An experimental design-driven hydrometallurgical process for selenium recovery from copper anode slime with optimization of nitric acid leaching and sulfide precipitation

This study establishes an optimized hydrometallurgical process for selenium recovery from Sarcheshmeh copper anode slime. The raw copper anode slime contained 9.27 wt.% selenium, along with a significant barite-rich gangue phase (~ 65 wt.%). Preliminary physical separation methods failed to reject this barite effectively, directing the focus toward direct acid leaching. Preliminary leaching confirmed high selenium solubility (> 99%) in nitric acid. To refine the process economically, critical parameters-nitric acid concentration, temperature, pulp density, and time-were optimized using a Central Composite Design within a Response Surface Methodology framework. The derived statistical model exhibited excellent predictive capability. The optimum leaching conditions were identified as 3.75 M HNO₃, 90 °C, a pulp density of 6.25%, and a 90-min duration, resulting in a 97.45% selenium extraction yield. Selenium was subsequently recovered from the resulting leachate via sodium sulfide precipitation. This secondary step was also optimized, with maximum selenium precipitation (97.80%) achieved at pH 2.5 using a 10 g/L Na₂S dose. The integrated process demonstrates an efficient and statistically-guided route for high-yield selenium recovery from complex anode slime matrices.