Gravity recovery is the first method family a tailings retreatment flowsheet reaches for, because it needs no reagents and no chemistry to work: it simply exploits the fact that gold is far denser than the silicate and sulphide gangue it sits alongside. This page is the sub-head for the whole gravity equipment family, covering the principle, the testwork question that determines whether it will actually work on a given dump, and where each piece of equipment fits.
How does gravity concentration actually separate gold?
Gravity concentration separates gold from gangue purely on density difference, with no chemical reaction involved at any stage. Gold has a specific gravity around 19, roughly seven times denser than the quartz and silicate gangue typically around 2.6 to 2.7 that makes up most tailings. When slurry is agitated or spun in a controlled way, that density gap causes gold particles to settle, concentrate, or separate from the lighter gangue, which is carried away instead. Because the mechanism is entirely physical, gravity concentration adds no reagent cost and creates no chemical byproduct, which is exactly why it is placed first in the flowsheet described in full on tailings gold recovery methods.
The underlying physics is governed by settling behavior in a fluid, the same relationship engineers describe with Stokes' law for fine particles: settling velocity scales with the square of particle diameter and directly with the density difference between the particle and the fluid it's settling through. That squared relationship is why particle size interacts so strongly with gravity recovery, a coarse gold particle settles disproportionately faster than a fine one even at the same density advantage, which is exactly why gravity equipment is tuned differently depending on the size range it's targeting. Every piece of gravity equipment covered in this set, whether centrifugal or conventional, is really just a different engineering answer to maximizing that settling-velocity gap under practical plant conditions.
What determines whether gravity works on a specific tailings dump?
Whether gravity concentration works on a specific dump comes down to one number: the proportion of gold in that material that is actually gravity recoverable, commonly referred to as GRG. Gold that is coarse and already liberated from surrounding minerals responds well to gravity separation. Gold that is fine, still locked inside sulphide grains, or bound to particle surfaces will not respond, no matter how well the gravity equipment is tuned. This is a testable, not assumed, property of a specific dump, established through staged grind-and-concentrate testwork rather than guessed from the grade alone.
It's worth being explicit about why grade alone can't answer this question. Two tailings dumps can report an identical 0.7 g/t residual grade and still behave completely differently in a gravity circuit: one dump might carry most of that gold as coarse, liberated particles that a centrifugal concentrator captures easily, while the other carries the same grade almost entirely as fine, sulphide-locked gold that gravity simply cannot reach. Grade tells you how much gold is present; GRG tells you how much of it a gravity circuit can actually get out. Conflating the two is one of the more common and costly mistakes in early-stage tailings project evaluation.
What equipment sits in a gravity circuit?
A gravity circuit on a tailings retreatment project typically uses one of two equipment families. Centrifugal concentrators, most commonly Knelson or Falcon units, spin the slurry to multiply the effective density difference and can capture gold considerably finer than older gravity technology managed. Conventional gravity equipment, spirals and shaking tables, uses helical or reciprocating flow geometry instead of centrifugal force and generally suits coarser, higher-tonnage duty or a final cleaning stage rather than the primary recovery step. Which family suits a given dump, and why, comes down to the same GRG testwork referenced below, covered unit by unit on centrifugal vs conventional gravity separation.
In practice, most tailings retreatment circuits don't choose one family exclusively. A common arrangement runs a centrifugal concentrator first to capture the widest possible range of gravity-recoverable gold, then routes that rough concentrate to a shaking table for final cleaning into a smeltable product. Spirals tend to appear on their own only where tonnage is high enough, and the gold coarse enough, that the lower capital cost outweighs the finer recovery a centrifugal unit would add.
Where does gravity sit in a tailings retreatment flowsheet?
