Effect of Basicity on Metallurgical Properties of Fluxed Pellets with High MgO Content

Abstract

The basicity (CaO/SiO₂) has an important influence on the metallurgical properties of fluxed pellet with high MgO content. This paper studied the effect and function of basicity on metallurgical properties of fluxed pellet, such as compressive strength, reduction disintegration, reduction swelling and the softening-melting behaviour systematically with simulating BF conditions. From the results, with increasing basicity from 0.6 to 1.3, the compressive strength increases from 2842 N to 3532 N, the pore structure become more compact, and the following phase transformation is observed from Fe₂SiO₄ → Ca₂SiO₄, Mg_XFe_2−XSiO₄ → MgFe₂O_4. Additionally, the reduction properties of high magnesium fluxed pellets are improved, the low temperature disintegration index increases from 89.8% to 97.78%, and the reduction swelling index decreases from 19.02% to 8.45%. With the increase of basicity in the pellets, the softening interval and melting interval of the pellets decrease slightly, and the position of the cohesive zone shift down and gradually become thinner, which improve the permeability of the cohesive zone. In the basicity range of 0.6–1.3, the optimum metallurgical properties of pellets are observed for the basicity of 1.1.

1. Introduction

The pelletization of iron ore is one of the widely practiced agglomeration techniques in iron and steel making because it enables the use of a very fine concentrate. Iron ore pellets offer several advantages over other ferrous burden materials in terms of compressive strength, reducibility, chemistry and porosity. The acid pellets are one of the most widely used iron-making burdens, however it has certain this includes high roasting temperature, poor reducibility (reduction disintegration index, reduction swelling, softening-melting behavior), and more resource consumption.^1,2,3) Some investigators have improved the metallurgical properties of pellets by improving the composition of iron ore, adjusting the roasting parameters (time, temperature, oxygen pressure), and adding fluxes. Among them, the chemical composition of the raw materials of mostly ironmaking enterprise has not changed much, and the roasting system is also relatively mature. So adding fluxes and adjusting the composition of gangue has become an important measure to improve the metallurgical properties of pellets. When Minoru Sasaki et al. studied the consolidation mechanism of iron ore during roasting process, they founded that the gangue played the role of flux in the process of combining hematite pellet slag.⁴⁾

Increasing the composition of MgO in pellets can improve the high temperature metallurgical properties of pellets during reduction process, including the reduction swelling index and softening-melting behavior. At the same time, increasing MgO in the pellets can meet the reasonable cohesive zone configuration, which is more conducive to Gas distribution in the blast furnace.⁵⁾ Singh et al. pointed out that conversion from magnetite (Fe₃O₄) to hematite (Fe₂O₃) was a key for pellet obtaining better strength during the high-temperature roasting process.⁶⁾ Besides, some research and practices had shown that MgO reduced compressive strength of the pellets. Gao Qiangjian et al. confirmed that the iron solution between the MgO and FeO would appear under high temperature conditions, the fluxed MgO participated in the conversion process to form MF ((Fe_xMg₁₋x)O∙Fe₂O₃) phase. The MF phase is also reverse spinel structure as the same as the structure of magnetite. Consequently, MgO can inhibit the oxidation process of Fe₃O₄ to a certain extent, making the recrystallization of Fe₂O₃ incomplete. Therefore, MgO is not beneficial for the densification process and strength of pellets. To counteract the adverse effects of MgO, pellets were fluxed with CaO by other researchers, in order to improve softening and melting properties. When the basicity was 0.3–0.8, the main binding phase in the pellets was high silica glass phase, and the pellets with basicity 1.15 were high calcium glass phases, the microstructure of the pellets changes, and the compressive strength and reducibility of the pellets improve.^8,9) Some Researchers conducted the isothermal reduction of acid pellets and basic pellets at 1023–1273 K. During the reduction process, it was found that due to the simultaneous presence of CaO and MgO, the reducibility of the pellets was improved.¹⁰⁾

In this paper, the effect of basicity on the metallurgical properties of high MgO fluxed pellets (compressive strength, reduction swelling index, reduction disintegration and softening-melting performance) were systematically studied with using flux addition. First, the compressive strength values of the fluxed pellets with different basicity (0.6 to 1.3) were tested, which were then combined with thermodynamic analysis, and the changes in phase migration of different basicity pellets were analyzed by XRD. Next, in the present research the reduction behavior of the pellets were also studied, including the reduction disintegration, reduction swelling and softening-melting properties. At the same time, SEM image was used to explain the growth of iron whiskers during the reduction swelling.

