The Recovery and Concentration of Spodumene Using Dense Media Separation

Article The Recovery and Concentration of Spodumene Using Dense Media Separation

Charlotte E. Gibson 1,* , Massoud Aghamirian 2, Tassos Grammatikopoulos 2, Darren L. Smith 3 and Lindsay Bottomer 4

1 Department of Mining, Queen’s University, 99 University Ave., Kingston, ON K7L 3N6, Canada 2 SGS Canada Inc., 185 Concession St., Lakeﬁeld, ON K0L 2H0, Canada; [email protected] (M.A.); [email protected] (T.G.) 3 Patriot Battery Metals Inc., Suite 500, 666 Burrard St., Vancouver, BC V6C 3P6, Canada; [email protected] 4 Far Resources Ltd., 580 Hornby St. #510, Vancouver, BC V3C 3B6, Canada; [email protected] * Correspondence: [email protected]

Abstract: In coming years, global lithium production is expected to increase as the result of widespread electric vehicle adoption. To meet the expected increase in demand, lithium must be sourced from both brine and hard-rock deposits. Heavy liquid separation (HLS) and dense media separation (DMS) tests were conducted on the pegmatites from Hidden Lake, NWT, Canada to demonstrate the poten-

tial role of this technology in the concentration of spodumene (LiAlSi2O6) from hard-rock sources. A continuously operated DMS circuit test, conducted on +840 µm material, produced a concentrate

grading 6.11% Li2O with ~50% lithium recovery. The circuit rejected 50% of the original mass to tailings, with only 8% lithium losses. Sensitivity analysis showed that minor changes (+/−0.05) in the DMS-specific gravity cut point resulted in significant changes to the mass rejected and to Citation: Gibson, C.E.; Aghamirian, the concentrate grade produced; this may limit the feasibility and operability of the downstream M.; Grammatikopoulos, T.; Smith, D.L.; Bottomer, L. The Recovery and grinding and flotation circuits. The results demonstrate the potential for DMS in the concentration Concentration of Spodumene Using of spodumene from the Hidden Lake pegmatites, and by extension, the potential for DMS in the Dense Media Separation. Minerals concentration of spodumene from other hard-rock occurrences. 2021, 11, 649. https://doi.org/ 10.3390/min11060649 Keywords: lithium; spodumene; beneficiation; dense media separation

Academic Editors: Yasushi Watanabe and Argyrios Papadopoulos 1. Introduction Received: 14 May 2021 As many countries move to reduce carbon dioxide emissions and curb dependence on Accepted: 11 June 2021 fossil fuels, the production of electric vehicles continues to reach new highs. The battery is Published: 18 June 2021 a critical component of electric vehicle technology; battery weight, reliability and longevity are very important to the widespread adoption of electric cars. As the lightest alkali Publisher’s Note: MDPI stays neutral metal, lithium (Li) has a high electrochemical potential, making it an ideal material for with regard to jurisdictional claims in rechargeable batteries. Consequently, global lithium production has increased rapidly— published maps and institutional affil- doubling over the past five years. In 2015, just 30% of the total lithium produced was iations. used in batteries; today, the majority (>70%) of lithium produced goes directly to battery applications [1–13]. Lithium occurs in three different deposit types: hard-rock pegmatite deposits, brine lake deposits containing lithium chloride, and lithium-rich sediments such as hectorite Copyright: © 2021 by the authors. clays. To date, all of the lithium produced has originated from hard-rock and brine lake Licensee MDPI, Basel, Switzerland. deposits [1–12]; sediment hosted/clay deposits were only recently identified as potential This article is an open access article economic sources of lithium and are yet to be commercially developed [14]. distributed under the terms and It is cheaper to produce lithium from brine lake deposits compared with hard-rock conditions of the Creative Commons Attribution (CC BY) license (https:// deposits and until recently, most of the lithium produced came from brines [15]. However, creativecommons.org/licenses/by/ as demand spiked over the past several years, lithium production from hard-rock deposits 4.0/). also increased. In 2020, over half the global lithium supply came from hard-rock deposits

Minerals 2021, 11, 649. https://doi.org/10.3390/min11060649 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 649 2 of 17

(5 operations in Australia, 1 in China). The remainder came from brine operations in Chile (2 operations), Argentina (2 operations), United States (1), and China (2 operations) [1]. While brine deposits are less expensive to operate, recent studies show that they have a significant impact on the environment. The process of lithium extraction from brines requires half a million liters of water to be evaporated per tonne of lithium carbonate produced—water consumption two orders of magnitude higher than the processing of hard- rock deposits [16,17]. Several companies are developing more environmentally compliant processes to extract lithium from brines. However, these processes have yet to be employed at an industrial scale [18]. Researchers believe that global lithium reserves are sufficient to meet future demand; however, this depends on supply from both brine and hard-rock deposits [18,19]. Approxi- mately 2.5% of total global lithium reserves are in Canada, contained almost entirely in hard-rock occurrences in Quebec, Manitoba, and the Northwest Territories [12]. In 2020, the Canadian and American governments announced a joint action plan to improve the security of critical metals, including lithium [20]. Canada’s ability to secure a competitive, domestic supply of lithium depends on the development its hard-rock resources; to move forward, projects must be economic, fully abide by environmental regulations, most importantly, be accepted by local communities. This paper focuses on technical means of improving efficiency and reducing energy consumption in the concentration lithium-bearing minerals from Canadian hard-rock deposits.

The Beneficiation of Hard-Rock Deposits Using Dense Media Separation There are nine lithium-bearing minerals considered to be economically favourable for recovery. Of these, spodumene (LiAlSi2O6) is the most significant, occurring with other silicate gangue minerals such as feldspars, micas, and quartz [13,14,21,22]. Reviews by Gibson et al. [21] and, more recently, by Tadesse et al. [13] detail the different separation techniques used to concentrate spodumene from associated gangue minerals. In general, spodumene separation is a multistage process, complicated by similarities in the physicochemical properties of the ore and gangue minerals. Spodumene beneficiation flowsheets can include a combination of dense media separation (DMS), magnetic separation, de-sliming, magnetic separation, and flotation, depending on the mineralogy of the deposit. DMS is commonly used as a preconcentration stage, intended to reject gangue minerals prior to further concentration by flotation. While numerous studies have been conducted on aspects of spodumene flotation systems [23–28], there are limited studies detailing spodumene concentration by DMS. DMS is a process that separates materials of different densities using a media with a known specific gravity (S.G.), greater than that of water (hence the name “heavy” or “dense” media), to produce a float and sink product; the process is effective for a feed size ranging from 500 to 850 µm [13,29]. An important parameter in DMS is the concentration criterion (Equation (1)). The concentration criterion, derived from Newton’s Law, is a unitless measure of the separa- bility of two particles with different specific gravities in a viscous fluid. The higher the concentration criterion, the more effective the separation of heavy particles from the light particles [29].

S.G. heavy − S.G. f luid Concentration Criterion, CC = (1) S.G. light − S.G. f luid

Equation (1) shows that the higher the specific gravity of the fluid in which the separation occurs, the higher the concentration criteria, and the more efficient the separation of the two particles. Similarly, the gap in specific gravity between the heavy and the light particles plays an important role in separation efficiency—operation of dense media circuits tends to be more efficient when there is a large difference between the S.G. value of the heavy and light minerals [30]. Further, like all mineral separation techniques, DMS efficiency is influenced by mineral liberation and association. Minerals 2021, 11, x FOR PEER REVIEW 3 of 17

Minerals 2021, 11, 649 3 of 17 the heavy and light minerals [30]. Further, like all mineral separation techniques, DMS efficiency is influenced by mineral liberation and association. Tadesse et al. [13]Tadesse outlined et al. three [13] outlinedmajor ch threeallenges major associated challenges with associated the dense with media the dense media separation of spodumene:separation of (1) spodumene: marginal diff (1)erences marginal in the differences S.G. of spodumene in the S.G. ofcompared spodumene compared with different withsilicate different gangue silicate minerals; gangue (2) minerals;the mineralogical (2) the mineralogical transformation transformation of spodu- of spodumene mene to micas toand micas clays and with clays lower with specific lower gravity, specific and; gravity, (3) the and; tendency (3) the of tendency spodumene of spodumene to to fracture intofracture acicular into particles acicular which particles are more which likely are to more report likely to the to report float fraction. to the float fraction. This paper aimsThis to paperdemonstrate aims to the demonstrate potential role the of potential DMS in role the ofconcentration DMS in the concentrationof of spodumene fromspodumene hard-rock from ores hard-rock, through studying ores, through its applic studyingability its to applicability the Hidden toLake the Hidden Lake pegmatites frompegmatites Northwest from Territories, Northwest Canada. Territories, We aim Canada. to demonstrate We aim to both demonstrate the benefits both the benefits and challengesand of this challenges processing of this techni processingque and techniquehighlight andimportant highlight flowsheet important design flowsheet design considerations.considerations.

