The Effects of Different Zn Forms on Sintering Basic Characteristics of Iron Ore

The micro-sintering method was used to determine the sintering basic characteristics of iron ore with Zn contents from 0 to 4%, the influence mechanism of Zn on sintering basic characteristics of iron ore was clarified by means of thermodynamic analysis and first-principles calculations. The results showed that (1) increasing the ZnO and ZnFe2O4 content increased the lowest assimilation temperature (LAT) but decreased the index of liquid phase fluidity (ILF) of iron ore. The addition of ZnS had no obvious effect on LAT but increased the LIF of iron ore. (2) ZnO and ZnFe2O4 reacted with Fe2O3 and CaO, respectively, during sintering, which inhibited the formation of silico-ferrite of calcium and aluminum (SFCA). The addition of ZnS accelerated the decomposition of Fe2O3 in the N2 atmosphere; however, the high decomposition temperature limited the oxidation of ZnS, so the presence of ZnS had a slight inhibitory effect on the formation of SFCA. (3) The Zn concentrated in hematite or silicate and less distributed in SFCA and magnetite in the form of solid solution; meanwhile, the microhardness of the mineral phase decreased with the increase in Zn-containing solid solution content. As the adsorption of Zn on the SFCA crystal surface was more stable, the microhardness of SFCA decreased more. The decrease in microhardness and content of the SFCA bonding phase resulted in a decrease in the compressive strength of the sinter.


Introduction
At present, the main iron-bearing charge of blast furnaces in China remains the sinter, accounting for about 70-80% of the blast furnace burden [1][2][3].Hence, the chemical composition of the sinter significantly affects the operation stability of the blast furnace.Zn compounds in sinter are reduced to Zn vapor in the high-temperature area of the blast furnace, most of Zn vapor is oxidized to ZnO, which is adsorbed on the surfaces of charges and furnace wall, resulting in the worsening of permeability of the charges and corrosion of refractories [4][5][6][7].In recent years, domestic steel enterprises have regarded metallurgical dust generated in the steel production process as sintering raw materials to control the cost and reduce the pollution [8,9].Metallurgical dust not only contains abundant Fe and C resources but also contains large amounts of harmful elements such as Zn [10][11][12].The recycling of metallurgical dust by sintering increases the Zn content in the sinter.Therefore, it is necessary to reveal the effect of Zn on sintering basic characteristics of iron ore, which provides theoretical guidance for optimizing sintering parameters to produce high-quality sinter.
The forms of Zn in sintering raw materials are various.Zn mainly presents in the form of sphalerite (ZnS) and zincite (ZnO) in natural ores [13].Wang et al. [14] adopted X-ray absorption spectroscopy (XAS) to examine the Zn-containing phase in metallurgical dust, and the results showed that Zn mainly existed in the form of ZnO and ZnFe 2 O 4 in metallurgical dust.In the last few years, previous studies have mainly focused on the sintering behavior of Zn compounds.Wang et al. [15] investigated the phase transformation of Zn during sintering by thermodynamic calculation, and the results identified that the Zn compounds such as ZnS, ZnO, and Zn 2 SiO 4 in industry sintering blends were mostly transformed into ZnFe 2 O 4 , only ZnCl 2 and a minimal amount of Zn reduced from ZnO were removed by vaporization during sintering.Lv et al. [16] studied the reaction behavior of ZnO under the reducing atmosphere, and the results showed that increasing the temperature and decreasing the CO content were conducive to the conversion of Fe 2+ to Fe 3+ , which correspondingly restrained the reduction of ZnO.Although revealing the sintering behavior of Zn compounds contributes to improving the removal rate of Zn during sintering, there is a threshold for the Zn removal rate.Over the last few decades, many metallurgists have begun to study the effect of gangue on the sintering basic characteristics of iron ore [17][18][19][20], nevertheless, there is little research on the effect of Zn on the sintering basic characteristics of iron ore, and relevant studies only stay at the level of pure reagent [21].The mechanism that influences the high-temperature sintering behavior of Zn compounds on the sintering basic characteristics of iron ore is unclear.
As mentioned above, Zn in sintering raw materials is mainly in the form of ZnO, ZnFe 2 O 4 , and ZnS.In this paper, the chemical reagents ZnO, ZnFe 2 O 4 , and ZnS were chosen as Zn resources to constitute the experimental samples with iron ore.Thermogravimetry (TG) was performed on the samples of iron ore containing different contents and forms of Zn to investigate the thermal decomposition behavior during sintering.The sintering basic characteristics of iron ore were measured by the micro-sintering method; meanwhile, firstprinciples calculations and thermodynamic calculations were used to clarify the influence mechanism of Zn on the sintering basic characteristics of iron ore.

Materials
The chemical composition of iron ore provided by a domestic sintering plant is shown in Table 1, and the ZnO content in iron ore is no more than 0.01%.The X-ray diffraction (XRD) pattern and Scanning electron microscope (SEM) photo of iron ore are shown in Figures 1 and 2, respectively.Figure 1 indicates that the main iron-bearing mineral is hematite.Before preparing the experimental sample, iron ore and chemical reagents were ground to powder with a particle size of less than 0.15 mm and heated in the oven at 100 • C for 3 h to eliminate the effects of particle size and moisture, respectively.dust, and the results showed that Zn mainly existed in the form of ZnO and ZnFe2O4 in metallurgical dust.In the last few years, previous studies have mainly focused on the sin tering behavior of Zn compounds.Wang et al. [15] investigated the phase transformation of Zn during sintering by thermodynamic calculation, and the results identified that the Zn compounds such as ZnS, ZnO, and Zn2SiO4 in industry sintering blends were mostly transformed into ZnFe2O4, only ZnCl2 and a minimal amount of Zn reduced from ZnO were removed by vaporization during sintering.Lv et al. [16] studied the reaction behav ior of ZnO under the reducing atmosphere, and the results showed that increasing the temperature and decreasing the CO content were conducive to the conversion of Fe 2+ to Fe 3+ , which correspondingly restrained the reduction of ZnO.Although revealing the sintering behavior of Zn compounds contributes to improving the removal rate of Zn during sintering, there is a threshold for the Zn removal rate.Over the last few decades, many metallurgists have begun to study the effect of gangue on the sintering basic characteristics of iron ore [17][18][19][20], nevertheless, there is little research on the effect of Zn on the sintering basic characteristics of iron ore, and relevant studies only stay at the level of pure reagent [21].The mechanism that influences the high-temperature sintering behavior o Zn compounds on the sintering basic characteristics of iron ore is unclear.
As mentioned above, Zn in sintering raw materials is mainly in the form of ZnO ZnFe2O4, and ZnS.In this paper, the chemical reagents ZnO, ZnFe2O4, and ZnS were cho sen as Zn resources to constitute the experimental samples with iron ore.Thermogravim etry (TG) was performed on the samples of iron ore containing different contents and forms of Zn to investigate the thermal decomposition behavior during sintering.The sin tering basic characteristics of iron ore were measured by the micro-sintering method meanwhile, first-principles calculations and thermodynamic calculations were used to clarify the influence mechanism of Zn on the sintering basic characteristics of iron ore.

