Development of a general acid method for the digestion of gold ore samples together with a comparison of extraction solvents for gold and determination by microwave-induced plasma-atomic emission spectrometry (MIP-AES)

Abstract

In this paper, we explore a variety of aqueous digestion techniques for the determination of gold, in a wide variety of mineral matrices. Acid digestion methods, based upon modified aqua regia in which a larger proportion of nitric acid (5 : 1 HNO₃ : HCl) is involved, competes effectively with fire assay to deliver accurate and precise values for gold in a fraction of the time. Using this ratio of acids, simple sand-bath digestions deliver recoveries of 95 to 98% of gold certified reference values, after a digestion time of approximately 35 minutes. Focused infrared digestions, however, take only 15 minutes, and deliver higher recoveries, in the range of 95 to 104% of gold reference values. An ultrasonic digestion with 1,3-dibromo-5,5-dimethylhydantoin acting as a source of bromine, delivered mediocre recoveries of gold. Gold determinations were conducted using inductively coupled plasma-mass spectrometry (ICP-MS). Solvent extraction of gold from digestion solutions effectively preconcentrate gold and significantly reduces interference from major elements, except for those samples containing high concentrations of iron. The use of 2-octanone as extraction solvent exhibited greater sensitivity than the use of the conventional solvent, methyl isobutyl ketone (MIBK). Although 2-ethylhexanol was also effective at extracting gold, its lengthy time for phase separation and memory effects, which increased wash-in and wash-out times, made it a less attractive extraction medium. For solvent extraction methods, microwave induced plasma-atomic emission spectrometry (MP-AES) was used for the determination of gold.