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Home , Gold extraction, Gossan, Lixiviant, Roasting (metallurgy), Saprolite

Article Effective Recovery from Near-Surface Oxide Zone Using Reductive Microwave and Magnetic Separation

Bong-Ju Kim 1, Kang Hee Cho 2, Sang-Gil Lee 1, Cheon-Young Park 3, Nag-Choul Choi 2,* and Soonjae Lee 1,* 1 Department of Earth and Environmental Sciences, Korea University, Anam-ro 145, Sungbuk-gu, Seoul 02841, Korea; [email protected] (B.-J.K.); [email protected] (S.-G.L.) 2 Department of Rural Systems Engineering/Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea; [email protected] 3 Department of Energy and Resource Engineering, Chosun University, Gwangju 61452, Korea; [email protected] * Correspondence: [email protected] (N.-C.C.); [email protected] (S.L.); Tel.: +82-62-230-7878 (N.-C.C.); +82-2-3290-3177 (S.L.)  Received: 23 October 2018; Accepted: 15 November 2018; Published: 16 November 2018 

Abstract: High content of gold in near-surface oxide zones above the gold deposit could be recovered using cyanidation. However, restricting the use of in mines has made it difficult to recover gold within the oxide zone. In this study, we investigated an application of the reductive microwave roasting and magnetic separation (RMR-MS) process for the effective gold recovery from in a near-surface oxide zone. Ore samples obtained from the near-surface oxide zone in Moisan Gold Mine (Haenam, South Korea) were used in RMR-MS tests for the recovery of and gold. The effect of the RMR process on the recovery of iron and gold was evaluated by given various conditions of the microwave irradiation as well as the dosages of reductant and additive. The microwave roasting resulted in a chemical reduction of non-magnetic iron oxide () to minerals, such as magnetite and maghemite. This phase change could induce the effective separation of iron minerals from the minerals by magnetic separation process. The increased iron recovery was directly proportional to the gold recovery due to the coexistence of gold with iron minerals. The RMR-MS process could be a promising method for gold recovery from the ores in near-surface oxide zones.

Keywords: gold recovery; oxide zone; reductive microwave roasting; magnetic separation; Fe-oxide; mineral phase change

1. Introduction Gossans (or iron cap) are highly oxidized and weathered , usually associated with the upper and exposed parts of an ore deposit or mineralized vein, and so it was an important guide to find the buried gold ore deposits in the 19th and 20th centuries. Gold is captured during the formation of Fe-oxide, either internally or where concentric structures are produced [1]. In the oxide zone, refractory gold is found together with hematite as coatings or internal cements in , saprolite, , Al-hydroxide, and ilmenite. The recovery of precious metals such as gold from refractory ores has received considerable attention [2–6]. To achieve a satisfactory recovery, a pretreatment stage is required to break down or at least to modify the matrix and release the precious metals before applying any conventional treatment [7]. A suitable pre-treatment process, such as roasting [8], pressure oxidation [9],

Metals 2018, 8, 957; doi:10.3390/met8110957 www.mdpi.com/journal/metals Metals 2018, 8, 957 2 of 11 bio-oxidation [10], alkaline [7] or alkaline sulfide [11] and, to a limited extent, ultrafine grinding [12], is often required to overcome refractoriness and render the encapsulated gold and accessible to the action of cyanide and oxygen. Several processes have been used for treating a sulfidic refractory gold ore, such as oxidation roasting, biological oxidation, Albion process, leachox process, pressure oxidation and flotation-preoxidation-cyanidation methods [13,14]. However, an improved method is needed to separate and recover high-grade Au from the mineralized oxidized zone, because a high percentage of Au in hematite cannot be recovered by the cyanidation process. Iron-oxide-bearing refractory gold yields low gold and silver extractions in cyanide leaching [15]. Under high pH of cyanide, the dissolved iron oxide adsorbs on the gold surface, inhibiting the cyanide dissolution of the gold (passivation) [16–18]. The commonly used flotation method cannot separate Au-containing hematite or clay minerals directly from gangue minerals, because the low hydrophobicity of the minerals coating clay particles reduces the efficiency of flotation separation [1,6,19,20]. Roasting processes have been applied for gold recovery as a pretreatment. Dunn and Chamberlain [8] used a two-step roasting process for gold recovery from hematite to exclude the generation of FeAsO4 and As in the air. After the two-step roasting, gold contained in hematite could be leached and extracted effectively with cyanide or non-cyanide solvent. But the two-step roasting required a large amount of cost and facilities [21]. Microwave roasting can shorten the processing time, in contrast with traditional methods that use electric furnaces. Several studies have used microwave reductive roasting to convert hematite or siderite (Fe-oxide minerals) and goethite (Fe-hydroxide mineral), into magnetite [22–24]. Since microwaves selectively react to specific iron minerals, thermal stress occurs at the boundary between gangue and iron minerals [4], and this thermal stress incurs microcracks at the boundary of minerals whose thermal expansion coefficient is different from each other, or inside the minerals [22]. Very high temperature obtained in the boundaries could also produce thermal fracturing [23,25]. The generated microcracks and faults are ultimately followed by thermal fracturing, and this thermal fracturing degrades the strength of the iron minerals, thereby facilitating the crushing and milling of ore minerals, resulting in increased Au liberation efficiency from gold ore [6,26]. After the mineral phase is changed through the microwave treatment, it is possible to use another technique for recovering the refractory gold from the oxide zone. However, few studies have considered recovery of refractory gold in the near-surface oxide zone and the applicability of the microwave roasting for concentrating gold. In this study, we investigated a method for gold concentration from the ores in near-surface oxide zone using the reductive microwave roasting and the magnetic separation processes. The mineralogical phase change of Fe-oxides was tested under various conditions of reductive microwave roasting (RMR). After magnetic separation (MS) of the treated concentrate, effect of RMR-MS process on enhancing Au content and Fe recovery was evaluated.