Materials
The chemical composition of iron ore provided by a domestic sintering plant is shown in Table 1, and the ZnO content in iron ore is no more than 0.01%.The X-ray dif fraction (XRD) pattern and Scanning electron microscope (SEM) photo of iron ore are shown in Figure 1 and Figure 2, respectively.Figure 1 indicates that the main iron-bearing mineral is hematite.Before preparing the experimental sample, iron ore and chemical re agents were ground to powder with a particle size of less than 0.15 mm and heated in the oven at 100 °C for 3 h to eliminate the effects of particle size and moisture, respectively.The chemical composition of iron ore with different contents and forms of Z shown in Table 2.The Zn content in the iron ore was limited to 0-4%; ZO, ZF, and represent ZnO (≥99.0%,XiLong Scientific, Chengdu, China), ZnFe2O4 (≥99.0%,Le Shanghai, China), and ZnS (≥99.0%,Rhawn, Shanghai, China), respectively.

Micro-Sintering Experiment
Referring to relevant literature [22,23], the micro-sintering experiment is as follo

Assimilation performance
Based on the data in Table 2, the ore powder and chemical reagents were thorou mixed with an appropriate amount of anhydrous ethanol for 30 min and then dried in electrothermal blowing dry box (101-0ASB, ±1 °C, Beijing Kewei Yongxing Instrument Ltd., Beijing, China) at 100 °C for 2 h.The experimental sample was obtained by mi the dried mixture in a ball milling for 20 min.The 1.0 g experimental sample and 2 CaO (≥98.0%,Damao, Tianjin, China) were weighed and filled into a steel mold and pressed into iron ore compact with a diameter of 10 mm and CaO compact with a diam of 20 mm under 20 MPa, respectively.The iron ore compact was placed on the CaO c pact and then sent into the micro-sintering device (BR-14AS-12, 1400 °C, Zhengzhou B Heat Kiln Co., Ltd., Zhengzhou, China) for roasting.The temperature was first raise The chemical composition of iron ore with different contents and forms of Zn is shown in Table 2.The Zn content in the iron ore was limited to 0-4%; ZO, ZF, and ZS represent ZnO (≥99.0%,XiLong Scientific, Chengdu, China), ZnFe 2 O 4 (≥99.0%,LeYan, Shanghai, China), and ZnS (≥99.0%,Rhawn, Shanghai, China), respectively.Referring to relevant literature [22,23], the micro-sintering experiment is as follows: 1. Assimilation performance Based on the data in Table 2, the ore powder and chemical reagents were thoroughly mixed with an appropriate amount of anhydrous ethanol for 30 min and then dried in the electrothermal blowing dry box (101-0ASB, ±1 • C, Beijing Kewei Yongxing Instrument Co., Ltd., Beijing, China) at 100 • C for 2 h.The experimental sample was obtained by milling the dried mixture in a ball milling for 20 min.The 1.0 g experimental sample and 2.5 g CaO (≥98.0%,Damao, Tianjin, China) were weighed and filled into a steel mold and then pressed into iron ore compact with a diameter of 10 mm and CaO compact with a diameter of 20 mm under 20 MPa, respectively.The iron ore compact was placed on the CaO compact and then sent into the micro-sintering device (BR-14AS-12, 1400 • C, Zhengzhou Bona Heat Kiln Co., Ltd., Zhengzhou, China) for roasting.The temperature was first raised to 1000 • C and then continued to rise at a heating rate of 3 • C/s until the assimilation characteristic appeared at the contact interface between the iron ore compact and CaO compact; the corresponding temperature was defined as the lowest assimilation temperature (LAT).During the heating stage, N 2 with 1 L/min was continuously injected into the micro-sintering device to promote the decomposition of Fe 2 O 3 to form Fe 3 O 4 , which was basically equivalent to the sum of the reduction and decomposition reaction of Fe 2 O 3 in the actual sintering process.The assimilation process is shown in Figure 3.
During the heating stage, N2 with 1 L/min was continuously injected into the micro-s tering device to promote the decomposition of Fe2O3 to form Fe3O4, which was basica equivalent to the sum of the reduction and decomposition reaction of Fe2O3 in the act sintering process.The assimilation process is shown in Figure 3.

Liquid Phase Fluidity
The ore powder and Zn compounds were configured into the experimental sam with a basicity of 4.0 by CaO, and the 0.8 g experimental sample was weighed and fil into a steel mold and then pressed into iron ore compact with a size of Φ8 × 5 mm.T iron ore compact was placed on the corundum sheet with a diameter of 30 mm and th sent into the micro-sintering device for roasting.The temperature was raised to 1280 and then held for 4 min.During the sintering process, N2 with 1 L/min (heating and ho ing stage) and O2 with 0.5 L/min (cooling stage) were continuously injected.When internal temperature of the device dropped to room temperature, the corundum sheet w taken out to calculate the index of liquid phase fluidity (ILF).The liquid phase flow p cess is shown in Figure 4, and the calculation of ILF is shown in Equation (1).

