Optimisation of silver purity and morphology in electro refining using central composite design and quantitative image analysis

This study examines the combined effects of operating parameters on silver purity and crystal morphology during electrorefining. Applied voltage (1.5–2.7 V), electrolyte silver concentration (50–100 g/L), copper ion concentration (10–50 g/L), and electrolyte pH (1–3) were systematically investigated. The results showed that compact deposits with high solidity values (≈0.87–0.88) were obtained at low voltage (1.5 V), low copper concentration (10 g/L), and moderate silver concentration (50 g/L). Increasing voltage or copper concentration promoted branching and dendritic growth, whereas higher pH values led to more uniform deposit structures. In contrast, silver purity was primarily governed by electrolyte composition. Analysis of Variance (ANOVA) revealed that silver concentration had a statistically significant positive effect on purity, while copper concentration had a statistically significant negative effect; pH exhibited no statistically significant effect on purity, and voltage showed only a minor influence. The highest purity of 99.9994% (≈0.6 ppm Cu) was achieved at 100 g/L Ag, 10 g/L Cu, and 1.5 V. These findings demonstrate that crystal morphology and purity are controlled by distinct electrochemical mechanisms, and that integrating CCD with quantitative image analysis provides an effective approach for optimising both deposit structure and ultra-high purity in silver electrorefining.