Pre-Concentration of Iron-Rich Sphalerite by Magnetic Separation

Article Pre-Concentration of Iron-Rich Sphalerite by Magnetic Separation

Soobok Jeong and Kwanho Kim * ID DMR Convergence Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-42-868-3580

Received: 9 May 2018; Accepted: 25 June 2018; Published: 27 June 2018

Abstract: With the rise in metal prices and the growing importance of metallic minerals in the South Korean economy, there has been a steadily increasing demand to redevelop metal mines that have been shut down since the 1990s. However, it is not possible to ensure that such plans are economically feasible by using conventional mining processes, mainly flotation, because of low ore grade and complex mineral compositions. To improve the efficiency, and to reduce the operating cost of the entire process, pre-concentration by magnetic separation of Pb–Zn deposits has been investigated to reduce the mass and improve the grade of feed samples that are loaded into the flotation system. The results show that the response of sphalerite to magnetic separation varied as a function of its iron content: iron-rich sphalerite was recovered at magnetic intensities below 0.65 T, and relatively pure sphalerite was recovered at magnetic intensities above 0.85 T. Therefore, Pb–Zn ore could be sufficiently pre-concentrated by magnetic separation between 0.65 and 0.85 T to remove low-grade target elements. As a result, the mass of the sample fed into the flotation system was reduced almost by half, and the grade of zinc, lead, and copper was enhanced by 65%, 55%, and 33%, respectively. Therefore, it is possible to improve the efficiency of the entire process by reducing the amount of the sample to be fed to subsequent processes, such as grinding and flotation, while minimizing loss of the target mineral through magnetic separation.

Keywords: sphalerite; Pb–Zn deposit; magnetic separation; beneﬁciation; pre-concentration

1. Introduction In South Korea, approximately forty domestic metallic mineral mines were in active operation until the 1980s. Nine of these mines were dedicated to the mining of Pb–Zn deposits, including the Gagok mine. All mineral processing circuits consisted of crushing, grinding, and flotation, which were not significantly different from those of Pb–Zn mines in the rest of the world [1–3]. However, ores in South Korean metal mines consist of complex metallic minerals and have various impurities. In particular, the deposits in the Gagok mine contain various sulfide minerals, such as zinc sulfide, lead sulfide, and copper sulfide, and the zinc sulfide mineral, known as sphalerite, contains an especially high amount of iron as an impurity. As the quality of extracted ores and the related profitability gradually deteriorated, the operators started to close the metal mines in the 1990s. Sphalerite (ZnS) is a representative zinc-containing sulfide mineral, and it is the most important resource for zinc metal production. Sphalerite is normally found with other sulfide minerals such as galena, chalcopyrite, and pyrite in Pb–Zn deposits, and they are separated from each other by froth flotation [3–5]. Therefore, for many decades, many researchers have focused on the flotation behavior of sphalerite when it is separated from other sulfide minerals [2,6–9]. Recent studies also investigated copper activation and depression of sphalerite with various other sulfide minerals for selective flotation. The adsorption of cupric ions from a solution on the mineral surface for activating

Minerals 2018, 8, 272; doi:10.3390/min8070272 www.mdpi.com/journal/minerals Minerals 2018, 8, 272 2 of 13 sphalerite depends on various factors, such as copper concentration, activation time, pulp potential, and galvanic interaction [10–14]. The effect of sphalerite depression using various reagents was investigated to float other sulfide minerals separately [15–17]. However, some studies reported that when sphalerite contains iron as an impurity, the adsorption of cupric ions on the sphalerite surface decreases with increasing iron content, leading to a low recovery of the mineral [18–20]. The chemical composition of sphalerite depends on the origin and area of ore deposits, and pure sphalerite rarely exists in nature because it invariably contains various impurities, with iron being the most abundant. When the iron content is more than about 10%, the zinc sulfide mineral is referred to as marmatite (ZnxFe1−xS), and the physical and electrochemical properties of sphalerite are known to vary with the iron content [21]. Among various properties of iron-containing sphalerite, the most representative feature is the magnetic susceptibility. According to previous studies on the magnetic susceptibility of sulfide minerals, sphalerite is a diamagnetic mineral in the pure state, and it becomes a paramagnetic mineral upon the addition of elemental iron; in other words, the magnetic susceptibility of sphalerite increases as the iron content increases in a sample [22–25]. However, there have not been many studies on the magnetic properties of iron-rich sphalerite, and it is not easy to find information on the actual beneficiation process of the sulfide mineral. As metal prices increase and metallic minerals play an increasingly important role in the South Korean economy, various efforts and attempts have been made to redevelop closed mines. However, if previously used conventional processes are applied, it is very likely that the same problems that caused the mines’ demise will emerge again. Therefore, an additional or enhanced process that can improve the efficiency and economy of the entire process should be developed. In this study, gravity separation and magnetic separation were investigated as possible means to pre-concentrate iron-rich sphalerite in Pb–Zn deposits and reduce the mass fraction introduced to the flotation process. If the mass fraction loaded to the flotation process is successfully reduced by pre-concentration, several advantages can be expected, such as the reduction of grinding cost, reagent consumption, wastewater treatment cost, and tailing treatment cost.

