A Process Mineralogy Approach to Gravity Concentration of Tantalum Bearing Minerals

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Article A Process Mineralogy Approach to Gravity Concentration of Tantalum Bearing Minerals

Yousef Ghorbani * ID , Rob Fitzpatrick, Melanie Kinchington, Gavyn Rollinson and Patrick Hegarty Camborne School of Mines, College of Engineering, Mathematics & Physical Sciences (CEMPS), University of Exeter, Penryn Campus, Exeter, Cornwall TR10 9FE, UK; [email protected] (R.F.); [email protected] (M.K.); [email protected] (G.R.); [email protected] (P.H.) * Correspondence: [email protected]; Tel.: +44-(0)759-603-4619

Received: 3 August 2017; Accepted: 12 October 2017; Published: 13 October 2017

Abstract: The historic Penouta mine in northwest Spain is the focus of efforts to extract tantalum from tin mining waste. This paper describes the characterisation of the tantalum mineralogy of waste material from the deposit. Characterisation was realised using quantitative mineralogy and geochemistry. This paper further identiﬁes other phases of interest and investigates the potential for extraction using gravity separation techniques. The gravity concentrate obtained through these tests was analysed using quantitative mineralogy and electron probe microanalysis. Following characterisation of the sample material to identify the key Ta-bearing mineral phases and assess liberation, a series of gravity separation trials were conducted using Heavy Liquid Separation (HLS), Mozley table, Knelson concentrator separation and shaking table. The laboratory shaking table used to conduct a rougher test and a rougher/cleaner test to simulate a spiral-table circuit using the Penouta material. Mass balance calculations were carried out to calculate the contained metal content of the feed material and concentrate products in order to assess recovery rates for Ta, Sn and Nb across a range of grains sizes. Ta was found to be present predominantly in the solid-solution columbite-group mineral, along with minor Ta present as microlite and as impurities within cassiterite. It was found that over 70% of the Ta is contained within the −125 µm fraction, with the Ta-bearing minerals tantalite and microlite being closely associated with quartz. Mozley table separation resulted in recoveries of 89% Ta and 85% Nb for the −125 µm fraction. The Knelson Concentrator trial was carried out on the −625 µm size fraction, thereby eliminating low grade material found in the coarsest fractions. Size analysis of the recovery rate for each product, shows that the Knelson concentrator is most efﬁcient for recovery of −125 µm particles.

Keywords: process mineralogy; tantalum bearing minerals; gravity concentration; Penouta mine

1. Introduction Tantalum has an average crustal abundance of 1.7 ppm [1] making it the 52nd most abundant element in the crust [2]. Its unique chemical and physical properties including superconductivity, corrosion-resistance, a very high melting temperature (2996 ◦C), shape memory properties, a high coefﬁcient of capacitance and bio-compatibility [3,4] make it an important metal in modern society. Speciﬁc industrial and medical applications range from metal alloys, carbides, chemicals, electronic components such as capacitors and equipment used for surgery. New technologies, leading to the miniaturisation of electronic devices have resulted in increased use of tantalum. These applications make tantalum an increasingly important metal which is considered of high importance for the European Union [5,6]. Tantalum is not currently produced in the European Union; the EU is entirely dependent on imports and due to concerns over reliability of future access to tantalum ore [1], up until 2014,

Minerals 2017, 7, 194; doi:10.3390/min7100194 www.mdpi.com/journal/minerals Minerals 2017, 7, 194 2 of 23 the European Commission had tantalum listed as a critical raw material [2]. The latest update to the critical raw materials list has removed tantalum from the list due to changes in primary tantalum production; however, despite the downgrade in its status, it is still considered a “strategic mineral” due to its widespread use and reliance on imports. Deposits are scarce, thus new methods have to been developed to exploit low grade deposits and also tailings from Sn deposits. Russia is currently the only country in Europe where tantalum is mined [5–7], although several potential projects can be also considered, such as alkaline intrusives of the central part of the Kola Peninsula, in Russia, Li–Sn–Ta–bearing pegmatites in Finland and the Beauvoir granite in the French Central Massif [8–10]. This is also the case of the Penouta Sn–Ta–Nb granite, in Spain, which contains 95.5 Mt of Measured and Indicated Mineral Resources with average grades of 77 ppm Ta and 443 ppm Sn [11]. The Penouta ore deposit is located in the Penouta village, municipality of Viana do Bolo, Ourense, Galicia, northeast of Spain. It is a Sn–Ta greisen-type ore deposit where mineralization occurs in quartz veins related to the greisen as in the surrounding leucogranites [12]. Historically, mining in the Penouta area has been carried out since Roman times, with small underground tunnels which followed cassiterite mineralized quartz veins hosted by a peraluminous albite leucogranite. Mining in the Penouta deposit is documented since 1906, but it was not extensively exploited until the 1970s, mainly to obtain cassiterite from a granitic cupola and a swarm of related hydrothermal quartz veins, while Ta was obtained as by-product. Processing during this time was by gravity separation and restricted to the −2 mm fraction. The +2 mm fraction was combined with gravity processing tailings and stored in dams. Thus, a great amount of cassiterite and columbite-group minerals (CGM) was not liberated from the hosting rock and was progressively accumulated in the tailings ponds. As a consequence, sands from tailings reached similar grades as those of the original granite [13–15]. Studies undertaken between 1961 and 1985 indicated 13 Mt of reserves with average grades of 750 ppm Sn and 90 ppm Ta [14,15]. However, the drop of metal prices led to the deﬁnitive closure of the mine in 1985, has meant that there has been scarce scientiﬁc research into the deposit since this time. However, the lack of detailed mineralogical information hindered separation of metals from sands. The revival of mining in recent years has encouraged the mining company Strategic Minerals Spain to carry out new exploration on the resources of the Penouta deposit [11]. This study describes a process mineralogy approach to gravity concentration of Tantalum bearing minerals. The implementation of chemical and mineralogical data is investigated to characterise material from tailings from Penouta and to link this to potential gravity separation methods to determine the extractability of tantalum from historical tailings material.

2. Materials and Methods

2.1. Ore Sample Samples were collected during January 2015 from the Penouta Mine site by representatives from OptimOre at the Universitat Politècnica de Catalunya (UPC). Sixty-four kilograms of material from the Balsa Grande tailing was shipped to the Camborne School of Mines (CSM), University of Exeter laboratories in Penryn, where it was split into representative 1 kg samples.

2.2. Chemical and Mineralogical Analysis All analysis mentioned below was conducted at the Camborne School of Mines, University of Exeter, Penryn Campus, Exeter, UK. A JEOL JSM-5400LV scanning electron microscope (SEM) was used in low vacuum mode alongside optical microscopy to qualitatively assess the mineralogy of the sample. A small number of particles were selected for analysis by Energy Dispersive Spectrometer (EDS) to aid in mineral identiﬁcation. The element composition of each particle was used to assist in mineral identiﬁcation. Quantitative mineralogical analysis was conducted on the samples using a QEMSCAN® 4300 (at Camborne School of Mines, University of Exeter, Penryn Campus, Exeter, UK) which is based on a Minerals 2017, 7, 194 3 of 23

Zeiss EVO 50 series SEM and consists of four light elements Bruker SDD (Silicon Drift Droplet) Energy Dispersive X-ray Spectrometers (EDS) and an electron backscatter detector [16]. Both the Fieldscan and PMA measurement modes were used on the samples depending on the sample particle sizes. iMeasure v. 4.2 was used for data acquisition, and iDiscover v. 4.2 and 4.3 were used for the data processing. Bulk geochemical analysis was carried out using a Bruker S4 Pioneer WDS X-ray Fluorescence (XRF) instrument. The results from QEMSCAN® were validated by comparison with XRF data. Portable X-ray Fluorescence analysis (PXRF) was carried out using an Olympus DP-6000C PXRF to provide more in depth analysis on sized fractions of gravity test products. PXRF results were calibrated using results from the WDS XRF analyser. Detailed spatially resolved chemical analysis of minerals was carried out by Electron Probe Microanalysis (EPMA) using a JEOL JXA-8200 Electron Microprobe.

2.3. Sample Preparation and Experimental Approach for Gravity Concentration

2.3.1. HLS and Mozley Table A 1 kg representative sample of the Penouta tailings was split into two sub-samples using a rifﬂe √ Splitter box. One sample was screened on a 2 system, splitting the sample into nine size fractions (Table1).

Table 1. Size fractions acquired through sieving of a representative sample from Penouta.

Sieve Size Fraction Mass (g) Mass (%) Separation Method +2800 µm 4.0 0.76 N/A (−2800/+1400) µm 66.1 12.51 (−1400/+1000) µm 75.8 14.35 (−1000/+710) µm 61.8 11.70 (−710/+500) µm 65.3 12.36 HLS (−500/+355) µm 62.6 11.85 (−355/+250) µm 57.6 10.90 (−250/+180) µm 46.0 8.71 (−180/+125) µm 39.4 7.46 Mozley Table −125 µm 49.7 9.41

The size fraction +2800 µm was not subjected to testing due to insufficient amounts of material. The size fractions (−180/+125) µm and −125 µm were not subjected to HLS due to practical difficulties in recovery from solution. Instead, these size fractions underwent separation by Mozley table. HLS of the coarse size fractions (−2800/+180 µm) was undertaken using sodium polytungstate (SPT) with a specific gravity of 2.86 as the medium. The fine fractions (−180/+125) µm and −125 µm were analysed by Mozley Table with a sand deck. Five products were collected for each size fraction: High Grade 1, High Grade 2, High Grade 3, Middlings and Tailings.

