Examination of copper electrowinning smoothing agents. Part II: Fundamental electrochemical examination of DXG-F7

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1 Examination of copper electrowinning smoothing agents. Part II: Fundamental electrochemical examination of DXG-F7 A. Luyima, M.S. Moats *, W. Cui and C. Heckman Postdoctoral research fellow, associate professor, graduate research assistant and undergraduate research assistant, respectively, Materials Research Center, Missouri University of Science and Technology, Rolla, MO, USA * Corresponding author moatsm@mst.edu Abstract Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and galvanodynamic and chronopotentiometric techniques were used to characterize the effects of DXG-F7, a commercial smoothing agent, on copper electrodeposition onto 316L stainless steel from a synthetic electrolyte containing 40 g/l copper (Cu 2+ ), 160 g/l sulfuric acid (H 2 SO 4 ) and 20 mg/l chloride (Cl ) at 40 o C. The findings were compared against results using HydroStar 4208, a modified polysaccharide. The nucleation overpotential and plating potential observed in cyclic voltammetry and galvanodynamic testing showed that neither DXG-F7 nor HydroStar significantly polarized acidic copper electrodeposition. Subtle differences between the additives were found during EIS testing and modeling, but more research will be needed to fully understand these differences. Surface-roughness measurements of two-hour deposits indicate that DXG-F7 and HydroStar produced smoother deposits as their concentrations in the electrolyte increased. No difference in the surface roughness of deposits was detected between the additives. Based on the experimental evidence resulting from the laboratory testing, no significant difference was found between DXG-F7 and HydroStar at 20 mg/l Cl concentration, with the exception that DXG-F7 dissolves more easily in water than HydroStar. Minerals & Metallurgical Processing, 2016, Vol. 33, No. 1, pp. XXX-XXX. (tbd) An official publication of the Society for Mining, Metallurgy & Exploration Inc. Key words: Copper, Electrowinning, Additives, DXG-F7, HydroStar, Cyclic voltammetry, Electrochemical impedance spectroscopy Introduction Cathode deposits obtained from acidified copper sulfate electrolyte(s) without organic additives are often soft, coarsely crystalline and nodular (Sun and O Keefe, 1992). Therefore, small amounts of organic additives are added to the electrolyte to produce level and dense cathodes during copper electrowinning. The action of organic additives is thought to be due to their adsorption on the surfaces of the newly formed copper grains. Hence, any copper grain that grows excessively fast adsorbs a layer of organic additive, which inhibits growth. The result is a smoother and harder deposit of suitable purity (Sun and O Keefe, 1992). In part I of our four-part series on the examining of copper electrowinning smoothing agents, a review of previously published work on organic additives used during copper electrowinning was presented (Moats, Luyima and Cui, 2016). In the review, it was noted that many tankhouses had switched from guar to other saccharide-based products such as HydroStar and DXG-F7. Since the introduction of Chemstar Products Company s HydroStar into the market, this organic polymer additive that acts as a smoothing agent has been used with success at several copper electrowinning tankhouses (Robinson et al., 2013). HydroStar is a modified polysaccharide that had been reported to produce level and dense deposits at lower cost compared with previously used guar (Sandoval, Morales and Bernu, 2010). The electrochemical fundamentals of HydroStar have been measured in synthetic copper electrowinning electrolytes (Moats and Derrick, 2012; Helsten and Moats, 2013; Moats et al., 2014). It was reported that HydroStar does not dramatically affect the nucleation or growth overpotentials of copper electrodeposition while leveling the copper electrodeposit surface. DXG-F7, a synthetic oligosaccharide product from G- Process, is another organic additive introduced into the copper electrowinning market (Robinson et al., 2013). DXG-F7 is claimed to have several advantages over other organic leveling agents, namely, (1) it is completely soluble at room temperature, (2) it does not form lumps during preparation, and (3) it decreases the chance of cathode rejection by avoiding both nodule shorting and unwanted occlusions. However, no fundamental electrochemical studies have been published thus far on the effect of DXG-F7. Paper number MMP Original manuscript submitted April Revised manuscript accepted for publication September Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications Dept. prior to Aug. 31, Copyright 2016, Society for Mining, Metallurgy & Exploration Inc. 8

