Hydrogen Fluoride Scrubber

Mechanism

Hydrogen Fluoride (HF) is a colorless gas. Hydrogen Fluoride in the liquid form is Hydrofluoric Acid. Hydrogen Fluoride emissions are present across a variety of industries including metal processing and finishing, agricultural, fertilizer, semiconductor, chemical production, and mining. HF is widely used in stainless steel and alloy industries for pickling and descaling various metals. HF is often used in the chemical etching process of aerospace components.

Exposure limit

The permissible exposure limit (PEL) for HF is 3 ppm (for an 8-hour time weighed average exposure)

The IDLH (immediately dangerous to life or health) limit for HF is 30 ppm. The odor detection threshold is about 1 ppm and is very noticeable at the PEL.

Highlights

Modern dry scrubbers operate with very high efficiency, typically above 99%, and all but the fugitive fluoride losses are returned into electrolytic cells.

The principle of purification of exhaust gases from electrolytic cells is adsorption of HF onto primary alumina in dry scrubber reactors, where gases from cells and flow of active (primary) alumina run into each other and react.

The scrubbers are filled with crushed limestone (CaCO₃) which reacts with HF to produce inert calcium fluoride (CaF₂):

CaCO3 + 2HF → CaF2 + H2O + CO2

There are three primary scrubbers operating in parallel plus a secondary scrubber following in series that is intended to further reduce the HF emissions to the environment.