Effect of Al₂O₃ on Sintering Reaction of Calcium Ferrite

Abstract

Calcium ferrite containing aluminum (CFA) has been widely used in blast furnace production because it is an important compound that affects the quality of sinter. The influence of Al₂O₃ on CFA preparation process was studied by the raw material of CaO-Fe₂O₃-Al₂O₃. Diffraction of x-rays (XRD), scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS) analysis were used to analyze the phase, micro morphology and the reducibility of the products. The results showed that CaO tended to combine more with Fe₂O₃ to form calcium ferrite with the increase of Fe₂O₃, whereas CaO tended to combine more with Al₂O₃ to form calcium aluminate with the content of Al₂O₃ above 45%. The micro morphology of sintered samples had a stable sintering skeleton structure due to the existence of calcium ferrite. Calcium ferrite was distributed between different particles in flakes, whereas calcium aluminate was attached to the Al₂O₃ particle surface. The large amount of calcium aluminate inhibited the emergence of calcium ferrite, and it had an adverse impact on the reducibility of sinter and the preparation of CFA.

1. Introduction

Sinter is sintered by powder ore and flux ingredients, and the quality of sinter directly affects the smooth production of blast furnace ironmaking [1,2]. Sinter requires characteristics of stable composition, high strength and good reducibility. The fundamental reason for the above characteristics is the formation of the binder phase calcium ferrite [3]. Calcium ferrite has been used in the production of blast furnace as a flux type sinter since 1930s [4]. There is little pure calcium ferrite in minerals, and the mineral phases containing magnesium, aluminum and silicon in the vein components. These oxides solidly dissolve into calcium ferrite during the sintering process, form calcium ferrite containing aluminum (CFA) and then transform into multi-component composite calcium ferrite (SFCA) [5,6]. SFCA is a low melting point binder phase mineral with high strength and good reducibility in sintered ore. Its Al₂O₃ composition is an important factor that affects the microstructure and macroscopic quality of sinter [7,8,9,10]. The stability of SFCA in sinter depends on the content of Al₂O₃ [11]. SFCA can stably exist with the mass fraction of Al₂O₃ exceeding 2.5%, otherwise, it will decompose into silicate and magnetite. Machida [12] found that the increase of alkalinity caused by Al₂O₃ promoted the formation of calcium ferrite at low temperatures, resulting in the appearance of pores in the sinter and the decrease of strength. CFA is a very important intermediate compound in the transition from calcium ferrite to SFCA, and the influence of Al₂O₃ content on its preparation process is worth studying. However, previous studies mainly focused on the range of low Al₂O₃ mass fraction (less than 20%), and the influence of using high Al₂O₃ mass fraction (more than 20%) to prepare sinter is not clear [13,14,15,16]. In order to provide more references for the preparation of CFA and the application of high-alumina sinter, it is necessary to study the influence of high Al₂O₃ ratio on the sintering process of calcium ferrite.

In this paper, Fe₂O₃, Al₂O₃ and CaO powder was used as raw material. The content of Al₂O₃ was 18%, 36%, 45% and 60% was used to sinter at 80 °C. The effect of alumina content on the phase and morphology of sintered products was characterized by XRD and SEM. The influence of the reducibility of the sintered material was analyzed by electrochemical impedance spectroscopy (EIS).

2. Experimental Section

The raw materials were Fe₂O₃, Al₂O₃ and CaO powders with a purity of 99.9%, produced by Shanghai Sinopharm. The average particle sizes of Fe₂O₃, Al₂O₃ and CaO were 2 μm, 8 μm and 3 μm, respectively. In order to prevent CaO from absorbing water, these powders were dried for 6 h at 200 °C, respectively. The raw material mass ratios of the four sintered samples were shown in

Table 1. Experimental conditions (Mass fraction, %).

