Effective recovery of HCl and metals from pickling solutions by cutting-edge membrane technologies

Abstract

The pickling process is an essential step in the hot-dip galvanizing process.It is performed by immersing manufacturedsteel pieces inacid bathsin order todissolve the oxidized layers. During the pickling process, acid attacks metal oxides on the surface, dissolving them in the pickling bath. Thus, the efficiency of the pickling liquor decreases due to the accumulation of metal salts and the consumption of free acid in the solution. When hydrochloric acid is used,ferrous chloride is producedduring the pickling treatment, reaching concentrations up to 250 g/l, while the acid concentration decreases by 75-85%. A pickling bath in this condition is considered spent [1] due to thevery low picklingrate, hence it needs to be replaced. In common industrial practice,part of the exhausted solution is withdrawn and replaced with fresh acid orwitha more concentrate solution to prolong the pickling bath life, while spentsolution is disposed asa waste.Disposal of the spentpickling solution strongly affects the hot-dip galvanizing industries footprintand costs, thus the recovery of acid is one of the most beneficial steps to reduce the environmental and economic impact.In particular, continuous regeneration of pickling solutions can enhance pickling rate and process performance, butalso reduce industrial wastewater disposal and chemicals consumption. The recovery and recycling of valuable compounds (e.g. acid, metals and water) can be accomplished by coupling two cutting-edge membrane technologies: diffusion dialysis (DD) and membrane distillation (MD) [2,3]. Membrane techniques are considered simple, effective and sustainable [1]and can be easily scaled from small to medium side installations.In the present work, a case study is presented, relevant tothe hot-dip galvanizing plant of TecnoZinco(Palermo, Italy).The site has a capacity of 20,000 tons per yearof treated steel. Starting from an accurate data mining, a Process Flow Diagram(PFD)of an integrated process that provides a continuous regeneration of pickling solution is proposed, within the framework of the EU-funded ReWaCEM project(www.rewacem.eu), with the final aim of buildinga pilot plant to be installedand operated in real environment. Asteady state process simulator for the integrated process has been developed, aiming to design, analyze and predict performance of the pilot unit. A parametric analysis of the model is performed varying hydrochloric acid in the stream going out from the pickling tank. Results presents the perspective operation of such plant, indicating the technical feasibility of the novel developed process.