Ultrafine gold: the limits of recovery

This page continues directly from fine gold recovery: the sub-75-micron problem, pushing the same question further down the size scale to where even the specialized equipment and chemistry described there begin to genuinely struggle. Being honest about where that limit actually sits, rather than implying every gram of contained gold is recoverable with enough investment, is what separates a credible technical resource from a promotional one.

Why does even leaching eventually struggle with ultrafine gold?

Leaching depends on cyanide solution physically reaching the gold surface and dissolved gold-cyanide complex then diffusing away from that surface back into the bulk solution, and both of those steps become harder as particle size drops toward the ultrafine range. Extremely fine gold particles have a very high surface-area-to-volume ratio, which in principle should favor fast dissolution, but in practice ultrafine particles are also more prone to surface passivation, a thin coating or oxidation layer that can form on the particle surface and physically block reagent access, and they are more likely to be entrained in fine clay or slime material that itself interferes with reagent contact. Colloidal gold, at the very bottom of this size range, can behave almost like a dissolved species suspended in the pulp rather than a discrete solid particle, complicating both its physical behavior and its assay.

The chemistry described on cyanidation basics: chemistry of gold leaching does not fail outright at these sizes, but the practical residence time and reagent concentration needed to push extraction meaningfully higher on the ultrafine fraction rises sharply, often well past the point where the marginal gold recovered is worth the marginal reagent and time cost.

What does "uneconomic" actually mean here, precisely?

Uneconomic means the cost of achieving additional recovery on the ultrafine fraction, more residence time, more reagent, finer grinding, exceeds the value of the extra gold that additional effort would recover, a testwork and economics threshold rather than a fixed physical wall that no technology could ever cross. This distinction matters because it means the practical recovery ceiling on a given dump's ultrafine fraction can shift over time as reagent costs, gold price, and technology all move, but it also means no responsible technical assessment should promise 100 percent recovery on any real tailings feed, since some genuinely uneconomic-to-recover fraction is the normal, expected outcome rather than a sign of poor plant design.

Where exactly do gravity and flotation hit their own ultrafine ceilings?

Gravity concentration, even the centrifugal, force-multiplied equipment covered elsewhere in this recovery methods set, loses effectiveness once particle size drops far enough that drag and turbulence overwhelm even an amplified density advantage, a physical relationship rooted in Stokes' law that applies regardless of how much centrifugal force a bowl generates. Beyond a certain point, adding more G-force to a centrifugal unit yields diminishing returns on the very finest particles while simultaneously dragging more fine gangue into the concentrate and lowering its grade, which is why gravity is never presented on this site as a complete answer to the fine end of the gold size distribution on its own.

Flotation faces a related but distinct ceiling. Very fine particles carry too little mass relative to their surface area to attach to a rising air bubble with enough force to stay attached through to the froth layer, and they are also more prone to entrainment, simply following water into the froth regardless of surface chemistry, which degrades concentrate selectivity rather than improving recovery. Both gravity and flotation, in other words, hand off responsibility for the finest fraction to leaching, which is itself the last method in the chain and the one this page's central argument concerns.

Does this mean some gold in every dump is permanently unrecoverable?

Practically, yes, a residual fraction of contained gold remains in the final tailings stream of essentially every real-world retreatment operation, and this is normal rather than a project failure. The benchmark that actually matters is not zero residual gold, an unrealistic standard, but a recovery rate consistent with, or better than, the honestly reported benchmarks for comparable methods and ore types, the kind of figure covered on the recovery rate benchmarking work elsewhere in this site's metallurgy and testwork section.

This residual fraction is, in a sense, the modern-day equivalent of what makes the site's own central topic possible in the first place: today's legacy Ghanaian tailings exist as a retreatment opportunity precisely because earlier generations of gravity-only and amalgamation-era equipment left a coarser, larger recoverable fraction behind than modern gravity, flotation and leaching technology can now capture. A well-run modern retreatment operation leaves behind a much smaller, much finer residual fraction than its predecessors did, but the pattern of some gold remaining unrecovered, at whatever the era's technological limit happens to be, has effectively repeated across generations of mining technology.

Framing residual ultrafine gold this way also has a second, practical implication worth stating plainly: because this residual fraction is a known, expected feature of tailings retreatment rather than a processing failure, a project's own retreated tailings, once redeposited, are not typically an attractive target for a third round of retreatment using the same methods, since whatever remains is disproportionately the genuinely uneconomic ultrafine fraction this page describes.

How should investors read a recovery percentage that isn't 100%?

A credible feasibility study states its assumed recovery ceiling explicitly and explains why, tying that figure back to GRG testwork, particle size and liberation data, and diagnostic leach results specific to the dump in question, rather than presenting a single headline recovery number without its supporting technical basis. An investor should treat a recovery claim that reads as suspiciously close to 100 percent, or one presented without supporting testwork, as a reason for closer scrutiny rather than reassurance, since the honest technical answer for real tailings feed always leaves some residual fraction unaccounted for.

A useful practical question for any due diligence conversation is simply to ask what specific size fraction and mineralogical form account for the gap between total contained gold and the recovery figure a project is presenting. A technical team that can answer that question specifically, pointing to named testwork results, is demonstrating real command of its own metallurgy; a team that cannot is worth pressing further before treating its recovery assumptions as reliable.

Is there active technology narrowing this limit further?

Research into fine-particle flotation reagents, novel lixiviant chemistry, and improved fine-particle gravity and sensor-based sorting technology continues to push the practical recovery boundary further down the size scale than it sat a generation ago, and that trend is likely to continue. None of this justifies overclaiming what is achievable today on a specific project, however; any technology still working through pilot or early commercial validation should be treated as exactly that, a genuine but unproven prospect, rather than folded into a current project's recovery assumptions before it has a real, tested track record on comparable feed.

The emerging lixiviant chemistry mentioned here specifically, including glycine-based and other alternative leaching approaches, is covered in its own right elsewhere in this recovery methods set, with the same honest framing about commercial maturity applied there as here: worth tracking, not yet a basis for today's project economics.