Formation of Calcium Ferrite Containing Aluminum (CFA) in Sintering of Iron Ore Fines

: Calcium ferrite containing aluminum (CFA) is a precursor of the low-temperature bonding phase in the sintering process of iron ore fines for blast furnace ironmaking. Thus, improving the formation of CFA at lower temperature is very important for saving energy, improving efficiency and production. In this paper, the formation process of CFA was investigated at 1200 ◦ C by reactions of alumina (Al 2 O 3 ), respectively with a mixture of calcium oxide (CaO) and hematite (Fe 2 O 3 ) and monocalcium ferrite (CF) as a recognized initial product, as well as reaction of Al-containing hematite (H ss ) with CF. The result confirmed that CF is an intermediate product formed easily in the sintering process

Patrick et al. [17] reported that SFCA is a stable phase possessing the chemical composition plane that connects the members of CaO•3Fe 2 O 3 (CF 3 ), CaO•3Al 2 O 3 (CA 3 ), and 4CaO•3SiO 2 (C 4 S 3 ), where the limitation of substitution ratio for Al 3+ to Fe 3+ was 31.5% of its mass.Scarlett et al. [18] investigated the formation process of SFCA using an in situ X-ray diffraction method, indicating that CaO•Fe 2 O 3 (CF) is a precursor of SFCA formation, and the formation of SFCA can be promoted by adding Al 2 O 3 .
Webster et al. [19] found that ternary calcium ferrite (CFA) is also a precursor of SFCA formation.Actually, CFA was discovered decades ago.It was observed earlier by Yamauchi [20] as a ternary compound in the CaO-Al 2 O 3 -Fe 2 O 3 -SiO 2 system.Lister et al. [21] defined this compound as "ternary phase" (TP) and pointed out that the chemical formula is CaAl 2 Fe 4 O 10 (CAF 2 ).Subsequently, Mumme et al. [22] determined the structure of CFA in Ca 5.1 Al 9.3 Fe 3+ 18.7 Fe 2+ 0.9 O 48 .As accordingly verified, CFA provides a basis for SFCA formation due to it having the same structure as triclinic crystal.According to previous studies, CFA is a solid solution formed by the reaction of Al 2 O 3 with CF [23].Webster et al. [24] also confirmed the formation of CFA using gibbsite (Al(OH) 3 ), kaolinite (Al 2 Si 2 O 5 (OH) 4 ), and aluminous goethite, respectively, in the sintering process of iron ore fines, and simultaneously observed that alumina in kaolinite or aluminous goethite provide a better condition for the formation of CFA at lower temperatures than gibbsite.In our previous works, it was reported [25] that alumina dissolved into hematite (H ss ) could promote the formation of CFA in a reaction with CaCO 3 .Moreover, it was found [26] that the reaction of Al 2 O 3 with CaFe 2 O 4 (CF) to form CFA is easier than using SiO 2 to form calcium ferrite containing silica (SFC), as a precursor for the formation of SFCA.Other work [27] also indicated that the simultaneous appearance of CFA and CF could be conducive to producing a liquid phase with a lower melting temperature than CF.So, it is very important to understand the formation of CFA in the sintering process of iron ore fines.
However, there still exist many questions, as follows: What are the reactions in the transformation process from CF to CFA?Why does the formation rate of CFA show a great difference in the sintering process of iron ore fines with different Al 2 O 3 materials?These have also been paid attention to in recent years [28][29][30][31].In this work, the mole ratio of Fe 2 O 3 to CaO with 3:1 and 4.0% Al 2 O 3 were used to simulate the chemical composition of sintering materials to investigate the transformation process of CF to CFA at 1200 • C for revealing the formation mechanism at low temperature; simultaneously, the reactions of CF with Al-containing hematite solid solution (H ss ), a mixture of Fe 2 O 3 and Al 2 O 3 , and only 4.0% Al 2 O 3 , respectively, at different times were investigated to ascertain the reason for different CFA formation rates by different Al-containing materials for improving the sintering process of iron ore fines.

