Comportamiento de las tierras raras en los drenajes ácidos de mina de la Faja Pirítica Ibérica

Abstract

The exposure of sulfides (mainly pyrite) to the action of water, oxygen, and microorganisms generates an acid leachate with sulfates and metal(oid)es that can seriously affects surface and/or groundwater. The most propitious context for the generation of these leachates occurs in historical sulphides or coal mining areas where, because of poor or no waste management, a large number of tailing dumps, ponds, galleries, or pit lakes have been abandoned, among others, favoring the exposure of sulfides. In these cases, the process is known as Acid Mine Drainage (AMD), and it generates great concern worldwide due to its longevity, continuing to be active for hundreds and even thousands of years after anthropic activity ceases. For all this, the treatment of AMD is an environmental necessity, driven by European regulations, but it also represents an opportunity for valorization due to the high concentrations of elements of economic interest that they present. Among the most notable elements are rare earth elements (REE), a group of elements with growing importance, due to the large number of applications they present and the supply risk derived from a strongly monopolized market. In this sense, in recent years the strategy of searching for new sources and substitutes for these elements has been motivated. As mentioned, one of these secondary sources could be the AMD. In addition to the high concentrations of REE they contain, when their pattern is normalized to crustal values, they are typically enriched in some of the most valuable REE. However, despite the aforementioned importance of these elements, their origin in AMD is still unclear. In this framework, the main objective of this doctoral thesis is to analyze the possible sources of REE to the leachate. To achieve this objective, firstly, samples of such leachates were taken in the Iberian Pyrite Belt (IPB), a historical mining region located in the south-west of Spain and severely affected by AMD. From the samples of the different AMD discharges in the region and from temporal datasets of specific discharges, it has been possible to study the hydrogeochemical variability and REE patterns in the IPB, in addition to establishing certain relationships between the REE and other elements, characteristic of the host rock. In addition, it was possible to estimate the economic potential of AMD in the IPB (up to 4.2-10.3 M$/year), which, although it cannot compete with the magnitude of active mining, represents an interesting option due to the longevity of the generation of this waste and the environmental requirements for its treatment. On the other hand, once the variability in the AMD had been studied and the possible relationship of origin of the REE with the host rock observed, chemical and mineralogical studies were carried out in two mining areas representative of said variability: the Perrunal mine and the Poderosa mine. Acid leaching tests were developed to simulate the interaction under AMD generation conditions between said leachates and the different representative lithologies. The comparison of REE patterns together with Ce and Eu anomalies between these leachates and the original rock seems to establish the relationship between the origin of the REE and the leaching of volcanic rocks and shales from the surroundings of both mines. On the other hand, mineralogical studies have confirmed the existence of phosphate and carbonate minerals with high REE concentrations in the rocks of both study areas, in addition to the existence of evidence of preferential leaching of some of these minerals. Finally, to support this hypothesis, a combined study of Sr and Nd isotopes has been carried out on the rock samples, samples from leaching tests similar to those described above, and a representative selection of the AMD in the IPB. These isotopic data are very useful to study processes that control the mobility and source of REE in systems such as AMD. In this sense, these data agree with the hypothesis of the preferential dissolution of REE from minerals contained in different rocks of the IPB, such as shales and volcanic rocks.