Recovery of Iron from Copper Slag Using Coal-Based Direct Reduction: Reduction Characteristics and Kinetics

Article Recovery of Iron from Copper Slag Using Coal-Based Direct Reduction: Reduction Characteristics and Kinetics

Hanquan Zhang 1, Chaojie Hu 1, Wangjie Gao 1 and Manman Lu 2,*

1 School of Resources and Safety Engineering, Wuhan Institute of Technology, Wuhan 430205, China; [email protected] (H.Z.); [email protected] (C.H.); [email protected] (W.G.) 2 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China * Correspondence: [email protected]

Received: 9 September 2020; Accepted: 29 October 2020; Published: 31 October 2020

Abstract: The Fe3O4 and Fe2SiO4 in copper slag were successfully reduced to metallic iron by coal-based direct reduction. Under the best reduction conditions of 1300 ◦C reduction temperature, 30 min reduction time, 35 wt.% coal dosage, and 20 wt.% CaO dosage (0.75 binary basicity), the Fe grade of obtained iron concentration achieved 91.55%, and the Fe recovery was 98.13%. The kinetic studies on reduction indicated that the reduction of copper slag was controlled by the interfacial reaction and carbon gasiﬁcation at 1050 ◦C. When at a higher reduction temperature, the copper slag reduction was controlled by the diﬀusion of the gas. The integral kinetics model research illustrated that the reaction activation energy increased as the reduction of copper slag proceeded. The early reduction of Fe3O4 needed a low reaction activation energy. The subsequent reduction of Fe2SiO4 needed higher reaction activation energy compared with that of Fe3O4 reduction.

Keywords: copper slag; direct reduction; reduction kinetics; pellet

1. Introduction In China’s copper industry, pyrometallurgy technology is presently the primary process for crude copper production [1,2]. In the copper pyrometallurgy process, 2–2.5 tons (metric) of copper slag byproducts are produced for every ton of crude copper produced [3–5]. As some papers have reported, in 2019, China’s copper output reached 9.8 million tons, and more than 20 million tons of copper slag were produced [6,7]. The accumulated stockpile of copper slag over the years exceeds 120 million tons [8]. The huge stockpile of copper slag encroaches on the land and pollutes the soil and groundwater [9]. Furthermore, the copper slag also contains valuable metals including 35–45 wt.% Fe [10], which suggests it is worth recycling and utilizing as a secondary resource. On the other hand, China lacks high-quality iron ore and imports over one billion tons of iron ore per year. In 2017, China’s import dependence ratio rose to 87.6% [11]. Therefore, utilizing the domestic low-grade Fe-bearing resource is necessary and urgent. Recently, many researchers have published studies on the utilization of copper slag, for example, as a cement additive [12], for preparing abrasive tools [13], for pavement materials [14], and for preparing glasses [15]. These processes could utilize the copper slag at low cost. However, these crude utilization methods neglect the recovery of valuable metals in copper slag, resulting in a waste of resources. Furthermore, the metal elements in copper slag (Fe, Ni, Zn, Cu, etc.) may cause environmental damage [16,17]. Therefore, the recovery of valuable metals is an important issue in the utilization of copper slag. Iron is the primary valuable metal in copper slag, and it mainly exists in copper slag in the form of fayalite (2FeO SiO )[18]. As previous research has reported, fayalite has high · 2

Minerals 2020, 10, 973; doi:10.3390/min10110973 www.mdpi.com/journal/minerals Minerals 2020, 10, 973 2 of 18 refractoriness and is difficult to be reduced by a reductant like carbon monoxide or carbon [19]. Therefore, fayalite is generally considered a kind of refractory iron compound [20]. At present, the mainstream iron recovery processes from copper slag mainly include the hydrometallurgy route [21], smelting iron alloy [22], and direct reduction (DR) process [23]. However, it is inefficient to utilize copper slag by the hydrometallurgy route due to the complex mineralogical properties of copper slag [24]. Moreover, as we know, the melting point of fayalite in the copper slag is very high, so it is hard to smelt [25]. Therefore, the energy consumption of the smelting process of copper slag is very high. In comparison, the direct reduction process may be the best process for recovering iron from copper slag considering the efficiency, cost, recovery rate, and operability [26]. Many scholars have published research on the direct reduction of fayalite for the purposes of iron recovery from copper slag [27–29]. Cao et al. [30] adopted rotary hearth furnaces to conduct the direct reduction process for the iron recovery from copper slag. Under optimum conditions of the reduction temperature of 1280 ◦C and the reduction time of 35 min, the direct reduction of iron (DRI) powder with an iron grade of 90.35% was obtained, and the iron recovery rate was 89.70%. A recent paper showed that the fayalite in the copper slag could be reduced by walnut shell char. The iron concentration with 73.20 wt.% iron grade and 95.56% recovery rate was obtained under a reduction temperature of 1300 ◦C[31]. For these reasons, Fe resources contained in copper slag can be recycled efficiently by the direct reduction method. The obtained metallic iron produced, with more 90% iron content, is an excellent raw material for the iron and steel industry [32]. Based on the above analysis, there is a lack of study on the reduction kinetics of copper slag. As the published papers report, the iron in copper slag mainly exists in the form of fayalite [33]. The structure and property of the fayalite are very stable during the reduction process [34], making it difficult to reduce to metallic iron. The reduction kinetics determines the efficiency and energy consumption of the reduction process and the direct reduction of iron (DRI) product quality. In this study, the effects of direct reduction temperature and time, coal dosage, and CaO dosage on the direct reduction–magnetic separation process were investigated. The kinetics of direct reduction was clarified by kinetic model calculations.

