Chemical Engineering Journal 153 (2009) 50–55 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Removal of iron fluorides from spent mixed acid pickling solutions by cooling precipitation at extreme temperatures M. Sartor ∗ , D. Buchloh, F. Rögener, T. Reichardt VDEh-Betriebsforschungsinstitut GmbH, Sohnstraße 65, 40237 Düsseldorf, Germany article info Article history: Received 13 February 2009 Received in revised form 20 May 2009 Accepted 3 June 2009 Keywords: Cooling precipitation Crystallisation Ferric fluoride crystals Mixed pickling acid Stainless steel abstract The precipitation crystallisation behaviour of spent mixed acid pickling solutions (HF/HNO 3 ) for stain- less 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 (FeF 3 ) precipitates at temperatures below -35 ◦ C under the conditions investigated which provides new approaches for the regeneration of spent pickling solutions. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Steel pickling processes require large amounts of acid and water. In order to keep the pickling efficiency constant a part of the pickling acid solution has to be discharged as waste and replaced by fresh solution or regenerated continuously. Spent pickling acid solu- tions contain not reacted acids (free acid) and metal salts (bound acid) [1]. Especially pickling acid solutions for stainless steel are highly hazardous because they consist of a mixture of hydrofluo- ric (HF) and nitric acid (HNO 3 ). In the metal finishing industry this acid mixture is commonly called “mixed acid”. Thus, the expres- sion “mixed acid” as a synonym for a HF/HNO 3 mixture will be applied in the following. In companies with very large pickling lines (>2000 L/h waste acid solution) total regeneration for regen- eration of both free and bound acids (e.g. by pyrohydrolysis) is state-of-the-art [2,3]. However, innovative and economic processes for the recycling of these solutions are still of interest for metal working companies dealing with pickling steps in the production chain. Previous investigations showed that the best way to crys- tallise metal fluorides from spent pickling acid solution is to pre-concentrate the solution. Pre-concentration is necessary since the metal salts - like any other solutions - precipitate only when the solutions are supersaturated. Depending on the concentration of the acids and the metals the precipitation process starts at tem- ∗ Corresponding author. Tel.: +49 211 6707 889; fax: +49 211 6707 923 889. E-mail addresses: miriam.sartor@bfi.de (M. Sartor), dirk.buchloh@bfi.de (D. Buchloh), frank.roegener@bfi.de (F. Rögener), tilo.reichardt@bfi.de (T. Reichardt). peratures significantly above 0 ◦ C [4,5]. Different experiments were conducted at 20 ◦ C and 4 ◦ C with an iron content of 29 and 37 g/L, respectively, at various HF and HNO 3 concentrations for a period between few hours and 150 days. Thus, an amount of about 1 to 10g/L salt crystals was gained. The experiments showed, that the higher the HF concentration and the longer the crystallisation time were the more crystals were formed. This result was anticipated since more fluoride for the formation of complexes is available at higher HF concentrations [4,5]. Furthermore, it is also possible to remove metal fluoride crystals from pre-concentrated solutions after heating and cooling them rapidly [6]. The sudden change of solubility causes a precipitation crystallisation. The precipitation crystallisation processes mentioned above need supersaturated solutions with a concentration near the max- imum solubility which can be achieved, e.g. by energy-intensive evaporation. The ferric-chloride-water-phase diagram indicates, that ferric chloride precipitates at temperatures below 0 ◦ C [7], please ref. Fig. 1. The diagram shows that for different concentra- tion conditions even significantly above 0 ◦ C crystallites of either FeCl 3 ·6H 2 O or 3.5H 2 O are contained in the liquid. For low ferric chloride concentrations at temperatures below 0 ◦ C considerably more crystallites can form. Due to this property ferric chloride can be precipitated from according solutions. Since this behaviour is typical for salt-containing solutions, for other ferric compounds a similar behaviour can be assumed. Thus, the target of the investiga- tions discussed in the following was to examine the precipitation crystallisation behaviour of ferric fluorides at temperatures signifi- cantly below 0 ◦ C. When it is possible to precipitate ferric fluorides at those temperatures a new processing approach for solutions con- taining fluorides may be developed. 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.06.008