Enhancement of thiourea leaching performance of refractory silver tailings by mechanical activation

Refractory silver tailings are both an environmental liability and a potential secondary resource for precious metals. However, strong mineralogical locking of silver-bearing phases within barite and aluminosilicate matrices severely limits silver recovery under conventional leaching conditions. In this study, mechanical activation was investigated as a pretreatment method to enhance the thiourea leaching performance of refractory silver tailings from the Kütahya Eti Silver Plant (Turkey). Mechanical activation was carried out in a planetary ball mill for 0, 30, 60, and 90 min. The resulting changes in particle size, specific surface area, and structure were evaluated by particle size analysis, BET, XRD, and SEM. Mechanical activation caused marked particle fragmentation, increased surface area, and partial amorphization, indicating enhanced structural disorder and improved accessibility of silver-bearing phases. Thiourea leaching experiments were performed using 0.5 M thiourea in acidic solution at pH 1.5, with 0.01 M ferric sulfate as the oxidizing agent. Under the optimum conditions identified in this study, namely 90 min mechanical activation, 50 °C, S/L = 1/10, and 600 rpm, silver recovery increased from about 68% for the non-activated sample to about 96% for the mechanically activated sample. Kinetic analysis under conditions minimizing external mass-transfer resistance showed that apparent rate constants increased with activation time, while the apparent activation energy decreased from 46.1 to 38.9 kJ·mol^-1. Post-leaching SEM and BET results revealed agglomerated and porous particle structures, indicating that diffusion through evolving porous particle assemblies contributes to the observed kinetic behavior. This work directly links mechanochemically induced structural evolution with apparent leaching kinetics in industrial refractory silver tailings and highlights mechanically assisted thiourea leaching as a promising cyanide-free alternative for refractory silver-bearing wastes.