Are abandoned mine tailings a valuable resource for recovering Rare Earth Elements? Life Cycle Assessment and Cost Analysis

Rare earth elements (REEs) are considered relevant metals in the modern technology because they are key components of electric and electronic products. Moreover, the mining of these metals is often environmentally problematic, resulting in a current global production being controlled by a small number of countries. Looking for alternative REE resources, their potential recovery from abandoned tailings by means of phytoextraction using a new native hyperaccumulator plant, Spergularia rubra, was analyzed from the environmental and economic points of view in a real study case carried out in an old Pb/Zn mine area in Spain. Photogrammetry combined with GIS calculations, soil and plant sampling and chemical analysis showed that the studied mining site has an approximate quantity of 1.7 million tons of mine tailings, which could represent up to 168 tons of REEs. In addition, approximate phytoextraction yields of Spergularia rubra of 50 g and 150 g REEs per hectare were experimentally obtained, being those values strongly influenced by the low concentration of REEs in the tailings. Life Cycle Assessment (LCA) was applied to two proposed scenarios for REEs recovery through phytoextraction and incineration processes to obtain an ash relatively concentrated in REEs oxides. The first scenario involves the cultivation, irrigation and harvesting of the biomass, and the other one only relying on the harvesting of spontaneous biomass growth in wild conditions. The environmental impacts resulting from phytoextraction were mainly associated with agricultural maintenance and biomass harvesting tasks, especially irrigation in the non-wild cultivation scenario. Lastly, the cost assessment of both scenarios to recover REEs showed that their cost did not appear to be offset by the economic value that could represent the metals recovered by this technique (except for Sc due to its high price). Nevertheless, it is necessary to emphasize that the main objective of these studies is the promotion of circular economy process restoring polluted lands, reducing environmental risks and recovering mineral ores that contain valuable metals such as REEs, as a secondary and sustainable alternative to recover them.