However tightening environmental standards and stringent air permit policies render it increasingly difficult to establish new pyrohydrolysis-based acid regeneration plants.
The following processes for the regeneration of HCl from spent pickle liquors have been adopted by the ferrous metals processing industry: Hydrothermal hydrolysis of hydrochloric SPL from carbon-steel pickling lines is a hydrometallurgical reaction, which takes place according to the following chemical formula: 12 FeCl2 + 3 O2 → 8 FeCl3 + 2 Fe2O3 2 FeCl3 + 3 H2O → 6 HCl + Fe2O3 Today hydrothermal hydrolysis, which operates at very low temperatures, consumes only a fraction of the energy other processes demand and produces virtually no emissions, is considered the most effective way to regenerate any given quantity of spent pickle liquor.
The liquid phase is re-circulated back to the venturi evaporator to increase mass and heat exchange performance.
In the reactor the conversion of droplets of preconcentrated waste acid into iron oxide powder and hydrogen chloride gas takes place.
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.