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Article Sulfation Roasting of Nickel Oxide–Sulﬁde Mixed Ore Concentrate in the Presence of Ammonium Sulfate: Experimental and DFT Studies

Guangshi Li 1, Xiaolu Xiong 1,*, Liping Wang 1, Lang Che 1, Lizhen Wei 1, Hongwei Cheng 1 , Xingli Zou 1, Qian Xu 1, Zhongfu Zhou 1,2, Shenggang Li 3,4 and Xionggang Lu 1,3,*

1 School of Materials Science and Engineering & State Key Laboratory of Advanced Special Steel, Shanghai University, 99 Shangda Road, Shanghai 200444, China; [email protected] (G.L.); [email protected] (L.W.); [email protected] (L.C.); [email protected] (L.W.); [email protected] (H.C.); [email protected] (X.Z.); [email protected] (Q.X.); [email protected] (Z.Z.) 2 Department of Physics, Institute of Mathematics, Physics and Computer Science, Aberystwyth University, Aberystwyth SY23 3FL, UK 3 School of Physical Science and Technology, ShanghaiTech University, 393 Middle Huaxia Road, Shanghai 201210, China; [email protected] 4 Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institue, Chinese Academy of Sciences, 100 Haike Road, Shanghai 201210, China * Correspondence: [email protected] (X.X.); [email protected] (X.L.); Tel.: +86-021-9692-8188 (X.X.)

Received: 31 October 2019; Accepted: 22 November 2019; Published: 25 November 2019

Abstract: Sulfation roasting, a common activation technique, is a potential method for cleaner production of nickel from complex low-grade ores. In this study, nickel oxide–sulfide mixed ore concentrate was roasted with the addition of ammonium sulfate under a static air atmosphere, and the roasted products were leached by water, in order to evaluate the extraction of metals. The ammonium sulfate activation roasting was investigated thoroughly and systematically by thermogravimetry–differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy. Particularly, the interface sulfation behavior and path were studied by the density functional theory (DFT) method. The results showed that a large amount of nonferrous metal sulfate (70% Ni, 89% Co, and 90% Cu) was generated, while iron was almost entirely transformed into iron oxide under appropriate roasting conditions of adding ammonium sulfate at a mass ratio of 200%, heating to 650 ◦C at 10 ◦C/min, and holding for 120 min. It was found that activation of ammonium sulfate can take two different paths: one in which ammonium sulfate directly reacts with raw ores below 500 ◦C and the other in which the SO2 decomposed from sulfates (ammonium sulfate, intermediate ammonium ferric sulfate, and ferric sulfate) reacts with the intermediate metal sulfides (NiS and Cu2S). The interface sulfation mechanism of NiS and Cu2S was investigated deeply by DFT method, which showed that there are two paths of sulfation for NiS or Cu2S, and both of them are thermodynamically favored. Thus, a thorough and systematic investigation of ammonium sulfate activation roasting of nickel oxide–sulfide mixed ore is provided; this might be a potential basis for future industrial applications of ammonium sulfate activation roasting techniques in complex mineral metallurgy.

Keywords: sulfation; roasting; nickel ore; ammonium sulfate; DFT; leaching

1. Introduction Nickel is a significant strategic metal, and sulfide and oxide ores are the main sources of nickel production around the world. Nickel sulfide minerals have comprised more than half of the resources

Metals 2019, 9, 1256; doi:10.3390/met9121256 www.mdpi.com/journal/metals Metals 2019, 9, 1256 2 of 15 for nickel metallurgy for decades. With the gradual consumption of high-grade nickel sulfide mineral resources, oxide ore (laterite) and low-grade nickel oxide–sulfide mixed ore are becoming more and more important in nickel metallurgy [1]. Highly efficient separation of nickel and iron is essential and difficult for the extractive metallurgy of either nickel laterite or nickel-oxide-sulfide mixed ore (NOSMO) [2]. The conventional matte smelting technique, which is famous for its high efficiency in sulfide ore metallurgy, can produce difficulties when processing oxide-based ores [3]. NOSMO is becoming a vitally important mineral resource for nickel production in China [4]. The specific features of nickel ore predetermine the necessity to develop novel pyro- or hydro-metallurgical techniques as alternatives. To date, researchers have made efforts to develop new metallurgical processes to treat laterite or oxide–sulfide mixed ore; these mainly include selective reduction [5–12] and selective leaching [1,2,13–19]. The selective reduction technique is more suitable for the treatment of nickel laterite, while selective leaching is a more appropriate process for NOSMO. The fundamental concept of selective leaching is to transform nickel oxides or nickel sulfides into water- or acid-soluble phases, while iron and other impurities remain in solid slag [20,21]. In order to obtain higher extraction rates of valuable nonferrous metals, different kinds of chemical additives have been used for activated roasting pretreatment before the selective leaching process [22–24]. Compared to strong corrosive additive media (such as H2SO4, Cl2, and SO2), salt solid additives (sulfate and chloride) are more environmentally friendly, which might result in potentially cleaner nickel production by using the roasting–leaching technique [21,25,26]. Because the sulfation roasting technique has a unique advantage in the process of nonferrous metal mineral, it has been studied by many researches [27–30]. Several kinds of salt solid additives have been reported as an effective sulfating agent during the roasting of polymetallic sulfide ore, such as sulfide [31], sodium sulfate [32], ferric sulfate [33], and ammonium sulfate [34]. As a potentially more economic and green metallurgical process, ammonium sulfate activation roasting has shown good applicability and has been widely investigated recently [19,20,26,30,35–38]. In particular, Mu et al. developed a low-temperature roasting-water leaching technique to treat complex low-grade nickel ore and achieved high extraction rates of nonferrous metals from raw minerals [19,20,39]. It was found that the gas products generated from ammonium sulfate thermal decomposition include NH3, SO2,N2, and H2O[40,41]. Therefore, the roasting reaction mechanism might be quite complex due to the variety of reactants. However, most researchers agree that the direct reactions between ammonium sulfate and minerals (metal oxides or sulfides) are mainly responsible for the effective sulfation of metals during the roasting process [2,19,42]. Clearly, there may be some truth to this, but it may be not the whole story of ammonium sulfate during the roasting process. Previous studies showed that the sulfation of nickel remained at a relatively low level after roasting with ammonium sulfate additive alone, but if other chemical media (sodium sulfate, sulfuric acid, etc.) were added, the sulfation yield of nickel reached 90% or more [19,20,39]. Additionally, similar studies have shown that incomplete sulfation of nickel is still exhibited during the roasting process, even after increasing the dosage of ammonium sulfate [2]. Therefore, there remain some ambiguities regarding the reaction mechanism of ammonium sulfate activation roasting of nickel ore, and there is a bottleneck to higher sulfation of nickel. It is necessary to clarify the situation and gain insight into the nature of ammonium sulfate’s action in the sulfation of nickel ores. Recently, density functional theory (DFT) calculations were applied to explore the oxidation of sulfide minerals, which is important for mineral processing and valuable metal extraction [43,44]. In our previous work, DFT calculations were performed successfully to investigate the oxidation mechanisms of chalcopyrite (CuFeS2) and pentlandite (Fe4.5Ni4.5S8)[45,46]. The computational results were in good agreement with our experimental observations [47]. Because the sulfation roasting of sulfides is a controllable oxidation process, DFT might be a suitable and powerful technique to study the behavior and mechanism of ammonium sulfate additives during the roasting process. As the content of main valuable mineral (pentlandite, chalcopyrite, etc.) in NOSMO is too low to be detected by XRD and other characterizations, a floatation concentrate of NOSMO was used as the Metals 2019, 9, 1256 3 of 15 rawMetals materials 2019, 9, x inFOR this PEER study. REVIEW The ammonium sulfate activation roasting of NOSMO concentrate3 of 15 was performed on a thermogravimetry–differential scanning calorimeter (TG–DSC) in an air atmosphere. Thewas sulfation performed roasting-water on a thermogravimetry leaching process–differential is discussed scanning with thecalorimeter evaluation (TG of– severalDSC) in parameters, an air includingatmosphere. the The roasting sulfation temperature, roasting-water dosage leaching of ammonium process is discussed sulfate, heating with the rate,evaluation and holding of several time. parameters, including the roasting temperature, dosage of ammonium sulfate, heating rate, and The transformation behaviors of the microstructure and crystal phase were characterized by X-ray holding time. The transformation behaviors of the microstructure and crystal phase were diffraction (XRD) and scanning electron microscopy (SEM), respectively. Furthermore, we used DFT to characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. explain the sulfation mechanism for NiS and Cu2S. Considering the atmosphere involved during the Furthermore, we used DFT to explain the sulfation mechanism for NiS and Cu2S. Considering the experiment, O2 and SO2 adsorption were both considered to elucidate the detailed reaction process. atmosphere involved during the experiment, O2 and SO2 adsorption were both considered to elucidate the detailed reaction process. 2. Materials and Methods