2. Materials and2. Methods Materials and Methods 2.1. The Hidden 2.1.Lake The Pegmatites Hidden Lake Pegmatites The spodumeneThe pegmatite spodumene occurrences pegmatite at occurrencesHidden Lake at are Hidden situated Lake 45 are km situated east- 45 km east- northeast of Yellowknifenortheast ofin Yellowknifethe Northwest in theTerritories Northwest of Canada Territories (Figure of Canada 1). (Figure1).

Yukon Northwest Territories

Hidden Lake Deposit Nunavut

Newfoundland & Labrador

British Quebec Alberta Manitoba PEI Columbia New Brunswick Saskatchewan Nova Ontario Scotia

FigureFigure 1. Location 1. Location of the of Hidden the Hidden Lake Lakelithium lithium deposit deposit in Northwest in Northwest Territories, Territories, Canada. Canada.

The mineral rightsThe mineralto the occurrences rights to the are occurrences held under are a Joint held Venture under a between Joint Venture Far between Far Resources Ltd.Resources and Patriot Ltd. Battery and PatriotMetals BatteryInc., at a Metals 60% and Inc., 40% at a interest, 60% and respectively. 40% interest, respectively. The lithium-bearingThe lithium-bearing pegmatite dykes pegmatite at Hidden dykes Lake at are Hidden part of Lake the arelarger part Yellowknife of the larger Yellowknife Pegmatite District,Pegmatite which District,have been which describ haveed been as one described of the largest as one lithium of thelargest resources, lithium resources, mainly as spodumene,mainly as in spodumene,Canada [31]. in Canada [31].

2.2. Mineralogy 2.2.and MineralogyHead Assays and Head Assays 2.2.1. Sample Preparation 2.2.1. Sample Preparation Four composite samples (Sample 1, Sample 2, Sample 3, and Sample 4) were collected Four composite samples (Sample 1, Sample 2, Sample 3, and Sample 4) were collected from the Hidden Lake pegmatites. The samples were ﬁrst crushed to 3 inches (7.62 cm). from the Hidden Lake pegmatites. The samples were first crushed to 3 inches (7.62 cm). The 2 kg charges were rifﬂe split from each sample and reduced to a P80 of 425 µm for the The 2 kg charges were riffle split from each sample and reduced to a P80 of 425 µm for the mineralogical analyses. Each sample was then screened and re-combined into four size mineralogical analyses. Each sample was then screened and re-combined into four size fractions, based on the weight (%) distribution: +425 µm, −425/+250 µm, −250/+106 µm, fractions, based on the weight (%) distribution: +425 µm, −425/+250 µm, −250/+106 µm, and −106 µm for the mineralogical work. Once the mineralogical analysis was completed, and −106 µm forit wasthe mineralogical decided that thework. samples Once the could mineralogical be treated asanalysis a single was composite completed, for the DMS and it was decided HLSthat andthe samples were combined could be in treated equal ratio as a (1:1:1:1).single composite for the DMS and HLS and were combinedThe composite in equal ratio sample (1:1:1:1). was crushed to −1/2” and a 9 kg subsample taken. This 9 kg subsample was further crushed to −1/4”, and subsamples of the −1/4” material submitted

Minerals 2021, 11, 649 4 of 17

for elemental and X-ray diffraction (XRD) analyses. A second 4 kg subsample of the −1/4” material was screened on an 840 µm (20 mesh) screen to prepare feed for heavy liquid separation (HLS) test. The remaining sample (approximately 400 kg) was used in DMS testing.

2.2.2. Elemental Analysis The composition of samples and separation products was determined by whole-rock analysis using X-Ray Fluorescence (XRF) for major elements and peroxide fusion with Inductively Coupled Plasma Emission Spectrometry (ICP-OES) ﬁnish for lithium. For whole-rock analysis, samples were prepared by borate fusion; 0.2–0.5 g of pulverized sample was mixed with lithium tetraborate/lithium metaborate, and fused to form a homogenous glass disk. The glass disk was then analyzed by X-Ray Fluorescence using a Bruker XRF Tiger Series 1 instrument (Bruker, Billerica, MA, USA). For lithium analysis, 0.1 g of pulverized sample was fused with sodium peroxide in a zirconium crucible. The resultant cake was then digested in hydrochloric acid and analyzed on an Agilent 725 ICP- OES instrument (Agilent, Santa Clara, CA, USA).

2.2.3. QEMSCAN Operational Modes and Quality Control QEMSCAN analysis was conducted at the Advanced Mineralogy Facility at SGS Canada, in Lakefield, ON, Canada. QEMSCAN is an EVO 430 automated scanning electron microscope, equipped with four light element energy dispersive X-ray spectrometers and iDiscover software capable of processing the data and images. QEMSCAN operates with a 25 kV accelerating voltage and a 5 nA beam current. The QEMSCAN measures, and the iDiscover software processes data from every pixel across a sample, with a measurement resolution defined based on the scope of the analysis. The software assigns each pixel a mineral name based on 1000 counts of energy dispersive X-ray spectral data and backscatter electron intensities. If the minerals or constituent phases comprising the sample are compositionally distinct, QEMSCAN is capable of reliably discriminating and quantifying them. The mode of the QEMSCAN analysis used for this project was the Particle Mineral Analysis (PMA). A predefined number of particles were mapped at a point of 3 to 8 µm pixel size to spatially resolve and describe mineral grain textures and associations. The PMA mode scans the polished section and provides a statistically robust population of mineral identifications based on the X-ray chemistry of minerals. Light elements such as lithium, boron, carbon, beryllium, oxygen, and hydrogen cannot be discriminated by the QEMSCAN analysis. Thus, the identification of spodumene was based on the aluminum–silica ratios since lithium cannot be detected with conventional instruments. The liberation and association characteristics of spodumene are given for each sample. For the purposes of this analysis, particle liberation is defined based on 2D particle area percent. Particles are classified in the following groups (in descending order) based on mineral-of-interest area percent: free (≥95% of the total particle area) and liberated (≥80%). The non-liberated grains have been classified according to association characteristics, where binary association groups refer to particle area percent greater than or equal to 95% of the two minerals or mineral groups. The complex groups refer to particles with ternary, quaternary, and greater mineral associations including the mineral of interest.

2.2.4. X-ray Diffraction Quantitative X-ray diffraction (XRD) by Rietveld Reﬁnement was conducted using a Bruker AXS D8 Advance Diffractometer (cobalt radiation, 35 kV, 40 mA). Results were interpreted using PDF2/PDF4 powder diffraction databases issues by the International Center for Diffraction Data (ICDD) with DiffracPlus Eva and Topas software version 4.2 (Karlsruhe, Germany). Mineral detection limits ranged from 0.5–2%, depending on the crystallinity. Minerals 2021, 11, x FOR PEER REVIEW 5 of 17

(Karlsruhe, Germany). Mineral detection limits ranged from 0.5–2%, depending on the Minerals 2021, 11, 649 crystallinity. 5 of 17

2.3. Heavy Liquid Separation (HLS) 2.3. Heavy Liquid liquid Separation separation (HLS) (HLS) is a laboratory test used to assess whether a hetero- genous material, such as an ore, is amenable to separation using DMS techniques. It is Heavy liquid separation (HLS) is a laboratory test used to assess whether a heteroge- conducted using a small sample and, if successful, can justify expenditure on a larger nous material, such as an ore, is amenable to separation using DMS techniques. It is dense media separation test. Further, heavy liquid test results help determine the optimal conducted using a small sample and, if successful, can justify expenditure on a larger dense specificmedia separation gravity set test. point Further, in dense heavy media liquid separation test results [29]. help determine the optimal speciﬁc gravityIn this set pointstudy, in HLS dense testing media was separation conducted [29]. using methylene iodide (density of 3.32 3 g/cm In) diluted this study, with HLS acetone. testing The was test conducted was conducted using methylene with heavy iodide liquid (density densities of 3.32 ranging g/cm3 ) 3 3 fromdiluted 2.60 with to 3.10 acetone. g/cm The in testincrements was conducted of 0.10 withg/cm heavy, according liquid to densities the flowsheet ranging shown from 2.60 in Figureto 3.10 2. g/cm 3 in increments of 0.10 g/cm3, according to the ﬂowsheet shown in Figure2.

