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Article Laboratory Testing of Flotation from Raw in Sangdong Mine for Process Development

Seongmin Kim , Sang-Ho Baek, Yosep Han * and Ho-Seok Jeon *

Mineral Resources Division, Korea Institute of Geoscience & Resources (KIGAM), 124, Gwahang-no, Yuseong-gu, Daejeon 34132, Korea; [email protected] (S.K.); [email protected] (S.-H.B.) * Correspondence: [email protected] (Y.H.); [email protected] (H.-S.J.); Tel.: +82-42-868-3181 (Y.H.); +82-42-868-3582 (H.-S.J.)  Received: 29 September 2020; Accepted: 28 October 2020; Published: 31 October 2020 

Abstract: is an essential metal for the manufacture of special alloys, which is in constant demand due to the development of the industry. The recovery of scheelite from undeveloped tungsten ore in South Korea was investigated to improve the flotation performance for high grade and recovery of concentrate. To investigate the interaction between the flotation reagents and the minerals, the adsorption experiments of oleic acid as a collector on Ca-bearing minerals, such as scheelite, , and fluorite were carried out. This reaction was confirmed chemical adsorption by analysis of zeta potential and FTIR analysis. The batch test was performed using a raw ore to enhance the grade and recovery of the scheelite concentrate. It was obtained at the optimal conditions for high WO3 grade and recovery of scheelite concentrate by using a simple process. In particular, the sodium used as a pH modifier was investigated to increase scheelite flotation performance by supporting the selective depression of Ca-bearing gangue minerals. Furthermore, a locked cycle test (LCT) was carried out based on batch test results for the design of a continuous pilot plant.

Keywords: scheelite; tungsten; flotation; Ca-bearing minerals; locked cycle test

1. Introduction Tungsten has been one of the important metals, which is widely used in various industries such as alloys, electron, chemical engineering, etc. [1–7]. Not only does it have the highest melting point among metals, but it also shows particularly high physical properties including moduli of compression and elasticity, thermal creep resistance and electrical conductivity [1,3,4,8]. Due to its significant economic importance and high supply risks, it has been classified as a strategic metal. According to a recent report, more than 80% of tungsten is produced in China, though a large amount of tungsten has been produced in Korea [1,6,9]. The Sangdong mine in South Korea was the powerhouse of the economy in the 1940s, contributing more than 50% of the export revenue in Korea [10]. This mine deposit acted as one of the largest tungsten resources across the globe. Furthermore, it had been the leading global tungsten producer for more than 40 years before it ceased to produce tungsten in 1992, not because of resource depletion [9]. Therefore, this indicates the current amount of undeveloped ore is expected to remain large. Additionally, the interest in the development of tungsten has been gradually increasing and the flotation characteristics of tungsten ore from the Sangdong mine are required for the re-development plan. There are various types of tungsten minerals. Among them, scheelite (CaWO4) is considered as an economically important mineral [2,4–7,9,11–17]. To be specific, scheelite is easily ground to fine particles during crushing and grinding processes due to its brittleness [1,18,19]. Generally, scheelite is partially composed of particles as fine as these particles, which is different from other types of minerals. This characteristic can be one of the reasons why many researchers have conducted researches about

Minerals 2020, 10, 971; doi:10.3390/min10110971 www.mdpi.com/journal/minerals Minerals 2020, 10, 971 2 of 13 froth flotation for the recovery of scheelite from tungsten ore [1,19,20]. Froth flotation is an extensively used separation technique in the minerals industry that is based on differences in the surface properties of particles [1,5,7,11]. A fatty acid collector is extensively used in scheelite flotation from the raw ore during the process of separation [2,8,13,15,21]. Scheelite is often associated with other Ca-bearing salt-type minerals such as calcite and fluorite, which has similar characteristics [5–7,22,23]. Their dissolved mineral species in the pulp might be some reactions such as hydrolysis, precipitation and adsorption which can affect the interaction between reagents and minerals [5,24]. Therefore, it is desirable to select suitable modifiers and depressants for selective separation which is available through the flotation method [5–8,12,16,21,24]. In particular, the studies on various kinds of flotation reagents have been carried out, and most of them have focused on the development of new collectors and depressants [8,11,14,15,21,25]. Nevertheless, this is still mostly basic research based on the reactivity of pure minerals and reagents, and conventional reagents are used in the field. Recently it has been carried out that sodium carbonate, which is known as a pH modifier, has a positive effect on the flotation of scheelite [25–28]. Based on this research, it is very important to improve and investigate the mechanism by applying it to the real complex ore system. This research mainly aimed to investigate the flotation characteristics for scheelite recovery from undeveloped tungsten ore in South Korea. Particularly, it focused on improving the selective separation performance of scheelite during the flotation process by using suitable flotation reagents. The effect of Na2CO3 on WO3 grade and recovery of scheelite concentrate have been investigated to improve flotation performance. Additionally, this mechanism was conducted, not a only batch test, but also a locked cycle test using raw ore in Sangdong mine.