2. Materials and Methods

2.1. Ore in the Near-Surface Oxide Zone Ore samples were collected from the near-surface oxide zone at 25 m below sea level at the Moisan Gold deposit in Haenam, South Korea. Moisan deposit is part of Sunshin mine, one of the several active mines in Korea. Sunshin mine has been developed since January 2002 with production from Eunsan deposit, and the production of Moisan deposit was followed in 2006. The combined mine production from the two deposits up to April 2008 was 1222 kg Au and 31,058 kg Ag [27–29]. A polished section was prepared of the ore sample and studied under reflected light microscopy to identify the ore minerals. The rest of the ore sample was crushed and pulverized to 0.075 mm size, using a disc . The mineral compositions were determined by X-ray diffraction (XRD) using an X’Pert Pro MRD (Philips, Amsterdam, The Netherland) from PANalytical in the Netherlands. For the determination of the chemical compositions, in particular Au, Ag, As, and total Fe, a sample was Metals 2018, 8, 957 3 of 11 Metals 2018, 8, x FOR PEER REVIEW 3 of 11 pretreatedpretreated by by aqua aqua regia regia digestion digestion and and analyzed analyzed using using atomic atomic absorption absorption spectrophotometryspectrophotometry (AAS)(AAS) usingusing aa Shimadzu Shimadzu AA-7000 AA‐7000 (Shimadzu, (Shimadzu, Kyoto,Kyoto, Japan)Japan) from from Japan. Japan. TheThe major major element element composition composition waswas determined determined byby X-rayX‐ray fluorescencefluorescence (XRF)(XRF) usingusing aa RigakuRigaku ZSXZSX PrimusPrimus IIII (Rigaku,(Rigaku, Tokyo,Tokyo, Japan)Japan) fromfrom Japan. Japan.

2.2.2.2. ReductiveReductive MicrowaveMicrowave Roasting Roasting (RMR)-Magnetic (RMR)‐Magnetic Separation Separation (MS) (MS) Process process TheThe overall overall processes processes of of RMR-MS RMR‐MS are are shown shown in in Figure Figure1 .1.

FigureFigure 1.1. ReductiveReductive microwavemicrowave roastingroasting (RMR) (RMR) and and magnetic magnetic separation separation (MS) (MS) processes. processes. 2.2.1. RMR Process 2.2.1. RMR Process RMR processing was conducted as a pre-processing to separate the Fe-oxides from the auriferous RMR processing was conducted as a pre‐processing to separate the Fe‐oxides from the ore in the near-surface oxide zone. The microwave roasting process, especially controlling irradiation auriferous ore in the near‐surface oxide zone. The microwave roasting process, especially controlling time, could be used to control the mineral phase change of ore sample [29]. For the reduction of irradiation time, could be used to control the mineral phase change of ore sample [29]. For the hematite, various reducing agents, such as activated carbon, sodium carbonate and charcoal, have reduction of hematite, various reducing agents, such as activated carbon, sodium carbonate and been used in the reductive roasting process [30–32]. In this study, the pulverized ore sample was mixed charcoal, have been used in the reductive roasting process [30–32]. In this study, the pulverized ore with activated carbon and sodium carbonate in a closed crucible (7.5 cm diameter by 25 cm height). sample was mixed with activated carbon and sodium carbonate in a closed crucible (7.5 cm diameter Here, the activated carbon was added to the ore sample as a reducing agent. The ore mixture was by 25 cm height). Here, the activated carbon was added to the ore sample as a reducing agent. The irradiated with a microwave using a microwave generator. The microwave generator was a batch-type ore mixture was irradiated with a microwave using a microwave generator. The microwave device operating at 2450 MHz with adjustable power of 1–6 kW. The microwave irradiation test was generator was a batch‐type device operating at 2450 MHz with adjustable power of 1–6 kW. The conducted under different conditions of reducing agent (Exps. A and B), microwave power (Exps. C) microwave irradiation test was conducted under different conditions of reducing agent (Exps. A and and irradiation time (Exps. D). Details of the different conditions during the experiment are given in B), microwave power (Exps. C) and irradiation time (Exps. D). Details of the different conditions Table1. during the experiment are given in Table 1. Table 1. Experimental conditions of the reductive microwave roasting (RMR). Table 1. Experimental conditions of the reductive microwave roasting (RMR). Experiments A B C D Experiments A B C D Ore Sample (g) 50 50 50 50 ActivatedOre Carbon Sample Mass (g) (g) 25–100 7550 7550 75 50 50 ActivatedSodium Carbonate Carbon (g) Mass (g) 6 2–1425–100 10 75 10 75 75 Microwave Intensity (kW) 5 5 3–6 5 Sodium Carbonate (g) 6 2–14 10 10 Irradiation time (min) 10 10 10 7–15 Microwave Intensity (kW) 5 5 3–6 5 Irradiation time (min) 10 10 10 7–15

