Free acids and chemicals recovery from stainless steel pickling baths

The conventional process used to treat waste acids from the stainless steel pickling baths is a direct neutralization with alkali. However, this process requires a high consumption of chemicals, produces a large amount of solid wastes, and furthermore, does not meet the recent European Waste Treatment Regulation because of the nitrogen contained in the final effluent. This paper presents an alternative treatment method based on a diffusion dialysis process for the free acid recovery and a neutralization process, followed by a salt splitting process, for the chemical recovery. This alternative eliminates waste disposal problems and, at the same time, it regenerates and recovers valuable by-products, which contributes to the economic feasibility of the process. Therefore, the work presented is focused on the technical and economical feasibility of electromembrane processes for the European spent pickling baths. Furthermore. bipolar electrodialysis and membrane electrolysis systems are compared for the salt splitting process. At the moment, there is a partial process in Washington Steel based on salt splitting; however, the recovery of free acid is not considered, and furthermore, the different concentration of the European pickling baths makes this process unsuitable for European stainless steel producers.