Gold Parting: Methods of Separating Gold from Silver and Base Metals.

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Parting is a crucial process in refining gold, specifically aimed at separating silver from gold and eliminating base metals. Given the potential harm of high base metal content, a preliminary refining step is often necessary. The acceptable level of base metals in alloys varies depending on the acid used—about 10% is permissible with sulphuric acid, while a slightly larger quantity is acceptable with nitric acid.

Gold parting involves separating gold from silver and other metallic impurities, challenging due to their chemical similarity and common extraction from the same ores. The alloy of gold and silver is known as electrum. Various methods of parting include cementation, melting with sulphide of antimony, melting with sulphur and precipitation, parting by nitric acid, parting by sulphuric acid (refining), a combination of the last two methods, the Gutzkow process, parting by chlorine gas, and parting by electrolysis. Each method plays a distinct role in achieving the desired separation and refinement of gold.

Contemporary Gold Refining Technologies

Contemporary gold refining relies on two dominant technologies, both starting with relatively pure gold.

  • The Miller process achieves up to 99.5% purity by passing chlorine through molten gold, forming impurity chlorides that create a floating slag.
  • Invented in 1874 by Emil Wohlwill, the Wohlwill process produces the highest purity gold (99.999%). This electrolytic method uses pure gold or titanium as the cathode and chloroauric acid as the electrolyte. Gold dissolves at the anode, and pure gold is plated onto the cathode through ion transfer. Insoluble silver chloride and soluble copper and platinum chlorides are removed. While highly efficient, the Wohlwill process incurs a substantial setup cost due to the extensive gold dissolution in the ore.

Historical Gold Parting Techniques

Summary: Various historical methods of gold parting, aimed at separating gold from silver and impurities, are detailed. These methods include cementation, melting with sulphide of antimony, melting with sulphur, parting by nitric acid, parting by sulphuric acid (refining), a combination of the last two methods, the Gutzkow process, parting by chlorine gas, parting by electrolysis, and parting by aqua regia.

  • Cementation, an ancient method, involves heating granulations of argentiferous gold mixed with a cement of brick-dust and common salt.
  • Parting by using sulphide of antimony, an obsolete process, includes repeated melting with sulphide of antimony to purify gold containing small quantities of silver.
  • Parting by means of sulphur was employed to concentrate gold in auriferous silver, enriching the alloy for later parting by nitric acid.
  • Parting by nitric acid, first mentioned by Albertus Magnus in the thirteenth century, gained widespread use in Venice in the fifteenth and sixteenth centuries and later in the Paris Mint around 1514.

These historical techniques, once conducted in secret, paved the way for the development of contemporary gold refining processes.

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