Float Float Float Float Float Float

Feed S.G. S.G. S.G. S.G. S.G. S.G. ~2 kg 3.10 3.00 2.90 2.80 2.70 2.60 +840 µm

Sink Sink Sink Sink Sink Sink

Figure 2. Figure 2. HeavyHeavy liquid liquid separation separation testing testing procedure. procedure. The test was initiated with approximately 2 kg of +840 µm feed material at the highest The test was initiated with approximately 2 kg of +840 µm feed material at the highest liquid density (3.00 g/cm3), using a separatory funnel to facilitate separation of heavy liquid density (3.00 g/cm3), using a separatory funnel to facilitate separation of heavy ma- material from light material, if required. Next, the heavy liquid was further diluted with terial from light material, if required. Next, the heavy liquid was further diluted with ac- acetone to achieve a lower liquid density (i.e., 3.00 g/cm3) and the float material from etone to achieve a lower liquid density (i.e., 3.00 g/cm3) and the float material from the the previous stage was tested again at the lower density. This procedure continued for previousall densities stage in thewas series, tested beginning again at the at the lowe highestr density. density This (3.10 procedure g/cm3) andcontinued ending for at theall 3 densitieslowest density in the (2.60series, g/cm beginning3). The at density the highest of the density heavy liquid (3.10 wasg/cm measured) and ending before at andthe low- after 3 esteach density test to (2.60 ensure g/cm the). target The wasdensity maintained. of the heavy All test liquid products was measured were washed before with and acetone after eachto dissolve test to ensure residual the methylene target was iodide maintained and then. All dried. test products The final were sink washed (6 total) with and acetone float (1) toproducts dissolve were residual weighed methylene and subsamples iodide and were then obtained dried. The for final analysis sink by(6 total) XRF and and AA. float (1) products were weighed and subsamples were obtained for analysis by XRF and AA. 2.4. Dense Media Separation 2.4. DenseThe specificMedia Separation gravity of spodumene is approximately 3.15, allowing it to be separated fromThe other specific silicate gravity minerals of spodumene (i.e., quartz is andapprox feldspar,imately S.G. 3.15, 2.5 allowing and 2.6, it respectively)to be separated by fromdense other media silicate separation. minerals If spodumene(i.e., quartz isand sufficiently feldspar, S.G. liberated 2.5 and at a2.6, coarse respectively) particle size, by denseDMS canmedia produce separation. a high-grade If spodumene mineral concentrateis sufficiently and liberated operate at as a the coarse sole concentrationparticle size, DMSstage. can However, produce mosta high-grade ores require mineral grinding concentr to finerate and particle operate sizes as the to achievesole concentration acceptable stage.liberation. However, Here, most DMS ores can berequire used asgrinding pre-concentration to finer particle stage sizes to recover to achieve coarse acceptable grains of liberation.liberated spodumeneHere, DMS atcan a coarsebe used size, as pre-conc and thusentration reduce thestage overall to recover mass processedcoarse grains in theof liberatedgrinding spodumene and flotation at circuits. a coarse size, and thus reduce the overall mass processed in the grindingDense and media flotation separation circuits. testing was conducted on a 400 kg sample of the Head CompositeDense media Sample separation at SGS in testing Lakefield, was ON, conducte Canada.d on The a 400 test kg was sample operated of the continuously Head Com- positeand consisted Sample at of SGS screening in Lakefield, the sample ON, onCanada. a 20 mesh The (840test wasµm) operated screen to continuously produce oversize and consisted(+840 µm) of and screening undersize the (sample−840 µ onm) a material. 20 mesh The(840 oversizeµm) screen material to produce was passed oversize through (+840 µm)two and dense undersize media separation (−840 µm) cyclones material. using The oversize a mixture material of ferrosilicon was passed and through magnetite two as densedense media media. separation The specific cyclones gravity using cut point a mixt ofure the of first ferrosilicon DMS stage and (DMS magnetite Pass 1) as was dense 2.70. media.The float The product specific formed gravity the cut final point silicate of the tailings first DMS product. stage The(DMS sink Pass fraction 1) was from 2.70. DMS The floatPass product 1 formed formed to feed the to final the silicate second tailings DMS stage product. (DMS The Pass sink 2), fraction conducted from atDMS a specific Pass 1 formedgravity to cut feed point to ofthe 2.90. second The DMS float productstage (DMS from Pass DMS 2), Pass conducted 2 was combined at a specific with gravity the screen cut pointundersize of 2.90. and The referred float product to as flotation from DMS feed. Pass The 2 sink was product combined from with DMS the Passscreen 2 formed undersize the final spodumene concentrate. The specific gravity cut points were selected based on the results obtained in heavy liquid separation testing; the details of cut point selection will be discussed in subsequent sections. After testing, the screen undersize, the DMS Pass 2 float, and the DMS Pass 2 sink products were filtered, dried, weighed and representative subsamples were collected for analysis by XRF and AA. The DMS Pass 1 float sample (i.e., Minerals 2021, 11, 649 6 of 17

silicate tailings) was subsampled periodically throughout operation; the subsamples were combined, ﬁltered, dried and subsamples for analysis by XRF and AA.

3. Results and Discussion 3.1. Mineralogy The geochemical analyses including major elements, lithium (Li), and rubidium (Rb) of the four head samples are shown in Table1. All four samples were similar in composition.

Table 1. Head analysis.

Element Li Li O Al Ca Fe K Mg Na P Rb Si (wt.%) 2 Sample 1 0.62 1.33 8.84 0.19 0.25 1.71 0.04 3.35 0.59 0.08 34.0 Sample 2 0.63 1.36 8.83 0.15 0.27 2.29 0.02 3.10 0.39 0.09 34.1 Sample 3 0.60 1.29 8.83 0.20 0.25 1.67 0.03 3.41 0.61 0.08 34.0 Sample 4 0.63 1.36 8.89 0.21 0.23 1.84 0.04 3.33 0.80 0.10 33.7

Table2 shows the modal mineralogy of the head samples, as determined by QEM- SCAN. The spodumene (LiAlSi2O6) grade (wt. %) was similar for all the four samples, ranging between 14.2% and 16.1%. Quartz (SiO2), plagioclase (albite: NaAlSi3O8), and muscovite [KAl2(AlSi3O10)(OH)2] showed very little variation among the samples. The grade of K-feldspars (KAlSi3O8) ranged from 8.7% to 9.7%, with the exception of Sample 3, which was 14.0% K-feldspar. Montebrasite [LiAl(PO4)(OH,F)] content of the samples varied from 1.5% to 3.69%.

Table 2. Modal mineralogy (wt. %)—sample determined by QEMSCAN.

Sample Sample 1 Sample 2 Sample 3 Sample 4 Spodumene 15.8 16.1 14.2 14.5 Quartz 27.9 26.5 28.5 27.3 Plagioclase 38.5 36.2 39.8 39.3 K-Feldspar 8.66 14.0 9.16 9.66 Muscovite 4.86 4.20 4.48 4.03 Biotite 0.02 0.03 0.01 0.01 Clays 1.09 0.91 0.93 1.07 Apatite 0.28 0.26 0.34 0.30 Montebrasite 2.68 1.54 2.36 3.69 Other 0.20 0.27 0.22 0.17 Total 100 100 100 100

Figure3 illustrates the mineral distribution by size fraction, where the mineral grade is a function of the weight percent mass distribution among the fractions. For all samples, spodumene preferentially deported to the +425 µm size fraction, compared with ﬁner fractions. Montebrasite and gangue minerals (quartz, feldspars, muscovite) were uniformly distributed across all size fractions. Further analysis showed that spodumene grain size (D50) ranged from 152 to 201 µm—coarser than the average grain size of other minerals. Figure4 illustrates the liberation and association proﬁle of spodumene at a P 80 of 425 µm. Free and liberated spodumene ranged from 86% (Sample 3) to 91% (Sample 4). The remainder of the spodumene occurred as complex particles 7% in Sample 4 to 10% in Sample 1, quartz/feldspars (1.4% to 3.9%), and other minerals (<1.5%). Minerals 2021, 11, x FOR PEER REVIEW 7 of 17

