Why flotation for copper

Copper sulphides such as chalcopyrite, bornite and chalcocite are too low-density to separate by gravity and too non-magnetic for magnetic methods, but their surfaces can be made water-repellent (hydrophobic) with a collector so they attach to air bubbles and float. Gangue minerals stay wetted and sink. Flotation is selective enough to separate copper minerals from pyrite and even, with the right reagents, to separate copper from molybdenum or lead-zinc. That selectivity is the core of the flowsheet.

The economic driver is the upgrade ratio. A typical porphyry copper ore grades only 0.5-1% Cu, far too low to smelt directly; freight and smelting of that diluted material would be ruinous. Flotation concentrates the copper roughly 20- to 50-fold into a 20-30% Cu product, so that only a small mass of high-value concentrate is shipped to the smelter while the bulk of the ore reports to tailings on site. Every percentage point of recovery lost is paid copper left in the tailing, which is why the flowsheet is engineered around recovery first and grade second, within smelter penalty limits.

The flowsheet, stage by stage

1. Crushing

Run-of-mine ore is crushed in two or three stages to a mill feed size, commonly below 10-15 mm. A jaw crusher handles primary reduction and a cone crusher takes secondary and tertiary duty. Feeders and screens, from the screening range, keep the circuit fed and closed.

2. Grinding and classification

Grinding liberates copper minerals from gangue, the single most important step for recovery. Typical flotation feed is 60-75% passing 75 micron, though finely disseminated ores need finer. A wet ball mill runs in closed circuit with a hydrocyclone cluster that returns coarse particles for regrinding and sends correctly sized pulp to flotation. Grind too coarse and copper stays locked; grind too fine and you waste power and generate slimes that hurt selectivity.

3. Flotation: rougher, scavenger, cleaner

The conditioned pulp enters the flotation circuit, the heart of the plant:

• Rougher: recovers the bulk of copper into a rough concentrate as fast as possible.
• Scavenger: treats rougher tailing to catch the last recoverable copper, improving overall recovery.
• Cleaner (often two or three stages): re-floats the rougher/scavenger concentrate to reject entrained gangue and lift grade to a saleable 20-30% Cu.

A copper flotation plant built from mechanical flotation cells provides the aeration and agitation each duty needs. Reagents, collector, frother, lime for pH control and depressants for pyrite, are dosed to the conditioning stage and tuned to the ore. See the full flotation equipment range for cell sizing.

Cell selection and arrangement matter as much as cell type. Roughers are usually larger cells run at higher pulp level to maximize froth recovery, while cleaners are smaller and run deeper to reject gangue. The reagent suite is dosed in stages: a primary collector such as a xanthate, sometimes supplemented by a dithiophosphate for selectivity, a frother to control bubble size and froth stability, lime to raise pH and depress pyrite, and where needed a specific depressant. A common error is over-dosing collector, which floats more pyrite and dilutes the concentrate; dosing to the rougher feed and staging additions through the bank gives cleaner separation at lower reagent cost. Froth depth, air rate and pulp level are the operator’s day-to-day levers for holding grade and recovery as feed changes.

4. Dewatering

The cleaner concentrate is thickened in a thickener to recover water, then filtered to a shippable cake, while flotation tailings are thickened and sent to storage with process water returned to the plant. Concentrate moisture is a real cost: every percent of water shipped is freight paid on water, so a filter that delivers a low-moisture cake earns its keep on long export hauls. On the tailings side, returning clarified water to the mill cuts freshwater make-up and the energy to pump it, which is significant given flotation circuits run at high water-to-solids ratios.

Typical performance figures

Design choices that drive results

• Grind size: set by liberation tests; the biggest lever on recovery and concentrate grade.
• Reagent scheme: collector and depressant selection determine how cleanly copper separates from pyrite.
• Circuit configuration: the number of cleaner stages trades grade against recovery; over-cleaning loses copper.
• Oxide content: oxidized copper does not float well and may need sulphidization or a leach route instead.
• By-products: gold, silver and molybdenum often report with copper and can add significant value if the flowsheet accounts for them.

Build the circuit around your ore

Two copper ores rarely respond identically, so the reagent scheme, grind and cell count must be set from testwork, not copied. Xinhai runs the ore tests, designs the flowsheet and delivers the complete copper processing plant under an EPC+M+O contract, so crushing, grinding, flotation and dewatering are balanced to your throughput and your mineralogy. For background on how flotation compares with other recovery routes, see our overview of recovery methods.

The first question: how does the gold occur?

Before comparing methods, characterize the ore. A few questions decide the flowsheet:

• Is the gold coarse and free-milling, or fine and disseminated?
• Is it associated with sulfides (pyrite, arsenopyrite) or free in quartz?
• Is there carbonaceous material that would re-adsorb dissolved gold?
• What is the head grade, and what recovery does the economics demand?