2. Experimental

2.1. Raw Material

The chemical compositions of raw materials use in this study consisted of 3 kinds of iron ore concentrate (the A magnetite concentrate, B magnetite concentrate and C magnetite concentrate), fluxes (limestone and dolomite), and bentonite in Table 1. The bentonite is used as binder in pelletizing, and the limestone and dolomite powder is added to adjust the basicity of high MgO flux pellets. In this study, the addition amount of bentonite is 0.5%, and the addition amount of limestone and dolomite powder is preset to six basicity levels (0.6, 0.8, 0.9, 1.0, 1.1, and 1.3), while the MgO content remains at 2.4%. The chemical compositions of the fired high MgO flux pellets with different basicity content are listed in Table 2.

Table 1. Chemical compositions of raw material (wt-%).

Materials	TFe	FeO	CaO	SiO2	Al2O3	MgO	TiO2	K	Na	P	S	LOI
magnetite iron ore A	66.63	28.69	0.54	4.98	1.08	0.84	0.11	0.16	0.06	0.015	0.304	…
magnetite iron ore B	66.04	26.31	0.37	5.90	0.24	0.43	0.03	0.18	0.07	0.008	0.020	…
magnetite iron ore C	63.92	27.92	1.49	1.16	3.53	0.61	2.48	0.048	0.216	0.160	0.063	…
Bentonite	2.30	0.12	4.44	59.88	12.10	2.61	0.42	1.07	2.27	0.00	0.00	14.79
Limestone	0.00	0.00	42.01	3.28	0.82	8.36	0.00	0.26	0.00	0.00	0.00	45.53
Dolomite	0.00	0.00	30.30	1.55	0.39	21.13	0.00	0.00	0.07	0.00	0.00	46.63

Table 2. The addition amount of each compositions of fluxing pellets with different basicity (wt-%).

Items	Bentonite	Limestone	CaO	SiO2	MgO	Al2O3	R(CaO/SiO2)
1#	1.44	5.51	2.79	4.62	2.40	0.78	0.6
2#	4.80	4.35	3.74	4.65	2.40	0.79	0.8
3#	6.40	3.80	4.19	4.66	2.40	0.80	0.9
4#	8.12	3.20	4.66	4.68	2.40	0.80	1.0
5#	10.00	2.50	5.15	4.70	2.40	0.81	1.1
6#	13.85	1.15	6.17	4.73	2.40	0.82	1.3

2.2. Experimental Methods

Three kinds of magnetite iron ore fines are the source of high-grade iron ore concentrates for the preparation of pellets, limestone and dolomite are used as the sources of CaO and MgO, and bentonite is used as the binder. The pellets are pelletized in pelletizing disc with a diameter of 1000 mm for 30 minutes. The green pellets of 10–12 mm are screened out and dried at 105°C for 5 h using a blast dryer. Oxidation roasting is carried out in the muffle furnace under an ample oxidation atmosphere. The temperature for the oxidation roasting process is increased from 900°C to 1250°C at the rate of 5°C/min, and roasting at 1250°C for 15 minutes by simulating field conditions. Figure 1 shows the flow chart of preparation process of magnesium fluxed pellets.

Fig. 1.

Flow chart of preparation process of high MgO fluxed pellets.

In this study, the metallurgical properties of fluxed pellets (compressive strength, reduction disintegration, reduction swelling index) are tested by these three standards (GB/T14201-1993, GB/T13242-91, GB/T13240-91). Among them, the compressive strength of the pellet is tested on a single pellet at a constant pressure by the compressive strength tester, the test is conducted till the pellet is completely broken. X-ray diffraction analysis is used to show case the phase transformations occurring in the pellets due to change in basicity, and in-turn the phase transformations are utilized to justify the difference in the compressive strength of the pellets.

