At Aurum Laboratories, we provide precise and efficient gold analysis using the aqua regia digest method.
This widely used technique is ideal for determining gold content in geochemical samples, offering a cost-effective and reliable alternative to fire assay for certain sample types.
The Aqua Regia Digest Method
Aqua regia digest is a chemical leaching process that dissolves gold from rock and core samples using a 2:1 mixture of hydrochloric acid (HCl) and nitric acid (HNO3).
This process allows for the extraction and measurement of gold using flame atomic absorption spectroscopy (AAS).
When to Use Aqua Regia Digest
This method is particularly effective for samples that do not contain excessive sulphides or highly mineralised materials, as these may not fully break down in the acid digest. For such samples, fire assay is recommended for a more comprehensive analysis.
It is suitable for bulk analysis of many samples in an exploration phase or to assess the general gold content in ores or concentrates.
The Gold Analysis Process
Sample Preparation
A 20g sample is weighed into a clean beaker to ensure accuracy. This weight provides a standardised baseline for analysis.
Acid Digestion
Pre-Boiling Stage - The sample is treated with approximately 50 mL of hydrochloric acid and gently heated to break down oxide coatings and free gold particles.
Nitric Acid Addition - 25 mL of nitric acid is added, and the sample is heated until boiling. Stirring ensures proper mixing and contact between the sample and acid mixture.
Digest Duration - The sample is left on the hot plate for about 20 minutes, allowing complete digestion.
Extraction and Filtration
A flocculant (Superfloc 573) is added to settle suspended particles, ensuring a clear solution for extraction.
The solution is diluted with tap water up to 200 mL and allowed to cool.
A 20 mL aliquot of the clear digest is taken and mixed with 2 mL of DIBK/Aliquat 336, an organic solvent that extracts gold.
Measurement Using Flame AAS
The organic extract is shaken to mix thoroughly and left to separate.
The solution is analysed using Flame Atomic Absorption Spectroscopy (AAS) against matrix-matched gold standards.
Over-range samples are diluted accordingly and reanalysed to ensure precise readings.
Challenges and Special Considerations
Carbonate-Rich Samples - These may react vigorously with the acid mixture, causing excessive frothing. To prevent this, a smaller sample weight is used with controlled acid addition.
High Iron Content Samples - Lateritic samples rich in iron require a pre-digest with hydrochloric acid to remove oxide coatings before proceeding with aqua regia digestion.
Sulphide-Rich Samples -These require an initial nitric acid treatment to break down sulphides and release gold particles.
If sulphides do not fully break down, a fire assay is recommended as a confirmatory test.
Standardisation and Quality Control
To ensure accuracy and consistency, all gold standards are prepared according to industry-standard protocols. The following dilution and extraction procedures are followed for calibration:
|
Standard Concentration |
Extraction Volume |
|
0.50 ppm Au |
take 30 ml of 1.00 ppm gold standard extracted with 40 ml DIBK |
|
1.00 ppm Au |
take 60 ml of 1.00 ppm gold standard extracted with 40ml DIBK |
|
2.00 ppm Au |
take 120 ml of 1.00 ppm gold standard extracted with 40 ml DIBK |
|
5.00 ppm Au |
take 60 ml of 5.00 ppm gold standard extracted with 40 ml DIBK |
|
10.00 ppm Au |
take 60 ml of 10.00 ppm gold standard extracted with 40 ml DIBK |
All higher concentration samples are diluted accordingly and recorded to maintain precision and traceability.
- Precision and Accuracy - Strict quality control ensures reliable gold analysis.
- Contamination Prevention - Our cleaning and dilution procedures eliminate cross-contamination risks.
- Expert Handling - Our trained professionals follow industry best practices to ensure high-quality results.
- Customisable Solutions - Whether using aqua regia digest or fire assay, and provide tailored solutions based on your sample type.