How gravity separation works

Every gravity device exploits the same principle: in a moving film of water, dense particles settle, lag or report differently than light particles. The usable density contrast is commonly expressed by the concentration criterion, (SG heavy – 1) / (SG light – 1). A value above about 2.5 means easy separation at almost any size; between 1.75 and 2.5 separation is feasible above roughly 0.15 mm; below 1.25 gravity alone struggles. Gold (SG 19), cassiterite (SG 7) and chromite (SG 4.5) separate readily from silica (SG 2.65); fine coal and some oxidized ores do not.

The two variables that decide everything

• Particle size: fine feed needs gentle, film-flow devices; coarse feed needs pulsing or tumbling action.
• Throughput vs. grade: high-tonnage roughing favors spirals and jigs; final cleaning to a sellable concentrate favors tables and centrifugal bowls.

The four main gravity devices compared

Shaking tables

A gold shaker table uses a riffled deck with asymmetric horizontal motion and a thin cross-flow of water. Dense particles migrate along the riffles to the concentrate end while light gangue washes over the edge. Tables give the sharpest separation of any gravity device and produce a clean, high-grade concentrate, which is why they are almost always the final cleaning stage in a small gold plant. The trade-off is low unit capacity, so tables are usually fed from a pre-concentrating device rather than raw ore. Browse the full range of gravity concentration equipment to see how tables pair with upstream units.

Spiral chutes

The spiral chute separator has no moving parts: pulp flows down a helical trough and centrifugal plus gravitational forces band the minerals across the channel, where splitters cut concentrate, middling and tailing. Spirals shine on fine, high-tonnage duties such as iron ore, chromite, ilmenite and zircon sands. They are cheap to run, easy to operate and scale simply by adding starts, making them a favorite roughing device ahead of magnetic or table cleaning.

Jigs

Jigs pulse water up through a bed of particles so dense grains stratify to the bottom and report through the screen. Because they handle coarse feed, jigs are ideal for alluvial deposits, coarse gold, tin and tungsten, and for pre-concentrating run-of-mine before grinding, which cuts downstream mill load. They tolerate variable feed and require little water relative to tonnage.

Centrifugal concentrators

A centrifugal concentrator applies many times the force of gravity in a fluidized, rotating bowl, capturing fine free gold that tables and spirals lose. Installed in or after the grinding circuit, it recovers gold as soon as it is liberated, often lifting overall plant recovery by several points and reducing the gold tied up in circulating load. It is the standard answer to the question, where does my fine gold go.

Centrifugal bowls run in two modes. Batch units periodically stop to rinse a high-grade concentrate and are common in small plants and for cleaning duty, while continuous units discharge concentrate on a timed cycle without stopping the feed, suiting larger throughputs. The captured concentrate is small in mass but high in grade, so it is almost always cleaned on a shaking table before smelting. Because the bowl applies tens of g, it recovers gold down to roughly 10-25 micron that a table would lose, which is exactly the fraction that otherwise reports slowly, or not at all, to a downstream leach.

Where gravity sits in the flowsheet

In most plants gravity is not the only step. It is a fast, cheap pre-concentration stage that pulls coarse and free values early, leaving a smaller, upgraded stream for flotation or leaching. A typical gold flowsheet runs the milled pulp through a centrifugal concentrator and table to recover free gold, then sends the gravity tail to a flotation circuit or a CIL/CIP leaching plant. This hybrid approach maximizes recovery while shrinking cyanide and reagent demand. For the full picture of how recovery routes combine, see our guide to gold recovery methods compared.

Wear, water and operating cost

Part of gravity’s appeal is low operating cost, but each device has its own wear and consumable profile worth budgeting. Spirals have no moving parts, so wear is limited to the polyurethane or rubber lining of the trough and the splitters, replaced periodically. Tables wear the deck surface and riffles and depend on a reliable head-motion drive. Jigs wear screens, diaphragms and the drive mechanism, and consume ragging where used. Centrifugal bowls wear the fluidization fittings and the bowl liner. None of these approaches the reagent and energy bill of flotation or leaching, which is precisely why gravity is placed first wherever the ore allows it.

Selection checklist

• Run a particle-size and SG analysis first; size dictates the device.
• Coarse and free values: jig or centrifugal up front.
• Fine, high tonnage: spirals for roughing, tables for cleaning.
• Always confirm the concentration criterion before committing to gravity alone.
• Test on a representative sample; Xinhai’s lab can recommend a flowsheet sized to your ore.