Regarding the measurement of reduction disintegration, in the reduction furnace, 500 g of fluxed pellets with a size range of 10 to 12.5 mm is loaded in a reduction furnace. Initially during heating the protective gas (N₂) is passed at a rate of 5 L/min through the reduction tube. Once the temperature reaches 500°C, the N₂ flow rate is increased to 15 L/min. For the nest 30 min, the samples are kept at 500 ± 10°C. Then the reducing atmosphere is adjusted to (N₂ 9 L/min, CO₂ 3 L/min, CO 3 L/min) and continued the reduction process for 1 hour. After the reduction reaction, the sample is poured out for screening, and then the sample is subjected to rotations in a drum for 300 r at the rate of 30 r/min. The reduction disintegration index (RDI_+3.15) of high MgO fluxed pellets is calculated by the following formula:   

$$RD{I}_{+3.15}=\frac{m_{D1}+m_{D2}}{m_{D0}}\times 100$$(1)

where m_D1 and m_D2 are the mass of the sample left by the fluxed pellets on the 6.30 mm and 3.15 mm sieve, and m_D0 is the mass of the sample after the reduction and before subjecting them to rotations in the drum.

For the measurement of the reduction swelling of the fluxed pellets, 18 pellets with a volume based particle size of 10–12.5 mm are required, which are then treated in the reduction furnace, and the total volume of the pellets is determined as V₀ before reduction. In the reduction furnace the temperature is increased to 900°C at the rate (mention rate), using N₂ as the protective gas. Then the temperature is kept constant at 900 ± 10°C for 30 mins.After the isothermal treatment in inert atmosphere, the samples are isothermally reduced with a gas composition of (N₂ 10.5 L/min, CO 4.5 L/min) for next 1 hour at 900°C. After the reduction reaction, the total volume of the pellets is determined to be V₁. The reduction swelling index (RSI) is defined as follows:   

$$RSI=\frac{V_1-V_0}{V_0}\times 100\%$$(2)

The SEM image analysis is used to investigate the mechanism behind the growth of iron whiskers which results into swelling.

In this study, the softening-melting behavior of pellets with different basicity (0.6, 0.8, 1.0, 1.3) was also tested. A graphite crucible with a bottom diameter of 75 mm (inner diameter) is used. In the crucible a 500 gm of pellet layer, with particle size of 10–12.5 mm, is sandwiched between two layers of coke. The bottom layer consists of coke particles with size in the range of 10–13 mm. The top layer constitutes the particles of size 8–10 mm. The pellet bed height is measured and the cake surface is flattened. Next, referring to the previous research,¹¹⁾ the softening-melting dripping experiment uses exact conditions by simulating the high temperature state of blast furnace, including temperature distribution, heating rate and reduction gases. During reduction the gas profile is varied with respect to the temperature. Initially the temperature is increased from 200°C to 400°C at a rate of 10°C/min with N₂ gas flow of 3 L/min. And at 400°C reducing gas of composition (3.9 L/min CO, 2.1 L/min CO₂, 9 L/min N₂) is introduced. The heating rate of 10°C/min is maintained till 900°C. At 1020°C, both gas composition and heating rate are changed, where heating rate is increased to 5°C/min and gas of composition of 4.5 L/min CO and 10.5 L/min N₂ is utilized in the furnace.

3. Results and Discussion

3.1. Effect of Basicity on Compressive Strength

The compressive strength (CS) of pellets is one of its main physical and chemical property indicator and it has an important influence on the progress of blast furnace smelting. Figure 2 shows the effect of basicity on the CS of high MgO fluxed pellets. The CS of high MgO fluxed pellets didn’t change significantly when basicity increased from 0.6 to 1.0, on the other hand the CS increased from 2800 N to 3500 N with the basicity increasing from 1.0 to 1.3. Figure 3 displays the effect of the basicity on the porosity of the fluxed pellets. The porosity was gradually decreasing with increasing basicity. Specifically, the porosity decreased from 20.48% to 19.05% with increasing basicity from 0.6 to 1.0, and further decreased to 18.85% and 18.18% with increasing in basicity to 1.1 and 1.3 respectively. These results indicated that the oxidation state and slag bonding of the fluxed pellets was greatly increased by the increasing basicity, in good agreement with the results of the compressive tests.

