Efficient separation of chalcopyrite and pentlandite is a core technical challenge in beneficiation of copper‑nickel sulphide ores. Sodium persulfate (Na₂S₂O₈), as a strong oxidising and environmentally friendly depressant, achieves selective depression of pentlandite through pulp potential control and selective surface oxidation, while preserving the natural floatability of chalcopyrite to the greatest extent. This enables low‑alkalinity, non‑toxic copper‑nickel flotation separation. This article systematically discusses the surface physicochemical properties of the minerals, depression mechanisms, influencing factors, and industrial applications, providing a sound theoretical and practical basis for the industrial use of sodium persulfate in copper‑nickel sulphide flotation separation.
I. Introduction
Chalcopyrite (CuFeS₂) and pentlandite ((Fe,Ni)₉S₈) are the most important coexisting valuable minerals in copper‑nickel sulphide deposits. They have similar crystal structures, closely comparable natural floatability, and fine intergrowth textures, making their separation a typical challenge.
Traditional copper‑nickel separation processes often use cyanide to depress nickel while floating copper, or lime high‑alkalinity depression. Cyanide is highly toxic, subject to strict environmental regulations, and incurs high wastewater treatment costs. The high‑lime system tends to cause excessive surface oxidation and passivation of chalcopyrite, resulting in copper recovery losses, along with slime coagulation, pipe scaling, and subsequent suppression of nickel mineral recovery.
Sodium persulfate, a persulphate‑type strong oxidant, has good water solubility, non‑toxic decomposition products, and excellent environmental compatibility. By precisely adjusting the pulp redox potential, it can selectively oxidise and hydrophilically depress pentlandite while having minimal impact on chalcopyrite floatability. It is an ideal green alternative to toxic cyanide and high‑alkalinity lime processes.
II. Surface Characteristics of Chalcopyrite and Pentlandite and Separation Difficulties
2.1 Crystal structure and surface electrochemical properties
Chalcopyrite has a tetragonal sulphide structure, with surface active sites mainly of Cu and Fe. In weakly alkaline to neutral pulps, its surface oxidation rate is low, and it readily forms stable hydrophobic metal xanthate complexes with xanthate collectors, giving it good natural floatability.
Pentlandite has an isometric structure with Fe and Ni ions undergoing isomorphous substitution in the lattice. Its surface oxidation activity is much higher than that of chalcopyrite, with a stronger tendency for spontaneous oxidation. The rest potentials and oxidation decomposition potentials of the two minerals differ significantly: pentlandite is more readily oxidised by moderate oxidants, while chalcopyrite is electrochemically more stable. This is the fundamental electrochemical basis for the selective depression achievable with sodium persulfate.
2.2 Core challenges in conventional flotation separation
- Their natural floatabilities are close, so under a single collector they tend to float together, causing high mutual contamination in copper and nickel concentrates.
- Pentlandite is highly sensitive to oxidation, and under conventional oxidising conditions it is easy to inadvertently affect chalcopyrite, making selective control difficult.
- Traditional depressants are either highly toxic or cause negative impacts under high‑alkalinity conditions, making it hard to simultaneously meet separation targets, environmental requirements, and production costs.
- When the ore contains associated pyrrhotite, pyrite and other impurity minerals, the pulp electrochemical environment becomes more complex, further increasing separation difficulty.
III. Mechanism of Sodium Persulfate in Depressing Pentlandite
Sodium persulfate dissociates in pulp to yield S₂O₈²⁻, which has strong oxidising ability and slowly decomposes to produce sulfate radicals. It achieves selective depression of pentlandite through three dimensions: selective oxidation, formation of a hydrophilic surface film, and blocking of collector adsorption.
3.1 Selective oxidation to differentially modify mineral surfaces
In a weakly alkaline low‑alkalinity pulp system, S₂O₈²⁻ preferentially oxidises the low‑valent Fe and Ni ions on the pentlandite surface, converting them to high‑valent oxides and hydroxides. The pentlandite surface becomes covered with hydrophilic species such as Fe(OH)₃ and Ni(OH)₂, which mask the active sites. Chalcopyrite, being electrochemically more stable, undergoes no significant oxidative phase transformation at its surface Cu and Fe sites under reasonable dosage and potential ranges, so its hydrophobic structure remains intact and its floatability is essentially unaffected.
3.2 Formation of a stable hydrophilic hydration film for depression
The oxidation products form a dense, stable inorganic hydrophilic film on the pentlandite surface. This film binds water molecules via hydrogen bonding to create a persistent hydration layer, greatly reducing surface hydrophobicity. As a result, pentlandite cannot attach to bubbles, loses its floatability, and is retained in the tailings. Simultaneously, the SO₄²⁻ generated from decomposition can react with metal ions leached from the pentlandite surface to form hydrophilic sulphate precipitates, further reinforcing the hydrophilic layer and making the depression stable and irreversible.
3.3 Blocking collector adsorption on the mineral surface
Xanthate collectors rely on chelation with exposed metal active sites on mineral surfaces to form hydrophobic xanthate adsorption layers. After oxidation and film formation by sodium persulfate, the active sites on pentlandite are completely covered by hydrophilic oxides, preventing effective collector adsorption. In contrast, the chalcopyrite surface remains intact and can normally adsorb xanthate, maintaining good floatability. Thus, selective separation of copper (floating) and nickel (depressed) is achieved.
IV. Key Factors Affecting Flotation Separation of Chalcopyrite from Pentlandite Using Sodium Persulfate
4.1 Pulp pH
pH determines the decomposition rate of persulfate, its oxidising strength, and the direction of surface reactions on both minerals:
- Weakly alkaline pH 7.0–8.5: moderate oxidation intensity, best selectivity—pentlandite is sufficiently depressed while chalcopyrite is not passivated; this is the optimal range.
- pH < 6.5 (acidic): persulfate oxidation becomes too strong, easily oxidising and passivating chalcopyrite, leading to lower copper recovery.
- pH > 9.5 (strongly alkaline): persulfate decomposes rapidly and ineffectively, reducing reagent utilisation; high alkalinity also tends to impair chalcopyrite floatability.
4.2 Sodium persulfate dosage
Insufficient dosage leads to incomplete oxidation and inadequate depression of pentlandite, resulting in higher nickel content in the copper concentrate. Excessive dosage raises the overall pulp oxidation potential beyond the optimal range, causing surface oxidation and deactivation of chalcopyrite, increasing copper losses. The commonly used industrial and experimental range is 200–500 g/t, which balances nickel depression with stable copper recovery.
4.3 Pulp potential and pre‑conditioning time
The pulp redox potential should be maintained in a moderate oxidising range suited to the selective oxidation of pentlandite. A pre‑conditioning time of 3–5 minutes is recommended to ensure sufficient interaction between the reagent and the pentlandite surface, while avoiding prolonged oxidation that could negatively affect chalcopyrite.
4.4 Collector type and reagent regime
It is advisable to use selective collectors such as butyl xanthate or ethyl thionocarbamate, avoiding long‑chain xanthates with strong collecting power that may break through the hydrophilic film on pentlandite and cause contamination. Proper control of collector and frother dosages further enhances the selectivity of copper‑nickel separation.
Post time: Jul-27-2026
