Browsing by Author "Masoga, Ivory"

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    Investigation of characteristics of polyacrylamide-based additives for copper electrowinning
    (Stellenbosch : Stellenbosch University, 2023-03) Masoga, Ivory; Tadie, Margreth; Dorfling, Christie; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The formation of nodulations during copper electrowinning remains a major operational concern because it increases the energy consumption of the operation and decreases the quality of the copper plates. Smoothing agents are introduced to mitigate the formation of nodulations and improve the efficiency of the process. One of the emerging prospects is polyacrylamides, research has shown that polyacrylamides have the potential to compete with the industry's leading additives, furthermore, it was also reported that molecular characteristics of polyacrylamides such as molecular weight can influence their grain refining efficacies i.e., reduce nodulation. This study investigated the structurally different polyacrylamide additives in terms of their viscosity in the solution and their percentage ionic charge. Their characteristics are compared in electrochemical methods and then related to electrowinning. Moreover, this study also investigated the compatibility of the additives with solvent extraction, an upstream process of copper electrowinning. This study was divided into three phases. The first phase was the fundamental study using cyclic voltammetry and involved three sections. The first section involved the investigation of the effect of additive concentration on polarisation as well as the investigation of additive molecular characteristics on polarisation. Furthermore, the results obtained were used to select the concentrations that were used for the rest of the work. Cyclic voltammetry experiments were conducted at 35 g/L copper at 40°C. The potentiostat was swept between 0.4 v vs Standard Hydrogen Electrode (SHE), and 0.07 v vs SHE, the scan rate was 5 mv/s and the electrolyte was stirred at 560 rpm using the rotating disk (cathode). The effects of additive concentrations of 1, 2, 10 and 20 mg/L on polarisation were investigated. For electrolytes at an additive concentration of 1 mg/L, polarisation was negligible. At 2 mg/L, polarisation increased with the decrease in molecular weight of the additive. However, at 10 and 20 mg/L, the inverse was reported. There was no significant difference between 10 and 20 mg/L, which prompted using 2 mg/L and 10 mg/L additive concentrations for further experiments. The second section involved the investigation of the effect of residence time on the polarisation behaviour of the additives as well as determining the additive dosage. At 2 mg/L, additive concentration polarisation decreased with time, and the additives were infective within 24 hours; however, at 10 mg/L, there was evidence of a degree of polarisation, prompting 10 mg/L as dosage for electrowinning experiments. The third section involved investigating the effects of copper at 35 g/L and 45 g/L and temperature at 40°C and 50°C. Copper concentration depolarised (increased) the copper reduction current density. However, introducing additives resulted in a less pronounced polarisation effect for 2 mg/L and 10 mg/L. Similar results were observed for the effect of temperature, where temperature depolarised the copper reduction process, however, the introduction of the additives resulted in a pronounced polarisation behaviour. The second phase involved bench electrowinning experiments with an electrolyte containing; 25 mg/L Cl- ,330 mg/L Mn2+, 2.0 g/ Fe3+, Co2+ 760 mg/L, H2SO4 170 g/L and 10 mg/L polyacrylamide. Effects of copper concentration (45-55 g/L) and temperature (40°C-50°C) were evaluated for current efficiency and specific energy consumption. Both factors positively affected efficiency and energy; this was evident by the increase in current efficiency and a decrease in specific energy consumption. Increasing temperature affected the molecular weight of the additives while increasing copper concentration affected the ionic charge of the additives. All the additives reduced the formation of nodulations. The last phase was stripping experiments which investigated the impact of the additives on phase disengagement time. were conducted at 25 mg/L Cl- , 330 mg/L Mn2+ , Fe3+ 2.0 g/L, Co2+ 760 mg/L, Cu2+ 40 g/L, 170 g/L H2SO4, and 10 mg/L PAM, the solutions were stirred at 700 rpm at A/0 ratio of 2.5. Phase disengagement time increased with the increase in additive concentration and molecular weight. The additive concentrations were varied from 2.5 mg/L to 10 mg/L. The results from this work can serve as a basis for polyacrylamide design for either direct electrowinning or the SX-EW integrated process. The molecular weight of the additives has a significant impact on the additives as well as their % ionic charge during electrowinning. It can be concluded that optimisation with respect to additive concentration is required for each operation to obtain an efficient process.