Characterizing the role of polyacrylamide additives in copper electrowinning

[摘要] ENGLISH ABSTRACT: Additives have been used for many years in the copper electrowinning process to improve cathode morphology. The industry has leaned towards polysaccharides in the form of Guar. Polyacrylamides however have been considered as possible alternatives. To get more value from the functionality of these additives it is necessary to understand the influence of structural chemistry on the resulting performance during electrowinning.This project investigated structurally different polyacrylamides (PAMs) and compared them to Guar and characterized their effect on copper electrodeposition. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were used to characterize the effect of molecular weight (MW) and ionic charge of polyacrylamides on electrodeposition. The findings were compared against results from a Guar product, utilized in the industry. During EIS and CV experiments copper was electrodeposited onto a platinum surface from a synthetic electrolyte containing 35 g/L copper (Cu2+), 160 g/L sulfuric acid (H2SO4), and 25 mg/L chloride (Cl-) at 45ᵒC. The EIS and CV experimental work were conducted at additive concentrations of 2 mg/L and 10 mg/L.EIS equivalent circuit modelling results indicate that an increase in polyacrylamide concentration and a decrease in additive molecular weight cause an increase in the overall system resistance. The charge transfer resistance (RCT) values at 2 mg/L additive concentration gradually increased from 3.36 Ω to approximately 7 Ω as the PAM additive MW decreased from very high to low. CV results of additives at 2 mg/L show that there was no significant effect on nucleation overpotential. However, significant plating polarization was observed for lower molecular weight additives, decreasing in significance as the molecular weight of the additive increased. The measured average current density at 0.22 V vs. SHE on the return sweep during CV experimental work systematically increased from 277.35 A/m2 for the Low MW PAM additive to 326.39 A/m2 for the Very High MW additive. At the same experimental conditions, an average current density of 337.00 A/m2 was measured in the absence of any additives. CV results at 10 mg/L additive show that there was significant nucleation polarization and plating polarization, correlating well with the EIS results. The industry standard organic additive Guar had no significant impact on the charge transfer resistance or the polarization behaviour at both 2 mg/L and 10 mg/L concentration. No correlation between the ionic content of the additives and system polarization were observed.Scanning electron microscopy (SEM) micrographs of copper deposits after 10 s and 30 s of electroplating on stainless-steel cathode tips at 2 mg/L additive concentration provided strong qualitative validation for the polarization trends observed on a fundamental level. The effect of polarization or increased charge transfer resistance was identified in the structure of the deposits. A spiky, dendrite like crystal structure was observed due to fast unrestricted deposition when no additives were added to the system, while smoother spherical 3-D crystal structures were obtained when using Guar. The polyacrylamide additives systematically produced a flatter 2-D compact crystal growth structure with a decrease in additive molecular weight.Electrowinning experiments on stainless steel plates in a laboratory (capacity) scale setup at 300 A/m2 and 40 ᵒC for 24 hours of electrodeposition were conducted to determine if microscale deposit structures will be sustained after multi-layers of deposition have occurred. Bench scale electrowinning results showed that PAM additives acted to radically reduce localized growth compared to copper cathode plates produced in the absence of any additive and in the presence of Guar. The copper plate in the absence of any additives developed abundant localized growth and dendrite structures. The mean surface deviation (Sa) value, which is a measure of surface roughness, was calculated for all copper plates that was produced in the presence of each organic additive. The calculated Sa value for the copper plate in the absence of any additive and in the presence of Guar was 82.22 μm and 32.87 μm respectively. The Low MW PAM additive produced the smoothest, brightest copper deposit of all the deposits and had the lowest calculated Sa value of 18.18 μm. The Very High MW PAM additive was the only additive to show inconsistency between the polarization behaviour trends, and the predicted deposit structure obtained in the bench scale experimental work. Current efficiency (CE) values between each electrowinning experiment varied with less than 1%. Such insignificant variation indicates that the organic additives had minor impact on the CE of the electrowinning experiments. It was concluded that both qualitative and quantitative analyses are necessary to evaluate the performance of organic additives in copper electrowinning and that they should be used in conjunction with one another.In future work, it is recommended that a molecular weight range should be proposed for optimal additive performance by determining the numerical molecular weights of the additives. Varying the most significant operating conditions like current density, temperature, and impurity type and concentration in the presence of the selected organic additives will provide insight into ideal operational ranges in the presence of these organic additives. SEM micrographs of cross sections of the resulting cathode plates in bench scale electrowinning work will aid in further characterization of the crystal structure that developed in the presence of each organic additive respectively.

[发布机构] Stellenbosch University