2.1.2. Materials and Methods

2.1.The Materials NOSMO concentrate sample used in this study originated from Jinchuan Group Co., Ltd. (Jinchang, Gansu, China). The concentrate sample was dried at 100 C for 12 h and then The NOSMO concentrate sample used in this study originated from Jinchuan◦ Group Co., Ltd. passed(Jinchang through, Gansu, 200 China). mesh prior The concentrate to experimentation. sample was Semiquantitative dried at 100 °C for estimation 12 h and of then all passed elements inthrough this powder 200 mesh sample prior was to experimentation. investigated by Semiquantitative X-ray fluorescence estimation technique of all (XRF), elements and in accurate this quantitativepowder sample analysis was ofinvestigated elements, by except X-rayO fluorescen and S, wasce technique measured (XRF) by inductively, and accurate coupled quantitative plasma massanalysis spectrometry of elements (ICP-AES)., except The O resultsand S, ofwas the measured chemical analysis by inductively of this powder coupled sample plasma are mass shown inspectrometry Table1. The (ICP nickel,-AES cobalt,). The results copper, of andthe chemical iron contents analysis were of this 8.93%, powder 0.24%, sample 6.51%, are and shown 28.26%, in respectively.Table 1. The The nickel, main cobalt, minerals, copper which, and were iron identified contents were from 8.93%, the X-ray 0.24%, diff raction 6.51%, and (XRD) 28.26 pattern%, (Figurerespectively.1), were The pentlandite main minerals, [(FeNi) which9S8, PDF#73-0515], were identified chalcopyrite from the (CuFeSX-ray d2iffraction, PDF#83-0983), (XRD) patternmagnetite (Fe(Figure3O4, PDF#72-2303), 1), were pentlandite pyrrhotite [(FeNi) 4M9S8 (Fe, PDF#737S8, PDF#29-0723),-0515], chalcopy pyriterite (CuFeS (FeS2,2 PDF#71-1680),, PDF#83-0983), quartzmagnetite (SiO 2, PDF#79-1912),(Fe3O4, PDF#72 willemseite-2303), pyrrhotite [(Ni,Mg) 4M3(Si (Fe2O75S)82, (OH)PDF#292, PDF#22-0711],-0723), pyrite (FeS and2, nepouite PDF#71-1680 [(Ni,Mg)), quartz3Si2O (SiO5(OH)2, 4, PDF#25-0524].PDF#79-1912), Ammonium willemseite sulfate[(Ni,Mg) (analytical3(Si2O5)2(OH) grade)2, PDF#22 was purchased-0711], and fromnepouite Sinopharm [(Ni,Mg) Group3Si2O5(OH) Co. Ltd.,4, Shanghai,PDF#25-0524] China,. Ammonium and ground sulfate into particles(analytical that grade) were was passing purchased through from 200 Sinopharm mesh before Group use. Co. Ltd., Shanghai, China, and ground into particles that were passing through 200 mesh before use. Table 1. Chemical composition of the NOSMO concentrate sample (wt.%). Table 1. Chemical composition of the NOSMO concentrate sample (wt.%). Fe O S Ni Mg Cu Si Ca Al Co Fe O S Ni Mg Cu Si Ca Al Co 28.26 22.43 19.61 8.93 6.66 6.51 6.45 0.52 0.39 0.24 28.26 22.43 19.61 8.93 6.66 6.51 6.45 0.52 0.39 0.24

FigureFigure 1.1. XRDXRD pattern of of the the NOSMO NOSMO concentrate concentrate sample sample.. 2.2. Experimental Procedure 2.2. Experimental Procedure AA quantity quantity of of 5 5 g g of of the theNOSMO NOSMO concentrateconcentrate sample sample was was weighed weighed and and finely ﬁnely powdered powdered in an in an agateagate mortar, mortar and, and the the ﬁne fine powder powder of the of the NOSMO NOSMO sample sample mixed mixed well well with with a desired a desired amount amount of ground of ammoniumground ammonium sulfate (100–200 sulfate mesh)(100–200 in eachmesh) experiment in each experiment by using anby agateusing mortar. an agate Then mortar the. mixtureThen the was mixture was added to a silica crucible. The sample was introduced into the cold furnace and then

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Metals 2019, 9, x FOR PEER REVIEW 4 of 15 added to a silica crucible. The sample was introduced into the cold furnace and then heated up to target heated up to target temperature with a certain rate. Sulfation roasting experiments were performed temperature with a certain rate. Sulfation roasting experiments were performed by using a muffle by using a muffle furnace. The roasted products were cooled inside the furnace to room temperature furnace. The roasted products were cooled inside the furnace to room temperature and then leached and then leached with hot deionized water (80 °C) with a liquid-to-solid ratio of 4:1 mL/g for 30 min. with hot deionized water (80 C) with a liquid-to-solid ratio of 4:1 mL/g for 30 min. The leaching The leaching slurry was then ◦filtered with a Buchner funnel. The effects of roasting temperature, slurry was then filtered with a Buchner funnel. The effects of roasting temperature, ammonium sulfate ammonium sulfate dosage, heating rate, and holding time were studied via the flowsheet shown in dosage, heating rate, and holding time were studied via the flowsheet shown in Figure2. Figure 2.

