Kinetics of K₂FeF₅·H₂O (s) and CrF₃·2H₂O (s) crystallization from stainless steel spent pickling baths

Stainless steel spent pickling bath is a hazardous waste composed mainly of iron, chromium, and nickel nitrates and fluorides and free nitric and hydrofluoric acids. A new process has been developed for the recovery of this waste. The first stage is the crystallization of K₂FeF ₅·H₂O and CrF₃·2H₂O by adding a mixture of potassium fluoride and hydroxide. Nickel is kept in solution and can be recovered in a further stage. In this work, crystallization of fluorides has been modeled by the population-balance method. Results show that crystal growth rate is linear-dependent with crystal size, while other population phenomena should be taken into account, such as aggregation or breakage. Temperature shows an anomalous influence on the crystals population balance, because of the stability decrease of CrF₃, although its solubility increases. Mass transfer modeling of the reaction allows supersaturation calculation. Mass transfer coefficient values at 40, 52.5, and 65°C are 4.1 × 10^-6, 1.7 × 10^-5, and 2.2 × 10^-3 L^4.5 mol^3.5 μm^-2 min^-1, respectively. A crystal growth global order of 4.5 has been observed. Moreover, it has been observed that steady-state supersaturation has a direct relationship with initial concentration of precipitating compounds, so a new simplified model has been proposed.