Samples	CaO	Fe2O3	Al2O3
S1	10	30	60
S2	10	45	45
S3	10	54	36
S4	10	72	18

. The CaO content selected in this experiment was fixed at 10%. The CaO content in steel making sinter is mostly between 8% and 14%. CaO will affect the basicity of sinter, and when the basicity increases, it is good for the strength of sinter, but it will affect the grade of sinter and reduce the smelting strength of the blast furnace. This CaO content is suitable for the needs of the blast furnace burden structure of steel plants. The powders were weighed and mixed, then XQM-2 vertical (Tencan Powder, Changsha, China) planetary for ball milling was used. Absolute ethanol was used as the dispersion medium, ball milling speed was 300 r/min and milling time was 6 h. After ball milling, the samples were placed in a DZF-6050 vacuum drying oven at 120 °C for 30 min. After drying, the powder mixture was pressed into cylindrical pellets (15 × 3 × 1 mm³) under a uniaxi-al pressure of 6 MPa for 3 min. Then, the pellets were put into a tubular furnace (BLMT-GB17810) (Bolemant Electric Furnace, Luoyang, China) to sinter at 800 °C for 5 h, with the heating rate of the furnace at 5 °C·min⁻¹.

The cooled samples were characterized by using X-ray diffraction (PANalytical, Amelo, Netherlands). The angle of X-ray diffraction is 5° to 90°, and the scanning speed is 10°/min. The acceleration voltage of the electron microscope is 5.0 kV. The microstructure and element analyses were carried out with SEM and EDS (Zeiss, Oberkohern, Germany). The electrochemical workstation (Chinstruments, Shanghai, China) was used to test the electrochemical impedance spectroscopy of the pellets to analyze the reduction differences caused by different ingredients. High frequency was set to 10⁶ Hz, and low frequency was set to 0.1 Hz. Scanning rate was 10 mV/s.

3. Results and Discussion

Thermodynamic calculation was analyzed by using Factsage7.3 software (Aachen, Germany), shown in Figure 1. Fe₂O₃ and Al₂O₃ will not react in theory, and there is almost no solid solution reaction [17]. The reaction in the sintering process is mainly on the Al₂O₃-CaO and Fe₂O₃-CaO systems, shown in Equations (1)−(4). CaO spontaneously reacted with Fe₂O₃ and Al₂O₃ to form calcium ferrite and calcium aluminate at 800 °C. Ca₂Fe₂O₅ was formed before calcium aluminate (CaAl₄O₇, CaAl₂O₄), and CaFe₂O₄ was finally formed.

CaO(s) + Fe₂O₃(s) = CaFe₂O₄(s)

(1)

2CaO(s) + Fe₂O₃(s) = Ca₂Fe₂O₅(s)

(2)

CaO(s) + 2Al₂O₃(s) = CaAl₄O₇(s)

(3)

CaO(s) + Al₂O₃(s) = CaAl₂O₄(s)

(4)

