sustainable supply chains and the development of efficient mineral processing technologies (Akyildiz et al., 2026). The Fen Deposit, located in Norway, has emerged as a promising European source of REEs. The deposit is hosted in a complex carbonatite system, with the ferruginous dolomite carbonatite (FDC) unit containing REE-bearing minerals such as bastnäsite, parisite, synchysite, and monazite (Silva et al., 2023). However, the beneficiation of this ore is challenging due to its complex mineralogy and the presence of calcium-bearing gangue minerals. In this context, achieving selective separation between REE-bearing minerals and associated gangue phases—particularly calcite and fluorite—remains a major challenge due to their similar surface chemistry and flotation behavior (Zhang and Honaker, 2017; Kim, Choi and Lee, 2025). The investigation and development of selective depressants are therefore essential to suppress gangue flotation and optimize REE recovery. In parallel, the identification of sustainable and biodegradable depressants is of fundamental importance to reduce the environmental footprint of mineral processing and to support the development of responsible and low-impact beneficiation routes. Sodium silicate (Na₂SiO₃) is widely used as a depressant for silicate minerals; however, its application in REE flotation may inadvertently depress bastnäsite, particularly in the presence of dissolved Ca²⁺ ions (Kim, Choi e Lee, 2025). Consequently, alternative and more selective depressants have been investigated. For instance, xanthan gum has been shown to selectively depress calcite due to its stronger interaction with calcium ions compared to REE ions (Wang et al., 2020). Similarly, sodium alginate (SA), a non-toxic and biodegradable polymer, has demonstrated high efficiency in depressing fluorite when used in combination with hydroxamate collectors (Guo et al., 2024; Kim, Choi and Lee, 2025). Phytic acid (PA) is another environmentally friendly reagent with low toxicity and high availability, has been successfully applied as a depressant in the flotation separation of scheelite and fluorite (Zhang et al., 2019). In contrast, no studies to date have reported the use of quinic acid (QA) as a depressant in rare earth flotation systems. Citric acid (CA), a water-soluble organic acid with strong metal-complexing capability, is also non-toxic and readily biodegradable, and has been reported as an efficient and selective depressant in mineral flotation systems (Dong et al., 2021). Within this context, this study—conducted under the Horizon Europe project REESource—aims to evaluate the performance of environmentally friendly depressants, namely quinic acid, phytic acid, sodium alginate, and citric acid, in the flotation of REEs from the Fen Deposit. The objective is to assess their effectiveness in improving gangue rejection and enhancing the selectivity of the flotation process. 2. MATERIALS AND METHODS 2.1 Samples preparation and characterization The samples used as feed material were originated from the Fen Deposit, which is classified as an iron–dolomite–carbonatite ore hosting rare earth element (REE)–bearing minerals, including
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