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• W75082009 abstract "In the copper electrodeposition industry, it is essential to dose very smallconcentrations of additives including chloride ions to control the physical and chemicalproperties of the copper deposit. It is also widely recognized in the industry that theadsorption of these additives at electrode surfaces significantly influences the currentpotentialrelationships. This thesis presents the development of a new organic additivefor copper electrowinning and electrorefining. This thesis also presents the comparisonbetween the new organic additive ‘activated polyacrylamide’ and Guartec (Guar), theindustry-standard organic additive for electrowinning under industrial conditions.The first half of the thesis presents the results of bulk electrolysis experimentsusing either a rotating cylinder electrode (RCE) or parallel plate electrodes. The RCEwas selected since it yields an uniform current distribution. In these experiments, theeffectiveness of both organic additives in the presence of chloride ions was determinedby directly measuring the surface roughness of the copper deposit. The second halfpresents Cyclic Voltammetry and Electrochemical Impedance Spectroscopy data usinga rotating cylinder electrode to measure and quantify the behaviour of Guar and APAMat the electrolyte/electrode interface also in the presence of chloride ions.It has been found from the literature review that polyacrylamide hydrolysis inweakly acidic solutions produces block copolymers. The diffusion layer thickness wasdetermined from experimental limiting current density data. In this thesis/work, a 15million Dalton molecular weight polyacrylamide was dissolved in full-strengthelectrolyte (Cu, 36g/L; H2SO4, 160 g/L and chloride ions, 25mg/L) and in solutionswith systematically halved concentrations, and in water and alkaline solutions at 50oCfor two hours under stirring. The effect of the preparation media of polyacrylamide wastested by measuring the surface roughness of the copper deposits obtained after 6-hourselectrowinning at 50oC using a rotating cylinder electrode. The statistically significantlowest surface roughness (6.59μm ± 0.49) was obtained when polyacrylamide wasprepared in 16-fold diluted electrolyte (10g/L sulfuric acid and 2.25g/L cupric ions and1.56mg/L chloride ions). The chemical structure of the polyacrylamide was examinedafter the preparation in 16-fold electrolyte using NMR. The NMR indicated that lessthan 10% of the non-ionic polyacrylamide was hydrolysed and it appears to consist of ablock copolymer of polyacrylamide and polyacrylic acid. The new organic additive wasnamed activated polyacrylamide (APAM)1.A comprehensive study on the effect of Guarfloc66 (Guar), the 40-year-oldindustry-standard organic additive, on copper electrowinning was lacking prior to thisthesis. Guar was used as a basis against which to compare the effectiveness of APAM.This study provides an industrial-relevant baseline against which the newadditive was compared. Therefore, the fundamental processes occurring at the stainlesssteel/copper metal-electrolyte interface in the presence and absence of Guar and APAMwere investigated. Fractional factorial experimental designs for copper electrowinning(EW) were conducted in which the effect of current density, temperature, diffusion layerthickness, Guar and APAM on surface roughness was studied. The evolution ofdendrite 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 wasobtained using APAM than Guar.Cyclic voltammetry (CV) was used to understand the polarization behaviour ofGuar 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 thatGuar depolarized the electrode. At 300A/m2 current density about 14mV maximumdepolarization was obtained at 2-3 hours residence time whether the working electrodewas stainless steel or freshly pre-plated copper metal. In contrast, CV tests showed thatthe presence of 2mg/L APAM polarized the electrode at 45oC and 65oC. At 300A/m2current density and 45oC the presence of APAM resulted in a significant polarization ofthe electrode on pre-plated copper over a 5-hour period, i.e., 13 mV at 5-hours residencetime. This polarization value at 65oC was achieved after 1-hour residence time and theelectrode remains significantly polarized over a 7-hour period.Electrochemical Impedance Spectroscopy (EIS) tests were also conducted tocharacterize the electrode