Highly efficient pretreatment for refractory gold ores using persulfate, catalyst and free radical based advanced oxidation processes to improve cyanidation

The extraction of gold (Au) from refractory ores poses major challenges due to the inclusion of gold particles in sulfide minerals. In this study, an alternative and highly efficient oxidative pretreatment for refractory gold ores and concentrates was developed using persulfate-based advanced oxidation process. This approach generates strong oxidizing radicals to oxidize sulfide minerals, thereby releasing the enclosed gold. For the studied gold concentrate, the extraction of gold by direct cyanidation reached only a yield of 62.6 %. The refractoriness behavior behind this performance has been examined using diagnostic leaching and mineralogical characterization using automated Scanning Electron Microscope (SEM) coupled with Energy Dispersive X-ray (EDX), and it was attributed to encapsulation of fine particles of gold within sulfide minerals mainly pyrite and arsenopyrite. To oxidize these sulfides by persulfate, four activation modes were examined to generate reactive radicals: ultraviolet (UV) activation, ultrasound (US) activation, thermal (T) activation and Ag⁺ and Fe²⁺ catalysts. The experimental results showed that UV and US methods are more efficient in generating reactive radicals, and the removal efficiency of Fe, As and S as well as the subsequent gold extraction by cyanidation varied in the following order: UV > US > T > Ag⁺ > Fe²⁺. The identification tests via utilization of phenol and nitrobenzene chemical probes revealed that the oxidizing radicals predominant and responsible for the oxidation of pyrite and arsenopyrite are sulfate radicals (${\mathrm{SO}}_4^{•-}$) and hydroxyl radicals (OH^•). These radicals are generated more effectively through UV and US activation modes. The US activation resulted in a gold extraction of 86.4 %, while UV activation achieved a slightly higher extraction of 88.1 %. The findings showed also that combining UV with Ag⁺ catalyst, US with heating at 80 °C, and US with heating at 80 °C in the presence of an Ag⁺ catalyst resulted in gold extractions of 91.1 %, 92.2 %, and 93.6 %, respectively. The results led to proposing a possible mechanism for the oxidation of sulfide minerals during the pretreatment process.