semi-smelting reduction and magnetic separation for the

minerals

Article

Semi-Smelting Reduction and Magnetic Separationfor the Recovery of Iron and Alumina Slag from IronRich Bauxite

Yingyi Zhang 1,2,*, Qiangjian Gao 3,*, Jie Zhao 1, Mingyang Li 1 and Yuanhong Qi 4

1 School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243002, China;[email protected] (J.Z.); [email protected] (M.L.)

2 College of Material Science and Engineering, Chongqing University, Chongqing 400030, China3 School of Metallurgy, Northeastern University, Shenyang 110819, China4 State Key Laboratory of Advance Steel Processes and Products, Central Iron and Steel Research Institute,

Beijing 100081, China; [email protected]* Correspondence: [email protected] (Y.Z.); [email protected] (Q.G.);

Tel.: +86-17375076451 (Y.Z.); Tel.: +86-13897923145 (Q.G.)

Received: 7 March 2019; Accepted: 4 April 2019; Published: 9 April 2019��

Abstract: This work presents a semi-smelting reduction and magnetic separation process for therecovery of iron and alumina slag from iron rich bauxite ore. The effect of the process parameterson the recovery rate of iron, maximum particle size of the iron nugget, and the Al2O3 content of thealumina slag was investigated and optimized. The results show that the iron nuggets and aluminaslag can be obtained in a short time through a semi-smelting reduction and magnetic separationprocess. The maximum particle size of iron nugget is about 15 mm, and the recovery rate of the ironand Al2O3 grade of the alumina slag are 96.84 wt % and 43.98 wt %, respectively. The alumina slagconsisted mainly of alumina (α-Al2O3), calcium hexaluminate (CaAl12O19), gehlenite (Ca2Al2SiO7),and small amounts of hercynite (FeAl4O7), and metallic iron (M.Fe).

Keywords: iron rich bauxite; reduction and smelting; iron nuggets; alumina slag; recovery rate

1. Introduction

At present, more than 0.5 billion tons of iron rich bauxite in western Guangxi, China, have beenexplored [1–3]. However, large quantities of iron rich bauxite are discarded as waste, and morethan 30% of iron rich bauxite resources have not been effectively developed and utilized inChina. Acid treatments are used to extract alumina from non-bauxite raw materials and silica-richbauxite or lateritic ore. Valeev et al. [4] and Zhao et al. [5] used the hydrochloric acid toautoclave the decomposition of bauxites, and analyzed the mechanism of the bauxites’ dissolution.In addition, hydrochloric acid leaching was used to remove the dolomite impurity from the flotationconcentrate of a low-grade bauxite ore, improve its Al2O3 grade and Al2O3/SiO2 (A/S) ratio [6].However, the extraction results of the acid treatments are influenced by the composition, mineralogy,and location of each ore [7]. Numerous studies show that the synchronous extraction of Al and Fe isdifficult to achieve using acid treatments, which can only be used for the selective chemical dissolutionof goethite (FeOOH) and gibbsite (Al(OH)3) [8,9]. Therefore, the precise location and content ofaluminum in laterites must be determined in order to predict whether this element can be economicallyextracted from lateritic bauxite deposits. The iron rich bauxite contains high iron oxide, but its recoveryrate has not been predicted because of the large quantities of Al2O3 and SiO2, as well as the complexityof the chemical and mineralogical characteristics [10].

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Minerals 2019, 9, 223 2 of 16

At present, there are few reports about the gas-based reduction of iron rich bauxite. Parhi et al. [11]reported the application of hydrogen for the reduction of the bauxite mineral, and found thatonly the iron oxide present in bauxite was reduced to iron, while other oxides remain unreduced.Samouhos et al. [12] reported the reduction of red mud by H2, and found that the maximum conversiondegree of hematite to magnetite that was achieved was above 87% at 480 ◦C. However, the formationamount of metallic iron is very insignificant (≤3 wt %). It is worth noting that natural gas is veryscarce in China, and a gas-based direct reduction process (such as MIDREX, HYL-III, FINMET) is notlikely to be the main direct reduced process in China [13,14].

However, non-coking coal resources are very rich in China, and coal-based direct reductionprocesses are short processes that have extensive attention in China [15]. This technique usesnon-coking coal to reduce iron ores and to produce high quality direct reduced iron (DRI) [16].In addition, DRI is an excellent feedstock for the electric furnaces or basic oxygen furnace, and canbe used to produce higher grades of steel [17,18]. Thus, this method is considered to be an effectivemethod for the treatment of complex iron ore and low-grade iron ore resources [19,20]. Chu et al. [21]used the coal-based direct reduction and magnetic separation process to recover iron from red mud,the results showed that the iron recovery rate and total iron content in the magnetic product couldbe up to 98.37% and 82.52%, respectively. Zhang et al. [22] reported a semi-smelting reduction andmagnetic separation process to recover the iron nuggets from bauxite under a higher basicity condition.They found that the highest recovery rate of the iron nuggets was 92.79 wt %, and the high qualityiron nugget consisted of iron (96 wt %–97 wt %) and carbon (2.5 wt %–3.0 wt %) with a low ganguecontent. In addition, the pig iron nugget is excellent for its ease of shipping, handling, and storage.Imanasa et al. [23] studied the characteristics of calcium aluminate slags and pig iron produced fromthe smelting reduction of low-grade bauxites. The studied results show that the recovery rate of ironcan reach from 94.8 wt % to 99.9 wt %, the amount of slag reached 70%–85%, and the unreacted cokeparticles can float over the molten slag phase at elevated temperatures. However, this kind of thick slagoperation process is against to the separation of slag and iron, while greatly increasing the smeltingenergy consumption. Therefore, a semi-smelting reduction and magnetic separation process is aneffective method for iron recovery from solid wastes and low-grade iron ore.

In this study, semi-smelting reduction and magnetic separation technology was employed torecover iron and alumina slag from iron rich bauxite under a lower basicity condition. The effect ofthe process parameters on the recovery of the iron nugget and Al2O3 was investigated and optimized.The properties of the iron nugget and the phase composition of the alumina slag were also investigated.

2. Experimental

2.1. Materials and Analysis

Iron rich bauxite, Ca(OH)2, anthracite, and CaF2 were thoroughly mixed and crushed by thePulverisette 5 type high-energy ball milling analyzer. The particle size distribution of the mixtures ispresented in Figure 1a, which was obtained by the Malvern Mastersizer 2000 laser particle size analyzer(Malvern Panalytical Inc., Westborough, MA, USA). The analysis results show that the average particlediameter and specific surface area of mixtures are 88.431 µm and 0.149 m2/g, respectively.

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Figure 1. The particle size distribution (a) of mixtures, and XRD pattern (b) of an iron rich bauxite sample.

The chemical composition of iron rich bauxite was determined using a chemical analysis method, and is shown in Table 1. It was found that the iron rich bauxite mainly consisted of 40.42 wt % Fe2O3, 11.70 wt % SiO2, and 26.53 wt % Al2O3. The mineral phase composition of the iron rich bauxite was identified by a X–ray diffractometer (D8ADVANCE, Bruker, Billerica, MA, USA) with CuKα (λ = 1.5406 Å), and is shown in Figure 1b. We found that the gibbsite (Al(OH)3), diaspore (AlO(OH)), goethite (FeO(OH)), hematite (Fe2O3), and magnesium trisilicate (Mg2Si3O8·2H2O) are the major minerals in the bauxite ore, and anatase (TiO2) and quartz (SiO2) are minor components.

Table 1. Major element compositions of the iron rich bauxite samples.

Raw Materials Fetot Fe2O3 FeO SiO2 Al2O3 TiO2 MnO MgO CaO LOI

Bauxite 28.29 40.42 0.20 11.77 26.53 1.57 1.21 0.48 1.38 16.42

The anthracite was from Yangquan in Shanxi province, it was used to reduce the iron rich bauxite in this work, and its major element compositions are shown in Table 2. Fixed carbon (78.73 wt %) was the main reducing reagent in the semi-smelting reduction process. Analytical-grade Ca(OH)2) and CaF2 fluxes were used as the fluxing agents.

Table 2. Major element compositions of anthracite from Yangquan in Shanxi province.

Anthracite Composition Moisture Ash Volatiles Fixed Carbon Sulfur Phosphorus Content (wt %) 2.56 10.16 8.55 78.73 0.28 0.023

2.2. Semi-Smelting Reduction and Magnetic Separation

In this work, the effect of the reduction temperature, reduction time, anthracite ratio, CaF2 ratio and w(CaO)/w(SiO2) ratio on the semi-smelting reduction behavior will be investigated. The anthracite ratio and CaF2 ratio refer to the mass percentages in the bauxite–coal composite briquettes, and the basicity refers to the ratio from w(CaO) to w(SiO2). The reduction experimental requirement is shown in Figure 2.

0 50 100 150 200 250 300 350 400-1

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51

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472

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1-Diaspore (AlO(OH))2-Gibbsite (Al(OH)3)3-Hematite (Fe2O3)4-Goethite (FeO(OH))5-Quartz (SiO2)6-Anatase (TiO2)7-Magnesium trisilicate (Mg2Si3O8·2H2O)

(b)

Figure 1. The particle size distribution (a) of mixtures, and XRD pattern (b) of an iron richbauxite sample.