2. Materials and Methods

2.1. Materials The copper slag used in this study came from Tongling Nonferrous Metals Group Holdings Co., Ltd. (Tongling, China). About 200 kg samples were collected from the different areas of the copper slag storage bin. The 200 kg samples were thoroughly mixed and bagged to ensure that the sample taken in each experiment was representative. The obtained copper slag was crushed into a fine powder with particle size less than 2 mm by a roll crushing mill. Subsequently, 1 g of the crushed copper slag powder continued to be ground to a particle size less than 30 µm in an agate mortar and was used to conduct the XRD test. The XRD spectrogram was recorded on Bruker AXS D8 advanceX diffractometer (Bruker, Billerica, MA, USA) using the Cu Kα. The scanning range of the 2θ angle was from 10◦ to 70◦, and the scanning step and measuring time were 0.02◦ and 0.2 s/step, respectively. The XRD pattern of copper slag is displayed in Figure1. Scanning electron microscope (SEM) (Quanta 200, FEI Corporation, Hillsboro, OR, USA) analysis was adopted to observe the micro appearance of copper slag, and energy-dispersive spectrometer (EDS) (Quanta 200, FEI Corporation) analysis was used to measure the atom content on the sample’s surface. Thirty milligrams of copper slag sample were applied to the conductive adhesive and put on the sample stage. The excess sample powder was purged by nitrogen. Then, a copper slag sample was observed by the SEM. The micrography and EDS analyses of copper slag are shown in Figure2. Minerals 2020, 10, 973 3 of 18 MineralsMinerals 2020 2020, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 33 of of 17 17

FigureFigure 1.1. The The XRD XRD pattern pattern of of copper copper slag. slag.

FigureFigure 2. 2. The TheThe SEM SEMSEM micrograph micrographmicrograph of of copper copper slag slag ( (aa),), the the EDS EDS result result of of point point A A ( (bb),), the the EDS EDSEDS result result of of point point B(BB ( (cc),), and and the the EDS EDS result result of of pointpoint point CC C (( d(d).).).

Additionally,Additionally, the the main main chemical chemical compositioncomposition ofof coppercopper slagslag andand bentonitebentonite waswas measuredmeasured byby anan X-rayX-ray fluorescence ﬂuorescencefluorescence spectrometer spectrometer (XRF, (XRF, Axios Axios Max, Max, Max, Ma Ma Malvernlvernlvern Panalytical, Panalytical, Panalytical, Malvern, Malvern, Malvern, UK). UK). UK). Five FiveFive grams grams grams of of ofthethe the samplesample sample waswas was firstfirst ﬁrst drieddried dried andand and ground.ground. ground. Then,Then, Then, thethe sa thesamplemple sample waswas was putput in putin aa moldinmold a mold andand tabletedandtableted tableted underunder under 20002000 2000MPaMPa MPapressurepressurepressure forfor 11 for min.min. 1 min. TheThe The samplesample sample tablettablet tablet waswas was usedused used toto to conductconduct conduct thethe the XRFXRF XRF test. test. TheThe detailed detailed analysisanalysis resultsresults ofof coppercopper slag,slag, coalcoal andand bentonite’sbentonite’s chemical chemicalchemical compositions compositionscompositions are are listed listed in in Tables Tables 1 1–3,1–3,–3, respectively. respectively.respectively. TheThe proximateproximate analysisanalysis ofof coalcoal waswas conductedconducted usingusing aa coalcoal proximateproximate analyzeranalyzer (5E-MAG6600,(5E-MAG6600, KaiyuanKaiyuan company,company, Changsha,Changsha, China).China). OneOne gramgram ofof coalcoal samplesample waswas putput intointo aa corundumcorundum crucible.crucible.

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The proximate analysis of coal was conducted using a coal proximate analyzer (5E-MAG6600, Kaiyuan company, Changsha, China). One gram of coal sample was put into a corundum crucible. The crucible was placed on a thermo-balance and heated to 1300 ◦C to detect the moisture, volatiles, ash, ﬁxed carbon, and sulfur contents of the coal sample. The detailed results are shown in Table2.

Table 1. Main chemical composition of copper slag (wt.%).

Fe2O3 CuO SiO2 Al2O3 CaO MgO Na2OK2O SO3 Cr2O3 68.20 0.98 24.72 3.38 0.30 0.72 0.48 0.65 0.40 0.08

Table 2. The proximate analysis of coal (wt.%).

Moisture Volatiles Ash Fixed Carbon Sulfur 7.65 28.67 10.08 52.39 0.64

Table 3. The chemical composition of bentonite (wt.%).

SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 SO3 57.32 15.1 7.29 3.41 2.72 1.23 1.21 1.39 0.02

P2O5 MnO ZnO SrO ZrO2 BaO Cl LOI 0.38 0.1 0.011 0.033 0.027 0.053 0.022 9.51

The diffraction peaks shown in Figure1 indicate that the iron in copper slag primarily exists in the form of fayalite and magnetite. There were also many stray interferential peaks near the baseline. The primary reason is that the copper slag adopted in this study is quenched slag. During the quenching process, some minerals are incompletely crystallized, forming the amorphous glass-phase substance. The stray interferential peaks belong to the glassy materials. The chemical composition analysis of copper slag shows that the iron grade of this copper slag sample is quite high, reaching 47.81 wt.%. The SiO2 content of the copper slag was 24.72 wt.%, indicating that this copper slag sample belonged to acid slag. Some extra CaO additive was needed to adjust the basicity in the direct reduction experiments [35]. The micrography of copper slag shown in Figure2 indicates that the particle size of copper slag was about 3–10 µm after grinding, which is favorable for the subsequent reduction roasting. The energy-dispersive spectrometer (EDS) analyses showed that the main elements of copper slag were Fe and Si. The EDS results showed that the fayalite, magnetite, and quartz in the copper slag were tightly bound together. It is difficult to separate them using conventional mineral processing methods. The SEM–EDS mapping shown in Figure3 shows that the Fe was evenly distributed inside the copper slag. A considerable part of the Fe also overlapped with Si in the mapping area, indicating that this part of Fe was closely combined with Si and belonged to the Fe2SiO4. In the direct reduction experiments, the additives include reductant coal, flux CaO, and binder bentonite. The additives used in this research were obtained from the Baowu Steel company (Wuhan, China). The physicochemical properties of coal and bentonite are shown in Tables2 and3, respectively. The results shown in Table2 indicate that the content of the volatile matters is quite high. The high content of volatile matter in the coal can improve the reduction process. The fixed carbon content of the coal is 52.39 wt.%, which is the main reductant in the direct reduction process. Bentonite is used as a binder to increase the mechanical strength of copper slag pellets in the pelletizing process. The main components of bentonite are SiO2 and Al2O3. Minerals 2020, 10, 973 5 of 18 Minerals 2020, 10, x FOR PEER REVIEW 5 of 17

