Copper electrodeposition in the presence of guar or activated polyacrylamide

Fabian, Cesimiro P. (2005) Copper electrodeposition in the presence of guar or activated polyacrylamide. PhD thesis, James Cook University.

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Abstract

In the copper electrodeposition industry, it is essential to dose very small concentrations of additives including chloride ions to control the physical and chemical properties of the copper deposit. It is also widely recognized in the industry that the adsorption of these additives at electrode surfaces significantly influences the currentpotential relationships. This thesis presents the development of a new organic additive for copper electrowinning and electrorefining. This thesis also presents the comparison between the new organic additive ‘activated polyacrylamide’ and Guartec (Guar), the industry-standard organic additive for electrowinning under industrial conditions. The first half of the thesis presents the results of bulk electrolysis experiments using either a rotating cylinder electrode (RCE) or parallel plate electrodes. The RCE was selected since it yields an uniform current distribution. In these experiments, the effectiveness of both organic additives in the presence of chloride ions was determined by directly measuring the surface roughness of the copper deposit. The second half presents Cyclic Voltammetry and Electrochemical Impedance Spectroscopy data using a rotating cylinder electrode to measure and quantify the behaviour of Guar and APAM at the electrolyte/electrode interface also in the presence of chloride ions. It has been found from the literature review that polyacrylamide hydrolysis in weakly acidic solutions produces block copolymers. The diffusion layer thickness was determined from experimental limiting current density data. In this thesis/work, a 15million Dalton molecular weight polyacrylamide was dissolved in full-strength electrolyte (Cu, 36g/L; H2SO4, 160 g/L and chloride ions, 25mg/L) and in solutions with systematically halved concentrations, and in water and alkaline solutions at 50oC for two hours under stirring. The effect of the preparation media of polyacrylamide was tested by measuring the surface roughness of the copper deposits obtained after 6-hours electrowinning at 50oC using a rotating cylinder electrode. The statistically significant lowest surface roughness (6.59μm ± 0.49) was obtained when polyacrylamide was prepared in 16-fold diluted electrolyte (10g/L sulfuric acid and 2.25g/L cupric ions and 1.56mg/L chloride ions). The chemical structure of the polyacrylamide was examined after the preparation in 16-fold electrolyte using NMR. The NMR indicated that less than 10% of the non-ionic polyacrylamide was hydrolysed and it appears to consist of a block copolymer of polyacrylamide and polyacrylic acid. The new organic additive was named activated polyacrylamide (APAM)1. A comprehensive study on the effect of Guarfloc66 (Guar), the 40-year-old industry-standard organic additive, on copper electrowinning was lacking prior to this thesis. Guar was used as a basis against which to compare the effectiveness of APAM. This study provides an industrial-relevant baseline against which the new additive was compared. Therefore, the fundamental processes occurring at the stainless steel/copper metal-electrolyte interface in the presence and absence of Guar and APAM were investigated. Fractional factorial experimental designs for copper electrowinning (EW) were conducted in which the effect of current density, temperature, diffusion layer thickness, Guar and APAM on surface roughness was studied. The evolution of dendrite formation was evaluated using Peaks-per-Centimeter and surface roughness. These above tests were verified by experimental design tests up to 12-hours EW time. The overall results indicated that a more uniform surface and lower roughness was obtained using APAM than Guar. Cyclic voltammetry (CV) was used to understand the polarization behaviour of Guar and APAM at the interface of the copper electrolyte/copper cathode. Cyclic voltammetry tests on the ageing of 2mg/L Guar in the electrolyte at 45oC indicated that Guar depolarized the electrode. At 300A/m2 current density about 14mV maximum depolarization was obtained at 2-3 hours residence time whether the working electrode was stainless steel or freshly pre-plated copper metal. In contrast, CV tests showed that the presence of 2mg/L APAM polarized the electrode at 45oC and 65oC. At 300A/m2 current density and 45oC the presence of APAM resulted in a significant polarization of the electrode on pre-plated copper over a 5-hour period, i.e., 13 mV at 5-hours residence time. This polarization value at 65oC was achieved after 1-hour residence time and the electrode remains significantly polarized over a 7-hour period. Electrochemical Impedance Spectroscopy (EIS) tests were also conducted to characterize