Rare earth minerals (REMs) contain rare earth elements (REEs) that are important in modern technologies due to their unique magnetic, phosphorescent, and catalytic properties. However, REMs are not only non-renewable resources but also non-uniformly distributed on the Earth’s crust, so the processing of REE-bearing secondary resources via recycling is one potential route to ensure the long-term sustainability of REE supply. Flotation—a method that separates materials based on differences in their surface wettability—is a process applied for both mineral processing and recycling of REEs, especially when the particles are fine and/or a high-purity product is required. In this review, studies about rare earth flotation from 2012 to 2021 were systematically reviewed using the PRISMA guideline. It was found that most REM flotation research works focused on finding better collectors and depressants while, for recycling, studies on advanced flotation techniques like froth flotation, ion flotation, solvent sublation, electroflotation, and adsorbing colloid flotation with an emphasis on the recovery of dissolved REEs from aqueous solutions dominated.

Huai Yuak area is located at Ban Kaeng Subdistrict, Sisatchanalai District, Sukhothai Province, Northern Thailand. The occurrences of gold are distributed along the Sukhothai Fold Belt from Phrae to Lampang though Sukhothai provinces. The Huai Yuak area is a one of the primary gold deposits in the Sukhothai Fold Belt. The geology of the Huai Yuak area is comprised of siltstone, sandstone, shale and tuff from the Permian to Triassic age. Quartz veins of the deposit were formed in siltstone, sandstone and shale. Based on the cross-cutting relationship and mineral assemblages, the quartz veins can be divided into three stages. Quartz veins of stage I are composed of a large amount of microcrystalline quartz with a minor amount of arsenopyrite and a trace amount of calcite, illite, pyrite, chalcopyrite, galena and electrum. Quartz veins of stage II consist of mainly quartz. Quartz veins of stage III are comprised of a large amount of dolomite and a small amount of quartz with a trace amount of illite and arsenopyrite. Electrum is associated with arsenopyrite, galena, pyrite, chalcopyrite and sphalerite. The electrum was formed as inclusion and filled in fractures of arsenopyrite and pyrite. Based on structure, texture and occurrence, pyrite and arsenopyrite can be classified into four types. Type 1 pyrite and arsenopyrite were disseminated in sandstone, siltstone and shale. Type 1 is characterized by euhedral to subhedral shape with size ranging from 0.1 to 3.0 mm. Type 2 pyrite and arsenopyrite were formed in a fracture as veins in shale and metashale. Type 3 pyrite and arsenopyrite occur in quartz veins. Type 3 is associated with galena, chalcopyrite, sphalerite and electrum. The electrum in Type 3 at Huai Yuak deposit was formed as inclusions and native gold associated with pyrite, arsenopyrite, chalcopyrite, galena, sphalerite and quartz of stage III. The concentration of electrum is related to arsenic content in pyrite. Type 4 of pyrite and arsenopyrite was formed in shale rock. Pyrite and arsenopyrite gains at the Huai Yuak gold deposit have had developed under a different structure and physical condition from uplift during volcanic arc setting.