Figure 2 shows the XRD patterns of samples in different Al₂O₃-Fe₂O₃ mass ratios. The content of Al₂O₃ decreased sequentially from S1 to S4. S1 obtained CaAl₂O₄ by Reaction (4). S2 obtained Ca₂Fe₂O₅ and CaAl₂O₄ by Reactions (2) and (4). S3 obtained Ca₂Fe₂O₅ by Reaction (2). The form of calcium ferrite in sample S4 was CaFe₂O₄, and due to the formed Ca₂Fe₂O₅ can react with excessive Fe₂O₃ to form CaFe₂O₄ [18]. With the decrease of Al₂O₃ content, calcium aluminate gradually disappeared, and calcium ferrite changed from Ca₂Fe₂O₅ to CaFe₂O₄. The reason was that reaction rate of the elementary reaction was proportional to the product of the power of the concentration of each reactant by the law of mass action [19]. The reaction rate of excessive Al₂O₃ was higher than that of Fe₂O₃ with a less amount, and CaO preferentially reacted with Al₂O₃ to produce CaAl₂O₄. As the Al₂O₃ content decreased, the influence of the law of mass action decreased gradually, and the reaction followed the laws of thermodynamics. The standard Gibbs free energy generated by Ca₂Fe₂O₅ was less than CaAl₂O₄, the content of Al₂O₃, Fe₂O₃ was same, and the law of mass action is comparable to the effect of thermodynamics on reactions, Ca₂Fe₂O₅ and CaAl₂O₄ coexisted in the sample S2. As the mass proportion of Fe₂O₃ further increased, there was no calcium aluminate in sample S3 and sample S4. On the one hand, when the content of Fe₂O₃ was more than Al₂O₃, the probability of collision between Fe₂O₃ particles and CaO particles was greater, and the quantity of activated molecules became more, and CaO reacted with Fe₂O₃ preferentially. On the other hand, the size of Al₂O₃ was significantly larger than Fe₂O₃, Fe₂O₃ and had a larger specific surface area than Al₂O₃. Therefore, the contact surface areas between CaO and Fe₂O₃ were larger, and the formation of calcium ferrite was more favorable. The kinetic conditions for the formation of the calcium iron oxide phase were better than those for the formation of the calcium aluminum oxide phase.

Figure 3 showed the SEM images of samples. The contact between different particles of the sintered sample was closer than that before sintering, and a stable sintering skeleton structure was formed. With the increase of Fe₂O₃ content, the number of pores in sintered samples decreased [20]. The smaller Fe₂O₃ particles can more fully fill the pores in the structure. In addition, the generated calcium ferrite had a bonding effect, the overall structure was more compact, and the contact among particles was better. Figure 4 was the SEM and energy spectrum point scan analysis of sample S3. It determined that particles with a smaller size were Fe₂O₃ at 003 point. These particles were densely and evenly distributed in the sample. The larger particles were Al₂O₃ according to the 001 point. The particles as chunk form or pellet form were calcium ferrite particles produced by sintering according to 002 in Figure 4, and they were evenly distributed between Fe₂O₃ and Al₂O₃ particles. Figure 5 showed the SEM and EDS surface scan images of sample S3. The main distribution position of Ca element was the same as that of the Fe element. Combined with the results of Figure 2, Figure 4 and Figure 5, it indicated that the particles in chunk form or pellet form were Ca₂Fe₂O₅ particles. Ca₂Fe₂O₅ and Fe₂O₃ particles were dispersed in the whole sample and closely contacted Al₂O₃ particles and formed a relatively dense structure.

Figure 6 showed the SEM images of samples S1–S4 with higher magnification. CaAl₂O₄ was formed by the reaction of adjacent CaO and Al₂O₃ particles, and it grew on larger Al₂O₃ particles in the microstructure. Combined with the XRD analysis of S1 and S2 in Figure 2, it was determined that the unique particle that formed the dendritic and leafy morphology in Figure 6a was CaAl₂O₄. According to Figure 4, the strip-shaped or needle-shaped particles interspersed between Al₂O₃ and Fe₂O₃ in Figure 6b–d was calcium ferrite. The Al₂O₃/Fe₂O₃/Ca₂Fe₂O₅/CaAl₂O₄ solid interface appeared in Figure 6b. According to the XRD results, the calcium ferrite in Figure 6d was CaFe₂O₄, and its needle structure was less obvious than that of Ca₂Fe₂O₅ in Figure 6c.

