Sulphide flotation before leaching

Sulphide flotation before leaching is the specific flowsheet configuration that puts the flotation mechanism described on flotation of gold tailings to practical use, and it is the direct answer to a question that page leaves open: once a sulphide concentrate exists, what actually happens to it, and why bother concentrating at all instead of leaching the whole feed directly. This is the configuration a technical reviewer should expect to see specified whenever a feasibility document mentions flotation for a sulphide-bearing Ghanaian tailings dump, since flotation on its own is never the final recovery step.

Why leach only the flotation concentrate rather than the whole feed?

Leaching only the flotation concentrate rather than the whole feed makes sense because a well-run flotation stage reduces a large tonnage of tailings down to a small, sulphide-rich concentrate representing a fraction of the original mass, while carrying the great majority of the sulphide-associated gold along with it. Sizing a leach circuit, tanks, reagent dosing systems, carbon inventory, for that reduced concentrate stream rather than the full plant throughput is dramatically cheaper in both capital and ongoing reagent cost, since every tank and every kilogram of cyanide only has to handle the smaller, upgraded volume rather than everything the plant processes.

This is the same principle already established for gravity concentrate cleaning on shaking tables for fine gold, applied here to a chemically rather than physically concentrated stream: process the bulk of the tonnage cheaply first, then apply the more expensive, more precise treatment only to the smaller fraction that actually needs it.

What does this do to overall cyanide consumption?

Concentrating sulphides via flotation before leaching cuts overall cyanide consumption meaningfully because base-metal sulphides, pyrite and arsenopyrite among them, consume cyanide through their own side reactions without contributing any gold to the leach solution in return, exactly as described on cyanidation basics: chemistry of gold leaching. Leaching the whole tailings tonnage directly on a sulphide-rich dump means paying that reagent cost across the entire feed mass. Floating the sulphides out first and leaching only the resulting concentrate confines that reagent-hungry chemistry to a small fraction of the total tonnage, which is where the flowsheet's real cost advantage comes from.

The flotation tailings, the much larger non-floated fraction left behind, typically carries a far lower sulphide content than the original feed, which means whatever residual leaching that stream still needs consumes proportionally less cyanide per tonne than the concentrate does. The net effect across the whole plant is a lower total reagent bill than a single, undifferentiated leach of the entire tonnage would produce, even though the concentrate itself is leached at a more reagent-intensive rate per tonne of concentrate. This is the specific mechanism by which flotation earns back its own capital and operating cost on sulphide-rich feed, not a general assumption that flotation always saves money regardless of mineralogy.

What happens to the concentrate between flotation and the leach tanks?

A flotation concentrate leaves the cells as a dilute slurry, since froth itself carries a large amount of water relative to solids, and it typically passes through a dedicated thickener to raise its solids content before entering the leach circuit, both because leach tanks are designed around a specific pulp density and because thickening reduces the volume of material the leach circuit's tanks and reagent dosing systems actually have to handle. A regrind stage often sits at this point too, since flotation feed is usually ground only fine enough to liberate the sulphide from gangue, a coarser target than the finer grind that maximizes gold exposure within the concentrate for the leach stage that follows.

This thickening and regrind sequence between flotation and leaching is not an optional convenience; skipping it and feeding a dilute, coarse concentrate directly to the leach tanks would undersize the leach circuit's effective residence time and likely leave gold locked inside particles too coarse for the leach reaction to reach efficiently, undermining much of the benefit the flotation stage was built to deliver. Sizing this intermediate thickening and regrind step correctly is therefore as much a part of a sound flotation-then-leach design as the flotation cells and the leach tanks themselves.

What happens to the flotation tailings, the non-floated fraction?

Flotation tailings, the fraction that does not float and therefore was not concentrated, usually carry low residual sulphide content and correspondingly modest cyanide demand, and depending on their assayed gold content, they may still warrant a lighter leaching pass of their own or, in genuinely low-grade cases, may bypass further chemical treatment entirely and proceed straight to final tailings disposal. Which of these applies to a specific project's flotation tailings stream is a metallurgical accounting question, comparing the assayed gold value remaining in that stream against the reagent cost of recovering it, rather than a default assumption in either direction.

Assaying the flotation tailings stream separately from the flotation concentrate, rather than only tracking a single combined plant recovery figure, is what actually lets a project make this call with real data. A stream that repeatedly assays near the project's cut-off grade is a candidate for closer attention, either a process adjustment upstream to capture more of its value or a formal decision that its remaining gold content does not justify further treatment cost.

When is this configuration worth the extra flotation capital?

Adding a flotation stage ahead of leaching is worth its extra capital and reagent cost specifically where testwork shows meaningful sulphide or preg-robbing content in the feed, the same conditions already identified on the flotation mechanism page as favoring flotation generally. On low-sulphide, already-oxidized tailings, the leach circuit alone handles the feed efficiently without flotation's added complexity, and inserting an unnecessary flotation stage would add cost without a corresponding recovery or reagent-saving benefit large enough to justify it.

The decision is made from a direct cost comparison: the capital and operating cost of the flotation stage itself, weighed against the cyanide savings and any recovery uplift it delivers on the specific feed. A feasibility study that proposes this configuration should show that comparison explicitly, using real diagnostic testwork data from the dump in question, rather than asserting flotation-then-leach as a default best practice that applies everywhere.

A useful rule of thumb for framing this decision, though never a substitute for the actual testwork, is that the cyanide cost saved scales with how much of the total tonnage's cyanide consumption the sulphide fraction is actually responsible for. A dump where sulphides make up a small share of total mass but a disproportionate share of cyanide consumption is exactly the profile where flotation's cost typically pays for itself fastest, while a dump with sulphides spread thinly and evenly through material that was already going to leach efficiently gains comparatively little from the added flotation step.