Dottorato in INGEGNERIA ELETTRICA, DEI MATERIALI E DELLE
NANOTECNOLOGIE

Titolo della tesi: Kinetics and thermodynamic study of the indium electrowinning process using different metal cathodic supports: operative conditions optimization and deposit characterization

The excessive indium demand of the existing markets for the manufacture of indium-based high-tech devices (solar panels, optoelectronic systems and LCD units) have promoted a shortage scenario of indium from primary supply routes. With emerging indium supply restrictions, the spotlight is on determining painful secondary sources and their respective efficient and eco-friendly treatment. Wet electrometallurgy, combined with separation/leaching /enrichment stages, has been highlighted as one of the most useful methodologies to recover indium from electronic waste (obsolete LCD screens) due to the operative parameter adaptation and high occupational and environmental safety. To date, chloride electrolytes have been widely used for indium electrowinning without considering critical challenges. In contrast, sulfate electrolyte shows lower environmental impact and higher efficiencies than conventional acidic solutions. This solution has been disregarded to carry out the indium electrowinning. In this doctoral investigation, the indium electrowinning process is studied from sulfate solutions considering the kinetic features of the indium reduction reaction near the cathodic surface. Also, it has been analyzed the influence of operative parameters on productivity and energy consumption. Initially, a critical outlook of the indium processing was performed considering different sources, electrolytes and methodologies to determine the main strengths, drawbacks, and targeting concerns that should be studied. Apart from this, the kinetic study of the indium reduction reaction near the cathodic surface was carried out using copper, titanium, aluminum, nickel and stainless-steel cathodes (Cu, Ti, Al, Ni and AISI 316L) as cathodic supports. These metal cathodes were assessed by cyclic voltammetry (CV) and chronoamperometry (CA) techniques. CVs described features of the indium cathodic curve and the HER influence. Since HER is a parasitic reaction, kinetic parameters, such as heterogeneous charge transfer rate constant (k0), charge transfer coefficient values (α), diffusion coefficient (D0), roughness factor (φ) and the exchange current density (i0), were calculated without HER interference. In the case of the i0, the values were equal to 1.20 mA/cm2, 0.30 mA/cm2 and 0.075 mA/cm2 for Cu cathode Ti and Al cathodes, respectively. Further correlations for previous parameters were held using both CV and CA. In fact, the k0 values for Cu, Ti and Al cathodes were 7.1·10-5 cm/s, 6.2·10-5 cm/s and 5.4·10-5 cm/s. Once indium reaction reduction kinetics near the cathodic support were analyzed, the indium electrowinning process was performed in long-lasting tests to achieve high and low energy consumption. In addition, operative conditions were optimized using five different metal supports. Therefore, the indium electrowinning was evaluated by varying operative conditions (current density, electrolyte composition, pH and temperature) using different metal supports (AISI 316L, Ni, Cu, Ti and Al). The optimization of the indium electrowinning process was focused on achieving high CE (Current Efficiency) and low SEC (Specific Energy Consumption). The performance of AISI 316L, Ni and Cu supports were significantly influenced by the electrolyte composition, while Al and Ti showed better results using merely the indium sulfate solution. Comparing AISI 316L, Ni and Cu support, nickel showed a significant increase in productivity (around 83% and 2.4 kWh/kg) working at 100 A/m2 with respect to AISI 316L and Cu supports, that can efficiently operate at 25 and 50 A/m2, respectively. Otherwise, Ti and Al depicted high CEs above 80% and reasonable SEC values using an etched cathode to improve the adhesion conditions between cathode and deposit. These electrowinning outputs for both cathodes demonstrated that the surface pretreatment improved the productivity in terms of high CE and low SEC for the indium recovery. The deposit showed very-well defined polygonal grains covered by a lamellar morphology. While in most cases, temperature increases negatively affect the crystallographic features of indium deposits.