Broekmans, MATM (editor) Proceedings, 10th International Congress for Applied Mineralogy (ICAM) 1-5 August 2011, TRONDHEIM, Norway ISBN-13: 978-82-7385-139-0
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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
*Correspondence to: [email protected]
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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 [1].
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 bastnäsite, monazite, and xenotime show prominence above all others [2]. Bastnäsite ((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 [1]. The phosphate mineral monazite ((Ce, La, Nd, Th)PO4) contains 4–12% ThO2, 20–30% Ce2O3, and 10–40% 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 60–67%. 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 [1]. 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 [3].
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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. Na–K 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 [4]. 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 [3]. The REE minerals that occur in carbonatites are almost entirely LREE bastnäsite, allanite, apatite, and monazite [1]. 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. Bastnäsite and barite in calcite-rich ore at Mountain Pass occur in the form of coarse grained phenocrysts with an average diameter of 300 µm in a carbonate matrix [5]. 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 [5] which are more complicated in texture and have a different paragenesis. The bastnäsite-barite dolomite-rich ore contains relatively fine ~90-µm sized bastnäsite 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 bastnäsite) and occurs in secondary calcite veins. Other ore types at this deposit include monazite-rich carbonatites, small late-stage lenses of bastnäsite- and calcite rich carbonatite, and a single thin sheet-like intrusion of parisite with barite, which is devoid of bastnäsite. Africa and in particular, the East African Rift Valley is rich in carbonatites, some of which contain high REE values [6]. At the Kangankunde Hill deposit in Malawi, the carbonatite is described as being peppered with finely disseminated monazite and strontianite and the entire assemblage is transected by crystal lined cavities and lenses in which coarse radiating hexagonal crystals are pseudomorphed by a mixture of strontianite, monazite, barite, and quartz [7]. 3.3 Metallurgical implications The primary mineral associations, late-stage ore processes and the resultant textures will have a direct effect on the physical up-grading of the ore during mineral processing. Infillings of fractures and vugs with exotic combinations of REE minerals, barite, strontianite, calcite, and quartz will not only create an inhomogeneous feed sample but liberation of the individual minerals will vary and be problematic. If fine-grained masking of the original rock texture occurs as a result of metasomatism (e.g., silicification, dolomitization) the process is further complicated. One of the methods used in the physical upgrading process of these ores is to exploit differences in density. Unless the individual minerals are liberated, particles will contain fine intergrowths of many minerals, and therefore the density of each and every coarsely crushed particle will differ. Dense media separation (DMS) is evaluated in scoping tests using heavy liquid separation (HLS) on a +1-mm feed size fraction whereas shaking table testwork is conducted on −1-mm feed size fraction. These methods target the lighter gangue minerals and in the case of carbonatite ores, removal of the
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carbonate is considered, as it is an acid consumer for the downstream leach application. The slurry material (100% −38 µm) is usually tested using a Falcon gravity concentrator. Flotation is then an option as a second upgrading step. 4 MATERIALS AND METHODS 4.1 Sample receipt and preparation For mineralogical and metallurgical scoping tests on carbonatite-hosted REE deposits approximately 100 kg of representative REE material at 15-mm top size is required. The material is weighed and stage crushed to 100% passing 6 mm. The −6-mm material is blended and split into two representative sub-samples weighing 60 kg and 40 kg. The 60 kg −6-mm sub-sample is further stage crushed to 100% −1.7 mm and split into several sub-samples of different weights in preparation for the scoping tests. Representative sub-samples are bagged for the following series of steps in the testwork programme: mineralogy, chemical head analysis, assay-by-size, Falcon gravity testwork, shaking table testwork, flotation analyses, and hydrometallurgical testwork. The second −6-mm sample is screened at 1 mm to generate a −6+1-mm sample, chemically analyzed and the remainder is retained for any HLS testwork. The −1-mm sample is representatively split and chemically analyzed. Ideally the mineralogical investigation should precede all other tests. 4.2 Mineralogy sample preparation The −1.7-mm feed sub-sample is dried at room temperature and prepared into a representative four sub-samples using a sample splitter. The first of these sub-samples is pulverized for XRD analysis using a Siemens D500 diffractometer with a step size of 0.02° 2θ (and a counting time of 1 s per step, applied over a range of 5–80° 2θ. The second sub-sample is prepared into polished sections for mineral and texture characterization using a Zeiss Evo MA 15 SEM with a Bruker detector for energy dispersive X-ray spectrometry (EDS). The third sub-sample is bottle- mounted and prepared into thin sections for petrographic studies.The last mineralogical sub-sample is stored for reference purposes. 5 RESULTS 5.1 Gangue mineralogy In carbonatite deposits with economic reserves of REE minerals, the characterization of the gangue is a vital part of the mineralogical investigation. In an example case study, the bulk mineralogy analysis using XRD identified major dolomite and quartz, subordinate calcite, strontianite, bastnäsite, and barite and accessory monazite, apatite, calcite, clay, and fluorite. The petrographic analysis of −1.7-mm particles, in transmitted light, showed that liberated dolomite occurs in minor amounts and the majority of particles consist of fine intergrowths of all minerals identified. Figure 1 shows a magnified carbonatite particle <1 mm in length with acicular shards of bastnäsite in a fine fabric with the carbonatite host rock; the particle is entirely silicified with an overprint of fine quartz. Significant liberation even with very fine milling is unlikely. The backscatter images in Figures 2 and 3 illustrate typical intergrowth textures of minerals such as bastnäsite, parisite, and barite. Early predictions immediately indicate to the metallurgist that liberation of REE minerals is not likely to be easy. 5.2 REE mineralogy
