Centrifugal vs conventional gravity separation

This page closes out the gravity equipment set covered on gravity recovery of gold from tailings by putting the centrifugal units, Knelson and Falcon, directly against the conventional options, spirals and shaking tables, attribute by attribute. Neither category is a universal answer; each covers a distinct part of the particle-size and economic range a tailings project needs to think through before committing capital.

What is the core mechanical difference between the two approaches?

Centrifugal separation adds a force multiplier, typically up to around 60G on tailings-scale units, on top of the same density-difference principle both approaches rely on. Conventional separation, spirals and tables, uses gravity plus simple flow geometry, helical channels or a reciprocating riffled deck, with no force multiplier at all. That multiplier is the entire reason centrifugal units can capture particles conventional equipment simply cannot settle fast enough to catch.

The physical reason comes down to settling velocity and the forces competing against it. As a particle gets smaller, drag and turbulence forces shrink more slowly than the gravitational force pulling it toward its density-determined settling path, so below a certain size the density advantage that makes gold recoverable by gravity in the first place gets swamped by drag. Centrifugal force does not change this relationship in kind, but it changes it in degree. Multiplying the driving force by tens of G effectively pushes the size threshold at which drag dominates much further down, into the fine and ultrafine range conventional equipment cannot reach regardless of how carefully it is operated. This relationship follows the same settling-velocity physics, rooted in Stokes' law, that governs particle behavior throughout gravity concentration generally, not a mechanism unique to any one equipment brand.

Attribute comparison

How do the two approaches compare on throughput and plant footprint?

Conventional equipment generally wins on throughput per dollar of capital and per square meter of plant footprint. A spiral bank has no moving parts beyond the feed pump and can be scaled up simply by adding parallel columns, while a centrifugal unit's bowl has a fixed practical throughput ceiling that requires installing additional complete units, motors, drives, and control systems included, to add capacity. For a project processing very large tonnages where the value per tonne is modest, that difference in cost-per-tonne-of-capacity can matter more than the recovery advantage centrifugal equipment offers on the margin.

Footprint matters practically on a Ghanaian retreatment site as well, since available flat land near a legacy tailings dump is not always abundant, particularly where a dump sits close to existing settlements or agricultural land. A conventional gravity circuit's larger footprint per tonne of capacity needs to be weighed against site constraints just as seriously as the equipment's purchase price.

Which approach recovers more fine gold, and why does that matter for tailings specifically?

Centrifugal separation recovers substantially more of the fine fraction, and that matters disproportionately for tailings because tailings are, by definition, the material historic plants already picked the coarse gold out of. A dump weighted toward fine gold will simply underperform on conventional equipment regardless of how well that equipment is operated, which is exactly the mechanism explored on fine gold recovery: the sub-75-micron problem.

This is also why comparing the two approaches on a single, generic recovery percentage is misleading. The right comparison always specifies particle size range, since a conventional spiral bank might match a centrifugal unit's recovery on coarse, well-liberated gold while trailing badly on the sub-75-micron fraction where the centrifugal unit's force multiplier does its real work. Any performance claim for either technology is only meaningful alongside the size distribution it was measured against, which is why a specific particle size and liberation profile for the feed in question, not a manufacturer's general specification sheet, should anchor any equipment recommendation for a real project.

How do the capital and operating economics actually compare?

Centrifugal units carry a higher upfront capital cost per unit of installed capacity and a continuous power draw for bowl rotation and fluidization, both of which conventional gravity equipment largely avoids. A spiral bank needs only enough power to pump feed to the top of the circuit, and a shaking table's reciprocating drive draws modestly compared with a spinning centrifugal bowl. Over a multi-year retreatment program, that difference in ongoing operating cost accumulates and belongs in the same economic comparison as the headline capital figure, not as an afterthought once equipment is already selected.

The economic case for the higher centrifugal cost rests entirely on the value of the additional fine gold recovered. Where GRG testwork shows a meaningful fraction of a dump's recoverable gold sits below the size conventional equipment can catch, the extra ounces a centrifugal stage recovers typically justify its added capital and operating cost many times over across the life of a project. Where the fine fraction is small, that same extra cost may not be justified, and a conventional-only gravity stage can be the financially sounder choice.

Maintenance cost also differs meaningfully between the two categories over a project's operating life. A spiral's lack of moving parts keeps its maintenance burden close to periodic trough-lining replacement, while a centrifugal unit's bowl, seals, and drive assembly need the more involved upkeep any rotating machinery requires, along with the spare-parts inventory and technical training that upkeep implies. A full economic comparison accounts for this ongoing maintenance difference across the life of the project, not just the initial purchase price of each option.

Can a flowsheet use both?

Yes, and on tailings retreatment this combination is common rather than exceptional: a centrifugal unit or spiral bank handles primary recovery across the full feed volume, while a shaking table cleans the resulting concentrate into a final, high-grade product. Using both plays to each technology's strength, throughput and fine-gold capture from the centrifugal or spiral stage, selectivity and final grade from the table.

A common staged configuration on larger Ghanaian retreatment projects runs spirals first for bulk, low-cost coarse recovery across the full tonnage, follows with a centrifugal stage sized only for the spiral tailings stream to catch the fine gold the spirals missed, and finishes with a shaking table cleaning the combined concentrate from both stages. This layering keeps the most expensive equipment, the centrifugal stage, sized for a smaller, already-reduced volume rather than the full plant throughput, which is often the most capital-efficient way to capture both the coarse and fine gold fractions in a single flowsheet.

Sequencing the two categories this way also phases capital spending in a way that suits a project ramping up production gradually. A conventional-only gravity stage can be commissioned first at lower upfront cost, begin generating revenue, and demonstrate the dump's actual recoverable grade before a project commits the larger capital a centrifugal addition requires, an approach that reduces early-stage financing risk for investors backing a retreatment operation from the ground up.