**4. Case Studies**

Beneficiation test work requires the investigation of comminution and separation characteristics of a given raw material. The first task should always be a thorough study of the unprocessed feed material, i.e. the ore itself in its pristine state [23]. Characterisation by SEM-based image analysis will provide important clues towards the development of a suitable beneficiation strategy. It will inform to the determination of the optimal grinding conditions and time. This is done to achieve a high degree of mineral liberation, whilst minimising the generation of fines. SEM-based image analysis data will provide important insight to assess the success of comminution. In the case of REE ores, comminution is often followed by a multi-stage flotation process [33]. The success of flotation test work can be monitored by SEM-based image analysis. Our case studies illustrate that the mineralogical properties, intergrowths, locking relationships and grain sizes of the REE minerals, as captured by automated mineralogy, are crucial in the critical assessment of the performance of beneficiation processes of fine grained and complex REE ores.

#### *4.1. Case Study 1: Run-of-Mine Ore*

The proper identification of the REE-bearing minerals, their mineral grain size and intergrowths is crucial to select suitable technologies and machine parameters for comminution and mineral separation. This is exemplified by the analysis of a polished thin section of a syenite from the Thor Lake Intrusion in Canada, Northwest Territories [34,35]. The sample has been taken from the mineralised T-Zone at the northern margin of the syenite intrusion. The abundance of gangue minerals such as fluorite and quartz in the sample attest to an intense hydrothermal overprint of the pluton. The SEM-MLA analysis (at 25 kV, 10 nA) has been performed in the GXMAP mode at 175 times magnification and with a greyscale trigger (25–255) that includes all minerals but excludes epoxy resin. The analysis took 15 h and is composed of 300 square frames of 1500 μm edge length, covering an area of 2.25 mm2. The analysed area is covered by 2,158,015 single EDX spectra, which means ~7,200 spectra per frame or 3,200 spectra per mm<sup>2</sup> with a stepsize of 18 × 18 μm. EDX spectra obtained from the REE-bearing minerals can be subdivided into 3 groups (Figure 2). Most abundant are the minerals of the REE-Ca-F group (resembling synchysite) with 3.58 area% (Figure 2b). REE-P-monazite is present, but of such low abundance that it has been ignored for the purpose of this study. A comparably large area proportion (4.16 area %) of the spectra related to the REE-Low-Mix group is obvious. This can be explained by the intense intergrowths of very fine-grained REE-bearing mineral grains with gangue minerals. Indeed, BSE images and GXMAPs sugges<sup>t</sup> that REE minerals are concentrated in 1 × 1.5 mm large aggregates composed of countless miniscule REE bearing mineral grains (Figure 2b). These minute REE mineral grains (0.2–5 μm) are tightly intergrown with phyllosilicates, feldspar and Fe-Ti-oxides. This leads to the conclusion that physical treatment of the ore for separation of REE mineral grains will require a very fine grind size, and that REE mineral grains—even if they are liberated—are unlikely to be recovered by a conventional flotation process properly.

**Figure 2.** (**a**) Backscattered electron image (BSE) of one frame (magnification 175 times) of an automated scanning electron microscopy-mineral liberation analysis (SEM-MLA) measurement of a polished thin section from a hydrothermally overprinted alkali plutonite. A complete measurement of the 25 × 40 mm sized thin section is composed of ~300 frames. The BSE image displays only a comparably low resolution due to technical reason. (**b**) Classified, grouped and color-coded presentation of the frame in (a) in an automated SEM-MLA measurement in the GXMAP routine; the modes in the mineral legend are in area percent and are related to the whole sample area. Stepsize is 18 μm. The images in (a) and (b) display fine-grained and heterogeneously composed parts and REE mineral grains in intimate intergrowth with phyllosilicates and Fe-oxides.

#### *4.2. Case Study 2: Comminution*

This case study concerns a carbonatite REE ore, with monazite as the most abundant REE mineral. Monazite is part of the REE-P-monazite group of EDX spectra with 3.5 wt % modal proportion (please note, in this case study we report wt %, different to the previous case study where data was reported as area %). All other groups of REE mineral spectra attain a total of only 0.7 wt % whilst the REE-Low-Mix group accounts for 0.5 wt %. The prevalent gangue minerals are dolomite (~70 wt %) and Fe-Mg carbonates (~13 wt %). Fluorite reaches ~7.0 wt % in abundance, whilst phyllosilicates, other silicates and quartz all together amount to a maximum of 2.5 wt % (Figure 3a).

