Every gold project reaches a defining technical question: how do we recover the gold from this ore most effectively and most economically?
The two principal tools available are gravity concentration and cyanide leaching — and choosing between them, or finding the right combination of both, is one of the most consequential process design decisions a gold project will make. Get it right, and you have a plant that performs efficiently for the life of the mine. Get it wrong, and the cost compounds with every tonne processed.
The good news is that this does not have to be a guess. With the right testwork, the answer is in your ore. This article explains how to read it.
The Fundamental Choice: Gravity, Leach, or Both?
Gravity concentration recovers gold by exploiting the significant density difference between gold and the surrounding rock. Gold is one of the densest naturally occurring minerals — roughly six to seven times denser than the silicate gangue minerals that make up most of the host rock. Gravity equipment uses this density contrast to separate gold particles from lighter material without the use of chemicals.
Cyanide leaching works differently. Rather than separating gold physically, it dissolves gold chemically — cyanide solution passes through the ore, selectively dissolving gold, which is then recovered from solution onto activated carbon or by zinc precipitation.
Most gold projects are not an either/or choice. The majority of operating African gold plants use both — a gravity circuit to capture the coarse, free-milling gold quickly and cheaply, followed by a leach circuit to recover the finer gold that gravity cannot efficiently capture. Understanding where your ore sits on this spectrum is what testwork is for.
How Ore Characteristics Drive the Decision
No two gold deposits are the same, and the processing route that is optimal for one may be entirely wrong for another. The key ore characteristics that drive the decision are gold liberation, grain size, and the nature of the surrounding rock.
Gold liberation refers to how easily gold particles are freed from the surrounding rock through crushing and grinding. Gold that liberates readily at coarse grind sizes — so-called free-milling gold — is an excellent candidate for gravity recovery. Gold that remains locked within sulphide minerals or other host phases until very fine grinding is applied will not respond well to gravity concentration regardless of how the circuit is configured.
Grain size matters enormously for gravity recovery. Coarse gold particles — those visible to the naked eye or easily seen under low magnification — are recovered very efficiently by modern gravity equipment. As gold grain size decreases into the fine and ultra-fine range, gravity efficiency drops off. Below a certain grain size threshold, gravity concentration becomes impractical and leaching becomes the primary recovery mechanism.
Gangue mineralogy — the mineralogy of the non-gold material in your ore — affects both routes. Carbonaceous material in the ore can adsorb dissolved gold during leaching, reducing recovery. High clay content affects slurry rheology and can reduce gravity separation efficiency. Sulphide minerals may contain locked gold that requires oxidation before leaching can be effective.
Understanding these characteristics is not a matter of analogy or assumption. It requires characterisation testwork on your specific ore.
The Economics of Gravity Concentration
Where gravity concentration is applicable, its economics are compelling.
The operating cost of a gravity circuit is low. Modern gravity concentrators are robust, relatively simple to operate, and consume modest amounts of energy and water compared to leach circuits. There are no reagent costs — no cyanide, no lime, no activated carbon to manage and regenerate.
The capital cost of gravity equipment is also modest relative to the leach circuit infrastructure — tanks, carbon management systems, elution and electrowinning circuits, and the associated reagent handling and storage facilities that a full CIL or CIP plant requires.
Critically, gravity circuits deliver fast gold recovery. Coarse gold is recovered early in the process, reducing the residence time of high-value material in the circuit and improving cash flow in the early years of operation — particularly important for projects that need to demonstrate performance to investors and lenders quickly.
For deposits with significant coarse free-milling gold, a well-designed gravity circuit can recover a substantial proportion of total gold at a fraction of the cost of the leach circuit that recovers the remainder.
When Cyanide Leaching Is the Right Answer
Cyanide leaching — typically in the form of Carbon-in-Leach (CIL) or Carbon-in-Pulp (CIP) circuits — becomes the primary or dominant recovery route when the ore’s characteristics make gravity concentration insufficient on its own.