Reactions of CFA Formation
Analytical reagents of CaCO 3 , Fe 2 O 3 , and Al 2 O 3 (purity above 98%) were used to prepare CF and Al-containing H ss , respectively, based on our previous work [25,32].The chemical compositions and sintering conditions for the synthesis of CF and H ss are shown in Table 1.The analytical reagents were first weighed and homogenized in a mortar grinder for 60 min under air atmosphere, and pressed into a tablet approximately 15 mm in diameter and 5 mm in height under 5 MPa pressure using an electric briquetting machine.Then, the samples were sintered in a tubular resistance furnace at the given temperature for different durations in air atmosphere and taken out, followed by air cooling.XRD patterns of synthesized CF and H ss are shown in Figure 1.It can be seen that the diffraction peaks of synthesized CF and H ss matched well with the stand card of CF and Fe 2 O 3 , without diffraction peaks of Al 2 O 3 .C for 60 min in the air atmosphere, followed by liquid nitrogen cooling.Similarly, based on the experimental results above, the reactions including CF with Fe 2 O 3 , Al 2 O 3 , and H ss with CF were also weighed, homogenized, pressed, and sintered at 1200 • C for different durations in the air atmosphere, followed by liquid nitrogen cooling.Afterwards, each sample was cut into two pieces along the radial line, where one was polished for microstructure observation and element quantification using SEM-EDS and optical microscope methods, and the other was grinded into small particles less than 50 µm for mineral composition identification using the XRD method.To investigate the generation order of intermediate products in the formation of CFA, three kinds of reactions, including Fe2O3-CaCO3-Al2O3, CF-Fe2O3-Al2O3, and Hss-CF, were conducted, and the corresponding mass fraction of reactants are listed in Table 2. To simulate the environment of the actual sintering process, the mole ratio of Fe2O3 to CaO was set at around 3:1.The reactants including Fe2O3, CaCO3, and Al2O3 were first weighed, homogenized in a mortar grinder (Fritsch Pulverisette 2, Idar-Oberstein, Germany) for 30 min, and pressed into a tablet of 5 mm in diameter and approximately 5 mm in height at 5 MPa, and sintered at 700 °C, 800 °C, 900 °C, 1000 °C, 1100 °C, and 1200 °C for 60 min in the air atmosphere, followed by liquid nitrogen cooling.Similarly, based on the experimental results above, the reactions including CF with Fe2O3, Al2O3, and Hss with CF were also weighed, homogenized, pressed, and sintered at 1200 °C for different durations in the air atmosphere, followed by liquid nitrogen cooling.Afterwards, each sample was cut into two pieces along the radial line, where one was polished for microstructure observation and element quantification using SEM-EDS and optical microscope methods, and the other was grinded into small particles less than 50 µm for mineral composition identification using the XRD method.Reactions of CF with Al2O3 and CF with Fe2O3 were also conducted to investigate the formation of calcium aluminate and alpha-calcium iron oxide.For the reactions of CF with Al2O3, 96 wt% of CF and 4 wt% Al2O3 were also weighed, homogenized, pressed, and sintered at 1200 °C for different durations, followed by liquid nitrogen cooling.For the reactions of CF with Fe2O3, the mole ratios of CF and Fe2O3 were set at 1:0.5, 1:0.9, 1:0.95, 1:0.98, 1:1, and 1:1.5.Similarly, samples of CF with different ratios of Fe2O3 were also weighed, homogenized, pressed, and sintered at 1200 °C for different durations, followed  Reactions of CF with Al 2 O 3 and CF with Fe 2 O 3 were also conducted to investigate the formation of calcium aluminate and alpha-calcium iron oxide.For the reactions of CF with Al 2 O 3 , 96 wt% of CF and 4 wt% Al 2 O 3 were also weighed, homogenized, pressed, and sintered at 1200 • C for different durations, followed by liquid nitrogen cooling.For the reactions of CF with Fe 2 O 3 , the mole ratios of CF and Fe 2 O 3 were set at 1:0.5, 1:0.9, 1:0.95, 1:0.98, 1:1, and 1:1.5.Similarly, samples of CF with different ratios of Fe 2 O 3 were also weighed, homogenized, pressed, and sintered at 1200 • C for different durations, followed by liquid nitrogen cooling.Afterwards, the synthesized alpha-calcium iron oxide was also homogenized with 5 wt% and 10 wt% Al 2 O 3 , pressed, and sintered at 1200 • C for 10 h in the air atmosphere, followed by liquid nitrogen cooling.Afterwards, the cooled samples were also analyzed using the XRD and SEM-EDS methods.

Minerals Determination
The mineral phase of samples was identified using a Rigaku Ultima IV X-ray diffractometer (Rigaku Corporation, Tokyo, Japan).Cu Kα was used as a radiation source (40 kV, 400 mA) with a graphite monochromator in the diffraction beam path.The XRD data were collected using continuous scanning mode, for which the scanning speed was 10 • /min.The XRD data were analyzed using Crystallographica Search-Match software 2.1.1.0(Oxford Cryosystems Ltd.Oxford, UK).
A piece of each sample was embedded with phenolic powders, molded by a metallographic molding machine, and polished for SEM and optical observations.Mineral morphology and microstructure observation were performed using an FEI Quanta 250 scanning electron microscope (FEI Corporation, Hillsboro, OR, USA) with an accelerating voltage of 20 kV.The chemical composition of elements was obtained on this instrument using an XFlash 5030 EDS detector (Bruker Nano GmbH, Berlin, Germany).Moreover, the element distribution was obtained through an EPMA detector (EPMA-1720H, Shimadzu, Kyoto, Japan).

Calculations of Formation Energy for Products
First-principles calculations were used to investigate the thermodynamical stability of the products of the above reactions [33,34]; where structural models were constructed, the formation energy (FE) of products was calculated using the density functional theory (DFT) in Materials Studio.The forces on every atom were converged within 0.05 eV/Å.The corresponding calculation parameters are presented in Table 3.