-106 μm Minerals 2021, 11Minerals, x FOR PEER2021, 11REVIEW, 649 7 of 17 7 of 17 -250/+106 μm Sample 4 -425/+250 μm

+425 μm

-106 μm -106 μm

-250/+106 μm -250/+106 μm Sample 4 Sample 3 -425/+250 μm -425/+250 μm

+425 μm +425 μm

-106 μm -106 μm

-250/+106 μm -250/+106 μm Sample 3 Sample 2 -425/+250 μm -425/+250 μm

+425 μm +425 μm Other -106 μm -106 μm Montebrasite Muscovite -250/+106 μm -250/+106 μm K-Feldspar Sample 2 Sample 1 -425/+250 μm-425/+250 μm Plagioclase Quartz +425 μm +425 μm Spodumene Other 0 10203040-106 μm Montebrasite Muscovite -250/+106Mineral μm Abundance (wt.%) Sample 1 K-Feldspar Figure 3. Mineral abundance (wt. %) by-425/+250 size fraction. μm Plagioclase Quartz +425 μm Spodumene Figure 4 illustrates the liberation and association profile of spodumene at a P80 of 425 0 10203040 µm. Free and liberated spodumene ranged from 86% (Sample 3) to 91% (Sample 4). The Mineral Abundance (wt.%) remainder of the spodumene occurred as complex particles 7% in Sample 4 to 10% in Figure 3. MineralSample abundanceFigure 1, quartz/feldspars 3. (wt.Mineral %) by abundance size fraction.(1.4% (wt. to 3.9%), %) by sizeand fraction. other minerals (<1.5%).

Figure 4 illustrates the liberation and association profile of spodumene at a P80 of 425 100 µm. Free and liberated spodumene ranged from 86% (Sample 3) to 91% (Sample 4). The remainder of the spodumene occurred as complex particles 7% in Sample 4 to 10% in Sample 1, quartz/feldspars (1.4% 80to 3.9%), and other minerals (<1.5%).

100 60

80 40 Mass (%Spodumene) 60 20

40 0

Mass (%Spodumene) Sample 1 Sample 2 Sample 3 Sample 4 Free Spodumene 1 66.8 72.3 68.1 65.8 20 Liberated Spodumene1 19.7 15.8 17.5 24.8 Spd:Quartz/feldspar 1 2.74 2.14 3.87 1.39 Spodumene:Other 1 0.75 1.19 1.30 1.36 Complex0 1 10.08.629.206.66 Sample 1 Sample 2 Sample 3 Sample 4 Free SpodumeneFigure 4. 1Summary66.8 of spodumene 72.3 liberationliberation 68.1 andand associationassociation 65.8 proﬁleprofile (mass%(mass% calculatedcalculated forfor thethehead Liberated Spodumene1 19.7 15.8 17.5 24.8 samples)head samples) at a P 80at ofa P 42580 ofµ 425m. µm. Spd:Quartz/feldspar 1 2.74 2.14 3.87 1.39 1 0.75 1.19 1.30 1.36 Spodumene:Other µ Complex The1 disproportionate disproportionate10.08.629.206.66 occurrence occurrence of of spodumen spodumenee in inthe the +425 +425 µm sizem sizefraction, fraction, the pres- the

presenceence of coarse of coarse spodumene spodumene grains, grains, and the and degree the degreeof liberation of liberation at a P80 of at 425 a P µm80 of suggested 425 µm Figure 4. Summarysuggested of spodumene that gravity liberation separation and associ techniquesation profile may (mass% be a suitablecalculated means for the of recovering and head samples) concentratingat a P80 of 425 µm. spodumene from associated gangue minerals. The liberation and association data are also critical to understanding ﬂotation behavior and interpreting ﬂotation test The disproportionate occurrence of spodumene in the +425 µm size fraction, the pres- results (outside the scope of the present study). ence of coarse spodumeneThe four grains, head samples and the were degree combined of liberation to produce at a P a80 Headof 425 Composite µm suggested sample for heavy liquid and dense media separation testing. Table3 reports geochemical analyses including

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major elements, lithium (Li), and loss on ignition for the combined Head Composite sample.

Table 3. Head analysis of HLS composite.

Element/Oxide, Li Li O SiO Al O Fe O MgO CaO Na OK OP O MnO LOI Sum % 2 2 2 3 2 3 2 2 2 5 Head Com- 0.64 1.38 72.5 16.7 0.23 0.04 0.26 4.18 2.41 1.40 0.02 0.82 98.6 posite

3.2. Heavy Liquid Separation Heavy liquid separation testing was performed on the Head Composite sample to evaluate the effectiveness of gravity separation techniques, specifically DMS. Table4 summarizes the abundance of each mineral in the Head Composite sample, the average specific gravity of each mineral, and the concentration criterion (CC; calculated using equation 1, where spodumene is the heavy mineral), in water and in a heavy liquid with a specific gravity of 2.5 (i.e., an arbitrary value higher than water but lower than the S.G. of most gangue minerals in the system).

Table 4. Summary of mineral abundance in the composite head sample; speciﬁc gravity; and concentration criterion (CC) in water and heavy liquid.

CC * in CC * in Abundance Speciﬁc Average Mineral Name Mineral Formula Water Heavy Liquid (wt. %) Gravity (S.G.) S.G. S.G. = 1.0 S.G. = 2.5

Spodumene LiAlSi2O6 16.9 3.1–3.2 3.15 1.00 1.00 Montebrasite LiAl(PO4)(OH) 1.40 2.98–3.04 3.01 1.07 1.27 Albite Na(AlSi O ) 38.6 2.6–2.65 2.63 1.32 5.20 (plagioclase) 3 8 Quartz SiO2 23.8 2.65–2.66 2.65 1.30 4.26 Microcline K(AlSi O ) 11.0 2.54–2.57 2.56 1.38 11.8 (k-feldspar) 3 8 Muscovite KAl2(AlSi3O10)(OH)2 6.28 2.77–2.88 2.83 1.18 2.00 Other - 2.81 - - - - * Concentration Criterion.

In water, the concentration criterion value for the separation of spodumene from other silicate minerals ranged from 1.18 (for muscovite) to 1.38 (for K-feldspar). It is generally accepted that gravity separation techniques are effective when the concentration criterion is greater than 2.5 [32]; indicating that spodumene and associated gangue minerals cannot be easily separated from each other in water even if minerals are perfectly liberated. However, raising the speciﬁc gravity of the liquid increases the concentration criterion for all of the silicate gangue minerals in the system. For example, when a heavy liquid (S.G. 2.5) was used, the concentration criterion between spodumene and various gangue minerals ranged from 2.00 to 11.8, suggesting that spodumene can be easily separated from albite (CC = 5.20), quartz (CC = 4.26), and K-feldspar (CC = 11.8); the separation of spodumene from muscovite may be less efﬁcient due to its lower concentration criterion value (i.e., <2.5). We note that the concentration criterion calculation considers an idealized environment where minerals are perfectly liberated from one another and the S.G. of the heavy liquid can be precisely controlled. In real ore systems, neither of these points is true, and some degree of imperfect separation is expected, regardless of the calculated CC values for each mineral. To prepare for testing, a 2.66 kg subsample of the Head Composite material was screened on a 20 mesh (840 µm) screen to remove undersize material: 23.5% of the mass, MineralsMinerals 20212021,, 1111,, 649x FOR PEER REVIEW 99 of 1717

containing 20% ofof thethe lithium,lithium, reportedreported toto thethe −−840 µmµm fraction and the remaining 76.5% (2.03(2.03 kgkg containingcontaining 80%)80%) waswas subjectedsubjected toto heavyheavy liquidliquid testing.testing. FigureFigure5 5 shows shows the the results results of heavy liquid testing. The plot on the left (a) shows the lithium and mass recovery toto thethe sink fraction basedbased onon thethe feed to the heavy liquidliquid test (‘stage recovery’), whilewhile thethe plotplot onon thethe rightright (b)(b) showsshows thethe lithiumlithium andand massmass recoveryrecovery toto thethe sinksink fractionfraction basedbased onon thethe wholewhole feedfeed (‘global(‘global recovery’,recovery’, i.e.,i.e., includingincluding thethe −−840 µmµm that was not subjected to heavyheavy liquidliquid separation testing.testing. Therefore, thethe globalglobal recoveryrecovery isis lowerlower thanthan stagestage recovery).recovery).