Fig. 2.

Effect of basicity on compressive strength of high MgO fluxed pellets. (Online version in color.)

Fig. 3.

Variations of the porosity of the fluxed pellets on function of the different basicity.

The empirical equation relating the CS and the porosity is¹²⁾   

$$\mathrm{S}=\mathrm{K}\cdot d^(-\alpha )\cdot e^{-\beta \rho }$$(3)

where S is the CS; K, α, and β are coefficients; d is the grain radius (m); and ρ is the porosity (vol%). Equation (3) shows that the CS increases with decreasing porosity.

The consolidation mechanism of flux pellets during the roasting process is closely related to the formation of the liquid phase. As shown in Fig. 4, by simulating the pellet composition and the corresponding roasting conditions, the phase diagram of the CaO–SiO₂–Fe₂O₃ system (P = 101325 Pa, P_O2 = 21278 Pa) was calculated by the phase diagram module in FactSage 7.2. During the calculation, the FToxid and FactPs database were used, and the possible species of solid phases were considered, such as spinel, monoxide, clinopyroxene, mellite, and orthopyoxene. It could be seen that the liquid slag phase could be formed when the oxidation roasting temperature exceeded 1200°C with the mass ratio of CaO to (CaO + SiO₂ + Fe₂O₃) from 0 to 0.05. When the mass ratio of CaO to (CaO + SiO₂ + Fe₂O₃) was greater than 0.05, the temperature higher than 1250°C could promote the formation of a liquid slag phase.

Fig. 4.

Phase diagram of CaO–SiO₂–Fe₂O₃ system (P = 101325 Pa, P_O2 = 21278 Pa).

Furthermore, the influence of basicity and temperature on the amount of liquid slag phase during the oxidation roasting process of pellet was investigated. Liquid phase content was calculated by FactSage 7.2 under different conditions. As shown in Fig. 5, the quantity of the liquid phase increases gradually with increasing the basicity and roasting temperature. According to the chemical compositions, the silicate minerals accounted for a large proportion in the iron ore concentrate. The silicate minerals were mainly connected to form the network structure by Si–O bonds, while CaO could be decomposed into Ca²⁺ and O²⁻. The O²⁻ destroyed the network structure in the form of network modifier and caused the complex silicate network. A smaller structural unit would be decomposed to reduce the melting point of silicate minerals, which indicated that more liquid phases would be produced. In addition, according to the thermodynamic equilibrium calculation and Fig. 6 XRD analysis of pellets with different basicity, it was founded that the Fe₃O₄ phase and the diffraction peak of CaFe₂O₄ phase increased slightly, which might be due to Ca²⁺ replacing the Fe³⁺ of silicate. This leaded to the content increase of composite calcium ferrite, and more low-melting-point substances would be formed, thereby increasing the liquid phase.

Fig. 5.

Effect of basicity and temperature on liquid phase during pellet roasting process. (Online version in color.)

Fig. 6.

XRD analysis of high MgO fluxed pellets with different basicity. (Online version in color.)

3.2. Effect of Basicity on Phase Components of Pellets

The compressive strength is closely related to the microstructure of the pellets, which mainly depends on its phase composition. In order to study the phase composition of high MgO flux pellets with different basicity during oxidative roasting, the XRD analysis was used to interpret the phase transformation. The percent basicity modification of this study ranges from 0.6 to 1.3, which indicated a change in phase during oxidative roasting of high MgO flux pellets.

