Recycling of waste pickle acid by precipitation of metal fluoride hydrates

Stainless steel is pickled in mixed acid solutions (1–3 M HNO3 and 0.5–4 M HF). The spent solution is usually neutralized with lime, and in Sweden about 18,000 tons/yr of metal hydroxide sludge is disposed as landfill waste. We are developing a cost-saving and environmentally friendly process, involving crystallization of β-FeF3 · 3H2O, where the metal content is recovered and the acid is recycled.Iron has been successfully separated from spent pickle bath solutions by precipitation of β-FeF3 · 3H2O in a continuous crystallizer (10 L scale) where the solution is concentrated by nanofiltration. The crystal growth rate of β-FeF3 · 3H2O has been determined in industrial pickle bath solutions at 50 °C and the results have been compared to previous measurements in pure HF/HNO3 solutions prepared in the laboratory. The growth rate of β-FeF3 · 3H2O crystals at 50 °C is in the order of 10−11 m/s in both industrial and pure acid mixtures.