The chemical composition of iron rich bauxite was determined using a chemical analysis method,and is shown in Table 1. It was found that the iron rich bauxite mainly consisted of 40.42 wt % Fe2O3,11.70 wt % SiO2, and 26.53 wt % Al2O3. The mineral phase composition of the iron rich bauxitewas identified by a X–ray diffractometer (D8ADVANCE, Bruker, Billerica, MA, USA) with CuKα

(λ = 1.5406 Å), and is shown in Figure 1b. We found that the gibbsite (Al(OH)3), diaspore (AlO(OH)),goethite (FeO(OH)), hematite (Fe2O3), and magnesium trisilicate (Mg2Si3O8·2H2O) are the majorminerals in the bauxite ore, and anatase (TiO2) and quartz (SiO2) are minor components.

Table 1. Major element compositions of the iron rich bauxite samples.

Raw Materials Fetot Fe2O3 FeO SiO2 Al2O3 TiO2 MnO MgO CaO LOI

Bauxite 28.29 40.42 0.20 11.77 26.53 1.57 1.21 0.48 1.38 16.42

The anthracite was from Yangquan in Shanxi province, it was used to reduce the iron rich bauxitein this work, and its major element compositions are shown in Table 2. Fixed carbon (78.73 wt %) wasthe main reducing reagent in the semi-smelting reduction process. Analytical-grade Ca(OH)2) andCaF2 fluxes were used as the fluxing agents.

Table 2. Major element compositions of anthracite from Yangquan in Shanxi province.

Anthracite Composition Moisture Ash Volatiles FixedCarbon Sulfur Phosphorus

Content (wt %) 2.56 10.16 8.55 78.73 0.28 0.023

2.2. Semi-Smelting Reduction and Magnetic Separation

In this work, the effect of the reduction temperature, reduction time, anthracite ratio, CaF2 ratioand w(CaO)/w(SiO2) ratio on the semi-smelting reduction behavior will be investigated. The anthraciteratio and CaF2 ratio refer to the mass percentages in the bauxite–coal composite briquettes, and thebasicity refers to the ratio from w(CaO) to w(SiO2). The reduction experimental requirement is shownin Figure 2.


Figure 2. The experimental requirement of the semi-smelting reduction process.

The experimental procedure of the semi-smelting reduction and magnetic separation process is shown in Figure 3. Firstly, the iron rich bauxite, anthracite, Ca(OH)2, and CaF2 fluxes were thoroughly blended at different ratios when the water mass percentage was 10 wt %. Second, the mixtures were placed in a roller press and squeezed into bauxite–coal composite briquettes with a 20 mm diameter by 25 mm length, then placed in an electric heating blast drying cabin at 200 °C for 3 h. At last, the composite briquettes were placed in a corundum crucible and introduced into the high temperature electric furnace, then reduced at a semi-smelting state with an Ar flow rate of 1.5 L/min, which was repeated three times.

After the semi-smelting reduction, the reduced briquettes were crushed to 1–10 mm by the jaw crusher, then the iron nugget and alumina slag were magnetically separated by the high strength roller dry magnetic separator (LONGi, Zhengzhou, China) with a magnetic intensity of 4000 Gs. These products were characterized by XRD, SEM, and energy dispersive X-ray spectroscope (EDS) so as to study the phase composition, microstructure, and smelting mechanism. The microstructure and element dissemination characteristics were characterized by scanning electron microscopy (SEM; FEI-Quanta 600, Munich, Germany) attached with a backscatter electron imaging (BSE) and energy dispersive X-ray spectroscope (EDS; BRUKER Nano, Berlin, Germany).

Figure 3. The experimental procedure of the semi-smelting reduction and magnetic separation process.


The experimental procedure of the semi-smelting reduction and magnetic separation process isshown in Figure 3. Firstly, the iron rich bauxite, anthracite, Ca(OH)2, and CaF2 fluxes were thoroughlyblended at different ratios when the water mass percentage was 10 wt %. Second, the mixtures wereplaced in a roller press and squeezed into bauxite–coal composite briquettes with a 20 mm diameterby 25 mm length, then placed in an electric heating blast drying cabin at 200 ◦C for 3 h. At last,the composite briquettes were placed in a corundum crucible and introduced into the high temperatureelectric furnace, then reduced at a semi-smelting state with an Ar flow rate of 1.5 L/min, which wasrepeated three times.

Minerals 2019, 9, x FOR PEER REVIEW 4 of 16


The experimental procedure of the semi-smelting reduction and magnetic separation process is shown in Figure 3. Firstly, the iron rich bauxite, anthracite, Ca(OH)2, and CaF2 fluxes were thoroughly blended at different ratios when the water mass percentage was 10 wt %. Second, the mixtures were placed in a roller press and squeezed into bauxite–coal composite briquettes with a 20 mm diameter by 25 mm length, then placed in an electric heating blast drying cabin at 200 °C for 3 h. At last, the composite briquettes were placed in a corundum crucible and introduced into the high temperature electric furnace, then reduced at a semi-smelting state with an Ar flow rate of 1.5 L/min, which was repeated three times.

After the semi-smelting reduction, the reduced briquettes were crushed to 1–10 mm by the jaw crusher, then the iron nugget and alumina slag were magnetically separated by the high strength roller dry magnetic separator (LONGi, Zhengzhou, China) with a magnetic intensity of 4000 Gs. These products were characterized by XRD, SEM, and energy dispersive X-ray spectroscope (EDS) so as to study the phase composition, microstructure, and smelting mechanism. The microstructure and element dissemination characteristics were characterized by scanning electron microscopy (SEM; FEI-Quanta 600, Munich, Germany) attached with a backscatter electron imaging (BSE) and energy dispersive X-ray spectroscope (EDS; BRUKER Nano, Berlin, Germany).



After the semi-smelting reduction, the reduced briquettes were crushed to 1–10 mm by the jawcrusher, then the iron nugget and alumina slag were magnetically separated by the high strengthroller dry magnetic separator (LONGi, Zhengzhou, China) with a magnetic intensity of 4000 Gs.These products were characterized by XRD, SEM, and energy dispersive X-ray spectroscope (EDS)so as to study the phase composition, microstructure, and smelting mechanism. The microstructureand element dissemination characteristics were characterized by scanning electron microscopy (SEM;FEI-Quanta 600, Munich, Germany) attached with a backscatter electron imaging (BSE) and energydispersive X-ray spectroscope (EDS; BRUKER Nano, Berlin, Germany).

Minerals 2019, 9, 223 5 of 16

The ion recovery rate and Al2O3 grade were used to evaluate the results of the semi-smeltingreduction and magnetic separation. The iron recovery rate (ηFe), the total alumina content of thebriquettes (T.Al2O3), and the Al2O3 grade of alumina slag (η′Al2O3

) can be mathematically expressedas follows:

ηFe =MFe

MT.Fe× 100% (1)

T.Al2O3 (in briquette) =Mo

briquete × ηbauxite × ηoAl2O3

M′briquette× 100% (2)

η′Al2O3(in slag) =

T.Al2O3

M′briquette − MFe× 100% (3)

where (ηFe ) is the recovery rate of the iron, MFe is the mass of iron in the nugget, MT.Fe is the mass ofthe total iron in the composite briquettes, Mo

briquete is the initial mass of the briquette, M′briquette is theend mass of the reduced briquette, is the mass fraction of bauxite in the composite briquettes, ηo

Al2O3is

the mass fraction of Al2O3 in the bauxite, η′Al2O3is the Al2O3 grade of the alumina slag, and T.Al2O3 is

the mass of the total alumina in the composite briquette.

2.3. Thermodynamic Mechanism

2.3.1. Thermodynamic Mechanism of Reduction Reaction

The reduction reaction equations and the Gibbs free energy that varies with the temperaturewere calculated using Factsage 7.0 (Thermfact/CRCT & GTT-Technologies) and are presented inFigure 4. The reduction of the iron oxides involves direct (Equations (4), (6), (8), and (10)) and indirect(Equations (5), (7), (9), and (11)) methods. The gas phase carburization reaction (Equation (14)) andindirect reduction (Equations (5), (9), and (11)) are predominant at temperatures below 973 K. On thecontrary, the initial reaction temperatures of the direct reduction (Equations (4), (6), (8), and (10)),carbon gasification reaction (Equation (12)), and solid-state carburization reaction (Equation (13))are above 973 K. The standard free energies of the direct reduction increase sharply with theincreasing temperature, and their standard free energies are more negative than the indirect reduction.Therefore, the direct reduction (Equations (4), (6), (8), and (10)) reaction is predominant when thetemperatures are above 973 K.


The ion recovery rate and Al2O3 grade were used to evaluate the results of the semi-smelting reduction and magnetic separation. The iron recovery rate (𝜂 ), the total alumina content of the briquettes (T.Al2O3), and the Al2O3 grade of alumina slag (𝜂 ) can be mathematically expressed as follows: 𝜂 = . × 100% (1) 𝑇. Al O (in briquette) = × ×

× 100% (2) 𝜂 (in slag) = . × 100% (3)

where (𝜂 ) is the recovery rate of the iron, 𝑀 is the mass of iron in the nugget, 𝑀 . is the mass of the total iron in the composite briquettes, 𝑀 is the initial mass of the briquette, 𝑀 is the end mass of the reduced briquette, 𝜂 is the mass fraction of bauxite in the composite briquettes, 𝜂 is the mass fraction of Al2O3 in the bauxite, 𝜂 is the Al2O3 grade of the alumina slag, and 𝑇. Al O is the mass of the total alumina in the composite briquette.