As a sinter, the reducibility of calcium ferrite is key property, and the reduction process is actually the process of electron loss. The reducibility is stronger, and the ability of electron loss is stronger. The reducibility can be expressed as its resistance. Figure 7 showed the electrochemical impedance spectroscopy (EIS) analysis diagram of samples S1~S4. In the equivalent circuit of the analysis diagram, RS (Ω) represented the solution resistance, RCT represented the charge transfer resistance, ZW was the diffusion impedance and C1 represented the capacitive element in the electrochemical measurement device. The Nyquist curve in the figure was composed of the high-frequency circular arc in the first half and the low-frequency line in the second half. The radius or area of the circular arc in the high-frequency area reflected the size of RCT, and it represented the internal resistance of the pellet. When the internal resistance was smaller, the radius of the circular arc was smaller. The order of arc radius in high frequency region was S1 > S2 > S3 > S4. With the increase of the proportion of Al₂O₃, the radius of the high-frequency circular arcs was larger. With the increase proportion of Fe₂O₃, the resistance of the sintered pellets was smaller. And the ability to obtain electrons was stronger and sinter was easier to be reduced. Combined with the phase of these pellets, the appearance of calcium aluminate prevented calcium ferrite from obtaining electrons, leading to the replacement of calcium aluminate with the poor reducibility of sinter.

Generally, the preparation of CFA required CaO and Fe₂O₃ to react at a low temperature to produce calcium ferrite. The crystal shape of calcium ferrite was mainly chunk and pellet. Calcium ferrite crystal had a fast crystallization speed and strong crystallization ability, was closely connected with Fe₂O₃, and the pellet formed a stable sintering skeleton in the microstructure. When the temperature reached above 1100 °C, the calcium ferrite reached the melting point and melt into Fe₂O₃ to achieve liquid-phase sintering. CaAl₂O₄ or Al₂O₃ was easy to dissolve into calcium ferrite to form CFA [21]. It was noted that the amount of Al₂O₃ had an important influence on the sintering process. An appropriate amount of Al₂O₃ improved the wettability and reducibility of the melt during liquid phase sintering [22,23,24], whereas excessive Al₂O₃ reduced the strength of the sinter and deteriorated the mineral structure. This statement was consistent with the results. It can be seen from Figure 2 that the content of Al₂O₃ was too high, and the competition between Al₂O₃ and Fe₂O₃ to capture CaO affected the formation of calcium ferrite. It showed that most of the CaO was used to form CaAl₂O₄ before reaching the CFA generation temperature, resulting in little calcium ferrite. Therefore, the content of Al₂O₃ should be reasonably controlled, so that the sintering system generates more calcium ferrite and less CaAl₂O₄ at low temperatures. When the solid Al₂O₃ or CaAl₂O₄ entered liquid calcium ferrite at high temperatures, it reduces the melting point of calcium ferrite and stabilizes its formation [25].

4. Conclusions

In this paper, the effect of Al₂O₃ content on the preparation process of CFA was studied by using CaO-Fe₂O₃-Al₂O₃ as raw materials. With the decrease of Al₂O₃ content, CaO was more inclined to combine with Fe₂O₃ to form calcium ferrite. When the Al₂O₃ content was higher, CaO was more inclined to combine with Al₂O₃ to form CaAl₂O₄. The simultaneous presence of Ca₂Fe₂O₅ and CaAl₂O₄ was detected with same content of Al₂O₃ and Fe₂O₃. After sintering, a stable sintering skeleton was formed. Calcium ferrite was distributed between different particles in chunks or pellets, whereas calcium aluminate was attached to the Al₂O₃ particle. The structure of samples containing more Fe₂O₃ were more compact. Due to the calcium ferrite generated by sintering being evenly distributed in the whole sample, it strengthened the connection between particles. A large amount of calcium aluminate brought by a high Al₂O₃ ratio significantly reduced the reducibility of sinter. The preparation of CFA required the formation of a considerable amount of calcium ferrite during the heating stage. It ensured that other elements were dissolved in calcium ferrite at high temperatures. The formation of calcium ferrite was hindered if there was more Al₂O₃. Therefore, in order to prepare CFA, it is necessary to control the proportion of raw materials to avoid the formation of calcium aluminate during the heating process, and made Al₂O₃ or calcium aluminate play a beneficial role at high temperatures.