Silico-Ferrite of Calcium and Aluminum (SFCA) Formation Characteristics
The ore powder and Zn compounds were configured into the experimental sam with a basicity of 2.0 by CaO, and the 2.0 g experimental sample was weighed and fil into a steel mold and then pressed into iron ore compact with a size of Φ15 × 8 mm.T iron ore compacts were placed into a corundum crucible with a volume of 20 mL and th sent into the micro-sintering device for sintering.The experimental temperature regi and atmosphere control were the same as the liquid phase fluidity experiment.When internal temperature of the device dropped to room temperature, the corundum cruci was taken out.The sintered compacts were analyzed by X-ray diffraction instrument ( Advance A25, Bruker AXS, Karlsruhe, Germany) with copper target, which was opera at 40 kV and 40 mA in step mode with 0.01°2θ step and a count time of 0.15 s per s over a 2θ range from 10° to 90°, to determine the mineral phase composition and inve gated by Scanning electron microscope-Energy dispersive spectroscopy instrum

Liquid Phase Fluidity
The ore powder and Zn compounds were configured into the experimental sample with a basicity of 4.0 by CaO, and the 0.8 g experimental sample was weighed and filled into a steel mold and then pressed into iron ore compact with a size of Φ8 × 5 mm.The iron ore compact was placed on the corundum sheet with a diameter of 30 mm and then sent into the micro-sintering device for roasting.The temperature was raised to 1280 • C and then held for 4 min.During the sintering process, N 2 with 1 L/min (heating and holding stage) and O 2 with 0.5 L/min (cooling stage) were continuously injected.When the internal temperature of the device dropped to room temperature, the corundum sheet was taken out to calculate the index of liquid phase fluidity (ILF).The liquid phase flow process is shown in Figure 4, and the calculation of ILF is shown in Equation (1).
During the heating stage, N2 with 1 L/min was continuously injected into the micro-sin tering device to promote the decomposition of Fe2O3 to form Fe3O4, which was basically equivalent to the sum of the reduction and decomposition reaction of Fe2O3 in the actua sintering process.The assimilation process is shown in Figure 3.

Liquid Phase Fluidity
The ore powder and Zn compounds were configured into the experimental sampl with a basicity of 4.0 by CaO, and the 0.8 g experimental sample was weighed and filled into a steel mold and then pressed into iron ore compact with a size of Φ8 × 5 mm.Th iron ore compact was placed on the corundum sheet with a diameter of 30 mm and then sent into the micro-sintering device for roasting.The temperature was raised to 1280 °C and then held for 4 min.During the sintering process, N2 with 1 L/min (heating and hold ing stage) and O2 with 0.5 L/min (cooling stage) were continuously injected.When th internal temperature of the device dropped to room temperature, the corundum sheet wa taken out to calculate the index of liquid phase fluidity (ILF).The liquid phase flow pro cess is shown in Figure 4, and the calculation of ILF is shown in Equation ( 1

Silico-Ferrite of Calcium and Aluminum (SFCA) Formation Characteristics
The ore powder and Zn compounds were configured into the experimental sampl with a basicity of 2.0 by CaO, and the 2.0 g experimental sample was weighed and filled into a steel mold and then pressed into iron ore compact with a size of Φ15 × 8 mm.Th iron ore compacts were placed into a corundum crucible with a volume of 20 mL and then sent into the micro-sintering device for sintering.The experimental temperature regim and atmosphere control were the same as the liquid phase fluidity experiment.When th internal temperature of the device dropped to room temperature, the corundum crucibl was taken out.The sintered compacts were analyzed by X-ray diffraction instrument (D Advance A25, Bruker AXS, Karlsruhe, Germany) with copper target, which was operated at 40 kV and 40 mA in step mode with 0.01°2θ step and a count time of 0.15 s per step over a 2θ range from 10° to 90°, to determine the mineral phase composition and investi gated by Scanning electron microscope-Energy dispersive spectroscopy instrumen (Gemini SEM 300, Carl Zeiss AG, Oberkochen, Germany) to observe the microstructur and determine the relative content of mineral phase.

Silico-Ferrite of Calcium and Aluminum (SFCA) Formation Characteristics
The ore powder and Zn compounds were configured into the experimental sample with a basicity of 2.0 by CaO, and the 2.0 g experimental sample was weighed and filled into a steel mold and then pressed into iron ore compact with a size of Φ15 × 8 mm.The iron ore compacts were placed into a corundum crucible with a volume of 20 mL and then sent into the micro-sintering device for sintering.The experimental temperature regime and atmosphere control were the same as the liquid phase fluidity experiment.When the internal temperature of the device dropped to room temperature, the corundum crucible was taken out.The sintered compacts were analyzed by X-ray diffraction instrument (D8 Advance A25, Bruker AXS, Karlsruhe, Germany) with copper target, which was operated at 40 kV and 40 mA in step mode with 0.01 • 2θ step and a count time of 0.15 s per step over a 2θ range from 10 • to 90 • , to determine the mineral phase composition and investigated by Scanning electron microscope-Energy dispersive spectroscopy instrument (Gemini SEM 300, Carl Zeiss AG, Oberkochen, Germany) to observe the microstructure and determine the relative content of mineral phase.

Compressive strength
The experimental procedures were the same as those used in the SFCA formation characteristic experiment.The compressive strength of the sintered compact was measured by an electronic pressure tester (LD-YB-2, ±1 N, Wuzhou Special Equipment Factory, Wuzhou, China).

