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Project Topic
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Rare Earth Elements and Yttrium (REY) are Critical Raw Materials (CRM) which are essential for moderntechnologies. They are conventionally mined from igneous rocks, and they are now a priority exploration target worldwide. The ultimate objective of the proposal is the extraction of REY from Acid Mine Water (AMD). This is an unwanted pollution that is expected to flow out from coal and sulphide mines for hundreds of years. AMD hosts REY concentrations much higher than those in continental waters and oceans. AMD treatment systems produce low crystalinity iron and aluminium hydroxides. Preliminary surveys have shown that these solids retain practically all REY dissolved in AMD. Therefore, the outputs may provide a new and almost inexhaustible natural source of REY that is sustainable and beneficial to the environment. The objective of the proposal technological: to test the feasibility and the optimum conditions to recover REY from AMD. Two methods of AMD neutralisation and precipitation of REY are proposed: the addition of controlled doses of alkaline reactives for high flow rate discharges; and the passive infiltration through a limestone filter. Both procedures aim to obtain a sequence of precipitates where REY can be obtained selectively. The systems will be tested at laboratory and field scale in the Spain and in South Africa. These hydrated precipitates are then dried and concentrated into oxides ready to be transported for extraction. Finally, REY concentrate with high purity will be obtained by a multi-step extraction process consisting of leaching and chemical separation. From these solutions, advanced REY materials could be obtained (coordination polymers, self-assembled lanthanide-organic frameworks) showing interesting application (catalyst, photonic or magnetic properties). Also a novel method based on a recently developed recyclable porous material, called CH Collector, will be tested to directly collect REY from AMD without any precipitation step.
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Project Results
(after finalisation)
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Acid Mine Drainage (AMD) is commonly considered an environmental pollution issue. However, Rare Earth Elements (Scandium, Yttrium and Lanthanides) concentrations in AMD can be several orders of magnitude higher than in naturally occurring water bodies. With respect to shale standards, the REY distribution pattern in AMD is convex and enriched in intermediateandvaluableREY.TraditionalAMDremediation
systems are based on the reaction of AMD with a base, such as lime or limestone, generating a large amount of iron and aluminum-rich sludge. The main objective of the AMDREY project was to characterize and recover REE from the treatmentsludgeandthereafterinvestigatetheefficiencyof
some chemical separation techniques. Project AMDREY
Extraction of Rare Earth Elements from Acid Mine Drainage
A main outcome of AMDREY project is to link the 100% recovering of REE in the sludge of AMD treatment. Thus, in active neutralization plants with lime or sodium carbonate, precipitation of the iron oxyhydroxide schwertmannite occurs at pH below 4 with no REE scavenge. On the contrary, the equivalent aluminum mineral basaluminite precipitates as neutralization progresses to pH 6.5. Practically all REE are retained in basaluminite. The same behavior is observed in passive remediation systems consisting of AMD circulating through a permeable substrate of limestone. There, schwertmannite and basaluminite successive fronts precipitate as the AMD is neutralized by calcite. The accumulation of REE in the treatment wastes is especially relevant in the Phalaborwa Industrial Complex (PIC) in South Africa. There, the waste rock dumps are composed mainly of carbonatite-material, with high REE content. Therefore, this alkaline material becomes an attractive solution to not only neutralize the acid drainage, but also to enrich them in REE. The results of AMDREY have demonstrated that commercial calcite, commonly used in treatment systems can be replaced by carbonatite (mining by-
product), which will decrease costs dramatically. Further studies need to be performed in order to characterize the REE-enriched wastes and its feasibility as a marketable product. Two chemical separation processes have been evaluated based on different technological systems based on “green chemistry” rules. Liquid/liquid extraction using solvating agent such as TODGA is efficient to recover selectively REE (Nd=Y>Gd>La >> Fe, Al, Mg, Ca) but the main drawback is the need to increase the concentration of H2SO4 until 7.5M. With respect solid-liquid systems with no solvent, one of the synthesized resin seems to be an efficient candidate to the treatment of AMD with high extraction (>90%) and stripping (>80%) behavior. First attempts to biorecovery have also been tested by means of investigating the metal resistant mechanisms of two bacteria. Clostridium sp. was able to reduce and accumulate Eu2+ intracellularly, whereas Thermus scotoductus SA-01 accumulated Eu extracellularly using two different strategies. The results have significant implications for REE biorecovery, probably as nanoparticles for Clostridium sp. and as insoluble carbonates for T. scotoductus SA-01. Rather than extracting REE from wastes, another objective of AMDREY was to extract REE straightforward from the AMD. Thus, the work led by Chemec Oy used the vegetal-based adsorber CH Collector to remove scandium from AMD. A pilot plant mounted in the Tharsis area (SW Spain) removed up to 81% Sc from the acidic waters without any pH control. The main constraint of recovering REE from AMD is the low annual tonnage of the ore. Compiling all the acid drainage produced in an entire region, such as the Iberian Pyrite Belt, a total annual reserve of 70 to 100 t REY2O3, with an average rate of 0.23% REY2O3 is obtained. The rate is in the lower range of those compiled for currently working mines and prospects. The annual reserves are less than 0.05% of the world annual production. However, despite the low rates and annual production expected for the IPB, the natural processes that generate AMD are expected to continue for centuries or thousands of years. In this sense, the IPB could function as a giant heap leaching process of regional scale, in which rain and oxygen act as natural driving forces with no energy investment. Furthermore, as the main objective of AMD treatment is to remove acidity and pollutants, the benefits for water reserves and ecosystems are obvious. Therefore, recovery from AMD can be considered a small but environmentally friendly and renewable source of REE.
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