Fluoride (F)

Fluoride as an air pollutant refers to gaseous hydrogen fluoride (HF) and particulate fluoride compounds released from certain high-temperature processes. It is highly reactive and bioaccumulates — it deposits on vegetation and is taken up by grazing animals and humans, causing fluorosis (mottling and weakening of teeth and bones) at chronic exposure. Indian general emission standards cap fluoride in stack gas at 25 mg/Nm³.

The classic fluoride sources are aluminium smelters (the electrolytic cells and anode bake ovens release HF from the cryolite bath), phosphate fertiliser and phosphoric acid plants (phosphate rock contains fluorine), brick kilns and ceramic industries, and glass manufacture. Among recycling sectors the direct relevance is limited, but it appears where aluminium dross or secondary aluminium is smelted (a common e-waste and metal-recycling activity) and wherever fluoride-bearing feedstock or flux is heated.

The damage is to the surrounding agricultural environment as much as to workers: fluoride emissions are a recurring cause of crop damage and cattle fluorosis disputes near aluminium and phosphate plants in India, leading to compensation claims and community conflict. On the worker side, HF is acutely corrosive and causes deep, painful burns on skin contact.

Control is alkaline scrubbing and good capture. Wet scrubbers and dry alkaline injection (alumina adsorption in aluminium plants) capture HF before the stack; particulate fluoride is removed with the baghouse. For metal recyclers smelting aluminium, the practical points are to control flux chemistry, capture and scrub the off-gas, and monitor stack fluoride against the 25 mg/Nm³ limit, while protecting workers from HF burns.