The Precious Metals’ “Guardian”: Why Dithiophosphates (Black Agents) Are Widely Used in Mineral Flotation

In flotation plants, xanthates are widely recognized as powerful collectors with strong general collecting ability. However, for improving recovery of gold and silver or separating lead from zinc, experienced metallurgists often recommend dithiophosphates, commonly known as “black agents” due to their dark, often viscous liquid appearance. Despite their unremarkable appearance, these reagents offer distinct advantages for treating precious metals and complex sulfide ores.

I. What Are Dithiophosphates (Black Agents)?

Dithiophosphates are a class of collectors chemically related to xanthates. While xanthates are strong, non-selective collectors that tend to float iron sulfides (pyrite), dithiophosphates are more selective. They are also more chemically stable and less prone to decomposition.

II. Key Advantages

1. Low Affinity for Pyrite: This is a major advantage for separating lead or copper sulfides from pyrite. Xanthates tend to float pyrite as well, whereas dithiophosphates show weak collecting ability toward pyrite, facilitating the separation.
2. Strong Collecting Ability for Gold and Silver: Dithiophosphates form stable chemical bonds with precious metal surfaces, making them highly effective for recovering gold, silver, and platinum. Many gold processing plants add dithiophosphates (e.g., Aero 238 or ammonium dibutyl dithiophosphate) in scavenger or cleaner circuits to maximize precious metal recovery.
3. Frothing Properties: Many dithiophosphates (e.g., Aero 238) possess inherent frothing ability, reducing or eliminating the need for additional frothers (such as MIBC or pine oil). The froth produced tends to be brittle and breaks down readily.

III. Common Types

• Aero 238 (Alkyl Dithiophosphate): A common liquid type with both collecting and frothing properties. Suitable for lead-zinc separation and copper-gold recovery. Has a strong odor and corrosive properties.
• Ammonium Dibutyl Dithiophosphate: Available as a white powder (despite the “black agent” name). Non-corrosive, low odor, water-soluble. Suitable for silver-bearing ores and refractory copper ores, especially where environmental concerns are prioritized.
• Diphenyl Dithiophosphate: Less common, with strong collecting ability. Used for oxidized or fine-grained refractory galena.

IV. Operational Considerations

• Corrosion Hazard: Liquid dithiophosphates (e.g., Aero 238) are acidic and corrosive. Use acid-resistant materials for dosing lines. Operators must wear appropriate PPE.
• Avoid Overdosing: Excessive dosing can produce excessive froth, leading to overflow (“carry-over”) from flotation cells. Adjust dosage carefully, especially when frother is also being used.
• Slower Kinetics: Dithiophosphates act more slowly than xanthates and typically require longer conditioning time for optimal performance.

V. Conclusion

In modern flotation practice, dithiophosphates are not substitutes for xanthates but rather complementary reagents. A common strategy is to use xanthates for bulk recovery in rougher flotation and dithiophosphates for cleaner stages to enhance selectivity and recover precious metals.