Figure 2. Ammonium sulfate activation roasting-water leaching process flowsheet. Figure 2. Ammonium sulfate activation roasting-water leaching process flowsheet. 2.3. Characterization Methods 2.3. Characterization Methods Thermogravimetry–differential scanning calorimetry (TG–DSC) analyses (NETZSCH STA 44 F3 Jupiter,Thermogravimetry Alden, German)–d wereifferential performed scanning on thecalorimet NOSMOry (TG concentrate–DSC) analyses sample, (NETZSCH ammonium STA sulfate, 44 F3 andJupiter their, A mixturelden, German (mass) ratiowere of performed 1:2) from 100on the to 1000 NOSMO◦C, at concentrate a heating rate sample, of 10 ◦ammoniumC/min under sulfate, an air atmosphereand their mixture (a flow (mass rate ratio of 40 of mL 1:/2min).) from A 100 quantity to 1000 of °C 20, at mg a heating of fine-powder rate of 10 sample °C/min was under weighed an air foratmosphere each TG–DSC (a flow test. rate The of 40 ion mL/min). concentrations A quantity of Fe, of Co,20 mg Ni, of and fine Cu-powder in the sample leachate was were weighed analyzed, for usingeach TG a inductively–DSC test. The coupled ion concentrations plasma–atomic of emission Fe, Co, Ni spectrometer, and Cu in (ICP-AES)the leachate (7300 were DV, analyzed Perkin, Elmer,using Waltham,a inductively MA, coupled USA). The plasma microstructure–atomic emission of the calcine spectromet waser observed (ICP-AES by) using (7300 a DV tungsten, Perkin filament Elmer, scanningWaltham, electron MA, USA microscope). The microstructure (TF-SEM) (SU-1500, of the calcine HITACHI, was observed Tokyo, Japan). by using The a XRD tungsten patterns filament were measuredscanning electron on an X-ray microscope diffractometer (TF-SEM (D8) ( Advance,SU-1500, Bruker,HITACHI Billerica,, Tokyo, MA, Japan USA),). The using XRD Cu patterns Kα radiation were withmeasured a 2θ range on an of X 5◦-ray–90 ◦ diffractometer, a step size of 0.02 (D8◦ , Advance, and a step Bruker time of, 1Billerica, s. MA, USA), using Cu Kα radiation with a 2θ range of 5°–90°, a step size of 0.02°, and a step time of 1 s. 2.4. Computational Details 2.4. ComputationalDFT calculations Details were performed as described previously [46,47]. The space group of NiS (millerite)DFT calculationsis R3m, and its were optimized performed cell parameters as described of previouslya = b = 9.564 [46 Å,47 and]. cThe= 3.126 space Å groupare consistent of NiS with(millerite) the experimental is R3m, and dataits optimized (a = b = 9.607 cell parameters Å, c = 3.143 of Å) a [=48 b]. = The9.564 p (1Å and2) c supercell = 3.126 Å of are the consistent NiS (100) × slabwith containing the experimental 24 atoms data and (a including = b = 9.607 a vacuum Å , c = 3.143 layer Å of ) [4 108]. Å T washe p employed, (1 × 2) supercell where theof the bottom NiS three(100) ofslab the containing six atomic 24 layers atoms were and fixed. including Cu2S hasa vacuum an orthorhombic layer of 10 cell Å withwas spaceemployed, group where Abm2 the [49 ].bottom The p (1three2) of supercell the six atomic of the layers (001) surface were fixed. for Cu Cu2S2S contains has an orthorhombic six layers with cell the with bottom space three group fixed. Abm2 [49]. × The p (1 × 2) supercell of the (001) surface for Cu2S contains six layers with the bottom three fixed.

3. Results and Discussion

3.1. Thermal Analysis of the Roasting Process The typical thermal analysis curves for NOSMO concentrate mixed with ammonium sulfate, as well as the blank contrast samples (ammonium sulfate and NOSMO concentrate, respectively), are

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3. Results and Discussion

3.1. Thermal Analysis of the Roasting Process

MetalsThe 2019, typical9, x FOR thermalPEER REVIEW analysis curves for NOSMO concentrate mixed with ammonium sulfate,5 of 15 as well as the blank contrast samples (ammonium sulfate and NOSMO concentrate, respectively), shownare shown in Figure in Figure 3. There3. There wa wass no no marked marked change change up up to to 285 285 °C◦C in in the the mixtu mixturere of of ammonium ammonium sulfate sulfate and NOSMO concentrate during the roasting process (Figure(Figure3 3a).a). Two Two well-defined well-defined endothermic endothermic peaks at 325 and 365 °C◦C,, respectively respectively,, were observed, and the simultaneous loss losseses in mass were also marked as 25 25%% (named S Steptep 1) and 6%6% (named(named StepStep 2),2), respectively.respectively. This phenomenon is largely attributable to the thermal decomposition of of ammonium sulfate sulfate,, as as shown in Figure 33b.b. However,However, with the increase in temperature, a small exothermic peak was first first observed at 425 ◦°C.C. This was accompanied byby aa correspondingcorresponding increase increase of of mass mass (7%) (7% in) in the the TG TG curve. curve. This This step step (named (named Step S 3)tep was 3) wasmainly mainly the combination the combination of the early of the oxidation early ofoxidation NOSMO of concentrate NOSMO andconcentrate the further and decomposition the further decompositionof ammonium sulfate.of ammonium The further sulfate. sulfation The further reactions sulfation (named reactions Step 4) resulted(named S intep a mass4) resu gainlted of in 2% a massand exothermal gain of 2% peaks and exothermal (460 and 530 peaks◦C) in(460 the and temperature 530 °C ) in range the temperature 450–600 ◦C. Thisrange simultaneous 450–600 °C . stageThis simultaneousalso occurred forstage the also nickel occurred concentrate for the sample nickel shown concentrate in Figure sample3c, which shown confirms in Figure the formation 3c, which confirmsof metal sulfates.the formation Above of 600metal◦C sulfates. (named Above Step 5), 600 a drastic°C (named mass Step loss, 5), as a welldrastic as mass endothermic loss, as well peaks, as wasendothermic observed, peaks, as shown was in Figureobserved3a,c., as This shown might in be attributedFigures 3a to,c. the This decomposition might be attributed of metal sulfates. to the decomposition of metal sulfates.