the electrode processes in the presence and absence of Guar and APAM. The electrochemical kinetics of the process was evaluated in terms of the chargetransfer resistance (or polarization resistance) and the double-layer capacitance. The electrochemical system of copper deposition in the presence and absence of Guar and APAM is described theoretically in terms of an equivalent circuit. The EIS experimental data was fitted to the equivalent circuit using complex nonlinear leastsquares (CNLS) technique with LEVM and ZSimpWin™ software packages. The electrode process in the presence of Guar or APAM was quantitatively characterized using the high frequency loop only since it represents the kinetic control of the process. The low frequency loop that represents the mass-transfer control was used qualitatively only. EIS tests indicate that Guar decreased the charge-transfer resistance values obtained from equivalent circuit modelling from about 0.83ohm.cm2 in its absence to 0.74ohm.cm2 at 2mg/L, -490mV versus the Hg/Hg2SO4 saturated in K2SO4, (MSE) for about 5-hours therefore increasing the electrochemical kinetics. The maximum reduction was determined to be 0.086ohm.cm2 at 2-3hours residence time of Guar in the electrolyte. Therefore, electrowinning using the RCE, CV and EIS indicated that the role of Guar during the deposition process is to act as a depolarizer. The presence of APAM in the electrolyte, in contrast, indicated that the chargetransfer resistance increased from about 0.79-ohm.cm2 to 1-ohm.cm2 for over 7-hours at 45oC and -490mV vs. MSE. The maximum increase in the charge-transfer resistance value was 0.23-ohm.cm2 at 3-5hours residence time. These set of tests with APAM were also repeated at -470mV vs. MSE and 45oC, the results also indicated that APAM increased the charge-transfer resistance under these conditions. At this potential, a maximum increase of 0.42-ohm.cm2 was determined at 3-5hours. When these tests were also repeated at electrorefining temperature (65oC) and -445mV vs. MSE, the presence of APAM in the electrolyte also increased the charge-transfer resistance from about 0.39 up to 0.69-ohm.cm2 for over 5-hours. The maximum increase of 0.34- ohm.cm2 was at 2-hours for 65oC. Electrowinning tests using the RCE, CV and EIS overall results with APAM were found to be consistent. In summary, APAM was found to decrease the electrochemical kinetics and that the role of APAM is as a levelling agent during the deposition process. Guar and APAM decreased the double-layer capacitance but the equivalent circuit simulation data indicated that while the maximum reduction for Guar was 0.46 x 10-5 μF/cm2, the maximum reduction for APAM was 1.2 x 10-5 μF/cm2 at 45oC and 6.8 x 10-5 μF/cm2 at 65oC. The overall EIS results may indicate that APAM is adsorbed more specifically than Guar. It was also shown that the effect of temperature on the ageing sequence of APAM is consistent with reaction kinetics, i.e., it is faster at 65oC than at 45oC as would be expected. Results were presented for the rotating disc electrode (RDE) also. The EIS data at 65oC and 45oC are consistent with the CV data whether the RCE or RDE was used. However, the EIS data with the RDE was not amenable to being modelled using the most commonly referred equivalent circuit for electrochemical systems. Bench-scale continuous electrowinning tests using parallel plate electrodes where APAM and Guar were also dosed continuously and independently also indicated that APAM produces smoother deposits than Guar. The cross sections of the copper deposits were examined using scanning electron microscopy (SEM) and showed that Guar produced porous deposits and APAM produced slightly columnar deposits. It is therefore concluded that the results of electrowinning, CV and EIS tests correlate: Guar depolarizes the electrode enhancing depolarization of the electrode by chloride ions and therefore increasing the growth rate with simultaneous formation of voids and porosity. APAM polarizes the electrode or decreases the rate of the deposition process and therefore assists the nucleation rate and produces purer copper deposits. Voids and porosity are probably reduced by the continuous formation of crystallites and their coalescence as indicated by x-ray diffraction data. The overall results indicate that APAM is more specifically adsorbed than Guar at the electrolyte/copper-metal interface and APAM acts as a levelling agent.

Item ID: 29
Item Type: Thesis (PhD)
Keywords: Copper, Electrodeposition, Guar, Activated polyacrylamide
Date Deposited: 20 Jun 2007
FoR Codes: 09 ENGINEERING > 0912 Materials Engineering > 091207 Metals and Alloy Materials @ 50%
09 ENGINEERING > 0904 Chemical Engineering > 090407 Process Control and Simulation @ 50%
SEO Codes: 86 MANUFACTURING > 8611 Basic Metal Products (incl. Smelting, Rolling, Drawing and Extruding) > 861102 Basic Copper Products @ 100%
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