Published · ANFA Chemical technical team
Separating minerals that look almost identical at the surface is one of the hardest problems in flotation. Feldspar and quartz, spodumene and other silicates, beryl and its gangue, or columbite–tantalite and quartz all share silicate or oxide surfaces that respond in similar ways to collectors. For decades, plants and laboratories have used fluoride reagents to open up a selectivity window between such minerals. Two fluoride sources that are frequently discussed are ammonium bifluoride (ABF, NH4HF2, CAS 1341-49-7) and fluorosilicic acid (H2SiF6, CAS 16961-83-4), alongside hydrofluoric acid and fluoride salts.
This article gives general, technically cautious guidance on where these reagents are used, how they are believed to work and what to consider before trialling them. It is not a process design: reagent choice and dosage always have to be established by testwork on the actual ore.
Why fluoride ions change flotation behaviour
Fluoride affects mineral surfaces in several ways, and which effect dominates depends on the mineral, the pH and the collector:
- Surface cleaning and etching. Fluoride in acidic solution attacks silicate surfaces, removing slime coatings, oxidised films or adsorbed contaminants and exposing fresh mineral. This can make a mineral more responsive to a collector.
- Complexing of surface cations. Fluoride forms strong complexes with aluminium, silicon, iron and other cations. On aluminosilicates such as feldspar, this changes surface charge and the sites available to cationic collectors.
- Depression of gangue. Fluorosilicate ions (SiF62−) and the products of their hydrolysis can adsorb on quartz and some silicates and make them more hydrophilic, reducing their recovery to the froth.
- Removal of activating ions. By complexing metal ions in the pulp, fluoride can limit unwanted activation of gangue minerals.
Because the same reagent can act as an activator for one mineral and a depressant for another, results are highly ore-specific.
Typical application areas
Feldspar–quartz separation
The classic route to separate feldspar from quartz is flotation of feldspar with a cationic (amine) collector in an acidic pulp in the presence of fluoride, traditionally supplied as hydrofluoric acid. Fluoride is understood to modify the feldspar surface so that the amine adsorbs preferentially on feldspar, while quartz stays in the tailings. Ammonium bifluoride is sometimes considered as a solid fluoride source that is easier to store and dose than HF solution, although it behaves differently and has to be evaluated on its own. Because of fluoride effluent concerns, fluoride-free and reduced-fluoride schemes have also been developed, and many operators compare both options.
Spodumene and lithium pegmatites
In spodumene flotation, fluoride pretreatment (for example with HF or fluoride salts) has been reported in the literature to clean and activate spodumene surfaces and to improve selectivity against feldspar, quartz and mica, particularly in reverse or cationic schemes. Most commercial spodumene circuits rely on fatty-acid collectors with careful desliming and conditioning; fluoride is an option to investigate rather than a default.
Beryl
Beryl separation from feldspar and quartz is notoriously difficult. Published flowsheets have used acid or fluoride conditioning to clean beryl surfaces before collector addition, and fluoride-based depression of associated silicates. These schemes are sensitive and should only be adopted after testwork.
Tantalum–niobium (columbite–tantalite) and related oxides
In flotation of columbite–tantalite and similar oxide minerals, with collectors such as hydroxamic or phosphonic acids, fluorosilicate reagents (fluorosilicic acid or its sodium salt) have been used as depressants for quartz and feldspar gangue. Similar depressant use is described for some cassiterite and other oxide flotation systems.
Other silicate systems
Fluoride reagents are also discussed for mica–feldspar–quartz separation and some industrial-mineral purification circuits, generally in combination with pH control and specific collectors.
ABF, fluorosilicic acid or HF?
| Ammonium bifluoride | Fluorosilicic acid | Hydrofluoric acid | |
|---|---|---|---|
| Form | White crystalline solid | Aqueous solution | Aqueous solution |
| Typical role discussed | Solid fluoride source; surface activation / cleaning | Depressant for quartz and silicate gangue | Fluoride activator (e.g. feldspar) |
| Handling | Weigh and dissolve; releases HF in acid | Corrosive liquid, pump dosing | Most hazardous; HF-specific controls |
| ANFA grades | Export ≥ 98.0% (HG/T 3586) | ≥ 40% / 35% / 30% H2SiF6 | Industrial 30–70% (GB/T 7744) |
The right choice depends on the mineral pair, the collector, the existing acid used for pH control, local regulations and how the site handles dangerous goods.
Practical considerations for testwork
- Start in the laboratory. Run bench tests with the plant's own ore, water and collector. Vary fluoride dosage, conditioning time and pH systematically.
- Control pH. Fluoride speciation (F−, HF, HF2−, SiF62−) changes strongly with pH, and so does its effect on surfaces.
- Watch the water chemistry. Calcium and magnesium in process water precipitate fluoride and change effective dosage; recycled water can accumulate fluoride.
- Consider materials of construction. Fluoride solutions attack glass, ceramics and some metals and concrete. Use compatible plastics or lined equipment.
- Plan the effluent. Fluoride in tailings water is regulated in most jurisdictions. Lime precipitation and monitoring are commonly needed.
Safety essentials
- All three reagents are corrosive and toxic; ammonium bifluoride and fluorosilicic acid can release hydrogen fluoride on contact with strong acids or heat.
- Use chemical goggles, face shield, fluoride-resistant gloves and protective clothing; ensure ventilation at mixing points.
- Fluoride burns may be painless at first. Keep calcium gluconate gel available and seek medical attention after any exposure.
- Follow the SDS and local dangerous-goods and environmental rules. For transport details see How to Safely Ship & Handle Hydrofluoric Acid (UN1790).
Supply and packaging
ANFA supplies ammonium bifluoride (EU REACH registered) in 25 kg woven bags with PE liner or 1000 kg jumbo bags; fluorosilicic acid in 25 kg and 250 kg PE drums, 1000 L IBCs or tank trucks; and hydrofluoric acid in PE drums, IBCs and ISO tanks. For a comparison of the two most common fluoride sources see Hydrofluoric Acid vs Ammonium Bifluoride.
Talk to a manufacturer
ANFA Chemical (安发化工（杭州）有限公司) has manufactured inorganic fluorides in its own factory in Hangzhou since 1995 and exports dual-use controlled items under export licences; an end-user declaration may be requested. Tell us the product, grade, estimated volume, packaging and destination port: email [email protected], WhatsApp +61 493 468 903, or use our contact page. SDS, TDS and COA are provided upon inquiry confirmation.