A non-toxic, non-volatile reagent does not gas off at low pHs
Pyrite (FeS2), is the most abundant sulphide mineral and is commonly found alongside valuable minerals like chalcopyrite, sphalerite, galena and gold. Pyrite is classified as a gangue mineral. Its economic insignificance means one thing in mineral processing: it needs to be removed.
However, separating pyrite from economically valuable minerals in flotation circuits is not straightforward, as understanding pyrite flotation is complex. Pyrite can be classified into three forms. Each behaves differently and needs its own strategy to depress it.
Normal pyrite
In traditional sulphide flotation, xanthates (ROCSS) are the most commonly used collectors. Interestingly, “normal” pyrite doesn’t float with xanthate. Instead, it floats when dixanthogen is formed – an oxygen-dependent process.
On the plant, two competing reactions for oxygen take place:
Reaction 1: Oxygen oxidises xanthate to form dixanthogen on the pyrite surface. It requires relatively high xanthate concentration and dissolved oxygen (high eH) to form dixanthogen. Note: dixanthogen is unstable at a pH > 11.
Reaction 2: Oxygen also oxidises the iron in pyrite; the result of this is pH-dependent:
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At high pH, iron precipitates as ferric hydroxide, coating the surface with a hydrophilic layer and thus the pyrite won’t float.
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At low pH, oxidation leaves behind a sulphur-rich, hydrophobic surface, which means that pyrite will float.
Whether oxygen helps or hinders pyrite flotation is a race between these two reactions and how fast the specific pyrite species oxidises. The takeaway is that it’s important to operate at high pHs to depress “normal pyrite”.
Locked pyrite
If the pyrite is locked in Cu-sulphides, Ni-sulphides or similar value minerals, it will float with the host mineral.
If pyrite recovery is still an issue after optimising pH, grinding media and reagents, it’s important to check for pyrite locking using a scanning electron microscope. Nasaco offers these services to determine the degree of locking and to propose possible solutions.
Copper-activated pyrite
Copper ions (Cu2+) adsorb onto reactive sulphur sites on pyrite, especially in low-oxygen (low-electropotential) conditions, like in the grinding mill. Once activated by copper, pyrite begins behaving like a Cu sulphide mineral. It will thus float with xanthate (low xanthate concentrations, low dissolved oxygen) and doesn’t need dixanthogen to form.
This copper activation can arise from:
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Galvanic interactions between pyrite and chalcopyrite in the mill.
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If there are copper ions in the process water.
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Contact with grinding media.
A lot of copper activation takes place in the mill, as the grinding media oxidises during the milling process, thereby consuming oxygen and creating a low electropotential environment where copper activation readily occurs. It’s important to limit copper activation. Otherwise, the pyrite will start acting like chalcopyrite and will float even at high pH.
Preventing and reversing copper activation
To stop pyrite from becoming copper-activated, the strategy starts at the source:
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Change grinding media type: Less oxidation and minimises low-potential environments where copper activation thrives.
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Add lime to the mill: The exact mechanism for why this is effective is not fully defined. Nevertheless, adding lime to the mill has been found to be an effective method in practice.
Prevention alone is rarely enough as copper ions are also often present in process water. Cyanide has traditionally been used to deactivate copper-activated sites. It forms stable complexes with the copper, thereby restoring it to “normal pyrite”.
The toxicity of cyanide is a significant drawback in processing.
A safe and effective solution
Nasmin 1032 is a non-toxic, non-volatile reagent that does not gas off at low pHs. Nasmin 1032 strips copper from pyrite surfaces, like cyanide, and restores it to “normal pyrite”. It also reduces pulp electropotential, limiting oxygen’s role in dixanthogen formation.
For more information visit: www.nasaco.ch
