Gold has always held a special place in human history. It’s not only the favorite metal of jewelers, who craft it into stunning pieces, but also a symbol of wealth and prosperity. Over centuries, this precious yellow metal has maintained its value due to its beauty and durability. Today, gold remains one of the most coveted metals by mining companies, especially with its price recently soaring to over $2,000 per ounce.
How is Gold Analyzed?
Regular gold assays are essential for exploration and mining projects. These analyses provide valuable information on various aspects of gold, including:
- Gold Grade: Determines ore reserves and helps optimize mill feed.
- Gold Status: Identifies whether the gold is free, attached, included, or refractory, helping optimize plant performance and minimize losses in tailings.
- Gold Speciation: Defines the mineralogical form in which the gold occurs, whether as native gold or in an alloy.
The method of analysis depends largely on the sample’s physical, chemical, and mineralogical characteristics. It also depends on the purpose of the analysis, such as mining exploration, optimizing extraction processes, or selling concentrates.
Before analysis, it’s crucial to note that gold ores vary greatly, so sample preparation must be handled carefully. Here are the most common methods used to analyze gold:
1. Fire Assay
Fire assay is a traditional and standardized method used to concentrate precious metals. The sample is mixed with fluxing agents and heated to a high temperature to collect metals via a lead process. The resulting mixture of precious metals is further processed to isolate gold, which is then measured using atomic absorption spectroscopy or inductively coupled plasma spectrometry.
Fire assay is highly effective for large gold samples, but certain elements like sulfides or chromium oxides can interfere with the process. In such cases, mining laboratories adjust the flux based on the mineralogy of the sample to ensure better recovery.
2. Acid Digestion with Aqua Regia
An alternative to fire assay, this method involves dissolving gold in a mixture of concentrated hydrochloric and nitric acids, known as aqua regia. The resulting solution is then analyzed using atomic absorption spectroscopy. While this method can deliver results comparable to fire assay, special sample preparation is required, especially for samples containing carbon, as gold may adsorb onto carbon and skew results.
3. Cyanidation
Cyanidation is an indirect gold assay method that involves dissolving gold in a dilute cyanide solution. It works best for free or attached gold grains. The concentration of gold in the cyanide solution is then measured to determine its content in the ore.
4. Mineralogical Analysis
Mineralogical analysis provides insights into the form and status of gold in the ore, offering information beyond total gold content. This analysis is done using microscopic observation techniques, including optical and electron microscopes, and elemental microanalysis with laser ablation coupled with ICP-MS.
The status of gold in the ore can be categorized as:
- Free Gold: Recoverable via gravity methods.
- Free and Attached Gold: Recoverable by cyanidation.
- Included Gold: Not recoverable by cyanidation but may be extracted gravimetrically if the particle size is larger than the host mineral.
- Refractory (Invisible) Gold: Embedded within the crystal structure, this form of gold cannot be seen by microscopy and requires elemental microanalysis for detection.
