DOI: 10.3303/CET2186175 Paper Received: 13 October 2020; Revised: 5 March 2021; Accepted: 21 April 2021 Please cite this article as: Randazzo S., La Corte D., Gueccia R., Cipollina A., Micale G., 2021, Metals Recovery from Waste Pickling Solutions by Reactive Precipitation, Chemical Engineering Transactions, 86, 1045-1050 DOI:10.3303/CET2186175 CHEMICAL ENGINEERING TRANSACTIONS VOL. 86, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. I SBN 978-88-95608-84-6; I SSN 2283-9216 Metals Recovery from Waste Pickling Solutions by Reactive Precipitation Serena Randazzo*, Daniele La Corte, Rosa Gueccia, Andrea Cipollina, Giorgio Micale Dipartimento di Ingegneria, Università degli studi di Palermo, Palermo, 90128, Italy serena.randazzo@unipa.it Pickling is one of the most important steps in steel manufacturing industry. During the process, an acid reacts with the surface oxides causing metal ions accumulation in the pickling solution. Disposal of the waste acid represents a critical issue for the hot-dip galvanizing industry in terms of environmental damage and high costs. Recovering of the main products by using an integrated process, with the perspective of a circular approach, could minimize the wastewater production, leading to reinvigorate this industrial sector economy. In this context, recovery of the metal ions mainly present in the pickling solution, such as Fe and Zn, becomes a critical issue. In this work, the reactive precipitation process reliability was proved through lab-scale experiments, in order to collect information for the design of a pilot-plant to be installed in the Tecnozinco SrL (Carini, Italy) hot-dip galvanizing plant. Experiments were carried out in a continuous stirred tank reactor by feeding the acidic metals-rich stream, an alkaline reactant and an oxidant. The quantity of Fe in the outlet solution and its speciation in the precipitate were detected to evaluate the recovery efficiency and the process quality, whereas Zn concentration in the precipitated was detected for determining the product purity. Some key parameters, such as temperature and pH, were studied by varying inlet streams flow rates. A very effective metals separation was observed by obtaining Fe(III) hydroxide at a high purity of 99 %. Zn ions were successfully separated by keeping them in solution with the aim of generating a zinc/ammonium chloride stream, to be reused in the fluxing baths of the hot-dip galvanizing plant. 1. Introduction The pickling process is one of the most important steps in the hot-dip galvanizing process as it allows the complete removal of oxidized layers from the surface of manufactured steel pieces. In fact, the quality of the covering zinc layer during the immersion of manufactured steel in the molten zinc bath strongly depends on the efficiency of the pickling step, as it is essential to have a good and uniform contact surface. In the pickling bath, the acid, typically HCl, reacts with metal oxides dissolving them in the solution. Thus, FeCl 2 is produced and Fe 2+ concentrations up to 200-250 g/L can be reached, whereas the concentration of the acid is reduced from values above 100 g/L to below 20 g/L. At this condition, a pickling bath is considered spent (Regel- Rosocka, 2010) because of the poor pickling rate, and its replacing is necessary. As pickled goods and hooks are commonly covered with zinc, also an accumulation of Zn 2+ ions can be observed, thus making the pickling solution even more ineffective (Regel et al., 2001). Thus, an improvement of the pickling process, with the aim of a sustainable development, could benefits the hot-dip galvanizing industry (Kong and White, 2010). With the perspective of a circular approach, beside the acid recovery, successfully obtained in a diffusion dialysis unit (Gueccia et al., 2019; Gueccia et al., 2020), also the recovery of the heavy metals present in pickling solutions should be taken in consideration. In fact, the pilot-plant installed at the Tecnozinco SrL (Carini, Italy) hot-dip galvanizing plant, which design and testing have been reported in previous authors’ papers (Culcasi et al., 2019; Gueccia at al., 2021), includes both the membrane technologies, to recover the acid, and the reactive precipitation, to recover the metals. The novelty of this innovative system proposed lies 1045