The Question Worth Asking First
Most small operators arrive assuming they need a leach circuit. Often they do not. Gravity recovery uses no chemicals, needs no permit for reagent handling, costs a fraction of a leach plant to build and run, and on genuinely free-milling ore it recovers 85 to 98 percent of the liberated gold.
The honest framing is not gravity versus cyanide as competing philosophies. It is a sequence. Gravity first, always, because it is cheap and it removes coarse gold before anything else touches it. Then cyanide, only if the ore has gold that gravity physically cannot reach and the economics justify the step up.
How Each One Works
Gravity
Gravity separation exploits the density difference between gold at about 19 grams per cubic centimetre and gangue minerals at around 2.7. A centrifugal concentrator spins the slurry at 60 to 200 G so dense particles are held in the bowl while light material overflows. A shaking table then cleans that concentrate to a directly smeltable product. No reagents, water only.
The limit is liberation and particle size. Gravity cannot recover gold that is still locked inside a sulphide or silicate grain, because the composite particle does not behave as gold. It also loses effectiveness below roughly 20 microns, where particles start behaving hydraulically rather than by density.
Cyanide leaching
Cyanide dissolves gold chemically into solution, so liberation matters far less and particle size matters much less. Gold too fine for gravity, and gold finely disseminated through the ore, both become recoverable. That chemistry is why leaching reaches 88 to 95 percent where gravity alone might reach 50.
What it costs is a different order of operation: reagent supply chain, storage and containment, pH control, gas monitoring, trained operators, tailings management, permitting and an emergency response capability.
Which Applies to Your Ore
- Alluvial and eluvial gold. Gravity, essentially always. The gold is already liberated and often coarse. A leach circuit here is capital spent for nothing
- Free-milling hard rock with coarse gold. Gravity recovers the bulk of it. Whether you add leaching depends on how much gold remains in the tailings and whether that residual pays for the circuit
- Fine gold, well disseminated. Gravity underperforms. Leaching is where the recovery is
- Sulphide-associated gold. Gravity will not reach it. Leaching, and possibly flotation or oxidation ahead of it
- Refractory ore. Neither route works without pre-treatment. This needs diagnostic test work and specialist advice before any equipment is bought
The Mercury Question
A great deal of small-scale gold in Tanzania is still recovered by mercury amalgamation. It should not be, and gravity is the reason it does not have to be.
A centrifugal concentrator followed by a shaking table recovers a comparable or higher proportion of free gold than amalgamation, produces a concentrate clean enough to smelt directly, and eliminates the mercury entirely. That removes the health exposure to operators and their families, removes the contamination of watercourses, and removes the gold that amalgamation loses to the tailings along with the mercury.
Tanzania is a party to the Minamata Convention on Mercury, and the direction of regulation is one way. Beyond the health and legal case, the commercial case usually stands on its own: mercury loses gold, and a gravity circuit recovers gold that amalgamation leaves behind.
Cost and Practicality Compared
Capital
A gravity circuit of crusher, mill, concentrator and table is a fraction of the cost of the same front end followed by a leach and elution plant. For a small operation the difference is frequently the difference between a project that can be funded and one that cannot.
Operating
Gravity consumes power, wear parts and water. Leaching adds cyanide, lime, activated carbon, elution fuel and a laboratory, plus the labour and compliance overhead of running a cyanide facility safely.
Regulatory
A gravity plant is a mechanical installation. A cyanide plant is a permitted chemical facility with monitoring and reporting obligations attached. Many small operators underestimate how much management attention that second category requires, and it is a common cause of plants running below capacity or being shut down.
Speed
A modular gravity plant can be running within weeks of arriving. A leach circuit needs commissioning of the reagent, elution and safety systems as well, and takes longer.
The Sensible Sequence
- Test the ore. Gravity recoverable gold, bottle roll leach, and grind size sensitivity on both
- Install gravity first. It is cheap, fast, and it recovers coarse gold that would otherwise report to a leach circuit where coarse gold behaves badly anyway
- Assay the gravity tailings. This tells you exactly what leaching would add, in grams per tonne, on your actual ore
- Do the arithmetic. Value the residual gold at realistic recovery against the full cost of a leach circuit, including compliance and labour
- Add leaching only if that arithmetic works. On many small Tanzanian operations it does not, and gravity alone is the right and profitable answer
Regions We Serve
Frequently Asked Questions
Can gravity recovery replace mercury completely?
Yes, for free gold. A centrifugal concentrator followed by a shaking table produces a concentrate clean enough to smelt directly, with no amalgamation step at any point. It typically recovers more gold than mercury does, because amalgamation loses fine gold to the tailings.
What percentage of gold can gravity recover?
85 to 98 percent of the gold that is liberated and coarser than about 20 microns. The critical word is liberated. If half the gold in your ore is locked in sulphides, gravity recovers 85 to 98 percent of the other half, and the headline recovery figure will look poor through no fault of the machine.
Is cyanide legal for small-scale mining in Tanzania?
Cyanide use is permitted subject to environmental regulation covering storage, containment, monitoring and emergency response. The obligations are real and are enforced. Obtain current advice on what applies to your licence type before procuring a leach circuit.
Should I run gravity ahead of a CIL plant?
Almost always yes. A gravity concentrator scalping the grinding circuit recovers coarse gold early, which improves overall recovery, reduces the gold inventory locked up in the leach circuit, and produces a high-grade concentrate you can smelt directly. It is standard practice on well-designed plants rather than an optional extra.