2. Materials and Methods

2.1. Materials and Reagents Pure minerals were used in adsorption experiments and analysis. They are as follows: scheelite, calcite, fluorite from China. It was analyzed through inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima-5300DV, Perkin Elmer, Waltham, MA, USA). The raw ore sample used in this study was obtained from the Sangdong mine in Korea (Almonty Korea Tungsten Corporation). They were initially crushed using a crusher (Cone crusher, The Hankook Powder Machine Engineering, Gyeonggi-do, Korea) and then pulverized in a mill (Rod mill, The Hankook Powder Machine Engineering, Gyeonggi-do, Korea). For the flotation the particle size of 80% passing 74 µm was used. Reagents (potassium amyl xanthate (KAX, C5H11KOS2), methyl isobutyl carbinol (MIBC, C5H14O), sodium carbonate (Na2CO3) sodium hydroxide (NaOH), sodium sulfide (Na2S), (CaO), sodium silicate (Na2SiO3), oleic acid (C18H34O2), Lankropol K-8300 (disodium oleamido MIPA-sulfosuccinate, Akzo Nobel Surface Chemistry LLC, Chicago, IL, USA)) used were supplied by Sigma Aldrich (Saint Louis, MO, USA) and were of technical and analytical grade in this research.

2.2. Experimental Methods and Analysis A total of 1 kg of real, raw ore sample in the Sangdong mine was subjected to the flotation experiments carried out in a flotation cell of a lab-scale Denver Sub-A flotation machine with 35% pulp density and 1500 rpm stirring. Batch type flotation experiments were conducted in accordance with the flow sheet shown in Figure1. Sulfides flotation is a process to remove sulfides mineral, such as (, MoS2) in before the scheelite flotation process, because they adversely affect the grade and recovery of concentrate in scheelite flotation. In this process, 150 g/t of KAX and 50 g/t of MIBC were used as a collector and frother, respectively. Scheelite rougher flotation was carried out using the remaining pulp with suspended sulfides, such as and molybdenite. Rougher flotation is a process to remove gangue minerals such as silicate minerals, calcite, and fluorite Minerals 2020, 10, x FOR PEER REVIEW 3 of 13

Na2S, and CaO were used as pH modifiers, 4–7 kg/t of Na2SiO3 as a depressant, 200–350 g/t of oleic acid as a collector and 75 g/t of K-8300 as a frother. Cleaner flotation is a process to float scheelite only Minerals 2020, 10, 971 3 of 13 for producing a high grade concentrate as possible without the addition of any reagent. Furthermore, a locked cycle test was carried out based on these results to calculate material balance to provide a anddesign increase of a continuous the recovery pilot of theplant. scheelite. Moreover, 1–5 kg/t of Na2CO3, NaOH, Na2S, and CaO were usedThe as pH zeta modifiers, potential 4–7 measurements kg/t of Na2SiO were3 as carried a depressant, out using 200–350 a zeta g/ tpotential of oleic acidanalyzer as a collector (ELS-Z Zeta and 75Potential g/t of K-8300 analyzer, as aOtsuka frother. Electronics Cleaner flotation, Osaka, is Japan a process). For tomeasurement, float scheelite the only samples for producing were ground a high to gradeparticles concentrate less than as5 μ possiblem using withoutan the mortar. addition The of mineral any reagent. suspensions Furthermore, with 0.01% a locked of the cycle samples test waswere carried stirred. out The based pH onvalue these of resultssuspension to calculate was adjusted material to balance a required to provide pH value a design using of0.1 a continuousN HCl and pilot0.1 N plant. NaOH [29].

Figure 1. Flowsheet of the batch type scheelite flotation from tungsten ore. Figure 1. Flowsheet of the batch type scheelite flotation from tungsten ore. The zeta potential measurements were carried out using a zeta potential analyzer (ELS-Z Zeta PotentialThe analyzer, FTIR analysis Otsuka was Electronics, carried Osaka, out using Japan). a ForFTIR measurement,-ATR spectrometer the samples (Nicolet were 6700 ground FTIR to particlesSpectrometer less than with 5 µ Attenuatedm using an Total agate mortar.Reflectance, The mineral Thermo suspensions Scientific, Waltham, with 0.01% MA, of the USA samples). For weremeasurement, stirred. The the pH samples value were of suspension ground to was particles adjusted less tothan a required5 μm using pH an value agate using mortar. 0.1 NThe HCl system and 0.1was N equipped NaOH [29 with]. a ATR , and then mineral particles were pressed and fastened towardsThe FTIR the ATR analysis crystal was carriedto collect out background using a FTIR-ATR spectra spectrometer first. All spectra (Nicolet were 6700 FTIR collected Spectrometer at room withtemperature Attenuated [30]. Total Reflectance, Thermo Scientific, Waltham, MA, USA). For measurement, the samplesThe mineralogy were ground and chemical to particles comp lessosition than of the 5 µ msamples using were an agate obtained mortar. using TheX-ray system diffraction was equippedspectrometer with (XRD,a diamond X’Pert ATR MPD, crystal, Philips and then, Amsterdam, mineral particles The were Netherlands pressed), and X- fastenedray towards thespectrometer ATR crystal (XRF, to collect MXF background-2400, Shimadzu spectra, Kyoto, first. All Japan spectra) and were scanning collected electron at room microscopy temperature-energy [30]. dispersiveThe mineralogy spectrometry and (SEM chemical-EDS, composition Tabletop Microscope of the samples TM3000, were Hitachi obtained, Tokyo, using X-rayJapan). di Theffraction WO3 spectrometercontent of samples (XRD, was X’Pert analyzed MPD, by inductively Philips, Amsterdam, coupled plasma The atomic Netherlands), emission X-ray spectrometer fluorescence (ICP- spectrometerAES, 5300DV, (XRF, Perkin MXF-2400, Elmer) Shimadzu, Kyoto, Japan) and scanning electron microscopy-energy After flotation, the floated and un-floated products were collected, filtered, dried in ovens for 24 h dispersive spectrometry (SEM-EDS, Tabletop Microscope TM3000, Hitachi, Tokyo, Japan). The WO3 contentat 90 °C of, weighed samples and was analyzed. analyzed The by inductivelyflotation performance coupled plasma is expressed atomic by emission Grade (G) spectrometer that is the (ICP-AES,content of 5300DV,a valuable Perkin component Elmer) in the product, recovery (R) of concentrate and enrichment ratio (ER).After The recovery flotation, is the the floated qualitative and un-floatedand quantitative products parameter were collected, of the product filtered, in driedwhich in Covens and c forare the weight and grade of concentrate, F and f are the weight and grade of the feed, respectively. 24 h at 90 ◦C, weighed and analyzed. The flotation performance is expressed by Grade (G) that is the contentRecovery of is a valuablethe percentage component of the in total the product, mineral recoveryor metal (R)contained of concentrate in the ore, and which enrichment is recovered ratio (ER). in Thethe concentrate. recovery is theThe qualitative recovery of and the quantitativeconcentrate is parameter given by ofthis the Equation product [18 in,3 which1]: C and c are the 푪 × 풄 weight and grade of concentrate, F and퐑f =are the weight × ퟏퟎퟎ and grade of the feed, respectively. Recovery(1) is 푭 × 풇 the percentage of the total mineral or metal contained in the ore, which is recovered in the concentrate.