2.3.2. Knelson Concentrator Approximately 20 kg of bulk material from Penouta was screened at 625 µm to obtain 10 kg of sample at −625 µm for centrifugal separation. A 10 kg sample was used because [17] states that for feed masses <10 kg an overestimation of gravity recoverable content can occur. A bulk separation trial was undertaken using a Knelson 3” laboratory concentrator. The test was run under the conditions of: feed rate of 0.49 kg/h, water pressure of 6 PSI (40 kpa) and a g force of 60 (chosen as this is the industry standard g force used in working plants). The test conditions were chosen based on the test conditions used by [17] during separation trials for gold. Six products were produced from the trial: a concentrate and ﬁve tailings products. The tailings products were collected over set time periods (Table2). A feed rate of 0.49 kg/min indicates a run time of two minutes per 1 kg of feed material. Minerals 2017, 7, 194 4 of 23

Minerals 2017, 7, 194 4 of 23 Table 2. Tailings collection times for Knelson concentrator trial, based on movement of feed through Table 2. Tailings collection times for Knelson concentrator trial, based on movement of feed through the concentrator. the concentrator.

TailingTailing Product Product Time Time (min) (min) Feed Feed (Kg) (Kg) 1 10–2 0–2 11 2 22–6 2–6 22 3 36–10 6–10 22 4 410–14 10–14 22 5 514–20 14–20 33

FollowingFollowing thethe trial,trial, eacheach productproduct waswas splitsplit intointo threethree samplessamples usingusing aa rifﬂeriffle splittersplitter box.box. OneOne halfhalf √ ofof thethe productsproducts were were screened screened into into eight eight size size fractions fractions using using a a2 √ sieve2 sieve system system and and analysed analysed for for Ta, Ta, Sn andSn and Nb Nb using using PXRF. PXRF. The The other other half half of the of productsthe products was was split split repeatedly repeatedly until until approximately approximately 50–60 50– g were60 g were obtained; obtained; this split this wassplit sentwas forsent analysis for analysis by Semi-Quantitative by Semi-Quantitative XRF (SQ-XRF).XRF (SQ-XRF).

2.3.3.2.3.3. Shaking Table TheThe laboratory shaking table table used used in in the the stud studyy was was a alaboratory laboratory Wilfley Wilfley table table model model 800. 800. A Arougher rougher test test and and a arougher/cleaner rougher/cleaner test test (used (used to to si simulatemulate a a spiral-table spiral-table circuit) circuit) were were run using thethe PenoutaPenouta material. To To generate generate suff sufficienticient feed feed material material for for the the roug rougher/cleanerher/cleaner test test approximately approximately 27 27kg kgof ofPenouta Penouta material material was was screened screened at 625 at 625 μm.µ m.A total A total of 10.77 of 10.77 kg of kg − of625− feed625 feedmaterial material was wasproduced. produced. The feed The feedfor the for thecleaner cleaner table table was was taken taken from from the the first first three three product streamsstreams ((concentrate/middlingsconcentrate/middlings = = 3.62 3.62 kg) kg) with with an an additional additional 1.38 1.38 kg kg of of material material from from product stream 44 added toto makemake thethe initialinitial feed feed weight weight up up to to 5.00 5.00 kg. kg. A A schematic schematic of of the the tests tests with with the the mass mass flows flows is shownis shown in Figurein Figure1. 1.

Figure 1. Experimental procedure andand massmass ﬂowflow ratesrates forfor auditaudit testingtesting usingusing PenoutaPenouta material.material.

3.3. Results and Discussions

3.1.3.1. Gravity Concentration UsingUsing HLSHLS SinkSink massmass fractionsfractions increasedincreased atat smallersmaller particleparticle sizes,sizes, particularlyparticularly inin thethe sizesize fractionsfractions ((−−500/+355)500/+355) μµmm and and ( (−−355/+250)355/+250) μm.µm. Mass Mass balance balance calculations calculations showed showed that that Ta, Ta, Sn Sn and Nb areare concentratedconcentrated in thethe finerfiner fractions.fractions. Table3 3 shows shows the the distribution distribution ofof sink sink and and float float for for each each particle particle sizesize fraction.fraction. ComparisonComparison of of this this study study to to the the work wo donerk done by Montero-Rby Montero-Ríosíos [18] show[18] show both studiesboth studies have foundhave found that the that majority the majority of contained of contained metal metal is located is located in the in finest the finest size fractions.size fractions. Montero-R Montero-Ríosíos [18] found[18] found that 83%that of83% Ta of and Ta Sn and are Sn located are located in the in− 630the −µ630m fraction, μm fraction, compared compared to 90% to Ta90% and Ta 62% and Sn 62% in theSn in− 500the µ−500m calculated μm calculated from from the mass the mass balance balance (Figure (Figure2). 2).

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Table 3. Products produced from heavy liquid separation using SPT with a speciﬁc gravity (SG) = 2.86. Sink and ﬂoat products are reported as weight per cent of that size fraction and weight per cent of the whole sample.

Original Sample Size (g) Lost Sink Product Float Product Size Fraction (µm) (g) (wt %) (g) (wt % Fraction) (wt % Total) (g) (wt % Fraction) (wt % Total) (g) (wt % Fraction) (wt % Total) +2800 4 0.76 ------(−2800/+1400) 66.1 12.51 0.75 1.13 0.14 0.25 0.38 0.05 65.10 98.49 12.32 (−1400/+1000) 75.8 14.35 0.80 1.06 0.15 0.20 0.26 0.04 74.80 98.68 14.16 (−1000/+710) 61.8 11.70 0.68 1.10 0.13 0.19 0.31 0.04 60.93 98.59 11.53 (−710/+500) 65.3 12.36 0.79 1.21 0.15 0.27 0.41 0.05 64.24 98.38 12.16 (−500/+355) 62.6 11.85 1.20 1.92 0.23 1.50 2.40 0.28 59.90 95.69 11.34 (−355/+250) 57.6 10.90 1.40 2.43 0.27 1.10 1.91 0.21 55.10 95.66 10.43 (−250/+180) 46 8.71 1.40 3.04 0.27 0.50 1.09 0.09 44.10 95.87 8.35 (−180/+125) 39.4 7.46 Too ﬁne for HLS—analysed with Mozley Table −125 49.7 9.41 Total 528.3 100.00 - 1.33 - 0.76 - 80.29 MineralsMinerals2017 2017, 7, ,7 194, 194 66 of of 23 23

Ta Nb Sn 40

Grade % 20

0 0 200 400 600 800 1000 1200

Size fraction (μm)

FigureFigure 2. 2.Size Size vs. vs grade grade for for HLS HLS concentration. concentration.