A gravity-recoverable gold (GRG) test and a mineralogical scan answer most of this and point to the right combination.

Gravity concentration

Gravity exploits the high density of gold (about 19 g/cm3) versus gangue (2.6-3 g/cm3). It is the cheapest method to run, uses no reagents, and recovers coarse free gold early so it does not get lost or over-ground. Centrifugal concentrators capture fine free gold down to tens of microns, while shaking tables and spiral chutes handle coarser fractions and produce a saleable concentrate. The limitation is that gravity cannot recover gold that is locked inside other minerals or too fine to settle.

Typical placement: a centrifugal concentrator in the grinding circuit to scalp free gold, with a shaking table to upgrade the gravity concentrate. See the full gravity concentration range for spirals and jigs.

Flotation

When gold is fine and associated with sulfides, flotation collects the gold-bearing sulfides into a concentrate that is a small fraction of the original mass. This is ideal for refractory or sulfide-hosted gold: instead of leaching the whole orebody, you leach (or smelt) only the concentrate, cutting reagent and energy cost dramatically. Flotation needs reagents and good liberation, and works best on sulfide minerals rather than free gold in oxide ore.

Cyanide leaching

Leaching dissolves gold chemically with dilute cyanide and recovers it onto activated carbon (CIL/CIP) or by zinc precipitation. It reaches the highest recoveries, 90-96% on amenable ores, and handles fine, disseminated gold that gravity and flotation miss. The trade-offs are reagent cost, residence time and the need for careful cyanide management. Leaching is applied either to the whole milled ore or, more economically, only to a flotation or gravity concentrate. See gold extraction equipment for leach tanks and the gold room.

Side-by-side comparison

Reagents and control in flotation

Flotation performance hinges on reagent selection and grind size. Collectors such as xanthates render the sulfide surfaces hydrophobic so they attach to air bubbles; frothers stabilize the bubble film; and modifiers (lime, copper sulfate, depressants) control pH and selectivity. Grind size must liberate the gold-bearing sulfides without overgrinding into slimes that float poorly. A typical sulfide gold flotation runs at a P80 around 75 microns, with the optimum confirmed by bench testwork. Because flotation rejects most of the gangue, the resulting concentrate is often 10-30 times higher grade than the feed, which is exactly what makes downstream leaching cheap.

Why most plants combine all three

A well-designed gold plant rarely relies on one method. A common high-recovery flowsheet runs gravity inside the grinding circuit to pull out coarse free gold first (which is hard to leach and easy to lose), then sends the rest to flotation or directly to leaching depending on mineralogy. For sulfide ores, gravity plus flotation concentrates the gold into a small mass that is then leached. This staged approach maximizes overall recovery while keeping reagent consumption proportional to the gold-bearing mass, not the whole orebody.

A typical free-milling flowsheet

1. Crush and grind to liberation size (often 75-106 microns).
2. Gravity scalp coarse free gold with a centrifugal concentrator.
3. Leach the gravity tailings by CIL/CIP.
4. Recover gold from loaded carbon by elution and electrowinning.

A typical refractory/sulfide flowsheet

1. Crush, grind and run gravity for any free gold.
2. Float the gold-bearing sulfides into a concentrate.
3. Treat the concentrate (leach, or oxidize then leach).

Reading recovery numbers correctly

Be careful comparing the recovery figures for each method, because they measure different things. Gravity recovery is quoted as a share of total contained gold and is inherently limited to the coarse free fraction, so 20-60% is normal and not a weakness – it simply reflects how much of the gold is recoverable by density. Flotation recovery is quoted to concentrate, where 85-95% is typical, but that concentrate still has to be treated to produce metal. Leaching recovery is the closest to a true overall figure. The number that ultimately matters is plant recovery across the whole flowsheet, which a well-staged combination maximizes by sending each gold form to the method best suited to it.

Cyanide management is part of the design

Any cyanidation circuit must address safety and environment from the outset: pH is held around 10.5-11 with lime to keep cyanide stable and avoid hydrogen cyanide release, and tailings are detoxified before discharge. These requirements influence reagent cost and permitting, and are a reason flotation pre-concentration is attractive – leaching a small high-grade concentrate uses far less cyanide than treating the whole orebody.

Matching method to ore is the whole game

Choosing gold recovery methods is really about reading the ore correctly. Coarse free gold to gravity, sulfide-locked gold to flotation, fine and disseminated gold to leaching, and almost always a combination. Xinhai runs the gravity, flotation and leach testwork in-house, then designs the integrated flowsheet and supplies the equipment under one EPC+M+O contract. For the CIL vs CIP vs heap leach decision specifically, see our gold processing plant comparison.