2.3. Thermodynamic Mechanism

2.3.1. Thermodynamic Mechanism of Reduction Reaction

The reduction reaction equations and the Gibbs free energy that varies with the temperature were calculated using Factsage 7.0 (Thermfact/CRCT & GTT-Technologies) and are presented in Figure 4. The reduction of the iron oxides involves direct (Equations (4), (6), (8), and (10)) and indirect (Equations (5), (7), (9), and (11)) methods. The gas phase carburization reaction (Equation (14)) and indirect reduction (Equations (5), (9), and (11)) are predominant at temperatures below 973 K. On the contrary, the initial reaction temperatures of the direct reduction (Equations (4), (6), (8), and (10)), carbon gasification reaction (Equation (12)), and solid-state carburization reaction (Equation (13)) are above 973 K. The standard free energies of the direct reduction increase sharply with the increasing temperature, and their standard free energies are more negative than the indirect reduction. Therefore, the direct reduction (Equations (4), (6), (8), and (10)) reaction is predominant when the temperatures are above 973 K.

Figure 4. The standard free energy (ΔGθ) with temperatures for Equations (4)–(14).

2.3.2. Thermodynamic Mechanism of Calcium Aluminate Slag

As is well known, within the reaction temperature ranges, the Slaked lime (Ca(OH)2), diaspore (AlOOH), gibbsite (Al(OH)3), goethite (FeOOH), and kaolinite (Al2Si2O5(OH)4·2H2O) are decomposed

400 800 1200 1600 2000 2400 2800 3200

-800

-600

-400

-200

0

200

400

ΔGθ (k

J/mol

)

Temperature /K

Eq.(4): 3Fe2O3+C=2Fe2O3

Eq.(5): 3Fe2O3+CO=2Fe3O4+CO2

Eq.(6): Fe3O4+C=3FeO+CO

Eq.(7): Fe3O4+CO=3FeO+CO2

Eq.(8): FeO+C=Fe+CO

Eq.(9): FeO+CO=Fe+CO2

Eq.(10): 2FeOOH+4C=2Fe+4CO+H2

Eq.(11): 2FeOOH+4CO=2Fe+4CO2+H2

Eq.(12): C+CO2=2CO

Eq.(13): C+3Fe=Fe3C

Eq.(14): 3Fe+2CO=Fe3C+CO2

Figure 4. The standard free energy (∆Gθ) with temperatures for Equations (4)–(14).

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2.3.2. Thermodynamic Mechanism of Calcium Aluminate Slag

As is well known, within the reaction temperature ranges, the Slaked lime (Ca(OH)2),diaspore (AlOOH), gibbsite (Al(OH)3), goethite (FeOOH), and kaolinite (Al2Si2O5(OH)4·2H2O) aredecomposed to CaO, Al2O3, Fe2O3, SiO2, and H2O(g), and the Fe2O3 is almost completely reduced tometallic iron (M.Fe). The slag mainly consists of Al2O3, SiO2, and CaO. In this work, the effectof the CaF2 ratio on the recovery rate of the iron and alumina slag from iron rich bauxite wasinvestigated. The phase diagrams of the Al2O3–SiO2–CaO and Al2O3–SiO2–CaO–CaF2 systemsare calculated using FactSage 7.0, and are presented in Figure 5. We can find that when the slagbasicity (R = w(CaO)/w(SiO2)) is 0.5, the slag mainly consists of liquid slag, aluminum oxide (Al2O3),and anorthite (CaAl2Si2O8). When the slag basicity is 1.0–1.25, the slag mainly consists of liquidslag, Al2O3, calcium dialuminate (CaAl4O7), and calcium hexaluminate (CaAl12O19). When theslag basicity is 1.5–2.0, the slag mainly consists of liquid slag, melilite (CaAl2SiO7), CaAl4O7,and CaAl12O19. In addition, FeO may react with SiO2 and Al2O3 to generate fayalite (Fe2SiO4)and hercynite (FeAl2O4). From Figure 4b, we can find that the CaF2 has little influence on the slagcomposition. However, the CaF2 can significantly reduce the slag viscosity and melt temperature.Stamboulis et al. [24] investigated the CaO–CaF2–Al2O3–SiO2 glasses and found that F can take partin the formation of the species (AlOxFy)n− (where x = 3–6, y = 6 − x, and n is the charge of the totalcomplex with the IVAl, VAl, and VIAl coordinate states). The IVAl, VAl, and VIAl species act as networkbreakers, and the O and F atoms are randomly distributed around the Al atoms [25].


to CaO, Al2O3, Fe2O3, SiO2, and H2O(g), and the Fe2O3 is almost completely reduced to metallic iron (M.Fe). The slag mainly consists of Al2O3, SiO2, and CaO. In this work, the effect of the CaF2 ratio on the recovery rate of the iron and alumina slag from iron rich bauxite was investigated. The phase diagrams of the Al2O3–SiO2–CaO and Al2O3–SiO2–CaO–CaF2 systems are calculated using FactSage 7.0, and are presented in Figure 5. We can find that when the slag basicity (R = w(CaO)/w(SiO2)) is 0.5, the slag mainly consists of liquid slag, aluminum oxide (Al2O3), and anorthite (CaAl2Si2O8). When the slag basicity is 1.0–1.25, the slag mainly consists of liquid slag, Al2O3, calcium dialuminate (CaAl4O7), and calcium hexaluminate (CaAl12O19). When the slag basicity is 1.5–2.0, the slag mainly consists of liquid slag, melilite (CaAl2SiO7), CaAl4O7, and CaAl12O19. In addition, FeO may react with SiO2 and Al2O3 to generate fayalite (Fe2SiO4) and hercynite (FeAl2O4). From Figure 4b, we can find that the CaF2 has little influence on the slag composition. However, the CaF2 can significantly reduce the slag viscosity and melt temperature. Stamboulis et al. [24] investigated the CaO–CaF2–Al2O3–SiO2 glasses and found that F can take part in the formation of the species (AlOxFy)n− (where x = 3–6, y = 6 − x, and n is the charge of the total complex with the IVAl, VAl, and VIAl coordinate states). The IVAl, VAl, and VIAl species act as network breakers, and the O and F atoms are randomly distributed around the Al atoms [25].

Figure 5. Thermodynamic phase diagrams of the (a) CaO–SiO2–Al2O3 system and (b) CaO–SiO2–Al2O3–CaF2 system.

The reaction equations and the Gibbs free energy that varies with the temperature are calculated using Factsage 7.0, and are presented in Figure 6. It can be seen that the diaspore (AlOOH) and gibbsite (Al(OH)3) are easily decomposed into alumina (Al2O3) and water (H2O) at low temperatures (Equations (16) and (17)). The kaolinite (Al2Si2O5(OH)4·2H2O) is decomposed into quartz (SiO2), alumina (Al2O3), and water (H2O) (Equation (18)) at temperatures over 623 K. Initially, the slaked lime (Ca(OH)2) gets decomposed into calcium oxide (CaO) and water (H2O) at temperatures over 1423 K. It can be seen that the reactions of Equations (18), (21), (22), and (23) are predominant at the indicated experimental temperature range, whereas the reactions of Equations (19) and (20) are negative. The standard free energies of Equations (18), (21), (22), and (23) are more negative than that of Equations (19) and (20). Therefore, the alumina slag is mainly composed of gehlenite (Ca2Al2Si2O7), anorthite (CaAl2Si2O8), and calcium hexaluminate (CaAl12O19), and a small amount of fayalite (Fe2SiO4) and hercynite (FeAl2O4).

Figure 5. Thermodynamic phase diagrams of the (a) CaO–SiO2–Al2O3 system and(b) CaO–SiO2–Al2O3–CaF2 system.

The reaction equations and the Gibbs free energy that varies with the temperature are calculatedusing Factsage 7.0, and are presented in Figure 6. It can be seen that the diaspore (AlOOH) andgibbsite (Al(OH)3) are easily decomposed into alumina (Al2O3) and water (H2O) at low temperatures(Equations (16) and (17)). The kaolinite (Al2Si2O5(OH)4·2H2O) is decomposed into quartz (SiO2),alumina (Al2O3), and water (H2O) (Equation (18)) at temperatures over 623 K. Initially, the slakedlime (Ca(OH)2) gets decomposed into calcium oxide (CaO) and water (H2O) at temperatures over1423 K. It can be seen that the reactions of Equations (18), (21), (22), and (23) are predominant atthe indicated experimental temperature range, whereas the reactions of Equations (19) and (20)are negative. The standard free energies of Equations (18), (21), (22), and (23) are more negativethan that of Equations (19) and (20). Therefore, the alumina slag is mainly composed of gehlenite(Ca2Al2Si2O7), anorthite (CaAl2Si2O8), and calcium hexaluminate (CaAl12O19), and a small amount offayalite (Fe2SiO4) and hercynite (FeAl2O4).



3. Results and Discussion

3.1. Recovery of Iron and Alumina Slag

3.1.1. Effect of Basicity

The briquettes basicity (w(CaO)/w(SiO2)) has an important influence on the slag melting point and phase composition. In order to obtain the optimal basicity, various basicities (0.5–2.0) were investigated with a constant process parameter, and the reduction temperature and time are 1400 °C and 20 min, respectively. The effect of the basicity on the recovery of the iron and alumina slag are presented in Figure 7a,b, respectively.

