Hydrochloric acid (HCl)

Hydrochloric acid (HCl) — hydrogen chloride gas, or its solution in water — is a strong, highly corrosive acid. As an air pollutant it appears as HCl gas or acid mist, sharply irritating and corrosive to the eyes, respiratory tract and skin. India's general emission standards cap HCl in stack gas at 35 mg/Nm³.

HCl is released from chemical manufacturing, metal pickling and surface treatment, and crucially from the thermal treatment of chlorinated materials. This last route is the key one for recyclers: when PVC or other chlorinated plastics are burned, pyrolysed or incinerated, their chlorine content is released largely as HCl. Any plastic or tyre pyrolysis or incineration stream contaminated with PVC will therefore generate HCl as an acid gas, and the same chloride is the precursor for dioxin and furan formation.

The hazard is corrosion and respiratory injury: HCl gas attacks the airways, corrodes process equipment and stacks (causing acid dewpoint corrosion when flue gas cools), and damages surrounding structures. In hydrometallurgical metal recovery, hydrochloric acid is also used as a leaching reagent, where handling generates corrosive mist.

Control is feedstock management plus alkaline scrubbing. The first defence in thermal recycling is to keep PVC and chlorinated material out of the feed through segregation and sorting. The second is alkaline (lime or caustic) scrubbing of the flue gas to neutralise HCl before the stack, which also helps suppress dioxin formation by removing chloride. Stack HCl is tested against the 35 mg/Nm³ limit, and acid-resistant materials are used where flue gas cools below the acid dewpoint.