2 Therefore, this paper, which is Part II of our series, presents recent experimental findings on the fundamental characterization of copper electrodeposition in the presence of DXG-F7. These results are compared against results obtained in the presence of HydroStar. Additionally, two-hour copper-electrodeposition experiments were performed using nonagitated synthetic electrolytes with DXG-F7 and HydroStar, respectively, to examine their effects on deposit surface roughness. Experimental Materials. The base electrolyte chemicals copper sulfate pentahydrate, sulfuric acid and sodium chloride were analytical-grade reagents. Distilled deionized water was used in all solution preparation. The organic compounds were provided by copper mining companies, and 316L stainless steel coupons were obtained from cathode producer T.A. Caid. Titanium mesh coated with a mixed metal oxide of iridium oxide (IrO 2 ) and tantalum pentoxide (Ta 2 O 5 ) and a double-junction reference electrode (0.218 V versus the standard hydrogen electrode, SHE) were used as the working electrode (cathode), counter electrode (anode) and reference electrode, respectively, in each experiment. All potentials are reported versus the SHE. Electrolyte solution. For each experiment, one liter of synthetic electrolyte was prepared. The organic additive concentration was varied while the starting copper, sulfuric acid and chloride (Cl ) concentrations were held constant at 40 g/l, 160 g/l and 20 mg/l, respectively. The organic additives (DXG-F7 and HydroStar) were predissolved in deionized water to form aqueous stock solutions of known concentrations. DXG-F7 dissolved faster and more thoroughly than HydroStar in water. Organics were added to the electrolyte 30 minutes prior to each experiment. Apparatus setup. A three-electrode cell was used for all electrochemical measurements. The electrowinning cell was a 450-mL glass vessel with a polycarbonate cover. The stainless steel working electrode was prepared by successively polishing with 180-, 400- and 600-grit silicon carbide sandpaper prior to each experiment. After polishing, the stainless steel sample was washed with distilled deionized water in an ultrasonic cleaner and then masked with chemically resistant nonconductive tape, leaving the top centimeter untaped (electrical connection) and a deposition area of 1 cm 2. Additional tape was applied to ensure the electrolyte contacted only the deposition area. The counter electrode was IrO 2 -Ta 2 O 5 -coated titanium mesh (15 cm by 2.5 cm) which was washed with distilled deionized water and acetone between experiments. The distance between the working electrode and counter electrode was kept at approximately 3.8 cm for all experiments. The reference electrode used was a double-junction reference electrode (Fisher Scientific model number B). The electrodes were immersed in 250 ml of electrolyte solution 15 minutes prior to each experiment to reach the target temperature of 40 C. For electrochemical testing, a Gamry Instruments Reference 3000 Potentiostat/ Galvanostat/ZRA connected to a computer and controlled by Gamry Framework software was used. Cyclic voltammetry (CV) experiments. In order to establish a reference base, a study of the nucleation and growth of copper on stainless steel in a solution that was free of organic additives was conducted. The working electrode potential was cycled between and V versus the SHE at a potential sweep rate of 1 mv/sec. The cyclic voltammogram was recorded as current, I, versus potential, E. The current density was calculated using the known area of the working electrode. Typical cyclic voltammograms are shown in Fig. 1, where the potential values are reported versus the SHE and (a) shows the cathodic current density plotted on a linear scale while (b) shows the plot on a logarithmic scale. The recorded voltage sweep was started at point 1, and the curve 1 2 is the initial decreasing potential portion of the curve in which the applied potential is greater than the copper reversible potential, therefore no copper electrodeposition occurs. Point 2 is the potential at which the curve 4 5 crosses the zero current line and approximates the copper reversible potential for the solution being tested. The line 2 3 corresponds to the nucleation/activation overpotential associated with copper deposition onto the stainless steel substrate. Point 3 indicates the initiation of copper deposition onto the cathode. The curve 3 4 reveals the increase in current density as the potential continues to decrease, caused by the nucleation and growth of copper on the stainless steel substrate. At point 4, the decreasing potential applied to the cell by the potentiostat is changed to an increasing potential. The curve 4 2 corresponds to the polarization overpotential associated with copper deposition on freshly deposited copper, which is normally less than the polarization overpotential for copper deposition onto stainless steel (3 4), which results in a higher current density. The curve represents a nonsteady-state condition in which the substrate is increasingly covered by copper, so the shape of the curve will vary depending on the voltage scanning speed. The Figure 1 Cyclic voltammograms for acidified copper sulfate electrolyte without additives, with cathodic current density plotted on (a) linear scale and (b) logarithmic scale. 