Gravity concentration sits at the front of a tailings retreatment flowsheet, immediately after re-mining and feed preparation and before flotation or leaching. Recovering the coarse, free gold fraction here first matters for a reason beyond simply capturing that gold early: every ounce gravity recovers is an ounce that never reaches the leach circuit, which means it never consumes cyanide or competes for carbon adsorption capacity. That knock-on cost saving is why gravity is placed first rather than left as an afterthought, a sequencing decision covered unit by unit on gold processing plants for tailings.
The economic logic compounds through the rest of the plant. A tonne of feed that gravity has already stripped of its coarse gold arrives at the leach tanks with a lower, more uniform residual grade, which makes leach performance more predictable and reduces the variability that complicates metallurgical accounting. Plants that skip or under-invest in the gravity stage don't just lose the gold gravity would have recovered directly, they also push a harder, more reagent-hungry job downstream onto circuits that are more expensive to run per ounce recovered.
What are the limits of gravity recovery?
Gravity recovery's central limitation is that it only captures gold that is both coarse enough and already liberated. Fine gold, generally anything below roughly 75 microns, settles too slowly relative to the surrounding gangue for the density advantage to matter much, and gold still locked inside a sulphide grain cannot be captured by density alone regardless of particle size. Both of those fractions need a different method family, flotation for locked sulphide gold or carbon-in-leach for fine and surface-bound gold, which is exactly the branching logic set out on tailings gold recovery methods.
A second, less obvious limit is that gravity performance degrades gracefully rather than failing outright, which can mislead an operator into over-crediting a poorly performing circuit. A gravity stage recovering well below its GRG-tested potential often isn't broken in any obvious mechanical sense, it's usually a feed density, fluidization, or wash-water setting quietly drifting out of tune, which is why routine metallurgical accounting against the GRG baseline matters as much as the initial testwork that set expectations in the first place.
How do you know if gravity alone is enough?
Knowing whether gravity alone is enough for a specific dump requires running the actual test rather than assuming an answer from grade or mineralogy alone. The GRG test grinds and gravity-concentrates successive sub-samples to build a cumulative recovery curve, and the resulting GRG percentage is the number a plant designer uses to decide how much of the flowsheet's total recovery gravity alone can be expected to deliver. A high GRG result justifies a gravity-first, gravity-heavy flowsheet; a low result shifts the design weight toward flotation and leaching instead, with gravity retained mainly to capture the easy fraction cheaply before those more expensive stages.
The practical answer is rarely a flat yes or no. Most Ghanaian tailings dumps land somewhere in between, GRG testwork showing that gravity alone recovers a meaningful but partial share of the contained gold, which is exactly the case the hybrid gravity-leach flowsheets on tailings gold recovery methods are built to handle. Treating the GRG result as a design input rather than a pass-fail verdict is what separates a well-specified plant from one that either over-invests in gravity capacity it doesn't need or under-invests and leaves recoverable gold in the leach tailings.
The testwork question this page sets up
The GRG percentage referenced above comes from the specific test procedure covered on the GRG test: gravity recoverable gold, not an estimate, and quantifying it properly is what actually tells a project whether to invest in gravity equipment at all before committing capital to it.
Why liberation matters as much as density
Gravity can only act on gold that grinding has already freed from its host mineral. A dump with excellent density contrast but poor liberation will still under-perform a gravity circuit, which is why particle size and liberation testwork is run alongside the GRG test rather than instead of it.
How this connects to the plant itself
Once GRG testwork confirms gravity is worth building around, the equipment selection and its position in the overall circuit are covered in full on gold processing plants for tailings and the complete tailings reprocessing equipment list.
Why Ghana's legacy tailings are a particularly relevant test case
Older Ghanaian tailings dams, produced before centrifugal concentrator technology was standard, often carry a higher proportion of coarse, gravity-recoverable gold than a modern plant's tailings would, simply because the original gravity equipment of the era was cruder. That historical pattern, covered in depth on why old tailings still contain gold, is part of why gravity recovery tends to play an outsized role in Ghanaian tailings retreatment economics specifically, rather than being a minor first stage before the "real" leach circuit takes over.