Microhardness Detection
The microhardness of the mineral phase in the sintered compact obtained by the compressive strength experiment was measured by a Semi-automatic micro-Vickers hardness tester (401 MAD, ±2 HV, JVC, Yokohama, Japan).Before measuring the microhardness of the mineral phase, the sintered compact was embedded in a rubber mold with a size of Φ24 × 12 mm by a mixture of epoxy resin curing agent and acrylic powder and then disposed by abrasive paper and a Semi-automatic polishing machine (XP 810E, Guangzhou Jingying Chemical Technology Co., Ltd., Guangzhou, China).The microhardness of the mineral phase was the average sum of the hardness values at 10 different points on the mineral phase surface.The Vickers hardness value (HV) calculation is shown in Equation (2) [24].F and d represent a fixed experimental load (100 g) and the arithmetic average of the diagonal of the diamond indentation, respectively. (2)

TG Experiment
The experimental samples configured in the compressive strength experiment were used in the Thermogravimetry (TG) experiment.The 50 mg experimental sample was filled into the corundum crucible and then placed into the synchronous thermal analyzer (STA449F3, Netzsch Instrument Inc., Selb, Germany).The temperature regime was set as room temperature-1350 • C, non-isothermal heating.Before heating up, O 2 at 20 mL/min and N 2 at 50 mL/min was injected to discharge the impurity gas, and then N 2 protection gas at 20 mL/min was injected during the heating stage.

Thermodynamic Calculation
The reaction thermodynamics of Zn compounds in this paper were investigated as follows: Firstly, primary compounds in the experimental samples and reaction equations were listed.Secondly, the reaction conditions were confirmed.Thirdly, the thermodynamic properties and reactions of Zn compounds were calculated by the "reaction" module in FactSage 7.1.Finally, the properties of the sintering equilibrium liquid phase of experimental samples containing different contents and forms of Zn were calculated by "Equilib" and "Viscosity" modules.

Results and Discussion
The TG-time relationship of experimental samples (R = 2.0) containing different contents and forms of Zn is shown in Figure 5.The experimental sample without Zn started to have a mass loss at about 250 • C, 380 • C, and 1200 • C during the heating process, which corresponded to the decomposition of crystalline water, Ca(OH) 2 , and hematite (Fe 2 O 3 ), respectively.When Zn content increased from 0 to 4%, Figure 5a,b

The Effects of Different Forms of Zn on Assimilation Performance
The results of the assimilation experiment of iron ore containing different contents and forms of Zn are shown in Figure 6.As can be seen from Figure 6, the lowest assimilation temperature (LAT) of iron ore without Zn is 1288 °C, which belongs to medium assimilation performance.With increasing the Zn content, the LAT of iron ore containing ZnO and ZnFe2O4 increased, while that of iron ore containing ZnS did not change significantly.When the Zn content increased from 0 to 4%, the LAT of iron ore containing ZnFe2O4 increased the most, from 1288 °C to 1298 °C.

The Effects of Different Forms of Zn on Assimilation Performance
The results of the assimilation experiment of iron ore containing different contents and forms of Zn are shown in Figure 6.As can be seen from Figure 6, the lowest assimilation temperature (LAT) of iron ore without Zn is 1288 • C, which belongs to medium assimilation performance.With increasing the Zn content, the LAT of iron ore containing ZnO and ZnFe 2 O 4 increased, while that of iron ore containing ZnS did not change significantly.When the Zn content increased from 0 to 4%, the LAT of iron ore containing ZnFe 2 O 4 increased the most, from 1288 • C to 1298 • C.
Figure 7 shows the thermochemical properties of ZnO, ZnFe 2 O 4 , and ZnS.As shown in Figure 7, ZnO, ZnFe 2 O 4 , and ZnS were all refractory in the sintering process.
The relationship between Gibbs free energy and temperature of partial reactions during sintering is shown in Figure 8. ZnO could react with

The Effects of Different Forms of Zn on Assimilation Performance
The results of the assimilation experiment of iron ore containing different conten and forms of Zn are shown in Figure 6.As can be seen from Figure 6, the lowest assimil tion temperature (LAT) of iron ore without Zn is 1288 °C, which belongs to medium a similation performance.With increasing the Zn content, the LAT of iron ore containin ZnO and ZnFe2O4 increased, while that of iron ore containing ZnS did not change signi cantly.When the Zn content increased from 0 to 4%, the LAT of iron ore containin ZnFe2O4 increased the most, from 1288 °C to 1298 °C. Figure 7 shows the thermochemical properties of ZnO, ZnFe2O4, and ZnS.As show in Figure 7, ZnO, ZnFe2O4, and ZnS were all refractory in the sintering process.The relationship between Gibbs free energy and temperature of partial reactions du ing sintering is shown in Figure 8. ZnO could react with Fe2O3 to form ZnFe2O4 durin sintering, which restrained the formation of low-melting calcium ferrite (CaFe2O4, 12 °C), and the LAT of iron ore increased.However, the ΔG of the ZnFe2O4 formation reactio was higher than that of the CaFe2O4 formation reaction.Therefore, the addition of Zn had limited influence on the formation of CaFe2O4 during sintering.The high-meltin dicalcium ferrite (Ca2Fe2O5, 1449 °C) was formed by a solid-phase reaction betwee ZnFe2O4 and CaO during sintering.The ΔG of the Ca2Fe2O5 formation reaction was low than that of the CaFe2O4 formation reaction.Therefore, the addition of ZnFe2O4 had a si nificant influence on the formation of CaFe2O4 during sintering.In the sintering proces the O2 produced by the decomposition of Fe2O3 oxidized ZnS to ZnO.However, the d composition temperature of Fe2O3 limited the oxidation process of ZnS, and ZnS hard affected the formation of CaFe2O4 at lower temperatures.Therefore, with the increase Zn content, the LAT of iron ore containing ZnFe2O4 increased the most, while that of iro ore containing ZnS had no obvious change.The relationship between Gibbs free energy and temperature of partial reactions du ing sintering is shown in Figure 8. ZnO could react with Fe2O3 to form ZnFe2O4 durin sintering, which restrained the formation of low-melting calcium ferrite (CaFe2O4, 12 °C), and the LAT of iron ore increased.However, the ΔG of the ZnFe2O4 formation reactio was higher than that of the CaFe2O4 formation reaction.Therefore, the addition of Zn had limited influence on the formation of CaFe2O4 during sintering.The high-meltin dicalcium ferrite (Ca2Fe2O5, 1449 °C) was formed by a solid-phase reaction betwee ZnFe2O4 and CaO during sintering.The ΔG of the Ca2Fe2O5 formation reaction was low than that of the CaFe2O4 formation reaction.Therefore, the addition of ZnFe2O4 had a si nificant influence on the formation of CaFe2O4 during sintering.In the sintering proces the O2 produced by the decomposition of Fe2O3 oxidized ZnS to ZnO.However, the d composition temperature of Fe2O3 limited the oxidation process of ZnS, and ZnS hard affected the formation of CaFe2O4 at lower temperatures.Therefore, with the increase Zn content, the LAT of iron ore containing ZnFe2O4 increased the most, while that of iro ore containing ZnS had no obvious change.) ) temperature/K

