A closed-loop system to recycle rare earth elements from industrial sludge using green leaching agents and porous β-cyclodextrin polymer composite

Industrial sludge is classified as hazardous waste as well as a resource of valuable metals and needs to be adequately treated for the sake of the environment and economy. This study reports a green approach for extracting and purifying rare earth elements (REEs) from industrial sludge using environmentally friendly washing solutions ((NH4)2SO4, N, N-Bis(carboxymethyl) glutamic acid (GLDA), tetrabutylammonium bromide ionic liquid (TBAB), and water) and a porous β-cyclodextrin polymer composite (PCDP-M-SHM) respectively. The results showed that there is no significant difference between used extracting solutions. However, the speciation study indicated that most REEs in the sludge sample were bounded to a water-soluble fraction. Therefore, for economic reasons, water was used for further studies. Furthermore, the recycling efficiencies after the purification of leached REEs with PCDP-M-SHM were in the range of 76.0 % (Gd) to 87.3% (Pr), except for Ce (8.29%). Moreover, cost evaluation and sensitivity analysis studies were performed to investigate the applicability of the reported method at the industrial level. The cost evaluation results estimated $3676 as the cost of processing 1 ton of sludge with revenue of $710/ton of sludge. Finally, the sensitivity analysis test affirmed the profitability to depend on the cost of PCDP-M-SHM. Based on the future market price of REEs, the proposed closed-loop recycling approach could be a promising green solution for environmental problems related to mining, processing, and conservation of REEs resources as well as industrial sludge disposal.