In the past few days PhD recipient Dr Steve Chingwaru’s groundbreaking research in geomatallurgy is turning heads. At just 26, he’s uncovered potentially the world’s largest invisible gold resource, valued at R450 billion. Questions have been asked on what invisible gold is and how it is extracted.
The below explainer seeks to give answers to the above:
Mining gold from pyrite stone, also due to its resemblance to real gold, is a challenging process that requires specialized techniques. Here’s a general overview of the process:
1. Identification of Potential Deposits: The first step in mining gold from pyrite stone is to identify potential deposits where pyrite is associated with gold. This involves geological surveys, exploration drilling, and sampling to determine the presence and extent of gold-bearing pyrite ores.
2. Crushing and Grinding: Once a viable deposit is identified, the pyrite-bearing rock is extracted from the earth and transported to a processing facility. The rock is then crushed into smaller particles and ground into a fine powder using crushers and grinding mills. This process increases the surface area of the pyrite particles, making it easier to extract gold.
3. Flotation: Flotation is a commonly used method to separate minerals from ore. In the case of gold-bearing pyrite ores, flotation can be used to separate the pyrite from the gold. In this process, the finely ground ore is mixed with water and a flotation reagent, such as cyanide or xanthate, which selectively attaches to the surface of the pyrite particles, causing them to float. The gold particles, which are often associated with the pyrite, are then recovered from the flotation concentrate.
4. Roasting: Roasting is another method used to extract gold from pyrite ores. In this process, the finely ground ore is heated in the presence of air or oxygen to oxidize the sulfide minerals, including pyrite, into soluble metal oxides and sulfur dioxide gas. The resulting calcine is then leached with cyanide or other chemicals to dissolve the gold, which can be recovered from the leach solution through various precipitation or adsorption methods.
5. Bioleaching: Bioleaching is an environmentally friendly alternative to traditional extraction methods that utilizes microorganisms to extract metals from ores. In the case of pyrite ores, certain bacteria, such as Acidithiobacillus ferrooxidans, can oxidize the sulfide minerals, releasing the gold and other metals into solution. The dissolved metals can then be recovered from the leach solution using conventional techniques.
6. Recovery and Refining: Once the gold is extracted from the pyrite ore, it undergoes further processing to purify and refine it into a marketable form. This may involve processes such as smelting, where the gold is melted and separated from impurities, or chemical refining, where the gold is dissolved and precipitated to remove any remaining impurities.
Explaining the process Dr Chingwaru said, “When sulphides become oxidised, they produce sulphuric acid, and when that goes into the groundwater, it increases the mobility of several toxic elements. It’s a big problem in some parts of Johannesburg where they’re scared that their groundwater is becoming polluted by tailings-related acid mine drainage. That’s why I’m passionate about highlighting the economic potential, as well as the environmental benefits of reprocessing tailings dumps efficiently.
“If you process the pyrite, you are taking out the key cause of acid mine drainage, plus you’re getting economic value from it. The process has the potential to recover additional valuable byproducts such as copper, cobalt and nickel, and reduce or even eliminate the heavy metal pollution and acid mine drainage associated with tailings dumping.”