How is Gold Cleaned and Refined?

Miller process

Gold is cleaned during the process of refining, which filters out impurities from the raw gold. The most common method of refining is called the Miller process. Using this method, gold is melted in an induction furnace. As it melts, chlorine gas is carefully and precisely added to the molten gold. This turns the by-products and base metals (such as silver and nickel) into solid chlorides.

The chlorides float to the top of the gold and are then skimmed off. They are called "slag." Gold is usually put through this refining process at least two times, each time taking two to four hours and resulting in a purity factor of from 99.0% to as much as 99.5%.

Skimming

The skimmed slag is then also processed to make sure no gold remains. After skimming, the slag is carefully poured by hand into something called a "de-golding" container, where it is slowly cooled. The container is a special crucible that can be set to different temperatures, allowing for the cooling temperatures to be monitored. During the cooling process, any remaining gold will drift to the bottom of the container and solidify. Once the gold solidifies, the remaining chloride is then poured off.

Gold melt

Next, the leftover solid gold particles are removed by hand from the cooling container and put in a ceramic filter, where they are washed. The gold from the process in Section 1 is also put into the filter and washed. If the gold is to be used for jewelry or gold plating, it is further refined through a chemical process where it is bathed in an electrolyte solution, which allows it to bond to other metals.

Resources

• More about how gold is refined.