FigureFigure 3.3. The SEM-EDS mapping ofof coppercopper slagslag sample:sample: (a) SEM micrograph; ((bb)) SEM-EDSSEM-EDS mappingmapping micrograph;micrograph; ( c)) Fe Fe element element distribution; distribution; (d ()d Si) Sielement element distribution; distribution; (e) (Ale) element Al element distribution; distribution; and and(f) Ca (f) element Ca element distribution. distribution. (a–f) ( ascale–f) scale bar is bar 3 μ ism. 3 µm. 2.2. Experimental Methods 2.2. Experimental Methods As shown in Figure4, the mixture of copper slag, CaO, coal, and bentonite were used for pelletizing. As shown in Figure 4, the mixture of copper slag, CaO, coal, and bentonite were used for The dosage of bentonite was 4.0 wt.%, and the dosages of CaO and coal depended on the conditions in pelletizing. The dosage of bentonite was 4.0 wt.%, and the dosages of CaO and coal depended on the the experiments. The pellets with diameters of 8–16 mm were sifted out as the qualiﬁed pellets. Then, conditions in the experiments. The pellets with diameters of 8–16 mm were sifted out as the qualified the pellets were dried in an air-dry oven at 105 C. During the drying process, the pellets were weighed pellets. Then, the pellets were dried in an air-dry◦ oven at 105 °C. During the drying process, the every hour until the weight of pellets stayed constant to ensure that all free moisture in the pellets was pellets were weighed every hour until the weight of pellets stayed constant to ensure that all free removed. The coal-bearing green pellets were roasted after drying. Subsequently, the roasted pellets moisture in the pellets was removed. The coal-bearing green pellets were roasted after drying. were crushed to a size less than 2 mm using a jaw crusher. The roasted pellet particle was ground Subsequently, the roasted pellets were crushed to a size less than 2 mm using a jaw crusher. The using a ball mill. The size of particles was controlled to be below 74 µm. The magnetic separation was roasted pellet particle was ground using a ball mill. The size of particles was controlled to be below conducted in a magnetic separator. The magnetic ﬁeld intensity was controlled at 80 KA/m. The direct 74 μm. The magnetic separation was conducted in a magnetic separator. The magnetic field intensity reduction of iron was obtained by the magnetic separation. was controlled at 80 KA/m. The direct reduction of iron was obtained by the magnetic separation. The reduction kinetics of copper slag was investigated by a thermo-balance, and the structure chart of the equipment is shown in Figure 5. Two hundred grams of dried pellets were placed on the salver of the balance and heated by shaft furnace. Nitrogen was used as the protective gas to avoid the oxidation of iron.

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Figure 4. The ﬂow chart of the direct reduction–magnetic separation of copper slag. Figure 4. The flow chart of the direct reduction–magnetic separation of copper slag. The reduction kinetics of copper slag was investigated by a thermo-balance, and the structure chart of the equipment is shown in Figure5. Two hundred grams of dried pellets were placed on the salver of the balance and heated by shaft furnace. Nitrogen was used as the protective gas to avoid the Figure 4. The flow chart of the direct reduction–magnetic separation of copper slag. oxidation of iron.

Figure 5. The diagram of the experimental equipment.

2.3. Reduction Kinetic Models Figure 5. The diagram of the experimental equipment. The reduction kineticsFigure 5.were The defineddiagram asof the experimentalrelationship equipment. between the reduction degree of copper slag and the reduction time [36]. The reduction degree α was obtained by the following Equation (1): 2.3. Reduction Kinetic Models WL m −m α= = (1) The reduction kinetics were defined as theWL relationshipm +m between +m the reduction degree of copper slag and the reduction time [36]. The reduction degree α was obtained by the following Equation (1):

WL m −m α= = (1) WL m +m +m

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2.3. Reduction Kinetic Models The reduction kinetics were deﬁned as the relationship between the reduction degree of copper slag and the reduction time [36]. The reduction degree α was obtained by the following Equation (1):

WLt m mt α = = 0 − (1) WL mO + mC + mV where α is the reduction degree of the copper slag; WLt is the weight loss of the copper slag pellet at time t, which is the diﬀerence between the initial weight m0 and the weight at time t mt; and WL is the theoretical weight loss during the reduction process, which is the sum of the O atom mass of iron-bearing compound (mO), the ﬁxed C mass of coal (mC), and the volatile matter mass of coal (mV). In this study, the single restricted-link kinetic models were used to analyze the reduction of iron-bearing compounds in copper slag. When the interfacial chemical reaction is the controlling step of the reduction reaction, the kinetic model can be expressed as follows [37–39]:

1 1 (1 α) 3 = kt (2) − − When the gasiﬁcation of carbon is the controlling step of the reduction reaction, the kinetic model can be expressed as follows: ln(1 α) = kt (3) − − When the diﬀusion of gas through the pellet is the controlling step of the reduction reaction, the kinetic model could be expressed as follows:

1 2 [1 (1 α) 3 ] = kt (4) − − where k is the reaction constant, t is time, and α is the reduction degree. The reaction activation energy was calculated according to the Arrhenius Equation, which can be expressed as follows [40]: Eα k = Aexp( ) (5) − RT Taking the logarithm of Equation (5), the linear formula of the Arrhenius Equation is expressed as follows: Eα 1 lnk = + lnA (6) − R × T 1 1 where A is the pre-exponential factor, s ;Eα is the reaction activation energy, J mol ; R is the ideal − · − gas constant 8.314 J mol 1 K 1; T is the temperature, K. · − · − In order to investigate the reduction kinetics of copper slag in the diﬀerent stages of the copper slag reduction process, integral kinetics research was introduced into this study. The details of this method are shown below: The relationship between α and t can be expressed as Equation (7):

dα = kf(α) (7) dt Substituting Equation (5) into Equation (7) and conducting integration, we could obtain the following Equation (8): Z Z 1 1 dt = dα (8) Aexp Eα f(α) − RT Minerals 2020, 10, 973 8 of 18

The Equation (8) could transfer to the following formula: Constant Eα tα = exp (9) A RT

Taking the logarithm of the Equation (9), the linear formula could be expressed by the following Equation (10): Constant Eα lntα = ln + (10) A RT 1 1 where A is the pre-exponential factor, s ;Eα is the reaction activation energy, J mol ; R is the ideal − · − gas constant 8.314 J mol 1 K 1; T is the temperature, K. · − · − 3. Results and Discussion

3.1. Thermomechanical Analysis The thermodynamics of the reduction reactions within a certain temperature range is an important indicator to judge whether the reaction can proceed. In the reduction roasting process, the reactions primarily included the carbon gasiﬁcation and the reduction of iron-bearing compounds. The reaction and thermodynamic Equations are as follows [30,36,41]:

