(Special session) Powder and Purification Engineering Division Committee "Student Award Session”

[1K0306-15-03] [Student presentation: Doctoral course] Trace analysis of gold in double refractory gold ores by ICP-OES

○Diego M. Mendoza¹, Keiko Sasaki¹ (1. Kyushu University)

司会：芳賀一寿（秋田大学）

Keywords:wet chemistry, gold determination, inductively coupled plasma optical emission spectrometry, reverse aqua regia, double refractory gold ore

Trace analysis by wet chemistry is inevitable to evaluate the gold grade in ores on a lab scale due to the limited sample amounts, while the fire assay is generally performed on an industrial scale. Gold (Au) is present as Au(0) in gold ores, especially encapsulated in sulfide grains of refractory gold ores, so aqua regia (3 HCl: 1 HNO₃) is generally used for oxidation of Au(0) and complexation with Cl^-. In addition, HF is necessary to digest the silicate minerals matrix that is often associated with sulfides. Generally, ICP-MS is a common measure for trace analysis in wet chemistry; however, “the memory effect” is known to diminish the accuracy of trace analysis for gold. Although the graphite furnace atomic absorption spectrometry (GF-AAS) is also used for trace analysis, it is known AuCl₄^- much reduces the absorbance. Therefore, the present work aims to establish the trace analysis of Au by inductively coupled plasma optical emission spectroscopy (ICP-OES). There are mainly two points to overcome. One is to examine the acid condition for stable digestion. Here, the different acid ratios of HCl and HNO₃ were examined to digest three types of gold ore concentrates with different mineralogical matrices. Another point is to avoid the influence of the large signals derived from coexisting Fe in ore samples. In ICP-OES, the primal emission peak from Au is observed at 242.795 nm; however, it is markedly interfered by coexisting Fe signals at 242.836 nm, which is generally coexisting with large concentrations in the ore samples. The obtained results were verified by comparison with the results from fire assay. The established methodology was applied to several types of gold ores with different gold contents, with higher/lower Fe contents, with/without carbonaceous matter, with/without silver (Ag). The detection limit was estimated up to 20 ppb in the established method. This is basically useful for exploring the mineral processing of gold ores in lab scale research.