Gold Assaying Procedures
Gold assaying at mine sites is a well tested technique that requires special procedures during sample preparation and analysis. The challenges are:
- Gold (Au) ore bodies have concentrations lower than 10ppm
- Gold (Au) is a malleable element that, during normal size reduction/milling, the gold (Au) can be “hammered out” into micro thin sheet sections
- When digested with Aqua regia, the matrix is very complex, with major elements such as Fe, Si, AI, Ti swamping the micro gold spectroscopic signal.
The common ore powder assay technique using XRF cannot be used because the Au ‘sheets’ distort the gold reading. Digestion of the ore powder removes the Au sheets and the extraction of the gold (Au) and silver (Ag) using DIBK (Diisobutyl Ketone) as the insoluble layer that has a chelating agent added for the extraction across the aqueous to organic layer. The ratio of the DIBK/aqueous digestion volume will be the concentration factor for gold.
For the analysis of gold on the Atomic Absorption Spectrometer (AAS) the instrument must have good sensitivity and very stable baseline. Normal elemental analysis by AAS will have a lower working limit (LWL) of 0.10 – 0.15 Absorbance units. With Au analysis the LWL will need to drop to 0.004 – 0.006 Abs. This is to ensure as much Au as possible is ‘stripped’ from the process circuit before the Au stripped solution is discharged to the tailings dam.
The AAS signal optimisation is critical to ensure high sensitivity and low baseline noise. There are many parameters that contribute to better sensitivity including:
- Zero burner rotation
- Correct burner position under the optic path
- Fuel / air ratio with the objective of a lean flame
- A stable nebuliser uptake rate
- Optimum positioning of nebuliser aspiration tube
For low baseline noise readings a double beam AAS is preferable to a single beam model. Assymetric modulation of the optic beam also provides a better signal / noise ratio. The Xplora AA model AAS has been specifically designed for gold analysis.
