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 FeCl + 4 HO + O = 8 HCl + 2 FeO
2 FeCl + 3 HO = 6 HCl + FeO
The process is an inversion of the chemical descaling (pickling) process.