From the XRD pattern shown in Fig. 6, it can be seen that the X-ray diffraction patterns of high MgO flux pellets with basicity of 0.6–1.3 were treated at 1250°C and 15 min. As shown in Fig. 6(a), diffraction peak intensities of (Ca_xFe_2−x)SiO₄ phase at 2θ = 24.219°, 30.201° and 33.321° could be observed. With the basicity increasing, the intensity of characteristic diffraction peaks of (Ca_xFe_2−x)SiO₄ and Fe₂O₃ phases increased gradually. The diffraction peaks of (Ca, Mg)SiO₄ group and Fe₂O₃ phases were observed in the standard card of diffraction peak between (Ca, Mg)SiO₄ group and Fe₂O₃ phases reference material in Figs. 7(a) and 7(c). As shown in Fig. 6(a), with the increase of basicity from 0.6 to 1.3, the diffraction peaks of CaFeSiO₄, Ca₂SiO₄ and Ca₂FeO₄ phases increased slightly. It might be that Ca²⁺ ions gradually replaced Fe³⁺ or Mg²⁺ ions of silicate, which leaded to the enhancement of characteristic diffraction peak (200) of (Ca_xFe_2−x)SiO₄ group. Theoretically, an appropriate amount of (Ca_xFe_2−x)SiO₄ group could improve the strength of pellets by forming slag bonds, which was beneficial to improve the metallurgical properties of pellets. However, the formation of too many low melting point groups such as (Ca_xFe_2−x)SiO₄ group would affect the recrystallization of Fe₂O₃, thus reducing the compress CS of flux pellets. From the experimental results, the CS of pellet did not decrease when the basicity R was 1.1, but increased. The (Ca_xFe_2−x)SiO₄ phase played a certain role in the process of pellet metallurgy, including improving the liquid phase adhesion in the pellet and promoting the recrystallization of Fe₂O₃ and increasing CS.

Fig. 7.

XRD patternsn of standard substances of (a) Ca_XFe_2−x(SiO₄) group; (b) Mg_XFe_2−XAlO₄ group. (Online version in color.)

The X-ray diffraction patterns of Mg_yFe_2−ySiO₄ (2 ≥ y ≥ 0) in high MgO flux pellets are shown in Fig. 6(b). From Fig. 6(b), the diffraction peaks of MgFe₂O₄, CaFe₂O₄ and Mg_yFe_2−ySiO₄ groups could be observed at 2θ = 35.757°, the diffraction peak of MgFe₂O₄ phase was observed at 2θ = 62.764°. With the basicity increasing, the diffraction peak intensity of MgFe₂O₄ phase increased slightly. The diffraction peaks of the standard substance Fe₃O₄, CaFe₂O₄ and MgFe₂O₄ were illustrated in Fig. 7(b). So it could be seen that the distribution of diffraction peaks of Fe₃O₄ and MgFe₂O₄ phases in the standard card was particularly similar due to their close lattice structures. With the increase of basicity, the content of Ca²⁺ in pellet increased continuously might replace Mg²⁺ in Mg_yFe_2−ySiO₄ group, which made the freely Mg²⁺ increase thus indirectly promoted the formation of MgFe₂O₄. On the other hand, it might be due to the increase of Ca²⁺ content in pellets that the formation of (Ca_xFe_2−x)SiO₄ group and CaFe₂O₄ phase, which promoted the diffusion of Mg²⁺. Because of the enhancement of Mg²⁺ migration ability, the probability of MgFe₂O₄ phase formation was increased. At the same time, it was possible to increase the formation of a small part of the MF phase and improve the high temperature metallurgical properties of flux pellets.

3.3. Effect of Basicity on RDI and RSI

Reduction disintegration property (RDI_+3.15) is one of the important physical and chemical indexes of blast furnace charge. Serious disintegration will lead to a large increase of furnace top dust, decrease the permeability and reducibility of material column, and affect the output of blast furnace and the quality of hot metal. The main cause of low temperature disintegration is the lattice transformation between Fe₂O₃ → Fe₃O₄, which leads to the fracture disintegration due to the expansion stress inside the pellets. Figure 8 shows the influence of pellet basicity on reduction disintegration of the pellets. When the basicity was in the range of 0.6 to 0.9, the RDI_+3.15 of pellets decreased slightly from 89.80% to 81.20%. However, in the process of increasing the pellet basicity from 0.9 to 1.3, the RDI_+3.15 increased significantly from 81.20% to 97.78%. Combined with the XRD image analysis of Fig. 6, it could be seen that CaFe₂O₄ did not remove obviously in the range of basicity 0.6–0.9. In the process of melting, it was possible that the existence of this CaFe₂O₄ phase reduced the stress caused by transformation from hematite to magnetite transformation.