It was found that the total iron content and Al2O3 grade of the metallized briquettes decreases gradually with the increasing w(CaO)/w(SiO2) ratio. However, the maximum size of the iron nuggets, the metallization rate of briquettes, the iron recovery rate, and the Al2O3 grade of the slag show a trend of first increasing and then decreasing, and the highest values are obtained at a w(CaO)/w(SiO2) ratio of 1.25. The CaO content of the metallized briquettes increases sharply with the increasing w(CaO)/w(SiO2) ratio, which leads to a decrease in the Al2O3 grade of the metallized briquettes. In addition, a reasonable basicity can reduce the slag melting point and viscosity; promote the effective separation of slag and iron; and improve the metallization rate, iron recovery rate, and Al2O3 grade in the slag.

Figure 7. Effect of the (a) CaO/SiO2 ratio on the reduction indexes and (b) Al2O3.

400 800 1200 1600 2000 2400 2800 3200

-300

-250

-200

-150

-100

-50

0

50

ΔGθ (k

J/mol

)

Temperature /K

Eq.(15): Ca(OH)2=CaO+H2O

Eq.(16): 2AlOOH=Al2O3+H2O

Eq.(17): 2Al(OH)3=Al2O3+3H2O

Eq.(18): Al2Si2O5(OH)4·2H2O=Al2O3+2SiO2+2H2O

Eq.(19): FeO+Al2O3=FeAl2O4

Eq.(20): 2FeO+SiO2=Fe2SiO4

Eq.(21): CaO+6Al2O3=CaAl12O19

Eq.(22): CaO+Al2O3+2SiO2=CaAl2Si2O8

Eq.(23): CaO+Al2O3+SiO2=Ca2Al2SiO7

Reaction mechanism equations:

0.50 0.75 1.00 1.25 1.50 1.75 2.004

8

12

16

20

24

28

32

Total iron content of metallized briquettes Maximum size of iron nuggets Metallization rate of briquettes Recovery rate of iron nuggets

CaO/SiO2

T.Fe

con

tent

/%, M

ax n

ugge

ts s

ize

/mm

40

50

60

70

80

90

100

Met

alliz

atio

n ra

te a

nd re

cove

ry ra

te /%

(a)

0.50 0.75 1.00 1.25 1.50 1.75 2.00

28

29

30

31

32

33

34

CaO/SiO2

T.Al

2O3 c

onte

nt /%

30

32

34

36

38

40

42

44

Total Al2O3 content of metallized briquettes Al2O3 content in slag Al

2O3 c

onte

nt in

sla

g

(b)

Figure 6. The standard free energy (∆Gθ) with temperatures for Equations (15)–(23).




The briquettes basicity (w(CaO)/w(SiO2)) has an important influence on the slag melting pointand phase composition. In order to obtain the optimal basicity, various basicities (0.5–2.0) wereinvestigated with a constant process parameter, and the reduction temperature and time are 1400 ◦Cand 20 min, respectively. The effect of the basicity on the recovery of the iron and alumina slag arepresented in Figure 7a,b, respectively.






The briquettes basicity (w(CaO)/w(SiO2)) has an important influence on the slag melting point and phase composition. In order to obtain the optimal basicity, various basicities (0.5–2.0) were investigated with a constant process parameter, and the reduction temperature and time are 1400 °C and 20 min, respectively. The effect of the basicity on the recovery of the iron and alumina slag are presented in Figure 7a,b, respectively.

It was found that the total iron content and Al2O3 grade of the metallized briquettes decreases gradually with the increasing w(CaO)/w(SiO2) ratio. However, the maximum size of the iron nuggets, the metallization rate of briquettes, the iron recovery rate, and the Al2O3 grade of the slag show a trend of first increasing and then decreasing, and the highest values are obtained at a w(CaO)/w(SiO2) ratio of 1.25. The CaO content of the metallized briquettes increases sharply with the increasing w(CaO)/w(SiO2) ratio, which leads to a decrease in the Al2O3 grade of the metallized briquettes. In addition, a reasonable basicity can reduce the slag melting point and viscosity; promote the effective separation of slag and iron; and improve the metallization rate, iron recovery rate, and Al2O3 grade in the slag.


400 800 1200 1600 2000 2400 2800 3200

-300

-250

-200

-150

-100

-50

0

50

ΔGθ (k

J/mol

)

Temperature /K

Eq.(15): Ca(OH)2=CaO+H2O

Eq.(16): 2AlOOH=Al2O3+H2O

Eq.(17): 2Al(OH)3=Al2O3+3H2O

Eq.(18): Al2Si2O5(OH)4·2H2O=Al2O3+2SiO2+2H2O

Eq.(19): FeO+Al2O3=FeAl2O4

Eq.(20): 2FeO+SiO2=Fe2SiO4

Eq.(21): CaO+6Al2O3=CaAl12O19

Eq.(22): CaO+Al2O3+2SiO2=CaAl2Si2O8

Eq.(23): CaO+Al2O3+SiO2=Ca2Al2SiO7

Reaction mechanism equations:

0.50 0.75 1.00 1.25 1.50 1.75 2.004

8

12

16

20

24

28

32

Total iron content of metallized briquettes Maximum size of iron nuggets Metallization rate of briquettes Recovery rate of iron nuggets

CaO/SiO2

T.Fe

con

tent

/%, M

ax n

ugge

ts s

ize

/mm

40

50

60

70

80

90

100

Met

alliz

atio

n ra

te a

nd re

cove

ry ra

te /%

(a)

0.50 0.75 1.00 1.25 1.50 1.75 2.00

28

29

30

31

32

33

34

CaO/SiO2

T.Al

2O3 c

onte

nt /%

30

32

34

36

38

40

42

44

Total Al2O3 content of metallized briquettes Al2O3 content in slag Al

2O3 c

onte

nt in

sla

g

(b)


It was found that the total iron content and Al2O3 grade of the metallized briquettes decreasesgradually with the increasing w(CaO)/w(SiO2) ratio. However, the maximum size of the iron nuggets,the metallization rate of briquettes, the iron recovery rate, and the Al2O3 grade of the slag show a trendof first increasing and then decreasing, and the highest values are obtained at a w(CaO)/w(SiO2)ratio of 1.25. The CaO content of the metallized briquettes increases sharply with the increasingw(CaO)/w(SiO2) ratio, which leads to a decrease in the Al2O3 grade of the metallized briquettes.In addition, a reasonable basicity can reduce the slag melting point and viscosity; promote the effectiveseparation of slag and iron; and improve the metallization rate, iron recovery rate, and Al2O3 grade inthe slag.

Minerals 2019, 9, 223 8 of 16

The above phenomena can be explained by the thermodynamic data of Figure 4. It can be seen thatthe w(CaO)/w(SiO2) ratio has an important effect on the growing up of iron nugget and the separationof the iron and slag. As shown in Figure 4, when the basicity is between 1.0 to 1.5, the alumina slagmainly consisted of low temperature eutectic mixtures (Al2O3–CaAl2Si2O8–Ca2Al2Si2O7), the amountof liquid phase of alumina slag is relatively high, and the highest amount of liquid phase is obtained ata w(CaO)/w(SiO2) ratio of 1.25. However, if excessive CaO is added into the bauxite–coal compositebriquettes, the amount of liquid phase of the alumina slag will decrease, and the metallizationrate of the briquettes and the recovery rate of the iron also will decrease. Therefore, the optimalw(CaO)/w(SiO2) ratio is 1.25 when the reduction temperature and time are 1400 ◦C and 20 min,respectively. Compared with the high basicity process [22], the amount of slag of this low basicitysemi-molten reduction process decreased to about 27.9 wt %–36.6 wt %, which will significantly reducethe smelting energy consumption.

3.1.2. Effect of Reducing Time

The reduction time has an important influence on the metallization rate of the briquettes and theiron recovery rate. In order to obtain an optimal reduction time, the various reduction times (5–20 min)were investigated with a constant process parameter, and the basicity and temperature are 1.25 and1400 ◦C, respectively. The effect of the reduction time on the recovery of the iron and alumina slag arepresented in Figure 8a,b, respectively.


The above phenomena can be explained by the thermodynamic data of Figure 4. It can be seen that the w(CaO)/w(SiO2) ratio has an important effect on the growing up of iron nugget and the separation of the iron and slag. As shown in Figure 4, when the basicity is between 1.0 to 1.5, the alumina slag mainly consisted of low temperature eutectic mixtures (Al2O3–CaAl2Si2O8–Ca2Al2Si2O7), the amount of liquid phase of alumina slag is relatively high, and the highest amount of liquid phase is obtained at a w(CaO)/w(SiO2) ratio of 1.25. However, if excessive CaO is added into the bauxite–coal composite briquettes, the amount of liquid phase of the alumina slag will decrease, and the metallization rate of the briquettes and the recovery rate of the iron also will decrease. Therefore, the optimal w(CaO)/w(SiO2) ratio is 1.25 when the reduction temperature and time are 1400 °C and 20 min, respectively. Compared with the high basicity process [22], the amount of slag of this low basicity semi-molten reduction process decreased to about 27.9 wt %–36.6 wt %, which will significantly reduce the smelting energy consumption.

3.1.2. Effect of Reducing Time

The reduction time has an important influence on the metallization rate of the briquettes and the iron recovery rate. In order to obtain an optimal reduction time, the various reduction times (5–20 min) were investigated with a constant process parameter, and the basicity and temperature are 1.25 and 1400 °C, respectively. The effect of the reduction time on the recovery of the iron and alumina slag are presented in Figure 8a,b, respectively.