Figure 3. TGTG–DSC–DSC curves curves of of a a mixture mixture of of ammonium ammonium sulfate sulfate and and NOSMO NOSMO concentrate concentrate ( (aa)),, ammonium ammonium sulfate alone ( b)),, and NOSMO concentrate alone (c).

3.2. Effect Effect of Roasting Temperature The e effectffect of of the the sulfation sulfation roasting roasting temperature temperature on the on recovery the recovery of metal of species metal speciesby water by leaching water wasleaching first was investigated. first investigated. For each For test, each a mixturetest, a mixture of 5 g of 5 NOSMO g of NOSMO concentrate concentrate sample sample and 10and g 10 of gammonium of ammonium sulfate sulfate was was heated heated to ditoff differenterent temperatures temperatures at at 20 20◦C °C/min/min and and maintained maintained forfor 22 h in static air. The w water-leachingater-leaching results of the roasted products are presented in Figure 4 4.. AsAs cancan be seen, the leaching yields of nonferrous metals (Cu, Ni Ni,, and Co) increased substantially with with a a rise in the roasting temperature, peaked at around 650 °C , and then decreased sharply when the temperature continued to rise. This is because a higher temperature may facilitate sulfation reactions between nonferrous metal sulfides and ammonium sulfate in the range 300–650 °C , under an air atmosphere. The decomposition of these sulfates (CuSO4, NiSO4, and CoSO4) contributes mainly to the descent of the nonferrous metal extraction. Similarly, the iron leaching recovery slightly increased at first and then decreased with the rising temperature. When the temperature reached 600 °C , the extraction of Fe gradually approached zero. As revealed in Figure 4, there was an optimum roasting temperature

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roasting temperature, peaked at around 650 ◦C, and then decreased sharply when the temperature continued to rise. This is because a higher temperature may facilitate sulfation reactions between nonferrous metal sulﬁdes and ammonium sulfate in the range 300–650 ◦C, under an air atmosphere. The decomposition of these sulfates (CuSO4, NiSO4, and CoSO4) contributes mainly to the descent of the nonferrous metal extraction. Similarly, the iron leaching recovery slightly increased at ﬁrst and then decreased with the rising temperature. When the temperature reached 600 ◦C, the extraction of MetalsFe gradually 2019, 9, x FOR approached PEER REVIEW zero. As revealed in Figure4, there was an optimum roasting temperature6 of 15 in the range 600–650 ◦C, at which we can achieve considerable extraction of nonferrous metals (~9% inCu, the ~79% range Co, 600 and–650 59% °C Ni), at andwhich complete we can iron achieve separation considerable (~0.1% extraction Fe); thus, thisof nonferrous process may metals be used (~9% to Cu,separate ~79% and Co,extract and 59 nonferrous% Ni) and complete metals from iron NOSMO. separation (~0.1% Fe); thus, this process may be used to separate and extract nonferrous metals from NOSMO.

FigureFigure 4. 4. EffectEﬀect of of roasting roasting temperature on the leaching recovery of metals.

3.3. Effect Effect of the Addition of Ammonium Sulfate As an essential factor, the dosage of ammonium sulfate was investigated for for its its effect effect on the sulfation of valuable metals metals.. A A mixture mixture of of 5 5 g of NOSMO concentrate sample and a selected amount of ammonium sulfate was was heated to to 650 °C◦C at 20 °C◦C/min/min and held for 2 h in static air. The leaching efficiencyefficiency is plotted against the weight percentage of ammonium sulfate to NOSMO concentrate in Figure 55.. ItIt cancan bebe seenseen thatthat thethe dosagedosage ofof ammoniumammonium sulfatesulfate hadhad aa significantsignificant eeffectffect on nonferrous metals. The The extraction extraction of ofNi was Ni was substantially substantially promoted promoted from 30% from to 60%, 30% withto 60 increasing%, with ammoniumincreasing ammoniumsulfate addition. sulfate The addition. extraction The ofextraction Cu and Coof Cu were and also Co improvedwere also toimproved a certain to degree. a certain However, degree. However,because iron because sulfates iron cannot sulfates stay cannot stable atstay around stable 650 at around◦C, the dosage650 °C , ofthe ammonium dosage of ammonium sulfate had littlesulfate or hanod influence little or over no influence the leaching over effi theciency leaching of iron. efficiency Therefore, of the iron. optimum Therefore, dosage the of optimum ammonium dosage sulfate of ammoniumwould be double sulfate the would mass of be NOSMO double concentrate, the mass of and NOSMO this dosage concentrate was consequently, and this chosen dosage for was the consequentlysubsequent experiments. chosen for the subsequent experiments.

Figure 5. Effect of the dosage of ammonium sulfate on the leaching recovery of metals.

3.4. Effect of Heating Rate Roasting experiments were conducted under different heating rates, in order to understand the effect of the heating process on the sulfation of the nonferrous metals. In these experiments, a mixture of 5 g NOSMO concentrate sample and 10 g ammonium sulfate was heated to 650 °C and held for 2 h under an air atmosphere. The hot-water extraction of valuable metals (Fe, Co, Ni, and Cu) is shown

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in the range 600–650 °C , at which we can achieve considerable extraction of nonferrous metals (~9% Cu, ~79% Co, and 59% Ni) and complete iron separation (~0.1% Fe); thus, this process may be used to separate and extract nonferrous metals from NOSMO.

Figure 4. Effect of roasting temperature on the leaching recovery of metals.

