Cyanide and Complex-Formation Extraction of Silver and Gold
Silver and gold, the two classic noble metals of this chapter, are extracted by forming a soluble complex with cyanide ion and then displacing the metal back out with zinc — a two-step complexation-then-reduction strategy quite different from carbon or electrolytic reduction, plus one older alternative method specific to silver found alongside lead.
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The cyanide (MacArthur-Forrest) process, step by step
Gold ore is treated with aqueous KCN (or NaCN) solution in the presence of air (O₂) and water: the gold is oxidised and simultaneously complexed by cyanide ion, forming the soluble dicyanoaurate(I) complex, [Au(CN)₂]-, while hydroxide ion is released as the other product (4Au + 8CN- + O₂ + 2H₂O → 4[Au(CN)₂]- + 4OH-). This soluble gold complex, X = [Au(CN)₂]-, is then reduced back to metallic gold by adding zinc dust: Zn displaces the gold from the complex, itself forming the soluble zinc-cyanide complex Y = [Zn(CN)₄]2- as the by-product (2[Au(CN)₂]- + Zn → [Zn(CN)₄]2- + 2Au). The identical process, with the same chemistry, extracts silver from silver-bearing ores too.
Cyanide is a complexing agent; zinc is the reducing agent
In this whole sequence, cyanide ion plays the role of a COMPLEXING agent (binding the metal ion to keep it in soluble form, not itself being oxidised or reduced), while zinc plays the role of the REDUCING agent (donating electrons to convert the complexed metal ion back to the neutral metal, while zinc itself is oxidised and stays in solution as its own cyanide complex). Do not swap these roles — cyanide does not reduce the metal, and zinc does not complex it; each performs a distinct, non-overlapping job in the two-step sequence.
Parkes process: an older, cyanide-free method for silver
Before cyanidation became standard, silver present in argentiferous lead (lead ore bullion containing traces of silver) was extracted by the Parkes process: molten zinc is added to the molten lead-silver mixture, and silver preferentially dissolves into the zinc layer (silver has a much higher affinity for zinc than for lead) rather than staying in the lead, forming a zinc-silver alloy that floats to the top (zinc being less dense than lead) and can be skimmed off, after which the silver is recovered from the zinc alloy separately. This is a completely different mechanism from cyanidation (no complexation involved at all, just differential metal solubility/alloying), and it specifically applies to silver recovered FROM LEAD, not to any cyanide-based process — pairing the cyanide process with lead reduction, rather than with silver or gold extraction, is an incorrect match.
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- ✓Gold cyanidation: 4Au + 8CN- + O₂ + 2H₂O to 4[Au(CN)₂]- + 4OH-, then 2[Au(CN)₂]- + Zn to [Zn(CN)₄]2- + 2Au
- ✓Cyanide ion is a COMPLEXING agent (keeps the metal soluble); zinc is the REDUCING agent (displaces the metal back to its elemental form)
- ✓The cyanide/complex-formation process is used to leach silver AND gold, not lead
- ✓The Parkes process (molten zinc added to molten argentiferous lead) is a separate, older method for recovering silver specifically from lead, using differential alloying rather than complexation
- ✓Pairing the cyanide process with lead reduction is an incorrect match — cyanide extraction is specifically for Ag/Au
Do not describe cyanide ion as the reducing agent in gold or silver extraction. Cyanide only complexes and solubilises the metal ion — it is zinc, added afterward, that actually donates electrons and reduces the complexed metal back to its elemental form.
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