Dry grinding experiments on the REE carbonatite ore were performed with a laboratory rodmill, starting with crushed (<2 mm particle size) feed material. For the determination of the optimal grinding fineness, two experiments at 45 min (sample M45) and at 90 min (sample M90) grinding time were conducted. The products (~10 g) were thoroughly mixed with an adequate amount (~10 g) of powdered graphite of pure and fine quality as a parting agent, and stirred into ~2 cm<sup>3</sup> of fast-hardening epoxy resin for the production of grain mount blocks of 30 mm in diameter [36,37]. The thickness of the epoxy layer containing sample grains is <5 mm to prevent severe gravity segregation effects. The horizontal block surfaces were polished after a thickness reduction of ~1 mm by grinding. MLA measurements in the XBSE analysis routine included 200,000 particles per sample which were examined within 3–4 h. The XBSE analysis routine is based on a single EDX spectrum within the barycentre of each mineral grain as identified by its grey colour in the BSE image. The cumulative bulk particle size distribution curves for both grinding tests display similar shapes. At 45 min grinding time the P50 (corresponds to cumulative 50 wt % of the distribution curve) is at ~19 μm, and at 90 min grinding time at ~16 μm. For the REE mineral monazite, the most important ore mineral in this case study, the corresponding grainsizes at P50 are 7.5 μm (M45) and 7.2 μm (M90), respectively. When compared at P50 the grainsizes of the carbonates are reduced from 20 to 17 μm, and those of the fluorite from 15 to 12 μm at the longer grinding time (not shown). This illustrates significant effects of selective comminution.

**Figure 3.** Results of comminution tests of REE carbonatite ore with 45 and 90 min length of time. (**a**) Modal mineralogy (in wt %, y-axis) of complete samples and of particle size classes by virtual sieving with the filter routine of equivalent circle (EC) diameter (see text). Proportion of the corresponding particle size class in wt % (x-axis). Dolomite (Dol); fluorite (Flu) and REE-P-monazite (Mnz) are labelled. (**b**) Mineral liberation of REE-P-monazite in terms of proportion in wt % of particle composition. Inset sketch displays the liberation class particle composition of 45 wt % REE-P-monazite in a schematic view. (**c**) Mineral liberation of REE-P-monazite in terms of proportion in contour% of free surface. Inset sketch displays the liberation class free surface of 50% REE-P-monazite in a schematic view. (**d**) Presentation of the intergrowths of non-liberated REE-P-monazite with fluorite (Flu) and dolomite (Dol). Proportions of fluorite and dolomite in wt % in the complete samples and in particle size fractions by virtual sieving with the filter routine EC diameter (see text).

With a longer grinding time one expects to achieve better mineral liberation of REE mineral grains. However, the problem of over-grinding of the REE mineral grains also increases with a longer grinding time. Over-grinding leads to a large proportion of very fine grains at <<10 μm that are known to usually float poorly [38], and will thus hamper separation by flotation [39]. Cumulative grain size distribution curves provide a first control of potential over-grinding. A subsequent sieve classification of the ground ore with subsequent study of further parameters (e.g., mode, mineral liberation, mineral locking) in the distinct grain size fractions would be of grea<sup>t</sup> interest [40]. At the given particle sizes, a mechanical sieve classification is only reliable at particle sizes >20 μm. An alternative is the virtual sieving by an electronic method. The shape classification parameter of the equal circle (EC) diameter turned out to give reasonable results for the 2-dimensional image, however, dependent on the overall particle shapes (e.g., rounded, cubic, platy, elongated, acicular, fibrous), distinct di fferences to the results of mechanical sieving are noted [23]. In the studied samples the rounded and cubic particles prevail. Virtual sieving of the XBSE data sets was performed in the particle size fractions 0–15 μm, 15–40 μm und 40–100 μm. As expected, increasing grinding time from 45 to 90 min resulted in a larger proportion of the smallest sieve size fraction from ~40 to 46 wt % (Figure 3a). It is obvious from the modal mineralogy that the REE mineral monazite is prominently enriched in the smallest sieve grain size fractions. In the smallest sieve size fraction 0–15 μm the mode of monazite slightly decreases with the longer grinding time while the mode of carbonates increases (Figure 3a). This is a consequence of the lower mechanical stability of the carbonates due to their cleavage planes, when compared to monazite.