2. Materials and Methods

2.1. Material The sample used in this study was a Pb–Zn deposit obtained from the Gagok mine in South Korea. The Gagok mine is one of the metal mines in the Mount Taebaek mining district and the second largest Pb–Zn mine in the country. According to a previous geological study [26], about 600,000 tons of ore were produced in 1978, and the average grades of zinc, lead, and copper were 3.9%, 0.2%, and 0.1%, respectively. This mine has been in a state of temporary shutdown since 1987, and a feasibility test is being planned to re-open the site for active operation. For the feasibility test, samples were obtained at various places on separate occasions, and they were mixed repeatedly to form a representative sample. This feed sample was used for the pre-concentration study presented here. The feed sample was crushed by a jaw crusher (JS-1, Jung Sin, Pohang, Korea) and a cone crusher (Marcy 10”, Svedala, Danville, PA, USA) for mineralogy and chemical composition analysis using X-ray diffraction (XRD; D/Max-2500, Rigaku, Tokyo, Japan), optical microscopy (CS-300, Leica, Wetzlar, Germany), scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS; JSM-6390, JEOL, Tokyo, Japan; 51-ADD0002, Oxford Instruments, Abingdon, UK), and inductive coupled plasma–optical emission spectrometry (ICP–OES; Optima-5300 DV, PerkinElmer, Waltham, MA, USA). For mineralogy analysis, the feed sample was first analyzed by optical microscopy and XRD. The type and condition of the constituent minerals were confirmed by optical microscopy and XRD, and the results were used as basic data to develop the beneficiation process. After confirming the target and gangue minerals, basic separation tests were conducted using gravitational and magnetic separation methods to concentrate the target minerals. Minerals 2018, 8, x FOR PEER REVIEW 3 of 13 Minerals 2018, 8, 272 3 of 13 2.2. Methods 2.2. MethodsTo recover high-density metallic minerals, a shaking table (No. 13 Wilfley table, Humphreys, Jacksonville,To recover FL, high-density USA) was used metallic for minerals,gravity separation. a shaking The table operating (No. 13 Wilfley conditions table, were Humphreys, adjusted Jacksonville,based on the FL,general USA) conditions was used forunder gravity which separation. the equipment The operating was operated. conditions The angle were adjustedof the shaking based ontable, the shaking general amplitude, conditions underand water which flowrate the equipment were varied was from operated. 1° to 5 The°, from angle 10 ofto the20 mm, shaking and table,from shaking8 to 12 L/min, amplitude, respectively. and water However, flowrate the were frequency varied fromwas fixed 1◦ to at 5◦ 300, from rpm 10 owing to 20 mm,to the and fixed from motor 8 to 12speed L/min, and respectively.reduction gear However, ratio. Under the frequency various operating was fixed conditions, at 300 rpm the owing optimum to the fixedconditions motor for speed the andbest reduction separation gear efficiency ratio. Under were various determined operating as follows: conditions, the angle the optimum of the shaking conditions table for was the 2.5 best°, separationshaking amplitude efficiency was were 15 determinedmm, and water as follows: flowrate the was angle 10 L/min. of the shakingThe results table of wasthe shaking 2.5◦, shaking table amplitudeexperiment was were 15 mm, divided and water into flowrate four was categories 10 L/min.—concentration The results of, the middling, shaking table tailing, experiment and wereslime divided—according into fourto the categories—concentration, area in which the sample middling,was recovered tailing, in andthe top slime—according-view of shaking to table, the area as inshown which in theFigure sample 1. was recovered in the top-view of shaking table, as shown in Figure1.

Figure 1. Categories of recovered samples in shaking table experiment.

In the magnetic separation process, a laboratory-scale cross-belt-type magnetic separator In the magnetic separation process, a laboratory-scale cross-belt-type magnetic separator (CBMS; (CBMS; Model EE112, Eriez, Erie, PA, USA) was employed to recover the magnetic minerals. By Model EE112, Eriez, Erie, PA, USA) was employed to recover the magnetic minerals. By changing the changing the current supplied to the separator, the applied magnetic field was modulated between current supplied to the separator, the applied magnetic field was modulated between 0.2 and 1.4 T. 0.2 and 1.4 T. The magnetic separation tests were conducted as the magnetic intensity was increased The magnetic separation tests were conducted as the magnetic intensity was increased from an initial from an initial low value of 0.2 T. After separation of the magnetic products at low intensity, the low value of 0.2 T. After separation of the magnetic products at low intensity, the remaining sample remaining sample was fed into a magnetic separator adjusted to a slightly higher magnetic intensity was fed into a magnetic separator adjusted to a slightly higher magnetic intensity to conduct further to conduct further separation tests. The magnetic separation tests were carried out sequentially in separation tests. The magnetic separation tests were carried out sequentially in this manner until the this manner until the magnetic intensity reached 1.4 T. magnetic intensity reached 1.4 T. The threshold particle size for magnetic separation was varied between 0.3 to 2 mm to The threshold particle size for magnetic separation was varied between 0.3 to 2 mm to determine determine the appropriate particle size for magnetic separation. Although a smaller particle size the appropriate particle size for magnetic separation. Although a smaller particle size generally generally improves the degree of liberation, excessively fine particles can deteriorate the separation improves the degree of liberation, excessively fine particles can deteriorate the separation efficiency in efficiency in magnetic separation. Moreover, an appropriate particle size can reduce the operating magnetic separation. Moreover, an appropriate particle size can reduce the operating cost resulting cost resulting from over-grinding. from over-grinding. A total of 10 kg of the feed sample was processed, and the results were confirmed to verify that A total of 10 kg of the feed sample was processed, and the results were confirmed to verify that the sample was successfully pre-concentrated in the magnetic separation process. the sample was successfully pre-concentrated in the magnetic separation process. 3. Results and Discussion 3. Results and Discussion

3.1.3.1. Chemical CompositionComposition of Feed SampleSample The XRD pattern of the feedfeed sample sample inin FigureFigure2 2 reveals reveals that that sphalerite sphalerite waswas the the major major sulﬁdesulfide mineral, and quartz, calcite, and muscovite were the majormajor ganguegangue minerals.minerals. However, galena and chalcopyrite, which which usually usually accompany accompan sphaleritey sphalerite in Pb–Znin Pb– deposits,Zn deposit weres, were not detected not detected by XRD by owing XRD toowing their tolow their content, low ascontent shown, as in Tableshown1. Tablein Table1 also 1. showsTable the1 also concentration shows the (wtconcentration %) of target ( elementswt %) of astarget a function elements of particleas a function size in of the particle feed sample. size in Thethe feed concentration sample. The of zinc concentration in the entire of sample zinc in was the 4.88entire wt sample %, and was the 4.88 lead wt and % copper, and the concentrations lead and copper were concentrations both 0.09 wt were %, which both are0.09 much wt % lower, which than are much lower than the zinc concentration. Therefore, the concentration of zinc, the main element in