3.3. Effect of the Addition of Ammonium Sulfate As an essential factor, the dosage of ammonium sulfate was investigated for its effect on the sulfation of valuable metals. A mixture of 5 g of NOSMO concentrate sample and a selected amount of ammonium sulfate was heated to 650 °C at 20 °C /min and held for 2 h in static air. The leaching efficiency is plotted against the weight percentage of ammonium sulfate to NOSMO concentrate in Figure 5. It can be seen that the dosage of ammonium sulfate had a significant effect on nonferrous metals. The extraction of Ni was substantially promoted from 30% to 60%, with increasing ammonium sulfate addition. The extraction of Cu and Co were also improved to a certain degree. However, because iron sulfates cannot stay stable at around 650 °C , the dosage of ammonium sulfate had little or no influence over the leaching efficiency of iron. Therefore, the optimum dosage of Metals 2019, 9, 1256 7 of 15 ammonium sulfate would be double the mass of NOSMO concentrate, and this dosage was consequently chosen for the subsequent experiments.

Figure 5. EffectEffect of the dosage of ammonium sulfate on the leaching recovery of metals. 3.4. Effect of Heating Rate 3.4. Effect of Heating Rate Roasting experiments were conducted under different heating rates, in order to understand Roasting experiments were conducted under different heating rates, in order to understand the the effect of the heating process on the sulfation of the nonferrous metals. In these experiments, a effect of the heating process on the sulfation of the nonferrous metals. In these experiments, a mixture mixture of 5 g NOSMO concentrate sample and 10 g ammonium sulfate was heated to 650 C and of 5 g NOSMO concentrate sample and 10 g ammonium sulfate was heated to 650 °C and held◦ for 2 Metalsheld for2019 2, 9 h, x under FOR PEER an airREVIEW atmosphere. The hot-water extraction of valuable metals (Fe, Co, Ni, and7 of Cu) 15 h under an air atmosphere. The hot-water extraction of valuable metals (Fe, Co, Ni, and Cu) is shown is shown in Figure6. The heating rate significantly influenced the leaching yields of nonferrous in Figure 6. The heating rate significantly influenced the leaching yields of nonferrous metals. As metals. As demonstrated in Figure6, the recovery of nonferrous metals showed slight increases demonstrated in Figure 6, the recovery of nonferrous metals showed slight increases with the with the increasing heating rate in the range from 1 to 10 ◦C/min. However, the leaching yields increasing heating rate in the range from 1 to 10 °C /min. However, the leaching yields started to started to decline sharply when the heating rate reached 10 ◦C/min. It is obvious that an overly rapid declineheating sharply rate may when inhibit the theheating sulfation rate reached of nonferrous 10 °C /min. metals It is and obvious result that in low an leachingoverly rapid efficiency heating of ratenonferrous may inhibit metals. the sulfation Indeed, this of nonferrous is due to incomplete metals and sulfation result in reactions,low leaching which efficiency are attributed of nonferrous to the metals.rapid decomposition Indeed, this is of due ammonium to incomplete sulfate. sulfation Our previous reactions, work which also reported are attributed that the to heating the rapid rate decompositionsignificantly influenced of ammonium the sulfation sulfate. of nickel Our previous sulfides [ 21work]. Additionally, also reported with that the increasing the heating heating rate significantlyrate, the amount influence of appreciabled the sulfation dissolution of nickel of sulfide iron decreaseds [21]. Additionally, gradually from with 4% the to increasing almost zero. heating As a rate, the amount of appreciable dissolution of iron decreased gradually from 4% to almost zero. As a result, the optimum sulfation roasting heating rate was 10 ◦C/min in this experiment, which was result,adopted the for optimum furtherinvestigations. sulfation roasting heating rate was 10 °C /min in this experiment, which was adopted for further investigations.

FigureFigure 6. 6. EffectEffect of of heating heating rate rate during during roasting roasting on on the the leaching leaching recovery recovery of of metals. metals. 3.5. Effect of Holding Time 3.5. Effect of Holding Time To study the effect of holding time on the sulfation of NOSMO concentrate, mixtures of 5 g NOSMO To study the effect of holding time on the sulfation of NOSMO concentrate, mixtures of 5 g samples and 10 g of ammonium sulfate were heated to 650 C at 10 C/min and held for 30 to 180 min. NOSMO samples and 10 g of ammonium sulfate were heated◦ to 650◦ °C at 10 °C /min and held for 30 to 180 min. The results of the hot-water extraction of valuable metals (Fe, Co, Ni, and Cu) are plotted in Figure 7. With the increasing holding time, the leaching yields of Ni, Co, and Cu gradually increased and then remained at a certain level at 120 min. More specifically, the extraction of Ni, Co, and Cu reached 70%, 89%, and 90%, respectively, while the extraction of Fe was about 2%. Thus, under appropriate roasting conditions (added ammonium sulfate with a mass ratio of 200%, heated to 650 °C at 10 °C /min and held for 120 min), a large amount of nonferrous metal sulfate was formed, while the iron was almost completely turned into iron oxide. Thus, excellent separation of the nonferrous metals and iron was achieved by the water leaching process.

Figure 7. Effect of holding time during roasting on the leaching recovery of metals.

Metals 2019, 9, x FOR PEER REVIEW 7 of 15 in Figure 6. The heating rate significantly influenced the leaching yields of nonferrous metals. As demonstrated in Figure 6, the recovery of nonferrous metals showed slight increases with the increasing heating rate in the range from 1 to 10 °C /min. However, the leaching yields started to decline sharply when the heating rate reached 10 °C /min. It is obvious that an overly rapid heating rate may inhibit the sulfation of nonferrous metals and result in low leaching efficiency of nonferrous metals. Indeed, this is due to incomplete sulfation reactions, which are attributed to the rapid decomposition of ammonium sulfate. Our previous work also reported that the heating rate significantly influenced the sulfation of nickel sulfides [21]. Additionally, with the increasing heating rate, the amount of appreciable dissolution of iron decreased gradually from 4% to almost zero. As a result, the optimum sulfation roasting heating rate was 10 °C /min in this experiment, which was adopted for further investigations.

Figure 6. Effect of heating rate during roasting on the leaching recovery of metals.

3.5.Metals Effect2019, of9, 1256Holding Time 8 of 15 To study the effect of holding time on the sulfation of NOSMO concentrate, mixtures of 5 g NOSMOThe results sample of thes hot-waterand 10 g of extraction ammonium of valuable sulfate were metals heated (Fe, Co,to 650 Ni, ° andC at Cu)10 °C are/min plotted and inheld Figure for 307. Withto 180 the min increasing. The result holdings of the time, hot-water the leaching extraction yields of valuable of Ni, Co, metals and Cu (Fe, gradually Co, Ni, and increased Cu) are and plotted then inremained Figure at 7. a With certain the level increas at 120ing min. holding More time, speciﬁcally, the leaching the extraction yields of of Ni, Ni, Co, Co and, and Cu Cu reached gradually 70%, increased89%, and 90%,and then respectively, remained while at a thecertain extraction level at of 120 Fe wasmin.about More 2%.specifically, Thus, under the extraction appropriate of roasting Ni, Co, andconditions Cu reach (addeded 70 ammonium%, 89%, and sulfate 90%, respectively, with a mass ratiowhile of the 200%, extraction heated of to Fe 650 wa◦Cs atabout 10 ◦ C2/%min. Thus, and underheld for appropriate 120 min), aroasting large amount condition of nonferrouss (added ammonium metal sulfate sulfate was formed,with a mass while ratio the ironof 200 was%, heated almost tocompletely 650 °C at turned10 °C /min into and iron held oxide. for 120 Thus, min excellent), a large separation amount ofof nonferrous the nonferrous metal metalssulfate andwas ironformed was, whileachieved theby iron the was water almost leaching completely process. turned into iron oxide. Thus, excellent separation of the nonferrous metals and iron was achieved by the water leaching process.

