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Limited natural resources and a continuous increase in the demand for modern technological products, is creating a demand and supply gap for rare earth elements (REEs) and Sc. There is therefore a need to adopt the sustainable approach of the circular economy system (CE). In this review, we defined six steps required to close the loop and recover REEs, using a holistic approach. Recent statistics on REEs and Sc demand and the number of waste generations are reported and studies on more environmentally friendly, economic, and/or efficient recovery processes are summarized. Pilot-scale recovery facilities are described for several types of secondary sources. Finally, we identify obstacles to closing the REE loop in a circular economy and the reasons why secondary sources are not preferred over primary sources. Briefly, recovery from secondary sources should be environmentally and economically friendly and of an acceptable standard concerning final product quality. However, current technologies for recovery from for secondary sources are limiting and technology needs will vary depending on the source type. The quality/purity of the recovered metals should be proven so that they do not result in any adverse effects on the product quality, when they are being used as secondary raw material. In addition, for industrial-scale facilities, process improvements are required that consider environmental conditions.

Rare earth elements or REEs are a vital and irreplaceable part of our modern technological and digital industries. Among the REEs that are the most critical to be recovered are Ce, La, and particularly, Nd, and Y, due to high demand and at a potential future supply risk. Innovative techniques must be considered to recover REEs from secondary resources. In this study, REEs are extracted from iron mining sludge from Central Anatolia in Turkey. Two different acid solutions were compared, one with a higher acid content (120 ml HCl and 80 ml HNO3 per liter) and one with lower acid content (20 ml HNO3 per liter). Nanofiltration, as a process to concentrate the acidic leachate and increase the REE concentration, was carried out at pH levels of 1.5, 2.5, and 3.5 and under 12, 18, and 24 bar operating pressures. SLM studies had been carried out using a PVDF membrane with a pore diameter of 0.45 mu m, with three different carriers to separate the REEs from other major elements in the concentrated leachate. Through this analysis, the optimum operating conditions for nanofiltration are at pH 3.5 at 12 bar, using the leach with low acidity, achieving about 90% recovery efficiency of the REEs. SLM studies using 0.3M D2EHPA, with a 3-h reaction time, showed the highest mass flux values for the REEs. Nanofiltration and SLM represent novel methods of REE concentration and extraction from iron mining sludge.