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Minerals 2018, 8, x FOR PEER REVIEW 4 of 13 the zinc concentration. Therefore, the concentration of zinc, the main element in sphalerite, was the mainsphalerite, consideration was the formain the consideration pre-concentration for the test. pre Theoretically,-concentration iron test. in Th sphaleriteeoretically, can iron be accompanied in sphalerite bycan zinc, be accompanied and when the by iron zinc content, and when is higher the thaniron 10content wt %, is it hashigher an opaquethan 10 blackwt % appearance, it has an opaque and is calledblack marmatite.appearance The and high is called iron content marmatite. (15.3 wtThe %) h ofigh the iron feed content sample (15.3 (Table wt1) can %) be of attributed the feed sample to the high(Table iron 1) contentcan be of attributed marmatite toor the the presencehigh iron of othercontent iron-bearing of marmatite minerals. or the The presence concentrations of other of theiron target-bearing elements—zinc, minerals. The lead, concentrations and copper—gradually of the target elements increased— aszinc, the lead, particle and size copper decreased.—gradually As a result,increased the totalas the concentration particle size of decreased. target elements As a result, in particles the total with concentration sizes below 74 ofµ mtarget was higherelements than in thatparticles in the with entire sizes feed below sample, 74 μ probablym was higher due to than the breakagethat in the property entire feed of metallic sample, minerals.probably Normally,due to the purebreakage metallic property minerals of metallic tend to beminerals. brittle, and Normally, ﬁne particles pure metallic are easily mineral produceds tend when to be they brittle, break. and In ourfine study,particle breakages are easily of the produced minerals when accounted they break for the. In high our concentration study, breakage of target of the elements minerals in accounted ﬁne particles for withthe high sizes concentration below 74 µm. of target elements in fine particles with sizes below 74 μm.

FigureFigure 2.2. X-rayX-ray diffractiondiffraction (XRD)(XRD) patternpattern ofof thethe feedfeed sample.sample.

TableTable 1. 1.Yield Yield (wt (wt %) % and) and concentration concentration (wt %)(wt of % the) of target the target elements elements in the feed in the sample feed as sample a function as a offunction size fractions. of size fractions.

Yield Concentration (wt %) Size FractionYield Concentration (wt %) Size Fraction (wt %) Fe Zn Pb Cu (wt %) Fe Zn Pb Cu >212 μm 12.5 14.6 2.90 0.05 0.06 >212 µm 12.5 14.6 2.90 0.05 0.06 106–212 μm 16.0 15.0 3.62 0.06 0.07 106–212 µm 16.0 15.0 3.62 0.06 0.07 74–10674µ–m106 μm 21.621.6 15.615.6 4.92 4.92 0.06 0.06 0.08 0.08 37–74 37µm–74 μm 13.113.1 16.116.1 5.69 5.69 0.10 0.10 0.10 0.10 <37 µm<37 μm 36.836.8 15.315.3 5.79 5.79 0.13 0.13 0.11 0.11 TotalTotal 100.0100.0 15.315.3 4.88 4.88 0.09 0.09 0.09 0.09

3.2.3.2. MicroscopeMicroscope AnalysisAnalysis ofof FeedFeed SampleSample ToTo determinedetermine the the presence presence of of not not only only major major target target minerals, minerals but, but also also trace trace minerals minerals that that were were not detectednot detected by XRD by XRDanalysis, analysis, the feed the sample feed sample was analyzed was analyzed by microscopy. by microscopy. Table2 shows Table the 2 shows result the of traditionalresult of traditional modal analysis modal ofanalysis 10 thin of sections 10 thin ofsections the feed of sample.the feed Modalsample. analysis Modal analysis is normally is normally used in geologyused in geology to estimate to estimat the relativee the proportionsrelative proportions of different of different minerals mineral from microscopes from microscop images,e andimage thes, proportionand the proportion of each mineral of each ismineral defined is asdefined the ratio as ofthe area ratio occupied of area occupied by a particular by a particular mineralto mineral the area to occupiedthe area occupied by all minerals. by all minerals. For our feedFor our sample, feed sphalerite sample, sphalerite was the mostwas the abundant most abundant target mineral, target andmineral, its area and proportion its area proportion was approximately was approximate 5–20%ly of 5– the20% total of the area total in variousarea in various thin sections. thin section Smalls. amountsSmall amounts (≈1%) of(≈ chalcopyrite1%) of chalcopyrite and galena and were galena identified were as identified the trace target as the minerals. trace target Pyrrhotite minerals. and pyritePyrrhotite were and identified pyrite aswere the identified iron-bearing as minerals,the iron-bearing which accounted minerals, forwhich the high accounted iron content for the in high the feediron sample.content in the feed sample.

Minerals 2018, 8, x FOR PEER REVIEW 5 of 13 Minerals 2018, 8, 272 5 of 13 Table 2. Area proportion (%) of each mineral identified by modal analysis of microscope images of the feed sample. Table 2. Area proportion (%) of each mineral identiﬁed by modal analysis of microscope images of the feed sample.Mineral 1 2 3 4 5 6 7 8 9 10 Sphalerite 14 15 17 7 20 - 18 20 5 8 MineralChalcopyrite 1 <0.5 2 1 31 41 1 5 1 61 7<0.5 8<0.5 9<0.5 10 SphaleriteGalena 14 <0.5 15 - 17- 7- <0.5 20 - -- 18- 20- 51 8 ChalcopyritePyrrhotite<0.5 <0.5 1 16 118 18 3 1 9 116 1<0.5 <0.5 3 <0.5<0.5 <0.5 GalenaPyrite <0.5 <0.5 - 1 -3 -2 <0.5 <0.5 <0.5 - 2 -2 -<0.5 -30 1 PyrrhotiteMarcasite <0.5 - 161 181 81 - 3 92 16- <0.5- 3- <0.5 Pyrite <0.5 1 3 2 <0.5 <0.5 2 2 <0.5 30 Magnetite - - - 1 ------Marcasite - 1 1 1 - 2 - - - MagnetitePb-Bi-S -- -<0.5 -<0.5 1- - - -<0.5 ------Pb-Bi-SIlmenite- <0.5 <0.5 - <0.5------<0.5------IlmenitePyroxenes <0.5 1 -56 -46 -58 37 - 61 - 22 -62 -88 -34 - PyroxenesGarnet 1- 56- 46- 58- 22 37 -61 13 22- 62- 882 34 Garnet - - - - 22 - 13 - - 2 AmphibolesAmphiboles 7 7 -- 22 33 <0.5 <0.5 12 12 1 1- -<0.5 <0.52 2 CalciteCalcite32 32 <0.5 <0.5 55 1111 8 8 6 69 9<0.5 <0.5 - -10 10 EpidoteEpidote ------3 312 121 1- - QuartzQuartz 36 36 -- -- 22 1 1 <0.5 <0.5 <0.5 <0.5 - -- -2 2 Biotite 7 5 4 4 4 4 4 <0.5 - 3 Biotite 7 5 4 4 4 4 4 <0.5 - 3 Fluorite ------5 - - - Fluorite ------5 - - -