It was found that the total iron content and Al2O3 grade of the metallized briquettes, the maximum size of iron nuggets, and the Al2O3 grade of the slag increases slightly with the increasing reduction time. The maximum size of the iron nuggets and the highest Al2O3 grade of slag are 15.55 mm and 43.47 wt %, respectively. However, the metallization rate of the briquettes and the iron recovery rate increased markedly, and when the time was over 15 min, the metallization rate of the briquettes and the iron recovery rate increased slightly. When the reduction time reaches 20 min, the highest metallization rate of the briquettes and the iron recovery rate are 98.9% and 94.7%, respectively.

Figure 8. Effect of reduction time on the (a) reduction indexes and (b) Al2O3 grade.

3.1.3. Effect of Reduction Temperature

Chu et al. [21] investigated the effect of the direct reduction temperature on the recovery of iron from red mud, and found that increasing the temperature can significantly increase the iron recovery rate and total iron content in the magnetic product. Therefore, the reduction temperature has an important influence on the slag melting point, metallization rate of the briquettes, and the iron recovery rate. In order to obtain the optimal reduction temperature, various reduction temperatures (1375–1450 °C) were investigated with a constant process parameter, and the basicity and time are 1.25 and 15 min, respectively. The effect of the reduction temperature on the recovery of the iron and alumina slag are presented in Figure 9a,b, respectively.

5.0 7.5 10.0 12.5 15.0 17.5 20.0

12

16

20

24

28

32

36

Reduction time /min

T.Fe

con

tent

/%, M

ax n

ugge

ts s

ize

/mm

80

84

88

92

96

100

Total iron content metallized briquettes Maximum size of iron nuggets Metallization rate of iron Recovery rate of iron nuggets

Sepe

ratio

n in

dexe

s / %

(a)

5.0 7.5 10.0 12.5 15.0 17.5 20.0

28.2

28.4

28.6

28.8

29.0

29.2

29.4

29.6

29.8

30.0

30.2

Total Al2O3 content of metallized briqurttes Al2O3 content in slag

Reduction time /min

T.Al

2O3 c

onte

nt /%

39

40

41

42

43

44

Al2O

3 con

tent

in s

lag

(b)

Figure 8. Effect of reduction time on the (a) reduction indexes and (b) Al2O3 grade.

It was found that the total iron content and Al2O3 grade of the metallized briquettes, the maximumsize of iron nuggets, and the Al2O3 grade of the slag increases slightly with the increasing reductiontime. The maximum size of the iron nuggets and the highest Al2O3 grade of slag are 15.55 mm and43.47 wt %, respectively. However, the metallization rate of the briquettes and the iron recovery rateincreased markedly, and when the time was over 15 min, the metallization rate of the briquettesand the iron recovery rate increased slightly. When the reduction time reaches 20 min, the highestmetallization rate of the briquettes and the iron recovery rate are 98.9% and 94.7%, respectively.

3.1.3. Effect of Reduction Temperature

Chu et al. [21] investigated the effect of the direct reduction temperature on the recovery ofiron from red mud, and found that increasing the temperature can significantly increase the ironrecovery rate and total iron content in the magnetic product. Therefore, the reduction temperaturehas an important influence on the slag melting point, metallization rate of the briquettes, and the ironrecovery rate. In order to obtain the optimal reduction temperature, various reduction temperatures(1375–1450 ◦C) were investigated with a constant process parameter, and the basicity and time are1.25 and 15 min, respectively. The effect of the reduction temperature on the recovery of the iron andalumina slag are presented in Figure 9a,b, respectively.

Minerals 2019, 9, 223 9 of 16


It was found that the total iron content and Al2O3 grade of the metallized briquettes, the metallization rate of briquettes, and Al2O3 grade of the slag increases slightly with the increasing reduction temperature. The highest metallization rate of the briquettes maximum and Al2O3 grade of slag are 98.81% and 43.35 wt %, respectively. However, the iron recovery rate increases markedly, and when the reduction temperature is over 1425 °C, the iron recovery rate increases slightly. In addition, the maximum iron nugget size follows a parabolic law with the increasing reduction temperature, and the maximum iron nugget size is about 9.1 mm, which is obtained at a temperature of 1425 °C.

Figure 9. Effect of reduction temperature on (a) reduction indexes and (b) Al2O3 grade.

3.1.4. Effect of the Anthracite Ratio

The anthracite ratio has an important influence on the direct reduction, carbon gasification reaction, and solid-state carburization reaction. In order to obtain the optimal anthracite ratio, the various anthracite ratios (from 8.82 wt % to 13.40 wt %) were investigated with a constant process parameter, and the basicity, temperature, and time are 1.25, 1425 °C, and 15 min, respectively. The effect of the anthracite ratio on the recovery of the iron and alumina slag are presented in Figure 10a,b, respectively.

It was found that the metallization rate of the briquettes, the maximum size of the iron nugget, and the Al2O3 grade of the slag increases slightly with the increasing anthracite ratio. The total iron content and total Al2O3 content and of the metallized briquettes varies slightly with the increasing anthracite ratio. The maximum size of the iron nuggets is 10.12 mm, and the highest Al2O3 grade of the slag is 43.07 wt %, which increased by 16.61 wt % compared with the Al2O3 in the bauxite ore. When the anthracite ratio further increases, the Al2O3 grade of the alumina slag decreases slightly. This observation is caused by the rapid increase in the carbon content with the increasing anthracite ratio, which leads to a decrease of Al2O3 grade in the metallized briquettes. However, the iron recovery rate increases markedly with increasing the anthracite ratio, and when the anthracite ratio is over 11.92 wt %, the iron recovery rate has almost no increases, and the highest iron recovery rate is 96.22%. Therefore, the appropriate increase in the anthracite ratio is beneficial to the growth of the iron nugget and the effective separation of the slag and iron. The optimal anthracite ratio is 11.92 wt % at the w(CaO)/w(SiO2) ratio, reduction time, and temperature, of 1.25, 15 min, and 1425 °C, respectively. Compared with the smelting reduction of the low-grade bauxites and red mud, the utilization rate of the pulverized coal of the semi-molten reduction process is very high, and the unreacted carbon content in the slag is only about 0.6 wt %–1.2 wt %, which is much lower than that of the unreacted coke of the smelting reduction process [26,27].

1375 1400 1425 1450

8

12

16

20

24

28

32

36

Temperature /℃

T.Fe

con

tent

/%, M

ax n

ugge

ts s

ize

/mm

87.5

90.0

92.5

95.0

97.5

100.0

Total iron content of metallized briquettes Maximum size of iron nuggets Metallization rate of iron Recovery rate of iron nuggets

Met

alliz

atio

n ra

te a

nd re

cove

ry ra

te /%

(a)

1375 1400 1425 145028.0

28.5

29.0

29.5

30.0

30.5

31.0

31.5

Total Al2O3 content of metallized briquettes Al2O3 content in slag

Temperature /℃

T.Al

2O3 c

onte

nt /%

42.0

42.2

42.4

42.6

42.8

43.0

43.2

43.4

Al2O

3 con

tent

in s

lag

(b)

Figure 9. Effect of reduction temperature on (a) reduction indexes and (b) Al2O3 grade.

It was found that the total iron content and Al2O3 grade of the metallized briquettes,the metallization rate of briquettes, and Al2O3 grade of the slag increases slightly with the increasingreduction temperature. The highest metallization rate of the briquettes maximum and Al2O3 gradeof slag are 98.81% and 43.35 wt %, respectively. However, the iron recovery rate increases markedly,and when the reduction temperature is over 1425 ◦C, the iron recovery rate increases slightly.In addition, the maximum iron nugget size follows a parabolic law with the increasing reductiontemperature, and the maximum iron nugget size is about 9.1 mm, which is obtained at a temperatureof 1425 ◦C.

3.1.4. Effect of the Anthracite Ratio

The anthracite ratio has an important influence on the direct reduction, carbon gasification reaction,and solid-state carburization reaction. In order to obtain the optimal anthracite ratio, the variousanthracite ratios (from 8.82 wt % to 13.40 wt %) were investigated with a constant process parameter,and the basicity, temperature, and time are 1.25, 1425 ◦C, and 15 min, respectively. The effect of theanthracite ratio on the recovery of the iron and alumina slag are presented in Figure 10a,b, respectively.Minerals 2019, 9, x FOR PEER REVIEW 10 of 16

Figure 10. Effect of anthracite ratio on the (a) reduction indexes and (b) Al2O3 grade.

3.1.5. Effect of the CaF2 Ratio

Recent semi-smelting reduction experimental studies show that increasing the content of CaF2 can significantly reduce the slag viscosity, and promote the separation of slag and iron [28,29]. However, the melting point of the alumina slag system is very high (shown in Figure 11a). When the basicity is 1.25 and the CaF2 ratios range from 0 wt % to 5.67 wt %, the melting points of the high alumina slag (CaO–Al2O3–CaF2–SiO2) are over 1470 °C, and the slag cannot be completely melted in the range of the experimental temperature (1375–1450 °C). So, it is impossible to obtain the true viscosity of the alumina slag using the experimental method. In order to investigate the effect of the CaF2 ratios (0 wt %–5.67 wt %) and the viscosity changeless on the semi-molten reduction, FactSage 7.0 thermodynamic software was used to simulate the viscosity of the CaO–Al2O3–CaF2–SiO2 system, and is presented in Figure 11b. It was found that the temperature and CaF2 ratio have an obvious effect on the slag viscosity. The higher the temperature and CaF2 ratio, the lower the slag viscosity. However, when the temperature is more than 1450 °C and the CaF2 ratio is over the 4.61 wt %, with the increasing CaF2 ratio and temperature, the slag viscosity decreased slightly. Therefore, it is necessary to further optimize the CaF2 ratio.