100 8 100 8

7 7 80 80 Mass Pull 6 6 Recovery Mass Pull O) O) Recovery 2 Grade 2 5 60 5 60 Grade 4 4 40 40 3 3 Grade (% Li (% Grade Grade (% Li (% Grade 2 2 20 20 Li Recovery / Mass Pull (%) / Mass Pull Li Recovery

Li Recovery / Mass Pull (%) 1 1

0 0 0 0 2.6 2.7 2.8 2.9 3.0 3.1 2.62.72.82.93.03.1 3 3 S.G. Cut Point (g/cm ) S.G. Cut Point (g/cm ) (a) (b)

FigureFigure 5. HeavyHeavy liquid liquid testing testing cumulative cumulative sink sink results—( results—(a) astage) stage recovery recovery and and mass mass pull pull and and(b) global (b) global recovery recovery and mass and masspull. pull.

Figure5 5 demonstrates demonstrates thethe grade–recoverygrade–recovery relationshiprelationship asas aa function function of of the the specific specific gravity cut point. When the cut point of the heavy media dropped,dropped, thethe concentrateconcentrate gradegrade dropped as gangue gangue and and middlings middlings particles particles repo reportedrted to the to the sink sink fraction, fraction, diluting diluting the con- the concentrate.centrate. A significant A significant change change in ingrade grade and and mass mass recovery recovery occurred occurred between between a cut pointpoint of 2.60 and 2.70, attributed attributed to to the the recovery recovery of of quartz quartz (SG (SG = =2.65) 2.65) and and albite albite (SG (SG = 2.63) = 2.63) to tothe the sink sink fraction fraction when when the thespecific specific gravity gravity of the of fluid the fluid fell below fell below the specific the specific gravity gravity of the ofmineral. the mineral. Aramante Aramante et al. [33] et al. performed [33] performed HLS testing HLS testing on a spodumene-bearing on a spodumene-bearing ore with ore withsimilar similar mineralogy mineralogy to the toHidden the Hidden Lake pegmatites. Lake pegmatites. In this study, In this the study, researchers the researchers obtained obtaineda sink product a sink grade product of 5.17% grade Li of2 5.17%O with Li 61%2O withLi recovery 61% Li recoveryat an S.G. at cut an point S.G. cutof 2.70 point on of a 2.70−2360/+2000 on a −2360/+2000 µm feed. Theseµm feed. results These are resultsin line arewith in those line with presented those presentedin Figure in5; the Figure com-5; theparatively comparatively improved improved grade–recovery grade–recovery performance performance in HLS testing in HLS of testing Hidden of Lake Hidden material Lake materialcan likely can be likelyattributed be attributed to a finer to feed a finer particle feed size particle distribution. size distribution. The products from heavy liquidliquid testing were analyzed by quantitative XRD to con- struct aa mineralmineral balance.balance. Figure 66 shows shows thethe mineralmineral distributiondistribution inin thethe sinksink fraction fraction at at each specificspecific gravitygravity cutcut point.point. XRD results confirmed that the increase in mass to the sink product and decrease in sink grade (% Li2O) at S.G. 2.60 resulted from more silicate gangue minerals reporting to the sink fraction, specifically quartz and albite. As was expected, the sink product at the highest specific gravity tests (3.10) had the highest lithium grade (7.33% Li2O or ~94% spodumene). Figure7 illustrates the recovery of silicate gangue minerals as a function of spodumene recovery, showing the selective separation of spodumene from muscovite, quartz, albite, and microcline. Selectivity (i.e., the preferential recovery of spodumene over gangue minerals) was a function of the gangue mineral-specific gravity (and by extension, the concentration criterion) and was as follows (in descending order): microcline > albite > quartz > muscovite.

Minerals 2021, 11, x FOR PEER REVIEW 10 of 17

100 Other Montebrasite 80 Microcline Albite Muscovite 60 Quartz Spodumene

20 Mineral Distribution of SinkProduct (wt.%) 0 2.60 2.70 2.80 2.90 2.95 3.00 3.10 Specific Gravity Figure 6. Heavy mineral distribution (wt. %) of sink fraction for different S.G. cut points.

MineralsMinerals 20212021,, 1111,, 649x FOR PEER REVIEW 1010 of 1717 XRD results confirmed that the increase in mass to the sink product and decrease in sink grade (% Li2O) at S.G. 2.60 resulted from more silicate gangue minerals reporting to the sink fraction, specifically quartz and albite. As was expected, the sink product at the highest specific gravity tests (3.10) had the highest lithium grade (7.33% Li2O or ~94% spodumene).100 Figure 7 illustrates the recovery of silicate gangue Other minerals as a function of spodu- Montebrasite mene80 recovery, showing the selective separation of spodumene Microcline from muscovite, quartz, albite, and microcline. Selectivity (i.e., the preferential Albite recovery of spodumene over Muscovite gangue minerals) was a function of the gangue mineral-specific gravity (and by extension, 60 Quartz the concentration criterion) and was as follows (in descending Spodumene order): microcline > albite > quartz > muscovite. 40In Figure 7, the closer the curve is to the x-axis, the better the selectivity. Even in the case of spodumene-muscovite separation—where the calculated concentration criterion suggested20 that efficient separation may be difficult—selectivity was high. For spodumene recovery values below ~85%, corresponding gangue mineral recovery was less than 15%. Mineral Distribution of SinkProduct (wt.%) As lithium0 recovery increased beyond 85%, gangue mineral recovery increased significantly, particularly2.60 2.70 2.80for minerals 2.90 2.95 with 3.00 higher 3.10 specific gravity (quartz and muscovite). These results demonstrateSpecific the Gravity potential for DMS to be used as a pre-concentration stage, despiteFigure 6.literature Heavy mineral reports distribution of the challenges (wt. %) of a sinkssociated fraction with for differentseparating S.G.S.G. minerals cutcut points.points. with similar densities [13]. XRD results confirmed that the increase in mass to the sink product and decrease in 100 sink grade (% Muscovite, Li2O) at SG S.G. = 2.83 2.60 resulted from more silicate gangue minerals reporting to the sink fraction, Quartz, specifically SG = 2.65 quartz and albite. As was expected, the sink product at the Albite, SG = 2.63 highest80 specific gravity tests (3.10) had the highest lithium grade (7.33% Li2O or ~94% Microcline, SG = 2.56 spodumene). Figure 7 illustrates the recovery of silicate gangue minerals as a function of spodu- 60 mene recovery, showing the selective separation of spodumene from muscovite, quartz, albite, and microcline. Selectivity (i.e., the preferential recovery of spodumene over gangue40 minerals) was a function of the gangue mineral-specific gravity (and by extension, the concentration criterion) and was as follows (in descending order): microcline > albite

> quartz (%) Recovery Mineral Gangue > muscovite. 20 In Figure 7, the closer the curve is to the x-axis, the better the selectivity. Even in the case of spodumene-muscovite separation—where the calculated concentration criterion suggested0 that efficient separation may be difficult—selectivity was high. For spodumene recovery20 values 40below ~85%, 60 corresponding 80 gangue 100 mineral recovery was less than 15%. Spodumene Recovery (%) As lithium recovery increased beyond 85%, gangue mineral recovery increased signifi- FigureFigurecantly, 7. 7. particularlyHeavyHeavy mineral mineral for distribution distribution minerals (wt. (wt.with %) %) higherof of sink sink fraction fractionspecific for for gravity different different (quartz S.G. S.G. cut cut andpoints. points. muscovite). These results demonstrate the potential for DMS to be used as a pre-concentration stage, despiteIn Figureliterature7, the reports closer of the the curve challenges is to the associated x-axis, the with better separating the selectivity. minerals Even with in sim- the caseilar densities of spodumene-muscovite [13]. separation—where the calculated concentration criterion suggested that efficient separation may be difficult—selectivity was high. For spodumene recovery100 values below ~85%, corresponding gangue mineral recovery was less than 15%. As lithium recovery Muscovite, increased SG = 2.83 beyond 85%, gangue mineral recovery increased significantly, Quartz, SG = 2.65 particularly for minerals with higher specific gravity (quartz and muscovite). These results 80 Albite, SG = 2.63 demonstrate the Microcline, potential SG = for 2.56 DMS to be used as a pre-concentration stage, despite literature reports of the challenges associated with separating minerals with similar densities [13]. 60The “sharpness” at which spodumene is separated from gangue minerals is an important parameter in beneficiation flowsheet design, and in assessing the performance of heavy liquid separation or the applicability of dense media separation. Ideally, the recovery of the40 mineral of interest to the sink product is complete (100%) when the heavy liquid density is less than the density of the mineral. As the density of the heavy liquid surpasses

the (%) Recovery Mineral Gangue density20 of the mineral, the recovery should theoretically drop to nil. If plotted, the ideal recovery of a mineral versus S.G. cut point forms a vertical line at the S.G. of the mineral, like the red line pictured in Figure8. 0 20 40 60 80 100 Spodumene Recovery (%) Figure 7. Heavy mineral distribution (wt. %) of sink fraction for different S.G. cut points.