Fig. 8.

Influnce of basicity on high MgO fluxed pellets reduction disintegration index. (Online version in color.)

Figure 9 shows the effect of the basicity on the RSI of fluxed pellet. As shown in Fig. 9, during the reduction process, the RSI of flux pellets decreased significantly with the increase of basicity from 0.6 to 1.3. And when the basicity was 1.3, the RSI decreased to a minimum of 8.45%. Combining XRD analysis of Fig. 6, it was founded that with the basicity increasing, the MgFe₂O₄ phase and Mg_yFe_2−ySiO₄ group were also formed accordingly. The low melting point groups (CaxFe_2−x)SiO₄ could promote the mineralization of MgO in fluxed pellets in the basicity range of 0.6–1.3. The flowing of more Mg²⁺ in the pellet promoted the formation of magnesioferrite phase. At the same time, because the Mg²⁺ radius (0.078 nm) was larger than the Fe³⁺ radius (0.064 nm), the lattice around solute atom would swell when part of Fe³⁺ was replaced by Mg²⁺, which resulted in the lattice volume of MgO·Fe₂O₃ was bigger than that of pure Fe₂O₃. It could be deduced that the lattice volume swelling during the transition of magnesio-ferrite to magnesio-wustite was smaller than that of Fe₂O₃ to FeO.¹³⁾ The increase in basicity in high MgO fluxed pellets promoted the fluidity of Mg²⁺, which might be one of the reasons for reducing reduction swelling.

Fig. 9.

Effect of basicity on the RSI of high MgO fluxed pellets. (Online version in color.)

Meanwhile, based on the above experimental results, pellets with basicity of 0.6 to 1.3 were reduced under 1173 K and 70% (volume percent) CO atmosphere. It could be observed from Fig. 10 that the iron whiskers of fluxed pellets with the basicity of 0.6 were more numerous and thick, and were mere spiral-shaped. Moreover, the surface of the pellet after reduction was also more cracked, showing an off-white phase, and the porosity was also increased accordingly. When the basicity was 0.8, a part of the flake Fe_XO phase was formed in the cause of forming iron whiskers. And a large sheet of Fe_XO phase could be observed from the SEM images of R1.1 and R1.3, and iron whiskers weren’t observed. Thus, in the course of Fe₂O₃ reduction after increasing the basicity, the nucleation of the iron whisker could be postponed with the increase of basicity. According to Fig. 10, it could be measured that the diameter of the iron whiskers was about 1–1.2 μm and the length was more than 10 μm when the basicity was 0.8. In the fluxd pellets having a basicity of 0.9, the iron whiskers were slightly shorter and had a diameter of about 0.8 μm and a length of about 3–5 μm. In the pellet having a basicity of 1.0, the iron whisker became finer, having a diameter of about 0.4 μm and a length of about 3–5 μm. In fluxed pellets with more than the basicity of 1.0, iron whisker became so tiny that iron whisker could not be identified using the stereo optical microscope. Therefore, the basicity increasing of the pellet and the lesser amounts of cracks with increase in basicity might inhibit the morphology of the iron whisker and make it fine.

Fig. 10.

SEM-EDS images of growth of iron whiskers in reduction of fluxed pellets on different basicity. (Online version in color.)

3.4. Effect of Basicity on Softening-melting Behaviour

Figure 11 shows the effect of basicity on the softening properties of high MgO flux pellets. As shown in Fig. 11, with increasing basicity in the pellets, the softening start temperature T₄ increased from 1106°C to 1157°C, whereas the softening end temperature T₄₀ increased from 1218°C to 1266°C. Thus, the softening interval (T₄₀–T₄) decreased from 112°C to 102°C and then to 109°C. Therefore, in the experimental range, the softening behavior of high MgO pellets slightly improved with the increase of basicity in the pellets.

