The GRG percentage has been referenced across this recovery methods set as the number that actually answers whether gravity recovery of gold from tailings is worth building around for a specific dump. This page covers the test procedure that produces that number, why it needs a specific minimum sample mass to be reliable, and how its result actually translates into an equipment and capital decision.
How is the GRG test procedure actually run?
The GRG test procedure, commonly associated with the Knelson/Laplante staged testwork method used industry-wide, grinds a sample to a coarse size first and gravity-concentrates it, then regrinds the remaining tailings from that stage to a finer size and concentrates again, repeating the cycle across several stages of progressively finer grind. Each stage's concentrate is measured, and the results are combined into a cumulative GRG recovery curve rather than reported as a single number, since how quickly recovery climbs across the stages is as informative as the final total.
The full Laplante procedure typically runs across three grind stages, using a laboratory-scale centrifugal concentrator, a smaller version of the same Knelson technology described elsewhere in this recovery methods set, to concentrate each stage's ground material. Because the test aims to measure the maximum gold that gravity could ever recover under near-ideal conditions, the laboratory equipment and grind stages are deliberately more thorough than a full-scale plant would practically run, which is why the GRG figure is described as a theoretical ceiling rather than a guaranteed plant recovery number.
How large a sample does a GRG test actually need?
A GRG test needs a substantially larger sample than a standard fire assay, commonly in the tens of kilograms rather than the grams a fire assay uses, because the staged grinding and regrinding procedure consumes material across multiple sequential steps and because Gy's sampling theory, referenced elsewhere on this site, demands a larger mass to keep sampling error acceptable when gold particles are coarse and unevenly distributed through the sample. Underestimating this mass requirement is a common and avoidable cause of unreliable GRG results, since a too-small sample can be dominated by the presence or absence of just one or two coarse gold particles rather than reflecting the dump's true average behavior.
Composite sampling across multiple locations and depths within a dump, rather than a single grab sample, is standard practice for exactly this reason, since a tailings dump's gravity-recoverable gold content can vary meaningfully across its volume depending on the original plant's processing history at different points in time. A GRG result from one unrepresentative sample location risks over- or understating what a full-scale gravity circuit would actually achieve across the whole dump.
What does a "good" GRG result look like for tailings?
A meaningfully high cumulative GRG result is what justifies a gravity-first, gravity-heavy flowsheet, while a low result shifts flowsheet weight toward flotation and leaching, with gravity retained mainly to capture the easy fraction cheaply rather than to carry the bulk of recovery. What counts as "good" is context-dependent rather than a fixed threshold, feeding directly into the centrifugal-versus-conventional equipment choice covered on centrifugal vs conventional gravity separation.
The value of even a modest GRG result also depends heavily on total contained ounces and processing cost, not on the percentage figure in isolation. A large-tonnage dump with a moderate GRG percentage can represent a genuinely attractive gravity opportunity in absolute ounce terms, while a small, high-grade sample with the same percentage might not justify a dedicated gravity circuit at all once fixed capital costs are spread across a smaller total tonnage. Reading a GRG result in isolation from the broader project economics is one of the more common analytical mistakes a technical review can catch.
How does GRG interact with particle size and liberation?
GRG is only recoverable if it is already liberated at the sample's current grind, which ties this test directly to the liberation question covered on particle size, liberation and recovery. A sample that shows low GRG at a coarse grind but a sharp jump after regrinding is telling a different story than one that shows consistently low GRG across every stage: the first suggests locked gold that finer grinding would liberate, while the second suggests gold that gravity simply won't reach regardless of grind size, which points toward leaching instead.
This is why the GRG test and a dedicated liberation study are run as complementary pieces of the same testwork program rather than as substitutes for one another. GRG measures what a gravity circuit actually captures at each grind stage; the liberation study explains why, by showing directly whether the remaining, uncaptured gold is still locked in gangue or is instead free but simply too fine for gravity separation to reach. A project relying on GRG data alone can identify that a problem exists at a given grind stage without necessarily understanding which of these two distinct causes is responsible.
What happens if GRG comes back low?
A low GRG result shifts the flowsheet's design weight toward flotation or carbon-in-leach rather than gravity, an honest branch-point rather than a failure of the test. Gravity is rarely abandoned entirely even in a low-GRG case, since it still captures whatever coarse fraction is present cheaply before the material reaches a reagent-consuming stage, but the project's recovery economics stop depending on it, a branching decision covered in full on tailings gold recovery methods. A low result is not a bad outcome for a project's overall economics either, provided the leach circuit is sized correctly for the fraction gravity leaves behind; it simply changes where the project's capital gets spent.
How does a lab-scale GRG result translate into full-scale equipment sizing?
The GRG percentage itself doesn't directly specify how many Knelson or Falcon units a plant needs; it establishes the ceiling that full-scale equipment, correctly matched to the feed's particle size and tonnage, should aim to approach. Actual plant-scale recovery typically falls somewhat short of the laboratory GRG figure, since full-scale equipment operates under less ideal conditions than a controlled laboratory test, and the gap between the two is itself a useful engineering benchmark: a plant achieving a recovery close to its GRG ceiling is running well, while a large, persistent gap points to equipment sizing, feed preparation, or operating tuning problems worth investigating.
Pilot-scale testwork, running a larger continuous sample through actual plant-representative equipment rather than a batch laboratory rig, is the usual next step once a GRG result justifies further gravity circuit investment, bridging the gap between the laboratory ceiling and what a real, continuously operating plant can be expected to achieve on the specific feed. A project that jumps straight from a laboratory GRG result to full-scale plant design, skipping pilot confirmation, is accepting real technical risk that a proper staged testwork program is specifically built to retire before major capital is committed.
Is a GRG test enough on its own to design a flowsheet?
No. GRG is one input alongside deportment work, mineral liberation analysis showing free, locked, and surface-bound proportions as covered on gold tailings composition, and reagent and leach testwork for whatever fraction gravity won't reach. A flowsheet designed from GRG data alone, without that broader deportment picture, risks under-building the leach circuit a project actually needs.
Why GRG testwork is one of the first line items in a serious feasibility budget
Because GRG results directly determine whether a project's flowsheet leans gravity-heavy or leach-heavy, and that determination cascades into nearly every subsequent capital cost estimate, GRG testwork is typically commissioned early in a feasibility program rather than added later as a confirmation step. A feasibility study for a Ghanaian tailings retreatment project that has not yet run this test on representative samples is still working from an assumption, not a technical foundation, on one of its most consequential flowsheet decisions.
How GRG results should be reported to investors
A credible technical disclosure reports the full cumulative GRG curve across grind stages, the sample mass and sampling protocol used, and the laboratory that performed the test, not a single headline percentage stripped of that supporting detail. Investors evaluating a retreatment opportunity should expect to see this level of detail before treating a stated GRG figure as a reliable basis for their own recovery and revenue assumptions.
Why an accredited, independent laboratory matters for this test
Because the GRG result carries so much weight in downstream flowsheet and capital decisions, running it through an accredited laboratory with documented chain of custody for the sample, rather than an in-house or unaccredited facility, is what gives the result the credibility an investor or lender needs to rely on it. The same standard applied to fire assay and JORC or NI 43-101 resource reporting elsewhere on this site applies equally to GRG testwork.