1: start of experiment; 2: crossover potential; 3: potential at which copper deposition is first observed; 4: reversing potential; 5: end of experiment (not shown in (b) as log of negative values cannot be displayed). (a) (b) 9 Vol. 33 No. 1 February 2016

3 curve 2 5 represents the anodic dissolution of the previously electrodeposited copper. The nucleation overpotential for copper deposition onto stainless steel, that is, the voltage difference between points 2 and 3, is a measurement used to examine the effect of organic additives and/or the plating substrate on the copper electrodeposition process. Point 2 was taken where line 4 5 crosses the zero current line, while point 3 was taken at the point where the slope of line 2 3 changes. Each CV test was performed at least twice and the values presented are the average of the tests. The tests were conducted with the addition of 1, 2, 5, 10 and 20 mg/l of DXG-F7 and 2.5, 5, 10 and 20 mg/l HydroStar in 40 g/l copper (Cu 2+ ), 160 g/l sulfuric acid (H 2 SO 4 ) and 20 mg/l chloride (Cl ) at 40 o C. Electrochemical impedance spectroscopy (EIS) experiments. The use of electrochemical impedance spectroscopy to characterize electrode processes and study the kinetics of electrochemical processes has been a mainstream electrochemical technique for many decades (Conway, Bockris and White, 1999; MacDonald, 1977). Recently, EIS has been used to study complex fluid solid interfaces and electrode processes as well as fundamental electrochemical characteristics, such as charge transfer resistance (R CT ) or polarization resistance (R P ), double-layer capacitance (C DL ), and mass-transfer resistance (Song and Seok, 2013). The application of EIS to understanding the electrodeposition of metals in the presence of additives has increased in recent years (Gabrielli et al., 2003; Fabian et al., 2009). The principle of EIS is to measure the impedance of an electrochemical reaction at a surface, usually at a fixed potential, while frequency is scanned to assess the mechanisms that regulate electrochemical kinetics. The results of EIS studies can be presented in the form of Nyquist plots, in which the imaginary and real impedances are plotted (Fabian et al., 2009). In an EIS experiment, the response analysis to a periodic, small-amplitude alternating-current signal applied to a target system provides electrochemical information about the system structure, electrode interface and relevant reactions (Song and Seok, 2013). Compared with other electrochemical methods such as galvanostatic measurement, CV and potentiometry, EIS can play a significant role in quantitatively characterizing electrochemical reactions (for example, their rate constants), which are key considerations in the design of electrochemical systems. In the current study, EIS experiments and corresponding equivalent circuit modeling were used to provide information about the charge transfer resistance and double-layer capacitance of copper electrodeposition without and with organic additives. EIS instrumentation. EIS data were recorded using a Gamry Instruments Reference 3000 Potentiostat/Galvanostat/ZRA. All EIS measurements were conducted in the three-electrode system described previously. The working electrode was polarized at V versus the SHE for 900 sec prior to the EIS measurement to plate fresh copper on the surface. The EIS was then conducted at V versus the SHE with a superimposed 5 mv rms sinusoidal potential with frequencies ranging from 0.1 to 100 khz, and the resulting current was recorded and impedance calculated by the software. The base electrolyte without organic additive was used for an initial impedance measurement during copper electrowinning. Each experiment was replicated to examine experimental reproducibility. EIS tests were conducted with the addition of 1, 5 and 10 mg/l of DXG-F7 or HydroStar in 40 g/l Cu, 160 g/l H 2 SO 4 and 20 mg/l Cl at 40 o C. Equivalent circuit modeling. To understand the effects of organic additives on copper electrodeposition, the EIS data were modeled using an equivalent circuit (Fig. 2) composed of electrical components. To model the current passing through the electrolyte in copper electrodeposition, a solution resistance (R S ) is inserted into an equivalent circuit as a series element along with a constant phase element, CPE, which is an equivalent electrical circuit