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After the processed sample was cooled sufficiently in ambient conditions, an aggregated ore mixture was obtained. Polished thin sections were prepared using a part of the aggregated samples. The RMR-processed sample was ground using a with a 10:1 material ratio at a rotational speed 300 r/min for 15 min. Part of the powdered sample was pretreated by aqua regia digestion and analyzed using AAS to determine the chemical content of the RMR-processed samples. Metals 2018, 8, x FOR PEER REVIEW 4 of 11

2.2.2. MS ProcessAfter the processed sample was cooled sufficiently in ambient conditions, an aggregated ore Magneticmixture separation was obtained. was Polished conducted thin sections for the were RMR-processed prepared using a samples part of the after aggregated milling samples. the aggregate The RMR‐processed sample was ground using a ball mill with a 10:1 material ratio at a rotational particles. For a comparison, magnetic separation was also conducted for the raw sample without RMR speed 300 r/min for 15 min. Part of the powdered sample was pretreated by aqua regia digestion and treatment.analyzed The magnetic using AAS separation to determine was the chemical carried outcontent using of the a Davis RMR‐processed tube magnetic samples. separator (XCGS-73, Changsha Research Institute of and Metallurgy Co., Changsha, China) for 20 min with a magnetic2.2.2. intensity MS Process of 4500 G. The weights of the magnetic concentrate and non-magnetic were measured toMagnetic calculate separation magnetic was fraction. conducted for the RMR‐processed samples after milling the aggregate Theparticles. chemical For composition a comparison, and magnetic iron grade separation of the was magnetic also conducted concentrate for the andraw sample non-magnetic without tailings were determinedRMR treatment. through The aquamagnetic regia separation digestion was andcarried AAS. out Inusing order a Davis to study tube magnetic the effect separator of MS process (XCGS‐73, Changsha Research Institute of Mining and Metallurgy Co., Changsha, China) for 20 min on the magneticwith a magnetic property intensity obtained of 4500 by G. the The transformation weights of the magnetic of hematite, concentrate the magneticand non‐magnetic hysteresis and magnetictailings intensity were were measured analyzed to calculate using magnetic Vibration fraction. Sample magnetometer (VSM, BHV-50HTI, Riken, Keiki, Japan).The chemical composition and iron grade of the magnetic concentrate and non‐magnetic tailings were determined through aqua regia digestion and AAS. In order to study the effect of MS process 3. Resultson and the magnetic Discussion property obtained by the transformation of hematite, the magnetic hysteresis and magnetic intensity were analyzed using Vibration Sample magnetometer (VSM, BHV‐50HTI, Riken. 3.1. CharacteristicsKeiki, Japan). of Ore in the Near-Surface Oxide Zone

The3. major Results element and Discussion composition and mineralogical characteristics of the ore sample are presented in Table2. The ore contains high concentrations of precious metals; 6.4 mg/kg of gold and 35.6 mg/kg 3.1. Characteristics of Ore in the Near‐Surface Oxide Zone of silver, as well as deleterious elements, such as 911.6 mg/kg of As. The composition of the total iron The major element composition and mineralogical characteristics of the ore sample are in the ore sample accounted for 28.6 wt. %. Hematite (Fe2O3) is the most abundant Fe-oxide mineral presented in Table 2. The ore contains high concentrations of precious metals; 6.4 mg/kg of gold and and accounts for 64% of the total iron content in the sample. The high concentrations of SiO and 35.6 mg/kg of silver, as well as deleterious elements, such as 911.6 mg/kg of As. The composition of 2 Al2O3 werethe total related iron to in the ore occurrence sample accounted of for and28.6 wt. muscovite %. Hematite in the (Fe2 oreO3) is sample the most from abundant the near-surface Fe‐ oxide zoneoxide (Figure mineral2). and accounts for 64% of the total iron content in the sample. The high concentrations of SiO2 and Al2O3 were related to the occurrence of quartz and muscovite in the ore sample from the Tablenear 2. ‐XRFsurface and oxide AAS zone analysis (Figure for 2). the ore sample from near-surface oxide zone in Moisan Gold deposit (Haenam, South Korea). The contents of the target elements such as Au, Ag, As and total Fe were Table 2. XRF and AAS analysis for the ore sample from near‐surface oxide zone in Moisan Gold analyzeddeposit using (Haenam, AAS. The South mineral Korea). compositions The contents of were the target determined elements XRFsuch as analysis. Au, Ag, As and total Fe were analyzed using AAS. The mineral compositions were determined XRF analysis. Element/Mineral Au Ag As T Fe Fe2O3 SiO2 Al2O3 Na2O SO3 K2O CaO TiO2 Contents (wt.Element/Mineral %) 6.4 a Au 35.6Aga 911.6Asa T28.6 Fe Fe2 26.2O3 SiO 24.22 Al2O3 6.8 Na2O 2.1SO3 6.8K2O CaO 2.4 TiO 3.12 0.9 Contents (wt. %) 6.4 a 35.6 a 911.6 a 28.6 26.2 24.2 6.8 2.1 6.8 2.4 3.1 0.9 a mg/kg. a mg/kg.