The recovery of the concentrate is given by this Equation [18,31]:

C c R = × 100 (1) F f × × Minerals 2020,, 1010,, 971x FOR PEER REVIEW 4 of 13

The enrichment ratio is the ratio of the grade of the concentrate (c) to the grade of the feed (f) The enrichment ratio is the ratio of the grade of the concentrate (c) to the grade of the feed (f ) and and is given by this Equation [32]: is given by this Equation [32]: 풄 퐄퐑 = c ER풇= (2) f

3. Results and D Discussioniscussion

3.1. Adsorption Mechanism between Mineral Surfaces and Flotation Reagents The adsorption mechanism of oleic acid with Ca Ca-bearing-bearing minerals waswas investigate investigatedd through zeta potential and FT FTIRIR analysis. analysis. The The surface surface charge charge of ofmineral mineral particles particles is changed is changed by pulp by pulp pH value pH value and andaffects aff ects the the adsorption adsorption of the of the collector collector on on mineral mineral surfaces surfaces [3 [333,3,344].]. To To investigate investigate the adsorption adsorption mechanism of the reagents on the mineral su surface,rface, the effects effects of zeta potential are shown in in Fig Figureure 22.. The zeta potentials of Ca-bearingCa-bearing minerals in tungsten ore such as scheelite, calcite and fluoritefluorite were measured at pH 2–12.2–12. It has been confirmed confirmed the scheelite has a negative electric charge in allall pHpH areas, and that the point of zero charge ( (PZC)PZC) of the calcite calcite is is 9 9–10–10 and and the the fluorite is is 4 4–5,–5, respectively. respectively. Oleic acidacid waswas usedused asas aa collectorcollector to to react react with with all all minerals minerals and and to to measure measure the the zeta zeta potential potential again again to confirmto confirm that that the the zeta zeta potential potential values valu ines all in minerals all minerals were were significantly significantly negatively negatively shifted. shifted. As a result, As a theresult, reaction the reaction between between these Ca-bearing these Ca- mineralsbearing minerals and oleic and acid oleic was found acid was to be found chemical to be adsorption, chemical notadsorption, physical not adsorption physical withadsorption different with pH differen and surfacet pH chargesand surface of minerals charges [ 34of ,minerals35]. [34,35].

Figure 2.2. Zeta potentials of scheelite, calcite and fluoritefluorite in the absence and presence of oleic acid as a collector.collector.

In order to further determination, the adsorption mechanism of flotation flotation reagents on Ca Ca-bearing-bearing mineral surfaces, the FTIR spectra of scheelite,scheelite, calcite and fluoritefluorite before and after treating with flotationflotation reagents werewere measured.measured. FigureFigure3 3 shows shows FTIRFTIR spectraspectra ofof scheelite,scheelite, calcite,calcite, and and fluoritefluorite beforebefore and after flotationflotation reagent treatment at pH 11. In the addition of the oleic acid as collector,collector, severalseveral new peaks are observed in the spectraspectra of (a)(a) scheelite,scheelite, (b)(b) calcite,calcite, andand (c)(c) fluorite.fluorite. The new peaks at 1 1 2852 cm−−1 and 2922 cm−1− are due to the –CH and –CH stretching bands in the oleic acid, indicating 2852 cm and 2922 cm are due to the –CH33 and –CH22 stretching bands in the oleic acid, indicating the chemicalchemical adsorptionadsorption ofof oleicoleic acidacid onon eacheach Ca-mineralCa-mineral surfacesurface [[3,17].3,17]. ThisThis chemicalchemical adsorptionadsorption consists of monolayer adsorption, allowing only one reagent to react to each available site on thethe mineral surface for adsorption.adsorption. Sodium carbonate as a selective depressantdepressant of Ca-bearingCa-bearing mineralsminerals waswas reactedreacted beforebefore the the oleic oleic acid acid to to confirm confirm the the adsorption adsorption mechanism mechanism between between flotation flotation reagents reagents and mineraland mineral surfaces. surfaces. The resultsThe results showed showed that oleic that acid oleic did acid not did react not to react mineral to mineralsurfaces surfacesafter reaction after withreaction sodium with carbonate. sodium carbonate. It is confirmed It is confirmed that the surface that the of Ca-bearing surface of Ca minerals-bearing occurred minerals monolayer occurred adsorptionmonolayer withadsorption flotation with reagents flotation by reagents chemical by adsorption. chemical adsorption.