Fine gold that cannot be efficiently captured by gravity equipment must be leached. Refractory gold locked in sulphide minerals requires either pre-treatment followed by leaching, or flotation to produce a concentrate for further processing. High head grade ores where maximum total recovery is critical to project economics typically require a leach circuit to capture every recoverable ounce.
Leaching is also more forgiving of variable gold grain sizes than gravity concentration. Where an ore contains a wide distribution of gold grain sizes — some coarse, some fine — a combined gravity-leach circuit captures the full range.
The trade-off is operating cost. Cyanide, lime, activated carbon, and the associated reagent management systems add meaningful cost per tonne processed. These costs must be weighed against the recovery benefit they deliver.
The Combined Circuit: Why Most African Gold Plants Use Both
The most common process configuration at African gold operations combines gravity concentration with CIL or CIP leaching — and for good reason.
A gravity circuit installed ahead of or within the leach circuit captures the coarse, free-milling gold quickly and cheaply, reducing the load on the more expensive leach circuit and improving overall economics. The leach circuit then recovers the fine gold and any gravity-untreatable gold that remains in the ore.
This combined approach delivers the best of both routes — the low operating cost and fast recovery of gravity concentration, and the comprehensive recovery of leaching — while managing the capital cost of each component appropriately.
The exact configuration of a combined circuit — how much gravity capacity, where it sits in the flowsheet, what leach retention time and carbon management approach is used — depends entirely on the ore’s characteristics as revealed by testwork.
How Testwork Informs the Decision
The process route decision should never be made by analogy. Assuming your ore behaves like a neighbouring deposit, or like a deposit in the same geological province, is one of the most common and costly mistakes in gold project development.
GRG testwork directly measures how much of your ore’s gold can be recovered by gravity concentration under defined conditions. The result — expressed as a percentage of total gold that is gravity-recoverable — is the primary input into the gravity circuit design decision. A high GRG value strongly supports gravity concentration. A low value suggests that leaching must carry the primary recovery burden.
EGRG testwork goes further, revealing whether additional gravity-recoverable gold is liberated through further comminution. For ores where the GRG result is moderate, EGRG testing can determine whether a more aggressive grinding approach justifies a larger gravity circuit.
Kinetic leach testwork determines how gold dissolves in cyanide solution under different conditions — grind size, retention time, reagent dosage. These results define the leach circuit parameters and feed directly into the operating cost model.
Comparing gravity and leach results side by side gives project developers the data to make a justified, defensible process route decision — one that stands up to technical review and informs a flowsheet that performs as designed.
Real-World Tradeoffs
Every process route decision involves tradeoffs, and understanding them honestly is part of good project development.
Gravity concentration has low operating cost but requires coarse, liberated gold to perform well. If your ore does not have it, adding gravity capacity adds capital cost without adding meaningful recovery.
Leaching recovers fine and locked gold that gravity cannot, but at higher operating cost and with the reagent management complexity that cyanide handling requires. For low-grade ores, the operating cost of a full leach circuit can be the difference between an economic and uneconomic project.
The combined circuit optimises both — but only if it is designed correctly from testwork data, not from assumed parameters. An oversized gravity circuit on a low-GRG ore wastes capital. An undersized leach circuit on a high-grade ore leaves gold behind.
Getting the balance right is what flowsheet development is for — and it starts with the testwork that tells you what your ore actually needs.
The choice between gravity recovery and cyanide leaching is not a philosophical one. It is a technical one, and your ore has the answer. The job of a testwork program is to find it — systematically, rigorously, and with results that are defensible to every stakeholder who will scrutinise your project.
At Peacocke & Simpson, we have been helping African gold projects make this decision correctly for over 40 years — through GRG and EGRG testwork, kinetic leaching programs, and integrated flowsheet development that translates testwork results into practical process designs.
Not sure which route is right for your ore? Our testwork team can help you find out.
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