Figure 11. Effect of (a) temperature and (b) CaF2 content on the slag viscosity.

In order to obtain the optimal CaF2 ratio, the various CaF2 ratios (0 wt %–5.67 wt %) were investigated with a constant process parameter, and the basicity, anthracite ratio, temperature, time are 1.25, 11.92 wt %, 1425 °C, and 15 min, respectively. The effect of the CaF2 ratios on the recovery of the iron and alumina slag are presented in Figure 12a,b, respectively.

It was found that the total iron content of the metallized briquettes decreases slightly with the increasing CaF2 ratio. The maximum size of the iron nuggets, the metallization rate of the briquettes,

8.8 9.6 10.4 11.2 12.0 12.8 13.68

12

16

20

24

28

32

36

Anthracite ratio /%

T.Fe

con

tent

/%, M

ax n

ugge

ts s

ize

/mm (a)

80

85

90

95

100


Met

alliz

atio

n ra

te a

nd re

cove

ry ra

te /%

8.8 9.6 10.4 11.2 12.0 12.8 13.6

27.8

28.0

28.2

28.4

28.6


Anthracite ratio /%

T.Al

2O3 c

onte

nt /%

42.0

42.2

42.4

42.6

42.8

43.0

43.2

Al2O

3 con

tent

in s

lag

(b)

0 2 4 6 8 10

1440

1480

1520

1560

1600

Mel

ting

tem

pera

ture

/℃

CaF2 /%

(a)

1470℃

1300 1325 1350 1375 1400 1425 1450 1475 15000

5

10

15

20

25(b)

Visc

/Pa·

s

T/℃

CaF2 ratios 0 wt% 4.61 wt% 8.82 wt% 12.67 wt% 16.21 wt%

SiO2-Al2O3-CaO-CaF2 system w(CaO)/w(SiO2)=1.25


It was found that the metallization rate of the briquettes, the maximum size of the iron nugget,and the Al2O3 grade of the slag increases slightly with the increasing anthracite ratio. The total ironcontent and total Al2O3 content and of the metallized briquettes varies slightly with the increasinganthracite ratio. The maximum size of the iron nuggets is 10.12 mm, and the highest Al2O3 grade ofthe slag is 43.07 wt %, which increased by 16.61 wt % compared with the Al2O3 in the bauxite ore.When the anthracite ratio further increases, the Al2O3 grade of the alumina slag decreases slightly.This observation is caused by the rapid increase in the carbon content with the increasing anthraciteratio, which leads to a decrease of Al2O3 grade in the metallized briquettes. However, the iron recovery

Minerals 2019, 9, 223 10 of 16

rate increases markedly with increasing the anthracite ratio, and when the anthracite ratio is over11.92 wt %, the iron recovery rate has almost no increases, and the highest iron recovery rate is 96.22%.Therefore, the appropriate increase in the anthracite ratio is beneficial to the growth of the iron nuggetand the effective separation of the slag and iron. The optimal anthracite ratio is 11.92 wt % at thew(CaO)/w(SiO2) ratio, reduction time, and temperature, of 1.25, 15 min, and 1425 ◦C, respectively.Compared with the smelting reduction of the low-grade bauxites and red mud, the utilization rateof the pulverized coal of the semi-molten reduction process is very high, and the unreacted carboncontent in the slag is only about 0.6 wt %–1.2 wt %, which is much lower than that of the unreactedcoke of the smelting reduction process [23,26].


Recent semi-smelting reduction experimental studies show that increasing the content of CaF2

can significantly reduce the slag viscosity, and promote the separation of slag and iron [27,28].However, the melting point of the alumina slag system is very high (shown in Figure 11a). When thebasicity is 1.25 and the CaF2 ratios range from 0 wt % to 5.67 wt %, the melting points of the highalumina slag (CaO–Al2O3–CaF2–SiO2) are over 1470 ◦C, and the slag cannot be completely meltedin the range of the experimental temperature (1375–1450 ◦C). So, it is impossible to obtain the trueviscosity of the alumina slag using the experimental method. In order to investigate the effect of theCaF2 ratios (0 wt %–5.67 wt %) and the viscosity changeless on the semi-molten reduction, FactSage 7.0thermodynamic software was used to simulate the viscosity of the CaO–Al2O3–CaF2–SiO2 system,and is presented in Figure 11b. It was found that the temperature and CaF2 ratio have an obviouseffect on the slag viscosity. The higher the temperature and CaF2 ratio, the lower the slag viscosity.However, when the temperature is more than 1450 ◦C and the CaF2 ratio is over the 4.61 wt %, with theincreasing CaF2 ratio and temperature, the slag viscosity decreased slightly. Therefore, it is necessaryto further optimize the CaF2 ratio.




Recent semi-smelting reduction experimental studies show that increasing the content of CaF2 can significantly reduce the slag viscosity, and promote the separation of slag and iron [28,29]. However, the melting point of the alumina slag system is very high (shown in Figure 11a). When the basicity is 1.25 and the CaF2 ratios range from 0 wt % to 5.67 wt %, the melting points of the high alumina slag (CaO–Al2O3–CaF2–SiO2) are over 1470 °C, and the slag cannot be completely melted in the range of the experimental temperature (1375–1450 °C). So, it is impossible to obtain the true viscosity of the alumina slag using the experimental method. In order to investigate the effect of the CaF2 ratios (0 wt %–5.67 wt %) and the viscosity changeless on the semi-molten reduction, FactSage 7.0 thermodynamic software was used to simulate the viscosity of the CaO–Al2O3–CaF2–SiO2 system, and is presented in Figure 11b. It was found that the temperature and CaF2 ratio have an obvious effect on the slag viscosity. The higher the temperature and CaF2 ratio, the lower the slag viscosity. However, when the temperature is more than 1450 °C and the CaF2 ratio is over the 4.61 wt %, with the increasing CaF2 ratio and temperature, the slag viscosity decreased slightly. Therefore, it is necessary to further optimize the CaF2 ratio.


In order to obtain the optimal CaF2 ratio, the various CaF2 ratios (0 wt %–5.67 wt %) were investigated with a constant process parameter, and the basicity, anthracite ratio, temperature, time are 1.25, 11.92 wt %, 1425 °C, and 15 min, respectively. The effect of the CaF2 ratios on the recovery of the iron and alumina slag are presented in Figure 12a,b, respectively.

It was found that the total iron content of the metallized briquettes decreases slightly with the increasing CaF2 ratio. The maximum size of the iron nuggets, the metallization rate of the briquettes,

8.8 9.6 10.4 11.2 12.0 12.8 13.68

12

16

20

24

28

32

36

Anthracite ratio /%

T.Fe

con

tent

/%, M

ax n

ugge

ts s

ize

/mm (a)

80

85

90

95

100


Met

alliz

atio

n ra

te a

nd re

cove

ry ra

te /%

8.8 9.6 10.4 11.2 12.0 12.8 13.6

27.8

28.0

28.2

28.4

28.6


Anthracite ratio /%

T.Al

2O3 c

onte

nt /%

42.0

42.2

42.4

42.6

42.8

43.0

43.2

Al2O

3 con

tent

in s

lag

(b)

0 2 4 6 8 10

1440

1480

1520

1560

1600

Mel

ting

tem

pera

ture

/℃

CaF2 /%

(a)

1470℃

1300 1325 1350 1375 1400 1425 1450 1475 15000

5

10

15

20

25(b)

Visc

/Pa·

s

T/℃

CaF2 ratios 0 wt% 4.61 wt% 8.82 wt% 12.67 wt% 16.21 wt%

SiO2-Al2O3-CaO-CaF2 system w(CaO)/w(SiO2)=1.25


In order to obtain the optimal CaF2 ratio, the various CaF2 ratios (0 wt %–5.67 wt %) wereinvestigated with a constant process parameter, and the basicity, anthracite ratio, temperature, time are1.25, 11.92 wt %, 1425 ◦C, and 15 min, respectively. The effect of the CaF2 ratios on the recovery of theiron and alumina slag are presented in Figure 12a,b, respectively.

Minerals 2019, 9, 223 11 of 16


the iron recovery rate, and the Al2O3 grade of the slag show a trend of first increasing and then decreasing, and the highest values are obtained at a CaF2 ratio of 1.96 wt %. The maximum size of the iron nuggets is 15.5 mm, the highest metallization rate of the briquettes, the iron recovery rate, and the Al2O3 grade of the slag are 98.84%, 96.84%, and 42.98%, respectively. When the CaF2 ratio further increases, the metallization rate of the briquettes and the iron recovery rate decrease rapidly, whereas the maximum particles size of the iron nugget decrease gradually. The Al2O3 grade of the metallized briquettes and alumina slag increase sharply with the increasing anthracite ratio. The highest Al2O3 grade of the metallized briquettes and the alumina slag are observed at an anthracite ratio of 1.96 wt %. When the CaF2 ratio further increases, the Al2O3 grade of the metallized briquettes and the alumina slag decrease sharply.

Therefore, increasing the CaF2 ratio by 1.96 wt % is beneficial to the growth of the iron nugget and for the effective separation of the slag and iron. However, if excessive CaF2 is added into bauxite–coal composite briquettes, the CaO content of the alumina slag will increase, and the metallization rate of the briquettes and the iron recovery rate will decrease. Therefore, the optimal CaF2 addition is 1.96 wt % when the w(CaO)/w(SiO2) ratio, reduction time, and temperature are 1.25, 15 min, and 1425 °C, respectively.