The Effects of Different Forms of Zn on Liquid Phase Fluidity
The results of the liquid phase fluidity experiment of iron ore containing differe contents and forms of Zn are shown in Figure 9.It can be seen from Figure 9 that wi increasing the Zn content, the index of liquid phase fluidity (ILF) of iron ore containin

The Effects of Different Forms of Zn on Liquid Phase Fluidity
The results of the liquid phase fluidity experiment of iron ore containing different contents and forms of Zn are shown in Figure 9.It can be seen from Figure 9 that with increasing the Zn content, the index of liquid phase fluidity (ILF) of iron ore containing ZnS increased while that of iron ore containing ZnO and ZnFe 2 O 4 decreased.The effect of ZnFe 2 O 4 on ILF was greater than ZnO when the Zn content was the same.The properties of the sintering equilibrium liquid phase of experimental samples con taining different contents and forms of Zn are shown in Table 3 and Figure 10.As can be seen from Table 3 and Figure 10a, with the Zn content increased, the Zn 2+ content in the liquid phase increased while the liquid phase content decreased, and the liquid phase content of the experimental sample containing ZnFe2O4 had a significant decrease.As shown in Figure 10b, with the increase of Zn content, the liquid phase viscosity of exper imental samples containing ZnO and ZnFe2O4 increased while that of the experimenta sample containing ZnS decreased.Research showed that Fe 2+ broke the chain of polymer macromolecules such as silico-oxygen complex anion groups, destroying the Si-O net work structure in the silicate liquid phase, so the viscosity of the liquid phase decreased [25].As can be seen from Table 3, with the increase of Zn content, Fe 2+ content in the liquid phase of experimental samples containing ZnO and ZnFe2O4 decreased, while that of the experimental sample containing ZnS increased.Therefore, under the combined effect o liquid content and viscosity, as the Zn content increased, the ILF of iron ore containing ZnS increased, while that of iron ore containing ZnO and ZnFe2O4 decreased.The properties of the sintering equilibrium liquid phase of experimental samples containing different contents and forms of Zn are shown in Table 3 and Figure 10.As can be seen from Table 3 and Figure 10a, with the Zn content increased, the Zn 2+ content in the liquid phase increased while the liquid phase content decreased, and the liquid phase content of the experimental sample containing ZnFe 2 O 4 had a significant decrease.As shown in Figure 10b, with the increase of Zn content, the liquid phase viscosity of experimental samples containing ZnO and ZnFe 2 O 4 increased while that of the experimental sample containing ZnS decreased.Research showed that Fe 2+ broke the chain of polymer macromolecules such as silico-oxygen complex anion groups, destroying the Si-O network structure in the silicate liquid phase, so the viscosity of the liquid phase decreased [25].As can be seen from Table 3, with the increase of Zn content, Fe 2+ content in the liquid phase of experimental samples containing ZnO and ZnFe 2 O 4 decreased, while that of the experimental sample containing ZnS increased.Therefore, under the combined effect of liquid content and viscosity, as the Zn content increased, the ILF of iron ore containing ZnS increased, while that of iron ore containing ZnO and ZnFe 2 O 4 decreased.

The Effects of Different Forms of Zn on SFCA Formation Characteristics
The XRD pa erns of sinter after sintering iron ore containing 2% Zn are shown Figure 11.It can be seen from Figure 11 that Zn mainly existed in the form of ZnFe2O4 the Zn-containing sinter.Moreover, the diffraction peaks of ZnFe2O4 and Fe3O4 coincid perfectly because both of them were spinel minerals.The relative area percentage of the mineral phase in the sinter was obtained by Ima J 1.54h software, as shown in Figure 12.It can be seen from Figure 12 that with the increa of ZnO and ZnFe2O4 contents, the contents of silico-ferrite of calcium and aluminu (SFCA), magnetite and silicate decreased, while the content of hematite increased; w the increase of ZnS content, the contents of SFCA and hematite decreased, while that magnetite and silicate increased.

The Effects of Different Forms of Zn on SFCA Formation Characteristics
The XRD patterns of sinter after sintering iron ore containing 2% Zn are shown in Figure 11.It can be seen from Figure 11

