## 4 Conclusions  
The production of REEs is a problem identified by authorities of every developed country due to its high consumption and monopolistic character. Because of this, industrial waste can be an interesting source for the recovery of the REEs contained within it, given the scarcity of minerals that possess these elements.  
Upon analyzing the most suitable materials from iron mining, it was observed that after a flotation process, REEs were found in higher proportions in the ore than in the tailings. Considering that REEs do not appear in the composition of steel, as the major product of the steel industry, this indicates that their presence must be considered in some of the waste from the iron and steel process, such as slag or sludge.  
The rest of the tailings analyzed showed that, except in the process of magnetic separation, the tailings resulting from iron mining did not cause a concentration of rare earths; on the contrary, they presented an impoverishment regardless of their origin. Only those from magnetic separation or roasting produced enrichment.  
The analysis of the ore concentrate from the deposit showed that the enrichment moved from the ore to the tailings as the materials became heavier, thus the tailings produced over-concentrations of HREEs.  
In summary, it was observed that the REE contents in these tailings were, on average, 1/10 ppm below those of the minerals. Although this quantity may seem prohibitive, it must be considered that the tailings are already prepared directly for loading and concentration, while in the exploitation of minerals, the waste/ore ratio must be considered, which in many cases is less than 10%. It is also necessary to consider the grade of the mineral itself. Although it is true that the process of concentration is more complex, it is evident that there is a pre-concentration of certain interest as a strategic reserve. This, combined with the positive environmental viability and the avoidance of the ongoing cost of storage, makes them the largest source of contingent resources of REE, as in the last 10 years, this waste has accumulated an amount of more than three million REEs, equivalent to 2-3% of the world's proven reserves.  
In order to assess its viability, not only must economic and financial aspects be taken into account, but it would be interesting to assess the Life Cycle Analysis (LCA) of the operation of REEs from these wastes in order to ensure that the process is environmentally positive.  
For this evaluation, a study of the rare earth cycle would be carried out throughout the steel production process, from the first stage of extraction in the mine until the final product is obtained.  
In the different phases, thermal, physical, and chemical treatments are produced, causing enrichment and impoverishment of these elements in the streams of materials, including waste, in which it would be necessary to determine the best time for their extraction.  
The recovery of REEs from these wastes can be economically feasible if the need for a material as well as its strategic viability allows novel techniques to emerge. If these techniques focus on the recovery of a particular element, extraction would be profitable.  
The authors contributed equally.  
This work has been subsidized through the Plan of Science, Technology and Innovation of the Principality of Asturias (Ref: FC-GRUPIN-IDI/2018/000225).  
The authors declare no conflict of interest.