The dataset of the XBSE measurement also allows the extraction of parameters of mineral liberation as (1) mineral liberation by particle composition, and (2) mineral liberation by free surface. For both parameters the particles are examined in liberation classes ranging from 0–100%. The liberation class 95–100% (fully liberated mineral grains) for mineral liberation by particle composition for the mineral group (REE-P-)monazite includes all particles that comprise of 95–100 wt % of (REE-P-)monazite. Correspondingly, the parameter mineral liberation by free surface, includes all particles with monazite where the (2D)-contour of the monazite grain is 95–100% free of inherent other mineral phases. The cumulative proportions of each liberation class in wt % are plotted along the Y axis as mass recovery (Figure 3b,c). For our case study, more than 90 wt % of the monazite of the grain size fraction 0–15 μm appear fully liberated. This is the best liberation among all (virtual) sieve grain size classes, as monazite is often locked by carbonates and fluorite in coarser particle size fractions (Figure 3d). Interestingly, the locking of monazite with fluorite is highest in the (virtual) sieve size fraction of 15–40 μm (Figure 3d). A longer comminution at 90 minutes resulted in no further improvement of monazite liberation in the size fraction 0–15 μm. For the larger sieve size fractions and the complete sample, the longer grinding time results in a moderate increase of the cumulative mass recovery of about 5% for the liberation class 95–100% (Figure 3b,c).

In the complete samples, the REE-P-monazite in the particles are often locked by carbonates (at 15–18 wt %), and with fluorite (at ~2 wt %, Figure 3d). With increasing sieve grain size fraction, the proportion of inherent carbonate minerals also increases, but with slightly lower values for the long comminution test at 90 min. At the locking of REE-P-monazite with fluorite the highest proportions are observed in the (virtual) sieve grain size fraction of 15–40 μm (Figure 3d). As a consequence of the results presented above, a multi-phase grinding process with only short periods of milling and intermittent classification has been established. This prevented the undesirable formation of fines and associated losses of REE-P-monazite. In addition, a sizing step by hydrocyclones was introduced to reject slimes (<5 μm particle size) prior to flotation.

#### *4.3. Case Study 3: Flotation*

The presented method for REE mineral classification was deployed in the evaluation of mineral processing tests for a further REE carbonatite ore. The studied samples were taken from multi-stage open cycle flotation tests (Figure 4). Previous to flotation, multi-stage comminution was performed with an interim classification step at 40 μm and recirculation of the >40 μm grain size fraction, followed by de-sliming with removal of the fraction <5 μm using a hydrocyclone. The de-slimed material was the feed to multi-step flotation including rougher flotation, scavenger flotation, two rougher-cleaner steps and one scavenger-cleaner flotation. Wet re-grinding was applied to the rougher concentrate prior to the cleaner stages, and also to the middling concentrate from the scavenger flotation prior to an additional scavenger-cleaner stage (Figure 4). This approach was chosen to accomplish a further improvement of the REE-liberation for boosting REE- and Y-recovery.

**Figure 4.** Flow scheme of multi-stage open cycle flotation tests with REE carbonatite ore, containing monazite and bastnaesite as principal REE minerals. Positions of analysed samples along the flow scheme are marked in yellow, flotation products which are not analysed are marked in grey.

Polished epoxy grain mounts with 30 mm in diameter were prepared from four flotation process samples (Figure 4) and analysed by automated MLA in the XBSE mode. Between 317,000 and 340,000 particles were analysed in each block during 4–5 h. Sample SPC2 represents the final concentrate after two cleaner flotation steps. Sample SPC1 is a middling concentrate produced by scavenger-cleaner flotation of a rougher-scavenger concentrate (following re-grinding). The sample BSC1 is a middling from the scavenger-cleaner stage that still contains liberated REE-mineral fines. The sample BS1 is the final tailing (Figure 4).