Figure 12. Effect of the CaF2 ratio on the reduction indexes (a) and Al2O3 (b).

3.2. Properties Characterization of Reduction Products

According to the above studies, the optimum process parameters should be a w(CaO)/w(SiO2) ratio of 1.25, reduction temperature of 1425 °C, time of 15 min, anthracite ratio of 11.92 wt %, and CaF2 ratio of 1.96 wt %. Under optimal conditions, the reduced briquettes were crushed to 1–10 mm by the jaw crusher, then the iron nugget and alumina slag were magnetic separated by the high strength roller dry magnetic separator, with a magnetic intensity of 4000 Gs. The iron nugget and alumina slag are obtained by a semi-smelting reduction and magnetic separation process, and are presented in Figure 13. The maximum particle size of the iron nuggets is about 15.5 mm, and the recovery rate of iron and Al2O3 content of alumina slag are 96.84 wt % and 43.98 wt %, respectively.

0 1 2 3 4 5 60

5

10

15

20

25

30

35

Total iron content Maximum size of iron nuggets Metallization rate of iron Recovery rate of iron nuggets

CaF2 Content /%

T.Fe

con

tent

/%, M

ax n

ugge

ts s

ize

/mm

82.5

85.0

87.5

90.0

92.5

95.0

97.5

100.0

Sepe

ratio

n in

dexe

s / %

(a)

0 1 2 3 4 5 6

28.0

28.5

29.0

29.5

30.0

30.5

Total Al2O3 content Al2O3 content in slag

CaF2 ratio /%

T.Al

2O3 c

onte

nt /%

(b)

36

37

38

39

40

41

42

43

44

Al2O

3 con

tent

in s

lag

Figure 12. Effect of the CaF2 ratio on the reduction indexes (a) and Al2O3 (b).

It was found that the total iron content of the metallized briquettes decreases slightly with theincreasing CaF2 ratio. The maximum size of the iron nuggets, the metallization rate of the briquettes,the iron recovery rate, and the Al2O3 grade of the slag show a trend of first increasing and thendecreasing, and the highest values are obtained at a CaF2 ratio of 1.96 wt %. The maximum sizeof the iron nuggets is 15.5 mm, the highest metallization rate of the briquettes, the iron recoveryrate, and the Al2O3 grade of the slag are 98.84%, 96.84%, and 42.98%, respectively. When the CaF2

ratio further increases, the metallization rate of the briquettes and the iron recovery rate decreaserapidly, whereas the maximum particles size of the iron nugget decrease gradually. The Al2O3 gradeof the metallized briquettes and alumina slag increase sharply with the increasing anthracite ratio.The highest Al2O3 grade of the metallized briquettes and the alumina slag are observed at an anthraciteratio of 1.96 wt %. When the CaF2 ratio further increases, the Al2O3 grade of the metallized briquettesand the alumina slag decrease sharply.

Therefore, increasing the CaF2 ratio by 1.96 wt % is beneficial to the growth of the iron nugget andfor the effective separation of the slag and iron. However, if excessive CaF2 is added into bauxite–coalcomposite briquettes, the CaO content of the alumina slag will increase, and the metallization rateof the briquettes and the iron recovery rate will decrease. Therefore, the optimal CaF2 additionis 1.96 wt % when the w(CaO)/w(SiO2) ratio, reduction time, and temperature are 1.25, 15 min,and 1425 ◦C, respectively.

3.2. Properties Characterization of Reduction Products

According to the above studies, the optimum process parameters should be a w(CaO)/w(SiO2)ratio of 1.25, reduction temperature of 1425 ◦C, time of 15 min, anthracite ratio of 11.92 wt %, and CaF2

ratio of 1.96 wt %. Under optimal conditions, the reduced briquettes were crushed to 1–10 mm bythe jaw crusher, then the iron nugget and alumina slag were magnetic separated by the high strengthroller dry magnetic separator, with a magnetic intensity of 4000 Gs. The iron nugget and aluminaslag are obtained by a semi-smelting reduction and magnetic separation process, and are presented inFigure 13. The maximum particle size of the iron nuggets is about 15.5 mm, and the recovery rate ofiron and Al2O3 content of alumina slag are 96.84 wt % and 43.98 wt %, respectively.

Minerals 2019, 9, 223 12 of 16


Figure 13. The photos of the iron nuggets and alumina obtained at the optimal condition.

3.2.1. X-Ray Diffraction Analysis

After magnetic separation, the alumina slag obtained at the optimal process were crushed to a particle size of 80% lower than 74 μm, then the phase composition of the alumina slag was investigated using XRD (Figure 14). It was found that the peaks of Fe2O3 disappear completely, and most of the metallic iron is separated through magnetic separation. A small number of intermediate products are observed in the XRD pattern, such as hercynite (FeAl4O7) and fayalite (Fe2SiO4). The alumina slag is mainly composed of gehlenite (Ca2Al2SiO7) and calcium hexaluminate (CaAl12O19), with small amounts of Fe2SiO4, FeAl2O4, manganese oxide (Mn3O4), and metallic iron. These results can be explained by the thermodynamic analysis of Figure 5, beecause the standard free energy changes of Equations (21), (22), and (23) are more negative than that of Equations (19) and (20).

Figure 14. The XRD pattern of the alumina slag.

3.2.2. SEM–EDS Analysis

The SEM micrographs and EDS analyses of the iron nugget obtained at the optimal condition are shown in Figure 15 and Table 3, respectively. We found that the iron nugget had numerous cracks and holes, and almost no micro inclusions. The iron nugget is mainly composed of Fe, C, and Mn, and the concentration of harmful elements such as S and P is very low. The concentration of Fe, C, and Mn are 94.48 wt %, 3.94 wt %, and 1.42 wt %, respectively. The concentrations of Si and P are 0.1 wt % and 0.06 wt %, respectively. Compared with the magnetic products obtained by coal-based direct reduction and magnetic separation technology, the iron nugget products have higher Fe and C contents, which have similar properties to the blast furnace pig iron and white cast iron [30–32].

10 20 30 40 50 60 70 80 90

500

1000

1500

2000

2500

3000

5

7

1 6

67

6

5

5

7

77

3

45

1

1

1

24

2

Inte

nsity

/Cou

nts

2θ(°)

23

1-Metallic iron (Fe)2-Alumina (Al2O3)3-Hercynite (FeAl2O4)4-Fayalite (Fe2SiO4)5-Manganese oxide (Mn3O4)6-Gehlenite (Ca2Al2SiO7)7-Calcium hexaluminate (CaAl12O19)



After magnetic separation, the alumina slag obtained at the optimal process were crushedto a particle size of 80% lower than 74 µm, then the phase composition of the alumina slag wasinvestigated using XRD (Figure 14). It was found that the peaks of Fe2O3 disappear completely,and most of the metallic iron is separated through magnetic separation. A small number of intermediateproducts are observed in the XRD pattern, such as hercynite (FeAl4O7) and fayalite (Fe2SiO4).The alumina slag is mainly composed of gehlenite (Ca2Al2SiO7) and calcium hexaluminate (CaAl12O19),with small amounts of Fe2SiO4, FeAl2O4, manganese oxide (Mn3O4), and metallic iron. These resultscan be explained by the thermodynamic analysis of Figure 5, beecause the standard free energy changesof Equations (21), (22), and (23) are more negative than that of Equations (19) and (20).




After magnetic separation, the alumina slag obtained at the optimal process were crushed to a particle size of 80% lower than 74 μm, then the phase composition of the alumina slag was investigated using XRD (Figure 14). It was found that the peaks of Fe2O3 disappear completely, and most of the metallic iron is separated through magnetic separation. A small number of intermediate products are observed in the XRD pattern, such as hercynite (FeAl4O7) and fayalite (Fe2SiO4). The alumina slag is mainly composed of gehlenite (Ca2Al2SiO7) and calcium hexaluminate (CaAl12O19), with small amounts of Fe2SiO4, FeAl2O4, manganese oxide (Mn3O4), and metallic iron. These results can be explained by the thermodynamic analysis of Figure 5, beecause the standard free energy changes of Equations (21), (22), and (23) are more negative than that of Equations (19) and (20).



The SEM micrographs and EDS analyses of the iron nugget obtained at the optimal condition are shown in Figure 15 and Table 3, respectively. We found that the iron nugget had numerous cracks and holes, and almost no micro inclusions. The iron nugget is mainly composed of Fe, C, and Mn, and the concentration of harmful elements such as S and P is very low. The concentration of Fe, C, and Mn are 94.48 wt %, 3.94 wt %, and 1.42 wt %, respectively. The concentrations of Si and P are 0.1 wt % and 0.06 wt %, respectively. Compared with the magnetic products obtained by coal-based direct reduction and magnetic separation technology, the iron nugget products have higher Fe and C contents, which have similar properties to the blast furnace pig iron and white cast iron [30–32].

