## 3 Results and Discussion
In the last 10 years, approximately 23,590 million tons of iron have been extracted worldwide [42], thus there are about 8492 million tons of stored tailings available as a possible source of REE recovery [81]. Taking sample A.2 as a reference of tailing concentrations as the most plausible for commercial exploitation (Table 10), the total amount of REEs contained in roasting and magnetic separation tailings is estimated to be 3.14 million tons of REEs worldwide over 10 years (an average of over 300,000 tons per year). The estimation implies an amount much higher than the annual production (130,000 tons of REEs in 2017). Proven reserves of REEs in 2017 were 120,000,000 tons, therefore the amount of REOs deposited on these residues is equivalent to 2.62% of world reserves [42]. Since there are no significant differences in composition between the current ores and the residues covered by this study (beyond the percentages that do not alter the conditions of production of rare metals, which always appear to be associated with such oxidic matrices), there seem to be no technical barriers that restrain their exploitation. Therefore, the technical feasibility of the benefit of iron mining waste is guaranteed, but it is always tempered by the financial viability. \begin{table}
\begin{tabular}{c c c} \hline
**Element** & **A.2 (ppm)** & **Quantity (t)** \\ \hline La & 96.51 & 819,601.5 \\ Ce & 137.14 & 1,164,647.7 \\ Pr & 13.31 & 113,033.8 \\ Nd & 54.87 & 465,977.9 \\ Sm & 12.31 & 104,541.4 \\ Eu & 3.49 & 29,638.5 \\ Gd & 13.51 & 114,732.3 \\ Tb & 2.00 & 16,984.8 \\ Dy & 12.44 & 105,645.4 \\ Ho & 2.98 & 25,307.3 \\ Er & 9.45 & 80,253.2 \\ Tm & 1.48 & 12,568.7 \\ Yb & 9.13 & 77,535.6 \\ Lu & 1.19 & 10,105.9 \\ Total & 369.81 & 3,140,574.4 \\ \hline \end{tabular}
\end{table} 
Table 10: Calculation of the amount of REEs contained in roasting and magnetic separation tailings. In order to compare economic feasibilities, the cost of mine-opening as well as maintenance and operation must be added to costs related to the handling of the waste produced once the activity of the mine is established, such as storage fees, treatment processes, landfill inertialization, etc. On this basis, and contextualizing the analysis of the waste market, it appears that the chances of obtaining REEs as a by-product of the tailings would bring about a new, profitable business. These tailings ponds must be monitored and controlled for decades due to the high pollution levels they produce, considering the economic cost that this entails. Therefore, if there are more than three million tons of REEs among this debris, the prospect that it contains REEs gives new added value to the businesses that generate them. With these data, it is evident that waste is a great potential source that is generated continuously and offers enormous potential for exploitation. Therefore, it must be considered, especially in the future context of increases in emerging technology markets. As a result, the recovery of REEs would not only be technically feasible but would have a substantial positive environmental impact by helping to reduce the volume of waste and pollution associated with them. This recovery would apply the principles of the circular economy, evaluating the opportunities around tailing valorization [82], since materials and substances are recovered from the waste generated that turn it into a potential raw material [83].