FigureFigure3 shows3 shows the the sink sink (on (on the the left) left) and and float float (on (on the the right) right) products products produced produced by by HLS HLS for for each each sizesize fraction. fraction. ItIt cancan bebe clearlyclearly observedobserved inin thethe photos photos fromfrom the the coarser coarser size size fractions fractions that that the the float float productproduct is is primarily primarily composed composed of of quartz, quartz, with with feldspar feldspar and and mica. mica. In comparison In comparison the sinkthe sink products products are composedare composed predominantly predominantly of micas of andmicas metal-oxides and metal- (Feoxides and Mn),(Fe and with Mn), minor with quartz minor and quartz cassiterite. and Thecassiterite. majority The of observed majority grains of observed are rounded grains in are shape rounded across allin sizeshape fractions. across Theall size exceptions fractions. to thisThe areexceptions the biotite to and this muscovite are the biotite grains, and which muscovite show the grains, distinctive which platy show shape the distinctive of mica. platy shape of mica.Twenty-six minerals grains were selected from the (−355/+250) µm Sink fraction for analysis by SEMTwenty-six to aid in mineralminerals identification. grains were selected The majority from the of grains (−355/+250) sampled μm were Sink identifiedfraction for to analysis be biotite. by TwoSEM grains to aid of in alkali mineral feldspar identification. were identified; The onemajority was iron-stained of grains sampled whilst the were other identified showed theto be presence biotite. ofTwo Al indicating grains of alterationalkali feldspar to kaolin were was identified; underway. one A 200 wasµm iron-stained× 100 µm columbite whilst the grain other was showed identified the onpresence the edge of ofAl a indicating quartz grain. alterati Theon Ta to and kaolin Nb was peaks underway. showed equal A 200 intensities μm × 100 μ indicatingm columbite the grain mineral was wasidentified columbite on the rather edge than of a thequartz Ta-rich grain. tantalite. The Ta Oneand Nb grain peaks was showed identified equal as Fe-oxide; intensities the indicating brightness the ofmineral the grains was and columbite presence rather of trace than Al the suggests Ta-rich hematite tantalite. rather One grain than magnetitewas identified or goethite. as Fe-oxide; Several the unusualbrightness grains of the were grains identified. and presence One contained of trace Al Ba, suggests Fe, Al, hematite Si, P and rather trace Kthan and magnetite Ca. The remainingor goethite. grainsSeveral contained unusual Mn,grains Si, Al,were Fe, identified. K and trace One Ti andcontained Zn. However, Ba, Fe, theAl, presenceSi, P and of trace Ti and K Znand suggests Ca. The thatremaining they could grains also contained be pieces Mn, of metal Si, Al, contamination Fe, K and trace from Ti previousand Zn. processing.However, the presence of Ti and Zn suggestsTo have adequate that they sample could also mass, be QEMSCAN pieces of metal® analysis contamination was carried from out previous on two combined processing. sized sink samplesTo withhave size adequate fraction sample of (−2800/+500) mass, QEMSCANµm and® (analysis−500/+180) was µcarriedm from out the on HLS two trial combined as described sized insink Table samples1. The (with−2800/+500) size fractionµmsink of ( fraction−2800/+500) is composed μm and primarily(−500/+180) of biotiteμm from (23.88%), the HLS Mn-oxides trial as (22.52%),described muscovite in Table (20.85%),1. The (−2800/+500) Fe-oxides (11.39%),μm sink quartzfraction (5.41%), is composed chlorite primarily (3.68%), cassiteriteof biotite (23.88%), (3.56%), tourmalineMn-oxides (3.34%)(22.52%), and muscovite K-feldspar (20.85%), (2.84%). TheFe-oxides Ta–Nb (11.39%), minerals makequartz up (5.41%), a minor chlorite fraction (3.68%), of the productcassiterite with (3.56%), columbite tourmaline (0.09%), (3.34%) tantalite and (0.02%) K-feldspar and microlite (2.84%). (<0.01%) The Ta–Nb all present.minerals The make (− 500/+180)up a minor µfractionm sink fractionof the product is composed with columbite primarily of(0.09%), muscovite tantalite (60.59%), (0.02%) biotite and (19.77%),microlite (<0.01%) Mn-oxides all(5.04%), present. Fe-oxidesThe (−500/+180) (3.58%), μm tourmaline sink fraction (2.97%), is composed quartz (2.29%),primarily chlorite of muscovite (1.70%) (60.59%), and K-feldspar biotite (19.77%), (1.41%). The Mn- Ta–Nboxides minerals (5.04%), Fe-oxides make up a(3.58%), minor tourmaline fraction of the(2.97%), product quartz with (2.29%), columbite chlorite (0.19%), (1.70%) tantalite and K-feldspar (0.07%) and(1.41%). microlite The (<0.01%)Ta–Nb minerals all present. make There up is a a minor marked fraction increase of in the the product amount with of muscovite columbite present (0.19%), in comparisontantalite (0.07%) to the coarserand microlite fraction (<0.01%) sample. all As present. presented There in Figure is a 4marked, this may increase be due in to the increasedamount of muscovite present in comparison to the coarser fraction sample. As presented in Figure 4, this may

Minerals 2017, 7, 194 7 of 23 be due to the increased grain size of the muscovite (120 μm compared to 70 μm in the (−2800/+500) μgrainm fraction) size of theincreasing muscovite particle (120 densityµm compared enough to to 70 caµusem inmore the particles (−2800/+500) to sinkµ mduring fraction) the separation increasing process.particle density enough to cause more particles to sink during the separation process.

Figure 3. PhotographsPhotographs of of sink sink and and float ﬂoat pr productsoducts side side by by side side for for comparison, comparison, across across all all size size fractions. fractions. All photos were taken under 6× magnification.magniﬁcation.

MineralsMinerals2017 ,20177, 194, 7, 194 8 of 23 8 of 23

Minerals 2017, 7, 194 8 of 23

FigureFigure 4. 4.Muscovite Muscovite liberation in in different different size size fraction. fraction. Figure 4. Muscovite liberation in different size fraction. CassiteriteCassiterite has has a largea large average average graingrain sizesize of 123 123 μµm,m, which which combined combined with with the mineral the mineral mass mass percentage,percentage,Cassiterite indicates indicates has thata thatlarge it isit averageis fairly fairly well wellgrain liberated.liberated. size of 123 The The μ m,products products which ofcombined ofHLS HLS show showwith a mineral the a mineral mineral association associationmass of cassiterite with quartz (12%–22%), muscovite (7%–12%) and K-feldspar (5%–7%). There is also a of cassiteritepercentage, with indicates quartz that (12%–22%), it is fairly well muscovite liberated. (7%–12%) The products and of K-feldspar HLS show a (5%–7%). mineral association There is also a strong association with kaolinite (7%–12%), present as an alteration product of K-feldspar, however strongof cassiterite association with with quartz kaolinite (12%–22%), (7%–12%), muscovite present (7%–12%) as an and alteration K-feldspar product (5%–7%). of K-feldspar, There is also however a this is not considered to be of importance as kaolinite makes up <1% of the product mineralogy. this isstrong not consideredassociation with to be kaolinite of importance (7%–12%), as present kaolinite as makesan alteration up <1% product of the of product K-feldspar, mineralogy. however this isFigure not considered 5 shows that to be only of importance 11% of the asTa–Nb kaolinite minerals makes are up well <1% liberated of the product in the mineralogy.(−2800/+500) μm Figure5 shows that only 11% of the Ta–Nb minerals are well liberated in the ( −2800/+500) µm HLSFigure sink product. 5 shows Almost that only 70% 11% of ofthe the Ta–Nb Ta–Nb phases minerals are poorlyare well liberated, liberated most in the likely (−2800/+500) due to small μm HLS sink product. Almost 70% of the Ta–Nb phases are poorly liberated, most likely due to small HLSgrain sink size product.(over 85% Almost of the 70%Ta–Nb of mineralthe Ta–Nb grains phases are −are30 μpoorlym). It isliberated, unlikely most to be likelypossible due to to recover small − µ graingrainthis size material size (over (over 85%without 85% of of theresorting the Ta–Nb Ta–Nb to mineralultrafine mineral grainsgrindigrainsng. are are In −30 comparison,30 μm).m). It is It unlikely is27% unlikely of the to beTa–Nb to possible be possiblemineral to recover grains to recover this materialthisin the material (−500/+180) without without μ resortingm resorting sink product to to ultraﬁne ultrafine are well grinding.grindi liberated,ng. In In comparison, with comparison, a further 27% 44% 27% of thepartially of Ta–Nb the Ta–Nb liberated. mineral mineral grains These grains in theinresults (the−500/+180) ( −suggest500/+180) thatµ μm mit sink wouldsink productproduct be preferable are well well to liberated,process liberated, material with with a furthersub a further 500 44%μm. 44% partially partially liberated. liberated. These These resultsresults suggest suggest that that it wouldit would be be preferable preferable toto process material material sub sub 500 500 μm.µ m.

Figure 5. Ta–Nb liberation. Columbite, tantalite and microlite have been combined to give an overall liberation rate. 80% liberation is required for mineral processing. FigureFigure 5. Ta–Nb 5. Ta–Nb liberation. liberation. Columbite, Columbite, tantalite and and micr microliteolite have have been been combined combined to give to givean overall an overall liberation rate. 80% liberation is required for mineral processing. liberationTable rate.4 shows 80% that liberation approximately is required 40% for mineralof the cassi processing.terite is well liberated in the two HLS sink products.Table In4 showsthe (− 2800/+500)that approximately μm sink 40%product, of the a cassifurtherterite 40% is wellis partially liberated liberated in the two in theHLS coarse sink products. Table4 shows In the that(−2800/+500) approximately μm sink 40% product, of the a cassiteritefurther 40% is is wellpartially liberated liberated in thein the two coarse HLS sink products. In the (−2800/+500) µm sink product, a further 40% is partially liberated in the coarse fraction, indicating the coarser material may be amenable to further grinding. In the (−500/+180) µm sink product, only a further 9% is partially liberated. Minerals 2017, 7, 194 9 of 23 Minerals 2017, 7, 194 9 of 23 fraction, indicating the coarser material may be amenable to further grinding. In the (−500/+180) μm sink product, only a further 9% is partially liberated. Table 4. Cassiterite liberation. 80% liberation is required for mineral processing.

Table 4. Cassiterite liberation. 80% liberation is required for mineral processing. Cassiterite Liberation Sample ≤10% ≤20% ≤30%Cassiterite≤40% Liberation≤50% ≤60% ≤70% ≤80% ≤90% ≤100% Sample ≤10% ≤20% ≤30% ≤40% ≤50% ≤60% ≤70% ≤80% ≤90% ≤100% (−2800/+500) µm 5.77 6.29 1.43 7.26 8.98 16.43 8.11 6.70 5.92 33.11 (−2800/+500) μm 5.77 6.29 1.43 7.26 8.98 16.43 8.11 6.70 5.92 33.11 (−500/+180) µm 12.69 8.69 13.83 14.77 9.23 0.00 0.00 0.00 13.20 27.58 (−500/+180) μm 12.69 8.69 13.83 14.77 9.23 0.00 0.00 0.00 13.20 27.58

FigureFigure 6 showsshows thethe spreadspread of of liberated liberated columbite columbite for for the the HLS HLS sink sink products. products. Grain Grain size size within within the thewell well liberated liberated particles particles is highly is highly variable. variable. As withAs with the cassiterite,the cassiterite, the poorlythe poorly liberated liberated material material in both in bothproducts products is very is very fine fine grained grained and and therefore therefore unlikely unlikely to beto recoverable.be recoverable. Figure Figure7 shows 7 shows the the spread spread of ofliberated liberated cassiterite cassiterite for for the the HLS HLS sink sink products. products. The partiallyThe partially liberated liberated grains grains tend towardstend towards the coarser the coarsersize fractions size fractions indicating indicating that with that further with further grinding grinding this material this material would bewo easilyuld be liberated, easily liberated, thereby therebyincreasing increasing potential potential recovery. recovery. The poorly The liberated poorly materialliberated in material both products in both is products very fine is grained very fine and grainedlocked upand within locked muscovite, up within biotite muscovite, and quartz biotite grains. and Ultrafine quartz grains. grinding Ultrafine would be grinding required would to liberate be requiredthis material, to liberate and therefore this material, it is unlikely and ther toefore be recoverable. it is unlikely to be recoverable. Mineral Name Mineral Others Kaolinite Tourmaline Chlorite Muscovite/Lepidolite Biotite/Zinnwaldite/Phlogopite Plagioclase feldspar K-Feldspar Quartz Mn Ox/silicates O3 Fe-Ox(Mn)/C Apatite Pyrite Topaz Zircon Ilmenite Rutile REE Tantalite Microlite Columbite Cassiterite Background

Figure 6. Image Grids of columbite liberation in the two HLS products. Figure 6. Image Grids of columbite liberation in the two HLS products.

