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

Abstract

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