FigureFigure 2. XRD 2. XRD pattern pattern for for the the ore ore sample sample from from the the near near-surface‐surface oxide oxide zone. zone.

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PolarizedPolarized light light microscopy microscopy identified identified spherical spherical hematite hematite particles particles associated associated with with cubic cubic pyrite crystalscrystals (Figure (Figure 3a,b)3a,b) that that formed formed through through the the leaching leaching of of pyrite pyrite by by acidic acidic water water (i.e., (i.e., supergene fluid). fluid). TheThe acidic acidic water water with with Fe Fe in in solution solution also also formed formed fine fine veins veins and and penetrated penetrated cubic cubic spaces spaces where where it it depositeddeposited the the spherical spherical hematite hematite particles particles (Figure (Figure 33c).c). The The dissolved iron also formed concentric hematitehematite structures structures within within the the veins veins (Figure (Figure 3d),3d), which which is is commonly commonly observed observed in in the the oxide oxide zone zone [33]. [33].

FigureFigure 3. 3. ReflectedReflected light light microphotographs microphotographs of of the the hematite hematite in in the the oxidation oxidation zone zone sample. sample. (a (a––cc) )are are in in crosscross-polarized‐polarized light light and and (d (d) )is is in in plane plane-polarized‐polarized light. light.

3.2.3.2. RMR RMR-MS‐MS for for Recovery Recovery of of Au Au Concentrate Concentrate from from Oxide Oxide Ore Ore TheThe raw raw sample sample had had sufficiently sufficiently high high levels levels of of gold gold and and silver, silver, but but a high a high content content of hematite of hematite in thein theFe‐oxide Fe-oxide reduced reduced the the effectiveness effectiveness of ofgold gold recovery recovery processes, processes, such such as as flotation. flotation. The The RMR RMR-MS‐MS processprocess was was tested tested as as a a method method for for the the efficient efficient separation separation of of Fe Fe-oxides‐oxides from from the the auriferous auriferous ore ore in in the the nearnear-surface‐surface oxide oxide zone. zone. The The effect effect of the of the RMR RMR-MS‐MS process process on the on composition the composition of the of concentrate the concentrate was evaluatedwas evaluated by comparing by comparing with the with compositions the compositions of the raw of the and raw MS and‐processed MS-processed samples samples (Table 3). (Table Here,3). theHere, RMR the was RMR conducted was conducted on 50‐g of on pulverized 50-g of pulverized ore using ore 75 g using of activated 75 g of carbon activated and carbon 10 g of andsodium 10 g carbonate.of sodium Microwave carbonate. irradiation Microwave took irradiation place at took 5 kW place for at10 5min. kW RMR for 10‐MS min.‐processed RMR-MS-processed ore sample showedore sample increased showed contents increased of Au, contents Ag and of Fe. Au, The Ag content and Fe. of Au The increased content of from Au 6.4 increased to 10.8 mg/kg from 6.4 and to Ag10.8 content mg/kg from and 35.6 Ag contentto 43.2 mg/kg. from 35.6 The to total 43.2 iron mg/kg. content The increased total iron from content 28.6 increased to 41.9 wt. from % but 28.6 the to 41.9 wt. % but the content of Fe O decreased from 26.2% to 2.7%. This indicates that Fe O reduced content of Fe2O3 decreased from2 26.2%3 to 2.7%. This indicates that Fe2O3 reduced and changed2 3 into magneticand changed minerals into during magnetic the minerals microwave during roasting the microwave process and roasting that the processmagnetic and fraction that the of the magnetic RMR treatedfraction ore of was the RMR successfully treated oreseparated was successfully by the MS separated process. The by the ratios MS of process. Au/total The Fe ratios and Ag/total of Au/total Fe showsFe and that Ag/total Au and Fe showsAg migrated that Au with and Agthe migratedFe‐oxide withminerals the Fe-oxide during the minerals MS process. during theThis MS suggests process. thatThis successful suggests thatseparation successful of Fe separation‐oxides could of Fe-oxides be an effective could be method an effective for increasing method for gold increasing recovery. gold In addition,recovery. as In content addition, decreased as content from decreased 911.6 to 10.5 from mg/kg 911.6 during to 10.5 RMR mg/kg‐MS during process RMR-MS by evaporation process of by evaporation of As O . The noticeable elimination of the penalty element could increase the value of As2O3. The noticeable2 3 elimination of the penalty element could increase the value of the ore the . The raw sample and magnetic fraction showed similar ratios of Au/Fe O and concentrate. The raw sample and magnetic fraction showed similar ratios of Au/Fe2O3 and Ag/Fe2 32O3. ThisAg/Fe supports2O3. This the supports concept thethat concept gold and that silver gold in and hematite silver inare hematite effectively are recovered effectively by recovered the magnetic by the separationmagnetic separationafter mineral after phase mineral change phase of hematite change of to hematite magnetic to minerals. magnetic minerals. The mineral phase change was observed in the image of reflected light microphotograph for the

