SELECTIVE SUSTAINABLE COBALT RECOVERING METHODS FROM HARDMETAL PRODUCTION BY-PRODUCTS

The aim of the present work is to study new sustainable solvometallurgical routes for the selective Co-leaching from WC-Co-based "HardMetals" (HM) recovery powders, with the view to turn these hazardous by-products of the HM manufacturing process in valued secondary materials to be directly re-employed for industrial purposes. An overview on HM, their peculiar properties as well as the criticalities related to their main constituents, W and Co, is provided in Chapter 1, besides the green chemistry-based approach this thesis would like to follow for pursuing the scope of a sustainable and profitable circular economy model in HM manufacturing, through the identification of sustainable leaching systems (PART I) and the design of sustainable processes for industrial application (PART II). Bio-derived organic acids for hydrometallurgy are described in Chapter 2. They are classified on the bases of the oxidizing specie involved in leaching process (H+ or O2) and tested on Co-metal powder as well as on the HM test specimens. Several of them demonstrated high leaching efficiency and selectivity in Co-dissolution from WC-Co powders, preserving WC for following application. A selection of acids working in non-water solvents like alcohols were studied in Chapter 3, maleic acid ethanolic solutions showing to be the most interesting for solvometallurgy. Chapter 4 investigates soft- and hard-donor ligand-based solvometallurgy in the presence of iodine as external, recyclable and safe oxidizing agent. S-donor organic ligand/I2 systems demonstrated to be the most powerful with Co-leaching times significantly shorter than those found in acidic-solvometallurgy. Very promising was lactate/I2 system that coupled high effectiveness and sustainability being based on the use of a low cost, biodegradable and bio-derived ligand. On the bases of the obtained results of PART I, PART II - Chapter 5 reports the scale-up of lactic, succinic and maleic acid-based leaching processes as well as the preliminary results on materials recovery for applicative HM production. Besides the confirmation of leaching efficiency and selectivity towards Co, the chapter points out the high quality of the recovered materials, prone to be used for industrial purposes. Supported by the successful experimentation, attempts of designing sustainable and profitable circular economy models in HM manufacturing, based on the use of dairy waste as source of lactic acid both for hydro- and solvo-metallurgical treatments, are proposed and discussed in Chapter 6 in the light of the idea that the greater the sustainability the greater the profitability of the model. The overall conclusions of the thesis are finally discussed in Chapter 7.