Nd, Pr and Dy are critical raw materials as major components for rare earth permanent magnets (REPM). These are integral for several components placed for example within electric vehicles and wind turbine generators. REE primary production is mainly realized in China (~80%) and no REPM recycling industry has been established. Hydrometallurgical recycling routes lead to iron dissolution (66 wt. % Fe in REPM), while pyrometallurgical approaches that utilize SiO₂ risk contaminating the produced iron phase. A two-step process is presented that (i) creates an FeO_x-CaO-Al₂O₃-REE₂O₃ molten slag at 1500 °C through oxidative smelting and (ii) separates an iron-depleted slag phase (CaO-Al₂O₃-REE₂O₃) and a molten iron phase via carbothermic or metallothermic reduction at 1700–2000 °C. The slag has been designed as a selective collector phase and the REE₂O₃ loading within the bulk slag can reach up 25 wt. % REE₂O₃ at 1700 °C. The contained minerals within the slag exhibit >40 wt. % REE (a higher REE concentration than in the initial REPM). The resulting phases are characterized via ICP-OES, CS and SEM-EDX. In addition, the first results with regard to the downstream hydrometallurgical processing of the CaO-Al₂O₃-REE₂O₃ slag are presented aiming at the recovery of REE₂O₃, as well as of CaO and Al₂O₃. The latter compounds are to be reused during the first process step, i.e., the oxidative smelting of REPM. Slag leaching with methane sulfonic acid (MSA) and separation with alternative methods, such as solvent extraction, seems promising. Future work will include slag filtration with the aim to separate REE-rich solid phases (minerals) from the slag and also molten salt electrolysis of the produced REE₂O₃ oxides.

Hydrometallurgical extraction of metals is generally considered eco-friendly compared to pyrometallurgical routes due to lesser energy requirements and the absence of hazardous gaseous emissions. Nonetheless, the leaching of SnO₂ (Sn⁴⁺) is challenging, considering that cassiterite has been found to be rather resistant to chemicals. Theoretically, it would be easier to dissolve tin in acid if it is first converted to SnO (Sn²⁺) by controlled reduction. Moreover, hydrogen, when produced through the use of renewable energy sources, is a favored reductant for metal oxides due to the resulting low CO₂ emission footprint compared to the use of carbonaceous alternatives. The concept of partial hydrogen reduction followed by hydrometallurgical treatment with regard to cassiterite concentrate was explored in this research. Literature review and FactSage™ modeling indicated that tin monoxide (SnO) is most stable in the slag phase. In addition, numerical modeling was used to assess maximum gas flow injection during experimentation without excessive splashing. Experiments were carried out in this regard to achieve SnO in the slag by varying reductant amounts at different temperatures. Slags generated from these reduction experiments exhibited different performances during acid leaching. Modeling values showed reasonable agreement with experimental observations. Reduction with 1.5 g H₂/100 g of a tin concentrate provided a leachable slag (also with regard to Sn) without producing a metal phase. On the other hand, reduction of the same tin concentrate with 3 g H₂/100 g of concentrate at 1300 °C yielded 50.4% Sn recovery (> 80% of the equilibrium value) in the metallic phase as a nugget with 99.60 wt% purity. However, ca. 12.51 wt% Sn was measured in a glassy slag phase from which complete dissolution of tin occurred in an acidic solution. Several process routes, including one-stage hydrogen reduction of cassiterite concentrate to produce raw tin, were explored based on achieved results to propose sustainable extraction methods for tin concentrates. Graphical Abstract

Pilot testing of pyrometallurgical processes is inherently difficult, with shortcomings in pilot testing almost always present. Despite these challenges, pilot testing is often critical for the development of new processes and confirming the technical viability of projects before they move ahead. This is usually only performed on pyrometallurgical processes without significant precedence in industry, or on very specific materials where there are gaps or uncertainties in the metallurgical knowledge. As such, these tests are non-routine and unique for each material. This has led to the development of a wide range of test equipment, methods, and approaches. Within the industry, there are no professional guidelines or manuals on what piloting is required for each stage of a project. In addition, for scientific projects the “Technology Readiness Levels” (TRLs) are rather unspecific as they are applied on a wide field far extending typical pyrometallurgical applications. Glencore Technology has developed new processes and flowsheets through this piloting process, developing guidelines for use on projects. This paper will review the purposes of pyrometallurgical piloting and provide an outline of the work carried out during each type of test.