Beneficiation of Rare Earth Minerals from Niobec Ore: Froth Flotation
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Beneficiation of Rare Earth Minerals from Niobec Ore: Froth Flotation Eileen Ross L. Espiritu Doctor of Philosophy Department of Mining and Materials Engineering McGill University Montreal, Quebec May 2018 A thesis submitted to McGill University in partial fulfilment of the requirements of the degree of Doctor of Philosophy © Eileen Ross L. Espiritu 2018 Dedication To my beloved mom and siblings. Whose love and support made me where I am today. To my dearest best friend and partner in life, Kelvin. Whose affection, love and encouragements made it possible for me to finish this work. Abstract Rare earth elements are the key components of the emerging green technologies such as wind turbines, hybrid vehicles and storage batteries. The expected increase in demand for rare earths and the export quota restriction that was imposed by China (the largest producer of rare earths) in 2008, have alarmed the rest-of-the-world consumers of rare earth elements. By 2025, the following rare earth elements: lanthanum, neodymium, praseodymium, dys- prosium, and terbium, are likely to be in short supply. This global demand can only be met with constant development of new mines outside of China. A number of advanced research projects outside China can be found in Canada. Among the projects in Canada, Niobec rare earth project of Magris Resources Inc. is found to contain the largest resource of rare earth elements with 1058.6 million tonnes at 1.73% total rare earth oxide (TREO). This corre- sponds to 18 million tonnes of TREO, which is almost half of China's rare earth elements reserve (43 million tonnes TREO). Detailed characterization conducted through Quantita- tive Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) reveals that the main rare earth minerals in the ore sample were monazite (1.43%) and bastn¨asite/synchisite (1.96%), while the main gangue is dolomite (50%). The fine grain sizes (-20 µm) of the rare earth minerals makes froth flotation the most applicable beneficiation technique for separa- tion. However, literature regarding rare earth mineral flotation is very limited. To process the rare earth ore, fundamental properties of the pure minerals (monazite, bastn¨asiteand dolomite) are first understood through surface characterization (e.g., ATR- FTIR and XPS), thermodynamic calculations (speciation diagrams), electrophoretic mea- surements (i.e., zeta potential measurements) and single-mineral microflotation tests using three different rare earth mineral collectors: benzohydroxamate, sodium oleate and organic phosphoric acid. These fundamental investigations enabled further understanding of the min- eral { reagent interactions and offered possible collector adsorption mechanisms. Benzohy- droxamate proved to be the most selective, while sodium oleate and organic phosphoric acid demonstrated strong affinity to the gangue. Several dolomite depressants were then investi- gated in the presence of these two strong collectors. Polymeric-type depressants (especially ii carboxymethylcellulose) showed promising depressing ability towards dolomite. However, the performance of these depressants is better with sodium oleate than with organic phos- phoric acid, indicating a high influence of collector's chemistry on their depressing ability. Since single-mineral flotation does not consider the mineral's solubility, the effect of dissolved mineral species (in the presence of benzohydroxamate and sodium oleate) was also explored through the mentioned techniques, complemented with molecular modeling based on den- 2+ − sity functional theory (DFT). XPS analyses indicated that Ca , CaCO3/MgCO3 or HCO3 from dissolved dolomite could be present at the rare earth mineral surface, providing new sites for collector adsorption. DFT-based molecular modeling found useful in validating the experimental results, provided a deeper understanding of the mineral { collector interaction and presented possible conformation of adsorbed species or collectors on the surface of the minerals. The DFT simulations were also able to provide explanation to phenomena that could not be explained with experimental observations alone. The insights acquired through fundamental investigations and the knowledge of the mineral- ogy of the Niobec rare earth ore provided guidance in the processing of the ore. The observed concentration of rare earth elements in the fine (-20 µm) fraction led to a decision to split the processing of the ore into two routes. This does not only prevent overgrinding of the -20 µm particles, but also achieves a more efficient grinding of the +20 µm particles. Although the work focuses on the feasibility of using the specially designed column cell in processing the fines, the coarse (+20 