at a PAM concentration of 2 mg/L the lower MW additives achieve greater surface coverage resulting in an increased polarization effect. Conversely the higher MW PAM additives achieved less surface coverage, resulting in a lower degree of polarization. Grchev et al. [19] also determined that it is more thermodynamically favourable for low MW PAM additives to adsorb onto gold and mild steel surfaces than for high MW PAM additives based on the $\Delta {\text{G}}_{\text{ads}}^{ \circ }$ values. The calculated $\Delta {\text{G}}_{\text{ads}}^{ \circ }$ values increased with an increase in PAM MW, meaning that spontaneous surface adsorption becomes more unfavourable with an increase in the PAM MW [19].

In the high frequency region, a significant increase in polarization/R_CT and C_DL was observed at an additive concentration of 10 mg/L compared to 2 mg/L, for all MWs. This dissimilar polarization behaviour at 2 mg/L and 10 mg/L of additive concentration suggests that there are interactions between the organic additive and the copper in the system, and this interaction is enhanced at higher additive concentrations due to excess additive. Such interactions may be a result of chemistry similar to that reported by Gabrielli et al. [11] and Fabian et al. [7], who suggested the formation of an Additive-Cl⁻–Cu⁺ complex via covalent bonding. The increased R_CT and C_DL values at 10 mg/L additive concentration are likely to be due to a passivating film of species like PAM-Cl⁻¹–Cu⁺ complexes formed over the freshly deposited copper. No significant increase in polarization/R_CT and C_DL (Table 1) was observed when guar (polysaccharide) was utilized as organic additive. This agrees with the work of Luyima et al. [8] where polysaccharide additive (HydroStar & DXG-F7) concentration was found to have had no significant effect on the R_CT and C_DL values.

The second capacitive loop can arise from diffusion of copper to the substrate surface (Warburg impedance) or because of the presence of a second electrical interface. A high degree of agitation was provided to the system to eliminate the possibility of the second capacitive loop arising because of copper diffusion. It is therefore deduced that the second capacitive loop arises from the interaction of the organic additive with the copper, forming a second electrical interface. At the low frequency region in Fig. 1a, a second capacitive loop (semi-circle) is observed specifically for the non-ionic additives (medium and very high MW) in contrast with the inductive loops (curling loop) observed for the Base Case, guar and anionic additive (low and high MW) scenarios. This suggests that the non-ionic additives interact with the copper surface in a way dissimilar to the anionic additives. Gabrielli et al. [11] reported that the inductive loop is related to the action of an accelerator additive and a capacitive loop to a suppressor additive, and to their degree of surface coverage. Guar acted as an accelerator (de-polarizer) and therefore the presence of an inductive loop correlates with the findings of Gabrielli et al. [11]. The anionic PAM additives in this work however do not behave as accelerators but rather as suppressors, and this does not correlate well with the findings of Gabrielli et al. [11].

Conclusion

This paper investigated polyacrylamide additives which are structurally different, and their effects on copper electrodeposition were characterized and compared to that of a base case scenario and guar. A RDE and potentiostat setup was utilized to conduct EIS experimental work to characterize the effects of molecular weight and ionic charge of the polyacrylamides on electrodeposition. These techniques were implemented to draw out impedance information, specifically relating to the system polarization behaviour (R_CT). The EIS data (Nyquist plots) and subsequent equivalent circuit modelling results were found to be reliable, since the base case R_CT value compared well with previous studies and all minor divergences could be accounted for. Polarization resistance and double layer capacitance increased for a decrease in PAM additive MW. The R_CT values at 2 mg/L additive concentration gradually increased from 3.4 to 6.92 Ω cm² as the additive MW decreased from very high to low. Increased polarization behaviour for lower PAM MW additives was attributed to a higher degree of surface adsorption/coverage achieved compared to high MW additives. No clear correlation between polarization behaviour and additive ionic content was observed. An increased additive concentration of 10 mg/L also enhanced the polarization resistance and double layer capacitance, and these occurrences were explained by the formation of PAM-Cl⁻–Cu⁺ complexes, adsorbing onto the cathode surface. It was determined that the formation of secondary loops in the low frequency region is because of a secondary electrical interface layer caused by the additives. Capacitive semi-circles were found for non-ionic PAM additives, being absent for the anionic PAM additives where evidence of inductive loops was observed. This phenomenon should be investigated further.

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