Browsing by Author "Mkhawana, Nhlamulo"

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    Effect of varying operating conditions on cathode surface roughness using guar as a smoothing agent in copper electrowinning
    (Stellenbosch : Stellenbosch University, 2021-12) Mkhawana, Nhlamulo; Tadie, Margreth; Dorfling, Christie; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Organic additives play a very important role in copper electrowinning to facilitate the production of cathode deposits with improved morphology. Guar has been used as the standard organic additive in copper electrowinning (EW) because of its compatibility with solvent extraction (SX) stage, as well as its ability to eliminate dendrite growth on the cathode surface. Dendrite formation produces an undesirable rough cathode surface, and a loss in current efficiency can be expected due to short circuits resulting from dendrite growth. In tankhouses, guar is dosed continuously since its performance reduces as it degrades by hydrolysis under acidic conditions. This project investigated the interaction of guar dosage with other electrowinning operating parameters and their effect on current efficiency and deposit quality. A bench scale electrowinning cell was used to conduct 24-hour electrowinning experiments by employing a mixed level factorial experimental design to investigate the interaction of guar dosage (2, 10 and 18 mg/l) with copper (35 and 45 g/l), sulphuric acid (150 and 180 g/l) and current density (180 and 300 A/m2) at a temperature of 40oC and their effect on the measured responses (current efficiency and cathode deposit quality). The electrolyte used was free of impurities with the addition of 25 mg/l chloride to act as de-polarizer and grain refiner in the copper deposition process. The mean surface deviation (Sa) value, which is a measure of surface roughness, was calculated for each copper plate produced. Acid digestion tests using aqua regia were conducted for 24 hours to investigate the chemical composition for all the copper plates. The amount of sulphur in the dissolved copper plates was analysed using the ICP analysis technique. A statistical analysis was performed to analyse the deposited surface roughness, current efficiency, and sulphur content. The electrowinning results showed an improvement in the elimination of dendrites, producing a smoother copper deposit in the presence of guar than a deposit achieved in the absence of guar in the electrolyte. The calculated Sa values and interaction plots shows that the individual and interactive effects of the operating parameters on the surface roughness in the presence of guar are dependent on the experimental conditions. The copper deposit achieved at 300 A/m2 current density, 45 g/l initial copper concentration, 180 g/l sulphuric acid concentration and 18 mg/l guar concentration had the lowest calculated Sa value (30.95 μm) and arguably the smoothest, brightest of all the deposits. Current density was observed to be the most influential parameter affecting the roughness of copper deposits while guar has an insignificant effect as predicted by statistical analysis. The interaction plots suggest that higher guar concentrations (10 mg/l or 18 mg/l) are more effective in reducing surface roughness than 2 mg/l guar concentration. Current efficiency (CE) values calculated for each experiment were high (98-99%) and varied by less than 1%. The little variance shows that varying guar concentrations had a minor impact on the CE for various operating conditions. Current density is the only influential parameter affecting current efficiency as predicted by statistical analysis. The chemical composition results show that the sulphur content in copper deposits increases at high guar concentrations and low current density. It was suggested that sulphate ions may be the source of sulphur entrapped in copper deposits. Statistical analysis of sulphur on each copper plate produced shows that current density and guar concentration significantly influences the amount of sulphur content. This study provides an industrially informed baseline against which new organic additives can be compared. Based on electrowinning experiments and statistical analysis results, it can be concluded that the adjustment of operating parameters in conjunction with an effective smoothing agent can reduce short-circuiting in copper electrowinning.