Minerals 2017, 7, 194 10 of 23 Minerals 2017, 7, 194 10 of 23 Mineral Name Mineral Others Kaolinite Tourmaline Chlorite Muscovite/Lepidolite Biotite/Zinnwaldite/Phlogopite Plagioclase feldspar K-Feldspar Quartz Mn Ox/silicates O3 Fe-Ox(Mn)/C Apatite Pyrite Topaz Zircon Ilmenite Rutile REE Tantalite Microlite Columbite assiterite C Background

Figure 7. Image Grids of cassiterite liberation in the two HLS products. Figure 7. Image Grids of cassiterite liberation in the two HLS products.

3.2. Gravity Concentration Using Mozley Table 3.2. Gravity Concentration Using Mozley Table The fine fractions (−180/+125) μm and −125 μm were separated using a Mozley concentrating The fine fractions (−180/+125) µm and −125 µm were separated using a Mozley concentrating table with a sand deck. Each sample was placed at the top of the table and water was added at flow table with a sand deck. Each sample was placed at the top of the table and water was added at rate of 700 mL/min. The table was operated with a stroke movement of 2.75 inches on the 3rd gear flow rate of 700 mL/min. The table was operated with a stroke movement of 2.75 inches on the 3rd speed, at the flattest inclination. A concentrate and tailings fraction were generated, with the gear speed, at the flattest inclination. A concentrate and tailings fraction were generated, with the concentrate being repassed to generate a cleaner concentrate. This two stage approach resulted in the concentrate being repassed to generate a cleaner concentrate. This two stage approach resulted in the generation of three products for each sized fraction: a rougher tails (RT); a cleaner tails (CT); and a generation of three products for each sized fraction: a rougher tails (RT); a cleaner tails (CT); and a cleaner concentrate (CC). XRF analyses of these products showed that separation had been successful cleaner concentrate (CC). XRF analyses of these products showed that separation had been successful for the elements of interest. Both cleaner concentrates showed upgraded Ta2O5, Nb2O5 and SnO2 for the elements of interest. Both cleaner concentrates showed upgraded Ta2O5, Nb2O5 and SnO2 abundances; in particular the −125 μm fraction, where the concentrate contains 161 times more Ta2O5, abundances; in particular the −125 µm fraction, where the concentrate contains 161 times more Ta2O5, 112 times more Nb2O5 and 93 times more SnO2 than the tailings. The −125 μm fraction concentrate 112 times more Nb2O5 and 93 times more SnO2 than the tailings. The −125 µm fraction concentrate also shows good concentration of rare earth elements Y2O3 (681 ppm), CeO2 (1344 ppm) and ThO2 also shows good concentration of rare earth elements Y O (681 ppm), CeO (1344 ppm) and ThO (288 ppm). There is no tungsten present at detectable levels2 3 in any of the samples.2 Both concentrates2 (288 ppm). There is no tungsten present at detectable levels in any of the samples. Both concentrates also showed a marked increase in TiO2, Cr2O3, MnO, Fe2O3, ZnO and As2O3. also showed a marked increase in TiO , Cr O , MnO, Fe O , ZnO and As O . QEMSCAN® analysis was carried2 out2 on3 the three 2products3 of the −21253 μm Mozley table trial. QEMSCAN® analysis was carried out on the three products of the −125 µm Mozley table trial. For the Mozley table products, >90% of both the tails products are <50 μm in size. In comparison, For the Mozley table products, >90% of both the tails products are <50 µm in size. In comparison, only 84% of the fine concentrate are <50 μm, indicating that the process has been more successful at only 84% of the fine concentrate are <50 µm, indicating that the process has been more successful at concentrating the coarser particle sizes. This may be due to poor liberation of the finer grains. The two tailings products of Mozley Table samples are composed primarily of quartz (50%–52%), K- feldspar (17%–23%), muscovite (11%–14%), plagioclase (7%–14%), kaolinite (1%–2%), and biotite

Minerals 2017, 7, 194 11 of 23 concentrating the coarser particle sizes. This may be due to poor liberation of the finer grains. The two tailings products of Mozley Table samples are composed primarily of quartz (50%–52%), K-feldspar (17%–23%), muscovite (11%–14%), plagioclase (7%–14%), kaolinite (1%–2%), and biotite (1%–2%). Columbite (0.01%–0.02%) and tantalite (≤0.01%) make up a minor fraction of the products. There is no microlite detected in the tailings products indicating that there was either a clean separation for this mineral or the grains were too small to be detected (this seems unlikely given the average grain size of microlite in the concentrate is 42 µm). In comparison, the fine concentrate product is primarily composed of quartz (47.99%), plagioclase (18.62%), K-feldspar (12.32%), muscovite (10.17%), Mn-oxides (2.11%), cassiterite (1.74%), kaolinite (1.4%), Fe-oxides (1.25%) and tourmaline (1.05%). The Ta–Nb minerals make up a minor fraction of the concentrate product, however are present in greater quantities than in the tailings, with columbite (0.58%), tantalite (0.15%) and microlite (0.04%) all present. The average grain size of cassiterite in the rough tailings is 95 µm, which decreases to 46–56 µm in the finer fractions. Average grain size for the columbite is 24–35 µm, whilst tantalite is finer (<15 µm). As shown in Table5, 50% of the cassiterite is poorly liberated in the finer fraction. Over 80% of the cassiterite grains in the Mozley table fine concentrate are well liberated. Cassiterite within the rough tailings is 20% well liberated. Over 75% of the material is only partially liberated, but, depending on grain size, may be amenable to further grinding. In the fine concentrate product from the Mozley table, 86% of the Ta–Nb mineral grains are well liberated. In both Mozley table tailings products, <50% is well liberated, with the remaining Ta–Nb grains very poorly liberated (<30%). This is due to the very fine grain size of the Ta–Nb minerals in these products: over 75% of grains in each product are <20 µm in size. In addition, 33% of the columbite and 80% of the tantalite grains within the rough tailings are associated with quartz.

Table 5. Cassiterite liberation. 80% liberation is required for mineral processing.

Cassiterite Liberation Sample ≤10% ≤20% ≤30% ≤40% ≤50% ≤60% ≤70% ≤80% ≤90% ≤100% FC1 (Fine 0.86 0.00 0.00 0.20 7.17 3.67 1.28 3.00 5.66 78.16 Concentrations) RT1 (Rough Tailings) 1.57 0.00 0.00 0.00 77.49 0.00 0.00 0.00 0.00 20.94 FT1 (Fine Tails) 0.27 0.00 0.00 0.00 0.00 0.32 0.00 0.00 0.00 99.41

For all samples, columbite and tantalite are closely associated, which is to be expected given they are end members of the same mineral series and occur as solid solution of (Fe,Mn)(Nb,Ta)2O6 within the same particles. Both minerals also show close association with quartz, particularly in the rough tailings from the Mozley table test (33% for columbite and 80% for tantalite), indicating that there is an amount of the minerals that remains unliberated and thus unrecoverable. In the (−2800/+500) µm fraction of the HLS, there is close association of both columbite and tantalite with cassiterite. Figure8 shows that the columbite is closely associated with quartz, muscovite, cassiterite and Mn-oxide. Columbite in the −500 µm fraction is better liberated. The concentrate product from the Mozley table test shows the same mineral associations as the HLS products, however almost twice as much cassiterite is liberated in the Mozley table products than in the HLS. In the tailings products, cassiterite is predominantly associated with K-feldspar (and kaolinite) and quartz. Figure9 shows that cassiterite mineralisation is closely associated with quartz, muscovite and K-feldspar. There is also 6%–10% association with other metal oxides in the +500 µm fraction. Minerals 2017, 7, 194 11 of 23

(1%–2%). Columbite (0.01%–0.02%) and tantalite (≤0.01%) make up a minor fraction of the products. There is no microlite detected in the tailings products indicating that there was either a clean separation for this mineral or the grains were too small to be detected (this seems unlikely given the average grain size of microlite in the concentrate is 42 μm). In comparison, the fine concentrate product is primarily composed of quartz (47.99%), plagioclase (18.62%), K-feldspar (12.32%), muscovite (10.17%), Mn-oxides (2.11%), cassiterite (1.74%), kaolinite (1.4%), Fe-oxides (1.25%) and tourmaline (1.05%). The Ta–Nb minerals make up a minor fraction of the concentrate product, however are present in greater quantities than in the tailings, with columbite (0.58%), tantalite (0.15%) and microlite (0.04%) all present. The average grain size of cassiterite in the rough tailings is 95 μm, which decreases to 46–56 μm in the finer fractions. Average grain size for the columbite is 24–35 μm, whilst tantalite is finer (<15 μm). As shown in Table 5, 50% of the cassiterite is poorly liberated in the finer fraction. Over 80% of the cassiterite grains in the Mozley table fine concentrate are well liberated. Cassiterite within the rough tailings is 20% well liberated. Over 75% of the material is only partially liberated, but, depending on grain size, may be amenable to further grinding. In the fine concentrate product from the Mozley table, 86% of the Ta–Nb mineral grains are well liberated. In both Mozley table tailings products, <50% is well liberated, with the remaining Ta–Nb grains very poorly liberated (<30%). This is due to the very fine grain size of the Ta–Nb minerals in these products: over 75% of grains in each product are <20 μm in size. In addition, 33% of the columbite and 80% of the tantalite grains within the rough tailings are associated with quartz.