FigureFigure3 3 shows shows images images of of various various ore ore textures textures in in the the feed feed sample. sample. SinceSince thethe mainmain concernconcern of of this this studystudy isis sulfidesulfide minerals,minerals, wewe mostlymostly identifiedidentified thethe typetype ofof thesethese mineralsminerals andand notednoted theirtheir physicalphysical appearance.appearance. C Chalcopyritehalcopyrite and and pyrr pyrrhotitehotite were were found within sphalerite, and the gangue mineral, pyroxene,pyroxene, was included included with with the the host host sphalerite sphalerite (Figure (Figure 3a3,a,b).b). The The particle particle size sizess of chalcopyrite of chalcopyrite and andgalena galena inside inside sphalerite sphalerite were were in the in approximate the approximate range range of 10 of–20 10–20 μm, µwhichm, which correspond correspond to very to very fine fineparticle particles,s, and andit was it was difficult difficult to toliberate liberate them them from from the the host host sphalerite sphalerite by by crushing and grindinggrinding (Figure(Figure3 3cc,d).,d). In In particular, particular, some some sphalerite sphalerite was was surrounded surrounded by by pyrite, pyrite as, as shown shown in in Figure Figure3d. 3d. ToTo analyze analyze the the elemental elemental composition composition of sphalerite, of sphalerite, SEM–EDS SEM–EDS analysis analysis was conducted; was conducted the results; the areresults shown are inshown Figure in4 Figureand Table 4 and3. TheTable zinc 3. andThe zinc iron and concentrations iron concentrations of sphalerite of sphalerite were about were 43 about and 1143 wtand %, 11respectively. wt %, respectively. This means This that evenmeans a region that even that appears a region relatively that appears pure in relatively a microscope pure image in a canmicroscop containe aimage considerable can contain amount a considerable of iron, and amount if sphalerite of iron could, and be if successfully sphalerite could recovered be successfully by various separationrecovered by methods, various the separation zinc concentrate methods, could the zincexhibit concentrat a high irone could content. exhibit Therefore, a high the iron high content iron. contentTherefore, of thethe feed high sample iron content (Table1 ) of can the be feed explained sample by (Tab thele iron-rich 1) can sphalerite be explained and by the the presence iron-rich of iron-bearingsphalerite and minerals the presence such as of pyrrhotiteiron-bearing and minerals pyrite. such as pyrrhotite and pyrite.

(a) (b)

Figure 3. Cont.

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(c) (d) (c) (d) Figure 3. Images of ore texture in various thin sections of the feed sample: (a) 400×; (b) 200×; (c) 50×; Figure 3.3. ImagesImages ofof oreore texturetexture inin variousvarious thin thin sections sections of of the the feed feed sample: sample (:a ()a 400) 400×;×;( b(b)) 200 200×;×;( (cc)) 5050×;×; (d) 100×. (Abbreviations:((dd)) 100100×.×. ( (Abbreviations:Abbreviations: Cu, chalcopyrite Cu Cu,, chalcopyrite chalcopyrite;; Zn; Zn Zn,, ,sphalerite sphalerite sphalerite;; Pho Pho,; Pho, pyrrhotite pyrrhotite;, pyrrhotite; Pb Pb,, galena galena;; Pb;, Bi Bi,galena, bismuth bismuth;; Bi; g g,,, bismuth; g, pyroxene; Pypyroxene;pyroxene, pyrite); Py,Py. , pyrite).pyrite).

1 1 2 2

Figure 4. Surface image of sphalerite obtained by scanning electron microscopy (SEM) and locations to be analyzed by energy dispersive X-ray spectroscopy (EDS).

Table 3. Energy dispersive X-ray spectroscopy (EDS) analysis results of the locations shown in Figure 4. Surface image of sphalerite obtained by scanning electron microscopy (SEM) and locations to Figure 4. SurfaceFigure 4image. of sphalerite obtained by scanning electron microscopy (SEM) and locations be analyzed by energy dispersive X-ray spectroscopy (EDS). to be analyzed by energy dispersive X-rayLocation spectroscopy (EDS). 1 2 Table 3. Energy dispersive X-ray spectroscopy (EDS) analysisS results 46.15 of the 45.37 locations shown in Figure4. Table 3. Energy dispersive Elemental X-ray spectroscopy composition ( wt(EDS) %) analysisFe 10.97 result10.99s of the locations shown in LocationZn 42.88 1 43.64 2 Figure 4. S 46.15 45.37 3.3. Using Gravity SElementaleparation compositionfor Pre-Concentration (wt %) Fe 10.97 10.99 Location Zn 42.881 43.642 To pre-concentrate valuable metallic minerals before the flotation process, various separation S 46.15 45.37 3.3.methods Using were Gravity applied Separation to the for feed Pre-Concentration sample. Since the density of a metallic mineral in this sample (sphalerite: 3.9–4.2Elemental g/cm3; galena: composition 7.2–7.6 g/cm (wt3; chalcopyrite: %) Fe 4.110.97–4.3 g/cm10.993; and pyrrhotite: 4.6 g/cmTo3) was pre-concentrate high when compared valuable to metallic that of mineralsa non-metallic before mineral, theZn ﬂotation 42.88density process, or43.64 gravity various separation separation was methodsexpected wereto yield applied good toseparation the feed result sample.s. To Since confirm the the density feasibility of a metallic of using mineral gravity inseparation this sample for 3 3 3 (sphalerite:pre-concentration, 3.9–4.2 a g/cm gravity; galena:-separation 7.2–7.6 test g/cm was conducted; chalcopyrite: using 4.1–4.3 a shaking g/cm table.; and Contrary pyrrhotite: to 3 3.3. Using Gravity4.6expectation, g/cm Separation) was both high the when forgrade P comparedre and-C oncentrationrecovery to that of ofthe a concentrate non-metallic were mineral, poor densityeven for or the gravity best separationseparation wasperformance expected shownto yield i goodn Table separation 4 for the results. test conducted To conﬁrm under the feasibility optimal of operation using gravity conditions separation. The To prefor-recoverconcentrate pre-concentration,y of every valuable target a element gravity-separation metallic mainly mineralsoccurred test was in thebefore conducted tailing the and using flotationslime a shakingfractions process, table. owing Contrary to various the high to separation methods wereexpectation,concentration applied both ofto target the the grade elementsfeed and sample. recovery among oftheSince the particles concentrate the withdensity were small poor of size, a even asmetallic shown for the inmineral best Table separation 1. Asin a this sample (sphalerite: result, 3.9–4.2 even g/cm though3; a galena: metallic mineral 7.2–7.6 had g/cm a high3; density chalcopyrite: when compared 4.1–4.3 to nong/cm-metallic3; and gangue pyrrhotite: 4.6 g/cm3) was high when compared to that of a non-metallic mineral, density or gravity separation was expected to yield good separation results. To confirm the feasibility of using gravity separation for pre-concentration, a gravity-separation test was conducted using a shaking table. Contrary to expectation, both the grade and recovery of the concentrate were poor even for the best separation performance shown in Table 4 for the test conducted under optimal operation conditions. The recovery of every target element mainly occurred in the tailing and slime fractions owing to the high concentration of target elements among the particles with small size, as shown in Table 1. As a result, even though a metallic mineral had a high density when compared to non-metallic gangue

