Rinse water regeneration in stainless steel pickling

Since stainless steel must be rinsed with water after pickling operation with mixtures of nitric and hydrofluoric acids, the present investigation was undertaken to define a major step towards the close loop circulation of water in steel plant. This can be done by a new procedure for removing also nitrates from rinse water in such a way that clean water can be recycled back to rinsing use. The concentrated part of used rinse water will be fed to pickling acid regeneration plant, where nitric and hydrofluoric acids can be separate and recycled back to the pickling use. The tests were carried out mainly in one of Outokumpu’s production plants in Sweden. Membrane technique was used for rinse water treatment. A filtering system based on reverse osmosis was developed to separate the acids and metals from the rinsing water. A pre-filtering method necessary for reverse osmosis was tested. Since the composition of permeate is not applicable for direct reusage in the rinsing zone, post-treatment methods were examined. Amberlite IRA67 ion exchange resin was tested for treatment of reverse osmosis filtrate (pH adjustment). For the recovery of the mixed acids, the concentrate of reverse osmosis was further treated with electrodialysis. On the basis of these results, a regeneration concept was developed, in order to close the rinse water loop. The proposed rinse water recycling concept reduces the overall usage of water.