10 20 30 40 50 60 70 80 90

500

1000

1500

2000

2500

3000

5

7

1 6

67

6

5

5

7

77

3

45

1

1

1

24

2

Inte

nsity

/Cou

nts

2θ(°)

23

1-Metallic iron (Fe)2-Alumina (Al2O3)3-Hercynite (FeAl2O4)4-Fayalite (Fe2SiO4)5-Manganese oxide (Mn3O4)6-Gehlenite (Ca2Al2SiO7)7-Calcium hexaluminate (CaAl12O19)



The SEM micrographs and EDS analyses of the iron nugget obtained at the optimal condition areshown in Figure 15 and Table 3, respectively. We found that the iron nugget had numerous cracksand holes, and almost no micro inclusions. The iron nugget is mainly composed of Fe, C, and Mn,and the concentration of harmful elements such as S and P is very low. The concentration of Fe, C,and Mn are 94.48 wt %, 3.94 wt %, and 1.42 wt %, respectively. The concentrations of Si and P are0.1 wt % and 0.06 wt %, respectively. Compared with the magnetic products obtained by coal-baseddirect reduction and magnetic separation technology, the iron nugget products have higher Fe and Ccontents, which have similar properties to the blast furnace pig iron and white cast iron [29–31].

Minerals 2019, 9, 223 13 of 16


Figure 15. The SEM micrographs (a) and energy dispersive X-ray spectroscope (EDS) spectrums (b) of the iron nugget obtained at the optimal condition.

Table 3. Energy dispersive X-ray spectroscope (EDS) results of the iron nuggets obtained at the optimal condition.

Fe C Si Mn S P 94.48 3.94 0.10 1.42 0.002 0.058

The SEM micrographs of the alumina slag are shown in Figure 15, and the EDS results are shown in Table 4. The SEM–EDS analysis indicates that the alumina (α-Al2O3), calcium hexaluminate (CaAl12O19), and gehlenite (Ca2Al2SiO7) are major minerals in the alumina slag. However, the contents of hercynite (FeAl4O7) and metallic iron (M.Fe) are lower in the slag, as confirmed by the previous XRD pattern (Figure 13). Most alumina (α-Al2O3) is needle shaped with a black color (Figure 16a-spot 1); furthermore, the crystal shape of hercynite (FeAl4O7) is rhombic or an irregular hexagon (Figure 16a,b-area 2), and the crystal shape of calcium hexaluminate (CaAl12O19) is a regular hexagon (Figure 16b-area 4).

Figure 16. SEM micrographs of the alumina slag with alumina phase (a) and the alumina slag (b) obtained at the optimal condition.

Table 4. EDS results of the alumina slag obtained at the optimal condition.

Test Positions Analysis Result/at % Mineral Names /Chemical

Structural Formula Fe C Si Al Ca O Mn Ti Co Ni Figure 16a-

spot 1 - - - 49.62 - 50.38 - - - - Alumina/α-Al2O3

Figure 15. The SEM micrographs (a) and energy dispersive X-ray spectroscope (EDS) spectrums (b) ofthe iron nugget obtained at the optimal condition.

Table 3. Energy dispersive X-ray spectroscope (EDS) results of the iron nuggets obtained at theoptimal condition.

Fe C Si Mn S P

94.48 3.94 0.10 1.42 0.002 0.058

The SEM micrographs of the alumina slag are shown in Figure 15, and the EDS results areshown in Table 4. The SEM–EDS analysis indicates that the alumina (α-Al2O3), calcium hexaluminate(CaAl12O19), and gehlenite (Ca2Al2SiO7) are major minerals in the alumina slag. However, the contentsof hercynite (FeAl4O7) and metallic iron (M.Fe) are lower in the slag, as confirmed by the previousXRD pattern (Figure 13). Most alumina (α-Al2O3) is needle shaped with a black color (Figure 16a-spot1); furthermore, the crystal shape of hercynite (FeAl4O7) is rhombic or an irregular hexagon(Figure 16a,b-area 2), and the crystal shape of calcium hexaluminate (CaAl12O19) is a regular hexagon(Figure 16b-area 4).


Test PositionsAnalysis Result/at % Mineral Names/Chemical Structural

FormulaFe C Si Al Ca O Mn Ti Co Ni

Figure 16a-spot 1 - - - 49.62 - 50.38 - - - - Alumina/α-Al2O3Figure 16a-area 2 12.60 - - 35.26 0.82 45.74 1.73 - - - Hercynite/FeAl4O7Figure 16a-area 3 2.91 - 9.72 18.01 15.25 51.61 0.64 1.22 - - Gehlenite/Ca2Al2SiO7Figure 16b-spot 1 90.41 - - - 0.63 - - - 1.18 2.51 Metallic iron/FeFigure 16b-area 2 15.13 - - 34.34 0.36 46.05 1.42 - - - Hercynite/FeAl4O7Figure 16b-area 3 2.93 - 9.57 17.78 15.01 52.43 0.44 1.20 - - Gehlenite/Ca2Al2SiO7Figure 16b-area 4 3.70 - 0.72 38.76 4.97 51.85 - - - - Calcium hexaluminate/CaAl12O19

Minerals 2019, 9, 223 14 of 16


Figure 15. The SEM micrographs (a) and energy dispersive X-ray spectroscope (EDS) spectrums (b) of the iron nugget obtained at the optimal condition.

Table 3. Energy dispersive X-ray spectroscope (EDS) results of the iron nuggets obtained at the optimal condition.

Fe C Si Mn S P 94.48 3.94 0.10 1.42 0.002 0.058

The SEM micrographs of the alumina slag are shown in Figure 15, and the EDS results are shown in Table 4. The SEM–EDS analysis indicates that the alumina (α-Al2O3), calcium hexaluminate (CaAl12O19), and gehlenite (Ca2Al2SiO7) are major minerals in the alumina slag. However, the contents of hercynite (FeAl4O7) and metallic iron (M.Fe) are lower in the slag, as confirmed by the previous XRD pattern (Figure 13). Most alumina (α-Al2O3) is needle shaped with a black color (Figure 16a-spot 1); furthermore, the crystal shape of hercynite (FeAl4O7) is rhombic or an irregular hexagon (Figure 16a,b-area 2), and the crystal shape of calcium hexaluminate (CaAl12O19) is a regular hexagon (Figure 16b-area 4).

Figure 16. SEM micrographs of the alumina slag with alumina phase (a) and the alumina slag (b) obtained at the optimal condition.


Test Positions Analysis Result/at % Mineral Names /Chemical

Structural Formula Fe C Si Al Ca O Mn Ti Co Ni Figure 16a-

spot 1 - - - 49.62 - 50.38 - - - - Alumina/α-Al2O3

Figure 16. SEM micrographs of the alumina slag with alumina phase (a) and the alumina slag(b) obtained at the optimal condition.

3.2.3. Chemical Composition Analysis

The major elements of the iron nuggets are listed in Table 5. It can be seen that the Fe, C, and Mn arethe major elements in the iron nuggets, and the S and P are minor components. The total iron contentis very high over 94.31 wt %, and the carbon and manganese contents are 3.86 wt % and 1.63 wt %,respectively. Almost no harmful elements were found in the nugget, such as S and P. The majorelement compositions of the alumina slag are presented in Table 6. We found that the slag basicity(w(CaO)/w(SiO2)) is about 1.25, and the alumina slag mainly consisted of 43.36 wt % Al2O3, 21.61 wt% SiO2, and 21.77 wt % CaO. According to the analysis results in Figure 4, the slag is mainly composedof liquid slag, Al2O3, calcium dialuminate (CaAl4O7), and calcium hexaluminate (CaAl12O19). As forthe application of the alumina slag, we mainly use it to synthesize the CaAl2Si2O8–Al2O3–CaAl12O19

composites, and the related research results are shown in previous research literature [32].

Table 5. Major element compositions of iron nuggets.

Fe C Si S P Mn

94.31 3.86 0.13 0.005 0.065 1.63

Table 6. Major element compositions of alumina slag.

FeO Al2O3 SiO2 CaO MnO TiO2 V2O5

2.26 43.36 21.75 27.11 2.56 2.65 0.31

4. Conclusions

(1) The semi-smelting reduction and magnetic separation technology provides the possibility ofthe comprehensive utilization of iron rich bauxite. Although the laboratory results are very good,there are still some problems that need to be solved in the industrial popularization and applicationof this technology. The precise control of the temperature and atmosphere temperature of industrialfurnaces are the key technologies for its industrial application.

(2) The optimum process of the parameters of the semi-smelting reduction and magneticseparation process should be w(CaO)/w(SiO2) ratio of 1.25, temperature of 1425 ◦C, time of 15 min,anthracite ratio of 11.92 wt %, CaF2 ratio of 1.96 wt %, and magnetic intensity of 4000 Gs.

Minerals 2019, 9, 223 15 of 16

(3) High quality iron nugget and high-grade alumina slag can be obtained by a semi-meltingreduction and magnetic separation process. The highest iron recovery rate and Al2O3 grade of aluminaslag are 96.84 wt % and 43.98 wt %, respectively.

Author Contributions: Conceptualization, Y.Z. and Y.Q.; methodology, Y.Z.; software, Y.Z. and J.Z.; validation,Y.Z. and M.L.; formal analysis, Q.G.; investigation, Y.Z.; resources, Y.Z.; data curation, Y.Z.; writing (original draftpreparation), Y.Z.; writing (review and editing), Y.Z. and Q.G.; supervision, Y.Z.; project administration, Y.Z.;funding acquisition, Y.Q.

Funding: This research was funded by the National Key R&D Program of China (2017YFB0603800 and2017YFB0603802); the National Natural Science Foundation (51604049); and the Fundamental Research Funds forthe Central Universities, China (N182504008).

Acknowledgments: The authors wish to acknowledge the contributions of associates and colleagues at AnhuiUniversity of Technology, Northeastern University and Central Iron and Steel Research Institute of China.The financial support of the National Key R&D Program of China, the National Natural Science Foundation andthe Fundamental Research Funds for the Central Universities, China are also appreciated.

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

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