Design of a novel membrane-integrated waste acid recovery process from pickling solution

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 membrane distillation (MD) technologies with a reactive precipitation unit where iron ions can be separated from the zinc-rich solution, in order to recover valuable products. To this purpose, a steady state process simulator was developed to predict the operation of the proposed integrated process.