What you are trying to achieve

The target is a chemical-grade concentrate, conventionally 6% Li2O (5.5-6.5% range), with iron kept low because Fe2O3 is a penalty in converter feed. Run-of-mine spodumene ore commonly assays 0.8-1.5% Li2O, so the plant must reject a large mass of gangue, mainly quartz, feldspar and mica, while not losing the relatively heavy, brittle spodumene grains. Two properties make this possible: spodumene is denser (SG ~3.1-3.2) than the silicate gangue (SG ~2.6), and its surface can be selectively floated after careful conditioning.

The mass balance is unforgiving. Upgrading a 1% Li2O ore to a 6% concentrate means the concentrate is only about a sixth of the feed mass at best, so a large tonnage of gangue must be rejected cleanly without dragging spodumene to tailings. Two complications make spodumene harder than a textbook density or flotation separation. First, spodumene weathers to less recoverable forms near surface, so feed mineralogy varies with depth. Second, the surface chemistry of spodumene and the feldspar it must be separated from is similar, so flotation demands tight control of pH, conditioning and reagent dosing. These realities are why testwork, not a generic flowsheet, drives the design.

The flowsheet, stage by stage

1. Crushing

Three-stage crushing typically reduces ore to below 10-12 mm. A jaw crusher takes the primary duty and a cone crusher handles secondary and tertiary reduction. Spodumene is brittle, so crushing is kept controlled to avoid over-generating fines that are harder to treat by density.

2. Grinding and classification

Ore is ground to liberate spodumene from gangue, usually to a flotation feed around 65-80% passing 150-200 micron. A wet ball mill in closed circuit with a hydrocyclone or classifier controls the product size. Over-grinding is avoided because ultrafine spodumene floats and separates poorly.

3. Dense-media separation (optional, for coarse feed)

Where the ore liberates coarse, dense-media separation upgrades the coarse fraction cheaply by exploiting the SG difference, rejecting a large mass of light gangue before grinding and shrinking the flotation plant. DMS commonly produces a coarse 4-6% Li2O pre-concentrate and is a major cost lever when liberation allows it.

4. Desliming, mica and iron removal

Fines (slimes) are removed before flotation because they consume reagents and depress selectivity. Mica is floated off or removed ahead of spodumene flotation, and magnetic separation pulls iron-bearing minerals to protect concentrate grade. A wet drum magnetic separator is the standard tool for iron removal; see the full magnetic separation range.

5. Spodumene flotation

The deslimed pulp is conditioned, typically at elevated pH with a fatty-acid collector after surface activation, and spodumene is floated away from quartz and feldspar in a bank of cells. A mechanical flotation machine rougher, scavenger and cleaner train produces the final 6% Li2O concentrate. Flotation is essential for the fine fraction that DMS cannot treat. Explore the flotation equipment options for circuit sizing.

Conditioning is the make-or-break step. Spodumene surfaces are activated, often with a cation such as calcium and at high pH, before a fatty-acid or hydroxamate collector is added, so that spodumene floats while feldspar and quartz stay depressed. Reagent dosage, conditioning time and water quality all shift the selectivity, and small changes can swing the grade-recovery balance noticeably. Because the separation is this sensitive, the rougher concentrate is almost always cleaned in two or three stages, with cleaner tailings recirculated, to lift grade to specification without throwing away recoverable lithium.

6. Dewatering

The concentrate is thickened and filtered to a shippable moisture. A thickener recovers process water and a filter produces cake, while tailings are dewatered for storage. Water recovery matters because many lithium projects sit in arid regions.

DMS vs flotation: where each fits

Most modern hard-rock plants combine the two: DMS handles the coarse, well-liberated fraction at low cost and flotation recovers the fines, giving the best overall recovery. The right split depends entirely on liberation, which is why a metallurgical test program comes first. As a rule of thumb, the coarser and better-liberated the spodumene, the more of the upgrade work DMS can do cheaply, shrinking the flotation plant; finely intergrown ores push more mass into flotation and raise reagent cost. A heavy-liquid separation test on sized fractions quickly shows how much DMS can achieve before any flotation testing begins.

Typical performance and the role of testing

• Feed grade: 0.8-1.5% Li2O run-of-mine.
• Concentrate grade: 5.5-6.5% Li2O, with Fe2O3 controlled below the converter penalty.
• Overall recovery: typically 65-85%, depending on fines content and liberation.
• Key losses: ultrafine spodumene to slimes and unliberated middlings.

Because reagent scheme, grind size and the DMS/flotation split are all ore-specific, Xinhai begins every lithium project with bench and pilot testing, then designs the complete spodumene processing plant under an EPC+M+O contract so comminution, separation and dewatering are balanced to your deposit rather than assembled from generic units.