MINERAL PROCESSING KNOWLEDGE · GOLD ORE PROCESSING
Learn how gold occurrence, sulfide association and mineral locking influence gravity recovery, flotation, leaching and refractory-ore pretreatment.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
The terms free-milling and refractory describe how readily gold can be recovered from an ore using conventional processing. They are not simply labels for high-grade or low-grade material. A free-milling ore may release gold through crushing and grinding and respond to conventional gravity recovery and/or cyanide leaching. In refractory ore, gold is locked within sulfide minerals or otherwise resists direct leaching, so additional treatment may be required.
Technical note: Gold processing can involve hazardous chemicals and specialized environmental controls. This article is a high-level technical explanation, not an operating procedure for cyanide handling or a design for a gold recovery plant. Any leaching or pretreatment route must be designed and operated by qualified professionals under applicable permits and safety controls.
1. What makes an ore free-milling?
Free-milling gold is generally amenable to conventional recovery after suitable comminution. The gold may occur as liberated or readily liberated particles, or be exposed sufficiently for leaching. Gravity concentration can recover coarse liberated gold before or within a grinding circuit. The remaining material may then be evaluated for leaching or another process, depending on mineralogy and test results.
- Gold particle size and shape.
- Proportion of coarse, free or gravity-recoverable gold.
- Gold association with quartz, sulfides or other minerals.
- Required grind size to expose gold.
- Leach response and reagent consumption, if leaching is being considered.
2. What makes an ore refractory?
A refractory ore does not respond adequately to direct cyanidation under conventional conditions. One common reason is that microscopic gold is encapsulated in sulfide minerals such as pyrite or arsenopyrite. Other causes can include preg-robbing carbonaceous matter, copper minerals or chemical constituents that interfere with leaching. The cause must be diagnosed before selecting a treatment route.
For sulfide-hosted gold, flotation may produce a smaller sulfide-rich concentrate for additional treatment. Pretreatment options reported in the technical literature include roasting, pressure oxidation and biological oxidation. These are specialized operations and are not interchangeable; selection depends on the refractory mechanism, concentrate characteristics, environmental requirements and economics.
3. Where gravity recovery fits
Gravity separation is useful when gold is present as liberated particles with a suitable size and density contrast. A jig, centrifugal concentrator, sluice or shaking table may be considered for different duties. Coarse free gold can sometimes be recovered early, reducing the amount of gold that proceeds to downstream processing. Very fine or locked gold may not respond to gravity separation in the same way.
The best place for gravity recovery is an ore-specific circuit decision. Testwork can investigate gravity-recoverable gold, size-by-size distribution and the value of placing a gravity unit on the mill discharge, circulating load or another stream.
4. Flotation and leaching serve different duties
| Process | Primary purpose | Important question |
|---|---|---|
| Gravity concentration | Recover liberated gold particles using density differences | What proportion of gold is liberated and recoverable at each size? |
| Flotation | Concentrate gold-bearing sulfide minerals where their surfaces respond to flotation | Does gold report with floatable sulfides, and what concentrate is produced? |
| Cyanide leaching / adsorption | Dissolve accessible gold and recover it from solution using a suitable flowsheet | Does the ore respond to leaching, and what are the consumption and residue implications? |
| Refractory pretreatment | Expose or transform gold-bearing minerals before downstream recovery | What mechanism causes refractoriness, and which pretreatment is technically and environmentally suitable? |
5. CIL and CIP are not the same arrangement
Carbon-in-pulp (CIP) and carbon-in-leach (CIL) both use activated carbon to adsorb dissolved gold from a leach slurry, but the process arrangement differs. In CIP, leaching and adsorption are generally carried out in separate stages: the slurry is leached first, then passed through adsorption tanks containing carbon. In CIL, leaching and adsorption occur together in the same series of tanks. The choice depends on ore behaviour, solution chemistry, carbonaceous material, plant design and testwork.
Neither CIL nor CIP should be selected solely from the word “gold” or from another plant’s configuration. Leach amenability, adsorption behaviour, reagent consumption, detoxification and tailings controls all require project-specific evaluation.
6. A practical decision sequence
- Confirm gold mineralogy and identify the main gold-bearing associations.
- Characterize gold particle size, liberation and gravity-recoverable gold.
- Run appropriate gravity tests where free gold is suspected.
- Evaluate flotation if gold is associated with floatable sulfide minerals.
- Conduct diagnostic leach and adsorption tests where cyanidation is being considered.
- If refractory behaviour is confirmed, compare specialist pretreatment options with qualified metallurgical and environmental review.
7. Equipment that may be relevant
A free-milling hard-rock circuit may include crushing, grinding, classification and gravity equipment, followed by a suitable downstream recovery process. A sulfide-associated route may also use flotation, thickening and filtration. The current LiCheng Tech catalogue includes examples such as a Ball Mill, Jigging Machine, Shaking Table and Flotation Machine. Specialist leach, oxidation, elution and refining equipment requires separate capability and supplier confirmation.
Conclusion
The key difference between free-milling and refractory gold is the accessibility of the gold to recovery. Mineralogical diagnosis and testwork should establish whether gravity, flotation, leaching or pretreatment is appropriate. A flowsheet copied from another gold mine can be misleading if the gold occurrence and mineral associations differ.
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