Minerals 2018, 8, 272 7 of 13 performance shown in Table4 for the test conducted under optimal operation conditions. The recovery of every target element mainly occurred in the tailing and slime fractions owing to the high concentration of target elements among the particles with small size, as shown in Table1. As a result, even though a metallic mineral had a high density when compared to non-metallic gangue minerals, it was not easy to apply a gravity separation method, such as a shaking table, for pre-concentration of metallic minerals in the feed sample. In addition, because pyroxene-group minerals, which have a high density, were present as non-metallic gangue minerals in this sample, the grade of the concentrate was not greatly improved.

Table 4. Results of gravity separation using a shaking table. (Angle of shaking table: 2.5◦; amplitude: 1.5 mm; water-ﬂow rate: 10 L/min; frequency: 300 rpm).

Fe Zn Pb Cu Yield (wt %) 1 G (%) 2 Re (%) G (%) Re (%) G (%) Re (%) G (%) Re (%) Concentrate 4.31 29.3 5.8 17.8 10.2 0.82 18.1 0.39 10.5 Middling 10.74 21.9 10.8 10.2 14.6 0.11 5.9 0.20 13.4 Tailing 65.61 14.3 42.9 3.2 28.0 0.06 20.1 0.07 27.0 Slime 19.35 13.5 40.5 5.4 47.2 0.17 55.9 0.12 49.1 1 G: Grade; 2 Re: Recovery.

3.4. Using Magnetic Separation for Pre-Concentration Next, the feasibility of using magnetic separation for pre-concentration of metallic minerals was investigated. Various minerals in the feed sample, such as pyrrhotite, iron-rich sphalerite, and gangue minerals, were expected to show different responses to the magnetic-separation process. If iron-rich sphalerite was separated from other minerals, the effectiveness of pre-concentration would be manifested as a significant mass reduction for the next stage. As mentioned in Section 2.2, the threshold particle size of the feed sample was adjusted on the basis of the maximum size, and only the coarse fraction (consisting of particles larger than 74 µm) was fed to the dry magnetic separation process. Owing to the low efficiency of separating the remaining fine particles with sizes below 74 µm in the dry magnetic-separation, direct migration of these fine particles to the next process was considered. Figure5 shows the grade of target elements of each magnetic product as a function of the magnetic intensity for particles of various sizes. Naturally, the iron content was the highest in the products extracted at 0.2 T from particles of all sizes, and the iron content did not change much (≈10%) in the products extracted at other magnetic intensities. The iron content of 0.2 T magnetic extracts increased as the maximum size of feed-sample particles decreased owing to the increased degree of liberation. These 0.2 T magnetic extracts mainly consisted of the iron-bearing ferromagnetic mineral, pyrrhotite, and some unliberated minerals, which accounted for the low iron content when compared to the theoretical iron content of pyrrhotite. High concentrations of zinc were expected in the 0.4–0.6 T magnetic products owing to the magnetic property of iron-rich sphalerite. However, high concentrations of zinc were also detected in products extracted at 0.85 T and higher, especially those extracted from feed-sample particles measuring 0.3 mm. This was probably due to the relatively pure sphalerite, which did not contain iron. As a result, the zinc content was high in products extracted at magnetic intensities below 0.6 T and above 0.85 T, and the zinc content was very low (≈1.5%) in magnetic products extracted at magnetic intensities in the range of 0.6–0.85 T. The grade of lead was slightly low in the products extracted at <0.6 T, and the variations in both copper and lead content showed trends similar to that of the zinc content, as shown in Figure5. Minerals 2018, 8, x FOR PEER REVIEW 8 of 13

As can be seen in Figure 3, fine chalcopyrite and galena particles exhibited behavior similar to that of sphalerite because of insufficient liberation during the crushing stage. However, the magnetic susceptibility of chalcopyrite and galena are also one of the factors that determined the outcome of this separation test. According to the study of Gaudin and Spedden, galena is classified as sulfide minerals with low magnetic susceptibility, and it is not sufficiently recovered even at high magnetic intensity [22]. Similar separation trends were observed for galena in this study. As shown in Figure 5, unlike zinc and copper, the grade of lead was only high at high magnetic intensity. According to a study by Jirestig, sphalerite and chalcopyrite were recovered at similar magnetic intensity from a copper–lead flotation concentrate [24]. This is one of the reasons why the grade of zinc and copper are Mineralssimilar2018 in this, 8, 272 study as a function of magnetic intensity. 8 of 13

FigureFigure 5.5. GradeGrade of of targettarget elementselements inin magnetic magnetic products products that that were were extracted extracted sequentially sequentially at at low low magneticmagnetic intensity intensity and and maximum maximum feed-sample feed-sample particle particle size. size.

