Development of a hydrometallurgical route for the production of highly pure indium.

Indium is essential for many electronic applications, e.g. photovoltaics and laptops. Due to the increasing demand for indium in high-tech applications and China’s dominance in the indium production, this metal is labelled as a critical raw material by the European Commission. To keep pace with the increasing demand, efficient industrial processes for the recovery of indium from ore-processing by-products and end-of-life consumer goods must be developed.

The Umicore Precious Metals Refining business unit at Hoboken (Belgium) produced 4N5 indium metal. This recycling process was quite energy- and time-consuming, had a low efficiency and yielded indium metal with fluctuating purity levels. Therefore, the production process is stopped halfway between the raw materials and pure indium metal, manufacturing an intermediate crude indium(III) hydroxide (In(OH)3, containing 80–90% indium). Given Umicore’s drive towards sustainable development, the aim of this PhD thesis is to design an alternative sustainable indium refinery process for the production of high-purity indium (5N) starting from crude In(OH)3. This will be done by developing several new hydrometallurgical unit processes and combining them into a new process flowsheet. With the aim of developing a sustainable refining process, ionic liquids are taken into account.

This PhD thesis shows how ionic liquids can be used as green alternative solvents to replace the conventional aqueous or organic solvents in the leaching, solvent extraction and electrowinning stages. In a first approach, the extraction of indium from chloride media by the commercially available ionic liquids Cyphos IL 101 and Aliquat 336 is presented. High percentages extraction, high loadings and fast kinetics make these solvent extraction systems very suitable for extraction of indium. Indium was recovered as In(OH)3 by precipitation stripping with NaOH, regenerating the ionic liquid at the same time. Moreover, the extraction process was selective for In(III), over many other metal ions (As(III), Mn(II), Ni(II), Cu(II)) that are commonly found as impurities in process solutions of indium refineries. Cd(II), Fe(III), Pb(II), Sn(IV) and Zn(II) were co-extracted to the ionic liquid phase. Speciation of indium complexes in the aqueous and ionic liquid phase can provide more insight in the extraction mechanism and allows proper tuning of conditions and selection of extractants. In an aqueous HCl solution (0–12 M), indium(III) exists as mixed octahedral complexes, [In(H2O)6–nCln]3–n (0 ≤ n ≤ 6), while in the ionic liquid phase indium(III) is present as the tetrahedral [InCl4]– unaffected by the HCl concentration in the aqueous phase. An extraction mechanism was proposed based on the speciation studies in which indium(III) can be extracted as a neutral complex, In(H2O)3Cl3.

In a second approach, a combined leaching/extraction system was proposed for the selective recovery of indium from crude In(OH)3 based on the thermomorphic and acidic properties of the ionic liquid [Hbet][Tf2N]. Efficient indium leaching was obtained by a 1:1 wt/wt [Hbet][Tf2N]–H2O mixture. The formation of a biphasic system induced metal separation where In(III) is extracted to the ionic liquid phase, whereas Al(III), Ca(II), Cd(II), Ni(II) and Zn(II) remain in the aqueous phase. Fe(III), As(V) and Pb(II) are co-extracted to the ionic liquid phase. Iron remained in the aqueous phase by addition of ascorbic acid to the aqueous phase, thereby reducing Fe(III) to Fe(II). A HCl solution was used to strip indium(III) to the aqueous phase, regenerating at the same time the ionic liquid. By combining a prehydrolysis and hydrolysis step on the aqueous phase obtained after stripping, the purity of the crude In(OH)3 was improved.

In a final approach, Cyphos IL 101 was used as an electrolyte for the recovery of indium by electrodeposition at elevated temperatures. Indium is electrochemically reduced from In(III) to In(I) and subsequently from In(I) to In(0). Droplet-like deposits were observed between 100 and 180 °C, but their origin is not clear yet: melting-point depression of very small primary indium particles in combination with undercooling or dewetting. Also, droplet-on-droplet deposition took place indicating that there is a surface indium oxide layer present preventing the droplets to coalesce. Moreover, the electrowinning process was selective for In(III) over Zn(II). High-temperature electrowinning requires thermally stable electrolytes. The thermal stability of Cyphos IL 101 was investigated, both by dynamic and static TGA. Dynamic TGA overestimated the real thermal stability, while addition of metal chlorides to the ionic liquid increased the thermal stability. It was shown that Cyphos IL 101 had a long-term thermal stability at 180 °C in an inert atmosphere.