Gold assay is a critical process for determining the purity and quality of gold samples. To ensure accurate results, a validated methodology must be used in the testing process.

Sample Preparation: The gold sample must be properly prepared before testing. This involves crushing and grinding the sample to a fine powder and ensuring that it is homogenous. The proper sieve size to use in sample preparation for gold assay in concentrates depends on the size distribution of the particles in the sample. The goal is to obtain a representative sample that contains particles of the appropriate size for accurate assay results. In general, a common practice is to use a set of sieves with different mesh sizes to obtain a representative sample. The size of the mesh on the sieve is measured in microns and indicates the size of the openings in the mesh.

For gold concentrates, the most commonly used sieves are the 100-mesh (150-micron) and 200-mesh (75-micron) sieves. These sieve sizes are suitable for most gold concentrates as they allow for a representative sample that contains particles of an appropriate size for the assay process.

Fire Assay: The fire assay method is the most commonly used method for gold assay. In this method, a small amount of the prepared sample is mixed with lead oxide, silver, and other reagents, and then heated to a high temperature to extract the gold. The resulting mixture is then cooled, and the gold is separated from the other elements using a process called cupellation. A minimum of three replicates we have to perform for the reliability of the result.

Assay Calculation: Once the gold has been separated from the other elements, its purity can be calculated. This is done by comparing the weight of the gold to the weight of the original sample, and applying a correction factor based on the specific gravity of gold.

Quality Control: To ensure the accuracy of the results, quality control measures must be in place. This includes the use of certified reference materials, replicate testing up to 5, and other quality control procedures.

Data Analysis and Reporting: Once the gold assay is complete, the results must be analyzed and reported. The results should include the purity of the gold sample, as well as any relevant information about the sample, such as its source and any associated impurities.

For ores or concentrates with more than 1 g/t gold content

The ASTM E1335-08(2018) standard provides a set of test methods for the determination of gold in bullion by fire assay cupellation analysis. The fire assay method is a type of gravimetric analysis that is widely used for the determination of the gold content in ores and other materials.

In the fire assay method, a small sample of the material to be tested is mixed with a fluxing agent and a collector, and then melted in a crucible at high temperatures. During the melting process, the gold and other precious metals are collected by the collector, which is typically made of lead, while the impurities are oxidized and absorbed by the flux. The resulting bead, which contains the precious metals, is then cupelled, or heated in a special furnace, to separate the gold from the lead.

The weight of the resulting gold bead is then measured, and the gold content of the original sample is calculated based on the weight of the bead and the weight of the original sample. This gravimetric method is highly accurate and is widely accepted as the standard method for gold assay in the mining and minerals industries.

Therefore, the fire assay method is the gravimetric method used for gold assay in the ASTM E1335-08(2018) standard.

For ores or concentrates with less or equal than 1g/t gold content

The fire assay-atomic absorption methodology is a two-step process for determining the gold content in a sample, as outlined in the ASTM E1335-08(2018) standard.

First, the fire assay method is used to separate the gold from other metals and impurities in the sample, and to produce a pure bead of gold. As described earlier, the fire assay involves melting the sample with a fluxing agent and a collector, and then cupelling the resulting bead to separate the gold from the lead.

Once the gold bead is obtained, it is dissolved in acid, and the resulting solution is analyzed using atomic absorption spectroscopy (AAS) to determine the gold content. AAS is a commonly used analytical technique for the determination of trace metal concentrations in a wide variety of samples.

In AAS, a sample is vaporized and atomized in a flame or graphite furnace, and the resulting atoms absorb light at a specific wavelength. The amount of light absorbed is proportional to the concentration of the metal in the sample, and can be used to determine the gold content of the original sample.

The fire assay-atomic absorption methodology is a widely accepted and reliable method for the determination of gold content in samples, and is used in many applications in the mining and minerals industries. It is described in detail in the ASTM E1335-08(2018) standard, and is often used in commercial testing laboratories for gold assay.