Removal of iron fluorides from spent mixed acid pickling solutions by cooling precipitation at extreme temperatures

The precipitation crystallisation behaviour of spent mixed acid pickling solutions (HF/HNO3) for stainless steel surface finishing was investigated. The main target was to extent the knowledge about the temperature dependency of these metal containing pickling acids at temperatures below 0 °C to induce precipitation by a temperature change instead of a salt supersaturation in the liquid.It was found, that ferric fluoride (FeF3) precipitates at temperatures below −35 °C under the conditions investigated which provides new approaches for the regeneration of spent pickling solutions.