METALLURGICAL PROCESS
❖ ORE PREPARATION
Objective: Increase surface area for cyanide contact.
- Crushing: Crush ore to ~10 mm or smaller using a jaw crusher & cone crusher.
- Screening: Ensure uniform size; oversized particles are recirculated.
- Weighing & stockpiling: 5,000 tonnes divided equally across 6 vats (~830 tonnes/vat).
❖ LIME ADDITION (pH CONTROL)
Purpose: Raise pH to prevent cyanide loss through HCN gas formation.
- Target pH: 10.5–11
- Lime dose: ~1–2 kg/tonne
Procedure
- Mix dry lime into ore on conveyor or manually before feeding to vat.
- Check pH with portable meter after wetting.
❖ VAT LOADING
- Load the pre-limed, crushed ore into concrete/leach tanks (vat).
- Ensure compact but permeable layering.
- Vats should have drainage systems and under-drains with filter medium.
❖ CYANIDE LEACHING
Objective: Dissolve gold into solution (Au → Au(CN)₂⁻)
- Cyanide solution: NaCN 0.05–0.1% (500–1000 ppm)
- Typical dose: ~1 kg NaCN/tonne of ore
- Leaching time: 48–72 hours
Procedure:
- Add cyanide solution evenly using drip pipes/spray system.
- Maintain saturation without flooding.
- Cover vats, if possible, to prevent cyanide degradation.
Chemical reaction:
Au + 2NaCN + ½O₂ + H₂O → NaAu(CN)₂ + NaOH
❖ OPTIONAL: OXIDIZING AGENT
If gold recovery is slow due to low dissolved oxygen:
- Add Hydrogen peroxide (H₂O₂) or Calcium hypochlorite in small quantities to enhance oxidation.
❖ PREGNANT SOLUTION COLLECTION
- Leach solution drains through bottom filters and is collected.
- Solution contains dissolved gold = “pregnant solution.”
❖ GOLD RECOVERY
Options:
- Zinc precipitation (Merrill–Crowe process):
- Add zinc dust to precipitate gold.
- Filter out solid gold.
- Activated carbon (CIL/CIC) (if available):
- Pass solution through carbon columns.
- Carbon absorbs gold → then eluted later.
❖ TAILINGS WASHING (OPTIONAL)
- Rinse vats with clean water to recover remaining gold.
- Water reused in new leaching batches.
❖ CARBON/SLUDGE TREATMENT (If used)
- Carbon stripping: Hot caustic solution.
- Electrowinning or smelting for final gold recovery.
❖ GOLD SMELTING
- Dry the gold sludge or precipitate.
- Mix with fluxes (borax, silica, soda ash).
- Melt in furnace → pour into molds → gold dore bars
❖ FINAL STEPS
- Waste handling: Neutralize spent ore (lime) & manage tailings.
- pH monitoring: Ensure cyanide destruction before discharge.
- Documentation: Record weights, pH, cyanide, and recovery for each batch.
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