RMR-MS-processedTable 3. Composition sample. of Au, Figure Ag, As,4a showsFe and ahematite core-rim (Fe structure2O3) in the with raw hematiteore sample along and theRMR boundary‐MS andprocessed magnetite samples. making up the core of the mineral. This structure could be generated through a number of processes: (1) the mineralogical transformation of Fe-oxide minerals from the interior site Sample Au Ag As T Fe (wt. %) Fe2O3 (wt. %) by energy focusing; (2) by the uniform distribution of temperature through the mineral with lower Raw 6.4 35.6 911.6 28.6 26.2 stresses at the grain boundaries [22]; or (3) during the microwave sintering of pyrite by the dissolution a re-precipitationRMR‐MS processed mechanism [310.8]. 43.2 10.5 41.9 1.9 a RMR was conducted on 50 g of pulverized ore using 75 g of activated carbon and 10 g of sodium carbonate. Microwave irradiation took place at 5 kW for 10 min.

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Metals 2018, 8, x FOR PEER REVIEW 6 of 11 Table 3. Composition of Au, Ag, As, Fe and hematite (Fe2O3) in the raw ore sample and RMR-MS processedMetals The2018 ,mineral 8 samples., x FOR PEERphase REVIEW change was observed in the image of reflected light microphotograph for6 of the 11 RMR‐MS‐processed sample. Figure 4a shows a core‐rim structure with hematite along the boundary Sample Au Ag As T Fe (wt. %) Fe O (wt. %) and Themagnetite mineral making phase changeup the was core observed of the mineral. in the image This ofstructure reflected could light microphotographbe generated2 3 through for the a RMRnumber‐MS of‐processed processes:Raw sample. (1) the Figuremineralogical 6.4 4a shows transformation 35.6 a core‐ 911.6rim structure of Fe‐oxide with 28.6 minerals hematite from along the 26.2 the interior boundary site a andby energy magnetiteRMR-MS focusing; making processed (2) byup thethe uniform core10.8 of distributionthe 43.2 mineral. of 10.5 This temperature structure 41.9 throughcould be the generated mineral 1.9 withthrough lower a anumberstressesRMR was atof conductedtheprocesses: grain boundaries on (1) 50 the g ofmineralogical pulverized [22]; or (3) ore during transformation using the 75 gmicrowave of activated of Fe‐oxide sintering carbon minerals and of pyrite 10 from g of by sodium the the interior dissolution carbonate. site Microwavebyre‐ precipitationenergy irradiationfocusing; mechanism took(2) by place the [3]. at uniform 5 kW for distribution 10 min. of temperature through the mineral with lower stresses at the grain boundaries [22]; or (3) during the microwave sintering of pyrite by the dissolution re‐precipitation mechanism [3].

FigureFigure 4. ( a4.) Reflected(a) Reflected light light microphotograph microphotograph showingshowing mineral mineral phase phase change change from from hematite hematite to to magnetite and maghemite. (b) X‐ray diffraction pattern of the magnetic fraction from the roasted magnetite and maghemite. (b) X-ray diffraction pattern of the magnetic fraction from the roasted Figuresample. 4. The (a) sample Reflected was light treated microphotograph with 5 kW of microwave showing powermineral in phaseExps. C.change from hematite to sample. The sample was treated with 5 kW of microwave power in Exps. C. magnetite and maghemite. (b) X‐ray diffraction pattern of the magnetic fraction from the roasted sample.The XRD The pattern sample for was the treated magnetic with 5fraction kW of microwave after RMR power shows in the Exps. generation C. of magnetic minerals The XRD pattern for the magnetic fraction after RMR shows the generation of magnetic minerals such as maghemite (γ‐Fe2O3), which has ferromagnetic properties, and magnetite (Figure 3b). γ suchMaghemite as maghemiteThe XRD can pattern be ( produced-Fe for2 Othe3), magneticalong which with fraction has magnetite ferromagnetic after by RMR the showsreduction properties, the generationof hematite and of at magnetite magnetic high temperature, minerals (Figure 3b). Maghemitesuchwith oxidationas can maghemite be producedupon (contactγ‐Fe along2O3 ),with which with air during magnetitehas ferromagnetic cooling by the[1]. reductionTheproperties, increase of and in hematite the magnetite magnetization at high (Figure temperature, of 3b). the withMaghemite oxidationroasted sample uponcan beis contactproducedillustrated with along by aircomparing with during magnetite the cooling magnetic by the [1]. reduction hysteresis The increase of loopshematite in of the theat magnetizationhigh raw temperature, and roasted of the roastedwithsamples sample oxidation (Figure is illustratedupon 5). The contact curves by with comparingshow air thatduring the the saturationcooling magnetic [1]. magnetization The hysteresis increase in loopssignificantly the magnetization of the increased raw and of from the roasted samplesroasted2.74 (Figure to sample26.915 ).emu/g Theis illustrated curves after reduction show by comparing that roasting. the saturationthe Themagnetic reduction magnetization hysteresis of hematite loops significantly ofusing the rawelectric and increased furnacesroasted from 2.74 tosamplesreported 26.91 emu/g (Figure an increase after5). The in reduction saturationcurves show roasting. magnetization that the The saturation reduction intensities magnetization of (M ) from 9 using significantlyto 37 emu/g electric byincreased furnaces the complete from reported an increase2.74conversion to in 26.91 saturation of emu/ghematite magnetizationafter to magnetite reduction [34]. intensitiesroasting. Good magneticThe (M s)reduction from separation 9 to of 37 hematite emu/g could be byusing achieved the completeelectric after furnaces roasting conversion reportedsince the anresidual increase magnetism in saturation (Mr) magnetization increased from intensities 0.5 to 3.9 (emu/gMs) from [34]. 9 to 37 emu/g by the complete of hematite to magnetite [34]. Good magnetic separation could be achieved after roasting since the conversion of hematite to magnetite [34]. Good magnetic separation could be achieved after roasting residual magnetism (Mr) increased from 0.5 to 3.9 emu/g [34]. since the residual magnetism (Mr) increased from 0.5 to 3.9 emu/g [34].

