Diffusion dialysis for HCl and heavy metals separation from highly concentrated pickling solutions

Disposal of pickling waste solutions strongly affects hot-dip galvanizing industries’ economics and environmental footprint. Thus, the recovery of acid is an important step to enhance the 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 work, two DD modules, a laboratory scale unit operating in batch and a continuous operated large unit, equipped with Fumasep anionic exchange membranes were tested. Results obtained show that zinc and iron affect HCl recovery in opposite way, however reaching high recovery values. In particular, zinc considerably tends to diffuse through the membrane because of the formation of negatively charged chloro-complexes. An already presented mathematical model, with a time and space distributed-parameters structure, was implemented to simulate operations in the presence of zinc and iron ions. A good fitting between experimental results and model predictions was achieved.