Gold ore processing involves several techniques to extract gold from its ore, depending on the type of ore (free-milling, refractory, or sulfide-rich) and economic factors. Here are the key methods:
1. Comminution (Crushing & Grinding)
– Purpose: Reduce ore size to liberate gold particles.
– Methods:
– Jaw Crusher / Cone Crusher (primary crushing)
– Ball Mill / SAG Mill (fine grinding)
– Rod Mill (coarse grinding)
2. Gravity Separation
– Used for free-milling gold (visible particles).
– Techniques:
– Jigging
– Shaking Tables
– Spiral Concentrators
– Centrifugal Concentrators (Knelson, Falcon)
– Works best with coarse gold before cyanidation.
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3. Flotation
– Used for sulfide ores where gold is locked in pyrite/arsenopyrite.
– Froth flotation separates gold-bearing sulfides from waste rock.
– Often followed by roasting or pressure oxidation before leaching.
4. Leaching Methods
# (a) Cyanide Leaching (Most Common)
– Gold dissolves in sodium cyanide (NaCN) solution with oxygen.
– Types:
– Heap Leaching (low-grade ores)
– Tank Leaching / CIL (Carbon-in-Leach) (higher-grade ores)
– CIP (Carbon-in-Pulp) – Activated carbon adsorbs dissolved gold.
– Recovery: Gold is stripped from carbon and electrowon/smelting.
# (b) Non-Cyanide Alternatives
– Thiosulfate Leaching – Less toxic, used for refractory ores.
– Thiourea Leaching – Fast but costly.
– Chlorination (Aqua Regia) – Rarely used industrially.
5. Refractory Ore Processing
Gold locked in sulfides or carbonaceous material requires extra steps:
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# (a) Roasting
– Burns sulfides to release gold for cyanidation.
– Produces SO₂ gas, requiring emission control.
# (b) Pressure Oxidation (POX)
– High-pressure autoclave oxidizes sulfides chemically.
# (c) Bio-Ox