Minerals 2021, 11, x FOR PEER REVIEW 11 of 17

The “sharpness” at which spodumene is separated from gangue minerals is an important parameter in beneficiation flowsheet design, and in assessing the performance of heavy liquid separation or the applicability of dense media separation. Ideally, the recovery of the mineral of interest to the sink product is complete (100%) when the heavy liquid density is less than the density of the mineral. As the density of the heavy liquid surpasses Minerals 2021, 11, 649 the density of the mineral, the recovery should theoretically drop to nil. If plotted,11 ofthe 17 ideal recovery of a mineral versus S.G. cut point forms a vertical line at the S.G. of the mineral, like the red line pictured in Figure 8.

100

40 Spodumene Recovery (%) 20 Actual Recovery Ideal Recovery

0 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3 S.G. Cut Point (g/cm ) FigureFigure 8. 8. IdealIdeal and and actual actual spodumene spodumene sepa separationration as as a a function function of of specific speciﬁc gravity gravity cut cut point in heavy heavyliquid liquid separation. separation.

InIn real real ore ore systems, systems, ideal ideal separation separation is is impossible impossible due due to to imperfect imperfect mineral mineral liberation. liberation. Therefore,Therefore, the the actual actual separation separation curve curve will will deviate deviate from from the theideal—this ideal—this deviation deviation was wasob- servedobserved in the in the behavior behavior of ofthe the Head Head Composite Composite during during heavy heavy liquid liquid separation separation testing testing (black(black line, line, FigureFigure8 ).8). Deviation Deviation from from the the ideal ideal complicates complicates the selectionthe selection of an of S.G. an cutS.G. point cut pointfor dense for dense media media circuit circuit operation operation as there as there will bewill a be tradeoff a tradeoff between between product product grade grade and andrecovery. recovery. TheThe minimum minimum spodumene spodumene concentrate concentrate grade grade required required for for downstream downstream processing processing dependsdepends on on the the end-use end-use of of the the lithium lithium and and downstream downstream processing processing techniques, techniques, varying varying fromfrom approximately approximately 6.0% 6.0% Li Li22OO to to 7.5% 7.5% Li Li22OO[ [30,34].30,34]. Because Because the the sharpness sharpness of of separation separation deviatesdeviates from from the ideal case,case, aa recoveryrecovery loss loss must must be be incurred incurred to to produce produce a final a final concentrate concen- tratethat meetsthat meets grade grade specifications. specifications. Figure Figure5 shows 5 shows that a cutthat point a cut of point 2.90 producedof 2.90 produced a concen- a trate grading 6.29% Li O; beyond this cut point, the grade increased and lithium recovery concentrate grading 6.29%2 Li2O; beyond this cut point, the grade increased and lithium recoverydecreased. decreased. At S.G. 2.90, At S.G. global 2.90, lithium global recoverylithium recovery to the sink to productthe sink product was 56.2% was in 56.2% 11.9% in of − µ 11.9%the global of the mass global (i.e., mass considers (i.e., considers that 23.5% that of 23.5% the mass of the reported mass reported to the 840to them −840 fraction µm fractionand was and not was subject not tosubject heavy to liquid heavy testing). liquid testing). While these While results theseare results positive are positive in terms in of concentrate grade, the lithium recovery was low compared to typical expected values for terms of concentrate grade, the lithium recovery was low compared to typical expected hard-rock processing by other separation techniques, such as flotation [27]. During HLS values for hard-rock processing by other separation techniques, such as flotation [27]. testing, the S.G. cut point that produces a high-grade, high-recovery sink product is a During HLS testing, the S.G. cut point that produces a high-grade, high-recovery sink good indicator of the degree of spodumene liberation; i.e., the lower the S.G. cut point at product is a good indicator of the degree of spodumene liberation; i.e., the lower the S.G. which a high-grade, high-recovery concentrate can be produced, the better the spodumene cut point at which a high-grade, high-recovery concentrate can be produced, the better liberation. the spodumene liberation. Examining Figure5 at a cut point of S.G. 2.70, global lithium recovery to the sink Examining Figure 5 at a cut point of S.G. 2.70, global lithium recovery to the sink product was high (>90% based on HLS feed). While the concentrate grade produced at product was high (>90% based on HLS feed). While the concentrate grade produced at this cut point (4.65% Li2O) did not meet minimum specifications, there was still significant this cut point (4.65% Li2O) did not meet minimum specifications, there was still significant mass rejection; nearly 80% of the global mass reported to the float fraction. The HLS mass rejection; nearly 80% of the global mass reported to the float fraction. The HLS re- results suggested that a multi-stage DMS circuit would be an effective way to recover and sultsupgrade suggested spodumene that a from multi-stage the Hidden DMS Lake circuit pegmatites; would be using an effective an initial way DMS to stage recover aimed and at upgraderejecting spodumene mass and silicate from the minerals Hidden (to Lake improve pegmatites; flotation using performance) an initial whileDMS stage maintaining aimed high lithium recovery, followed by a second DMS stage to produce a concentrate that meets minimum grade specifications.

3.3. Dense Media Separation Testing Using the results of heavy liquid separation, a DMS continuous test was designed and conducted according to the ﬂowsheet shown in Figure9. Minerals 2021, 11, x FOR PEER REVIEW 12 of 17

at rejecting mass and silicate minerals (to improve flotation performance) while maintaining high lithium recovery, followed by a second DMS stage to produce a concentrate that meets minimum grade specifications.

3.3. Dense Media Separation Testing Using the results of heavy liquid separation, a DMS continuous test was designed and conducted according to the flowsheet shown in Figure 9. Minerals 2021, 11, x FOR PEER REVIEW 12 of 17

3.3. Dense Media Separation Testing Minerals 2021, 11, 649 Using the results of heavy liquid separation, a DMS continuous test was designed12 of 17 and conducted according to the flowsheet shown in Figure 9.

Figure 9. DMS circuit flowhseet.

Figure 10 compares the results of the DMS with those of the HLS testing. The DMS test successfully produced a final spodumene concentrate (i.e., sink product at S.G. 2.90) grading 6.11% Li2O; similar to that produced in HLS. Lithium recovery to the spodumene concentrate was 6.7 percentage points lower in DMS compared with HLS (49% vs. 56% recovery), attributed to process scale-up; HLS is a simulated environment that is easily controlled, whereas DMS is operated continuously, using solid particulate matter to control fluid-specific gravity. In particular, control of the dense media set point is difficult at scale; minor fluctuations in the media density tend to occur, resultingFigure in changes 9. DMS in circuitmass ﬂowhseet.distribution to each of the products. The magnitude of Figure 9. DMS circuit flowhseet. these changes is a function of the mineralogy of ore. The closer the specific gravities of the ore andFigure gangue 10 compares minerals, the and results the more of the middlings DMS with particles those of with the intermediate HLS testing. Thedensities, DMS test successfullyFigure 10 compares produced the a results ﬁnal spodumene of the DMS concentrate with those (i.e.,of the sink HLS product testing. at The S.G. DMS 2.90) testthe moresuccessfully sensitive produced the process a final will spodumene be to changes co ncentratein dense media-specific(i.e., sink product gravity. at S.G. 2.90) grading 6.11% Li2O; similar to that produced in HLS. grading 6.11% Li2O; similar to that produced in HLS.

