## 1 Introduction
On one hand, they are materials that have already been extracted and concentrated in all elements except in those that were the objects of exploitation, which increases the relative percentage of the rest. On the other hand, they are already separated from their matrix structure and have undergone frequent milling and crushing, meaning that these energy-intensive stages have been previously performed. They are also materials that are accumulated in landfills and are consequently causing harm to the environment. Their use does not create an environmental problem but rather can be part of its solution. In the case of iron mining, an extra factor is also added--the majority of oxides exist in iron sources with similar structures to those of rare earths. These extremely abundant waste products are difficult to treat [60; 61]. A very interesting example of REE tailing ponds in the case of iron ore mining is taking place in the Bayan Obo Inner-Mongolia Mine (China). This polymetallic iron-niobium-REE deposit, in addition to other elements of interest [62], has been in operation since 1927, and the main activity of the mine has been the exploitation of iron ore that contains up to 6% rare earth oxides (REO) and 35% Fe. About 4-7% of the total REEs are disseminated in iron oxides, with 90% occurring as independent REE minerals [2]. The separation techniques of these REEs carried out in recent decades have been low-intensity magnetic separation and high-intensity flotation and magnetic separation [63; 64]. Although many studies have been developed into possible recovery techniques [65; 66; 67; 68], currently, the recovery of REEs in this exploitation is very inefficient. The deposit is mainly exploited for its iron ore, and only about 10% to 15% of the rare earth content is recovered; due to this low percentage, the average REE law of tailing has been increased from 6.8% to 8.85%, close to the grade of raw ore [63]. The rest are stored in tailing dams as a secondary resource, causing pollution and environmental risks. The generation of tailings from iron mining has been increasing in recent years, reaching an output of more than 500 million tons in 2015 [69] due to the recent growth of the steel industries that generate this type of waste during the beneficiation process of iron ore concentrate. The same problem that occurs in Bayan Obo is common in other mining facilities. Although a wide range of methods for iron recovery from the iron ore tailings have been developed [70; 71; 72; 73], in practice, these processes create new waste streams [70], making these inefficient. In addition to this, some authors have highlighted the low ratio of reuse of tailings, the poor knowledge of treatment technologies, the paucity of supporting data, and the low political and economic support in China's industry [71]. The aim of this work is to analyze the effects of iron ore beneficiation on the concentration of rare earths. For this purpose, three samples of iron tailings from different deposits and treated by different technologies were collected. Once its content in REE was known, the feasibility to use its tailings as a strategic reserve of rare earths was evaluated. The effects of the concentration can redistribute the generally low presence of REE in both fractions (ore/tailing), creating an enriched material that has been previously processed. Considering the distribution of iron ore mining and the size of the tailings, this approach would reduce the need for new mines, save energy from crushing, and avoid landfilling new tailings of REE beneficiation.