MINERAL PROCESSING KNOWLEDGE · GOLD ORE PROCESSING
Gold is dense, soft and easy to over-grind, so the best place to recover it is usually the circulating load of the grinding circuit rather than the tailings end of the plant.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Gravity equipment is often added to a gold plant as an afterthought, at whatever point in the flowsheet has room for it. Positioned that way, it recovers gold that has already been through the mill, sometimes many times over. Placed deliberately, the same unit works on gold that is still coarse and still easy to capture. This article explains why the grinding circuit is the right home for gravity recovery, where inside that circuit the unit belongs, and what the decision changes downstream.
Technical note: The following is a principle-level explanation of gravity recovery within a grinding circuit. It is not an engineering design, a mass balance or a guarantee of recovery performance. Placement, sizing and water balance must be confirmed through testwork and detailed design for each project.
1. Two valid positions in a closed grinding circuit
Most gold grinding circuits are closed. Mill discharge is pumped to a hydrocyclone, the overflow passes forward as finished product and the underflow returns to the mill as circulating load. Gravity units are installed inside that loop, and there are two recognised positions for them: between the ball mill discharge and the cyclone feed, or on the cyclone underflow itself.
The distinction matters because the two streams are not the same material. Between discharge and cyclone feed the unit sees the whole mill product before classification. On the underflow it sees only the coarse fraction the cyclone has already rejected, which is where dense liberated particles have concentrated.
| Position | What the unit sees | Point to check |
|---|---|---|
| Mill discharge to cyclone feed | Full mill product, before classification | Enough head and flow without disturbing cyclone feed |
| Cyclone underflow | Coarse fraction already enriched in liberated gold | Return of gravity tails without short-circuiting the cyclone |
2. Why gold leaves a ball mill in poor condition
Native gold is exceptionally dense at 19.3 g/cm³ and highly malleable, and neither property helps inside a ball mill. Gold particles are struck repeatedly, and instead of breaking they flatten into flakes. Flakes resist further size reduction while continuing to travel around the circuit, and dense gold settles in the mill, staying in the grinding charge longer than the gangue around it.
The result is over-grinding of exactly the material the circuit is trying to recover. A flattened flake is harder to recover by gravity than the rounded particle it came from, and it is harder to dissolve as well, because the surface available to cyanide is small relative to its mass.
3. The circulating load concentrates what gravity can catch
Cyclone underflow is where liberated coarse gold accumulates. Gold is far denser than the silicates it is liberated from, so it reports preferentially to the coarse underflow rather than to the overflow that becomes finished product. A gravity unit on that stream receives a feed already enriched in the particles it handles best.
Recovered at that point, the concentrate can account for 30%-70% of the free gold in the ore, and it is recovered early, before the material has had a chance to be over-ground or to reach the leach tanks.
4. What early recovery takes off the downstream circuit
Every gram recovered by gravity is a gram that does not have to be dissolved, adsorbed, eluted and electrowon. Removing coarse free gold from the mill circuit reduces the load on downstream cyanidation or flotation and reduces reagent consumption with it.
The effect on tailings matters more than the operating saving. Coarse gold that reaches a cyanide tank leaches slowly, and particles that are still partly locked or flattened may not dissolve within the residence time available, so they leave with the tailings. A gravity unit in the circulating load intercepts that material while it is still coarse, without demanding a longer leach or more reagent.
5. Choosing between a jig and a centrifugal concentrator
The two devices usually considered for this duty work on different size ranges. A jig is applied to coarser liberated gold, typically in the 0.5-15 mm range, and stratifies particles by density in a pulsating water bed. A centrifugal concentrator works on much finer material, typically 0.010-0.1 mm, using a centrifugal field to accelerate the settling of dense particles. Because liberated gold in a closed circuit is generally fine, the centrifugal concentrator is the more common choice for circulating-load duty, with a jig considered where the grind is deliberately coarse.
Whatever is chosen, the unit has to fit the circuit rather than interrupt it: a feed taken without starving the cyclone, a concentrate that can be handled, and a tailings return that puts material back where it came from.
6. Checks before installation
- Confirm from testwork how much of the gold is gravity-recoverable and at what size.
- Check that the chosen stream has the head and flow to feed the unit without upsetting classification.
- Plan concentrate handling and the gravity tailings return before the unit is positioned.
- Allow for the water balance, since gravity units add process water inside the grinding circuit.
Conclusion
Gravity recovery in a gold circuit is a placement decision first and an equipment decision second. Its value comes from intercepting liberated gold while it is still coarse, in the stream where it concentrates, before repeated impact in the mill flattens it or a leach that is too short loses it. A unit at the end of the plant can only work on gold that has already survived the whole circuit; the same unit in the circulating load works on it at the first opportunity. Context for the wider plant is set out under gold ore processing.
How LiCheng Tech can help
LiCheng Tech matches equipment to your material, feed size and target capacity from our product catalogue, and connects you with suitable Chinese suppliers where a specification falls outside our standard range. For gravity recovery inside a grinding circuit that means the ball mill and the classifier that close the loop, plus the gravity units themselves: a jig machine for coarser feed or a centrifugal concentrator for fine liberated gold, with a vibrating screen where the concentrate needs sizing.
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