Lithium recovery to the spodumene concentrate7 was 6.7 percentage points lower in

DMS60 compared with HLS (49% vs. 56% HLS recovery Recovery ), attributed to process scale-up; HLS is a simulated environment that is easily controlled, DMS Recovery whereas6 DMS is operated continuously, HLS Assay using50 solid particulate matter to control fluid-specific gravity. In particular, control of the DMS Assay 5 dense media set point is difficult at scale; minor fluctuations in the media density tend to O) occur,40 resulting in changes in mass distribution to each4 2 of the products. The magnitude of these changes is a function of the mineralogy of ore. The closer the specific gravities of the ore 30and gangue minerals, and the more middlings3 particles with intermediate densities, the more sensitive the process will be to changes in dense media-specific gravity.

Li Recovery (%) 2 20 Assay (% Li

1 10 7

60 HLS Recovery 0 0 DMS Recovery 6 Spodumene Flotation Feed HLSTailings Assay 50 Concentrate (DMS Midds & -850 μm) DMS Assay 5 O)

Figure 10. Comparison of heavy liquid separation and dense2 media separation results. Figure40 10. Comparison of heavy liquid separation and dense4 media separation results. Lithium recovery to the spodumene concentrate was 6.7 percentage points lower in 30 3 DMS compared with HLS (49% vs. 56% recovery), attributed to process scale-up; HLS is a

Li Recovery (%) 2 simulated20 environment that is easily controlled, whereas Assay (% Li DMS is operated continuously, using solid particulate matter to control fluid-specific gravity. In particular, control of the 1 dense10 media set point is difficult at scale; minor fluctuations in the media density tend to

occur, resulting in changes in mass distribution to each0 of the products. The magnitude of these0 changes is a function of the mineralogy of ore. The closer the specific gravities of the Spodumene Flotation Feed Tailings ore and gangueConcentrate minerals,(DMS Midds and & -850 the μm) more middlings particles with intermediate densities, the more sensitive the process will be to changes in dense media-specific gravity. FigureThe 10. Comparison results showed of heavy higher-grade liquid separation and lithiumand dense distribution media separation to the results. DMS middlings fraction (i.e., the float product at S.G. 2.90), with only a minor increase in lithium reporting to the silicate final tailings. As expected, DMS operation was slightly less efficient than heavy liquid separation. However, lithium units lost during DMS may still be recovered

by ﬂotation, should it be implemented to treat the DMS middlings and −840 µm fraction.

The results showed higher-grade and lithium distribution to the DMS middlings fraction (i.e., the float product at S.G. 2.90), with only a minor increase in lithium reporting to the silicate final tailings. As expected, DMS operation was slightly less efficient than heavy liquid separation. However, lithium units lost during DMS may still be recovered by flotation, should it be implemented to treat the DMS middlings and −840 µm fraction.

3.4. Effect of the S.G. Cut Point in DMS Operation A sensitivity analysis (based interpolation of the heavy liquid separation results) was used to quantify the effect of changes in dense media S.G. cut point on overall process performance. First, the specific gravity cut point of DMS Pass 1 was increased from 2.60 to 2.90 in increments of 0.05. It was assumed that the S.G. cut point of DMS Pass 2 remained constant at 2.90 (meaning that the grade of the sink fraction was constant at ~6.3% Li2O), and that all DMS middlings and −840 µm material could be subjected to flotation with an expected lithium (stage) recovery of 80% and concentrate grade of ~6.4% Li2O; grade and recovery values were selected based on typical flotation test results for Hidden Lake material (the details of flotation testing will be discussed in a subsequent paper, cur- Minerals 2021, 11, 649 13 of 17 rently under preparation). We acknowledge that flotation performance would likely vary with changes in feed composition. However, this variation would not materially change the conclusions drawn fromperformance. the analysis, First, and the thus specific was gravity assumed cut to point be constant of DMS Passfor the 1 was sake increased of discussion. from 2.60 Like- to wise,2.90 in the increments values of ofthe 0.05. flotation It was grade assumed and thatrecovery the S.G. selected cut point will ofnot DMS affect Pass the 2 trends remained ob- servedconstant in atthe 2.90 analysis. (meaning The thatresults the of grade the first of the sensitivity sink fraction analysis was constant(Figure 11) at ~6.3%showed Li that2O), changesand that in all the DMS operation middlings of the and first−840 DMSµm stage material (i.e., could fluctuations be subjected in specific to flotation gravity with cut point)an expected influenced lithium recovery (stage) relatively recovery ofequally 80% and with concentrate each incremental grade of ~6.4%change Li in2O; specific grade gravity.and recovery values were selected based on typical flotation test results for Hidden Lake materialSince (the the detailsfirst DMS of flotation stage directly testing controls will be the discussed mass and ina lithium subsequent reporting paper, tocurrently the final tailings,under preparation). the lower the S.G. cut point, the less mass (and therefore, less lithium) reported to theWe tailings. acknowledge However, that more flotation mass was performance retained in would the middlings likely vary fraction with changes and sent in to feed the flotationcomposition. circuit, However, along with this the variation −840 µm would material. not materiallyFigure 11 shows change that the at conclusions a specific gravity drawn lessfrom than the analysis,2.70, the andrelationship thus was assumedbetween tolithium be constant recovery for theand sake mass of diverges—minor discussion. Likewise, in- crementalthe values gains of the in flotation recovery grade corresponded and recovery to a selected significant will increase not affect in thethe trendsmass reporting observed toin grinding the analysis. and The flotation. results A of drop the first in S.G. sensitivity cut point analysis (whether (Figure by 11design) showed or due that to changes opera- tionalin the variation) operation of of 0.1 the from first 2.70 DMS to stage 2.60 caused (i.e., fluctuations the mass flow in specific to the gravitygrinding/flotation cut point) circuitinfluenced to increase recovery by relatively44% percentage equally points with each(from incremental 34% to 78%). change in specific gravity.

100 100

90 90

80 80

70 70

60 60

50 50 Li recovery (%)

40 40

30 30

20 20 Projected* global Mass (%) to grinding/flotation circuit 10 Mass (%) to grinding/flotation 10 Li recovery to combined concentrate 0 0 2.60 2.65 2.70 2.75 2.80 2.85 2.90 S.G. Cut Point in DMS Pass 1 FigureFigure 11. 11. TheThe effect effect of of S.G. S.G. cut cut point point in in DMS DMS Pass Pass 1 1 on on mass mass recovery recovery and and global global lithium lithium recovery recovery (combined(combined DMS DMS and and flotation). ﬂotation).

FromSince a the process first DMS design stage perspective, directly controls the im theplementation mass and lithiumof DMS reporting as a pre-concentra- to the final tiontailings, stage the ahead lower of theflotation S.G. cut presents point, an the oppo lessrtunity. mass (and The therefore, size of the less equipment lithium) reportedrequired to the tailings. However, more mass was retained in the middlings fraction and sent to the flotation circuit, along with the −840 µm material. Figure 11 shows that at a specific gravity less than 2.70, the relationship between lithium recovery and mass diverges—minor incremental gains in recovery corresponded to a significant increase in the mass reporting to grinding and flotation. A drop in S.G. cut point (whether by design or due to operational variation) of 0.1 from 2.70 to 2.60 caused the mass flow to the grinding/flotation circuit to increase by 44% percentage points (from 34% to 78%). From a process design perspective, the implementation of DMS as a pre-concentration stage ahead of flotation presents an opportunity. The size of the equipment required would be nearly cut in half, reducing capital costs. Grinding costs and energy consumption would also be significantly reduced. However, from an operational perspective, the sensitivity of the process to minor changes in specific gravity can be challenging and requires tight control of media density. Without control of the specific gravity of the dense media, the variability in mass reporting to downstream processes will be unmanageable. Constant changes in mill throughput will make grinding circuit optimization nearly impossible and without achieving the target grind size, flotation performance will suffer significantly. This may require that the DMS and flotation circuits be “decoupled” from each other with buffer capacity included in the process design. Minerals 2021, 11, x FOR PEER REVIEW 14 of 17