TheAccording magnetic to separation the results results shown of in this Figure study 5 are, the in magnetic good agreement product withs could those be of divided previous into studies three onranges the magnetic: 0.6 T magneticsusceptibility product of sulfides, 0.6 minerals T non- [magnetic/22–24]. Gaudin0.85 T and magnetic Spedden product found thats, and sphalerite 0.85 T (marmatite)non-magnetic showed product as; wide the rangeaverage of grade magnetic of each susceptibility element in asthese a function products of is the shown iron in content Figure [ 226. ],It awas trend confirmed that was alsothat observedthe grade by of Keysall target et al. [elements23]. The varyingin the medium grade of-magnetic elements- recoveredintensity range at different (0.6 T magneticnon-magneti intensities,c/0.85 T represented magnetic product by thes results) was reduced, shown in whereas Figure5 , the was grade the result of other of different products iron was concentrationsenhanced when in compared the sphalerite; with these the grade results of confirmed these elements the finding in the offeed previous sample studies. regardless The of iron-rich particle sphaleritesize. Therefore, was recovered if the products at a magnetic extracted intensity at medium of around magnetic 0.4 T, and intensity relatively with pure low sphalerite grade were was recoveredseparated, at they high would magnetic demonstrate intensity the above effect 1.2 of T. pre-concentration by dry magnetic separation. AsThe can next be seen consideration in Figure3 , for fine magnetic chalcopyrite separation and galena was particlesthe particle exhibited size of behavior the feed similar sample. to thatAlthough of sphalerite there were because no significant of insufficient difference liberations in the during grade thes of crushing target elements stage. However, with variations the magnetic in the susceptibilitymaximum particle of chalcopyrite size, the recovery and galena and are the also yield one varied of the factorsgreatly thatdepending determined on the the particle outcome size. of thisMoreover, separation since test. the According dry magnetic to the- studyseparation of Gaudin process and is Spedden, not designed galena is to classified produce asthe sulfide final mineralsconcentrate, with the low recovery magnetic and susceptibility, the yield should and be it is considered not sufficiently in order recovered to ensure even the atoverall high magneticeconomic intensity [22]. Similar separation trends were observed for galena in this study. As shown in Figure5, unlike zinc and copper, the grade of lead was only high at high magnetic intensity. According to a study by Jirestig, sphalerite and chalcopyrite were recovered at similar magnetic intensity from a copper–lead flotation concentrate [24]. This is one of the reasons why the grade of zinc and copper are similar in this study as a function of magnetic intensity. According to the results shown in Figure5, the magnetic products could be divided into three ranges: 0.6 T magnetic products, 0.6 T non-magnetic/0.85 T magnetic products, and 0.85 T non-magnetic products; the average grade of each element in these products is shown in Figure6. It was confirmed that the grade of all target elements in the medium-magnetic-intensity range (0.6 T Minerals 2018, 8, 272 9 of 13

non-magnetic/0.85 T magnetic products) was reduced, whereas the grade of other products was enhanced when compared with the grade of these elements in the feed sample regardless of particle size. Therefore, if the products extracted at medium magnetic intensity with low grade were separated, they would demonstrate the effect of pre-concentration by dry magnetic separation. Minerals 2018The, 8 next, x FOR consideration PEER REVIEW for magnetic separation was the particle size of the feed sample.9 of 13 Although there were no significant differences in the grades of target elements with variations in efficiency.the maximum It is obvious particle size, that the a recoveryhigh mass and fraction the yield and varied a low greatly loss depending ratio of ontarget the particle elements size. to be removedMoreover, from since the the sample dry magnetic-separation are more advantageous process for is not the designed overall toprocess. produce Figure the final 7 shows concentrate, the mass fractionthe recovery of the medium and the yield-magnetic should-intensity be considered product in orderand the to ensureloss ratio the of overall target economic elements efficiency. as functions of Itthe is obviousmaximum that feed a high-sample mass fractionparticle and size. a lowThe lossmass ratio fraction of target of elements the medium to be-magnetic removed- fromintensity the sample are more advantageous for the overall process. Figure7 shows the mass fraction of the products was 28.3% when the maximum particle size was 0.3 mm, and it increased to about 45–48% medium-magnetic-intensity product and the loss ratio of target elements as functions of the maximum as the particle size increased to 1 mm or higher. As the particle size increased, the loss ratio of target feed-sample particle size. The mass fraction of the medium-magnetic-intensity products was 28.3% elementswhen the increase maximumd with particle increase size wass in 0.3 mass mm, fraction and it increased of the medium to about- 45–48%magnetic as- theintensity particle product size s. Whenincreased the maximum to 1 mm or particle higher. Assize the reached particle and size increased,exceeded the1.4 lossmm, ratio the of loss target ratio elements of target increased elements, especiallywith increases that of in lead mass, was fraction high. of Therefore, the medium-magnetic-intensity based on the mass fraction products. and When loss theratio maximum of the target elements,particle sizeit can reached be concluded and exceeded that 1.4 the mm, separation the loss ratioeffici ofency target wa elements,s the highest especially when that the of maximum lead, particlewas high. size Therefore,was 1 mm. based on the mass fraction and loss ratio of the target elements, it can be concluded that the separation efficiency was the highest when the maximum particle size was 1 mm.

FigureFigure 6. 6.GradeGrade of of elements elements as as a a function function of thethe magneticmagnetic intensity intensity during during the the separation separation process. process. (Abbreviations:(Abbreviations: mag., mag., magnetic; magnetic; non non-mag.,-mag., non-magnetic).non-magnetic).

Minerals 2018, 8, 272 10 of 13 Minerals 2018, 8, x FOR PEER REVIEW 10 of 13

FigureFigure 7. Mass 7. Mass fraction fraction (wt (wt %) %) of of products products extracted at at mediummedium magnetic magnetic intensity intensity (0.6 ( T0.6 T non-non-magnetic/0.85magnetic/0.85 T magnet T magneticic products products)) and and loss loss ratio ofof target target elements elements as functionsas function ofs maximum of maximum size sizeof feed of feed-sample-sample particles. particles.