Figure 7. EffectEﬀect of of holding holding time during roasting on the leaching recovery of metals.

3.6. The Transformation Behaviors of the Crystal Phase and Microstructure of Roasting Products Recently, studies of the sulfation roasting of nickel sulﬁdes in air revealed strong additive-dependent behavior [19,21,39]. In order to gain insight into the sulfation roasting process, the microscopic morphology and crystal structure of the roasting products were investigated thoroughly and systematically by XRD (Figure8) and SEM (Figure9), respectively, which was conducive to analysis of the behavior of ammonium sulfate and to deducing the reaction mechanism of ammonium sulfate roasting. As shown in Figure8a, the product roasted at 450 ◦C mainly consisted of NH4Fe(SO4)2, Fe2(SO4)3, CuSO4, NiSO4, MgSO4, CuFeS2, Cu2S, NiS, Ni3S4, (Ni,Mg)3(Si2O5)2(OH)2, and SiO2. First, ammonium sulfate was gradually decomposed into a gas mixture (SO2, NH3,N2, SO2, and H2O). The thermal decomposition stages can be represented by Equations (1)–(3) [41]:

(NH4)2SO4 = NH4HSO4 + NH3 (1)

2NH4HSO4 = (NH4)2S2O7 + H2O (2)

3(NH4)2S2O7 = 2NH3 + 2N2 + 6SO2 + 9H2O (3)

Meanwhile, ammonium sulfate reacted with metal sulﬁdes in the presence of O2. The corresponding chemical reactions can be summarized as follows [20]:

4Fe7S8 + 54(NH4)2SO4 + 69O2 = 28(NH4)3Fe(SO4)3 + 24NH3 + 14H2O + 2SO2 (4)

4CuFeS2 + 22(NH4)2SO4 + 17O2 = 4(NH4)3Fe(SO4)3 + 4CuSO4 + 4NH3 + 2H2O (5)

8(Ni,Fe)9S8 + 112(NH4)2SO4 + 93O2 = 36NiSO4 + 36(NH4)3Fe(SO4)3 + 58NH3 + 29H2O (6)

2FeS2 + 3(NH4)2SO4 + 7O2 = 2(NH4)3Fe(SO4)3 + 2SO2 (7) Metals 2019, 9, x FOR PEER REVIEW 8 of 15 Metals 2019, 9, x FOR PEER REVIEW 8 of 15 3.6. The Transformation Behaviors of the Crystal Phase and Microstructure of Roasting Products 3.6. The Transformation Behaviors of the Crystal Phase and Microstructure of Roasting Products Recently, studies of the sulfation roasting of nickel sulfides in air revealed strong additive- Recently, studies of the sulfation roasting of nickel sulfides in air revealed strong additive- dependent behavior [19,21,39]. In order to gain insight into the sulfation roasting process, the dependent behavior [19,21,39]. In order to gain insight into the sulfation roasting process, the microscopic morphology and crystal structure of the roasting products were investigated thoroughly microscopic morphology and crystal structure of the roasting products were investigated thoroughly and systematically by XRD (Figure 8) and SEM (Figure 9), respectively, which was conducive to and systematically by XRD (Figure 8) and SEM (Figure 9), respectively, which was conducive to Metalsanalysis2019 of, 9 ,th 1256e behavior of ammonium sulfate and to deducing the reaction mechanism of ammonium9 of 15 analysis of the behavior of ammonium sulfate and to deducing the reaction mechanism of ammonium sulfate roasting. sulfate roasting.

Figure 8. The XRD phase identification of roasted products at different temperatures (adding mass Figure 8.8. The XRD phase identiﬁcationidentification of roasted productsproducts atat didifferentﬀerent temperaturestemperatures (adding(adding massmass ratio of ammonium sulfate to NOSMO concentrate sample: 200%, heating rate: 10 °C/min, holding ratioratio ofof ammoniumammonium sulfatesulfate toto NOSMONOSMO concentrateconcentrate sample:sample: 200%,200%, heatingheating rate:rate: 1010 ◦°C/min,C/min, holdingholding time: 2 h): (aa) 450 °C , (bb) 550 °C , (cc) 650 °C , and (dd) 750 °C . time:time: 2 h): ( a)) 450 °C,◦C, ( b)) 550 550 °C,◦C, ( (c)) 650 650 °C,◦C, and and ( (d)) 750 750 °C.◦C.

Figure 9. EffectEﬀect of temperatures on the micromorphology of roasted products (static air air atmosphere, Figure 9. Effect of temperatures on the micromorphology of roasted products (static air atmosphere, addition of 200% ammonium sulfate, heating at 10 ◦°CC//min,min, and holding for 2 h at each temperature). addition of 200% ammonium sulfate, heating at 10 °C/min, and holding for 2 h at each temperature).