Table 5. Cassiterite liberation. 80% liberation is required for mineral processing.

Cassiterite Liberation Sample ≤10% ≤20% ≤30% ≤40% ≤50% ≤60% ≤70% ≤80% ≤90% ≤100% FC1 (Fine Concentrations) 0.86 0.00 0.00 0.20 7.17 3.67 1.28 3.00 5.66 78.16 RT1 (Rough Tailings) 1.57 0.00 0.00 0.00 77.49 0.00 0.00 0.00 0.00 20.94 FT1 (Fine Tails) 0.27 0.00 0.00 0.00 0.00 0.32 0.00 0.00 0.00 99.41

Minerals 2017, 7, 194 12 of 23

The concentrate product from the Mozley table test shows the same mineral associations as the HLS products, however almost twice as much cassiterite is liberated in the Mozley table products than in the HLS. In the tailings products, cassiterite is predominantly associated with K-feldspar (and kaolinite) and quartz. Figure 9 shows that cassiterite mineralisation is closely associated with quartz, muscovite and Figure 8. QEMSCAN false colour image of Columbite-group and associated minerals. K-feldspar. ThereFigure is 8. alsoQEMSCAN 6%–10% false associat colourion image with ofother Columbite-group metal oxides in and the associated +500 μm minerals. fraction.

Figure 9. QEMSCAN false colour image of Cassiterite minerals associations (Legend is the same as Figure8 8).).

3.2.1Quantification Quantification of the of Chemistrythe Chemistr ofy Tantalumof Tantalum Bearing Bearing Minerals Minerals EPMA of of the the columbite-tantalite columbite-tantalite grains grains provided provided accurate accurate quantification quantification of the of thechemistry chemistry of the of columbite-tantalitethe columbite-tantalite series. series. However, However, EPMA EPMA resu resultslts are are variable variable regarding regarding both both accuracy accuracy and reproducibility. In total, 141 points were analysed by spot analysis across 67 grains from one carbon coated polished block of the cleaner concentrate (CC) produced by Mozley table separation. Of Of those, those, 60 spot analyses fall within ±1.5%±1.5% of of the the expected expected 100% 100% element element total total and and are are summarised summarised in in Table Table 66.. ® The grains are divideddivided intointo thethe mineralmineral typestypes asas defineddefined inin thethe QEMSCANQEMSCAN® mineralmineral list.list. For the remaining 81 81 points, points, there there is is no no clear clear reason reason for for the the low low totals totals and, and, as a asresult, a result, these these points points have havebeen excludedbeen excluded from further from further analysis. analysis. Standard Standard deviations deviations show the show variation the variation on chemistry on chemistry for each mineral. for each Lowmineral. standard Low standard deviations deviations for microlite for microlite indicate indicate chemistry chemistry is consistent is consistent across across microlite microlite grains. grains. Cassiterite also has consistent chemistry except for SnO2 and Ta2O5, where standard deviations indicate Cassiterite also has consistent chemistry except for SnO2 and Ta2O5, where standard deviations indicate variable chemistry. This is most likely due to substitution of Ta forfor SnSn withinwithin thethe cassiteritecassiterite mineralmineral structure. Columbite-tantalite show highly variable chemistry (particularly regarding Ta2O5 and Nb2O5 structure. Columbite-tantalite show highly variable chemistry (particularly regarding Ta2O5 and content), which is expected as these minerals are part of a solid solution series. Nb2O5 content), which is expected as these minerals are part of a solid solution series. 3.2.1.1 Columbite-Tantalite Grains The Ta:Nb and Fe:Mn oxide ratios were calculated for 73 points identified as columbite or tantalite. Each point and plotted on a quadrilateral plot of mineral composition (Figure 10). The quadrilateral plot shows a bimodal distribution, that spans the full Mn-end of the columbite mineral series. Each analysed point was assigned a mineral name according to its Ta:Nb and Fe:Mn ratios. A Ta:Nb ratio >0.5 is classed as tantalite. An Fe:Mn ratio >0.5 is classed as Mn-rich, and assigned the prefix mangano- (as opposed to the Fe-rich end members, which are assigned the prefix ferro-). Of the 73 points analysed across 32 columbite-tantalite grains, 25 fell within ±2% of 100% range. Of these, 24 points were identified as mangano-columbite and 1 as mangano-tantalite. A further 14 points lay within ±3% and 70 of the 73 points lay within ±5% of 100%. The data show that the grains are manganese rich and cover a range from Nv-rich mangano-columbite and Ta-rich mangano-tantalite. Although the totals are relatively low for most of the points, they fit well with the expected stochiometric relationship for the columbite-tantalite series ((Fe,Mn)(Nb, Ta)2O6). A plot of the molar ratio of Fe + Mn to Nb + Ta shows that the points lie along the 2:1 line (Figure 11).

Minerals 2017, 7, 194 13 of 23

Columbite-Tantalite Grains The Ta:Nb and Fe:Mn oxide ratios were calculated for 73 points identified as columbite or tantalite. Each point and plotted on a quadrilateral plot of mineral composition (Figure 10). The quadrilateral plot shows a bimodal distribution, that spans the full Mn-end of the columbite mineral series. Each analysed point was assigned a mineral name according to its Ta:Nb and Fe:Mn ratios. A Ta:Nb ratio >0.5 is classed as tantalite. An Fe:Mn ratio >0.5 is classed as Mn-rich, and assigned the prefix mangano- (as opposed to the Fe-rich end members, which are assigned the prefix ferro-). Of the 73 points analysed across 32 columbite-tantalite grains, 25 fell within ±2% of 100% range. Of these, 24 points were identified as mangano-columbite and 1 as mangano-tantalite. A further 14 points lay within ±3% and 70 of the 73 points lay within ±5% of 100%. The data show that the grains are manganese rich and cover a range from Nv-rich mangano-columbite and Ta-rich mangano-tantalite. Although the totals are relatively low for most of the points, they fit well with the expected stochiometric relationship for the columbite-tantalite series ((Fe,Mn)(Nb, Ta)2O6). A plot of the molar ratio of Fe + Mn to Nb + Ta showsMinerals that2017, the7, 194 points lie along the 2:1 line (Figure 11). 13 of 23 Columbite-Tantalite Ferro-tapiolite Magnano-tantalite 1.0 100% ± 2% 100% ± 3% 100% ± 5%

0.5 Ta/(Ta+Nb)

0.0 0.0 0.5 1.0 Ferro-columbite Magnano-columbite Mn/(Mn+Fe) Figure 10. Quadrilateral plot of columbite-tantalite elemental composition. Plotted based on Figure 10. Quadrilateral plot of columbite-tantalite elemental composition. Plotted based on normalised moles of cations. normalised moles of cations.

Columbite-Tantalite 0.30 100% ± 2% 100% ± 3% 100% ± 5%

0.25 Fe + Mn

0.20 0.40 0.45 0.50 0.55 0.60 Nb + Ta

Minerals 2017, 7, 194 13 of 23 Columbite-Tantalite Ferro-tapiolite Magnano-tantalite 1.0 100% ± 2% 100% ± 3% 100% ± 5%

0.5 Ta/(Ta+Nb)

0.0 0.0 0.5 1.0 Ferro-columbite Magnano-columbite Mn/(Mn+Fe)

Minerals 2017, 7, 194Figure 10. Quadrilateral plot of columbite-tantalite elemental composition. Plotted based on 14 of 23 normalised moles of cations.

Columbite-Tantalite 0.30

Minerals 2017, 7, 194 100% ± 2% 14 of 23 100% ± 3% Figure 11. Plot of molar ratio of cations Fe + Mn and Nb + Ta. The 2:1 ratio expected based on the 100% ± 5% stochiometry of columbite-tantalite (Fe,Mn)(Nb, Ta)2O6 is included.