component that models the behavior of an electrolyte double layer on an irregular surface, that is, an imperfect capacitor, or double-layer capacitance (C DL ). Both of these are nearly ideal circuit elements independent of frequency (Fabien et al., 2009). However, the components of the faradaic impedance are nonideal. As a result, the faradaic impedance consists of pure resistance, R CT, the charge transfer resistance, also known as polarization resistance (Fabien et al., 2009). In the present electrochemical cell, the diffusion impedance may also include two finite diffusion layers physically located in series: the organic additive layer and the Nernst layer. The equivalent circuit components used in this study for the organic additive layer were chosen based on previous research conducted on guar and polyacrylamide additives in copper electrowinning (Fabien et al., 2009). The equivalent circuit design, relevant data processing, and impedance parameter estimation of the electrochemical components in Fig. 2 were conducted using Gamry Echem Software Galvanodynamic experiments. The galvanostatic staircase method published by Moats and Derrick (2012) for copper electrowinning was also used to evaluate the nucleation and plating potentials of copper in the presence and absence of organic additives. The galvanostatic staircase method uses three sequential galvanodynamic ramps, as shown in Fig. 3a. Figure 2 Electrochemical-cell equivalent circuit with reference electrode (R.E), solution resistance (R S ), double-layer capacitance (C DL ), charge transfer or polarization resistance (R CT or R P ), capacitance (C 2 ), resistance (R 2 ) and working electrode (W.E). 10

4 (a) (1600, 600) (b) (2351, 0) (1500, 12.5) Figure 3 (a) Schematic diagram of galvanodynamic ramps used in the galvanostatic staircase method. (b) Results using the galvanostatic staircase method, with arrows showing the scan directions. Galvanodynamic ramp values used were t 1 = 1,500 sec, i 1 = 12.5 A/m 2, t 2 = 1,600 sec, i 2 = 600 A/m 2, t 3 = 2,351 sec, i 3 = 0 A/m 2. Nucleation (E n ) and plating (E p ) potentials are identified. In the first ramp, the current density was increased from 0 to 12.5 A/m 2 in 1,500 sec. The second ramp increased the current density to 600 A/m 2 in an additional 100 sec. The third ramp returned the current density back to 0 A/m 2 over the final 751 sec. Moats and Derrick (2012) explained that this method results in a well-behaved plot with easily identifiable nucleation (E n ) and plating (E p ) potentials. An example of a resultant plot is shown in Fig. 3b with E n and E p identified. The difference between E n and E p (ΔE) can also be computed. Galvanodynamic experiments were conducted with the addition of 1, 5 and 25 mg/l of DXG-F7 or HydroStar in 40 g/l Cu, 160 g/l H 2 SO 4 and 20 mg/l Cl at 40 o C. Surface-roughness (chronopotentiometry) experiments. Two-hour electrodeposition tests with and without organic additives were performed in the cell setup. Direct-current electrical power was supplied to the electrowinning cells by a Gamry Instruments Reference 3000 Potentiostat/Galvanostat/ ZRA, which was operated in galvanostatic control. Current and potential values were recorded once every second during each experiment. Surface-roughness experiments were conducted with the addition of 1, 5 and 25 mg/l of DXG-F7 or HydroStar in 40 g/l Cu, 160 g/l H 2 SO 4 and 20 mg/l Cl at 40 o C. After each two-hour test, cathodes were rinsed with distilled deionized water and dried. The surface roughness of the electrodeposited copper was analyzed using a Hirox KH digital microscope with 3D profile imaging software. All analyses were performed at the same microscope settings of 35x magnification and maximum lighting to minimize variation in the measurement technique. Surface roughness was measured from the 3D scans of the surface. The surface-roughness parameter R Z was used as the measure of surface roughness in this study. R Z examines the maximum peak-to-valley height for a line scanned across the surface. Nine lines were scanned for each deposit: three parallel lines in the vertical direction of the cathode, four parallel lines in the horizontal direction, and two diagonal lines. A spline filter with a cutoff wavelength (Raja, Muralikrishnan and Fu, 2002) of 2.5 mm was used to reduce some waviness caused by the thicker deposits at the edges resulting from the larger anode area to cathode surface area ratio used in the experiments. The reported R Z values are the averages of the nine line scans. Figure 4 Cyclic voltammograms in acidified copper sulfate electrolyte with and without addition of DXG-F7 organic additive. Figure 5 Cyclic voltammograms in acidified copper sulfate electrolyte with and without addition of HydroStar organic additive. 11 Vol. 33 No. 1 February 2016

