Superiority of Thiourea Au Extraction from Low Graphiticity Carbonaceous Gold Ores

Abstract

Double refractory gold ores (DRGOs), in which few percentages of carbonaceous matter are present and gold grains are encapsulated in sulfides, have been abandoned to develop due to economic loss caused by adsorption of Au(CN)₂⁻ on carbonaceous matter after cyanidation in spite of the relatively higher Au contents than in usual gold ores. To avoid using extremely toxic reagent like cyanide, alternative ligand was examined to extract Au and the affinity between Au complex and carbonaceous matter was concerned in the present work. Two double refractory gold ores with different graphitic degrees were selected to pretreat for sulfide and carbonaceous matter decomposition, and then thiourea and cyanide extraction were performed to compare the leaching characteristics. Bio-oxidation with iron-oxidizing bacteria and chemical oxidation with FeCl₃ were used as pretreatment for sulfide degradation, followed by carbonaceous degradation with laccase. For DRGO with a high graphiticity carbonaceous matter and complete removal of sulfides by pretreatment, the extraction rate was over 80% for both cyanidation and thiourea leaching. Meanwhile, for a lower graphiticity carbonaceous matter-bearing gold ores, cyanidation showed a decrease in gold extraction rate due to preg-robbing of Au(CN)₂⁻ complexes, however, thiourea extraction showed the stable extraction. Thiourea leaching was suitable and superior in gold extraction from low graphitc degree carbonaceous matter-bearing gold ores after sulfide decomposition.