1. Receiving and Sampling: The raw gold bars are received at the refinery. They are checked for authenticity and purity. A small sample is taken from each batch for testing and analysis to determine its composition and impurities.
2. Melting: The raw gold bars are melted in a furnace at high temperatures to liquefy the metal. This process separates the gold from other metals and impurities that might be present in the raw material.
3. Chemical Treatment (Acid Digestion): The molten gold is then treated with various chemicals, typically acids like nitric acid or aqua regia, to dissolve impurities such as silver, copper, and other base metals. This step helps to separate the gold from other elements.
4. Precipitation: After the impurities are dissolved, the remaining solution containing gold ions is treated with a precipitant, usually sodium metabisulfite or another reducing agent. This causes the gold ions to solidify and form small particles or flakes, which can then be filtered out of the solution.
5. Smelting: The gold precipitate obtained from the previous step is dried and then melted again to form gold doré, which is a semi-pure alloy containing gold and other metals. This doré is then further refined to achieve the desired purity.
6. Electrolysis (Electrowinning): Electrolysis is used to further refine the gold to 99.9% purity. The gold doré is dissolved in a solution containing cyanide or other chemicals, and then an electric current is passed through the solution. This causes the gold to deposit onto electrodes, effectively separating it from any remaining impurities.
7. Casting and Forming: The refined gold, now at 99.9% purity, is cast into molds to form bars, ingots, or other desired shapes for easy handling and transportation.
Equipment needed for a gold refinery typically includes:
Overall, the process involves a combination of physical and chemical processes to purify the raw gold bars into high-purity refined gold suitable for various industrial and investment purposes.