Development of recycling process for stainless steel pickling acids

The waste pickling solution used with stainless steel, which consists of mixed fluoric acid and nitric acid, is treated by pH adjustment, and the larger part of the neutralized sludge product is dewatered and disposed of in landfills. Thus, with recent pressure on landfill sites and stricter controls on the fluorine and nitrogen concentrations in waste water, the development of a recycling technology for this waste pickling solution has become an urgent task. In the present research, useful metal components and fluorine were separated and the separated fluorine solution was recycled as a pickling solution on both beaker and bench scale levels with the aim of developing a 100% recycling process for stainless steel pickling solutions. As a result, a multi-step recycling process was developed at the bench scale level. The features of the process are as follows. (1) Fluorine is separated from the neutral sludge by neutralizing the waste pickling solution to pH 10 or higher with alkali. (2) The neutral sludge from which fluorine has been separated is washed and dried, and useful metals such as Ni, Cr, Fe, etc. are then recovered using smelting reduction technology. (3) On the other hand, the acids and alkali are separated by electrodialysis. The regenerated fluoro-nitric acid is concentrated by again applying the electrodialysis technique, and is recycled and used as a pickling solution for stainless steel.