Extraction of Silver and Gold
Master the Mac-Arthur Forrest Cyanide Process and the role of Oxygen.
Silver ($Ag$) and Gold ($Au$) are noble metals. Because they are highly unreactive, they are often found in nature in their free, native state. However, they are intimately mixed with massive amounts of rocky impurities (gangue). To extract them efficiently, industry uses a brilliant hydrometallurgical technique known as the Mac-Arthur Forrest Cyanide Process.
1. Step 1: Leaching (Dissolution)
The finely powdered ore is treated (leached) with a dilute aqueous solution of Sodium Cyanide ($NaCN$) or Potassium Cyanide ($KCN$) while a continuous current of air ($O_2$) is blown into the mixture.
The Reaction for Native Gold (or Silver)
The native metal is oxidized by the oxygen and simultaneously complexes with the cyanide ions to form a soluble coordination complex. The rocky gangue remains undissolved and is filtered out.
Gold and Silver will not dissolve in cyanide alone. Oxygen gas ($O_2$) acts as the oxidizing agent, oxidizing the $Au^0$ to $Au^+$ so it can form the soluble dicyanoaurate(I) complex, $[Au(CN)_2]^-$.
Figure 1: The Cyanide Process. Tank 1 uses Oxygen to oxidize Gold into a soluble complex. Tank 2 uses Zinc to displace it.
2. Special Case: Silver from Argentite ($Ag_2S$)
While Gold is mostly found native, Silver is frequently extracted from its sulfide ore, Argentite ($Ag_2S$). The leaching reaction here is slightly different and introduces a powerful application of Le Chatelier's Principle.
Notice that this reaction is reversible. To prevent the reaction from going backward, a continuous blast of air ($O_2$) is passed through the solution. The oxygen oxidizes the Sodium Sulfide ($Na_2S$) into Sodium Sulfate ($Na_2SO_4$), removing a product and thereby driving the equilibrium to the right!
3. Step 2: Displacement (Cementation) by Zinc
Once the gangue is filtered away, we have a clear solution containing the soluble precious metal complex, $[Au(CN)_2]^-$ or $[Ag(CN)_2]^-$. To get the solid metal back, we perform a redox displacement reaction by adding Zinc dust.
Zinc is significantly more electropositive (more reactive) than Gold or Silver. Therefore, Zinc readily oxidizes (loses electrons to form $Zn^{2+}$) and acts as the reducing agent. It displaces the precious metal from the complex, forming the highly stable tetracyanozincate(II) ion $[Zn(CN)_4]^{2-}$, forcing the pure Gold or Silver to precipitate as a solid sponge.
Mastery Check: Silver & Gold Extraction
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