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Figure 3. FTIR spectra of (a) scheelite, (b) calcite, and (c) fluorite in the absence and presence of flotation Figure 3. FTIR spectra of (a) scheelite, (b) calcite, and (c) fluorite in the absence and presence of reagents (OA: Oleic Acid, SC: Sodium Carbonate). flotation reagents (OA: Oleic Acid, SC: Sodium Carbonate). 3.2. Characterization of the Tungsten Ore from Sangdong Mine in South Korea 3.2. Characterization of the Tungsten Ore from Sangdong Mine in South Korea Figure4 shows X-ray powder di ffraction patterns of run-of-mine tungsten ore from the SangdongFigure mine 4 shows in South X-ray Korea. powder According diffraction to these patterns results, of tungsten run-of-mine mineral tungsten in the ore ore consistsfrom the Sangdong mine in South Korea. According to these results, tungsten mineral in the ore consists of of scheelite, and major gangue minerals consist of (SiO2), hedenbergite (Ca(Fe,Mg)(SiO3)2), scheelite, and major gangue minerals consist of quartz (SiO2), hedenbergite (Ca(Fe,Mg)(SiO3)2), biotite biotite (K(Mg,Fe)3(AlSi3O10)(F,OH)2), magnesiohornblende (Ca2(Mg,Fe)5(Si,Al)8O22(OH)2), calcite (K(Mg,Fe)3(AlSi3O10)(F,OH)2), magnesiohornblende (Ca2(Mg,Fe)5(Si,Al)8O22(OH)2), calcite (CaCO3), (CaCO3), and fluorite (CaF2)). It was confirmed that the chemical composition of scheelite ore was and fluorite (CaF2)). It was confirmed that the chemical composition of scheelite ore was SiO2 57.20 SiO2 57.20 wt.% and Al2O3 13.61 wt.%, Fe2O3 10.48 wt.%, CaO 6.79 wt.% and small amounts of WO3 0.41wt.% wt.% and by Al XRF2O3 13.61 and ICP wt.%, analysis Fe2O3 (Table10.48 1wt.%,). CaO 6.79 wt.% and small amounts of WO3 0.41 wt.% by XRF and ICP analysis (Table 1.). Table 1. Chemical composition of the tungsten ore from Sangdong mine in South Korea. Table 1. Chemical composition of the tungsten ore from Sangdong mine in South Korea. 1 Elements WO3 SiO2 CaO Fe2O3 Al2O3 MgO K2O TiO2 Na2OP2O5 LOI. Elements WO3 SiO2 CaO Fe2O3 Al2O3 MgO K2O TiO2 Na2O P2O5 LOI.1 % 0.41 57.20 13.61 10.48 6.79 3.05 3.19 0.97 0.62 0.30 2.72 % 0.41 57.20 13.61 10.48 6.79 3.05 3.19 0.97 0.62 0.30 2.72 1 1Loss Loss on on ignition ignition

