Experimental investigation and modeling of diffusion dialysis for HCl recovery from waste pickling solution

Hydrochloric acid recovery from pickling solutions was studied by employing a batch diffusion dialysis (DD)laboratory test-rig equipped with Fumasep membranes. The effect of main operating parameters such as HClconcentration (0.1–3 M) and the presence of Fe2+ (up to 150 g/l) was investigated to simulate the system operationwith real industrial streams. The variation of HCl, Fe2+ and water flux was identified. When only HCl ispresent, a recovery efficiency of 100% was reached. In the presence of FeCl2, higher acid recovery efficiencies,up to 150%, were observed due to the so-called “salt effect”, which promotes the passage of acid even against itsconcentration gradient. A 7% leakage of FeCl2 was detected in the most severe conditions. An original analysison water flux in DD operation has indicated that osmotic flux prevails at low HCl concentrations, while adominant “drag flux” in the opposite direction is observed for higher HCl concentrations.A comprehensive mathematical model was developed and validated with experimental data. The model has atime and space distributed-parameters structure allowing to effectively simulate steady-state and transient batchoperations, thus providing an operative tool for the design and optimisation of DD units.