Gold is refined using electrolytic refining, chlorination, or acid leaching processes to remove impurities after initial separation and smelting.
Here's a more detailed look at these refining methods:
Electrolytic Refining (The Wohlwill Process)
This is a common method used to achieve very high purity gold.
- Process: Gold bullion is cast into anodes and placed in an electrolytic cell containing a solution of hydrochloric acid and gold chloride. A thin sheet of pure gold serves as the cathode.
- Electrolysis: When an electric current is passed through the cell, gold dissolves from the anode and is deposited on the cathode as highly pure gold (typically 99.99% purity). Impurities either dissolve in the electrolyte or settle to the bottom as "anode slime."
- Anode Slime: This slime often contains valuable metals like silver and platinum, which can be recovered in further refining processes.
- Advantages: High purity, recovery of valuable byproducts.
- Disadvantages: Relatively slow, more expensive than some other methods.
Chlorination (The Miller Process)
This is a faster, but less pure, refining method.
- Process: Chlorine gas is bubbled through molten gold. The chlorine reacts with impurities, forming chloride compounds that separate from the gold.
- Observation: The process is visually monitored by observing the different colors of the fumes produced.
- Advantages: Faster than electrolytic refining.
- Disadvantages: Lower purity (typically 99.5% to 99.9%), not suitable for refining gold containing significant amounts of silver, as silver chloride can form a surface layer that hinders the process. Handling of chlorine gas poses hazards.
Acid Leaching
Acid leaching uses acids to dissolve the base metals, leaving the gold undissolved. Different acids may be used, depending on the specific impurities present. Aqua regia, a mixture of nitric acid and hydrochloric acid, is famously used to dissolve gold, but it isn't usually used to refine gold from base metals as much as to dissolve already relatively pure gold for analysis or other purposes. For refining, specific acids that target base metal impurities are more common.
- Process: The gold-containing material is treated with the selected acid(s). The acid dissolves the unwanted metals, leaving the gold behind.
- Advantages: Can be effective for removing certain impurities.
- Disadvantages: May require multiple leaching steps with different acids, can be environmentally problematic due to the use of corrosive chemicals.
In summary, gold is refined using several methods to achieve different purity levels. Electrolytic refining is used for the highest purity, while chlorination offers a faster, less expensive alternative. Acid leaching is another viable method, often tailored to the specific impurities that need to be removed.
