## 5 Conclusions  
Flortation technology is widely employed in mineral processing, particularly for the concentration of REMs, because of its high efficiency in separating particles based on their contrasting surface properties (hydrophilic and hydrophobic). The findings of this systematic review indicate that flotation technology as applied to REEs can be categorized into two distinct groups: (i) flotation technology for primary resources, and (ii) flotation technology applied to secondary resources. For the first group, flotation proves to be advantageous in the separation of REMs from associated gangue minerals, especially at finer liberation sizes. This technique also offers higher selectivity for REMs recovery than other separation methods and ensures that the products have higher grades. In the second group, conventional and advanced flotation techniques were employed to recover REE-bearing materials from secondary sources, such as wastewater generated by hydrometallurgical processes and other industries, including end-of-life products like phosphors in fluorescent lamps. The application of flotation technology in these secondary resources allows for the effective extraction and recovery of REEs, contributing to not only resource recycling but also environmental conservation. Recent research works highlight that in the flotation of primary resources, significant attention was given to the selection of flotation reagents, including collectors, brothers, and depressants. Among the various collectors used, three types are frequently employed: (i) carboxylate, (ii) hydroxamate, and (iii) organo-phosphoric acid. It was observed that, during flotation, hydroxamate and organo-phosphoric acid collectors exhibited higher affinities for REMs than carboxylate collectors. Hydroxamate, however, has the highest price among the three common collectors for REMs. Meanwhile, organo-phosphoric acid collector was the most sensitive in terms of flotation conditions and was reported to be effective only under acidic conditions. When economics is considered for the selection of a collector for REMs flotation, carboxylate is the best candidate, due to its lower cost compared with the other collector groups. Over the past decade, numerous studies have investigated the performance-enhancing effects of hydroxamate collectors. Additionally, alternative collectors, such as ionic liquids, sodium dodecyl sulphate, flotigam EDA, R845N, combinations of kerosene and fatty acids, and combinations of octanohydroxamic acid and sodium oleate, have been introduced to further improve flotation efficiency. The literature consistently demonstrates that the most effective approach to enhance flotation recovery and grade is through the depression of associated gangue minerals. Several depressants have been employed in REMs flotation. Notably, the combination of sodium silicate and starch was beneficial for depressing all types of gangue minerals, particularly silicate gangue minerals. Meanwhile, ammonium lignosulfonate was suitable for inhibiting iron-bearing minerals. Dissolved ions were also highlighted as having significant effects on REMs flotation. Extensive research works have elucidated the consumption of collectors when semi-soluble gangue minerals like calcite, fluorite, dolomite, and barite were present in the flotation pulp. The inevitable presence ofalkaline earth metal ions (e.g., Ca\({}^{2+}\), Mg\({}^{2+}\), Ba\({}^{2+}\), Sr\({}^{2+}\)) inhibited the adsorption of collectors on REM surfaces, resulting in lower floatability. In contrast, certain metal cations such as Pb\({}^{2+}\), La\({}^{3+}\), Fe\({}^{3+}\), and Al\({}^{3+}\) have been applied to enhance the floatability of REMs. These cations exhibited a strong affinity to activate REM surfaces by providing more active sites for collector adsorption. For secondary resources, flotation technologies for REE recovery can be classified into five main techniques: (i) froth flotation, (ii) ion flotation, (iii) solvent sublation, (iv) electroflotation, and (v) absorbing colloid flotation.