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Figure 4. X-ray powder diffraction patterns of the run-of-mine tungsten ore from the Sangdong mine Figure 4. X-ray powder diffraction patterns of the run-of-mine tungsten ore from the Sangdong mine in South Korea. in South Korea. 3.3. Effect of Flotation Reagent on the Selective Depression of Ca-Bearing Minerals in Real Ore System 3.3. Effect of Flotation Reagent on the Selective Depression of Ca-Bearing Minerals in Real Ore System According to the investigation results, flotation experiments using a real ore have been conducted in theAccording Denver Sub-A to the type investigation flotation cell. results, The pH flotatio controln during experiments flotation using is deemed a real as ore critical have because been theconducted mineral in surface the Denver characteristics Sub-A type in the flotation pulp and cell. the The reactivity pH control with during the reagent flotation vary is as deemed the pH as of thecritical pulp because changes the [ 1mineral,19]. Corresponding surface characteristics to the preliminary in the pulp and experiment the reactivity result, with the the pH reagent of the pulpvary wasas the adjusted pH of the to 11pulp and changes the experiments [1,19]. Corresponding were conducted to the using preliminary NaOH, experiment Na2S, CaO, result, and Na the2CO pH3 asof pHthe modifiers.pulp was adjusted For a depressant to 11 and and thea experiments collector of scheelite were conducted flotation, using Na2 SiO NaOH,3 and Na oleic2S, acidCaO, were and selectedNa2CO3 as due pH to modifiers. the preliminary For a depressant experimental and results a collector and theyof scheelite are widely flotation, used Na in many2SiO3 and conventional oleic acid processeswere selected [5,23 , due24,34 to,36 – the38 ]. preliminary Figures S1–S3 experiment show theal results results of the and experiment they are widely with the used variable in many type ofconventional depressant, processes collector, [5,23,24,3 and frother4,3 in6–3 this8]. Fig study.ures S1–S3 show the results of the experiment with the variableFigure type5a of shows depressant, the WO collector3 grade, and and frother recovery in this of the study. concentrate according to the type of pH 3 modifierFigure with 5a.optimal shows the dosage, WO respectivelygrade and recovery (Na2CO of3; 4the kg concentrate/t, NaOH; 2 according kg/t, CaO; to 3 kgthe/t, type and of Na pH2S; 2 3 2 2modifier kg/t at Nawith2SiO optimal3; 4 kg dosage,/t, oleic acid;respectively 300 g/t, (Na K-8300;CO 75; 4 gkg/t,/t). NaNaOH;2CO3 2was kg/t, used CaO; as 3 akg/t, pH and modifier; Na S; it2 waskg/t 2 3 2 3 foundat Na SiO that; the4 kg/t, performance oleic acid; was300 maximizedg/t, K-8300; with75 g/t) the. Na WOCO3 grade was used of 70.2% as a andpH modifier; the recovery it was of 83.15%found andthat smallthe performance amounts of was Mo 0.002%,maximized Fe 0.2%, withand the SnWO 5.23 grade mg/kg. of This70.2% result and confirmedthe recovery that of the 83.15 tendency% and wassmall similar amounts to of that Mo of 0.002%, other research, Fe 0.2%, and and Sn the 5.2 mechanism mg/kg. This for result this confirmed was determined that the as tendency follows was [26]. 2 Insimilar comparison to that ofto otherthe case research, that NaOH and the used mechanism as a general for pH this modifier, was determined CO3 − in as Na follows2CO3 seemed [26]. In 32– 2 3 2 tocomparison influence theto the selective case that separation NaOH of used scheelite. as a general Na2CO pH3 is modifier, ionized in CO the pulp, in Na andCO CO seemed3 − reacts to 2+ 2 3 32– withinfluence Ca theon selective the surface separation of Ca-based of scheelite minerals. Na andCO precipitates is ionizedas in CaCOthe pulp,3 like and surface CO carbonation. reacts with InCa2+ this on process,the surface the of Ca-bearing Ca-based gangueminerals minerals and precipitates such as calciteas CaCO and3 like fluorite, surface which carbonation. have relatively In this highprocess, Ca surfacethe Ca- sitebearing density, gangue have minerals been employed such as tocalcite precipitate and fluorite, on the surfacewhich have by reacting relatively earlier high than Ca scheelitesurface site with density, low Cahave surface been employed site density to (Fluorite:precipitate 11.1–12.9 on the surfaceµmol /bym 2reacting, Calcite: earlier 8.24 µthanmol scheelite/m2 and Scheelite:with low Ca 5.06–6.58 surfaceµ sitemol density/m2)[24 (Fluorite:,26,39]. To 11.1 support–12.9 μmol/m this, there2, Calcite: is a Ca–O 8.24 (Ca–F) μmol/m bond2 and length. Scheelite: It is known5.06–6.58 that μmol/m the shorter2) [24,26,3 the9 bond]. To support length,the this, larger there the is a bond Ca–O energy. (Ca–F) Relativelybond length. short It is bond known lengths that ofthe calcite shorter and the fluorite bond length, reacted the with larger Na 2theCO bond3, which energy. is likely Relatively to be depressed short bond before lengths scheelite of calcite due and to thefluorite enhanced reacted reactivity with Na due2CO to3, which the larger is likely bond to energy be depressed (Calcite: before 2.36 Å, scheelite Fluorite: due 2.37 to Å the and enhanced Scheelite: reactivity due to the larger bond energy (Calcite:2+ 2.36 Å , Fluorite: 2.37 Å and Scheelite: 2.442+ –2.48 Å ) 2.44–2.48 Å) [15,24,26,40]. Furthermore, Na in Na2CO3 reduces the probability that Ca activates 2+ 2 3 2+ quartz[15,24,26, and40]. hereby Furthermore, silicates Na [26,41 in]. Na ToCO sum reduces up, after the the probability addition of that Na Ca2CO activates3 to the pulp, quartz based and onhereby kinetics, silicates fluorite [26,41 is]. theTo sum first up, mineral after the to be addition depressed, of Na then2CO3calcite, to the pulp, silicates, based and on finallykinetics, scheelite, fluorite is the first mineral to be depressed, then calcite, silicates, and finally scheelite, if Na2CO3 overdosed. if Na2CO3 overdosed. When the oleic acid is added after this reaction, scheelite reacted with less When2 the oleic acid is added after this reaction, scheelite reacted with less CO32− among Ca-bearing CO3 − among Ca-bearing minerals, is adsorbed with oleic acid as a collector. It is confirmed that the availableminerals, adsorptionis adsorbed site with on oleic the scheeliteacid as a surfacecollector. is limitedIt is confirmed due to the that reaction the available between adsorption the scheelite site surfaceon the scheelite and the surface flotation is reagentlimited due is chemical to the reaction adsorption. between As the a result, scheelite the surface surface and of thethe scheeliteflotation reagent is chemical adsorption. As a result, the surface of the scheelite becomes hydrophobic, adheres to the bubbles, and floats in the pulp. Figure 6 shows the selective depressing mechanism of Ca-

