Modeling and Design of Membrane Process Recovery of HCl and Metals from Pickling Solutions

Abstract Hydrochloric acid pickling is one of the key steps in the hot-dip galvanizing process. It is a process widely used as a chemical pre-treatment method for cleaning, where metal surfaces with oxides are immersed into an acid solution. During the pickling process, the acid concentration decreases with time while the metal is accumulated. Thus, the efficiency of the pickling solution decreases and fresh solution must be used. Continuous regeneration of pickling solutions enhances pickling rate and process performance, but also minimises industrial wastewater disposal and chemicals consumption. The recovery and recycling process of valuable substances (e.g. acid and metals) can be accomplished by coupling diffusion dialysis (DD) and membrane distillation (MD) technologies [1,2]. The integrated process is based on a recovery of more than 80% of the free acid exiting from the pickling bath by passing through a selective anionic exchange membrane (in the DD), and then, its concentration by water evaporation through a hydrophobic membrane (in the MD) in order to be recycled in the pickling tank. The outlet stream from the diffusion dialysis, concentrated of metal salts, is fed to a reactive precipitation unit to recover iron as valuable product (iron hydroxide) by adding an alkaline reactant, whereas zinc salt is kept in the solution to be reused. In the present work, a steady state process simulator for the integrated process has been developed, in order to analyze and predict performances of a small pilot-scale unit to be installed and operated within a hot-dip galvanizing plant. A parametric analysis of the model is performed varying hydrochloric acid and iron concentration in the pickling tank. In this way, usual operations of withdrawing of partially exhausted solutions and refilling with fresh acid is avoided allowing to continuously operating under the optimal pickling conditions.