3.5. Pre3.5.-C Pre-Concentrationoncentration Process Process of ofSphalerite Sphalerite Using the above separation results, a pre-concentration process of Pb–Zn deposits was developed; Using the above separation results, a pre-concentration process of Pb–Zn deposits was a flowchart of the process is shown in Figure8. First, the sample is to be crushed using a jaw crusher developedand a cone; a flowchart crusher, and of thenthe process screened is to shown limit the in maximum Figure 8. particle First, the size sample to 1 mm. is Theto be portion crushed of fine using a jaw crusherparticles and with a sizes cone below crusher 74 µ, mand should then bescreened transferred to limit directly the to maximum the flotation particle process siz owinge to 1 to mm. the The portionlow separation of fine particles efficiency with of these sizes particles below in 7 the4 μ drym magnetic-separationshould be transferred process. directly The coarse to the portion flotation processof particles owing withto the sizes low above separation 74 µm should efficiency be fed of to these the dry particles magnetic-separation in the dry magnetic process, and-separation the process.paramagnetic The coarse products portion (0.85 T of magnetic particles products) with are sizes to be above removed 74 to μ reducem should the mass befraction fed to fed the to dry magneticthe grinding-separation process. process, The recovered and the ferromagnetic paramagnetic and product diamagnetics (0.85 products T magnetic are to be product ground ands) are fed to be removedto the to flotation reduce process the mass with fraction the portion fed of to fine the particles. grinding The process. pre-concentration The recovered test results ferromagneti are shownc and diamagneticin Table5 . product It can bes seen are thatto be about ground 48.5% and of the fed feed to samplethe flotation was removed process in the with pre-concentration the portion of fine particlesstage,. The and 73–82%pre-concentration of the recovered test target result elementss are shown were movedin Table to the5. It next can stage, be seen i.e., flotation, that about for final48.5% of concentration. Consequently, the grade of the target elements was enhanced from 4.88% to 8.1% for the feed sample was removed in the pre-concentration stage, and 73–82% of the recovered target zinc, from 0.09% to 0.14% for lead, and from 0.09% to 0.12% for copper in the pre-concentration stage. elements were moved to the next stage, i.e., flotation, for final concentration. Consequently, the grade of the target elementsTable wa 5. Resultss enhanced of pre-concentration from 4.88% processto 8.1% of for target zinc elements., from 0.09% to 0.14% for lead, and from 0.09% to 0.12% for copper in the pre-concentration stage. Yield Fe Zn Pb Cu (wt %) 1 G (%) 2 Re (%) G (%) Re (%) G (%) Re (%) G (%) Re (%) Ferromagnetic product 23.1 27.5 36.3 8.93 42.0 0.12 31.0 0.15 42.3 Paramagnetic product 48.5 14.3 39.6 1.55 15.3 0.04 21.1 0.04 22.5 Diamagnetic product 15.2 12.7 11.1 7.3 22.7 0.16 27.3 0.10 17.5 Fine particle (<74 µm) 13.2 17.2 13.0 7.45 20.0 0.14 20.6 0.11 17.7 1 G: Grade; 2 Re: Recovery.

Minerals 2018, 8, 272 11 of 13 Minerals 2018, 8, x FOR PEER REVIEW 11 of 13

FigureFigure 8. 8.FlowchartFlowchart of of pre pre-concentration-concentration process usingusingmagnetic magnetic separation. separation.

4. Conclusions Table 5. Results of pre-concentration process of target elements. Yield Fe Zn Pb Cu This paper describes the pre-concentration of Pb–Zn deposits to reduce the mass of the load for subsequent processes, such(wt as %) grinding 1 G (%) and 2 flotation.Re (%) G The (%) conclusions Re (%) G of (%) the studyRe (%) are G as (%) follows: Re (%) Ferromagnetic product 23.1 27.5 36.3 8.93 42.0 0.12 31.0 0.15 42.3 1. Sphalerite was the major sulfide mineral in the Pb–Zn deposits obtained from the Gagok mine, Paramagnetic product 48.5 14.3 39.6 1.55 15.3 0.04 21.1 0.04 22.5 and galena and chalcopyrite were not detected by XRD analysis owing to their low content. Diamagnetic product 15.2 12.7 11.1 7.3 22.7 0.16 27.3 0.10 17.5 The grades of zinc, lead, copper, and iron were 4.88%, 0.09%, 0.09%, and 15.3%, respectively. Fine particle (<74 μm) 13.2 17.2 13.0 7.45 20.0 0.14 20.6 0.11 17.7 The high content of iron in the feed sample can be attributed to the iron-rich sphalerite and iron- 1 2 bearing minerals, such as pyrrhotite. G: Grade; Re: Recovery. 2. Because target minerals (sphalerite, chalcopyrite, and galena) were concentrated by crushing 4. Conclusions in the portion of fine particles with sizes below 74 µm, the gravity separation method using a Thisshaking paper tabledescribes was not the effective pre-concentration for this sample. of Pb–Zn deposits to reduce the mass of the load for subsequent3. The dryprocess magnetices, such separation as grinding experiment and flotation. confirmed The that conclusions concentrations of the of study target are elements as follows: were low in the products separated at intermediate magnetic intensity (0.6–0.85 T magnetic products), 1. Sphalerite was the major sulfide mineral in the Pb–Zn deposits obtained from the Gagok mine, and the maximum particle size of 1 mm was appropriate from the viewpoint of mass reduction and galena and chalcopyrite were not detected by XRD analysis owing to their low content. The and loss ratio of target elements. grades of zinc, lead, copper, and iron were 4.88%, 0.09%, 0.09%, and 15.3%, respectively. The 4. The pre-concentration process was developed using the results of magnetic separation. After the high content of iron in the feed sample can be attributed to the iron-rich sphalerite and iron- crushing stage, to limit the maximum particle size to 1 mm, the portion of coarse particles bearingwith sizesminerals above, such 74 µ mas waspyrrh fedotite. to the magnetic separation process to remove low-grade target 2. Becauseelements target from minerals the products (sphalerite, extracted chalcopyrite, at intermediate and galena) magnetic were intensity. concentrated About by 48.5% crushing of the in thefeed portion sample of wasfine removedparticles in with the pre-concentration sizes below 74 μ stage,m, the and gravity the grades separation of the target method elements using a shakingwere enhanced: table was 65%not effective for zinc, 55% for this for lead, sample. and 33% for copper. Consequently, it is expected that 3. Thethe dry economic magnetic efficiency separation of the experiment entire process confirmed will be that significantly concentrations improved of target because elements the cost were of lowgrinding in the and products flotation separated can be greatly at reducedintermediate by removing magnetic the low-grade intensity (fraction0.6–0.85 of T separated magnetic products), and the maximum particle size of 1 mm was appropriate from the viewpoint of mass reduction and loss ratio of target elements. 4. The pre-concentration process was developed using the results of magnetic separation. After the crushing stage, to limit the maximum particle size to 1 mm, the portion of coarse particles

Minerals 2018, 8, 272 12 of 13

products in the pre-concentration stage. However, it is difficult to estimate the process costs from a simple flowchart. Generally speaking, the grinding and flotation process account for more than 50% of the total process cost. Therefore, cost saving of 20% or higher can be expected for the entire process by adopting the pre-concentration process.

