Milling ore for gold mining involves several steps to break down the ore and extract the gold. Here’s a general process:
1. Crushing the Ore
– Primary Crushing: Use a jaw crusher or gyratory crusher to reduce large chunks of ore into smaller pieces (~6 inches or smaller).
– Secondary Crushing: Further crush the ore using a cone crusher or impact crusher to achieve finer particles (~1 inch or smaller).
2. Grinding (Milling) the Ore
– Ball Mill or Rod Mill: The crushed ore is fed into a rotating mill with steel balls or rods to grind it into a fine powder (typically 100-200 mesh). Water is often added to create a slurry.
– SAG Mill (Semi-Autogenous Grinding): Some operations use large rotating drums where ore and grinding media break down the material.
3. Classification (Sizing)
– A spiral classifier or hydrocyclone separates the ground material into coarse and fine fractions, ensuring proper particle size for gold recovery.
4. Gold Extraction Methods
# A. Gravity Separation (For Free-Milling Gold)
– Shaking Tables, Sluices, or Centrifuges: Used to separate gold from lighter materials based on density.
# B. Froth Flotation (For Sulfide Ores)
– Chemicals are added to make gold-bearing minerals hydrophobic, allowing them to attach to air bubbles and float for collection.
# C. Cyanidation (Most Common for Fine Gold)
– The finely ground slurry is mixed with a weak cyanide solution (NaCN or KCN), which dissolves the gold.
– The gold-laden solution is then processed through:
– Carbon-in-Pulp (CIP): Activated carbon adsorbs gold from the cyanide solution.
– Carbon-in-Leach (CIL): Similar to CIP but with leaching and adsorption happening simultaneously.
– Merill-Crowe Process: Zinc dust precipitates gold from the cyanide solution.
# D. Heap Leaching (For Low-Grade Ores)
– Crushed ore is piled on a lined pad and irrigated with cyanide solution, which percolates through and dissolves gold.
5. Refining & Smelting
– The collected gold concentrate is further refined by:
– Smelting in a furnace with fluxes to remove impurities.