As shown in Equations (4) and (7), ammonium sulfate can react with iron sulﬁdes to produce SO2 gas, and the intermediate ammonium ferric sulfate ((NH4)3Fe(SO4)3) can be decomposed further to generate SO3 gas by Equation (8) [40]:

2(NH4)3Fe(SO4)3 = Fe2(SO4)3 + 4NH3 + 2SO3 + 3H2O (8) Metals 2019, 9, 1256 10 of 15

Additionally, polymetallic sulﬁdes can be decomposed into monometallic sulﬁdes, for instance, Cu2S, NiS, and Ni3S4. This has been systematically studied and proved by previous work [47], and the reaction equations were deduced and shown as follows:

2CuFeS2 + 6O2 = Cu2S + Fe2(SO4)3 (9)

8(Ni,Fe)9S8 + 51O2 + 38SO3 = 12Ni3S4 + 18Fe2(SO4)3 (10)

Ni3S4 + O2 = 3NiS + SO2 (11)

Particularly, the (Ni,Mg)3Si2O5(OH)4 disappeared at 450 ◦C, while (Ni,Mg)3(Si2O5)2(OH)2 remained. It is suggested that (Ni,Mg)3Si2O5(OH)4 may have reacted with (NH4)3Fe(SO4)3, as illustrated by Equation (12):

2(Ni,Mg)3Si2O5(OH)4 + 6(NH4)2SO4 = 3NiSO4 + 3MgSO4 + 4SiO2 + 12NH3 + 10H2O (12)

Based on Equations (1)–(8), we conclude that the direct reaction between ammonium sulfate and metal sulfides can result in the formation of intermediate ammonium ferric sulfate and SO2 gas. Because the decomposition temperature of (NH4)3Fe(SO4)3 might be higher than that of pure ammonium sulfate, there may be a higher concentration of SO2/SO3 gas on further sulfation interface. For that reason, the sulfation of nickel and copper can be improved by ammonium sulfate activation roasting. In addition, the oxidation of metal sulfides could also occur in this process, and the reaction paths were studied and presented in our previous work [39]. As the temperature increased to 550 ◦C, the roasting product mainly consisted of Fe2(SO4)3, CuSO4, NiSO4, MgSO4, Cu2S, NiS, Fe3O4, Fe2O3, (Ni,Mg)3(Si2O5)2(OH)2, and SiO2 (Figure8b). The decomposition of (NH4)3Fe(SO4)3 and Fe2(SO4)3 during this process may result in increasing Fe2O3 and SO2, and, thus, higher pressure of SO2 on the reaction interface could accelerate the sulfation of nickel and copper sulfides. This shows good agreement with the water leaching results in Figure4. When the temperature increased to 650 ◦C (Figure8c), the NiS was turned into NiSO 4 and NiFe2O4 (or NiO), while Cu2S was almost entirely transformed into CuSO4. This means that the sulfation of nickel sulfide is more difficult than that of copper sulfide. Meanwhile, the (Ni,Mg)3Si2O5(OH)4 disappeared at 650 ◦C (Figure8c). This finding can be explained by the possibility that Fe 2(SO4)3 can react with (Ni,Mg)3Si2O5(OH)4, as illustrated by Equation (13):

2(Ni,Mg)3Si2O5(OH)4 + 2Fe2(SO4)3 = 3NiSO4 + 3MgSO4 + 2Fe2O3 + 4SiO2 + 4H2O (13)

However, the peaks of NiSO4 and MgSO4 were not detected in Figure8d when the roasting temperature further increased to 750 ◦C. This may be attributed to the decomposition of sulfates at a higher temperature. Combined with Figures4 and8d, it can be concluded that the sulfation of nickel and copper sulfide can be suppressed by excessive temperature. For that reason, the sulfation temperature should be kept at 650 ◦C or below. As demonstrated in Figure9, the SEM images of the roasted products show great diversity among different temperatures, ranging from 300 to 700 ◦C. The surface of the roasted product was characterized by floccules at 300 ◦C, which is attributed to the decomposition of the ammonium sulfate additive. When the roasting temperature increased to 500 ◦C, the flocculent-shaped surface appearance gradually faded away. This result shows excellent agreement with the TG–DSC test in Figure3. As the roasting temperature rose to the range between 500 and 650 ◦C, the roasted products gradually became loose and porous. This is owing to inner diffusion of O2 and outer release of SO2/SO3 during inner oxidation of the sulfide ore particles. When the roasting temperature was above 700 ◦C, the surface of the roasted products presented a characterization of a molten sintering state (Figure9). It may be that large amounts of eutectic sulfates (CuSO4 and NiSO4) formed at a higher roasting temperature ( 700 C). ≥ ◦ Metals 2019, 9, 1256 11 of 15

3.7. The Sulfation Mechanism for the NiS and Cu2S Surfaces by DFT Studies As mentioned, ammonium sulfate can promote the sulfation of nickel and copper by direct reaction and SO2 gas decomposition. The indirect reaction plays a significant role in the sulfation of NiS and Cu2S, both of which are the intermediate products during the NOSMO roasting process. Thus, in an effort to increase our understanding of the sulfation route and behavior, DFT was used to calculate the interaction between O2/SO2 and metal sulfide intermediates. As shown in Figure 10, the (100) surface of NiS was adopted to investigate the sulfation mechanism. The (100) slab of NiS contains five layers, with the bottom two layers fixed and the top three layers relaxed. After the optimization of the clean surface, the oxygen dissociative adsorption on the NiS (100) surface was studied, as shown in Figure 10c. The two oxygen atoms bind with two nickel atoms, leading to an adsorption energy of 1.81 eV. The adsorbed oxygen atoms provide the sites for SO − 2 interaction. The interaction between SO2 and the NiS (100) surface via two paths was studied; these are clearly shown in Figure 10c. In Path 1, SO2 binds with only one adsorbed oxygen atom with an S–O bond length of 1.59 Å, resulting in a thermodynamically more stable structure with adsorption energy of 0.75 eV. Based on the formed SO 2 , the desorption of SO was further explored. The SO − 3 − 3 3 desorption leads the relative energy to be more positive by 1.26 eV, implying that this process is exothermic. In addition, the adsorption energy of SO3 on the adsorbed oxygen atom of the NiS (100) 2 surface was predicted to be 3.23 eV, finally leading to the formation of SO4−. In Path 2, SO2 directly − 2 Metalsinteracts 2019, with9, x FOR two PEER oxygen REVIEW atoms, leading to the formation of SO4− directly. 11 of 15

Figure 10. (a) Bulk structure of NiS. (b) The (100) surface of NiS (the top image represents the top view, Figureand the 10. bottom (a) Bulk image structure represents of NiS the. ( sideb) The view (100) of thesurface NiS (100)of NiS surface). (the top (c )image Sulfation represents mechanism the top for viewthe NiS, and (100) the bottom surface. image represents the side view of the NiS (100) surface). (c) Sulfation mechanism for the NiS (100) surface.

