Functionalized nanoporous silicon for extraction of Sc from a leach solution

Production of scandium (Sc) is a complicated process largely because the Sc concentration in ores is typically low in comparison to other types of metals. Therefore, typical extraction processes, such as solvent extraction and precipitation, that are used for separation and purification of leach liquors are inefficient. Adsorption/desorption is a good alternative method for extraction, but it is challenging to prepare affordable adsorbents with high stability, good selectivity, and high adsorption capacity. Most Sc sources are currently unexploited, and new, efficient, and environmentally sustainable methods for Sc extraction are needed. In the present study, bisphosphonates (BPs) immobilized on the surface of nanoporous silicon were used as metal ion chelators. Thermal carbonization was used to passivate the porous silicon surfaces with silicon carbide, on which the BP molecules were grafted. The BP grafted thermally carbonized porous silicon (BP-TCPSi) was used to extract Sc from a leach solution of Sc ore obtained from Rautalampi, Finland. First, Sc was leached from the ore using sulfuric acid. An oxidative precipitation method was employed to reduce the concentrations of the interfering metal ions; this treatment decreased the Fe, Ti, Al, and Mn concentrations by 99 ± 2%, 100 ± 14%, 35 ± 1%, and 76 ± 2%, respectively, while only 12 ± 1% of Sc was lost. The selectivity of BP-TCPSi was studied in a flow-through setup to adsorb Sc from the treated leach solution; then, in a two-step desorption process, Sc was separated from the other metal ions with a separation factor of 440 ± 13. The reusability of BP-TCPSi was examined in a flow-through setup with 10 adsorption/desorption cycles using a treated leach solution showing good stability of the adsorbent. The developed method provides a fast and convenient way to extract, purify, and enrich Sc with total recovery of 69 ± 1% in one cycle of adsorption/desorption of Sc from the leach solution with a low Sc concentration and a relatively high concentration of the other metal ions.