The Effects of Different Forms of Zn on SFCA Formation Characteristics
The XRD pa erns of sinter after sintering iron ore containing 2% Figure 11.It can be seen from Figure 11 that Zn mainly existed in the fo the Zn-containing sinter.Moreover, the diffraction peaks of ZnFe2O4 and perfectly because both of them were spinel minerals.The relative area percentage of the mineral phase in the sinter was o J 1.54h software, as shown in Figure 12.It can be seen from Figure 12 that of ZnO and ZnFe2O4 contents, the contents of silico-ferrite of calcium (SFCA), magnetite and silicate decreased, while the content of hematit the increase of ZnS content, the contents of SFCA and hematite decreas magnetite and silicate increased.The relative area percentage of the mineral phase in the sinter was obtained by Image J 1.54h software, as shown in Figure 12.It can be seen from Figure 12 that with the increase of ZnO and ZnFe 2 O 4 contents, the contents of silico-ferrite of calcium and aluminum (SFCA), magnetite and silicate decreased, while the content of hematite increased; with the increase of ZnS content, the contents of SFCA and hematite decreased, while that of magnetite and silicate increased.
Materials 2024, 17, x FOR PEER REVIEW 10 of 16 adhesion layer, which promoted the formation of SFCA.Therefore, the content of SFCA in the sinter after the sintering of iron ore containing ZnS decreased slightly.The microstructure of the sinter after sintering iron ore containing 2% Zn is shown in Figure 13, and the EDS analysis results of points in Figure 13 are shown in Table 4.It can be seen from Table 4 that Zn was mainly distributed in hematite or silicate.
During sintering, ZnO reacted with Fe2O3 to form ZnFe2O4, and then, ZnFe2O4 reacted with CaO to form Ca2Fe2O5 and ZnO; however, the content of CaO in the experimental sample was much lower than that of Fe2O3.Moreover, SiO2 reacted with CaO to form Ca2SiO4, which restrained the formation of Zn2SiO4.Therefore, most of Zn in the sinter eventually existed in the hematite in the form of ZnFe2O4.ZnS began to be oxidized by O2 generated by the decomposition of Fe2O3 at about 1100 °C; CaO was almost exhausted at this temperature.As shown in Figure 11, there was still residual SiO2 in the sinter, and the ΔG of the Zn2SiO4 formation reaction was lower than that of the ZnFe2O4 formation reaction.Therefore, part of Zn existed in the form of Zn2SiO4 in the sinter after sintering iron ore containing ZnS. Zn in SFCA and magnetite existed in the form of a solid solution.Spinel polysomes were commonly contained in SFCA mineral structures, which meant that both SFCA and magnetite contained spinel structures [26].Compared with the Mg 2+ radius (0.65 Å), the Zn 2+ radius (0.74 Å) was closer to the Fe 2+ radius (0.76 Å); hence, Fe 2+ in SFCA and magnetite was mainly substituted by Zn 2+ .ZnO and ZnFe 2 O 4 had a solid-phase reaction with Fe 2 O 3 and CaO, respectively, during sintering, which inhibited the formation of calcium ferrite, a reactant in the SFCA formation process, and ZnFe 2 O 4 had a stronger inhibition effect.In addition, the viscosity of the liquid phase increased because of the decrease of Fe 2+ content in the liquid phase, so the permeability of the liquid phase to the surrounding adhesion layer was weakened, which also inhibited the formation of SFCA.Therefore, the content of SFCA in sinter after sintering iron ore containing ZnFe 2 O 4 decreased greatly.Although ZnS promoted the decomposition of Fe 2 O 3 and inhibited the formation of SFCA, there was more Fe 2+ content in the liquid phase formed by iron ore containing ZnS during sintering.The penetration range of the liquid phase with stronger fluidity was wider than that of the surrounding adhesion layer, which promoted the formation of SFCA.Therefore, the content of SFCA in the sinter after the sintering of iron ore containing ZnS decreased slightly.The microstructure of the sinter after sintering iron ore containing 2% Zn is shown in Figure 13, and the EDS analysis results of points in Figure 13 are shown in Table 4.It can be seen from Table 4 that Zn was mainly distributed in hematite or silicate.The element distribution of Zn-bearing sinter in Figure 13 is shown in Figu can be seen from Figure 14a,b that Zn in sinter ZO2 and ZF2 was concentrated in h and less distributed in SFCA. Figure 14c shows that Zn in the sinter ZS2 was conce in the silicate and overlapped be er with the Si element, uniformly distributed bearing minerals.
To further reveal the mechanism of Zn deviation distribution in SFCA and ma based on first-principles calculations, the adsorption energies of Zn on SFCA and m ite crystal surfaces were calculated by the Vienna ab initio simulation package 6.3.2) [27].The generalized gradient approximation by Perdew-Burke-Ernzerho was employed to describe the exchange-correlation functional [28].The ion-elec teraction was estimated by the projector-augmented wave model [29].The energ for the plane wave expansion was set to 450 eV.A 3 × 3 × 3 Monkhorst Pack k-poi was used for geometry optimization.The energy convergence for terminating t tronic self-consistent field was 10 −6 eV/atom, and the force convergence on each a   11, there was still residual SiO 2 in the sinter, and the ∆G of the Zn 2 SiO 4 formation reaction was lower than that of the ZnFe 2 O 4 formation reaction.Therefore, part of Zn existed in the form of Zn 2 SiO 4 in the sinter after sintering iron ore containing ZnS. Zn in SFCA and magnetite existed in the form of a solid solution.Spinel polysomes were commonly contained in SFCA mineral structures, which meant that both SFCA and magnetite contained spinel structures [26].Compared with the Mg 2+ radius (0.65 Å), the Zn 2+ radius (0.74 Å) was closer to the Fe 2+ radius (0.76 Å); hence, Fe 2+ in SFCA and magnetite was mainly substituted by Zn 2+ .
The element distribution of Zn-bearing sinter in Figure 13 is shown in Figure 14.It can be seen from Figure 14a,b that Zn in sinter ZO2 and ZF2 was concentrated in hematite and less distributed in SFCA. Figure 14c shows that Zn in the sinter ZS2 was concentrated in the silicate and overlapped better with the Si element, uniformly distributed in ironbearing minerals.The cell parameters calculated after optimizing the crystal structure of SFCA and magnetite are shown in Table 5; the deviations between calculation values and experience values of cell parameters are lower than 1%, belonging to a reasonable range [30].According to the corresponding strongest diffraction peaks of SFCA and magnetite in Figure 11, SFCA (420) and Fe3O4 (220) crystalline planes were selected to construct the crystal surface adsorption models, as shown in Figure 15.To further reveal the mechanism of Zn deviation distribution in SFCA and magnetite, based on first-principles calculations, the adsorption energies of Zn on SFCA and magnetite crystal surfaces were calculated by the Vienna ab initio simulation package (VASP 6.3.2) [27].The generalized gradient approximation by Perdew-Burke-Ernzerhof (PBE) was employed to describe the exchange-correlation functional [28].The ion-electron interaction was estimated by the projector-augmented wave model [29].The energy cutoff for the plane wave expansion was set to 450 eV.A 3 × 3 × 3 Monkhorst Pack k-point setup was used for geometry optimization.The energy convergence for terminating the electronic selfconsistent field was 10 −6 eV/atom, and the force convergence on each atom for geometric optimization was 10 −2 eV/Å.
The atomic site and cell structure of the 15 × 15 × 15 Å cell with Zn atom in the central location were optimized by the VASP program under the above parameter setting, and the energy of the Zn atom was finally obtained to be −0.2647eV.
The cell parameters calculated after optimizing the crystal structure of SFCA and magnetite are shown in Table 5; the deviations between calculation values and experience values of cell parameters are lower than 1%, belonging to a reasonable range [30].According to the corresponding strongest diffraction peaks of SFCA and magnetite in Figure 11, SFCA (420) and Fe 3 O 4 (220) crystalline planes were selected to construct the crystal surface adsorption models, as shown in Figure 15.The adsorption energies of Zn on the SFCA and magnetite crystal surfaces are shown in Figure 16. Figure 16 shows that the SFCA crystal released more energy during the adsorption of Zn, which meant that the SFCA crystal was more stable after the adsorption of Zn.Therefore, more Zn-containing solid solutions were formed in SFCA.Materials 2024, 17, x FOR PEER REVIEW 13 The adsorption energies of Zn on the SFCA and magnetite crystal surfaces are sho in Figure 16. Figure 16 shows that the SFCA crystal released more energy during the sorption of Zn, which meant that the SFCA crystal was more stable after the adsorp of Zn.Therefore, more Zn-containing solid solutions were formed in SFCA.The results of microhardness detection are shown in Table 6.Table 6 shows that w the increase of Zn content, the microhardness of silicate had no obvious change; am the iron-bearing minerals, the microhardness of hematite decreased slightly, while tha SFCA in sinter after sintering iron ore containing ZnO and ZnFe2O4 decreased sig cantly.
Combined with the data in Tables 4 and 6, it can be deduced that the microhard of the mineral phase was inversely proportional to the amount of Zn-containing solid lution contained in the mineral phase.Therefore, the micromechanical properties of SF and magnetite deteriorated because of the formation of Zn-containing solid solution addition, the formation of ZnFe2O4 broke the continuity of the Fe2O3 crystal bond, res ing in a decrease in the microhardness of hematite.The results of microhardness detection are shown in Table 6.Table 6 shows that with the increase of Zn content, the microhardness of silicate had no obvious change; among the iron-bearing minerals, the microhardness of hematite decreased slightly, while that of SFCA in sinter after sintering iron ore containing ZnO and ZnFe 2 O 4 decreased significantly.Combined with the data in Tables 4 and 6, it can be deduced that the microhardness of the mineral phase was inversely proportional to the amount of Zn-containing solid solution contained in the mineral phase.Therefore, the micromechanical properties of SFCA and magnetite deteriorated because of the formation of Zn-containing solid solution.In addition, the formation of ZnFe 2 O 4 broke the continuity of the Fe 2 O 3 crystal bond, resulting in a decrease in the microhardness of hematite.