Figure 5. Magnetic hysteresis loops of the raw sample and magnetic fraction from the roasted sample. The roasted sample was treated with 5 kW of microwave power in Exps. C. FigureFigure 5. Magnetic 5. Magnetic hysteresis hysteresis loops loops of of the the raw raw sample and and magnetic magnetic fraction fraction from from the roasted the roasted sample. sample. The roastedThe roasted sample sample was was treated treated with with 5 5 kW kW of of microwave microwave power in in Exps. Exps. C. C.

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3.3. EffectsMetals of2018 RMR, 8, x FOR Conditions PEER REVIEW on Iron Recovery 7 of 11

The3.3. effectiveEffects of RMR recovery Conditions of gold on Iron from Recovery the near-surface oxide zone using RMR and magnetic separation (MS), requires the efficient reduction of hematite. The RMR-MS tests were conducted The effective recovery of gold from the near‐surface oxide zone using RMR and magnetic underseparation different conditions(MS), requires of reducingthe efficient agents reduction (contents of hematite. of activated The RMR carbon‐MS tests and sodiumwere conducted carbonate) and microwaveunder different operation conditions (microwave of reducing power agents and (contents irradiation of activated time), tocarbon optimize and sodium the RMR carbonate) process for gold recovery.and microwave In the operation following, (microwave the effects power of the and operation irradiation conditions time), to on optimize the recovery the RMR of process iron minerals for coexistinggold withrecovery. gold In were the investigatedfollowing, the to effects better of understand the operation the conditions gold recovery. on the recovery of iron minerals coexisting with gold were investigated to better understand the gold recovery. 3.3.1. Effect of Activated Carbon and Sodium Carbonate 3.3.1. Effect of Activated Carbon and Sodium Carbonate Figure6 shows the results of RMR experiments that were conducted using different concentrations of activatedFigure carbon 6 andshows sodium the results carbonate. of RMR Hematite experiments was not that sufficiently were conducted reduced throughusing different microwave irradiationconcentrations without activatedof activated carbon, carbon which and sodium clearly carbonate. illustrates Hematite that a reducing was not agent, sufficiently such asreduced activated through microwave irradiation without activated carbon, which clearly illustrates that a reducing carbon is essential for the reduction of hematite by microwave roasting (Figure6a). The results show agent, such as activated carbon is essential for the reduction of hematite by microwave roasting an increase(Figure in 6a). grade The andresults recovery show an of increase iron associated in grade and with recovery an increase of iron in associated the concentration with an increase of activated in carbon,the but concentration a rapid decrease of activated in both carbon, indices but when a rapid the decrease carbon-to-ore in both ratioindices is lesswhen than the 1.5.carbon This‐to‐ mayore be explainedratio byis less the incompletethan 1.5. This reduction may be explained of hematite by whenthe incomplete the activated reduction carbon of ishematite insufficient, when andthe an over-reductionactivated carbon of hematite is insufficient, to ferrous and oxide an over when‐reduction activated of hematite carbon to is ferrous excessive. oxide This when illustrates activated the importantcarbon effect is excessive. of the ratio This of illustrates activated the carbon-to-iron-oxide important effect of the mineralsratio of activated on iron carbon recovery.‐to‐iron The‐oxide optimal ratio ofminerals activated on iron carbon-to-ore recovery. The was optimal determined ratio of toactivated be 1.5. carbon At this‐to ratio,‐ore was the determined activated carbon to be 1.5. acted At as a microwavethis ratio, susceptor, the activated as well carbon as aacted reducing as a microwave agent [4,35 susceptor,]. When as activated well as a carbon reducing is irradiatedagent [4,35]. with When activated carbon is irradiated with microwaves, CO2 and CO gases are generated sequentially microwaves, CO2 and CO gases are generated sequentially by heat (Equation (1)) and by Boudouard’s by heat (Equation (1)) and by Boudouard’s reaction (Equation (2)), respectively [25,36]. The generated reaction (Equation (2)), respectively [25,36]. The generated CO can reduce hematite to magnetite by CO can reduce hematite to magnetite by the reaction in Equation (3) [37]. the reaction in Equation (3) [37]. CO CO (1) C + O2 = CO2 (1)

C +CCOCO2 = 2CO2CO (2) (2)

3Fe2O3Fe3 +OCO CO2Fe= 2Fe3O4O+ COCO2 (3) (3)

FigureFigure 6. Effects 6. Effects of (a ,ofc) ( activateda,c) activated carbon carbon and and (b (,bd,d)) sodium sodium carbonatecarbonate concentrations concentrations on on iron iron (a,b ()a and,b) and gold (cgold,d) recovery. (c,d) recovery.

