compound of the saturated roast gas leaving the preconcentrator is adiabatically absorbed in water (which in many cases is acid rinse water from a carbon steel pickling line). Regenerated acid (typical strength: 18% wt/wt) is collected at absorption column bottom. Exhaust gas treatment The roast gas is conveyed through the system by means of an exhaust gas fan (VI). Fans in plants provide pressure increases of approx. 200 mbar and are feedback-controlled to maintain a relative pressure of -3 mbar between reactor and atmosphere to avoid any overpressure-related leakage of acid gas. To rinse the impeller and cool the gas as well as to remove remaining traces of HCl from the roast gas, the exhaust gas fan is commonly supplied with quenching water, which is separated from the exhaust gas stream by means of a mist eliminator (VII) at the pressure side of the fan. In a final scrubber, commonly consisting of a combination of wet scrubbers such as venturi scrubbers (IX) and scrubber columns (X), remaining traces of HCl and dust are removed. In some plant, absorption chemicals such as NaOH and Na2S2O3 are used to bind HCl and Cl2 (which is created under certain circumstances in several, but not all spray roasting reactors). Environmental impact Pyrohydrolysis based acid regeneration processes produce a considerable amount of stack emissions containing HCl, particles and chlorine, which has led to numerous violations of the U.S. clean air act in the past. Notes External links Minimizing Fuel Cost during Regeneration of the HCl Lixiviant (by Hatch) 3D Animation of Spray Roaster Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies) 3D Animation of Fluidized Bed Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies) 3D Animation of Hydrothermal Hydrochloric Acid Regeneration Plant (by SMS Siemag Process Technologies) Inorganic reactions Chemical processes