The sulfation mechanism of Cu2S was also investigated through O2 and SO2 adsorption on the The sulfation mechanism of Cu2S was also investigated through O2 and SO2 adsorption on the (001) surface of Cu2S. Figure 11b,c shows the top view and the side view of the (001) surface of Cu2S, (001)respectively, surface of showing Cu2S. Figure that the 11b,c copper shows atoms the top of the view ﬁrst and layer the are side all view equivalent of the (001) and thesurface bottom of Cu three2S, respectively, showing that the copper atoms of the first layer are all equivalent and the bottom three layers are ﬁxed. After the relaxation, oxygen dissociation on the (001) surface of Cu2S leads to an layersadsorption are fixed. energy After of the1.09 relaxation, eV with a Cu–Ooxygen bond dissociation length of on 1.81 the Å. (001) Similar surface to the of sulfation Cu2S leads process to an of adsorption energy of –−1.09 eV with a Cu–O bond length of 1.81 Å . Similar to the sulfation process of NiS, the sulfation mechanism of Cu2S was also elucidated in two paths. In Path 1, the SO2 interacts NiS, the sulfation mechanism of Cu2S was also elucidated in two paths. In Path 1, the SO2 interacts with the Cu2S surface through the adsorbed oxygen atom, leading to an adsorption energy drop of – 0.77 eV, with an S–O bond length of 1.58 Å . When the adsorbed species of SO2 and the adsorbed oxygen atom desorb simultaneously from the Cu2S (001) surface, the adsorption energy becomes more positive, indicating that this process is also exothermic. Eventually, the SO3 adsorption leads to 2 the formation of the SO4 species on the Cu2S (001) surface. In Path 2, when the SO2 directly interacts with the two adsorbed oxygen atoms, can also be formed successfully.

Metals 2019, 9, 1256 12 of 15

with the Cu2S surface through the adsorbed oxygen atom, leading to an adsorption energy drop of 0.77 eV, with an S–O bond length of 1.58 Å. When the adsorbed species of SO and the adsorbed − 2 oxygen atom desorb simultaneously from the Cu2S (001) surface, the adsorption energy becomes more positive, indicating that this process is also exothermic. Eventually, the SO3 adsorption leads to the 2 formation of the SO4− species on the Cu2S (001) surface. In Path 2, when the SO2 directly interacts 2 Metalswith the2019, two 9, x FOR adsorbed PEER REVIEW oxygen atoms, SO4− can also be formed successfully. 12 of 15

Figure 11. (a) Bulk structure of Cu S. (b) Top view and (c) side view of the (001) surface of Cu S. Figure 11. (a) Bulk structure of Cu2S2. (b) Top view and (c) side view of the (001) surface of Cu2S. (d2 ) (d) Sulfation mechanism for the Cu S (001) surface. Sulfation mechanism for the Cu2S (001)2 surface. 4. Conclusions 4. Conclusions Ammonium sulfate activation roasting-water leaching was studied as a suitable and green Ammonium sulfate activation roasting-water leaching was studied as a suitable and green technique to extract nonferrous metals from NOSMO concentrate. The TG–DSC tests indicated that technique to extract nonferrous metals from NOSMO concentrate. The TG–DSC tests indicated that the decomposition of ammonium sulfate and sulfation of NOSMO concentrate was consistent in the decomposition of ammonium sulfate and sulfation of NOSMO concentrate was consistent in terms of the temperature range. The effects of several parameters, including the roasting temperature, terms of the temperature range. The effects of several parameters, including the roasting temperature, dosage of ammonium sulfate additive, heating rate, and holding time, were investigated by the control dosage of ammonium sulfate additive, heating rate, and holding time, were investigated by the variable method. The results demonstrated that 70% Ni, 89% Co, 90% Cu, and 2% Fe were recovered control variable method. The results demonstrated that 70% Ni, 89% Co, 90% Cu, and 2% Fe were under appropriate conditions (adding ammonium sulfate at a mass ratio of 200%, heating to 650 ◦C recovered under appropriate conditions (adding ammonium sulfate at a mass ratio of 200%, heating at 10 ◦C/min, and holding for 120 min). It was found that ammonium sulfate cannot only directly to 650 °C at 10 °C /min, and holding for 120 min). It was found that ammonium sulfate cannot only react with metal sulfides at a lower roasting temperature but can also promote the sulfation of metal directly react with metal sulfides at a lower roasting temperature but can also promote the sulfation sulfides by increasing the interface partial pressure of SO2 at a higher temperature. This increase in of metal sulfides by increasing the interface partial pressure of SO2 at a higher temperature. This SO2 can be attributed to the decomposition of ammonium sulfate and intermediate ammonium ferric increase in SO2 can be attributed to the decomposition of ammonium sulfate and intermediate sulfate. In addition, DFT calculations were performed, focusing on the respective detailed sulfation ammonium ferric sulfate. In addition, DFT calculations were performed, focusing on the respective mechanisms of NiS and Cu2S. Based on our calculation results, sulfation of both NiS and Cu2S can be detailed sulfation mechanisms of NiS and Cu2S. Based on our calculation results, sulfation of both achieved by the interaction between O2 and SO2. When the SO2 directly interacts with two oxygen NiS and Cu2S can be achieved by the interaction between O2 and SO2. When the SO2 directly interacts atoms, sulfation can be achieved directly. Besides this, sulfation can also be promoted by the formation with two oxygen atoms, sulfation can be achieved directly. Besides this, sulfation can also be of SO3 species. It is concluded that both paths can promote the sulfation of NiS and Cu2S, which are promoted by the formation of SO3 species. It is concluded that both paths can promote the sulfation both thermodynamically favored. of NiS and Cu2S, which are both thermodynamically favored.

Funding: This work was financially supported by the China Postdoctoral Science Foundation, the Super Postdoctoral Incentive Program in Shanghai, the Steel Joint Research Foundation of National Natural Science Foundation of China–China Baowu Iron and Steel Group Co. Ltd. (Grant No. U1860203), the National Natural Science Foundation of China (Grant No. 51576164), National Basic Research Program of China (973 Program) (Grant 2014CB643403), and the CAS Interdisciplinary Innovation Team.

Conflicts of Interest: The authors declare no conflict of interest.

Metals 2019, 9, 1256 13 of 15

Author Contributions: G.L., X.X., and X.L. conceived of and designed the experiments; G.L. performed the experiments and analyzed the data; X.X. and S.L. conducted the DFT calculations; L.W. (Liping Wang), L.C., and L.W. (Lizhen Wei) participated in the roasting and leaching experiments; H.C., Q.X., X.Z., and Z.Z. contributed reagents/materials/analysis tools; G.L. and X.X. wrote this paper. Funding: This work was financially supported by the China Postdoctoral Science Foundation, the Super Postdoctoral Incentive Program in Shanghai, the Steel Joint Research Foundation of National Natural Science Foundation of China–China Baowu Iron and Steel Group Co. Ltd. (Grant No. U1860203), the National Natural Science Foundation of China (Grant No. 51576164), National Basic Research Program of China (973 Program) (Grant 2014CB643403), and the CAS Interdisciplinary Innovation Team. Conflicts of Interest: The authors declare no conflict of interest.

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