5 Table 1 Summary of nucleation potentials and current densities from the copper electrowinning tests. Organic additive Organic additive (mg/l) Overpotential (mv) Current density at 0.22 V vs SHE on return sweep (A/m 2 ) None DXG-F Hydro- Star Figure 6 EIS curves at V versus SHE in acidified copper sulfate electrolyte with addition of DXG-F7 organic additive. Figure 7 EIS curves at V versus SHE in acidified copper sulfate electrolyte with addition of HydroStar organic additive. Results and discussion Dissolution of additives. During the preparation of the stock solutions of DXG-F7 and HydroStar, it was readily noticed that the DXG-F7 dissolved much easier than Hydro- Star. To quantify this difference, a simple dissolution test was conducted where 1.75 g of additive were placed into 500 ml of distilled deionized water at 40 o C. Agitation was provided by a magnetic stirrer bar rotating at 500 rpm. The DXG-F7 was visibly dissolved in 4 min while the HydroStar required between 9 and 21 hr. CV results. Figures 4 and 5 contain plots of the cyclic voltammograms produced without and with organic additives. Figure 4 depicts the data collected for DXG-F7 while Fig. 5 illustrates the data collected for HydroStar. Visually, the CV data collected with the different additives at various concentrations are similar in shape to the data generated in the presence of no additive. This indicates no gross differences in the polarization behavior. The cyclic voltammograms were further analyzed by extracting the nucleation overpotential and the plating current density at a fixed potential of 0.22 V versus the SHE during the return sweep. These selected values are presented in Table 1. The data indicate that neither a substantial decrease nor increase in the nucleation overpotential or current density at a fixed potential was observed with either additive relative to the experiments with no additives. The current density measurements at a fixed potential were on average higher with HydroStar than with DXG-F7, which would indicate that perhaps HydroStar has a slight depolarizing effect. However, as will be shown, the reverse could be true based on the galvanodynamic data. Therefore, the cyclic voltammogram data are interpreted as: neither DXG-F7 nor HydroStar significantly affects the nucleation overpotential or plating potential of copper electrodeposition. EIS results. The EIS results are shown in Nyquist plots presented in Figs. 6 and 7, with Fig. 6 showing the data for DXG-F7 and Fig. 7 showing the data for HydroStar. Each plot is characterized by a large quasi-half circle followed by either a line or a hump. In EIS, the high-frequency data occur on the left in a Nyquist plot and as the frequency decreases the data move to the right (higher real resistance). Visual examinations of the plots indicate that the DXG-F7 results are similar to those generated without additive. Hydro- Star produces a different shape at low frequency, with a 45 o line as opposed to a small hump for DXG-F7 at the right of the plot. A 45 o line is a typical indicator of a diffusion-controlled process, which is likely given that the experiments were performed with no external agitation. For each EIS experiment, equivalent circuit modeling was performed. The resulting calculated component values are presented in Table 2. The R CT, C DL and C 2 values obtained are more consistent (reasonable reproducibility) and errors associated with their calculation indicate that the values reported can be examined. The EIS data reveal that R CT and its associated error do not change significantly with the addition of DXG-F7 or HydroStar at any of the concentrations examined. This would indicate that neither additive is directly affecting electron transfer during the reduction of copper. This supports the previous interpretation of CV results, where neither additive affects the polarization of copper reduction. An examination of the C DL values indicates statistical differences between the data sets for no additive, with DXG-F7 12

6 Table 2 Summary of equivalent circuit parameters and their errors determined from fitting of EIS data. Additive mg/l R CT (Ω) R CT error (Ω) C DL (S.sec a ) C DL error (S.sec a ) R 2 (Ω) R 2 error (Ω) C 2 (S.sec a ) C 2 error (S.sec a ) None x x DXG-F x x x x x x HydroStar x x x x x x *Note: S = siemens; sec = second; a = CPE exponent with value between 0 and 1. and with HydroStar. HydroStar produced larger differences in the capacitance values relative to the no-additive value than DXG-F7. Muresan and Varvara (2005) indicated that additive molecules often decrease the double-layer capacitance by partially blocking the surface of the electrode. Thus, it appears that these saccharide molecules are absorbing onto the electrode surface but do not affect the kinetics of electron transfer. Increasing the concentration of the saccharide additive did not