Minerals 2020, 10, x FOR PEER REVIEW 7 of 13 bearing gangue minerals using Na2CO3. Figure 5b shows the WO3 grade and recovery of the concentrate according to the dosage of Na2CO3. The more pH modifier was added, the more the grade and recovery of the concentrate were showed until 4 kg/t of Na2CO3. This is because the concentration of CO32− in the pulp increased and CO32− precipitated as a form of CaCO3 onto the surface of scheelite as well as calcite and fluorite [26]. Na2SiO3 is widely used as a reagent to depress gangue minerals that consists of silicate minerals [11,14]. Figure 5c shows the WO3 grade and recovery of the concentrate according to the addition amount of Na2SiO3 used as a depressant. The recovery decreases as the reagent addition amount increases, and the grade increases continuously, but decreases at more than 5 kg/t of Na2SiO3. When the depressant is added in the pulp, silicate minerals are strongly hydrophilized and depressed, but when excess is added, HSiO3- reacts with the scheelite to form (HSiO3)2Ca salt to make it hydrophilic [16,19,36,37]. Therefore, it is explained that scheelite did not react with oleic acid used as a collector, is depressed, and remains in the flotation cell without floating. Oleic acid is one of the fatty acids with aliphatic tails and is known to have excellent performance in collecting the scheelite in [1,34]. Figure 5d shows the WO3 grade and recovery of the concentrate with respect to the additional amount when oleic acid was used as a collector. As the addition amount of the collector increased, the grade gradually increased but the recovery decreased Minerals 2020, 10, 971 7 of 13 at more than 300 g/t of oleic acid. This is because the hydrophobicity of the particle surface and the possibility of adhesion to bubbles for flotation is increased as the amount of the collector added increasesbecomes [19,3 hydrophobic,8]. When oleic adheres acid to is theadded bubbles, in an excess and floats amount, in the it is pulp. considered Figure 6that shows not only the selectivescheelite butdepressing also other mechanism gangue of minerals Ca-bearing are gangue collected, minerals which using Na 2 toCO a3 .decrease Figure5b in shows the theWO WO3 grade3 grade of concentrate.and recovery of the concentrate according to the dosage of Na2CO3. The more pH modifier was added, the moreFigure the 7 shows grade andthe X recovery-ray diffraction of the concentrate patterns of wereraw ore showed sample until and 4 each kg/t ofprocess Na2CO product3. This in is 2 2 Figurebecause 1. theIn the concentration case of tailings, of CO silicate3 − in theminerals, pulp increased such as quartz, and CO ,3 − precipitated and mica, as aare form contained of CaCO in3 largeonto thequantities. surface In of scheeliteFigure 7b, as well as are calcite the andproducts fluorite obtained [26]. by using KAX, a typical collector of sulfideNa minerals2SiO3 is widely [1,27]. used Therefore, as a reagent it can to be depress confirmed gangue that minerals most of that the consists suspended of silicate minerals minerals are [pyrite11,14]. (FeSFigure2) and5c shows molybdenite the WO (MoS3 grade2). In and Figure recovery 7c, tailings of the concentrate were found according to be quite to similar the addition in composition. amount of MostNa2SiO of3 theseused asproducts a depressant. contain The quartz, recovery a silicate decreases mineral, as the some reagent magnesiohornblende addition amount increases, and biotite. and theIn Figuregrade increases7a., the concentrate continuously, consists butdecreases mostly of atscheelite, more than and 5 some kg/t ofof Nathe2 SiOsame3. Ca When-minerals the depressant calcite and is fluoriteadded inexist. the pulp,Based silicateon the mineralsXRD analysis are strongly results in hydrophilized Figure 7, SEM and-EDS depressed, analysis butwas whencarried excess out to is - determineadded, HSiO the3 morereacts accurate with the character scheeliteistics to form of the (HSiO final3 )concentrate,2Ca salt to make and it it was hydrophilic confirmed [16 that,19,36 most,37]. ofTherefore, the composition it is explained was Ca, that W scheeliteand O in didthe notscheelite react (CaWO with oleic4) and acid very used small as a impurities collector, is such depressed, as Mo andand Si remains existed in (Figure the flotation 8.). cell without floating.

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Figure 5. Effects of operation parameters on the scheelite flotation (a) kinds of pH modifier, (b) dosage Figure 5. Effects of operation parameters on the scheelite flotation (a) kinds of pH modifier, (b) dosage of sodium carbonate as pH modifier, (c) dosage of sodium silicate as depressant, (d) dosage of oleic of sodium carbonate as pH modifier, (c) dosage of sodium silicate as depressant, (d) dosage of oleic acid as collector, respectively. acid as collector, respectively.

Figure 6. Schematic diagram of the selective depressing mechanism using sodium carbonate on the Ca-bearing mineral surface for recovery of scheelite.

Figure 7. X-ray diffraction patterns and image of flotation products (a) concentrate, (b) sulfides, and (c) tailings, respectively.

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Figure 5. Effects of operation parameters on the scheelite flotation (a) kinds of pH modifier, (b) dosage of sodium carbonate as pH modifier, (c) dosage of sodium silicate as depressant, (d) dosage of oleic Minerals 2020, 10, 971 8 of 13 acid as collector, respectively.

FigureFigure 6. 6. SchematicSchematic diagram diagram of of the the selective selective depressing depressing mechanism mechanism using using so sodiumdium carbonate carbonate on on the the CaCa-bearing-bearing mineral mineral surface surface for for recovery recovery of of scheelite scheelite..

