The chemistry of the Hispanic-American amalgamation process

Abstract

The chemistry of the Hispanic-American amalgamation or patio process for the extraction of silver has been studied through a laboratory model in which a suspension of silver sulfide in concentrated sodium chloride solution reacts with copper(II) chloride before being treated with mercury. Approximately 90% of the silver was extracted and separated as a liquid amalgam. The products of the reaction have been studied by X-ray powder diffraction. The process seems to have incorporated a very early example of oxidative chloride leaching. Copper(II) chloride oxidizes silver sulfide to elemental sulfur, and silver chloride in the form of both a precipitate and solubilized complexes. Copper(I) complexes such as [CuCl₃]^2–(aq) are produced, and then re-oxidized by atmospheric oxygen to copper(II), resulting in the precipitation of insoluble Cu₂(OH)₃Cl, principally in the form of paratacamite, with smaller amounts of atacamite. Mercury then reduces silver chloride to silver amalgam by forming calomel, and combines with some of the free sulfur to form HgS. In contrast to suggestions made by some previous investigators, no sulfides of copper were detected amongst the products.