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Broekmans, MATM (editor) Proceedings, 10th International Congress for Applied Mineralogy (ICAM) 1-5 August 2011, TRONDHEIM, Norway ISBN-13: 978-82-7385-139-0
MINERALOGICAL STUDIES IN ASSISTING BENEFICIATION OF RARE EARTH ELEMENT MINERALS FROM CARBONATITE DEPOSITS
Wendy Thompson*, Annegret Lombard, Eunice Santiago, Ashma Singh
Mintek, Private Bag X3015, Randburg 2125, South Africa
Abstract Rare earth elements (REE) are either housed in exotic REE minerals that sporadically populate the host rock or they occur as ionic substitutions in the crystal structure of existing rock-forming minerals. The genesis of REEs in carbonatite rocks can result in intricate mineralogical textures between REE minerals, host rock minerals, and minerals formed by later processes. Upfront mineralogical assessment discloses these associations, in turn leading to correct metallurgical planning and providing early predictions in beneficiation testwork. The method recommended in this study utilizes X-ray diffraction (XRD), optical microscopy, and a conventional scanning electron microscope (SEM) with energy dispersive spectroscopy. The aim of the REE up-grading process is to provide the hydrometallurgist with a REE mineral concentrate where the total rare earth oxides (TREOs) exceed 40 mass%. Upgrading is based on firstly achieving the best liberation possible for all minerals and secondly using physical differences in minerals to concentrate the REE minerals. Keywords: REE, gangue, mineralogy, mineral processing, carbonatites 1 INTRODUCTION
Today, the exploration and evaluation of mineral deposits has the benefit of advanced techniques, the outcome of which is that deposits can be accurately mapped, modeled, delineated, and characterized in terms of grade, mineralogy, and geology. This information gives direction to the extraction metallurgist such that mineral recovery can be optimized and ultimately impact the production of metals.
The process of assessing whether economic minerals can be successfully beneficiated from ore should realistically encompass a multi-disciplinary approach utilizing expertise in all sectors of the minerals industry. Of significant importance is a comprehensive mineralogical and metallurgical laboratory scoping study that should be undertaken in conjunction with careful sampling and research. All of these should be undertaken in carefully prioritized steps to avoid unnecessary expenditure.
The extent of the mineralogy undertaken depends on a number of variables: the budget constraints, the extent to which previous mineralogical work has been undertaken, the genesis of the ore body, the secondary geological changes to the ore body, and the commodity being sought. If little is known about the mineralogy of the deposit, a preliminary study using X-ray diffraction (XRD), the optical microscope, and the conventional scanning electron microscope (SEM) can suffice. These methods identify all the minerals present, provide relative modal abundances, and describe the in-situ texture of minerals and their associations, which in turn better defines the planning of the metallurgical testwork programme.
This contribution provides mineralogical assessment for physical up-grading and downstream hydrometallurgical treatment of carbonatite REE ores. In order to convert rare earth element (REE) minerals
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into REE metals, the hydrometallurgist requires at least a 40-mass% total rare earth oxide (TREO) concentrate and this can only be achieved if the REE minerals are successfully separated from the gangue minerals. As the upgrading process utilizes differences in the physical properties of both the REE minerals and the host rock minerals, the importance of the gangue mineralogy is emphasized. 2 RARE EARTH ELEMENT MINERALS
The TREO grade obtained from analytical assays will define whether the REE deposit contains light rare earth elements (LREE) or heavy rare earth elements (HREE). LREE include all elements from La through to Eu and HREE include all elements from Gd through to Lu. The identification of the REE minerals is obtained from mineralogical studies, and some of the challenges with REE mineral characterization are as follows: REE minerals are usually in carbonate, oxide or phosphate form, never in elemental form, and in most cases REE minerals can contain several REEs within their chemical composition. The REE minerals contain enrichments or depletions of certain REEs in their composition, which create variations in the TREO chemical analyses. Discrete REE minerals invariably occur in accessory amounts within the host rock and they are frequently sporadically disseminated as micron sized grains or as radial accumulations of needle-shaped grains. Few instances record REE minerals concentrated in natural bands in the host rock. REEs can also occur as ionic substitutions within the crystal lattice of a variety of rock forming minerals rather than in discrete mineral form. This is because of similarities in ionic radii and oxidation states between REEs and other ions, most commonly Ca .
