592 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 bastnäsite, parisite, synchysite, and monazite. The material was subjected to a grinding unit with a roll diameter of 200 mm, aiming to achieve a particle size below 80 μm, suitable for direct flotation tests. Following the crushing and grinding stages, the samples were characterized in terms of particle size distribution, semiquantitative elemental chemical composition by X-ray fluorescence (XRF) using a portable X-MET7500 analyzer (Oxford Instruments), and gamma-emitting radionuclide activities (40K, 232Th, and 238U), measured with a GT-40 gamma spectrometer (Georadis). In addition, the rare earth element content of the sample was quantitatively determined by inductively coupled plasma mass spectrometry (ICP-MS). 2.2 Flotation tests Flotation experiments were conducted in two sequential stages, aiming to maximize the recovery of rare earth elements (REEs). The tests were performed using an adapted laboratory-scale flotation setup consisting of a 600 mL beaker-based flotation cell immersed in a water bath to maintain the pulp temperature at 50 °C, as illustrated in Figure 1. This configuration ensured adequate mineral surface activation and collector dissolution throughout the experiments. Agitation was maintained at 400 rpm, and the pulp pH was controlled under mildly alkaline conditions, at approximately 8.3. In the first flotation stage, 180 g of feed material was processed, corresponding to a solids concentration of approximately 25 wt.%. In the second stage, 100 g of the underflow material obtained from the first stage was used, resulting in a solids concentration of about 15 wt.%. This two-stage configuration was adopted to enhance REE recovery from both the initial feed and the partially depleted material. Hydroxamate-based collectors, specifically octanohydroxamic acid, were selected due to their higher selectivity towards REE-bearing minerals compared to other reagent classes, such as carboxylates and phosphoric acid esters (Kim, Choi and Lee, 2025). Quinic acid, phytic acid, sodium alginate, and citric acid were evaluated as depressants. Identical collector concentrations were applied across all tests, while the performance of the different depressants was comparatively assessed. Sodium chloride (NaCl) and sodium hydroxide (NaOH) were used for pH adjustment and control at approximately 8.3, which has been reported to favor flotation performance (Zhang et al., 2019).
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