CO2 + C = 2CO θ 1 (11) ∆G = 166550 171T J mol− 11 − − ·

Fe3O4 + C = 3FeO + CO θ 1 (12) ∆G = 196720 199.38T J mol− 12 − · FeO + C = Fe + CO θ 1 (13) ∆G = 149600 150.36T J mol− 13 − ·

Fe2SiO4 + 2C = 2Fe + SiO2 + 2CO θ 1 (14) ∆G = 353924 338.91T J mol− 14 − ·

Fe3O4 + CO = 3FeO + CO2 θ 1 (15) ∆G = 30170 29.38T J mol− 15 − ·

FeO + CO = Fe + CO2 θ 1 (16) ∆G = 16950 + 20.64T J mol− 16 − ·

Fe2SiO4 + 2CO = 2Fe + SiO2 + 2CO2 θ 1 (17) ∆G = 12510 + 10.54T J mol− 17 ·

Fe2SiO4 + CaO = 2FeO + CaSiO3 θ 1 (18) ∆G = 82600 32.39T J mol− 18 − − · The Gibbs equation lines of the reactions were showed in Figure6. From Equations (11)–(18), it can be found that under experimental temperature conditions (1050–1300 ◦C), Fe3O4 and FeO can both be reduced by carbon and carbon monoxide. Equation (17) shows that Fe2SiO4 cannot be reduced directly by carbon monoxide, but in the presence of CaO, Fe2SiO4 can decompose easily and produce FeO, which can be reduced by carbon and carbon monoxide simultaneously. Equation (14) indicates that Fe2SiO4 can be reduced by carbon directly. However, this reaction proceeds very slowly on the kinetic level. Therefore, it can be inferred that carbon monoxide was still the primary reductant of the copper slag reduction. The above thermodynamic analysis indicates that the iron-bearing compound in the copper slag can be reduced to metallic iron by carbon and carbon monoxide. Furthermore, the premise that the Gibbs function can be established is when ∆Cp is zero. Therefore, the above analyses of the Minerals 2020, 10, x FOR PEER REVIEW 8 of 17

3. Results and Discussion

3.1. Thermomechanical Analysis The thermodynamics of the reduction reactions within a certain temperature range is an important indicator to judge whether the reaction can proceed. In the reduction roasting process, the reactions primarily included the carbon gasification and the reduction of iron-bearing compounds. The reaction and thermodynamic Equations are as follows [30,36,41]:

CO +C=2CO (11) ΔG = −166550 − 171T J · mol

Fe0 +C=3FeO+CO (12) ΔG = 196720 − 199.38T J · mol

FeO + C = Fe + CO (13) ΔG = 149600 − 150.36T J · mol

FeSiO +2C=2Fe+SiO +2CO (14) ΔG = 353924 − 338.91T J · mol

Fe 0 + CO = 3FeO + CO (15) ΔG = 30170 − 29.38T J · mol

FeO + CO = Fe + CO (16) ΔG = −16950 + 20.64T J · mol FeSiO +2CO=2Fe+SiO +2CO (17) Minerals 2020, 10, 973 9 of 18 ΔG = 12510 + 10.54T J · mol

FeSiO +CaO=2FeO+CaSiO Gibbs Equations were under theoretical conditions. In the real reaction process, whether a reaction(18) ΔG = −82600 − 32.39T J · mol can occur also depends on the system pressure, reactant concentration, reaction rate, etc. Therefore, the reaction kinetic model is an important tool to investigate the reaction process. The thermodynamic lines of each reduction reaction are shown in Figure 6.

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From Equations (11)–(18), it can be found that under experimental temperature conditions (1050–1300 °C), Fe3O4 and FeO can both be reduced by carbon and carbon monoxide. Equation (17) shows that Fe2SiO4 cannot be reduced directly by carbon monoxide, but in the presence of CaO, Fe2SiO4 can decompose easily and produce FeO, which can be reduced by carbon and carbon monoxide simultaneously. Equation (14) indicates that Fe2SiO4 can be reduced by carbon directly. However, this reaction proceeds very slowly on the kinetic level. Therefore, it can be inferred that carbon monoxide was still the primary reductant of the copper slag reduction. The above thermodynamic analysis indicates that the iron-bearing compound in the copper slag can be reduced to metallic iron by carbon and carbon monoxide. Furthermore, the premise that the Gibbs function can be established is when ΔCp is zero. Therefore, the above analyses of the Gibbs Equations were under theoretical conditions. In the real reaction process, whether a reaction can occur also depends on the system pressure, reactant concentration, reaction rate, etc. Therefore, the reaction kinetic model is an importantFigure toolFigure to 6. investigate6.The The thermodynamic thermodynamic the reaction lines lines process. of of each each reduction reduction reaction. reaction.

3.2. The Direct Reduction Characteristics 3.2.1. The Effects of Reduction Temperature and Time 3.2.1. The Effects of Reduction Temperature and Time As can be seen that from the above thermomechanical analysis, the reduction temperature is an As can be seen that from the above thermomechanical analysis, the reduction temperature is an important factor for direct reduction. The Fe3O4 and Fe2SiO4 could be reduced to metallic iron in important factor for direct reduction. The Fe3O4 and Fe2SiO4 could be reduced to metallic iron in a a certain temperature range. The reduction time also influences the reduction effect of copper slag. certain temperature range. The reduction time also influences the reduction effect of copper slag. The The Fe grade and recovery are listed in Figure7 under di fferent reduction temperatures and times. Fe grade and recovery are listed in Figure 7 under different reduction temperatures and times. The The coal and CaO dosage were set at 35 wt.% and 20 wt.%, respectively. The Fe grade refers to the iron coal and CaO dosage were set at 35 wt.% and 20 wt.%, respectively. The Fe grade refers to the iron content in the obtained direct reduction iron, and the Fe recovery is the ratio of iron content in direct content in the obtained direct reduction iron, and the Fe recovery is the ratio of iron content in direct reduction iron to that in copper slag. reduction iron to that in copper slag.

Figure 7. The effect eﬀect of reduction temperature and time on Fe grade ((aa)) andand recoveryrecovery ((bb).).

