An integrated approach for the HCl and metals recovery from waste pickling solutions: pilot plant and design operations

Continuous regeneration of industrial pickling solutions and recovery of valuable materi-als are implemented in a pilot-scale plant including diffusion dialysis (DD), where HCl isrecovered, membrane distillation (MD), where HCl is concentrated, and reactive precipita-tion (CSTR), where metal ions are recovered in different forms. The integration of the threeprocesses allows to minimize waste streams generation and to accomplish a closed-loopprocess, thus increasing the environmental sustainability and economic impact of the gal-vanizing industry. Process reliability was proved through the operation of a demonstratorin the real industrial environment of the Tecnozinco SrL hot-dip galvanizing plant (Carini,Italy), assessing the actual performance in fully reducing spent pickling solution disposaland recovering useful compounds. Tests were conducted firstly with synthetic solutionsand then with real waste liquors from the pickling plant. A high acid recovery (80%) can beachieved in the diffusion dialysis unit and quantitative metals separation was accomplished,with iron hydroxide produced at 99% purity. The membrane distillation performance sufferswhen metal salts are present in large quantities, due to the “salting out” effect, resultingin reduced water vapor pressure, though the use of available low grade waste heat allowsenergy-sustainable operation of the MD.