Leaching characteristics of Sc-enriched, Fe-depleted acidic slags

Scandium is currently classified as a critical raw material for the European Union, and several research projects focus on the search for new sources to supply to its expected increasing demand. The Kiviniemi mafic intrusion in Finland is a potential primary source for Sc; at Kiviniemi, Sc occurs mainly within the lattices of ferrous silicates, clinopyroxene and amphibole. Some of the main challenges in leaching of Kiviniemi-type feed material are related to either the complexity of Fe and Ti separation from Sc-containing solutions or a possible gelation problem when leaching a material with a high SiO2 content. According to preliminary beneficiation tests, direct acid leaching of the Kiviniemi feed material was not successful mainly due to extensive leaching of Fe from the ferrous concentrate. Therefore, a processing scenario with conventional magnetic separation followed by py-rometallurgical reduction of the FeO component to tackle the Fe issue prior hydrometallurgical stage was studied in the previous parts of our project. Leaching experiments in this study focus on the possibilities to extract Sc from low-FeO, Sc2O3-enriched amorphous slag. Both a synthetic slag and slags produced from Kiviniemi con-centrates were submitted to H2SO4-based leaching experiments applying dry digestion, H2O2-assisted leaching, and high-pressure acid leaching (HPAL). The formation of silicic acid (gelation) under acidic conditions was avoided with all these methods. With H2O2-assisted leaching, efficiencies reached 40???50%, whereas leaching with dry digestion was very limited. With HPAL, the leaching efficiency remained at 30???45% under experimental conditions of 2.5 M H2SO4, 60 bar, and 150 ???C. These levels of leaching efficiencies are due to an amorphous structure and high SiO2 content of the slag. Future options for hydrometallurgical processing of Kiviniemi-type slags could include controlled cooling of the slag to produce crystalline structures, combined potentially with further chemical modification to enable selectivity for leaching.