Although there are many minerals that contain REEs in their composition, almost all production of REEs has come from less than ten minerals, and in particular bastnsite, monazite, and xenotime show prominence above all others . Bastnsite ((Ce, La, Pr)CO3F )is considered the most economic REE mineral and contains approximately 70% REO in its structure. It is the major REE mineral found at Mountain Pass in California, Bayan Obo in China, and Brockman in Australia . The phosphate mineral monazite ((Ce, La, Nd, Th)PO4) contains 412% ThO2, 2030% Ce2O3, and 1040% La2O3 in its structure. Monazite is present in acidic igneous rocks and vein deposits, where it frequently occurs as micron sized equidimensional grains and is a common beach placer mineral. Xenotime (YPO4) is dominated by HREEs namely Y, Dy, Er, and Ho with a TREO content of approximately 6067%. The phosphate mineral apatite Ca5(PO4)3(F, Cl, OH) is a calcium fluorophosphate associated with carbonatite deposits and alkaline igneous rocks. Apatite is not a rare earth mineral but REEs substitute for similar sized Ca ions in the crystal lattice. Perovskite CaTiO3, zircon ZrSiO4, and pyrochlore (Na, Ca)2Nb2O6(OH, F) are rock forming minerals that commonly incorporate REE as substitutions into their crystal structure . 3 CARBONATITES AND REE FORMATION 3.1 Carbonatites
Carbonatites and alkaline igneous rocks are the common primary host rocks for REE deposits but isolated concentrations of REE minerals are known to occur in other igneous rocks. Many carbonatites and alkaline igneous rocks occur in the form of ring complexes or intrusions concentrically placed into the country rock in a pipe or funnel-shaped structure. Alkaline rocks contain significant amounts of sodium and potassium minerals such as feldspathoids, Na-rich pyroxenes, and amphiboles. Carbonatites are often simplistically described as mono-mineralic rocks as more than half the minerals present in these rocks consist of carbonate minerals. Classification of carbonatites considers the actual chemistry of the dominant carbonate minerals (e.g., calcium, magnesium or iron-rich carbonate minerals). Most carbonatite deposits occur in close proximity to alkaline igneous complexes, but are not always mineralogically linked to the alkaline rocks .
Many carbonatite deposits have been altered by metasomatic processes and hydrothermal solutions, which introduce ions such as strontium, lanthanum, barium, fluorine, phosphorous, niobium, uranium, and thorium into the system and result in the formation of new minerals. NaK rich fluids permeate surrounding country rocks in a process known as fenitization, and this process results in the formation of potassium-rich feldspars and sodium rich pyroxenes and amphiboles, which frequently form in a concentric alteration zone around a carbonatite intrusion. Large amounts of silica are released in the hydrothermal process . The minerals and mineralogical textures produced by these processes are varying and complicated. 3.2 REE mineralization in carbonatite rocks
The REE minerals rarely form during the primary crystallization of a carbonatite rock, yet one of the characteristic features of carbonatites and associated rocks is their enrichment of a range of elements that include REE . The REE minerals that occur in carbonatites are almost entirely LREE bastnsite, allanite, apatite, and monazite . Hydrothermal solutions, metasomatism, and metamorphism can all occur after magma crystallization, resulting in remobilization of REEs, and changes in the original mineralogy. The ore processes that bring about REE mineralization in carbonatite rocks vary both locally and regionally from deposit to deposit and two examples are given to illustrate this. Bastnsite and barite in calcite-rich ore at Mountain Pass occur in the form of coarse grained phenocrysts with an average diameter of 300 m in a carbonate matrix . These simply-locked mineral textures would likely result in fairly easy liberation and upgrading of REE minerals. Several other types of REE mineralization at Mountain Pass are documented  which are more complicated in texture and have a different paragenesis. The bastnsite-barite dolomite-rich ore contains relatively fine ~90-m sized bastnsite as a late stage mineral in ferroan dolomite; the highly mixed dolomite-calcite type ore contains microcrystalline monazite with secondary REE mineral replacement (e.g., synchisite after bastnsite) and occurs in secondary calcite veins. Other ore types at this deposit include monazite-rich carbonatites, small late-stage lenses of bastnsite- and calcite rich