Automated SEM Mineral Liberation Analysis (MLA) with Generically Labelled EDX Spectra in the Mineral Processing of Rare Earth Element Ores
Abstract
:1. Introduction
- REE-bearing minerals occur in many mineral classes, including REE-phosphates, REE-carbonates, REE-halogenides, REE-oxides, REE-silicates, REE-arsenates, as well as combinations of it as REE-fluoro-carbonates [24].
- Many REE minerals have a complex mineral chemistry with light REE (LREE, elements La to Eu), heavy REE (HREE, elements Gd to Lu), Th, U, Si, Al, Ca, F, CO3, PO4, Nb, Y, As, S and others.
- Most of the REE minerals are solid solutions with single and coupled substitution involving LREE, HREE, Y, Si, Al, Ca, F, P, Nb, Th, U and others. Compositional variations are widespread.
- Many REE minerals are hydrated phases. This considerably hinders their identification by chemical analysis.
- The often complex and multistage geological processes of REE enrichment lead to mineral intergrowths, pseudomorphs, partial and complete replacement, hydration and dehydration. This often results in a complex REE mineral assemblage.
2. Approach and Analytical Methods
3. Energy-Dispersive X-ray (EDX) Spectra of Rare Earth Element (REE)-Bearing Minerals
- During a first automated SEM-MLA measurement, all EDX spectra in a given sample are captured (XBSE-STD measurement mode). The discrimination of these spectra is provided by a high reliability value of 1℮−10 which means a high degree of conformance. The spectra that fall within this limit of conformance receive consecutive numbers (Mineral 1, Mineral 2, …). Dependent on the mineralogical diversity of the ore, ca. 100–150 different EDX spectra can be collected from REE-bearing and gangue minerals (e.g., carbonates and silicates). A certain fraction of these spectra will be from grain boundaries or artefacts of preparation effects. These can be ignored during further assessment by a tentative classification of the measurement.
- The MLA software functions allow driving the SEM stage to the mineral grains (e.g., Mineral 1, Mineral 2, …) where the consecutively numbered spectra were obtained for the first time during the measurement. A quantitative element analysis by EDS is performed from these grains. The EDX spectra from gangue minerals can then be labelled by corresponding mineral names (e.g., calcite, dolomite, fluorite, …). It is recommended labelling several slightly differing spectra from the same gangue minerals (e.g., calcite1, calcite-mix).
- The EDX spectra from REE-bearing mineral grains receive a generic label that matches the normalised results of EDS element analysis. An EDX spectrum from an REE-bearing mineral suggesting e.g., 3.9 wt % Si, 4.7 wt % Ca, 14.3 wt % F and 2.4 wt % P (when totals are normalised to 100) will be labelled as REE-04-05-14-02 (Figure 1g–i). The range of the labelled elements should be the same for all REE-bearing minerals in a particular study, e.g., REE-Si-Ca-F-P, to assure consistency and comparability. When P and F are purposefully positioned at the end of the label, this will facilitate the subsequent step of spectra grouping (see below). Due to the carbon coating and the analytical uncertainty related to peak interferences, the elements C and the REE are not used in this generic labelling process. In a similar manner EDX spectra from REE-bearing minerals with Y, Nb or further elements such as As can also be labelled. A similar approach has previously been applied to automated SEM-MLA measurements of sewage sludge ashes [29], soils [30] or to zoned metamorphic garnets [31].
- A labelling of the EDX spectra based on quantitative EDS analysis of the REE is not reasonable because the relative REE concentrations have only secondary relevance for the distinction of mineral classes. Also, a considerable analytical uncertainty is caused by the REE peak interferences, which could yield erroneous absolute concentrations of REE. Therefore, when the totals are normalised to 100 wt %, the analysis of Si, Ca, F and P will provide at best the relative proportions of these elements in a REE bearing mineral grain.
- EDX spectra from REE-bearing minerals with high content of P are summarised under the group REE-P-monazite. A typical mineral of this group would be monazite.
- EDX spectra from REE-bearing minerals with low content of P but elevated contents of Si and Ca are summarised as the REE-Ca-Si-P group. A typical mineral of this group would be britholite.
- The EDX spectra from REE-bearing minerals without P but high contents of Ca and F, and intermediate to low contents of Si are summarised as the REE-Ca-F group. A typical mineral of this group would be synchysite.
- The EDX spectra from REE-bearing minerals with dominant F at low Ca and Si concentrations are combined as the REE-F group. Typical minerals of this group are bastnaesite and parisite. The element carbon cannot be used for labelling due to the carbon-coating of the samples.
- EDX spectra from grains with detectable but low REE contents are merged to form the REE-Low-Mix group. In contrast to the groups 1–4 with high cts/s on the numerous lines of REE and corresponding elevated element contents, the REE-Low-Mix group integrates spectra with low counts on the REE lines and apparently low REE contents. Minerals containing low REE concentrations (for example allanite) can generate such spectra, but similar may be true for small acicular and fibrous crystals of REE minerals enclosed in gangue minerals. In the latter case, the excitation bulb of the electron beam captures gangue minerals beside and below, which will led to such mixed spectra, with low counts on the REE.
4. Case Studies
4.1. Case Study 1: Run-of-Mine Ore
4.2. Case Study 2: Comminution
4.3. Case Study 3: Flotation
5. Analytical Uncertainties
6. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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MLA-Group | Mineral | General Formula | Dens. | ∑REE | P | Ca | Si | C | F | O |
---|---|---|---|---|---|---|---|---|---|---|
REE-P-monazite | monazite | (LREE,Y,Th,Si,Ca)PO4 | 5.10 | 59.49 | 13.21 | 0.00 | 0.00 | 0.00 | 0.00 | 27.29 |
REE-Al-P-phases | florencite | (LREE)Al3(PO4)2(OH)6 | 3.58 | 27.31 | 12.07 | 0.00 | 0.00 | 0.00 | 0.00 | 43.66 |
REE-Ca-Si-P-phases | britholite | (LREE,Ca)5(SiO4,PO4)3(OH,F) | 4.45 | 46.44 | 2.00 | 14.70 | 9.40 | 0.00 | 0.50 | 26.85 |
REE-Ca-F-phases | synchysite | (LREE,Ca)(CO3)2F | 4.02 | 43.89 | 0.00 | 12.56 | 0.00 | 7.53 | 5.95 | 30.07 |
REE-F-phases | bastnaesite | (LREE)(CO3)F | 4.97 | 63.94 | 0.00 | 0.00 | 0.00 | 5.48 | 8.67 | 21.90 |
REE-F-phases | fluocerite | (LREE)F3 | 6.13 | 71.07 | 0.00 | 0.00 | 0.00 | 0.00 | 28.93 | 0.00 |
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Schulz, B.; Merker, G.; Gutzmer, J. Automated SEM Mineral Liberation Analysis (MLA) with Generically Labelled EDX Spectra in the Mineral Processing of Rare Earth Element Ores. Minerals 2019, 9, 527. https://doi.org/10.3390/min9090527
Schulz B, Merker G, Gutzmer J. Automated SEM Mineral Liberation Analysis (MLA) with Generically Labelled EDX Spectra in the Mineral Processing of Rare Earth Element Ores. Minerals. 2019; 9(9):527. https://doi.org/10.3390/min9090527
Chicago/Turabian StyleSchulz, Bernhard, Gerhard Merker, and Jens Gutzmer. 2019. "Automated SEM Mineral Liberation Analysis (MLA) with Generically Labelled EDX Spectra in the Mineral Processing of Rare Earth Element Ores" Minerals 9, no. 9: 527. https://doi.org/10.3390/min9090527