Compression Strength
The compressive strength of the sinter after sintering iron ore containing different contents and forms of Zn is shown in Figure 17. Figure 17 shows that the compressive strength of the sinter decreased with the Zn content increased, and the compressive strength of the sinter after sintering iron ore containing ZnFe 2 O 4 decreased more sharply.
Materials 2024, 17, x FOR PEER REVIEW 14 strength of the sinter decreased with the Zn content increased, and the compres strength of the sinter after sintering iron ore containing ZnFe2O4 decreased more shar As the microhardness of iron-bearing minerals in the Zn-bearing sinter decrea the compressive strength of the sinter decreased accordingly.However, it can be from Table 6 that ZnO and ZnFe2O4 had an approximate influence on mineral microh ness, which was inconsistent with the results in the compressive strength experim Meanwhile, the effect of Zn on SFCA formation was consistent with that of Zn on compressive strength of the sinter.Therefore, Zn affected the bonding phase strengt the formation of a solid solution and decreasing the SFCA content.

Conclusions
In this paper, the micro-sintering method was adopted to measure the sintering b characteristics of iron ore, and the influence mechanism of ZnO, ZnFe2O4, and ZnS sintering basic characteristics of iron ore was determined by first-principles calculat and thermodynamic analysis.The conclusions are as follows: 1.After the addition of ZnO and ZnFe2O4, the LAT of iron ore increased while the of iron ore decreased.The addition of ZnFe2O4 consumed limited CaO, so the and ILF of iron ore containing ZnFe2O4 had a greater variation.As the ZnS con increased, the LAT of iron ore had no obvious change, while the ILF of iron ore creased; 2. The addition of ZnO and ZnFe2O4 resulted in the formation of ZnFe2O4 and Ca2Fe respectively; the content of reactants that participated in the formation of SFCA creased.Hence, the presence of ZnO and ZnFe2O4 had a serious impact on SFCA mation during sintering.The addition of ZnS promoted the decomposition of Fe however, the process was limited by temperature.Hence, the presence of ZnS h As the microhardness of iron-bearing minerals in the Zn-bearing sinter decreased, the compressive strength of the sinter decreased accordingly.However, it can be seen from Table 6 that ZnO and ZnFe 2 O 4 had an approximate influence on mineral microhardness, which was inconsistent with the results in the compressive strength experiment.Meanwhile, the effect of Zn on SFCA formation was consistent with that of Zn on the compressive strength of the sinter.Therefore, Zn affected the bonding phase strength by the formation of a solid solution and decreasing the SFCA content.

Conclusions
In this paper, the micro-sintering method was adopted to measure the sintering basic characteristics of iron ore, and the influence mechanism of ZnO, ZnFe 2 O 4 , and ZnS on sintering basic characteristics of iron ore was determined by first-principles calculations and thermodynamic analysis.The conclusions are as follows:

Figure 1 .
Figure 1.The XRD pattern of iron ore.

Figure 2 .
Figure 2. The SEM photo of iron ore.

Figure 2 .
Figure 2. The SEM photo of iron ore.

Figure 3 .
Figure 3.The schematic diagram of the assimilation process.

Figure 4 .
Figure 4.The schematic diagram of the liquid phase flow process.

Figure 3 .
Figure 3.The schematic diagram of the assimilation process.

Figure 3 .
Figure 3.The schematic diagram of the assimilation process.

Figure 4 .
Figure 4.The schematic diagram of the liquid phase flow process.

