MINERAL PROCESSING KNOWLEDGE · COPPER ORE PROCESSING
Pyrite is the most common diluent in copper flotation, and suppressing it is a question of pulp chemistry rather than of cell design.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Copper sulfide ores are rarely single-mineral ores. Chalcopyrite (CuFeS₂) usually arrives with pyrite (FeS₂) in quantities large enough to dominate the concentrate unless the pyrite is deliberately suppressed. Because both are sulfides, they respond to the same collector, and the selectivity that separates them has to come from the chemistry of the pulp rather than from the flotation machine.
Technical note: The pH values and reagent combinations below are principles only. They are not engineering designs or performance guarantees. Depression schemes must be established by testwork on representative ore samples and reviewed by qualified metallurgists.
1. Why pyrite floats with the copper
The main sulfide copper minerals, chalcopyrite (CuFeS₂), chalcocite (Cu₂S) and bornite (Cu₅FeS₄), have good natural floatability. A typical circuit uses xanthate collectors such as butyl xanthate or ethyl thionocarbamate with pine oil (2# oil) as frother, at a mildly alkaline pulp pH of 8–10, and can produce a copper concentrate grading 20%–30% Cu.
Xanthate is not selective on its own. Pyrite is a sulfide as well and responds to the same collector chemistry that recovers the copper minerals. Where pyrite is abundant, the amount that reports to the froth can dominate the mass balance of the concentrate and drag the copper grade down while copper recovery still looks acceptable. Grade and recovery pull in opposite directions, and the lever that resolves them is the surface chemistry of pyrite.
2. High-pH lime suppression
The most direct strategy is to raise the pH with lime. Large quantities of lime (CaO) are added to the pulp to bring it to pH 10.5–12.0.
The effect is an oxidation and precipitation reaction at the pyrite surface. At high pH the concentrated OH⁻ in solution reacts with oxygen from the air, and the pyrite surface develops a hydrophilic film of ferric hydroxide, Fe(OH)₃, together with calcium sulphate precipitate. Both are water-wetting, and the film hinders the chemical adsorption of xanthate on pyrite. Pyrite therefore stays hydrophilic and does not attach to bubbles, while the copper minerals continue to float.
Lime suppresses pyrite by keeping it wet; it does not remove pyrite from the pulp. The pyrite still reports to the tailings with the gangue, and its behaviour in thickening and tailings handling should be considered alongside the flotation result.
3. The cost of running at high pH
Lime is a blunt instrument. It does not distinguish between pyrite and copper sulfides. When the pulp is held at the upper end of the range, copper minerals such as chalcocite can be suppressed as well, and the loss appears as reduced copper recovery and copper reporting to the tailings, which costs more than a slightly lower concentrate grade.
High pH also carries operational consequences. Lime addition must be stable, pH must be controlled at the points where it matters, and the pulp chemistry must be consistent enough that the circuit does not swing between suppressing pyrite and suppressing copper. The set point is therefore a compromise, and for high-sulfur ores where that compromise is poor the answer is usually not more lime but a different combination of reagents.
4. Combined depressant schemes
For high-sulfur and difficult copper ores, the lime dosage is reduced and the pulp is held at pH 9–10 instead. Selectivity is then carried by a combination of depressants rather than by pH alone.
Two combinations are used. One is sodium sulphite (Na₂SO₃) together with zinc sulphate (ZnSO₄). The other is potassium ferrocyanide, known as yellow prussiate. Both work by selectively desorbing collector from the pyrite surface: pyrite that has already been coated with collector becomes hydrophilic again and stops floating, while the copper minerals keep their coating and their floatability.
| Approach | Pulp pH | Key reagents | Where it fits |
|---|---|---|---|
| High-pH lime suppression | 10.5–12.0 | Lime (CaO) | Circuits where pyrite is abundant and the copper minerals tolerate the alkalinity |
| Sulphite and zinc sulphate | 9–10 | Sodium sulphite (Na₂SO₃) with zinc sulphate (ZnSO₄) | High-sulfur difficult ores where high pH suppresses copper minerals as well |
| Ferrocyanide addition | 9–10 | Potassium ferrocyanide (yellow prussiate) | Alternative selective desorption of collector from the pyrite surface |
5. What to verify before fixing a reagent scheme
A depression scheme that looks convincing on paper behaves differently on a real pulp. The following should be established first:
- The copper mineral assemblage and the pyrite content, because both decide how much pH the circuit can afford.
- How copper grade and copper recovery respond as pH is stepped through the range, rather than at a single set point.
- The collector scheme, because the suppression achieved by pH depends on which collector is present on the pyrite surface.
- Mass pull and concentrate grade at each condition, so that grade is not bought with unreported copper losses.
- Locked-cycle behaviour, because circulating streams change the reagent demand of the circuit as a whole.
On a running plant the equivalents are pH at the conditioning stage and at each flotation bank, reagent addition stability, froth appearance, and assays of concentrate and tailings. A scheme that is working shows a stable concentrate grade with copper losses held down, not merely a lower pyrite assay in a single sample.
Conclusion
Pyrite depression is a pulp chemistry problem. Lime suppression at pH 10.5–12.0 builds a hydrophilic ferric hydroxide and calcium sulphate film that keeps xanthate off the pyrite surface, but it is blunt and can suppress copper minerals such as chalcocite along with the pyrite. Where the ore is high in sulfur and that suppression costs more copper than it saves, reducing lime to pH 9–10 and adding sodium sulphite with zinc sulphate, or potassium ferrocyanide, desorbs collector selectively from pyrite. The right combination is specific to the ore and should follow from testwork, not from a fixed recipe.
How LiCheng Tech can help
LiCheng Tech supplies the equipment that makes a depression strategy practical: conditioning and reagent mixing tanks where pH is established before flotation, flotation machines for roughing, scavenging and cleaning, and the thickening and filtration stages that follow. Share your copper assay data, mineralogy and concentrate specification, and we can discuss how the reagent scheme you are considering affects the duty of each stage and prepare an equipment proposal for the circuit you intend to run.
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