After EDX spectra classification and grouping, the REE-P-monazite and the REE-F-phases are the dominant groups among the REE-bearing minerals, and are denoted as monazite and bastnaesite for simplification (Figure 5a). The gangue minerals are the carbonate minerals dolomite, siderite and ankerite (regrouped as siderite-ankerite) but also apatite and quartz. The mineral grain sizes for monazite and bastnaesite in the pristine ore do not exceed 30 μm. In the cumulative grain size distribution curves the P50 values are 8 μm for monazite and 11 μm for bastnaesite. In all samples from this ore type, the P50 grain sizes for monazite are lower than those for bastnaesite (Figure 5b).

Virtual sieving based on the parameter equivalent circle (EC) diameter was performed at several sieve grain size classes. Coarse sieve grain size fractions (>40 μm) are not further considered here, as they account only for ~5.2 wt % in the sample BS1 and less than 0.5 wt % in the other three samples. In the final concentrate SPC2 a grade of 33.28 wt % of monazite and of 18.47 wt % of bastnaesite is achieved (Figure 5a). In contrast, only 0.59 wt % monazite and 0.29 wt % of bastnaesite report to the final tailings (sample BS1). The contents of the REE-Low-Mix spectra group in sample BSC1 is at 2.24 wt % fairly elevated when compared to the other samples with modes markedly below 1.0 wt %.

The REE minerals in the flotation tests reach their highest grades in the virtual sieve grain size class of 0–15 μm (Figure 5a,c). As expected, the multi-stage grinding and de-sliming process produced a narrow range of particle sizes in the virtual sieve grain size classes 0–15 μm and 15–40 μm. The cumulative grain size distribution curves display P50 particle sizes between 9 μm in sample BSC1 and 18 μm in sample BS1 (Figure 5b). The concentrates SPC2 and SPC1 display intermediate P50 particle sizes of ~12 μm.

**Figure 5.** Results of mineral processing tests with the steps SPC2-SPC1-BSC1-BS1 during multistage flotation of REE carbonatite ore (see Figure 4 for positions of samples in the flow scheme. (**a**) Modal mineralogy (in wt %) of complete samples and selected particle size fractions after virtual sieving with the filter mode equivalent circle diameter. Ap—apatite; Bas—bastnaesite; Dol—dolomite; Fe-Ti—Fe-Ti-minerals; Flu—fluorite; Mnz—REE-P-monazite; Sil—silicate minerals. SPC2—second cleaner concentrate; SPC1—scavenger cleaner concentrate; BSC1—scavenger cleaner middlings; BS1—tailings. (**b**) Cumulative particle size distributions (cumulative passing) of the step samples of the mineral processing tests in (a), for all particles, for (REE-P)-monazite (Mnz) and bastnaesite (Bas). (**c**) Modal mineralogy of virtual sieve fractions of concentrate SPC2. Proportions of fractions are listed below the columns. Maximum modes of REE minerals are found in the fraction 0–5 μm. Numbers are grain counts. (**d**) Mineral liberation of (REE-P)-monazite in terms of proportion in wt % of particle composition. Data for complete samples (labelled as all) in thick lines; data for selected particle size classes in microns after virtual sieving in thin lines. Same legend as in (b). (**e**) Intergrowth relationships of non-liberated (REE-P)-monazite with other REE-minerals, fluorite and carbonates.

SEM-based image analysis is currently the only available routine analytical method to quantify parameters such grain sizes or liberation of distinct minerals in fine-grained material without a previous mechanical mineral separation. Thus, the effects of selective comminution in mineral processing can be critically assessed [32]. In this case study, the monazite has a P50 value of 8.5 μm, whereas the bastnaesite has a P50 of 12 μm in the concentrate sample SPC2 (Figure 5b). Furthermore, it is noted that the P50 grain sizes of the REE minerals in the concentrate samples are always higher than in the tailings (Figure 5b).

Unsurprisingly, the liberation of monazite in the parameter particle composition is for all samples always best in the (virtual) sieve grain size class 0–15 μm. In this grain size class, the liberation class 95–100% in the final concentrate sample SPC2 has a value of cumulative mass recovery of 93% (Figure 5d). For the complete sample SPC2 the cumulative mass recovery of the monazite and the other REE minerals has a very high value of 88% for monazite and 82% for bastnaesite for the liberation class 95–100% (Figure 5d). For sample BS1 these values are minimal at 60% monazite and 56% bastnaesite for the liberation class 95–100%. This illustrates the need for an efficient comminution and liberation.