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Na2CO3 was added along with the activated carbon to increase the reduction of ore during the Metals 2018, 8, x FOR PEER REVIEW 8 of 11 RMR process [38]. In our study, different amounts of Na2CO3 were added for microwave roasting. Figure6b shows the grade and recovery of iron as a function of the Na 2CO3 dosage. The maximum Na2CO3 was added along with the activated carbon to increase the reduction of ore during the recovery of iron increased up to 77% by adding 20% of sodium carbonate and an additional supply of RMR process [38]. In our study, different amounts of Na2CO3 were added for microwave roasting. ◦ CO2, whichFigure is 6b essential shows the for grade the reductive and recovery roasting of iron of hematiteas a function (Equation of the Na 2).2CO Na32 dosage.CO3 melts The atmaximum 874 C and producesrecovery CO2 (ofNa iron2CO increased3 → Na up2O to+ 77%CO2 by). Therefore,adding 20% theof sodium production carbonate of CO and2 gasan additional by the addition supply of Na2COof3 facilitatedCO2, which the is essential reduction for of the hematite reductive to roasting magnetite of hematite [39,40]. (Equation This, in turn, 2). Na improves2CO3 melts the at efficiency874 °C of magneticand produces separation CO2 [ (5Na,41CO,42].→NaOCO). Therefore, the production of CO2 gas by the addition of FigureNa2CO6a,b3 facilitated shows reduced the reduction iron recovery of hematite at high to levelsmagnetite of activated [39,40]. carbonThis, in and turn, sodium improves carbonate. the This canefficiency be explained of magnetic by the separation reduction [5,41,42]. process of hematite. According to the principle of Baykova, Figure 6a,b shows reduced iron recovery at high levels of activated carbon and sodium reduction of Fe-oxides, such as hematite, takes place in three steps: Fe2O3 → Fe3O4 → FeO → Fe [43]. carbonate. This can be explained by the reduction process of hematite. According to the principle of Due to the non-magnetic properties of FeO, the generation of FeO by high levels of reduction inhibits Baykova, reduction of Fe‐oxides, such as hematite, takes place in three steps: Fe2O3 → Fe3O4 → FeO the efficient→ Fe recovery[43]. Due ofto iron.the non Figure‐magnetic6c shows properties the increase of FeO, ofthe gold generation content of by FeO increasing by high levels dosage of of activatedreduction carbon, inhibits but the the addition efficient of recovery sodium of carbonate iron. Figure did 6c not shows show the any increase significant of gold change content in by gold contentincreasing (Figure6 d).dosage of activated carbon, but the addition of sodium carbonate did not show any significant change in gold content (Figure 6d). 3.3.2. Effect of Microwave Power and Irradiation Time 3.3.2. Effect of Microwave Power and Irradiation Time The microwave process was optimized through different operation conditions of the microwaves in the RMRThe for oremicrowave from the near-surfaceprocess was oxideoptimized zone. through Figure7 a,cdifferent shows thatoperation both grade conditions and recovery of the of iron increasedmicrowaves rapidly in the when RMR thefor ore microwave from the powernear‐surface increased oxide fromzone. 3Figure to 5 kW, 7a,c butshows decreased that both when grade the microwaveand recovery power increased of iron increased to 6 kW. rapidly In Figure when7b,d, the the microwave grade and power recovery increased of iron from and 3 goldto 5 kW, increased but decreased when the microwave power increased to 6 kW. In Figure 7b,d, the grade and recovery of with increasing irradiation time, but prolonged irradiation inhibited the efficient recovery of iron and iron and gold increased with increasing irradiation time, but prolonged irradiation inhibited the gold. This could be due to over-reduced hematite to weakly magnetic ferrous oxide, such as FeO. In efficient recovery of iron and gold. This could be due to over‐reduced hematite to weakly magnetic this RMRferrous condition, oxide, 5such kW as of FeO. microwave In this powerRMR condition, and 10 min 5 kW of irradiation of microwave are thepower optimum and 10 conditionsmin of for theirradiation effective recovery are the optimum of iron andconditions gold. for the effective recovery of iron and gold.

FigureFigure 7. Effect 7. Effect of reduction of reduction roasting roasting conditions conditions ((a ((,ca),c microwave) microwave intensityintensity and and (b (b,d,)d irradiation) irradiation time) time) on the Fe (a,b) and Au (c,d) contents in the ores from the near‐surface oxide zone. on the Fe (a,b) and Au (c,d) contents in the ores from the near-surface oxide zone.

