Determination of iron and chromium fluorides solubility for the treatment of wastes from stainless steel mills

Stainless steel pickling waste bath is one of the most important environmental problems in stainless steel mills because of its high toxicity and economic losses due to conventional neutralization treatment of this waste. Nitric and hydrofluoric acids reclamation solves only a part of the problem while metals still are considered as a waste and the whole content of hydrofluoric acid is not recovered because fluoride forms very stable complexes with metals. In this work metal-fluoride complexes solubility is determined in order to design a process for metals recovery, consisting of separating iron and chromium, precipitated as K2FeF5(s) and CrF3(s), from nickel that remains in solution. Iron and chromium would be further hydrolyzed for oxides recovery, while fluoride release to solution and is recycled to selective precipitation stage. Results are obtained by the application of a novel equilibrium model for spent pickling liquors (SPL) called in this work “UCM-Model”. Its applicability is evaluated by comparison with other literature models and it is used in the determination of thermodynamic solubility products. Obtained values are Ks, K2FeF5(s) = exp(−4.1 ± 0.5 − (1400 ± 200)/T (K)) and Ks, CrF3(s) = exp(−7.8 ± 1.0 − (5800 ± 500)/T (K)).