0.25 As Figure 9 shows, the columbite-tantalite grains are highly zoned, commonly with Nb-rich

cores movingFe + Mn to Ta-rich rims. It must be noted that the point analysis was carried out across these zoned minerals with several analyses per grain. Thus, any bimodal distribution previously commented on is within the grains rather than between the grains. As shown in Figure 12, crystals often are zoned with the core rich in Nb and the rim rich in Ta. Columbite group grains also exhibit a convoluted zoning.0.20 0.40 0.45 0.50 0.55 0.60 3.2.1.2 Microlite Nb + Ta FigureThree points, 11. Plot analysed of molar ratioacross of cationsone grain Fe +were Mn andidentified Nb + Ta. as Themicrolite. 2:1 ratio Microlite expected is based a sub-group on the of mineralsstochiometry within the of columbite-tantalitepyrochlore supergroup. (Fe,Mn)(Nb,Ta) The pres2Oence6 is included. of F within the mineral grain suggests this is the fluorcalciomicrolite species of the microlite group. As Figure9 shows, the columbite-tantalite grains are highly zoned, commonly with Nb-rich cores moving3.2.1.3 Cassiterite to Ta-rich Grains rims. It must be noted that the point analysis was carried out across these zoned mineralsCassiterite with several was also analyses found per to contain grain. Thus, some anytantalum. bimodal Tantalum distribution in cassiterite previously is due commented to inclusions on is withinof microcrystals the grains of rather columbite than between group minerals the grains. and As microlite shown in and, Figure probably, 12, crystals to substitutions often are zoned of Sn with by theNb coreand richTa. Thirty-nine in Nb and the point rim analyses rich in Ta. across Columbite 27 grains group were grains identified also exhibit as cassiterite. a convoluted SnO2 zoning. content within the cassiterite grains was variable from 91% to 100%. For grains that reported low SnO2 content Microlite the totals were still within the acceptable ±1.5% of 100% limit. Analysis of the data revealed that SnO2 was beingThree substituted points, analysed for by across Ta2O5 one (Figure grain 13). were Ta identiﬁed2O5 withinas cassiterite microlite. was Microlite observed is a in sub-group two forms: of mineralsas inclusions within of theTa-bearing pyrochlore minerals supergroup. (Figure The 14) presence and as of substitution F within the within mineral the grain cassiterite suggests crystal this is thestructure. ﬂuorcalciomicrolite species of the microlite group.

Figure 12. Columbite-tantalite grains analysed by EM PA with clear mineral zoning; (a,b,d) Nb-rich Figure 12. Columbite-tantalite grains analysed by EM PA with clear mineral zoning; (a,b,d) Nb-rich cores and Ta-rich rims; (c,e,f), convoluted zoning. cores and Ta-rich rims; (c,e,f), convoluted zoning.

Minerals 2017, 7, 194 15 of 23

Cassiterite Grains Cassiterite was also found to contain some tantalum. Tantalum in cassiterite is due to inclusions of microcrystals of columbite group minerals and microlite and, probably, to substitutions of Sn by Nb and Ta. Thirty-nine point analyses across 27 grains were identiﬁed as cassiterite. SnO2 content within the cassiterite grains was variable from 91% to 100%. For grains that reported low SnO2 content the totals were still within the acceptable ±1.5% of 100% limit. Analysis of the data revealed that SnO2 was being substituted for by Ta2O5 (Figure 13). Ta2O5 within cassiterite was observed in two forms: as inclusions of Ta-bearing minerals (Figure 14) and as substitution within the cassiterite crystalMinerals structure.2017, 7, 194 16 of 23 Minerals 2017, 7, 194 16 of 23

1010 Ta2O5Ta2O5 Nb2O5Nb2O5 FeOFeO MnOMnO

22 Elemental association (wt %) association Elemental Elemental association (wt %) association Elemental 00 9090 92 92 94 94 96 96 98 98 100 100 SnO (wt %) SnO (wt %) Figure 13. Elemental association within cassiterite. Figure 13. Elemental association within cassiterite.

Figure 14. Cassiterite grains analysed by EPMA, that show Ta-rich compositions. Figure 14. Cassiterite grains analysed by EPMA, that show Ta-rich compositions. Figure 14. Cassiterite grains analysed by EPMA, that show Ta-rich compositions.

3.3.3.3. GravityGravity ConcentrationConcentration UsingUsing KnelsonKnelson TheThe Knelson Knelson Concentrator Concentrator trial trial was was carried carried out out on on the the − −625625 μ μmm size size fraction, fraction, thereby thereby eliminating eliminating thethe lowlow gradegrade coarsercoarser fraction.fraction. TheThe productsproducts fromfrom ththee KnelsonKnelson concentratorconcentrator trialtrial werewere splitsplit intointo two;two; halfhalf ofof thethe productsproducts werewere sentsent forfor WDS-XRFWDS-XRF ananalysisalysis ofof thethe wholewhole product,product, thethe otherother halfhalf waswas screenedscreened intointo differentdifferent sizesize fractionsfractions andand analysedanalysed byby PXRF.PXRF. ToTo determinedetermine thethe accuracyaccuracy ofof thethe PXRF,PXRF, thethe pelletspellets analysedanalysed byby WDS-XRFWDS-XRF forfor thethe wholewhole sasamples,mples, werewere analysedanalysed andand thethe resultsresults compared.compared. TheThe PXRFPXRF resultsresults followfollow thethe samesame patternspatterns ofof rerecoverycovery asas thethe WDS-XRF,WDS-XRF, however,however, forfor Ta,Ta, thethe recoveriesrecoveries are are much much lower, lower, indicating indicating that that the the PXRF PXRF is is not not measuring measuring Ta Ta content content accurately. accurately. Previous Previous comparisonscomparisons ofof PXRFPXRF andand SQ-XRFSQ-XRF datadata indicatesindicates thatthat thethe PXRFPXRF isis overestimatingoverestimating thethe TaTa content,content, whichwhich isis believedbelieved toto bebe thethe reasonreason forfor thethe lowlow recorecoveries.veries. FigureFigure 1515 showsshows thatthat therethere isis positivepositive correlation between the PXRF and SQ-XRF for Ta2O5, Nb2O5 and SnO2. However, the ratio between correlation between the PXRF and SQ-XRF for Ta2O5, Nb2O5 and SnO2. However, the ratio between thethe PXRF PXRF results results and and the the semi-quantitative semi-quantitative results results is is different different for for each each element. element. This This remains remains true true for for the majority of elements. The PXRF consistently measured high for both Ta2O5 and Nb2O5 in the majority of elements. The PXRF consistently measured high for both Ta2O5 and Nb2O5 in comparison to the SQ-XRF. In particular, the Ta2O5 content was measured at over 100 ppm greater comparison to the SQ-XRF. In particular, the Ta2O5 content was measured at over 100 ppm greater forfor allall butbut thethe concentrateconcentrate whichwhich measuredmeasured low.low.

Minerals 2017, 7, 194 16 of 23

Table 6. Summary table of EPMA results for point analysis (wt %).

Mineral Na2O F Ta2O5 Nb2O5 MgO SnO2 MnO CaO FeO TiO2 Totals No. of Samples Mean b.d. * b.d. 20.3 59.9 0.1 0.1 15.0 b.d. 3.8 0.1 99.1 Mangano-columbite 13 Std. Dev - - 5.6 5.3 0.0 0.0 1.0 - 1.1 0.0 0.5 Mean b.d. b.d. 50.2 32.1 b.d. 0.3 13.4 b.d. 3.0 0.1 99.0 Mangano-tantalite 4 Std. Dev - - 8.9 8.7 - 0.1 1.0 - 1.3 0.0 0.4 Mean 6.0 4.0 72.6 4.4 b.d. 1.5 0.1 10.7 b.d. 0.2 99.2 Microlite 3 Std. Dev 0.1 0.2 0.2 0.1 - 0.0 0.0 0.1 - 0.0 0.3 Mean 0.1 0.2 2.2 0.6 0.1 98.3 0.1 b.d. 0.4 0.1 100.8 Cassiterite 39 Std. Dev 0.0 0.0 1.8 0.3 0.0 2.7 0.1 - 0.3 0.1 0.6 * (b.d. = below detection limit). Minerals 2017, 7, 194 17 of 23

3.3. Gravity Concentration Using Knelson The Knelson Concentrator trial was carried out on the −625 µm size fraction, thereby eliminating the low grade coarser fraction. The products from the Knelson concentrator trial were split into two; half of the products were sent for WDS-XRF analysis of the whole product, the other half was screened into different size fractions and analysed by PXRF. To determine the accuracy of the PXRF, the pellets analysed by WDS-XRF for the whole samples, were analysed and the results compared. The PXRF results follow the same patterns of recovery as the WDS-XRF, however, for Ta, the recoveries are much lower, indicating that the PXRF is not measuring Ta content accurately. Previous comparisons of PXRF and SQ-XRF data indicates that the PXRF is overestimating the Ta content, which is believed to be the reason for the low recoveries. Figure 15 shows that there is positive correlation between the PXRF and SQ-XRF for Ta2O5, Nb2O5 and SnO2. However, the ratio between the PXRF results and the semi-quantitative results is different for each element. This remains true for the majority of elements. The PXRF consistently measured high for both Ta2O5 and Nb2O5 in comparison to Minerals 2017, 7, 194 17 of 23 the SQ-XRF. In particular, the Ta2O5 content was measured at over 100 ppm greater for all but the concentrate which measured low.