As shown shown in in Figure Figure 7a,7a, the the increasing increasing reduction reduction temperature temperature significantly signiﬁcantly improved improved the Fe the grade Fe gradeof the ofconcentrate. the concentrate. The higher The highertemperature temperature made Fe made2SiO Fe4 and2SiO Fe4 3andO4 be Fe reduced3O4 be reduced rapidly rapidlyby carbon by and carbon monoxide. On the other hand, the FeO was reduced to metallic Fe under a high temperature. The formation of metallic Fe greatly increased the Fe grade of the concentrate. From the Figure 7b, it was found that the Fe recovery increased first and then declined. Excessively extending the reduction time would cause the generated metallic Fe, CaSiO3, and glass-phase gangue to be intercalated and adhered to each other. The metallic iron wrapped in the glass phase was difficult to dissociate, leading to the decline of the Fe recovery. The optimum conditions were determined as 1300 °C and 30 min from the above results. Under this condition, the max Fe grade and recovery were 91.55% and 98.14%, respectively.

Minerals 2020, 10, 973 10 of 18 carbon and carbon monoxide. On the other hand, the FeO was reduced to metallic Fe under a high temperature. The formation of metallic Fe greatly increased the Fe grade of the concentrate. From the Figure7b, it was found that the Fe recovery increased ﬁrst and then declined. Excessively extending the reduction time would cause the generated metallic Fe, CaSiO3, and glass-phase gangue to be intercalated and adhered to each other. The metallic iron wrapped in the glass phase was diﬃcult to dissociate, leading to the decline of the Fe recovery. The optimum conditions were determined as 1300 ◦C and 30 min from the above results. Under this condition, the max Fe grade and recovery were Minerals91.55% 2020 and, 10 98.14%,, x FOR PEER respectively. REVIEW 10 of 17

3.2.2.3.2.2. The The Effect Effect of of Coal Coal Dosage Dosage TheThe reductantreductant dosagedosage is is crucial crucial to theto copperthe copper slag reductionslag reduction process. process. During During the reduction the reduction roasting, roasting,the carbon the gasified carbon and gasified produced and carbonproduced monoxide, carbon thusmonoxide, keeping thus a reducing keeping atmosphere a reducing inatmosphere the reactor. inThe the e ffreactor.ects of The the coaleffects dosage of the oncoal the dosage Fe grade on th ande Fe recovery grade and are recovery shown inare Figure shown8. inThe Figure reduction 8. The reductiontemperature temperature and time and were time set atwere 1300 set◦ Cat and1300 30 °C min, and respectively,30 min, respectively, and the CaOand the dosage CaO wasdosage set wasat 20 set wt.%. at 20 wt.%.

FigureFigure 8. 8. TheThe effects eﬀects of of coal coal dosage dosage on on the the Fe Fe grade grade and and recovery. recovery.

InIn order order to to keep keep a a strong strong reducing reducing atmosphere atmosphere in in the the roasting roasting furnace, furnace, an anexcessive excessive amount amount of reductantof reductant is generally is generally added. added. The The bituminous bituminous used used in inthis this study study began began to to decompose decompose at at 450 450 °C,◦C, therefore,therefore, at at the the experimental temperature, temperature, coal coal had decomposed and and produced sufficient sufficient reducing gas.gas. However, However, when when coal coal dosage dosage increased increased to 40 40 wt.%, wt.%, the the Fe Fe recovery recovery slightly slightly declined. declined. The The primary primary reasonreason was was that that the the unreacted unreacted coal coal powder powder remained remained inside inside the the pellet, pellet, preventing preventing fine fine metallic metallic iron iron grainsgrains from from aggregating and growing. The The residual residual coal coal powder powder and and gasifica gasificationtion products products of of coal couldcould be be removed removed in the the magnetic magnetic separation proces process.s. Furthermore, some carbon could combine with with ironiron to to form non-magnetic iron iron ca carbiderbide lost lost in in the magnetic separation. separation. Therefore, Therefore, the the 35 35 wt.% wt.% coal coal additionaddition was the optimal condition for the copper slag direct reduction.

3.2.3.3.2.3. The The Effect Eﬀect of of CaO CaO Dosage Dosage

AsAs shown shown in in Table Table 11,, thethe CaOCaO andand SiOSiO2 contentscontents of of copper copper slag slag were were 3.41 3.41 wt.% wt.% and and 57.32 57.32 wt.%, wt.%, respectively.respectively. The The basicity basicity is is calculat calculateded based based on on the the ratio ratio of of CaO CaO and and SiO SiO22 contentcontent in in the the copper copper slag. slag. Therefore, the natural basicity of copper slag was 0.0595. Such Such a a low low basicity basicity is is disadvantageous for for thethe reduction reduction of of iron-bearing iron-bearing compounds compounds in the the coppe copperr slag [42]. [42]. Therefore, Therefore, some some extra extra CaO CaO additive additive was needed needed to to increase increase the the basicity basicity of of the the copper copper slag slag pellet. pellet. The The primary primary purpose purpose of ofadding adding CaO CaO is tois reduce to reduce the the decomposition decomposition temperature temperature of fayalite of fayalite and andincrease increase the liquid the liquid phase phase of the of system, the system, thus promotingthus promoting the separationthe separation of slag of an slagd iron and grains iron grains [42]. The [42 ].effects The e ofﬀects the ofCaO the dosage CaO dosage on the onFe thegrade Fe and recovery are shown in Figure 9. The reduction temperature and time were set at 1300 °C and 30 min, respectively, and the coal dosage was 35 wt.%. As displayed in Figure 9, the CaO dosage had little effect on the Fe grade of concentrate. This result indicated that the metallic iron had been produced with low basicity. However, the Fe recovery increased first and then declined with the increase of CaO dosage. The max Fe recovery was obtained with the 20 wt.% CaO addition. When the CaO dosage continued to increase, the calcium ferrite liquid phase was easily generated inside the copper slag pellets, which caused the metallic iron grains to be wrapped and difficult to dissociate, thus reducing the Fe recovery. The optimal CaO addition should be 20 wt.%.

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grade and recovery are shown in Figure9. The reduction temperature and time were set at 1300 ◦C andMinerals 30 2020 min,, 10 respectively,, x FOR PEER REVIEW and the coal dosage was 35 wt.%. 11 of 17

Figure 9. TheThe effects eﬀects of the CaO dosage on the Fe grade and recovery.

