Hydrochloric acid regeneration or HCl regeneration is a chemical process for the reclamation of bound and unbound HCl from metal chloride solutions such as hydrochloric acid. Field of application The commercially most relevant field of application for HCl regeneration processes is the recovery of HCl from waste pickle liquors from carbon-steel pickling lines. Other applications include the production of metal oxides such as, but not limited, to Al2O3 and MgO, as well as rare-earth oxides, by pyrohydrolysis of aqueous metal chloride or rare-earth chloride solutions. A number of different process routes are available. The most widely used is based on pyrohydrolysis and adiabatic absorption of hydrogen chloride in water, a process invented in the 1960s. However tightening environmental standards and stringent air permit policies render it increasingly difficult to establish new pyrohydrolysis-based acid regeneration plants. Known processes The following processes for the regeneration of HCl from spent pickle liquors have been adopted by the ferrous metals processing industry: Regeneration Pyrohydrolysis Spray roaster pyrohydrolysis Fluidised bed pyrohydrolysis Hydrothermal regeneration Electrolytic Fe precipitation Recovery of free HCl Retardation Dialysis Ion exchange Transformation of FeCl2 to FeCl3 Electrolytic oxidation Chemical oxidation Hydrothermal regeneration Hydrothermal hydrolysis of hydrochloric SPL from carbon-steel pickling lines is a hydrometallurgical reaction, which takes place according to the following chemical formula: Step 1: oxidation 12 FeCl2 + 3 O2 → 8 FeCl3 + 2 Fe2O3 Step 2: hydrolysis 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. Advantages low energy consumption (about 1300 kJ per litre waste acid) no gaseous emissions wide operating range (10 to 100% of nominal capacity) high-value byproduct (>20 m3/g BET specific surface; >2 kg/L specific weight; <0.05% water-soluble chlorides) theoretically unlimited operating capacity Known implementations Known implementations of the hydrothermal HCl regeneration processes include the PORI process (1974 for J&L Steel, dismantled) and the optimized SMS Demag wet process (2008 for ThyssenKrupp Steel, under construction). Pyrohydrolysis of spent pickle liquor Pyrohydrolysis of hydrochloric spent pickle liquor from carbon steel pickling lines is a hydrometallurgical reaction which takes place according to the following chemical formulae: 4 FeCl2 + 4 H2O + O2 = 8 HCl + 2 Fe2O3 2 FeCl3 + 3 H2O = 6