0.10

0.09

0.08

0.07 Ta2O5 wt%

Nb2O5 wt% 0.06

SnO2 wt% 0.05 Linear (Ta2O5 wt%) 0.04 Linear (Nb2O5 wt%) SQ-XRF (wt %) 0.03 Linear (SnO2 wt%)

0.02

0.01

0.00 0.00 0.01 0.02 0.03 0.04 0.05

pXRF (wt %)

Figure 15. Scatter plot showing correlation of results from PXRF and SQ-XRF analysis of Knelson FigureConcentrator 15. Scatter products plot forshowing for Ta, correlation Nb and Sn. of results from PXRF and SQ-XRF analysis of Knelson Concentrator products for for Ta, Nb and Sn. The semi-quantitative results for the whole samples show little variation in major element The semi-quantitative results for the whole samples show little variation in major element chemistry between the different tailings products and the feed. However the tailings are depleted in chemistry between the different tailings products and the feed. However the tailings are depleted in Ta2O5, Nb2O5 and SnO2 when compared to the feed material. These elements have been upgraded in Ta2O5, Nb2O5 and SnO2 when compared to the feed material. These elements have been upgraded in the concentrate; Ta2O5 has upgraded from 55 ppm to 4320 ppm (a factor of 79), Nb2O5 has upgraded the concentrate; Ta2O5 has upgraded from 55 ppm to 4320 ppm (a factor of 79), Nb2O5 has upgraded from 104 ppm to 4545 ppm (a factor of 44) and SnO2 has increased from 661 ppm to 2.9% (also a factor from 104 ppm to 4545 ppm (a factor of 44) and SnO2 has increased from 661 ppm to 2.9% (also a factor of 44). TiO2, Cr2O3, MnO, Fe2O3, ZnO, As2O3,Y2O3 and ThO2 have also upgraded in the concentrate. of 44). TiO2, Cr2O3, MnO, Fe2O3, ZnO, As2O3, Y2O3 and ThO2 have also upgraded in the concentrate. The calculated recoveries for each stage based on the WDS XRF results are shown in Figure 16, The calculated recoveries for each stage based on the WDS XRF results are shown in Figure 16, with feed mass for each stage. For all target metals, recovery was best in the ﬁrst minute of the trial, with feed mass for each stage. For all target metals, recovery was best in the first minute of the trial, likely due to the build-up of the particle bed within the concentrator cone. For Ta, whilst recovery dropped by almost 20% it then stayed constant during the next six minutes. Recovery dropped off again after entering the final stage. Overall, 63.5% of Ta was recovered from the feed material. For Sn, recovery dropped by 10% over the first five minutes of the trial, before then increasing almost back to original recovery levels. Overall, Sn recovery was 40%. As with the other metals, Nb recovery dropped by 10% after the first minute. It then remained relatively constant (±3%) for the rest of the trial to give an overall recovery of 36.9%. Overall, for Ta and Nb, recovery decreased by 20% after the first minute, whilst, for Sn, recovery decreased by 10%. No mineralogical work was done on the products of this trial, however the chemistry of the products is dominated by SiO2, Al2O3 and K2O, indicating that there is still a large proportion of silicate minerals within the concentrate. The drop in recovery is believed to be due to the replacement of lower density quartz particles (containing unliberated columbite-tantalite and cassiterite) with higher density mica grains in the concentrate bed.

Minerals 2017, 7, 194 18 of 23 likely due to the build-up of the particle bed within the concentrator cone. For Ta, whilst recovery dropped by almost 20% it then stayed constant during the next six minutes. Recovery dropped off again after entering the final stage. Overall, 63.5% of Ta was recovered from the feed material. For Sn, recovery dropped by 10% over the first five minutes of the trial, before then increasing almost back to original recovery levels. Overall, Sn recovery was 40%. As with the other metals, Nb recovery dropped by 10% after the first minute. It then remained relatively constant (±3%) for the rest of the trial to give an overall recovery of 36.9%. Overall, for Ta and Nb, recovery decreased by 20% after the first minute, whilst, for Sn, recovery decreased by 10%. No mineralogical work was done on the products of this trial, however the chemistry of the products is dominated by SiO2, Al2O3 and K2O, indicating that there is still a large proportion of silicate minerals within the concentrate. The drop in recovery is believed to be due to the replacement of lower density quartz particles (containing unliberated Mineralscolumbite-tantalite 2017, 7, 194 and cassiterite) with higher density mica grains in the concentrate bed. 18 of 23

Figure 16. 16. PlotPlot of of Ta, Ta, Sn Sn and and NbNb recovery recovery rates rates throug throughh time timeversus versus volume volume of feed of measured feed measured using PXRF.using PXRF.

Figure 17 shows how the metal grade of the tailings changed over the course of the trial. Ta and Figure 17 shows how the metal grade of the tailings changed over the course of the trial. Ta and Nb both increased as would be expected, given the drop in recovery. Ta grade increased from 10 ppm Nb both increased as would be expected, given the drop in recovery. Ta grade increased from 10 ppm to 30 ppm, whilst Nb increased from 59 ppm to 73 ppm. Sn grade in the tailings products through to 30 ppm, whilst Nb increased from 59 ppm to 73 ppm. Sn grade in the tailings products through time also matches the recovery pattern, with Sn grade increasing in the tails from 366 ppm to 421 time also matches the recovery pattern, with Sn grade increasing in the tails from 366 ppm to 421 ppm ppm during the first half of the trial, before dropping back down to 372 ppm. during the first half of the trial, before dropping back down to 372 ppm. Figure 18 shows the average recovery of each particle size. For Ta, recovery significantly Figure 18 shows the average recovery of each particle size. For Ta, recovery significantly improved improved below −125 μm from <2% recovery in the +125 μm fraction to over 20% recovery in the below −125 µm from <2% recovery in the +125 µm fraction to over 20% recovery in the (−90/+63) (−90/+63) μm fraction. For Sn recovery significantly improved below −180 μm from <10% in the +180 µm fraction. For Sn recovery significantly improved below −180 µm from <10% in the +180 µm μm fractions to over 40% in the (−90/+63) μm fraction. Of the +180 μm fractions the greatest recovery fractions to over 40% in the (−90/+63) µm fraction. Of the +180 µm fractions the greatest recovery for Sn took place in the +500 μm material, averaging 8.8%. Nb recovery also significantly improved for Sn took place in the +500 µm material, averaging 8.8%. Nb recovery also significantly improved below −180 μm from <5% in the +180 μm fractions to over 40% in the (−90/+63) μm fraction. Size below −180 µm from <5% in the +180 µm fractions to over 40% in the (−90/+63) µm fraction. Size analysis of the recovery rate for each product shows that the Knelson concentrator is most efficient analysis of the recovery rate for each product shows that the Knelson concentrator is most efficient for for recovery of −180 μm particles. This is likely a reflection of the physics of the machine and the recovery of −180 µm particles. This is likely a reflection of the physics of the machine and the change change in liberation with particle size. For all three elements, greatest recovery was enjoyed by the in liberation with particle size. For all three elements, greatest recovery was enjoyed by the (−90/+63) (−90/+63) μm fraction size. µm fraction size. Comparison of recovery rates from this study to previous work by Montero-Ríos [18] reveals very similar recovery rates (Table 7).

Table 7. Comparison of results of centrifugal separation measured using SQ-XRF.

Product Recovery Recovery [18] Ta 63.6% 61.9% Sn 40.0% 46.4%

The work carried out by [18] only tested material −100 μm in size. For the recovery to show no significant change, despite the change in particle size (material −625 μm was used in this study), is encouraging and indicates that centrifuge separation could potentially be used to treat a wider variety of grain sizes than just the fines, with no significant loss to recovery.

Minerals 2017, 7, 194 19 of 23

Comparison of recovery rates from this study to previous work by Montero-Ríos [18] reveals very similar recovery rates (Table7).

Table 7. Comparison of results of centrifugal separation measured using SQ-XRF.

Product Recovery Recovery [18] Ta 63.6% 61.9% Sn 40.0% 46.4%

The work carried out by [18] only tested material −100 µm in size. For the recovery to show no significant change, despite the change in particle size (material −625 µm was used in this study), is encouraging and indicates that centrifuge separation could potentially be used to treat a wider MineralsMinerals 2017 2017, ,7 7, ,194 194 1919 of of 23 23 variety of grain sizes than just the fines, with no significant loss to recovery.

TaTa NbNb SnSn 8080 430430 420 7070 420 410410 6060 400400 50 50 390390 Sn Grade (ppm) 4040 380380 Sn Grade (ppm) 370 3030 370 360360 Ta and Nb Grade (ppm) Ta and Nb Grade (ppm) 2020 350350 10 10 340340 00 330330 0-10-1 (1-3) (1-3) (3-5) (3-5) (5-7) (5-7) (7-10) (7-10) TimeTime IntervalsIntervals (min)(min)

FigureFigure 17.17. GradeGrade of of Ta, Ta, NbNb andand SnSn inin tailingstailings throughthrough time.time.

4545 4040 TaTa SnSn NbNb 3535 3030 2525 2020 Recovery (%) Recovery (%) 1515 1010 55 00 -63-63 -90-90 -125-125 -180-180 -250-250 -355-355 -500-500 -625-625

ParticleParticle SizeSize FigureFigure 18.18. AverageAverage recoveryrecovery byby particleparticle sizesize during during separationseparation byby aa KnKnelsonKnelsonelson concentrator.concentrator. RecoveriesRecoveries calculatedcalculated fromfrom PXRFPXRF results.results.