3.2.4.As Product displayed Analysis in Figure 9, the CaO dosage had little e ffect on the Fe grade of concentrate. This result indicated that the metallic iron had been produced with low basicity. However, the Fe Figure 10 consists of the micrographs and EDS analyses of the DRI product obtained from the recovery increased first and then declined with the increase of CaO dosage. The max Fe recovery was direct reduction–magnetic separation processes. The XRD pattern of the metallic iron product was obtained with the 20 wt.% CaO addition. When the CaO dosage continued to increase, the calcium shown in Figure 11. The chemical component of metallic iron product was shown in Table 4. In the ferrite liquid phase was easily generated inside the copper slag pellets, which caused the metallic SEM micrographs, the bright area represents the metallic iron and the grey area represents the slag iron grains to be wrapped and difficult to dissociate, thus reducing the Fe recovery. The optimal CaO phase. The primary component of the slag phase is non-magnetic calcium metasilicate. Therefore, the addition should be 20 wt.%. iron could be easily recycled using magnetic separation. Furthermore, there is also a small amount of 3.2.4.anorthite Product and amorphous Analysis phase in the slag. It can be seen that the large metallic iron particles formed after direct reduction. Meanwhile, some slag phase wrapped around the metallic iron particles and enteredFigure the 10DRI consists product, of thethereby micrographs reducing and the EDS Fe grade analyses of the of theproduct. DRI product The XRD obtained pattern from of DRI the directproduct reduction–magnetic showed that the main separation XRD peak processes. is the iron. The Furthermore, XRD pattern there of the are metallic some interferential iron product peaks was shownbelonging in Figureto the 11amorphous. The chemical phase. component Table 4 indicate of metallics that ironthe iron product content was in shown the DRI in Tableproduct4. In could the SEMachieve micrographs, 91.55 wt.%. the Meanwhile, bright area the represents harmful theelements metallic in ironthe DRI and product the grey such area as represents alkalis, S, the and slag P phase.were very The low. primary It could component be seen from of the the slag chemical phase component is non-magnetic of the calciumDRI product metasilicate. that this Therefore,product is thean excellent iron could raw be material easily recycled for steelmaking. using magnetic separation. Furthermore, there is also a small amount of anorthite and amorphous phase in the slag. It can be seen that the large metallic iron particles formed after direct reduction. Meanwhile, some slag phase wrapped around the metallic iron particles and entered the DRI product, thereby reducing the Fe grade of the product. The XRD pattern of DRI product showed that the main XRD peak is the iron. Furthermore, there are some interferential peaks belonging to the amorphous phase. Table4 indicates that the iron content in the DRI product could achieve 91.55 wt.%. Meanwhile, the harmful elements in the DRI product such as alkalis, S, and P were very low. It could be seen from the chemical component of the DRI product that this product is an excellent raw material for steelmaking.

Minerals 2020, 10, x FOR PEER REVIEW 11 of 17

Figure 9. The effects of the CaO dosage on the Fe grade and recovery.

3.2.4. Product Analysis Figure 10 consists of the micrographs and EDS analyses of the DRI product obtained from the direct reduction–magnetic separation processes. The XRD pattern of the metallic iron product was shown in Figure 11. The chemical component of metallic iron product was shown in Table 4. In the SEM micrographs, the bright area represents the metallic iron and the grey area represents the slag phase. The primary component of the slag phase is non-magnetic calcium metasilicate. Therefore, the iron could be easily recycled using magnetic separation. Furthermore, there is also a small amount of anorthite and amorphous phase in the slag. It can be seen that the large metallic iron particles formed after direct reduction. Meanwhile, some slag phase wrapped around the metallic iron particles and entered the DRI product, thereby reducing the Fe grade of the product. The XRD pattern of DRI product showed that the main XRD peak is the iron. Furthermore, there are some interferential peaks belonging to the amorphous phase. Table 4 indicates that the iron content in the DRI product could achieve 91.55 wt.%. Meanwhile, the harmful elements in the DRI product such as alkalis, S, and P Mineralswere2020 very, 10 low., 973 It could be seen from the chemical component of the DRI product that this product12 is of 18 an excellent raw material for steelmaking.

Minerals 2020, 10, x FOR PEER REVIEW 12 of 17

FigureFigure 10. 10.The The micrography micrography of of the the directdirect reductionreduction of iron (DRI) (DRI) product product (a (a,b,b);); the the EDS EDS result result of ofpoint point A(Ac); (c the); the EDS EDS result result of of point point B B (d (d).).

Table 4. Chemical component of DRI product (wt.%).

TFe* CuO SiO2 Al2O3 CaO MgO Na2OK2OSP Figure91.55 10. The micrography 0.25 0.32 of the 0.57direct reduction 0.31 of 0.08 iron (DRI) 0.08 product 0.02 (a,b); the 0.07 EDS result 0.01 of point A (c); the EDS result of pointTFe* isB the(d). total iron content in the metallic iron product.

Figure 11. The XRD pattern of DRI product.

Table 4. Chemical component of DRI product (wt.%).

TFe* CuO SiO2 Al2O3 CaO MgO Na2O K2O S P 91.55 0.25 0.32 0.57 0.31 0.08 0.08 0.02 0.07 0.01 TFe* is the total iron content in the metallic iron product. FigureFigure 11.11. The XRD patternpattern ofof DRIDRI product.product. 3.3. The Direct Reduction Kinetic Model Analysis According to EquationTable (1), 4. theChemical reduction component degrees of underDRI product various (wt.%). reduction temperatures and times were calculated and are depicted in Figure 11. TFe* CuO SiO2 Al2O3 CaO MgO Na2O K2O S P As shown in Figure 12, the reduction degree was significantly influenced by the reduction 91.55 0.25 0.32 0.57 0.31 0.08 0.08 0.02 0.07 0.01 temperature, and the high temperature improved the reduction of iron-bearing compounds in the TFe* is the total iron content in the metallic iron product. copper slag. The fayalite is a kind of silicate mineral whose smelting point is about 1220 °C [43]. According to thermodynamic calculation, the addition of CaO could decrease the decomposition 3.3. The Direct Reduction Kinetic Model Analysis temperature of fayalite. When reduction temperature was lower than 1250 °C, the solid fayalite was hardlyAccording reduced to by Equation the fixed carbon(1), the in reduction the coal by degr solid–solidees under reaction. various The reductionreaction efficiency temperatures between and timessolid were fayalite calculated and solid and CaO are is depicted also very in low. Figure When 11. at a higher temperature, especially higher than 1250As °C, shown the fayalite in Figure began 12, to themelt, reduction producing degree liquid waphase,s significantly and the liquid influenced fayalite could by the react reduction with temperature,CaO more andefficiently. the high The temperature produced improvedFeO was thereduced reduction by fixedof iron-bearing carbon and compounds reducing gasin the copper slag. The fayalite is a kind of silicate mineral whose smelting point is about 1220 °C [43]. According to thermodynamic calculation, the addition of CaO could decrease the decomposition temperature of fayalite. When reduction temperature was lower than 1250 °C, the solid fayalite was hardly reduced by the fixed carbon in the coal by solid–solid reaction. The reaction efficiency between solid fayalite and solid CaO is also very low. When at a higher temperature, especially higher than 1250 °C, the fayalite began to melt, producing liquid phase, and the liquid fayalite could react with CaO more efficiently. The produced FeO was reduced by fixed carbon and reducing gas