would be nearly cut in half, reducing capital costs. Grinding costs and energy consumption would also be significantly reduced. However, from an operational perspective, the sensitivity of the process to minor changes in specific gravity can be challenging and requires tight control of media density. Without control of the specific gravity of the dense media, the variability in mass reporting to downstream processes will be unmanageable. Constant changes in mill throughput will make grinding circuit optimization nearly im- Minerals 2021, 11, 649 14 of 17 possible and without achieving the target grind size, flotation performance will suffer significantly. This may require that the DMS and flotation circuits be “decoupled” from each other with buffer capacity included in the process design. A second sensitivityA second analysis sensitivity was conducted analysis was to assess conducted the effect to assess of changes the effect in the of S.G. changes in the S.G. cut point in thecut second point stage in the of second DMS stage(DMS of Pa DMSss 2) (DMS on the Pass process 2) on performance. the process performance. The spe- The specific cific gravity cutgravity point of cut DMS point Pass of DMS2 was Passincreased 2 was from increased 2.70 to from 3.10 2.70in increments to 3.10 in of increments 0.05. of 0.05. It It was assumedwas that assumed the S.G. thatcut point the S.G. of DMS cut point Pass of1 remained DMS Pass constant 1 remained at 2.70, constant and that at 2.70, and that again, all DMSagain, middlings all DMS and middlings −840 µm material and −840 couldµm materialbe subjected could to be flotation subjected with to flotationan with an expected lithiumexpected (stage) lithiumrecovery (stage) of 80% recovery and concentrate of 80% and grade concentrate of ~6.4% grade Li2O. ofFigure ~6.4% 12 Li 2O. Figure 12 shows the effectshows of S.G. the cut effect point of in S.G. DMS cut Pass point 2 inon DMS lithium Pass recovery 2 on lithium and mass recovery flow andto the mass flow to the grinding/flotationgrinding/flotation stage (left); and stage the effect (left); of and S.G. the cut effect point of on S.G. the cut DMS point final on concentrate the DMS final concentrate grade (right). grade (right).

100 100 7.5

80 80 7.0

6.5 O) 60 60 2 6.0

40 40 5.5 Grade (% Li Grade

5.0 20 20 Minimum

Mass (%) to grinding/flotation (%) recovery Li global Projected* Mass (%) to grinding/flotation circuit to grinding/flotation (%) Mass 4.5 DMS Concentrate Li recovery to combined concentrate 0 0 2.72.82.93.03.1 2.7 2.8 2.9 3.0 3.1 S.G. Cut Point in DMS Pass 2 S.G. Cut Point in DMS Pass 2 (a) (b)

Figure 12. TheFigure effect 12. ofThe changes effect of in changes the speciﬁc in the gravity specific cut gravity point ofcut DMS point Pass of DMS 2 on Pass (a) mass 2 on recovery (a) mass andrecov- global lithium recovery, andery (b and) DMS global concentrate lithium recovery, grade. and (b) DMS concentrate grade.

In the second Instage the of second DMS, stagechanges of DMS, in specific changes gravity in specific cut point gravity showed cut pointlimited showed limited variability in thevariability mass reportin in theg mass to downstream reporting to processes, downstream as this processes, was largely as this a function was largely a function DMS Pass 1 (asDMS discussed Pass 1 above, (as discussed drastic above,mass recovery drastic masschanges recovery occurred changes when occurred the S.G. when the S.G. dropped belowdropped 2.70 in DMS below Pass 2.70 1). in However, DMS Pass DMS 1). However, Pass 2 controlled DMS Pass the 2 controlledgrade of the the grade of the final DMS concentrate.final DMS Figure concentrate. 12 shows Figure that 12 when shows DMS that Pass when 2 DMSwas Passoperated 2 was below operated an below an S.G. S.G. cut point 2.85cut point (very 2.85 clos (verye to the close operational to the operational cut point cutof 2.90), point ofthe 2.90), DMS the concentrate DMS concentrate failed failed to meet tominimum meet minimum concentrate concentrate grade specifications grade specifications (approximately (approximately 6.0% Li2 6.0%O), in- Li2O), indicated dicated by the bydashed the dashed horizont horizontalal line. Therefore, line. Therefore, although although there was there limited was impact limited on impact on mass mass distributiondistribution (assumed (assumed steady operation steady operation of DMS ofPass DMS 1), Passthe concentrate 1), the concentrate grade was grade was highly highly sensitivesensitive to minor to variation minor variation in dense in media-specific dense media-specific gravity gravityin DMS in Pass DMS 2. PassIn prac- 2. In practice, it is tice, it is possiblepossible that variations that variations in the in cut the points cut points of both of both DMS DMS Pass Pass 1 and 1 and 2 2may may occur. occur. These would These would resultresult in in changes changes in in the the mass mass reporting reporting to to the the grinding grinding circuit circuit and and the the DMS DMS concentrate concentrate grade.grade. Further, Further, due due to tothe the genera generall heterogeneity heterogeneity of of ores, ores, changes changes in in the the modal modal mineralogy of mineralogy of the feed toto thethe DMSDMS circuit circuit would would also also be be expected expected to to cause cause changes changes to theto the mass distribution mass distributionthrough through the the DMS DMS circuit. circuit. 4. Summary and Conclusions Dense media separation was shown as an effective means to separate spodumene from associated gangue minerals from the Hidden Lake pegmatites in NWT, Canada, despite similarities in mineral-specific gravities. Even for minerals with a relatively low concentration criterion, such as muscovite mica, HLS results and subsequent DMS testing showed the selective recovery of spodumene over other silicate minerals. A two-stage, continuously operated dense media separation test conducted on +840 µm material produced a concentrate grading 6.11% Li2O with ~50% lithium recovery; 50% of the original mass was rejected to the DMS tailings with only 8% of the lithium. The remaining mass (“DMS middlings”) was combined with the −840 µm fraction to form flotation circuit feed; a paper discussing the details of flotation testing in under preparation. These results illustrate the benefit of dense media separation in spodumene beneficiation circuits, including the Minerals 2021, 11, 649 15 of 17

opportunity to reject a significant quantity of mass prior to grinding, reducing the required capital and operating expenditures of the grinding and flotation circuits. Sensitivity analysis of the specific gravity cut point in the dense media circuit showed minor changes (+/−0.05) in the specific gravity of the dense media resulted in significant changes to the mass rejected by DMS and to the grade of the DMS concentrate. The downstream effect of these changes, particularly in mass flow to the grinding circuit, may limit process feasibility and operability. To mitigate these effects, several things might be considered in process design: (1) a robust process control system that closely monitors and adjusts the specific gravity of the dense media, as required; (2) decoupling of the density media and grinding/flotation circuits (i.e., inclusion of buffer capacity in the process design); (3) ore sorting/blending systems that classify feed to the DMS circuit based on its modal mineralogy and expected behavior in DMS to minimize the need for frequent changes in media density; and (4) elimination of flotation circuits, towards a DMS-only flowsheet (as long as the process produces a concentrate of sufficient grade and the reduced risk and costs with this flowsheet option offset reduced lithium recoveries). This may include the implementation of a magnetic separation circuit on the crushed feed to remove well-liberated, magnetic mineral particles that may tend to report to the sink fraction in dense media separation. Overall, the results of this study demonstrate the importance of dense media separation in the concentration of spodumene from the Hidden Lake occurrence, and by extension, the potential importance to the concentration of lithium from other hard-rock deposits. It is hoped that careful consideration of the benefits of this separation technique—and its operational challenges—will assist to develop beneficiation processes that use as little energy and water as possible and yield maximum lithium recoveries.

Author Contributions: Conceptualization, M.A.; methodology; T.G., M.A., D.L.S., L.B.; validation, M.A., T.G.; formal analysis, C.E.G., T.G., M.A., writing—original draft preparation, C.E.G.; writing— review and editing, C.E.G., T.G., M.A., D.L.S., L.B.; supervision, D.L.S., L.B., project administration, M.A., T.G.; funding acquisition, D.L.S., L.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by Patriot Battery Metals Inc. and Far Resources as a part of the process development program for the pegmatites from Hidden Lake, NWT. Personnel from Far Resources and Gaia Metals Corp. (L. Bottomer and D. Smith, respectively) were involved in various aspects of program design and program management. They were not directly involved in the collection, analysis, and interpretation of data. Data Availability Statement: The data presented in this study are available on request from authors L. Bottomer and D. Smith. The data are not publicly available due to the terms of the project. Acknowledgments: We would like to acknowledge Far Resources and Patriot Battery Metals Inc. for their input and support of the laboratory test work and for permission to share results. We would also like to acknowledge SGS Canada (Lakefield) for their ongoing support of this research. Conflicts of Interest: The authors declare no conflict of interest.

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