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 HCl concentration (0.1-3 M) and the presence of Fe 2+ (up to 150 g/l) was investigated to simulate the system operation with real industrial streams. The variation of HCl, Fe 2+ and water flux was identified. When only HCl is present, 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 its concentration gradient. A 7% leakage of FeCl2 was detected in the most severe conditions. An original analysis on water flux in DD operation has indicated that osmotic flux prevails at low HCl concentrations, while a dominant "drag flux" in the opposite direction is observed for higher HCl concentrations.

Hydrochloric acid pickling is a common practice in steel manufacturing industry. During the process, acid is consumed to dissolve surface oxides and metals ions are accumulated in the solution, which becomes less effective with time. In addition, the costly and risky waste acid disposal is another issue affecting the hot-dip galvanizing industry. In this work, a novel sustainable waste acid recovery process from pickling solutions based on circular approach is proposed to tackle these issues. The innovative system allows (i) the continuous regeneration of pickling solutions to enhance process rate and performance and (ii) minimise the highly expensive and environmentally risky wastewater disposal. In this way, refilling pickling baths with fresh acid, as done in conventional operation, can be avoided and can be carried out continuously under optimal working conditions. Moreover, the recovery of valuable substances (e.g. metal hydroxide or salts solution) can be obtained as an additional benefit. Continuous treatment and regeneration of pickling solution can be accomplished by coupling diffusion dialysis (DD) and

Acid recovery from pickling waste solutions is an important step to enhance hot-dip-galvanizing industry process sustainability. Diffusion dialysis (DD) can be used to separate acids and heavy metals (e.g., iron and zinc) from pickling waters, promoting the circular use of such raw materials. In the present study, a laboratory scale unit operating in batch and a continuous large scale unit, both equipped with Fumasep anionic exchange membranes, were tested. Results obtained show that zinc and iron concentration affect the HCl recovery in opposite ways. Iron chlorides enhance acid recovery, while zinc chlorides considerably tend to diffuse through the membrane because of negatively charged chloro-complexes formation and slightly reduce the acid diffusion. A multi-components mathematical model, with a time-dependent and distributed-parameters architecture, was adopted enabling the prediction of operations with hydrochloric acid, zinc, and iron metals both in batch and in continuous dialyzers. As a result, a good comparison between model simulations and experiments was achieved in both configurations.

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