Oleic acid is one of the fatty acids with aliphatic tails and is known to have excellent performance in collecting the scheelite in froth flotation [1,34]. Figure5d shows the WO 3 grade and recovery of the concentrate with respect to the additional amount when oleic acid was used as a collector. As the addition amount of the collector increased, the grade gradually increased but the recovery decreased at more than 300 g/t of oleic acid. This is because the hydrophobicity of the particle surface and the possibility of adhesion to bubbles for flotation is increased as the amount of the collector added increases [19,38]. When oleic acid is added in an excess amount, it is considered that not only scheelite but also other gangue minerals are collected, which leads to a decrease in the WO3 grade of concentrate. Figure7 shows the X-ray di ffraction patterns of raw ore sample and each process product in Figure1. In the case of tailings, silicate minerals, such as quartz, feldspar, and mica, are contained in large quantities. In Figure7b, sulfides are the products obtained by using KAX, a typical collector of sulfide minerals [1,27]. Therefore, it can be confirmed that most of the suspended minerals are pyrite (FeS2) and molybdenite (MoS2). In Figure7c, tailings were found to be quite similar in composition. Most of these products contain quartz, a silicate mineral, some magnesiohornblende and biotite. In Figure7a., the concentrate consists mostly of scheelite, and some of the same Ca-minerals calcite and fluorite exist. Based on the XRD analysis results in Figure7, SEM-EDS analysis was carried out to determine the more accurate characteristics of the final concentrate, and it was confirmed that most of the compositionFigure 7. X-ray was diffraction Ca, W and patterns O in and the scheeliteimage of flotation (CaWO 4products) and very (a) smallconcentrate, impurities (b) sulfides such as, and Mo and Si existed(c) tailings, (Figure respectively8). . Additionally, to investigate the separation mechanism of the sodium carbonate-sodium silicate-oleic acid system, the grade and recovery of concentrates were measured by the change of the added reagent order. Adding Na2SiO3 followed by Na2CO3, the grade and recovery of concentrate were 0.45%, 0.02%, and it is explained that scheelite was not separated from the tungsten ore at all. This can be attributed to the fact that the reactivity of the mineral with the collector was influenced by the reaction order between the reagent and the surface of the minerals (Figure S4) [26]. Na2CO3 prevents the depressing effect caused by adsorption of Na2SiO3 on the surface of scheelite. The addition of Na2CO3 before the addition of Na2SiO3 leads to the formation of CaCO3 precipitates on the surface through the process of surface carbonation reaction of Ca-bearing minerals such as calcite and fluorite before scheelite [25,26,42]. It activated the depression caused by Na2SiO3 in gangue minerals [3,25,43]. Conversely, when Na2SiO3 is added first, Na2SiO3 depressed all minerals and scheelite was not been able to be separated from the raw ore. Minerals 2020, 10, x FOR PEER REVIEW 8 of 13

Figure 5. Effects of operation parameters on the scheelite flotation (a) kinds of pH modifier, (b) dosage of sodium carbonate as pH modifier, (c) dosage of sodium silicate as depressant, (d) dosage of oleic acid as collector, respectively.

Figure 6. Schematic diagram of the selective depressing mechanism using sodium carbonate on the Minerals 2020, 10, 971 9 of 13 Ca-bearing mineral surface for recovery of scheelite.

Figure 7. X-ray diffraction patterns and image of flotation products (a) concentrate, (b) sulfides, Figure 7. X-ray diffraction patterns and image of flotation products (a) concentrate, (b) sulfides, and Mineralsand 2020 (c), tailings,10, x FOR respectively. PEER REVIEW 9 of 13 (c) tailings, respectively.

Figure 8. (a) SEM image, (b) energy dispersive spectrometry (EDS) spectrum, and element mapping Figure 8. (a) SEM image, (b) energy dispersive spectrometry (EDS) spectrum, and element mapping images for (c) Ca, (d) W, (e) O, and (f) Mo of the final scheelite concentrate, respectively. images for (c) Ca, (d) W, (e) O, and (f) Mo of the final scheelite concentrate, respectively. 3.4. Locked Cycle Test (LCT) Additionally, to investigate the separation mechanism of the sodium carbonate-sodium silicate- oleicA acid total system, of six cyclesthe grade of LCTand recovery was carried of concentrates out to calculate were material measured balance by the based change on of the the batch added test resultsreagent (70.2 order WO. Adding3%, 83.15%). Na2SiO The3 followed flotation bycircuit Na2 includingCO3, the grade one sulfide and recovery rougher, of two concentrate sulfide scavenger, were one0.45%, scheelite 0.02%, rougher, and it is twoexplained scheelite that scavenger scheelite was and not four separated scheelite from cleaner. the Thetungsten flotation ore at flowsheet all. This is presentedcan be attributed in Figure to9 the, and fact the that results the reactivity are listed of in the Figure mineral 10, and with Table the collector2. The WO was3 influencedgrade and recoveryby the inreaction scheelite order concentrate between the attained reagent to and 71.2% the and surface 83.37% of the (average mineral values (Figure of 4–6 S4) cycles).[26]. Na The2CO3 enrichment prevents ratiothe depressing (ER) is more effect than caused 170 with by aadsorption high recovery. of Na The2SiO concentrate3 on the surface grade of and scheelite. yield of The the addition final 3 cycles of inNa the2CO 63 cycle before test the are addition almost constantof Na2SiO (Grade;3 leads to0.1%, the formation Recovery; of0.1% CaCO and3 precipitates Yield; 1.4%). on the These surface results ± ± ± showthrough that the the process performance of surface of LCTcarbonation with the reaction optimal of condition Ca-bearing was minerals good mass such conservationas calcite and fluorite and good stability.before scheelite This suggests [25,26,4 that2]. It the activated developed the depression flotation process caused ebyffectively Na2SiO3 enriched in gangue the minerals tungsten [3,25,4 minerals3]. Conversely, when Na2SiO3 is added first, Na2SiO3 depressed all minerals and scheelite was not been able to be separated from the raw ore.