Author Contributions: S.J. designed and performed the experiment and wrote the paper, and K.K. analyzed the date and wrote the paper. Funding: This research was supported by the Convergence Research Project (CRC-15-06-KIGAM) funded by the National Research Council of Science and Technology (NST) Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Fuerstenau, M.C.; Flotation, A.M. Gaudin Memorial Volume 2; American Institute of Mining, Metallurgical and Petroleum Engineers: New York, NY, USA, 1976; pp. 1029–1231. 2. Gómez, F.L.; Sierra, J.R.; Carcedo, F.G.; Garcia, F.M.; Garcia, J.M. Influence of ultrafine grain size in differential flotation of galena and sphalerite. Int. J. Miner. Process. 1986, 17, 303–316. [CrossRef] 3. Sehlotho, N.; Sindane, Z.; Bryson, M.; Lindvelt, L. Flowsheet development for selective Cu-Pb-Zn recovery at Rosh Pinah concentrator. Miner. Eng. 2018, 122, 10–16. [CrossRef] 4. Forssberg, K.E.; Subrahmanyam, T.; Nilsson, L.K. Influence of grinding method on complex sulphide ore flotation: A pilot plant study. Int. J. Miner. Process. 1993, 38, 157–175. [CrossRef] 5. Mehrabani, J.; Noaparast, M.; Mousavi, S.; Dehghan, R.; Ghorbani, A. Process optimization and modelling of sphalerite flotation from a low-grade Zn–Pb ore using response surface methodology. Sep. Purif. Technol. 2010, 72, 242–249. [CrossRef] 6. Pérez-Garibay, R.; Ramírez-Aguilera, N.; Bouchard, J.; Rubio, J. Froth flotation of sphalerite: Collector concentration, gas dispersion and particle size effects. Miner. Eng. 2014, 57, 72–78. [CrossRef] 7. Fosu, S.; Skinner, W.; Zanin, M. Detachment of coarse composite sphalerite particles from bubbles in flotation: Influence of xanthate collector type and concentration. Miner. Eng. 2015, 71, 73–84. [CrossRef] 8. Wang, J.; Xie, L.; Liu, Q.; Zeng, H. Effects of salinity on xanthate adsorption on sphalerite and bubble–sphalerite interactions. Miner. Eng. 2015, 77, 34–41. [CrossRef] 9. Ejtemaei, M.; Nguyen, A.V. A comparative study of the attachment of air bubbles onto sphalerite and pyrite surfaces activated by copper sulphate. Miner. Eng. 2017, 109, 14–20. [CrossRef] 10. Finkelstein, N. The activation of sulphide minerals for flotation: A review. Int. J. Miner. Process. 1997, 52, 81–120. [CrossRef] 11. Laskowski, J.; Liu, Q.; Zhan, Y. Sphalerite activation: Flotation and electrokinetic studies. Miner. Eng. 1997, 10, 787–802. [CrossRef] 12. Rao, S.; Nesset, J.; Finch, J. Activation of sphalerite by Cu ions produced by cyanide action on chalcopyrite. Miner. Eng. 2011, 24, 1025–1027. [CrossRef] 13. Albrecht, T.; Addai-Mensah, J.; Fornasiero, D. Critical copper concentration in sphalerite flotation: Effect of temperature and collector. Int. J. Miner. Process. 2016, 146, 15–22. [CrossRef] 14. Yang, B.; Tong, X.; Lan, Z.; Cui, Y.; Xie, X. Influence of the interaction between sphalerite and pyrite on the copper activation of sphalerite. Minerals 2018, 8, 16. [CrossRef] 15. Dávila-Pulido, G.; Uribe-Salas, A.; Espinosa-Gómez, R. Comparison of the depressant action of sulfite and metabisulfite for Cu-activated sphalerite. Int. J. Miner. Process. 2011, 101, 71–74. [CrossRef] 16. Huang, P.; Cao, M.; Liu, Q. Selective depression of sphalerite by chitosan in differential Pb–Zn flotation. Int. J. Miner. Process. 2013, 122, 29–35. [CrossRef]

17. Liu, J.; Wang, Y.; Luo, D.; Zeng, Y. Use of ZnSO4 and SDD mixture as sphalerite depressant in copper ﬂotation. Miner. Eng. 2018, 121, 31–38. [CrossRef] 18. Solecki, J.; Komosa, A.; Szczypa, J. Copper ion activation of synthetic sphalerites with various iron contents. Int. J. Miner. Process. 1979, 6, 221–228. [CrossRef] 19. Buckley, A.; Woods, R.; Wouterlood, H. An XPS investigation of the surface of natural sphalerites under ﬂotation-related conditions. Int. J. Miner. Process. 1989, 26, 29–49. [CrossRef] Minerals 2018, 8, 272 13 of 13

20. Boulton, A.; Fornasiero, D.; Ralston, J. Effect of iron content in sphalerite on flotation. Miner. Eng. 2005, 18, 1120–1122. [CrossRef] 21. Tong, X.; Song, S.; He, J.; Rao, F.; Lopez-Valdivieso, A. Activation of high-iron marmatite in froth flotation by ammoniacal copper (ii) solution. Miner. Eng. 2007, 20, 259–263. [CrossRef] 22. Gaudin, A.; Spedden, H. Magnetic separation of sulphide minerals. Trans. Am. Inst. Min. Metall. Eng. 1943, 153, 563–575. 23. Keys, J.; Horwood, J.; Baleshta, T.; Cabri, L.; Harris, D. Iron-iron interaction in iron-containing zinc sulphide. Can. Miner. 1968, 9, 453–467. 24. Jirestig, J.; Forssberg, E. Magnetic characterization of sulphide ores: Examples from Sweden. Phys. Sep. Sci. Eng. 1992, 4, 31–45. [CrossRef] 25. Pearce, C.I.; Pattrick, R.A.; Vaughan, D.J. Electrical and magnetic properties of sulfides. Rev. Miner. Geochem. 2006, 61, 127–180. [CrossRef] 26. Yun, S.; Einaudi, M.T. Zinc-lead skarns of the Yeonhwa-Ulchin district, South Korea. Econ. Geol. 1982, 77, 1013–1032. [CrossRef]

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