Match the plant to the ore first

Before any equipment is bought, two questions decide the whole design: is the gold free-milling or refractory, and is it coarse or fine. Free-milling, coarse gold can be recovered largely by gravity alone, which is the cheapest plant to build and run. Finely disseminated or sulphide-locked gold needs leaching, which adds tanks, reagents and an elution circuit but lifts recovery from perhaps 40-60% (gravity only) to 88-95% (gravity plus CIP). A 1-2 tonne metallurgical test on a representative sample is the single best investment you can make before ordering steel.

Two diagnostic tests are worth commissioning early. A gravity-recoverable-gold (GRG) test tells you how much of the gold can be won by gravity alone, which sets the size of the gravity stage and the cyanide you can avoid. A bottle-roll cyanidation test on the gravity tailing tells you the leach recovery and reagent consumption to expect. Together they convert the vague question of which plant into hard numbers: recovery, reagent cost per tonne and the split between gravity and leach. Skipping this step is the most expensive shortcut in small-scale gold, because it tends to surface only after the plant is built and underperforming.

The equipment list, stage by stage

1. Crushing and feeding

Run-of-mine ore is reduced to a millable size, usually below 15-25 mm. A small plant typically uses a single-stage or two-stage crusher with a feeder ahead of it.

• PE jaw crusher for primary reduction; a PE-400×600 handles roughly 15-60 t/h.
• Vibrating grizzly feeder to meter ore and scalp fines.
• A spring cone crusher as a second stage if a finer, tighter product is needed.

2. Grinding and classification

Grinding liberates the gold, typically to 60-80% passing 75 micron for leaching. A closed circuit of mill plus classifier keeps the product size consistent.

• Wet ball mill sized to your tonnage and target grind.
• Spiral classifier or a hydrocyclone to close the grinding circuit.

3. Gravity recovery

Free gold should be captured as soon as it is liberated, before it dissolves slowly or is lost. A gravity stage is cheap insurance and often recovers a third or more of the gold.

• Centrifugal concentrator in the grinding circuit to catch fine free gold.
• Shaking table to clean the gravity concentrate to a smeltable grade.

4. Leaching and recovery (for refractory or fine gold)

The gravity tail goes to a cyanide leach. A CIP plant adsorbs dissolved gold onto activated carbon in a train of agitated tanks, then an elution and electrowinning system strips the carbon and recovers gold for smelting. For the choice between CIP, CIL and heap leach, see our guide to CIL vs CIP vs heap leach.

A small leach circuit is usually a string of six to eight agitated tanks giving a total residence time of 18-30 hours, sized so the gold is fully dissolved and adsorbed before the pulp leaves the last tank. Lime is added to hold pH around 10-10.5, air or oxygen is sparged to supply the dissolved oxygen leaching needs, and carbon is advanced counter-current to the pulp so the loaded carbon is drawn from the first tank. Getting the tank count and residence time right is what separates a plant that hits its recovery target from one that chronically leaves gold in the tailing.

Indicative budget by plant size

The ranges below are equipment-only and indicative; they exclude civil works, freight, duties and installation. Frame them as starting points to scope your project, not quotes.

Hidden costs to budget for

• Civil and installation: often 20-40% on top of equipment cost.
• Power and water: grinding dominates power draw; secure supply or a genset early.
• Reagents: cyanide, lime and carbon are ongoing operating costs in a leach plant.
• Tailings and water return: a thickener pays back in recovered water and is increasingly required for permitting.
• Spares and wear parts: mill liners, balls, crusher jaws and pump parts.

Phasing a project on a tight budget

Cash-constrained projects often build in phases. A common approach is to commission a gravity-only plant first to generate early cash flow from the free gold, then add the leach circuit once the deposit and the cash position are proven. This works only if the layout is planned for the leach from the start, with tank pads, power and water sized for the final configuration; retrofitting a leach into a plant laid out for gravity alone wastes money. Plan the full flowsheet on paper even if you build it in stages.

Build it as one system

The most common small-plant mistake is buying machines piecemeal that do not balance, leaving a bottleneck that caps throughput. Because Xinhai supplies turnkey mineral processing plants under an EPC+M+O model, the crusher, mill, gravity and leach stages are sized against one another and against your ore test, and the package includes installation, commissioning and operator training so the plant reaches nameplate faster.

