Patio process

The patio process was a process used to extract silver from ore. It was developed by Bartolomé Medina in Pachuca, Hidalgo, Mexico in 1557 for the Pachuca-Real del Monte mines. The patio process was the first process to use mercury amalgamation to recover silver from ore. Other amalgamation processes were later developed, most importantly the pan amalgamation process, and its variant, the Washoe process.

Silver ores were crushed (typically either in arrastras or stamp mills) to a fine slime which was mixed with salt, water, "magistral" (essentially an impure form of copper sulfate), and mercury, and spread in a one- to two-foot thick layer in a shallow-walled, open enclosure or patio. Horses were driven around on the patio to further mix the ingredients, and, after weeks of mixing and soaking in the sun, a complex reaction converted the silver to native metal, which formed an amalgam with the mercury and was recovered. [W.H. Dennis (1963) "100 Years of Metallurgy" Chicago: Aldine]

The patio process solved a crisis in the silver-mining districts of the Spanish colonies in the western hemisphere, where the high-grade silver ore that could be economically smelted ("direct-smelting" ore) was being rapidly exhausted. By recovering silver from the more common lower-grade silver ore, the patio process and later amalgamation processes allowed silver mining to continue for centuries in the great silver mining districts of Mexico, Peru, and Bolivia. However, the high cost of mercury became a limiting factor in treating marginally economic silver ores.

The amount of salt and copper sulfate varied from one-quarter to ten pounds of one or the other, or both, per ton of ore treated. The decision of how much of each ingredient to add, how much mixing was needed, and when to halt the process depended on the skill of an "azoquero" (English: "quicksilver man"). The loss of mercury in amalgamation processes is generally one to two times the weight of silver recovered.

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