Figure 4 .
Figure 4.The schematic diagram of the liquid phase flow process.

Figure 6 .
Figure 6.The effects of different forms of Zn on the LAT of iron ore.
Fe 2 O 3 to form ZnFe 2 O 4 during sintering, which restrained the formation of low-melting calcium ferrite (CaFe 2 O 4 , 1216 • C), and the LAT of iron ore increased.However, the ∆G of the ZnFe 2 O 4 formation reaction was higher than that of the CaFe 2 O 4 formation reaction.Therefore, the addition of ZnO had limited influence on the formation of CaFe 2 O 4 during sintering.The high-melting dicalcium ferrite (Ca 2 Fe 2 O 5 , 1449 • C) was formed by a solid-phase reaction between ZnFe 2 O 4 and CaO during sintering.The ∆G of the Ca 2 Fe 2 O 5 formation reaction was lower than that of the CaFe 2 O 4 formation reaction.Therefore, the addition of ZnFe 2 O 4 had a significant influence on the formation of CaFe 2 O 4 during sintering.In the sintering process, the O 2 produced by the decomposition of Fe 2 O 3 oxidized ZnS to ZnO.However, the decomposition temperature of Fe 2 O 3 limited the oxidation process of ZnS, and ZnS hardly affected the formation of CaFe 2 O 4 at lower temperatures.Therefore, with the increase of Zn content, the LAT of iron ore containing ZnFe 2 O 4 increased the most, while that of iron ore containing ZnS had no obvious change.

Figure 6 .
Figure 6.The effects of different forms of Zn on the LAT of iron ore.

Figure 6 .Figure 7 .
Figure 6.The effects of different forms of Zn on the LAT of iron ore.Materials 2024, 17, x FOR PEER REVIEW 7 of

1 )Figure 7 .Figure 7 .
Figure 7.The relationship between Gibbs free energy and temperature of different forms of Zn.

Figure 8 .
Figure 8.The relationship between Gibbs free energy and temperature of partial reactions during sintering.

Figure 9 .
Figure 9.The effects of different forms of Zn on the ILF of iron ore.

Figure 9 .
Figure 9.The effects of different forms of Zn on the ILF of iron ore.

Figure 10 .
Figure 10.The properties of the equilibrium liquid phase at 1280 °C of experimental samples co taining different contents and forms of Zn: (a) Contents; (b) Viscosities.

Figure 10 .
Figure 10.The properties of the equilibrium liquid phase at 1280 • C of experimental samples containing different contents and forms of Zn: (a) Contents; (b) Viscosities.

Figure 10 .
Figure 10.The properties of the equilibrium liquid phase at 1280 °C of experim taining different contents and forms of Zn: (a) Contents; (b) Viscosities.

Figure 12 .
Figure 12.The variation of mineral phase content of sinter after sintering iron ore containing different contents and forms of Zn: (a) SFCA; (b) Hematite; (c) Magnetite; (d) Silicate.3.4.The Effects of Different Forms of Zn on Bonding Phase Strength 3.4.1.The Distribution Characteristics of Zn and Microhardness

Figure 12 .
Figure 12.The variation of mineral phase content of sinter after sintering iron ore containing different contents and forms of Zn: (a) SFCA; (b) Hematite; (c) Magnetite; (d) Silicate.

3. 4 .
The Effects of Different Forms of Zn on Bonding Phase Strength 3.4.1.The Distribution Characteristics of Zn and Microhardness

Figure 16 .
Figure 16.Adsorption energies of Zn on the SFCA and magnetite crystal surfaces.

Figure 16 .
Figure 16.Adsorption energies of Zn on the SFCA and magnetite crystal surfaces.

Figure 17 .
Figure 17.The effects of different forms of Zn on the compressive strength of sinter.

Figure 17 .
Figure 17.The effects of different forms of Zn on the compressive strength of sinter.

Table 2 .
Chemical Composition of Iron Ore with Different Contents and Forms of Zn (%).

Table 2 .
Chemical Composition of Iron Ore with Different Contents and Forms of Zn (%).
shows that the TG curve increased from 92.19% to 92.69% (ZnO) and 92.77% (ZnFe 2 O 4 ) at the end of the Fe 2 O 3 decomposition stage, respectively; hence, ZnO and ZnFe 2 O 4 inhibited the decomposition of Fe 2 O 3 .Figure5cshows that the TG curve decreased from 92.19% to 91.61% (ZnS) at the end of the Fe 2 O 3 decomposition stage; hence, ZnS promoted the decomposition of Fe 2 O 3 .

Table 3 .
The Compositions of Equilibrium Liquid Phase at 1280 °C of Experimental Samples Con taining Different Contents and Forms of Zn (%).

Table 3 .
The Compositions of Equilibrium Liquid Phase at 1280 • C of Experimental Samples Containing Different Contents and Forms of Zn (%).
that Zn mainly existed in the form of ZnFe 2 O 4 in the Zn-containing sinter.Moreover, the diffraction peaks of ZnFe 2 O 4 and Fe 3 O 4 coincided perfectly because both of them were spinel minerals.

Table 4 .
EDS Analysis Results (at%).ZnO reacted with Fe 2 O 3 to form ZnFe 2 O 4 , and then, ZnFe 2 O 4 reacted with CaO to form Ca 2 Fe 2 O 5 and ZnO; however, the content of CaO in the experimental sample was much lower than that of Fe 2 O 3 .Moreover, SiO 2 reacted with CaO to form Ca 2 SiO 4 , which restrained the formation of Zn 2 SiO 4 .Therefore, most of Zn in the sinter eventually existed in the hematite in the form of ZnFe 2 O 4 .ZnS began to be oxidized by O 2 generated by the decomposition of Fe 2 O 3 at about 1100 • C; CaO was almost exhausted at this temperature.As shown in Figure

Table 6 .
Microhardness values of mineral phases in sinter after sintering iron ore containing di ent contents and forms of Zn (HV).

Table 6 .
Microhardness values of mineral phases in sinter after sintering iron ore containing different contents and forms of Zn (HV).