µm) fraction is also processed using a conventional bench-scale flotation cell. It was presented that significant enrichment and recovery of the rare earth elements can be achieved even with only one stage of rougher flotation in both the column and the conventional cell, which was typically obtained through several stages of cleaning or scavenging stages. An improvement in the column cell parameters (physical aspect) and fur- ther investigations regarding the reagent scheme (chemistry aspect) are suggested for future work. iii R´esum´e Les terres rares sont les ´el´ements cl´esdes technologies vertes ´emergentes telles que les ´eoliennes, les v´ehicules hybrides et les batteries d'accumulateurs. L'augmentation atten- due de la demande en terres rares et la restriction des quotas d'exportation impos´eepar la Chine (le plus grand producteur de terres rares) en 2008 ont alarm´eles consommateurs du reste du monde d'´el´ements de terres rares. D'ici 2025, les ´el´ements des terres rares suivants: lanthane, n´eodyme, pras´eodyme, dysprosium et terbium, seront tr`esrares. Cette demande mondiale ne peut ^etresatisfaite qu'avec le d´eveloppement constant de nouvelles mines en dehors de la Chine. Un certain nombre de projets de recherche avanc´eeen dehors de la Chine peuvent ^etretrouv´esau Canada. Parmi les projets au Canada, le projet Niobec des terres rares de Magris Resources Inc. contient la plus grande ressource d'´el´ements de terres rares avec 1058,6 millions de tonnes `a1,73% d'oxyde de terres rares totaux (OTRT). Cela correspond `a18 millions de tonnes d'OTRT, soit pr`esde la moiti´ede la r´eserve d'´el´ements de terres rares de la Chine (43 millions de tonnes d'OTRT). La caract´erisation d´etaill´eer´ealis´ee par QEMSCAN r´ev`eleque les principaux min´erauxdes terres rares dans l'´echantillon sont la monazite (1,43%) et la bastn¨asite/ synchisite (1,96%), tandis que la gangue principale est la dolomite (50%). Les granulom´etriesfines (<20 µm) des min´erauxdes terres rares font de la flottation par mousse la technique d'enrichissement la plus applicable pour la s´eparation. Cependant, la litt´eratureconcernant la flottation des min´erauxdes terres rares est tr`eslimit´e. Pour traiter le minerai de terres rares, les propri´et´esfondamentales des min´eraux purs (monazite, bastn¨asiteet dolomite) sont d'abord comprises par la caract´erisationde sur- face (p.ex., ATR-FTIR et XPS), calculs thermodynamiques (diagrammes de sp´eciation), mesures ´electrophor´etiques(c.-`a-d.,mesures du potentiel z^eta)et test de microflottation mono-min´eraleen utilisant trois diff´erents collecteurs de min´erauxde terres rares, le ben- zohydroxamate, l'ol´eatede sodium et l'acide phosphorique organique. Ces investigations fondamentales ont permis de mieux comprendre les interactions min´eral{ r´eactifet ont offert des m´ecanismesd 'adsorption collecteurs possibles. Le benzohydroxamate s'est r´ev´el´e^etrele plus s´electif,tandis que l'ol´eate de sodium et l'acide phosphorique organique ont montr´eune iv forte affinit´epour la gangue. Plusieurs d´epresseursde dolomite ont ensuite ´et´e´etudi´esen pr´esencede ces deux collecteurs forts. Les d´epresseursde type polym´erique(en particulier la carboxym´ethylcellulose) pr´esentaient une capacit´ed´eprimante prometteuse vis-`a-visde la dolomite. Cependant, la performance de ces d´epresseursest meilleure avec l'ol´eatede sodium qu'avec l'acide phosphorique organique, ce qui indique une forte influence de la chimie du collecteur sur leur capacit´e`ad´eprimer. Puisque la flottation mono-min´eralene tient pas compte de la solubilit´edu min´eral,l’effet des esp`ecesmin´erales dissoutes (en pr´esencede benzohydroxamate et d'ol´eatede sodium) a ´egalement ´et´e´etudi´epar les techniques men- tionn´ees,compl´et´eespar une mod´elisationmol´eculairebas´esur la th´eoriefonctionnelle de la 2+ − densit´e(DFT). Les analyses XPS ont indiqu´eque Ca , CaCO3/MgCO3 ou HCO3 provenant de la dolomite dissoute pourraient ^etrepr´esents `ala surface des min´eraux des terres rares, fournissant de nouveaux sites pour l'adsorption des collecteurs. La mod´elisationmol´eculaire bas´eesur la DFT s'est av´er´eeutile pour valider les r´esultatsexp´erimentaux, a permis une compr´ehensionplus approfondie de l'interaction min´eraleet a montr´eune ´eventuelle confor- mation des esp`ecesadsorb´eesou des collecteurs `ala surface des min´eraux.Les simulations DFT ont ´egalement permis de fournir des explications sur des ph´enom`enesqui ne pouvaient pas ^etreexpliqu´espar des observations exp´erimentales uniquement. Les connaissances acquises gr^ace`ades ´etudesfondamentales et la connaissance de la min´eralogie du minerai de terres rares Niobec ont fourni des conseils pour le traitement du minerai. La concentration observ´eed'´el´ements des terres rares dans la fraction fine (-20 µm) du minerai a conduit