dramatically change the double-layer capacitance. The values calculated for the low-frequency data are not precise enough to be analyzed. The calculated errors associated with fitting R 2 and C 2 indicate that no discussion can be made. As there may be some differences between DXG-F7 and HydroStar based on visual examination of the low-frequency data, further studies could be conducted where the hydrodynamics of the system are better controlled, for example, by conducting the experiments with a rotating disk electrode. Galvanodynamic test results. The outputs of the galvanodynamic experiments are presented in Figs. 8 and 9 for DXG-F7 and HydroStar, respectively. As with the other electrochemical tests, the data appear visually similar with and without additives. However, the potential peak related to nucleation appears at lower current densities when organic additives are present, for both DXG-F7 and HydroStar. The average nucleation and plating potentials are summarized in Table 3. Previous research (Moats and Derrick, 2012) indicates that the nucleation potential appears to be controlled by the substrate. The freshly polished 316L stainless steel appears to control the nucleation potential both in the galvanodynamic and CV experiments. Glue and thiourea additives used in electrorefining (Moats and Derrick, 2012) do affect nucleation, but DXG-F7 and HydroStar do not affect the nucleation potential dramatically and the differences were determined to be within experimental error. While the nucleation potential did not change with the addition of either additive, the current density needed to achieve that potential was lower when the additives were present (2-3 A/m 2 ) than in the additive-free electrolyte (5 A/m 2 ). Based on the EIS data, it is believed that the additives are adsorbing onto the stainless steel surface. The resulting decrease in double-layer capacitance causes the electrode potential to Figure 8 Results of tests using galvanostatic staircase method with addition of DXG-F7 organic additive. Figure 9 Results of tests using galvanostatic staircase method with addition of HydroStar organic additive. 13 Vol. 33 No. 1 February 2016

7 Table 3 Summary of nucleation (E n ) and plating (E p ) potentials from the galvanodynamic tests. mg/l E n (V vs SHE) E p (V vs. SHE) at E = E E n p 300 A/m 2 (mv) None DXG-F Organic additive Hydro- Star increase more quickly with increasing current density until the nucleation potential of copper is reached. Thus, copper nucleates at lower current density in the presence of the additives than in their absence. The plating potential as measured at 300 A/m 2 during the return sweep was slightly different in the presence of the organic additives than in their absence. While the plating potentials were slightly more positive on average, the differences were not statistical differences relative to the experimental errors. Surface roughness of two-hour deposits. Figure 10 shows selected 3D renderings of the surfaces of two-hour electrodeposits. The surface of the deposit grown in the absence of smoothing agents exhibits cavities and protrusions (Fig. 10a). The surfaces of the deposits grown in the presence of DXG-F7 (a) (b) (c) Figure 10 Selected 3D renderings of the surfaces of 2-hr copper electrodeposits for (a) no additives, (b) 1 mg/l DXG-F7 and (c) 1 mg/l HydroStar. 14

8 Figure 11 Average surface roughness parameter (R Z ) for 2-hr copper electrodeposits as a function of concentration for DXG-F7 and HydroStar. or Hydrostar were smoother (Figs. 10b and 10c). The use of higher concentrations of either smoothing agent did not appear to change the roughness of the surface. To confirm these visual observations, surface-roughness profiles were collected. Figure 11 shows a plot of the average roughness parameter R z, calculated from the 3D scans taken with the Hirox KH-8700 digital microscope of the two-hour electrodeposits produced in our laboratory, against the additive concentration. The data indicate each additive regardless of concentration smooths to a similar roughness at 300 A/m 2 in nonagitated electrolytes. The two-hour electrowinning experiments indicate that both DXG-F7 and HydroStar smoothen or level the electrodeposited copper. Besides average roughness, the standard deviation of R z across each deposit (using the nine measurement lines) was also evaluated. The addition of additives decreased the standard deviation of R z from 66 micrometers with no additives to 27 micrometers with 25 mg/l of DXG-F7 and 31 micrometers with 25 mg/l of Hydrostar. The difference between the additives was not statistically significant. The roughness data indicate that enough additive effectively levels the copper deposit. From the electrochemical data, it appears that both additives are present near and/or on the surface (for example, lower double-layer