The dewatering chain: coarse to fine water removal

Water removal gets harder and more expensive the drier you go. A sensible circuit removes the easy water first by sedimentation, then tackles the bound water by pressure or vacuum filtration. The typical sequence is thickener then filter, with cyclones sometimes splitting coarse sand for separate dewatering. Each stage has a job:

• Thickening: bulk water recovery and slurry densification, the cheapest water you will ever remove.
• Filtration: final moisture reduction to a handleable or stackable cake.

Thickeners vs filter presses vs vacuum filters

Thickeners: the workhorse first stage

A deep cone thickener uses gravity sedimentation, aided by flocculant, to settle solids into a dense underflow while clarified overflow water returns to the plant. A deep-cone or high-rate design produces underflow at 45-65% solids and reclaims the bulk of process water, often 70-85% of incoming water, which is decisive in arid regions. Thickeners are the cheapest water removal per tonne and almost always come first. Where space allows, a thickener alone can feed a conventional tailings dam; where dry stacking is required, it pre-densifies feed for filtration and dramatically cuts the load on the downstream filter. See the full thickening and dewatering range for sizing options.

Thickeners are sized on settling-flux testwork, not rules of thumb, because settling rate and achievable underflow density depend heavily on particle size and flocculant response. The flocculant itself is a key operating cost and a key performance lever: the right type and dose can double the settling rate and add several percent to underflow solids, while an under-dosed or poorly mixed feed produces a dilute underflow that overloads the downstream filter. A deep-cone design pushes underflow density toward the high end, approaching paste consistency for some ores, which is why it is favored where dry or paste tailings are the goal.

Filter presses: the route to dry stack

A plate-and-frame filter press clamps filter cloths between plates and forces slurry through under pressure, producing a firm cake at 75-85% solids that can be trucked and stacked without a dam. The press operates in batches: fill, pressurize, optional membrane squeeze and air blow, then discharge. It delivers the lowest residual moisture and the clearest filtrate, which is why it is the standard for dry-stack tailings and for high-value concentrate where every percent of moisture costs freight. The trade-offs are higher capital cost, cloth replacement and batch cycle management.

Vacuum filters: continuous concentrate dewatering

A disc vacuum filter draws slurry onto rotating discs under vacuum, forming and discharging cake continuously at roughly 80-88% solids. Continuous operation suits steady, high-tonnage streams such as iron or copper concentrate, and ceramic-disc versions cut energy use sharply versus conventional cloth designs. Vacuum filters generally leave slightly more moisture than a pressure filter and depend on a reliable vacuum system, but their continuous output and lower per-tonne energy make them attractive where a stackable but not bone-dry cake is acceptable.

The ceramic-disc variant deserves a note because it changes the economics. Its microporous ceramic plates hold vacuum within the plate, so only a small vacuum pump is needed and air is not drawn through the cake as in a conventional cloth filter. The result is markedly lower power per tonne and very clear filtrate, at the cost of careful plate maintenance and acid cleaning to prevent blinding. For steady concentrate streams, the energy saving over the plant life can be substantial, which is why ceramic vacuum filters have become common on iron and copper concentrate duties.

How to choose and sequence

Start from the discharge requirement and work backward. If a permitted dam is available and water recovery is the goal, a thickener may be enough. If dry stacking is mandated, plan for a thickener plus filter press. For continuous concentrate at high tonnage, a thickener plus vacuum filter is often the lower-cost continuous option.

• Always thicken first. Feeding a filter at 50%+ solids instead of 25% can halve filter area and cost.
• Match flocculant to ore. Settling rate and underflow density depend heavily on flocculant type and dose; bench-test before sizing.
• Mind the fines. Clay-rich tailings settle slowly and filter slowly; a deep-cone thickener and paste-capable press handle them better.
• Recover water deliberately. Pair dewatering with slurry pumps and a closed water loop to minimize freshwater make-up.
• Plan for variability. Tailings characteristics shift as the orebody and grind change, so size equipment with headroom rather than at the average case.

One more consideration is increasingly decisive on new projects: the regulatory and closure picture. Dry-stack tailings remove the standing-water dam that drives the worst tailings-failure risks, and many jurisdictions now favor or require filtered tailings for new permits. That regulatory pressure, combined with the water-recovery savings, is why the thickener-plus-filter route is steadily displacing conventional dam-only schemes even where a dam would be technically adequate. Factoring closure cost and permitting risk into the comparison usually tilts the economics further toward filtration than capital cost alone suggests.

Dewatering is a system, not a single machine. Because Xinhai delivers the full circuit under one EPC+M+O contract, the thickener, filter and water-return are sized together against your actual tailings sample rather than bolted on at the end.