3.4. Locked Cycle Test (LCT) A total of six cycles of LCT was carried out to calculate material balance based on the batch test results (70.2 WO3%, 83.15%). The flotation circuit including one rougher, two sulfide scavenger, one scheelite rougher, two scheelite scavenger and four scheelite cleaner. The flotation flowsheet is presented in Figure 9, and the results are listed in Figure 10, and Table 2. The WO3 grade and recovery in scheelite concentrate attained to 71.2% and 83.37% (average value of 4–6 cycles). The enrichment ratio (ER) is more than 170 with a high recovery. The concentrate grade and yield of the final 3 cycles in the 6 cycle test are almost constant (Grade; ±0.1%, Recovery; ±0.1% and Yield; ±1.4%). These results show that the performance of LCT with the optimal condition was good mass conservation and good stability. This suggests that the developed flotation process effectively enriched the tungsten minerals in the raw ore of Sangdong mine. LCT results can be used to provide a baseline for the design and operation of a continuous pilot plant.

Minerals 2020, 10, x FOR PEER REVIEW 10 of 13 Minerals 2020, 10, 971 10 of 13

in the raw ore of Sangdong mine. LCT results can be used to provide a baseline for the design and Mineralsoperation 2020 of, 10 a, x continuous FOR PEER REVIEW pilot plant. 10 of 13

Figure 9. Flowsheet of the locked cycle test for scheelite flotation from tungsten ore . Figure 9. Flowsheet of the locked cycle test for scheelite flotation from tungsten ore. Figure 9. Flowsheet of the locked cycle test for scheelite flotation from tungsten ore.

Figure 10. Mass and tungsten (W) output distribution of each cycle product by locked cycle test, Figure 10. Mass and tungsten (W) output distribution of each cycle product by locked cycle Figurerespectively 10. Mass . and tungsten (W) output distribution of each cycle product by locked cycle test, test, respectively. respectivelyTable .2. Concentrate grade, enrichment ratio (ER), recovery and yield of each cycle product of locked cycle test. Table 2. Concentrate grade, enrichment ratio (ER), recovery and yield of each cycle product of locked Cycle Grade (WO3%) ER Recovery (%) Yield (%) cycle test. 1 67.51 164.66 81.24 0.68 2 72.25 176.22 85.16 0.77 Cycle Grade (WO3%) ER Recovery (%) Yield (%) 3 71.94 175.46 83.78 0.75 1 67.51 164.66 81.24 0.68 4 71.17 173.59 83.45 0.73 2 5 72.2571.23 176.22173.73 83.3785.16 0.71 0.77 3 6 71.9471.2 175.46173.66 83.2983.78 0.72 0.75 Average4 (4–6) 71.1771.2 173.59173.66 83.3783.45 0.72 0.73 5 71.23 173.73 83.37 0.71 6 71.2 173.66 83.29 0.72 Average (4–6) 71.2 173.66 83.37 0.72

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Table 2. Concentrate grade, enrichment ratio (ER), recovery and yield of each cycle product of locked cycle test.

Cycle Grade (WO3%) ER Recovery (%) Yield (%) 1 67.51 164.66 81.24 0.68 2 72.25 176.22 85.16 0.77 3 71.94 175.46 83.78 0.75 4 71.17 173.59 83.45 0.73 5 71.23 173.73 83.37 0.71 6 71.2 173.66 83.29 0.72 Average (4–6) 71.2 173.66 83.37 0.72

4. Conclusions In this study, flotation experiments were carried out to investigate the flotation mechanism and improve the scheelite flotation performance of undeveloped tungsten ore from the Sangdong mine in South Korea. The tungsten ore used in this study contains scheelite minerals with a WO3 grade of 0.41%. Moreover, it was confirmed that calcite, fluorite, and other silicate minerals are present as major gangue minerals that can retard selective separation of scheelite. To reveal the adsorption mechanism of the reagents on the mineral surface, basic adsorption experiments of reagents and pure minerals were conducted. This reaction was confirmed chemical adsorption by analysis of zeta potential and FTIR analysis. Then, kinds of modifiers and dosages, and the addition order of flotation reagent experiments were conducted to determine the factors affecting the grade and recovery of scheelite concentrate. Corresponding to the experiment results with various types of pH modifiers, it was confirmed that Na2CO3 performed not only pH control, but also the depressing of Ca-bearing gangue minerals based on kinetics. After the addition of Na2CO3 to the pulp, it is ionized in the pulp and 2 2+ CO3 − reacts with Ca on the surface of Ca-based minerals and precipitates, as CaCO3 like surface carbonation. The fluorite is the first mineral to be depressed, then calcite, silicates, and finally scheelite. Furthermore, the flotation performance was greatly different according to the added order of the reagent due to the reaction order between the flotation reagent and mineral surface. Based on the locked cycle test, the optimal conditions for recovering high grade and recovery (71.2 WO3%, 81.37%) with good mass conservation and stability were obtained.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/10/11/971/s1, Figure S1: Effect of types of depressant on scheelite concentrate grade and recovery of WO3, Figure S2: Effect of types of collector on scheelite concentrate grade and recovery of WO3, Figure S3: Effect of types of frother on scheelite concentrate grade and recovery of WO3, Figure S4: Effect of reaction order on the scheelite flotation (a) pH modifier-depressant-collector-frother and (b) depressant-pH modifier-collector-frother. Author Contributions: Conceptualization, S.K., and H.-S.J.; methodology, S.K., and S.-H.B.; formal analysis, S.K., and Y.H.; investigation, S.K., Y.H., and S.-H.B.; writing—original draft preparation, S.K.; writing—review and editing, S.K., Y.H., and H.-S.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Basic Research Project (20-3212-1) of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science, ICT and Future Planning of Korea and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Korean Ministry of Trade, Industry& Energy (MOTIE), Republic of Korea (No. 20172510102220). Conflicts of Interest: The authors declare no conflict of interest.

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