Acid pickling of carbon steel Decapado ácido de aceros al carbono

This study reviews the possibilities of recovering the pickling waters from carbon and galvanised steel. Acid pickling with hydrochloric acid (HCl) is the most widely used chemical process to remove iron oxides from the metal surface without any significant attack on the steel itself. The acid pickling bath contains mainly ferrous chloride (FeCl) produced by the reaction between the steel and free hydrochloric acid. However, zinc chloride (ZnCl) is also found in the pickling of carbon steel parts prior to galvanisation, as the hooks and tools used to hang the carbon steel parts are also galvanised and reuse again polluting with Zn the pickling waters. Pickling water recovery or recycling technologies primarily seek the reuse of HCl in two ways. Partially by recovering the unreacted HCl or fully by breaking the FeCl bond through Pyrolysis technologies such as fluidised bed and spray roasting which in turn produces another iron oxide by-product. However, the most common by-product produced by pickling water recovery and recycling technologies is ferric chloride (FeCl), as it is a coagulant widely used in wastewater treatment. However, if the pickling water contains ZnCl or other metals, the production of FeCl becomes unattractive and the pickling water is neutralised and deposited in landfill sites. This study also discusses a wide range of technologies capable of recovering all or part of the pickling water, including galvanic pickling water, that are usually excluded from circular economy strategies.

This work would not have been possible without the financial support of Hierros y Aplanaciones S.A, (Hiasa), which is part of Gonvarri group and the scientific research of Centro Nacional de Investigaciones Metalúrgicas (CENIM-CSIC) within DECAPVALE project.