AMDREY project

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 intermediate and valuable REY. Traditional AMD remediation 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 treatment sludge and thereafter investigate the efficiency of some chemical separation techniques. 

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 byproduct), 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. 

Recycling of mining and smelting residues 

• CSIC (Spain)
• Oy Chemec Ab (Finland)
• ICSM (France)
• UFS (South Africa)
• UHU (Spain) 

24 months (2016-2018)