Collector Agents in Flotation
A collector agent is a flotation reagent that alters the hydrophobicity of mineral surfaces, enabling floating mineral particles to adhere to bubbles.
It is one of the most critical types of flotation reagents, possessing two fundamental properties:
- Selective adsorption onto the mineral surface.
- Increasing the hydrophobicity of the mineral surface, thereby enhancing its attachment to bubbles and improving flotation efficiency.
Xanthate: A Key Collector Agent
Xanthate is one of the most important collector agents.
Chemically known as alkyl dithiocarbonate, xanthates can be considered derivatives of carbonates, where one metal ion is replaced by an alkyl group, and two oxygen atoms are substituted with sulfur atoms.
The general formula is R-OCSSMe, such as sodium ethyl xanthate.
Chemical Structure & Classification
- R represents an alkyl hydrocarbon group (CnH2n+1), typically with n = 2 to 6. Less commonly, R can be an aromatic, cycloalkyl, or alkylamine group.
- Me is usually Na(+) or K(+), with sodium xanthate being the industrially predominant form.
- Sodium xanthate vs. Potassium xanthate:
- Potassium xanthate is more stable than sodium xanthate.
- Sodium xanthate is more hygroscopic and absorbs moisture easily, whereas potassium xanthate does not.
- Sodium xanthate is cheaper than potassium xanthate.
- Both dissolve easily in water, alcohol, and acetone.
Based on the number of carbon atoms in the R group, xanthates are classified as:
- Lower xanthates: Methyl and ethyl xanthates.
- Higher xanthates: Butyl xanthate and above.
Physical and Chemical Properties
- Appearance: Crystalline or powdered form.
- Color: Impure xanthates often appear yellow-green or orange-red.
- Density: 1.3–1.7 g/cm³.
- Odor: Pungent and irritating.
- Toxicity: Moderately toxic.
- Solubility:
- Easily soluble in water, acetone, and alcohol.
- Slightly soluble in ether and petroleum ether.
- Purification: Recrystallization using a mixed solvent of acetone and ether.
Xanthate’s Collecting Ability & Selectivity
The collecting ability and selectivity of xanthates are closely related to the solubility product (Ksp) of the corresponding metal xanthates.
Based on the Ksp of ethyl xanthate salts, minerals are classified into three groups:
1. Copper-Loving Minerals (Ksp < 4.9 × 10⁻⁹)
This group includes metals such as Au, Ag, Hg, Cu, Pb, Sb, Cd, Co, and Bi.
- Xanthates have the strongest collecting ability for native metals (e.g., Au, Ag, Cu) and their sulfide minerals.
2. Iron-Loving Minerals (4.9 × 10⁻⁹ < Ksp < 7 × 10⁻²)
This category includes metals such as Zn, Fe, and Mn.
- Xanthates have a weaker collecting ability for sulfide minerals of these metals.
- When used as a collector, flotation separation between copper-loving and iron-loving minerals is more effective.
- Co and Ni have a Ksp less than 10⁻¹, making them copper-loving. However, they usually coexist with iron sulfide minerals, so they are floated together.
3. Stone-Loving Minerals (Ksp > 4.9 × 10⁻²)
Metals in this category include Ca, Mg, and Ba.
- Their high Ksp prevents the formation of hydrophobic films under standard flotation conditions.
- Xanthates have no collecting ability for these minerals.
- As a result, xanthates are not used for separating alkali metals, alkaline earth metals, oxides, or silicate minerals.
Mechanism of Xanthate Adsorption
For xanthates to act effectively, the metal sulfide mineral’s solubility must be lower than that of the corresponding metal xanthate salt.
Pure metal sulfides do not naturally react with xanthate anions (X⁻) to replace S²⁻.
However, when metal sulfide surfaces undergo slight oxidation, S²⁻ is replaced by OH⁻, SO₄²⁻, S₂O₃²⁻, or SO₃²⁻. This oxidation allows xanthate anions (X⁻) to replace these ions, forming a hydrophobic layer that enhances flotation efficiency.
Conclusion
Xanthates are widely used in flotation as effective collector agents.
Their efficiency depends on:
- The type of metal mineral.
- The solubility product of metal xanthate salts.
- Surface oxidation of metal sulfide minerals.
Understanding these factors allows for the optimal selection of xanthates in mineral processing, leading to enhanced flotation performance and increased recovery rates.
