What is the name of the gold refining process that uses chlorine gas?

The gold-refining process that uses chlorine gas is the Miller process.

Developed and patented by Francis Bowyer Miller in 1867, the process treats molten, impure gold with a stream of chlorine. Most accompanying metals react with chlorine to form metal chlorides. These compounds either vaporize or collect in a relatively low-density slag that can be removed from the surface.

The Miller process typically produces gold about 99.5% pure, making it efficient for large-scale refining when extreme purity is unnecessary. It is faster and simpler than electrolytic refining, although it does not usually reach the highest purity grades.

A common mix-up is the Wohlwill process. Wohlwill refining is electrolytic rather than chlorine bubbling and can produce gold up to about 99.999% pure. Amalgamation uses mercury, while cyanidation dissolves gold in a cyanide solution; neither is the chlorine-based refining method described here.