In short
- HF has a UK workplace exposure limit of 1.8 ppm over 8 hours and 3 ppm over 15 minutes.
- HF attacks glass and silica, so material choice in the wetted path is critical.
- Caustic scrubbing captures HF well, but the fluoride it produces has to be managed.
Hydrogen fluoride turns up in more processes than most people expect: semiconductor and glass etching, stainless steel pickling, battery recycling, and some chemical synthesis. It is also one of the most dangerous acid gases to get wrong.
This article covers what makes HF different, and the design decisions that follow.
Why HF is treated with extra care
In the UK, HSE guidance EH40 sets a long-term workplace exposure limit for hydrogen fluoride (as F) of 1.8 ppm over 8 hours, and a short-term limit of 3 ppm over 15 minutes. For comparison, hydrogen chloride has a long-term limit of 1 ppm and a short-term limit of 5 ppm.
Beyond its toxicity, HF is unusual in attacking glass and silica. That rules out glass-reinforced components, glass-based packing and silica-containing materials anywhere the gas or its liquor can reach.
Materials that survive HF
Thermoplastics are the usual choice. Published chemical resistance tables rate polypropylene for hydrofluoric acid up to around 80°C and PVDF up to around 100°C, with reduced service life at the upper end. In practice that means:
- Polypropylene for most ambient-temperature HF duties.
- PVDF where the gas is hotter, the concentration is higher, or a longer service life is worth the cost.
- A quench ahead of the scrubber when gas comes from a hot process such as pyrolysis.
- No glass, silica or glass-filled parts in the wetted path, including sight glasses and some packings.
Have an HF duty in mind?
Send us the source, flow and temperature. An engineer will come back within 4 working hours.
Liquor and water quality
Sodium hydroxide is the usual reagent. It reacts with HF to form sodium fluoride, which stays in solution and is removed with the bleed. pH control holds the liquor in the working range.
Water quality matters more with HF than with most acid gases. Fluoride readily forms insoluble salts with calcium, so hard make-up water or calcium-based reagents can lead to scale on packing and nozzles. Softened or treated make-up water is often specified for that reason.
Engineer's note
Where HF scrubbers give trouble, it is often not the chemistry but the small parts: a glass level gauge, a ceramic-filled gasket or a standard spray nozzle. We check every wetted component against HF before anything is ordered, down to the instrument seals.
Where HF comes from, and what else comes with it
| Source | Usually present with HF | Design point |
|---|---|---|
| Semiconductor and glass etching | Other acids, sometimes NOx | Keep HF streams separate from incompatible exhausts |
| Stainless steel pickling | Nitric acid and NOx | NOx needs its own approach; HF and nitric mist scrub readily |
| Battery recycling | Fluorinated organics, solvents, CO | Oxidiser upstream for organics, then quench and caustic |
| Chemical synthesis | Depends on process | Review every vent before combining |
Working with HF on a pickling, plating or etch line? Our acid fume page covers single-acid and mixed-acid designs.
See acid fume scrubbersData your permit will ask for
Whatever regime you operate under, expect to provide predicted inlet and outlet HF concentrations, liquor consumption and bleed rate, and a sampling location that meets the relevant measurement standard. We include these on every quotation datasheet, so the numbers are ready before the application is drafted.
For battery recycling lines, our battery recycling scrubbers are designed around exactly this combination of HF, heat and scale-up.
Common questions
Can a standard acid scrubber handle HF?
Often yes, if the materials are right. The main differences are no glass or silica in the wetted path, attention to temperature, and water quality to avoid fluoride scaling.
Should HF and HCl go to the same scrubber?
They can share a caustic scrubber if they come from compatible processes. The deciding factor is usually what else is in each stream, not the two acids themselves.
Is PVDF always better than polypropylene for HF?
Not always. PVDF handles higher temperatures and concentrations, but costs more. For ambient, dilute duties polypropylene is usually the right choice.