Gold extraction involves various chemical processes depending on the type of ore and environmental considerations. Here are the most common methods:
1. Cyanidation (Most Common Method)
– Chemicals Used: Sodium cyanide (NaCN) or potassium cyanide (KCN), oxygen (O₂), lime (CaO).
– Process:
1. Crushed gold ore is mixed with a dilute cyanide solution (~0.01–0.05% NaCN).
2. Oxygen is added to form a soluble gold-cyanide complex:
\[
4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH
\]
3. Gold is recovered from the solution via adsorption (activated carbon) or zinc precipitation (Merrill-Crowe process).
– Advantages: High efficiency, works for low-grade ores.
– Disadvantages: Highly toxic; requires strict environmental controls.
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2. Aqua Regia (Used for Refining & Small-Scale Extraction)
– Chemicals Used: Hydrochloric acid (HCl) + nitric acid (HNO₃) in a 3:1 ratio.
– Process:
\[
Au + HNO₃ + 4HCl → HAuCl₄ + NO + 2H₂O
\]
Gold dissolves into chloroauric acid, which can be precipitated using reducing agents like sodium metabisulfite or oxalic acid.
– Used For: Recovering gold from electronics, jewelry scrap, and refining.
3. Thiosulfate Leaching (Non-Toxic Alternative to Cyanide)
– Chemicals Used: Sodium thiosulfate (Na₂S₂O₃), copper(II) sulfate (CuSO₄), ammonia (NH₃).
– Process: Forms a soluble gold-thiosulfate complex:
\[
4Au + 8S₂O₃²⁻ + O₂ + 2H₂O → 4[Au(S₂O₃)₂]³⁻ + 4OH⁻
\]
– Advantages: Safer than cyanide, used for carbonaceous ores.
4. Halogen-Based Methods