3.4.3.4. GravityGravity ConcentrationConcentration UsingUsing ShakingShaking TableTable FiguresFigures 1919 andand 2020 summarisesummarise thethe recoveryrecovery ofof elementselements ofof interestinterest acrossacross thethe rougherrougher andand cleanercleaner tabletable separations.separations. InIn bothboth rougherrougher andand cleanercleaner tabletable concentration,concentration, therethere isis aa similarsimilar trendtrend forfor NbNb andand SnSn whichwhich areare recoveredrecovered preferentiallypreferentially toto Ta.Ta. ThisThis isis perhapsperhaps unexpectedunexpected asas thethe densitydensity ofof tantalitetantalite isis greatergreater thanthan columbitecolumbite andand therethere isis closeclose associationassociation ofof thethe twotwo minerals.minerals. TheThe mostmost likelylikely explanationexplanation isis thatthat thethe resultresult isis duedue toto aa combinationcombination ofof particleparticle sizesize anandd liberation.liberation. TheThe particleparticle sizesize ofof thethe columbitecolumbite andand cassiteritecassiterite (30(30 toto 8080 μμm)m) isis largerlarger thanthan thethe MicroliteMicrolite andand TantaliteTantalite (20(20 toto 4040 μμm),m), soso thisthis willwill affectaffect howhow itit behavesbehaves onon thethe gravitygravity table.table. FigureFigure 2121 showsshows thethe theoreticaltheoretical gradegrade recoveryrecovery curvescurves forfor columbitecolumbite andand tantalitetantalite recoveredrecovered toto thethe firstfirst twotwo productsproducts whichwhich givegive anan indicationindication ofof liberation.liberation. TheThe curvescurves areare generatedgenerated byby combiningcombining thethe massmass andand gradegrade ofof individualindividual particlesparticles inin descendingdescending orderorder ofof grade.grade. TheThe figuresfigures showshow thatthat thethe gradegrade ofof TaTa isis consistentlyconsistently lowerlower thanthan columbitecolumbite forfor aa givengiven recovery.recovery. ThisThis suggestssuggests greatergreater lockinglocking whichwhich isis furtherfurther illustratedillustrated inin imageimage gridsgrids ofof thethe QEMSCANQEMSCAN particles,particles, asas shownshown inin FigureFigure 22.22.

Minerals 2017, 7, 194 20 of 23

3.4. Gravity Concentration Using Shaking Table Figures 19 and 20 summarise the recovery of elements of interest across the rougher and cleaner table separations. In both rougher and cleaner table concentration, there is a similar trend for Nb and Sn which are recovered preferentially to Ta. This is perhaps unexpected as the density of tantalite is greater than columbite and there is close association of the two minerals. The most likely explanation is that the result is due to a combination of particle size and liberation. The particle size of the columbite and cassiterite (30 to 80 µm) is larger than the Microlite and Tantalite (20 to 40 µm), so this will affect how it behaves on the gravity table. Figure 21 shows the theoretical grade recovery curves for columbite and tantalite recovered to the ﬁrst two products which give an indication of liberation. The curves are generated by combining the mass and grade of individual particles in descending order of grade. The ﬁgures show that the grade of Ta is consistently lower than columbite for a given recovery. This suggests greater locking which is further illustrated in image grids of the QEMSCAN particles,Minerals as 2017 shown2017,, 77,, 194194 in Figure 22. 2020 ofof 2323

100100

TaTa Nb SnSn 8080

6060

4040 Mass Pull % Mass Pull Mass Pull % Mass Pull

2020

00 00 20406080100 20406080100

Recovery % FigureFigureFigure 19. Mass 9.9. Mass pull pull (%) (%) and and recovery recovery (%) (%) for for WilﬂeyWilfley shaking table table rougher rougher test test on onPenouta Penouta sample, sample, measuredmeasured using using PXRF. PXRF.

100100 TaTa Nb SnSn

8080

6060

4040 MASS PULL % MASS PULL MASS PULL % MASS PULL

2020

00 00 20406080100 20406080100

RECOVERY %

FigureFigureFigure 20. Mass 20.20. MassMass pull pullpull (%) (%)(%) and andand recovery recoveryrecovery (%) (%)(%) for forfor Wilﬂey WilfleyWilfley shakingshakingshaking tabletable table cleanercleaner cleaner testtest test onon on PenoutaPenouta Penouta sample,sample, sample, measured using PXRF. measured using PXRF.

100100

8080

6060

4040 Grade, % Grade, Grade, % Grade, 2020

00 00 102030405060708090100 102030405060708090100 Recovery, % Columbite Concentrate 1 TantaliteTantalite ConcentrateConcentrate 11 Columbite Concentrate 2 TantaliteTantalite ConcentrateConcentrate 22 FigureFigure 21.21. ColumbiteColumbite andand tantalitetantalite theoreticaltheoretical gradegrade rerecoverycovery curvescurves forfor thethe firstfirst twotwo concentrateconcentrate products of Wilfley separation.

Minerals 2017, 7, 194 20 of 23

100

Ta Nb Sn 80

40 Mass Pull % Mass Pull

0 0 20406080100

Recovery % Figure 9. Mass pull (%) and recovery (%) for Wilfley shaking table rougher test on Penouta sample, measured using PXRF.

100 Ta Nb Sn

40 MASS PULL % MASS PULL

0 0 20406080100

RECOVERY % Minerals 2017, 7, 194Figure 20. Mass pull (%) and recovery (%) for Wilfley shaking table cleaner test on Penouta sample, 21 of 23 measured using PXRF.

100

40 Grade, % Grade, 20

0 0 102030405060708090100 Recovery, % Columbite Concentrate 1 Tantalite Concentrate 1 Columbite Concentrate 2 Tantalite Concentrate 2 Figure 21. Columbite and tantalite theoretical grade recovery curves for the first two concentrate Figure 21. Columbite and tantalite theoretical grade recovery curves for the ﬁrst two concentrate products of Wilfley separation. Mineralsproducts 2017 , 7, of194 Wilﬂey separation. 21 of 23

Background Cassiterite Columbite Microlite Tantalite REE Rutile Ilmenite Zircon Topaz Al Ox/OH Apatite Fe-Ox/C O3 Mn Ox/silicates Quartz K-Feldspar Plagioclase feldspar Biotite/Zinnwaldite/Phlogopite Muscovite/Lepidolite Chlorite Ca Fe Al silicates Tourmaline Kaolinite Others

Figure 22. Ta–Nb vs.vs Size, Size, Ta–Nb Ta–Nb combined combined and and all all other other minerals minerals invisible invisible for for ease ease of of viewing viewing ( (toptop);); and Ta–Nb combinedcombined andand allall otherother mineralsminerals includedincluded ((bottombottom).).

Minerals 2017, 7, 194 22 of 23

4. Conclusions Mangano-tantalite and mangano-columbite were the main tantalum hosting phases in the feed material. These minerals occur in solid solution together. Minor tantalum was present in the pyrochlore group mineral microlite. The tailings material contains 200–500 ppm Sn in cassiterite. The cassiterite averages 50–150 µm in size and is well liberated in the +500 µm size fraction. Mineralogy is dominated by silicate phases, particularly quartz and muscovite, which average 20–225 µm in size across the samples analysed. All ore minerals are most abundant in the −125 µm size fraction (73% Ta, 44% Sn and 38% Nb), which accounts for 9% of the feed material. Less than 30% of the ore mineralogy is well liberated, although liberation increases as grain size decreases. Ore minerals are closely associated with quartz and muscovite. The mineralogical characterization of tantalum ore is necessary to determine the grade that could be obtained during the ore processing. Nb and Ta occur as a solid solutions in different oxide minerals. In some cases, as in the Penouta leucogranite, the Nb-rich and the Ta-rich zones of the mineral particle could be separated with a conventional degree of milling, obtaining a high degree of T2O5 content in the concentrate. In other cases, these metals are completely mixed or distributed in very narrow zones, preventing separation by physical methods. The high radioactive content (elements) in the ore from the alluvial Bolivian deposits require treatment to reduce them before being placed on the market. Separation trials indicate that gravity processing is an effective means of upgrading the Penouta tin mining tailings. Recoveries were greatest at ﬁne sizes, which is expected due to the greater concentration and liberation of the minerals of interest in these fractions. It can be expected that courser liberated material has been artiﬁcially depleted as the material is tailings from historic tin processing. There were encouraging results using conventional gravity separation techniques and centrifugal concentration over a wide size range.

Acknowledgments: This work is part of the OptimOre project. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 642201. Authors are thankful from the Strategic Minerals enterprise for their help in sampling, visit and information provided. Author Contributions: Yousef Ghorbani (preparing paper, analyzing data, help in the experimental design and performance), Rob Fitzpatrick (help in preparing the paper, analyzing & obtaining data, help in the experimental design and performance), Melanie Kinchington (this paper was based on her master’s thesis), Gavyn Rollinson (mineral characterization, analyzing results and checking drafts of the paper) and Patrick Hegarty (help in experimental design and performance). Conﬂicts of Interest: Authors declare no conﬂicts of interest.

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