Automated SEM Mineral Liberation Analysis (MLA) with Generically Labelled EDX Spectra in the Mineral Processing of Rare Earth Element Ores

Abstract

Many rare earth element (REE) deposits have experienced multistage geological enrichment processes resulting in REE bearing mineral assemblages of considerable complexity and variability. Automated scanning electron microscopy (SEM) mineral liberation analysis of such REE ores is confronted by the difficult assignment of energy-dispersive X-ray (EDX) spectra to REE mineral names. To overcome and bypass this problem, a generic and reliable labelling of EDX reference spectra obtained from REE-bearing minerals based on their contents of Si, Ca, F and P in a bulk normalised analysis is proposed. The labelled spectra are then combined into groups of REE-P (~monazite), REE-Ca-Si-P (~britholite), REE-Ca-F (~synchysite) and REE-F (~bastnaesite, parisite, fluocerite). Mixed spectra with low counts for REE from minute REE mineral grains are combined into a separate group. This classification scheme is applied to automated SEM mineral liberation analysis (MLA) data from beneficiation products by comminution and multistage flotation of REE carbonatite ores. Mineral modes, mineral grain size distribution, mineral liberation, mineral locking and mineral grade versus recovery curves based on the analysis of >200,000 particles in a sample can be recognised and interpreted in virtual grain size fractions. The approach as proposed here will allow future process mineralogical studies of REE deposits to be robust and comparable. REE ores are not only marked by geological complexity, but REE ore mineralogy is also very complex, with different minerals having complex chemical and crystallographic properties. More than 200 REE minerals are known [9]. Actually, in the largest REE mineral deposit of Bayan Obo in Mongolia one can distinguish three ore types with Fe-REE-, dolomite-REE-and silicate-REE-ores. The economic REE minerals are bastnaesite and monazite [10]. Other important REE mineral deposits are associated with carbonatite intrusions, as Mountain Pass (USA) and Mount Weld (Australia). The ore minerals in these deposits are bastnaesite, allanite (part of the epidote mineral group), monazite, apatite and pyrochlor. In lateritic clay deposits, REE released during chemical weathering of igneous host rocks may occur adsorbed in clay minerals [11][12][13][14].The beneficiation of REE ores poses major technological challenges [15][16][17][18]. These may be understood-and then overcome-by applying modern and quantitative analytical methods that yield not only chemical but also mineralogical and microfabric data. Bulk chemical analysis of ore and processing products by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) will readily provide elemental concentrations, especially of the REE [19]. X-ray diffraction (XRD) analysis, in turn, allows the identification of minerals, but is afflicted with considerable uncertainty and error when mineral modal abundance is below~1 wt %. However, both analytical methods require powdered samples at grainsizes <2 µm and provide no tangible information on particle and min...

Fe-Al Phosphate Microcrystals in Pedogenic Goethite Pisoliths

“…Silicon drift energy dispersive X-ray detector (SDD) technology, with its high count rates, good energy resolution, and large effective area, permits characterizing minerals within seconds. Extended atomic databases included in the EDS software improve the identification and quantification of elements with overlapping peaks, due to the thin active layer which suppresses the Compton background [38][39][40].…”

Editorial for Special Issue “Applications of SEM Automated Mineralogy: From Ore Deposits over Processing to Secondary Resource Characterization”

“…The contribution of generically labelled EDS spectra in the mineral processing of rare earth element ores by Schulz et al [21] provides a possibility for how to relate the EDS spectra from the complex and variable REE mineral phases in an ore to mineral names. The labelling of the spectra obtained from REE-bearing minerals is based on their contents of Si, Ca, F and P in a bulk normalised analysis.…”

Towards Identification of Zircon Populations in Permo-Carboniferous Rhyolites of Central Europe: Insight from Automated SEM-Mineral Liberation Analyses

“…These EDX spectra have characteristic peaks at distinct positions in the keV scale and distinct relative cts/s, allowing identification of the major elements present and providing a semiquantitative indication of their concentrations. The SEM-MLA software (version MLA 3.1 by FEI) recognizes each mineral phase after its EDX spectrum by comparison to a reference set of EDX spectra from identified mineral phases with known compositions [7,8]. The mode of recognition is straightforward for phases larger than several microns, but this is not always the case for the rhyolitic matrix.…”

“…To achieve this, automated detection is helpful to obtain a representative catalogue of zircon populations. So far, automated Scanning Electron Microscope Mineral Liberation Analysis (SEM-MLA) has been applied in the industry for solving problems of ore mineral processing and extraction [7]. However, there are other assets of automated SEM methodology, including the detailed modal analysis of different rocks [8,9] or quantification of environmental weathering [10,11].…”