Mineralogical studies undertaken at Mintek from various carbonatite ores concur with other research, that LREEs dominate in carbonatites, occurring in minerals such as bastnäsite, synchisite, and parisite with
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apatite and monazite also being fairly consistent [1]. The habit of bastnäsite in carbonatites appears to be acicular or needle-shaped forming either in radial accumulations or intricate cross-cutting grids within a variety of minerals. Where hydrothermal alteration is evident, late stage bastnäsite and synchisite occur as grain boundary infillings and coatings around calcite or dolomite. Exotic minerals such as strontianite, barite, are added to the assemblage by these processes, further complicating the texture. Where metasomatism occurs, secondary dolomitization , or alternatively fine grained anhedral quartz during silicification can occur. Silicification occurs either as pervasive flooding of fine quartz throughout the carbonatite host rock or as a crosscutting network of veinlets [5]. Monazite and xenotime occur most commonly in the form of microcrystalline, sporadic, isolated equidimensional crystals. Pseudomorphism of the primary calcium rich minerals (e.g., carbonates, apatite) results in a range of minerals that form the new inner structure leaving the outer crystal form intact as a home or casing. 5.3 Physical up-grading The mineralogical study predicted that upgrading of REE minerals at the two prepared grinds (−6+1 mm and −1.7 mm) would not succeed at achieving a 40-mass% TREO grade. As REE minerals are <20 µm in width, it seemed unlikely that the size by assay results would show any preferential upgrading of REE minerals and additionally it seemed highly likely that the −20-µm fraction would yield a significant mass of TREO which would be lost immediately. Metallurgical scoping testwork attempted to up-grade the ore by using differences in minerals density in various tests including HLS, shaking table testwork, and Falcon gravity testwork. The specific gravity of the various minerals is as follows: bastnäsite 4.97, synchisite 4.02, dolomite 2.9, quartz 2.6, barite 4.5, fluorite 3.2, and strontianite 3.78.The HLS testwork marginally rejected some of the acid consuming carbonate minerals to the floats at 3.1, but enhancement of LREE to the sinks fraction was disappointing. Shaking table and Falcon testwork showed no preferential upgrading of the REE-bearing minerals to the concentrate even at a very fine grind of 100% −38 µm. It was noted that the composite particles containing higher ratios of dolomite and quartz compared to REEs reported to the gravity tailings, causing high REE losses. 6 DISCUSSION Different rougher mineral upgrading techniques operate best either on +1-mm grind sizes or −1-mm grind sizes. These grind sizes appear too coarse in the upgrading of REE minerals from carbonatites. If REE minerals report equally to all size classes in size by assay tests, they show no preferential natural concentration within any narrow size range. The Mountain Pass deposit is unusual in that the main ore body of REE minerals appear to have crystallised concurrently with the carbonatite magma, and REE minerals are therefore “coarse” enough within this carbonatite to be physically concentrated [5]. Where REE minerals have formed from processes such as metasomatism, hydrothermal alteration, and ionic substitution, the grain size of the REE minerals is likely to be fine (~20 µm) and secondary intricate textures and associations are likely to form between REE minerals, the new minerals that have formed, and the primary carbonate minerals. These factors will have a direct impact on the upgrading testwork results. 7 CONCLUSIONS REE occur in different modes of occurrence, even if similar host rocks are considered. This is dependent on post-crystallization of hydrothermal or metasomatic processes that cause widespread remobilization and recrystallization within the parent rock. An understanding of the impact of these processes on the textural features in the rocks allows assessment of suitability for up-grading and downstream processing. The results of the case
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study presented here show the importance of upfront mineralogical assessment in determining whether potentially viable grades of REE are metallurgically feasible for extraction. 8 REFERENCES [1] Gupta, CK, and Krishnamurthy, N (2005): Extractive Metallurgy of Rare Earths. CRC Press, London:
pp. 484. [2] Castor, SB, and Hedrick, JB (2006): Rare earth elements. In: Kogel, JE, Tivedi, NC, Barker, JM, and
Krukowski, ST (editors): Industrial Minerals and Rocks Commodities, Markets and Uses, 7th edition, Society for Mining, Metallurgy and Exploration, Littleton, Colorado: pp. 1555.
[3] Rankin, AH (2005): Carbonatite associated rare metal deposits: composition and evolution of ore-forming fluids-the fluid inclusion evidence. In: Linnen, RL, and Samson, IM (editors): Rare-Earth Geochemistry and Mineral Deposits, Geological Association of Canada, GAC Short Course Notes 17: 299–314.
[4] McKie, D (1962): Goyasite and florencite from two African carbonatites. Mineralogical Magazine (33): 281–297.
[5] Castor, SB (2008): The Mountain Pass rare-earth carbonatite and associated ultrapotassic rocks, California. The Canadian Mineralogist (46): 779–806.
[6] Woolley, AR, and Kjarsgaard, BA (2008): Carbonatite occurrences of the world: map and database. Geological Survey of Canada (Open file report 5796).
[7] Heinrich, EWm (1966): The Geology of Carbonatites, Rand McNally and Company, Chicago.
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Figure1A: Silicification of carbonatite particle containing REE mineral (plane polarized light).
Figure1B: Silicification of carbonatite particle containing REE mineral (crossed polars).
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Bastnasite
Barite
Carbonate
Parisite
10 µm
Figure 2: SEM BSE image of bastnäsite, barite, and parisite interstitial to carbonate grains.
Bastnasite
Parisite
Carbonate
20 µm
Figure 3: SEM BSE image of bastnäsite and parisite forming rims and radial aggregates
between calcite particles.