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3.4.3.4. Au Concentration during RMR-MSRMR‐MS RMR-MS-processedRMR‐MS‐processed samples samples showed showed an increase an increase in iron gradein iron and grade gold contents. and gold The contents. enhancements The ofenhancements the gold content of the andgold ironcontent grade, and whichiron grade, were which obtained were through obtained RMR-MS through RMR test under‐MS test various under conditionsvarious conditions of RMR, of are RMR, shown are together shown together in Figure in8. Figure The result 8. The shows result that shows the gold that contentthe gold increase content isincrease closely is related closely to related the increase to the increase of the iron of the grade. iron Thisgrade. is This because is because gold and gold silver and showsilver theshow same the behaviorssame behaviors with ironwith atiron the at gossan the gossan and and the the oxidation oxidation zone zone [44 [44].]. This This suggests suggests that that the the separationseparation ofof iron-bearingiron‐bearing mineralsminerals obtained through reduction of oxideoxide mineralsminerals is applicableapplicable as anan effectiveeffective methodmethod forfor goldgold recoveryrecovery fromfrom thethe oresores inin near-surfacenear‐surface oxide oxide zone. zone.

FigureFigure 8. (a) Increases ( (∆)) of of gold gold contents contents and and (b (b) )silver silver contents contents during during the the increase increase of of iron iron grade grade in inRMR RMR-MS‐MS processed processed concentrates. concentrates. All All data data obtained obtained in in the the overall overall RMR RMR-MS‐MS tests tests in in this study waswas plotted.plotted. (Raw oreore samplesample containedcontained 6.46.4 andand 35.635.6 g/tg/t of gold and silver and 28.6% of iron).

4.4. Conclusions ThisThis researchresearch focusedfocused onon thethe recoveryrecovery ofof high-gradehigh‐grade AuAu associatedassociated withwith aa near-surfacenear‐surface oxideoxide zonezone throughthrough RMR-MS.RMR‐MS. TheThe mainmain mineralmineral phasesphases in the ore sample from the near-surfacenear‐surface oxide zone areare hematite,hematite, quartz,quartz, andand muscovite.muscovite. Microwave irradiation enhanced the recovery of ironiron throughthrough magneticmagnetic separation by transformingtransforming hematite to magneticmagnetic iron minerals,minerals, such asas magnetitemagnetite andand maghemite.maghemite. Gold and silver that coexist with hematite are also also recovered recovered efficiently efficiently by this operation. operation. TheThe microwavemicrowave treatment treatment also also removed removed , arsenic, a deleterious a deleterious element. element. Reductive Reductive microwave microwave roasting needsroasting to beneeds optimized to be becauseoptimized the over-reductionbecause the over of hematite‐reduction could of hematite generate weaklycould generate magnetic weakly ferrous minerals,magnetic whichferrous inhibit minerals, the recovery which inhibit of iron. the Efficient recovery iron of recovery iron. Efficient (which couldiron recovery also be an (which indicator could of Aualso recovery), be an indicator is influenced of Au byrecovery), the microwave is influenced process by conditions, the microwave as well process as the reducingconditions, agents, as well such as asthe activated reducing carbon agents, and such sodium as activated carbonate. carbon The and RMR-MS sodium process carbonate. is a promising The RMR method‐MS process to extract is a goldpromising from a method near-surface to extract oxide gold zone. from This a near process‐surface allows oxide for zone. most This of the process gold in allows a near-surface for most oxideof the zonegold toin bea near extracted‐surface during oxide mining. zone to be extracted during mining.

Author Contributions: Conceptualization, N.-C.C. and S.L.; methodology, N.-C.C.; software, K.H.C.; validation, K.H.C.Author and Contributions: S.-G.L.; formal Conceptualization, analysis, B.-J.K.; investigation,N.‐C.C. and S.L.; B.-J.K.; methodology, resources, N. B.-J.K.;‐C.C.; data software, curation, K.H.C.; B.-J.K. validation, and S.L.; writing—originalK.H.C. and S.‐G.L.; draft formal preparation, analysis, B. B.-J.K.;‐J.K.; investigation, writing—review B.‐J.K.; and resources, editing, S.L.; B.‐ visualization,J.K.; data curation, S.-G.L.; B.‐J.K. supervision, and S.L.; C.-Y.P.;writing—original project administration, draft preparation, N.-C.C.; fundingB.‐J.K.; acquisition,writing—review C.-Y.P. and editing, S.L.; visualization, S.‐G.L.; Funding:supervision,This C. research‐Y.P.; project was funded administration, by the Korean N.‐C.C.; Ministry funding of Environmentacquisition, C. (MOE),‐Y.P. grant number 2016000140010. Acknowledgments:Funding: This researchThis studywas funded was supported by the byKorean the KoreanMinistry Ministry of Environment of Environment (MOE), (MOE) grant as number part of the2016000140010. “Advanced Technology Program for Environmental Industry” (Technologies for the Risk Assessment and Management 2016000140010). Acknowledgments: This study was supported by the Korean Ministry of Environment (MOE) as part of the Conflicts“Advanced of Interest:TechnologyThe Program authors declare for Environmental no conflict of interest.Industry” (Technologies for the Risk Assessment and Management 2016000140010). References Conflicts of Interest: The authors declare no conflict of interest. 1. Wu, Y.; Fang, M.; Lan, L.D.; Zhang, P.; Rao, K.V.; Bao, Z.Y. Rapid and direct magnetization of goethite ore roasted by biomass fuel. Sep. Purif. Technol. 2012, 94, 34–38. [CrossRef]

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