Minerals 2020, 10, 973 13 of 18

3.3. The Direct Reduction Kinetic Model Analysis According to Equation (1), the reduction degrees under various reduction temperatures and times were calculated and are depicted in Figure 11. As shown in Figure 12, the reduction degree was significantly influenced by the reduction temperature, and the high temperature improved the reduction of iron-bearing compounds in the copper slag. The fayalite is a kind of silicate mineral whose smelting point is about 1220 ◦C[43]. According to thermodynamic calculation, the addition of CaO could decrease the decomposition temperature of fayalite. When reduction temperature was lower than 1250 ◦C, the solid fayalite was hardly reduced by the fixed carbon in the coal by solid–solid reaction. The reaction efficiency Minerals 2020between, 10, x solidFOR PEER fayalite REVIEW and solid CaO is also very low. When at a higher temperature, especially higher 13 of 17 than 1250 ◦C, the fayalite began to melt, producing liquid phase, and the liquid fayalite could react with CaO more efficiently. The produced FeO was reduced by fixed carbon and reducing gas simultaneously. Therefore, a higher temperature is favorable for copper slag reduction. However, the simultaneously. Therefore, a higher temperature is favorable for copper slag reduction. However, energy consumptionthe energy consumption and equipment and equipment life should life should also also be beconsidered. considered.

Figure Figure12. The 12. reductionThe reduction degree degree under under various various reduction reduction temperatures temperatures and times. and times. Subsequently, the reduction degree under experimental temperatures was fitted according to the Subsequently,Equations (2)–(4). the Thereduction calculated degree lines of under direct reductionexperimental kinetics temperatures are shown in Figure wa s13 fitted. For kinetic according to the Equationsmodel fitting,(2)–(4). the The reduction calculated degree lines data of after dire equilibriumct reduction are meaningless,kinetics are so shown in the nextin Figure fitting 13. For kinetic modelcalculations, fitting, the timethe reduction range was set degree from 0 todata 20 min.afte Subsequently,r equilibrium the are calculation meaningless, result was so fitted in the next according to the Arrhenius Equation to calculate the reaction activation energy. fitting calculations, the time range was set from 0 to 20 min. Subsequently, the calculation result was fitted according to the Arrhenius Equation to calculate the reaction activation energy. From the calculated correlation coefficient (R2) shown in Table 5, it can be seen that the reduction kinetics of copper slag at 1050 °C are more consistent with the first and second kinetic models that belong to the interfacial reaction and carbon gasification controlling. This result indicated that at a lower temperature, the copper slag’s reduction was controlled by the interfacial reaction between carbon powder and copper slag and the gasification reaction of carbon. However, when the reduction temperature increased, the controlling step changed to the diffusion of the gas. At this time, the diffusion of CO in the copper slag pellet controlled the reduction rate. The above analyses indicated that, at a lower temperature, the Fe2SiO4 began to decompose to FeO. The Fe3O4 and FeO in copper slag were mainly reduced by carbon. When reduction temperature increased, the carbon was gasified to carbon monoxide. The Fe3O4 and FeO in copper slag were reduced by carbon monoxide. The reduction rate was improved.

Table 5. The fitting parameters of different kinetic models under various temperatures.

𝟏 𝟏 ( )𝟑 𝐥𝐧(𝟏 − 𝛂)𝐭 =−𝐤 ( )𝟑 𝟐 T/°C 𝟏− 𝟏−𝛂 =𝐤𝐭 [𝟏 − 𝟏−𝛂 ] =𝐤𝐭 k R2 k R2 k R2 1050 0.01012 0.976 0.03283 0.983 0.00146 0.973 1100 0.01093 0.967 0.03564 0.976 0.00169 0.982 1150 0.01191 0.964 0.03914 0.974 0.00199 0.984 1200 0.01292 0.956 0.04277 0.967 0.00232 0.990 1250 0.01446 0.940 0.04839 0.954 0.00287 0.996 1300 0.01661 0.918 0.05648 0.936 0.00372 0.994

Minerals 2020, 10, 973 14 of 18 Minerals 2020, 10, x FOR PEER REVIEW 14 of 17

Figure 13. 13. TheThe calculated calculated lines lines of different of diﬀerent kinetic kinetic models—( models—(a) Equationa) Equation (2), (b) (2), Equation (b) Equation (3), and (3),(c) Equationand (c) Equation (4)—under (4)—under various varioustemperatures. temperatures.