processes in the presence and absence of Guar and APAM.The electrochemical kinetics of the process was evaluated in terms of the chargetransferresistance (or polarization resistance) and the double-layer capacitance. Theelectrochemical system of copper deposition in the presence and absence of Guar andAPAM is described theoretically in terms of an equivalent circuit. The EISexperimental data was fitted to the equivalent circuit using complex nonlinear leastsquares(CNLS) technique with LEVM and ZSimpWin™ software packages. Theelectrode process in the presence of Guar or APAM was quantitatively characterizedusing 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 qualitativelyonly.EIS tests indicate that Guar decreased the charge-transfer resistance valuesobtained from equivalent circuit modelling from about 0.83ohm.cm2 in its absence to0.74ohm.cm2 at 2mg/L, -490mV versus the Hg/Hg2SO4 saturated in K2SO4, (MSE) forabout 5-hours therefore increasing the electrochemical kinetics. The maximumreduction was determined to be 0.086ohm.cm2 at 2-3hours residence time of Guar in theelectrolyte. Therefore, electrowinning using the RCE, CV and EIS indicated that therole of Guar during the deposition process is to act as a depolarizer.The presence of APAM in the electrolyte, in contrast, indicated that the chargetransferresistance increased from about 0.79-ohm.cm2 to 1-ohm.cm2 for over 7-hours at45oC 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 APAMwere also repeated at -470mV vs. MSE and 45oC, the results also indicated that APAMincreased the charge-transfer resistance under these conditions. At this potential, amaximum increase of 0.42-ohm.cm2 was determined at 3-5hours. When these testswere also repeated at electrorefining temperature (65oC) and -445mV vs. MSE, thepresence of APAM in the electrolyte also increased the charge-transfer resistance fromabout 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 EISoverall results with APAM were found to be consistent. In summary, APAM was foundto decrease the electrochemical kinetics and that the role of APAM is as a levellingagent during the deposition process.Guar and APAM decreased the double-layer capacitance but the equivalentcircuit simulation data indicated that while the maximum reduction for Guar was 0.46 x10-5 μF/cm2, the maximum reduction for APAM was 1.2 x 10-5 μF/cm2 at 45oC and 6.8x 10-5 μF/cm2 at 65oC. The overall EIS results may indicate that APAM is adsorbedmore specifically than Guar.It was also shown that the effect of temperature on the ageing sequence ofAPAM is consistent with reaction kinetics, i.e., it is faster at 65oC than at 45oC as wouldbe expected. Results were presented for the rotating disc electrode (RDE) also. TheEIS data at 65oC and 45oC are consistent with the CV data whether the RCE or RDEwas used. However, the EIS data with the RDE was not amenable to being modelledusing the most commonly referred equivalent circuit for electrochemical systems.Bench-scale continuous electrowinning tests using parallel plate electrodeswhere APAM and Guar were also dosed continuously and independently also indicatedthat APAM produces smoother deposits than Guar. The cross sections of the copperdeposits were examined using scanning electron microscopy (SEM) and showed thatGuar produced porous deposits and APAM produced slightly columnar deposits.It is therefore concluded that the results of electrowinning, CV and EIS testscorrelate: Guar depolarizes the electrode enhancing depolarization of the electrode bychloride ions and therefore increasing the growth rate with simultaneous formation of voids and porosity. APAM polarizes the electrode or decreases the rate of thedeposition process and therefore assists the nucleation rate and produces purer copperdeposits. Voids and porosity are probably reduced by the continuous formation ofcrystallites and their coalescence as indicated by x-ray diffraction data. The overallresults indicate that APAM is more specifically adsorbed than Guar at theelectrolyte/copper-metal interface and APAM acts as a levelling agent." @default.
• W75082009 created "2016-06-24" @default.
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• W75082009 date "2005-01-01" @default.
• W75082009 modified "2023-09-27" @default.
• W75082009 title "Copper electrodeposition in the presence of guar or activated polyacrylamide" @default.
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