Grain size dependent trends are exemplified for three particle size fractions (Figure 5a). At grain size fractions below 15 μm these trends are continued, as exemplified for sample SPC2, so that at grain size fractions below 5 μm, more than 70 wt % mode of REE minerals are observed (Figure 5c). In the grain size fraction <15 μm the 95–100% liberation of monazite is well above 90% cumulative mass recovery (Figure 5d). This emphasises that high modes in distinct grain size fractions *in combination* with a high degree of liberation are important for a later enrichment of REE minerals. However, this positive effect is partly counteracted by the very fine grain size of the liberated grains.

In the concentrate sample SPC2 the monazite is mostly in contact with fluorite (3.1 wt %) and carbonates (2.4 wt %) but also often intergrown with other REE minerals (2.5 wt %). In sample SPC1 the intergrowth with carbonates has the highest value at 7.7 wt %. In the tailings BS1 the target mineral group monazite is mostly locked by carbonates (31.2 wt %) and rarely locked within fluorite (Figure 5e). Liberation data can be used to determine recovery curves, also known as mineral grade vs. recovery curves [41,42]. These curves allow the comparison of efficiencies in mineral-processing schemes. The values for the curves are defined by the proportions of the given mineral in wt % in the various liberation classes. As can be expected, the final concentrate SPC2 displays the best curve for monazite, whereas the tailings BS1 illustrate the worst case (Figure 6a). This is confirmed by the (virtual) sieve grain size fractions, where curves for the class 0–15 μm are more favourable than those observed for bulk (i.e. unsieved) samples. For monazite, the larger (virtual) sieve size fraction 15–40 μm shows a more advantageous curve for the concentrate SPC2 when compared to SPC1. In the coarse (virtual) sieve grain class in sample SPC2 one can recognise a potential for a partition of more monazite. The curve for bastnaesite displays a more advantageous trend than that for monazite (Figure 6a).

This assessment of potential recoveries is especially interesting for the scavenger and cleaner sample BSC1 and the tailings in sample BS1. For these samples, the (virtual) sieve grain size fraction 0–15 μm displays a quite favourable recovery curve for the REE-bearing minerals (Figure 6b,c). In contrast, the recovery curve for the (virtual) sieve grain size fraction 15–40 μm displays a potential for further recovery of REE minerals by improving their liberation, possibly by re-grinding. This potential can be also evaluated and visualised by a simple line-up of the particles with REE-bearing minerals (Figure 6d).

The line-up function is an important tool within the MLA processing software packages. Even in the tailings sample BS1 numerous well-liberated REE mineral grains remain. Most of these have grain sizes <10 μm, i.e. they can be expected to not float well. The loss of such REE mineral grains is undesirable, but technically induced. The reasons of non-floating fines may be: (1) insufficiently adapted hydrodynamics for fines flotation; (2) a too short flotation time (kinetic problem); (3) insufficiently adapted bubble size distribution; and (4) an insufficiently optimized reagen<sup>t</sup> dosage. Another important observation in the line-up view is the presence of REE mineral grains locked in coarse-grained carbonate particles. In this case the liberation of REE mineral grains may be improved by finer grinding, so that these grains will also float. However, further grinding will inevitably also result in more fines, a typical trade-off when developing a process flow sheet.

**Figure 6.** (**<sup>a</sup>**–**<sup>c</sup>**) Mineral grade vs recovery curves of REE carbonatite ore samples in mineral processing tests involving multistage flotation. Data of 100 wt % cumulative recovery is given by the mode of bastnaesite (Bas—broken lines) and REE-P-monazite (Mnz). Data of 100 wt % mineral grade is given by the proportion in wt% of fully liberated grains, extracted from the mineral liberation data in Figure 5d. SPC2—second cleaner concentrate; SPC1—scavenger cleaner concentrate; BSC1—scavenger cleaner middlings; BS1—scavenger tailings. Data for the complete samples in thick lines; data for particle size classes (in microns, virtual sieving) in thin lines. (**d**) Particle line-up of monazite and bastnaesite (REE) in the BS1 scavenger tailings sample. Note that many REE mineral particles are fully liberated but did not float. REE mineral grains of same size are mostly enclosed in carbonate (ankerite Ank, dolomite Dol), fluorite (Flu) and titanite (Ttn) particles.
