GREEN METHODS BASED ON ECO-FRIENDLY LIXIVIANTS FOR CRITI-CAL METAL RECOVERY FROM TUNGSTEN INDUSTRIAL SCRAPS

This PhD thesis aims to develop new recycling methods for the recovery of critical metals from hard metal (HM) wastes, based on the use of functional and eco-friendly organic ac-ids in a hydro- or solvometallurgical leaching system. The thesis is presented as a collection of research products (3 articles and 1 patent), com-plemented by sections covering the aspects studied but still under investigation, through a logical construction. Firstly, tungsten and HMs are introduced in Chapter 1, as the feedstock and the primary industrial tungsten application, respectively. Section 1.2 is addressed to highlight the cru-cial role played by secondary sources valorization and provides a critical review of current recycling methods, highlighting the hydrometallurgical studies with organic and inorganic acids and their strengths and weaknesses (Cera M. et al. DOI: 10.1016/j.ijrmhm.2023.106249). Then, the thesis is divided into two parts. PART I describes metal leaching studies by bio-derived chemicals. Specifically, Chapter 2 describes the results in selective Co leaching by a variety of organic acids in water. Two classes of acids were identified depending on the redox and complexing capabilities demonstrated towards Co. Among them, lactic acid proved its efficacy and efficiency in mild operative conditions (Cera M. et al. DOI: 10.3389/fenvc.2023.1216245). The production of a leaching mixture rich in lactic acid, starting from agro-industrial wastes is described in Chapter 3. The described biotechnological process consists in a low cost and easy to manage dark fermentation of cheese whey or second cheese whey, ex-ploiting the action of the indigenous biomass. The process is appropriately controlled with the view to obtaining a mixture with a desired pH and lactic acid concentration and minimizing the addition of chemicals and separative and purification steps (Co-inventor Cera M. patent WO 2023/199263 A1). PART II focuses on the valorization of real tungsten industrial scraps through a Chemical Recycling based on the use of the bio-derived reagent lactic acid, named BioCR. The scraps treated in this work are two powder products (POX and REW) derived from a ther-mal regenerative process of HM scraps (ORC process), and a powder product (sludge) de-rived from the grinding of sintered HM tools. Specifically, a combined ORC-BioCR process is proposed for hard scraps valorization. A va-riety of operative conditions of the chemical leaching phase were tested on the two re-generated powders, namely POX (Chapter 4, Cera M. et al. DOI: 10.1016/j.ijrmhm.2025.107062) and REW (Section 5.1), with the aim to define the best thermal-chemical integrated process which can combine sustainable leaching conditions with high quality recycled materials. Based on the results of previous chapters, a BioCR process was proposed to treat sludge powders, as described in Section 5.2. The process aims to recover and separate Co and W from the sludge rich in SiO2. POX and REW treated powders were sintered to produce bar samples of recycled HM. Their microstructural and mechanical assessment is described in Chapter 6, while the main routes investigated to recover Co and W from the leaching solutions are described in Chapter 7.