Gold (Au) Metallurgy and Industrial Extraction

Gold (Au) Metallurgy and Industrial Extraction

Gold (Au) is a noble metal known for its excellent resistance to corrosion and high electrical conductivity. Its extraction primarily focuses on recovering the native metal.

Natural Occurrence & Major Ores

Gold is predominantly found in its native (free) state due to its extremely low reactivity.

• Native Gold: Occurs as nuggets, flakes, or fine particles disseminated in quartz veins and alluvial (placer) deposits.
• Alloys: Often found alloyed with silver, forming electrum.
• Tellurides: Less common, but gold can be found as tellurides, e.g., Calaverite (AuTe₂) and Sylvanite ((Ag,Au)Te₂). For industrial extraction, native gold is the primary target.

Concentration of the Ore

The concentration method depends on the nature of the gold ore.

1. Gravity Separation

• Principle: Utilizes the high specific gravity of gold (19.3 g/cm³) compared to gangue materials (e.g., quartz, specific gravity ~2.65 g/cm³).
• Process: For alluvial deposits or finely crushed ores, methods like panning, sluicing, and jigging are employed. Water washes away lighter gangue particles, leaving behind heavier gold particles.

2. Chemical Leaching (Cyanide Process / MacArthur-Forrest Process)

• Principle: Gold, even in its native state, can be dissolved selectively by dilute alkali cyanides in the presence of air (oxygen). This is the most common method for low-grade ores and finely disseminated gold.
• Process:
  • Finely powdered gold ore is treated with a dilute solution (0.1-0.3%) of sodium cyanide (NaCN) or potassium cyanide (KCN) in the presence of air (as an oxidizing agent) and water.
  • Gold dissolves to form a soluble dicyanoaurate(I) complex.
  • Reaction: 4Au(s) + 8CN-(aq) + O2(g) + 2H2O(l) → 4[Au(CN)2]-(aq) + 4OH-(aq)
  • This process effectively concentrates gold by separating it from insoluble gangue.

Reduction to Crude Metal

Following the cyanide leaching, gold needs to be recovered from the dissolved complex.

1. Displacement by Zinc (Merrill-Crowe Process)

• Principle: Gold is precipitated from the dicyanoaurate(I) complex solution by adding an electropositive metal like zinc dust. Zinc acts as a reducing agent, displacing gold.
• Process: The clear solution containing [Au(CN)₂]⁻ ions is de-aerated (to prevent zinc oxidation) and treated with zinc dust.
• Reaction: 2[Au(CN)2]-(aq) + Zn(s) → [Zn(CN)4]2-(aq) + 2Au(s)
• The precipitated gold is filtered, washed, and dried, yielding crude gold (often called “gold slime” or “gold precipitate”).

Refining and Purification

The crude gold obtained from the reduction process contains impurities (e.g., Ag, Cu, Zn, Fe, Te). Several methods are used for refining.

1. Miller Process

• Principle: Impurities are removed by reacting them with chlorine gas at high temperatures.
• Process: Gaseous chlorine is blown through molten crude gold (above its melting point of 1064 °C). Impurities like Ag, Cu, Zn, and Te react with chlorine to form their respective chlorides.
  • Reaction for Silver: 2Ag(l) + Cl2(g) → 2AgCl(l)
• Silver chloride (AgCl) and other metal chlorides are less dense and either float as a separate molten layer or are volatile and escape, leaving highly pure gold. The purity achieved is typically 99.5-99.9%.

2. Electrolytic Refining (Wohlwill Process)

• Principle: Gold is purified using electrolysis, which allows for extremely high purity (up to 99.999%).
• Setup:
  • Anode: Cast blocks of crude gold containing impurities.
  • Cathode: Thin sheets of pure gold foil.
  • Electrolyte: An aqueous solution of gold chloride (AuCl₃) containing hydrochloric acid (HCl) to maintain conductivity and prevent hydrolysis.
• Process:
  • When electric current is passed, gold and less noble metals (e.g., Cu, Zn, Fe) from the anode dissolve into the electrolyte as positive ions.
  • Anode Reaction: Au(s) → Au³⁺(aq) + 3e⁻ (and similar for less noble metals)
  • Only gold ions from the solution are selectively deposited as pure gold onto the cathode.
  • Cathode Reaction: Au³⁺(aq) + 3e⁻ → Au(s)
  • More noble metals (e.g., platinum, palladium, rhodium, silver) do not dissolve at the anode potential and fall to the bottom as “anode sludge” (anode mud), which is later processed to recover these valuable metals.
  • Less noble metals remain dissolved in the electrolyte.
• This method yields gold of very high purity.