capacitance). However, the additives are not affecting the charge transfer kinetics (for example, overpotential or charge transfer resistance). Based on these, the additives appear to be acting as interfacial and morphological inhibitors, which lead to deposit smoothing. Conclusions CV, EIS and galvanodynamic testing along with analysis of the surface roughness of two-hour deposits were used to examine the effects of two commercial saccharide-based copper electrowinning additives (DXG-F7 and HydroStar). The CV and galvanodynamic method indicate the nucleation overpotentials and plating potentials for copper electrodeposition in the presence of either additive were similar, and no significant electrochemical polarization was observed. In EIS modeling, both additives affected the double-layer capacitance calculated from an equivalent circuit model. The lower double-layer capacitance was also revealed in the galvanodynamic testing data. This is interpreted as the additives being at or near the electrode surface, which decreases the concentration of ions in the double layer. There were also differences observed in the low-frequency EIS data between HydroStar and DXG-F7, but more experiments with controlled hydrodynamics are needed to further understand this difference. Analysis of the roughness of two-hour copper electrodeposits indicated that both DXG- F7 and HydroStar produce smoother surfaces and to similar extent with increasing concentration. Based on these results, the additives appear to be acting as interfacial and morphological inhibitors, which lead to deposit smoothing. A potentially noteworthy difference from an operational perspective was that DXG-F7 dissolved much faster (4 min as opposed to 9-21 hr) in distilled deionized water at 40 o C than HydroStar. Acknowledgments The authors thank the Materials Research Center, Missouri S&T, for its support of this work as well as the copper mining companies that supplied the organic reagents. We would also like to thank Mr. Kevin Rudolph for conducting the additive dissolution experiments. References Ashford, B., Clayton, C., and Sandoval, S., 2012, Improved electrowinning process, Patent Application No. WO A2, 19 April Conway, B., Bockris, J., and White, R., 1999, Electrochemical Impedance Spectroscopy and Its Applications in Modern Aspects of Electrochemistry, Kluwer Academic/Plenum. Fabian, C.P., Ridd, M.J., Sheehan, M.E., and Mandinc, P., 2009, Modeling the charge-transfer resistance to determine the role of guar and activated polyacrylamide in copper electrodeposition, Journal of The Electrochemical Society, Vol. 156, No. 10, pp Gabrielli, C., Mocoteguy, P., Perrot, H., and Zdunek, A., 2003, Copper interconnects, low-k inter-level dielectrics, and new contact metallurgies/structures, The Electrochemical Society Proceedings Series, 2003, The Electrochemical Society, Pennington, NJ. Helsten, T., and Moats, M.S., 2013, An investigation of modified polysaccharide and polyacrylamide on plating polarization and surface roughness in copper electrowinning, Copper 2013 Conference Proceedings, Vol. V, Santiago, Chile, IIMCh pp Vol. 33 No. 1 February 2016

9 MacDonald, D., 1977, Transient Techniques in Electrochemistry, Plenum Press. Moats, M.S., and Derrick, A., 2012, Investigation of nucleation and plating overpotentials during copper electrowinning using the galvanostatic staircase method, Electrometallurgy 2012, pp Moats, M.S., Luyima, A., and Oliveria, T., 2014, Examination of selected copper electrowinning additives, Proceedings of Hydrometallurgy Moats, M.S., Luyima, A., and Cui, W., 2016, Examination of copper electrowinning smoothing agents. Part I: A review, Mineral & Metallurgical Processing, Vol. 33, No. 1. Muresan, L.M., and Varvara, S.C., 2005, Leveling and brightening mechanisms in metal electrodeposition, Metal Electrodeposition, Nova Science Publishers, Hauppauge, NY, pp Raja, J., Muralikrishnan, B., and Fu, S., 2002, Recent advances in separation of roughness, waviness and form, Precision Engineering, Vol. 26, No. 2, pp Robinson, T.G., Sole, K.C., Sandoval, S., Moats, M.S., Siegmund, A., and Davenport, W.G., 2013, Copper electrowinning: 2013 world tankhouse operating data, Proceedings of Copper 2013 Conference, Vol. V, Book 1, IIMCh, pp Sandoval, S., Morales, C., and Bernu, C., 2010, Development and commercialization of modified polysaccharide smoothing agent for copper electrowinning, SME Annual Conference & Expo 2010, Phoenix, AZ, Society for Mining, Metallurgy and Exploration, Englewood, CO. Song, S.J.B., and Seok, Y., 2013, Correlation between internal structure and electrochemical impedance spectroscopy of multiphase slurry systems, Analytical Chemistry, Vol. 85, pp Sun, M., and O Keefe, T.J., 1992, The effect of additives on the nucleation and growth of copper onto stainless steel cathodes, Metallurgical Transactions B,Vol. 23B, pp