DEVELOPMENT AND MODELING OF MEMBRANE PROCESSES FOR THE REGENERATION OF ACID PICKLING SOLUTIONS

The doctoral thesis focuses on a novel integrated process for the recovery and valorisation of acid and metal salts present in the waste solutions of the pickling process. The proposed process is based on the integration of two innovative membrane technologies, the Diffusion Dialysis and the Membrane Distillation, coupled with a reactive precipitation section. This hybrid process allows the recovery and the enhancement of waste solutions, as well as optimal operating conditions for the continuous pickling process, thus improving its efficiency. Hydrochloric acid recovery was assessed through a detailed study on the Diffusion Dialysis process by implementing a wide experimental campaign using artificial solutions produced in laboratory, in order to understand and characterize the system. A mathematical model was developed with time and space distributed-parameters structure for the effective simulation of steady state and transient batch operations, thus providing an operative tool for the design and optimization of DD units. Selective separation of metal salts was reached by precipitating ferric hydroxide and maintaining ammonium and zinc chlorides in solution, which can be used as fluxing solution in the galvanizing process itself, thus implementing the typical Circular Economy concept. The feasibility of the proposed process is demonstrated through the use of a purposely developed process simulator able to predict steady state operation of the integrated process and to perform sensitivity analysis for the identification of the best operating conditions of the system. An experimental campaign was carried out with a demonstrator unit, jointly designed and constructed by Fraunhofer ISE (Freiburg, Germany), eventually installed in the real industrial environment of Tecnozinco s.r.l hot-dip galvanizing plant in Carini, Sicily. The campaign assessed the integration performance of the different units and the process reliability, resulting in a minimization of spent pickling solution disposal and in high quality recovered compounds. An engineering economic analysis was carried out in order to assess the economic feasibility of the proposed process. The economic model was implemented in the gPROMS simulation platform and was used to conduct an optimization analysis, defining the optimal operational and design conditions for which the process is more profitable and performing. The process simulator was also used to provide a scale-up of the demonstrator plant. The results have shown that the process has a good potential for industrial implementation, thanks to the economic and environmental benefits.