Fig. 11.

Effect of the basicity in the high MgO fluxed pellets on the softening behavior. (Online version in color.)

There were two reasons for the increase of both the softening start temperature T₄ and the softening end temperature T₄₀. First, when the basicity in the pellets increased, the presence of a high MgO content of 2.4% promoted the mineralization of MgO and produced magnesium ferrite. As it could be seen from the XRD diffraction pattern of Fig. 6, some Mg²⁺ ions in the Mg_2−yFe_ySiO₄ group were replaced, so the MgFe₂O₄ phase showed an upward trend. The MgFe₂O₄ phrase could lead to reduced reduction relatively and the shrinkage of the pellets. Second, the high-melting-point substance containing MgO increased in the primary slag phase with increasing CaO content in the pellets. Based on these two effects, the softening start temperature T₄ and the softening end temperature T₄₀ both increased with the basicity of the pellets increasing, which could promote the gas-solid reaction of the pellets in the material column.

Figure 12 shows the effect of basicity on the melting behavior of pellets. As shown in Fig. 12 with increasing basicity in the pellet, the melting start temperature T_S increased from 1296°C to 1316°C and the dripping temperature T_D increased from 1394°C to 1410°C. The melting interval (cohesive zone) T_D–T_S decreased from 98°C to 94°C. In addition, as shown in Fig. 13, the location of the cohesive zone shifted down slightly and the cohesive zone becomes mildly narrowed, which was beneficial to the better permeability of the cohesive zone. Therefore, the melting behavior of pellets was improved to some extent with increasing basicity in the pellet.

Fig. 12.

Effect of the basicity in the high MgO fluxed pellets on the melting behavior. (Online version in color.)

Fig. 13.

Effect of the basicity in the high MgO fluxed pellets on the location of the cohesive zone. (Online version in color.)

The existence of high MgO content in pellets was the main reason for the slight increase in T_S and T_D, which was mainly explained by the slag phase and magnesium ferrite phase. First, when the basicity of the pellets increased, the liquid phase of calcium ferrite also increased, causing the flow of Mg²⁺, promoting the formation of the high-temperature substance magnesium ferrite, and increasing the melting point of the burden. Second, the content of Mg²⁺ in the slag was also increased accordingly, which promoted the formation of merwinite (Ca₃MgSi₂O₈),¹²⁾ thereby increasing the viscosity of the slag phase, and correspondingly increasing the melting point of the high magnesium pellet slag phase.

4. Conclusions

(1) With the increase of basicity of high MgO fluxed pellets, the CS increases slowly at first, and then increased sharply for the basicity was 1.1. The increasing of basicity improved the liquid phase adhesion, and increased the slag phase and decreased the porosity.

(2) With the increase of the basicity, the reduction disintegration index of the fluxed pellets decreased first and then increased, which indicated that the increase of the liquid phase promoted the flow of Mg²⁺ and inhibited the crystal from transition of Fe₂O₃→Fe₃O₄, thereby increasing the reduction disintegration index.

(3) With the basicity increasing, the reduction swelling index continued to decrease. The increasing of basicity inhibited the formation of iron whiskers and made it fine; on the other hand, the generation of liquid phase promoted the flow of Mg²⁺, which inhibited the crystal form transition of Fe₂O₃→Fe₃O₄ to reduce the swelling index.

(4) With increasing basicity in the fluxed pellets, the softening interval T₄₀–T₄ decreased from 112°C to 109°C, which facilitated the gas–solid reaction for the burden. The melting interval T_D–T_S decreased from 98°C to 94°C. The location of the cohesive zone was down slightly, and the cohesive zone became moderately thinner. The cohesive zone was beneficial to the better permeability of the cohesive zone. The softening–melting characteristic was better when the basicity in the pellets was 1.1.

Acknowledgement

The authors wish to express their thanks to the National Natural Science Foundation of China-Liaoning Joint Funds (U1808212) and National Natural Science Foundation of China (52074080), the Fundamental Research Funds of the Central Universities of China (N182504010), and Xingliao Talent Plan (XLYC1902118).

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