martes, 28 de enero de 2014

Bartolomé de Medina. Silver amalgamation or Patio process

Born in Seville, Bartolomé de Medina (1497-1585) became a prosperous merchant of that city. Best known for his method for amalgamation of silver with mercury. His process of obtaining silver represented the main source this metal until 1900 when it was replaced by the cyanidation process.

He embarked on the expedition of Francisco Montejo after discovering the secrets of how to extract by the best way gold and silver by a german chemist (called master Lorenzo or Leonard). He traveled and tried to apply the method described by the german. He arrived in New Spain in 1554 and started to apply this method.

He experienced the treatment unsuccessfully but continued investigating until he discovered that it was necessary another substance, "the magistral", catalyst of copper sulfate which produced the expected reaction and revolutionized the system of obtaining silver by the method of amalgamation, known as "beneficio de patio (benefit of silver)". This method was applied for more than 300 years.

What probably happened is that the german transmited a method of the ancient Greeks who used chlorargyrite (AgCl) while Pachuca ore mine presented silver in the form of silver sulfide (AgS). Medina discovered the ​​leaching process in which the sulfur is oxidized to sulfate in the presence of chlorine, hence the use of the brine) . The silver chloride formed react with mercury producing the metallic silver that would be later separated by heat.

according to: 

CuSO+  2NaCl -> CuCl2 + NaSO4
Ag2S +2NaCl + 2O2 -> Na2SO4 + 2AgCl
2AgCl + 2Hg-> 2Ag + Hg2Cl2
Ag + Hg -> amalgam.

The method consist of several steps. First, the mineral was crushed and ​​mixed with brine, mercury and deposited in a 0.3-0.6 m-thick layer in a patio. The "magistral" or a form of impure copper or iron sulfide was mixed, waiting several weeks. Copper or iron chlorides produced after reacting with sulfides in present of salt made possible the reaction between mercury and silver during the formation of silver chloride. Later, the amalgams were heated to recover silver.

In 1555 the Viceroy Luis de Velasco awarded to Bartolomé de Medina with the patent for the silver amalgamation process and endowed him with exploitation rights by this method. Many sites did not recognize the patent rights and he started having financial problems, so he asked Philip II an amount for his discovery. Medina died poor in Pachuca.

Thus the mercury became a strategic mineral for the empire thanks to the Almadén and Huancavelica ore mines in Spain and Peru, respectively.

In 1571, Pedro Fernández de Velasco introduced in the Vyceroyalty of Peru this system. It is believed that high levels of mercury in the southern hemisphere could be caused by the use of this technique.


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