The Washoe process of amalgamation was developed in the 1860s. They used iron tanks with mechanical agitators. Each tank ("pan") was circular, and commonly held 1,200 to 1,500 pounds of ore that had been crushed to sand size. Water was added to make a pulp, and 60 to 70 pounds of mercury, along with one-half to three pounds each of salt and copper sulfate were also added. A circular iron plate called a muller was mounted on a vertical shaft and lowered into the pan, and was rotated to provide both agitation and additional grinding. Heat was delivered to the pans by steam pipes. The iron filings worn from the muller and pan proved to be an essential ingredient in the process.
https://en.wikipedia.org/wiki/Pan_amalgamation#Washoe_process

The valuable part of the ore would then "dissolve" in the mercury, which would be removed, leaving the worthless rock. The mercury would then be boiled away leaving the valuable metal, and also, poisoning the workers, streams, air, etc. They later switched to a cyanide process which was much more efficient and safer.