2 TheFrom reaction the calculated activation correlation energy calculations coefficient (R displa) shownyed inin TableFigure5, it14 can and be Table seen that6 showed the reduction that the reductionkinetics of reaction copper slagwas atmainly 1050 ◦ Ccontrolled are more by consistent interfacial with reaction the first and and carbon second gasification kinetic models at a thatlow temperature.belong to the At interfacial that moment, reaction the and primary carbon reac gasificationtion that occurred controlling. was the This conversion result indicated of Fe3O that4 to atthe a FeO,lower which temperature, needed thethe copperlow reaction slag’s activation reduction energies. was controlled Meanwhile, by the the interfacial Fe2SiO4 reactioncould react between with CaO,carbon decomposing powder and to copper FeO. slagWhen and the the reduction gasification temperature reaction ofincreased, carbon. However,the primary when reaction the reduction was the reductiontemperature of increased,the FeO to the the controlling metallic stepiron, changed which ne toeded the di theffusion high of reaction the gas. Atactivation this time, energies.the diffusion The aboveof CO results in the copper illustrated slag that pellet the controlled reduction the characteristics reduction rate. of the The copper above slag analyses at various indicated temperatures that, at a werelower different. temperature, The themain Fe 2reasonSiO4 began was that to decompose the compos toition FeO. of The copper Fe3O slag4 and is FeO complex, in copper and slagdifferent were iron-bearingmainly reduced compounds by carbon. inWhen copper reduction slag have temperature different reduction increased, characteristics. the carbon was In gasifiedgeneral, to the carbon high reductionmonoxide. temperature The Fe3O4 and can FeOimprove in copper the reductio slag weren of reduced iron-bearing by carbon compounds monoxide. in copper The reduction slag. rate was improved. Table 6. The fitting parameters of the Arrhenius Equation of various kinetic models (Equations (2)–(4)). Table 5. The fitting parameters of different kinetic models under various temperatures. 𝟏 𝟏 𝟏−(𝟏−𝛂)𝟑 =𝐤𝐭 𝐥𝐧(𝟏 − 𝛂)𝐭 =−𝐤 [𝟏 − (𝟏−𝛂)𝟑]𝟐 =𝐤𝐭 Parameters 1 1 2 α 1 (1 α-1) 3 =kt 2 ln(1 α α)= kt-1 [ 2 α 3 ] α -1 2 E /k J·mol R E−/k J·mol− 1R(1 ) E=/kkt J·mol R T/◦C − − − − Copper slag pellet 33.370k R2 0.966 k 36.508R 2 0.963k R 63.0482 0.967 1050 0.01012 0.976 0.03283 0.983 0.00146 0.973 1100 0.01093 0.967 0.03564 0.976 0.00169 0.982 1150 0.01191 0.964 0.03914 0.974 0.00199 0.984 1200 0.01292 0.956 0.04277 0.967 0.00232 0.990 1250 0.01446 0.940 0.04839 0.954 0.00287 0.996 1300 0.01661 0.918 0.05648 0.936 0.00372 0.994

Minerals 2020, 10, 973 15 of 18

The reaction activation energy calculations displayed in Figure 14 and Table6 showed that the reduction reaction was mainly controlled by interfacial reaction and carbon gasification at a low temperature. At that moment, the primary reaction that occurred was the conversion of Fe3O4 to the FeO, which needed the low reaction activation energies. Meanwhile, the Fe2SiO4 could react with CaO, decomposing to FeO. When the reduction temperature increased, the primary reaction was the reduction of the FeO to the metallic iron, which needed the high reaction activation energies. The above results illustrated that the reduction characteristics of the copper slag at various temperatures were different. The main reason was that the composition of copper slag is complex, and different iron-bearing compounds in copper slag have different reduction characteristics. In general, the high reductionMinerals 2020 temperature, 10, x FOR PEER can REVIEW improve the reduction of iron-bearing compounds in copper slag. 15 of 17

Figure 14. The linear ﬁttingfitting of the Arrhenius Equation of various kinetic modelsmodels (Equations(Equations (2)–(4)).(2)–(4)).

Table 6. The ﬁtting parameters of the Arrhenius Equation of various kinetic models (Equations (2)–(4)). 4. Conclusions

1 1 2 3 ln(1 α)= kt 3 In summary, the Fe3O4 and1 (1 Feα2)SiO=kt4 in the copper slag could be reduced to[1 the(1 metallicα) ] =kt iron by Parameters − − − − − − 1 2 1 2 1 2 coal-based direct reductionEα/k and J mol recoveredR by magnEα/ketic J mol separation.R The bestEα/ kreduction J mol conditionsR · − · − · − wereCopper those slag in which pellet the reduction 33.370 temperature 0.966 was 36.508 1300 °C, the 0.963 reduction 63.048time was 30 0.967min, the dosage of coal addition was 35 wt.%, and the dosage of CaO addition was 20 wt.% (0.75 binary basicity). Under this optimum condition, the Fe concentration with 91.55% iron grade was obtained 4. Conclusions after grinding and magnetic separation progresses, the Fe recovery could achieve 98.13%. InResearch summary, on the the reduction Fe3O4 and kinetics Fe2SiO indicated4 in the copper that the slag reduction could be of reduced copper slag to the was metallic controlled iron byby coal-basedthe interfacial direct reaction reduction and and carbon recovered gasification by magnetic at 1050 separation. °C. When Thereduction best reduction temperature conditions increased, were thosethe reduction-controlling in which the reduction step temperature of copper was slag 1300 change◦C, thed to reduction the diffusion time wasof the 30 gas. min, The the dosageresults ofof coal the additionintegral kinetics was 35 wt.%, model and illust therate dosage that of the CaO reduction addition of was Fe3 20O4wt.% in the (0.75 copper binary slag, basicity). which Underneeds thislow optimumreaction activation condition, energy, the Fe occurred concentration first. Meanwhile, with 91.55% the iron Fe2SiO grade4 could was react obtained with afterCaO, grindingdecomposing and magneticto FeO. Then, separation the FeO progresses, began to be the reduced Fe recovery to metallic could iron achieve by carbon 98.13%. monoxide. The controlling steps of eachResearch reduction on the reaction reduction stage kinetics of copper indicated slag that were the reductiondifferent. ofThe copper main slag reason was controlledwas that the by thecomposition interfacial of reaction copper andslag carbonis complex, gasification and different at 1050 iron-bearing◦C. When reduction compounds temperature in copper increased,slag have thedifferent reduction-controlling reduction characteristics.step of copper In general, slag changed the high to the reduction diffusion temperature of the gas. Thecan resultsimprove of thethe integralreduction kinetics of iron-bearing model illustrate compounds that the in reduction copper slag. of Fe 3O4 in the copper slag, which needs low reaction activation energy, occurred first. Meanwhile, the Fe2SiO4 could react with CaO, decomposing to FeO. Then,Author the Contributions: FeO began to Conceptualization, be reduced to metallic H.Z. and iron M.L.; by carbon methodology, monoxide. H.Z.; The investigation, controlling C.H. steps and of W.G.; each reductionwriting—original reaction draft stage preparation, of copper M.L.; slag funding were di ffacquiserent.ition, The H.Z. main All reason authors was have that read the and composition agreed to the of copperpublished slag version is complex, of the manuscript. and different iron-bearing compounds in copper slag have different reduction Funding: This research was funded by the National Natural Science Foundation of China, grant number 51974204.

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

References

Minerals 2020, 10, 973 16 of 18 characteristics. In general, the high reduction temperature can improve the reduction of iron-bearing compounds in copper slag.

Author Contributions: Conceptualization, H.Z. and M.L.; methodology, H.Z.; investigation, C.H. and W.G.; writing—original draft preparation, M.L.; funding acquisition, H.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51974204. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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