Effect of Temperature on CFA Formation
The XRD patterns of samples of Fe 2 O 3 , CaCO 3 , and Al 2 O 3 reacted for 60 min at different temperatures lower than 1200 • C are shown in Figure 2a.It can be seen that CaCO 3 corresponding to PDF 5-586 decomposed completely at 900 • C after sintering for 60min, while CaO corresponding to PDF 37-1497 had also not appeared, which indicates that the CaO had already participated in the reaction to generate other products.As the temperature increased to 1100 • C, Ca 2 Fe 2 O 5 (C 2 F) corresponding to PDF 47-1744, CFA (Ca 3.18 Fe 15.48 Al 1.34 O 28 ) corresponding to PDF 52-1258, and CF corresponding to PDF 32-168 successively appeared.However, it can easily be seen that the diffraction peak intensity of CF decreased, while the diffraction peak intensity of CFA and Ca 2 Fe 15.5 O 25 corresponding to PDF 78-1669 increased when the temperature increased to 1200 • C. In more detail, for diffraction angles ranging from 33.5 • to 35.4 • , as shown in Figure 2b, the first strong peaks of CF (3 2 0), CFA (0 -2 4), and Ca 2 Fe 15.5 O 25 (1 1 18) clearly exited, and this phenomenon had already appeared above 1000 • C. Amazingly, the diffraction peak of CaO (2 0 0) appeared at 1200 • C, corresponding to the first strong peak, as shown in Figure 2a.According to above result, it can be considered that CF transformed to Ca 2 Fe 15.5 O 25 and CFA at 1200 • C, as follows: However, a large amount of Fe 2 O 3 corresponding to PDF 33-664 still remained in samples at 1200 • C. In addition, CaAl 2 O 4 possibly also appeared at 1200 • C, corresponding to the first strong peak (2 2 0) of PDF 23-1036, as shown in Figure 2a, but it was very week.
However, a large amount of Fe2O3 corresponding to PDF 33-664 still remained in samples at 1200 °C.In addition, CaAl2O4 possibly also appeared at 1200 °C, corresponding to the first strong peak (2 2 0) of PDF 23-1036, as shown in Figure 2a, but it was very week.

Effect of Different Types of Al-Containing Materials on CFA Formation
The XRD patterns of samples sintered at 1200 °C for different durations in reactions of CF with mixtures of hematite and alumina (Fe2O3-Al2O3) and Al-containing hematite solid solution (Hss), respectively, are shown in Figure 3.It can be seen that the diffraction peaks of Ca2Fe15.5O25 and CFA appeared in the CF-Fe2O3-Al2O3 sample sintered for 30 min; then, the corresponding diffraction peak intensity increased as time went on, but the former increased much more than the latter, indicating that the reaction is conducive to promoting the formation of Ca2Fe15.5O25, while the diffraction peak of CFA appeared in the CF-Hss sample sintered for 5 min, which was far earlier than that of the CF-Fe2O3-Al2O3 sample; the intensity increased with time; simultaneously, the diffraction peak intensity of CF decreased, but the diffraction peak of Ca2Fe15.5O25 was not observed, indicating the reaction is evidently conducive to promoting the formation of CFA.
In order to reveal the reason for the obvious difference in the formation of CFA in the two reactions, XRD patterns of the CF-Fe2O3-Al2O3 sample and CF-Hss sample ranging from 34° to 34.8° were investigated.It was obvious that the diffraction peaks of CaAl4O7 and CaAl2O4 appeared in the CF-Fe2O3-Al2O3 sample, respectively, corresponding to PDF 23-1037 and PDF 23-1036.To verify the reason, the geometrical structure characteristic of Al-containing materials was further investigated.The results of SEM and EDS on the section of the CF-Hss sample sintered at 1200 °C for 60 min and the CF-Fe2O3-Al2O3 sample sintered at 1200 °C for 120 min, respectively, are shown in Figure 4 and Table 4.As shown in Figure 4a, combined with the EDS results as shown in Table 4, it can be confirmed that there are samples of Hss with light gray and CF and CFA with dark gray  As shown in Figure 4a, combined with the EDS results as shown in Table 4, it can be confirmed that there are samples of H ss with light gray and CF and CFA with dark gray in which the CFA and CF were hard to identify by SEM, and we could only rely on EDS due to having a near-reflected electron color.The CF was directly surrounded by the CFA, and so it can be deduced that CFA was generated between CF and H ss directly, as follows: Then, the amount of CFA increased via the diffusion of Fe 3+ , Al 3+ , and Ca 2+ in the CFA layer between the CF and H ss .
As shown in Figure 4b    Reactions ( 4) and ( 5) first occurred, respectively, due to the strong binding force and low Gibbs free energy for the formation of CA and CA 2 .Afterwards, the Fe 3+ in Fe 2 O 3 and the Al 3+ in Al 2 O 3 diffused in opposite directions; simultaneously, the Ca 2+ and Fe 3+ in the CF diffused in the Fe 2 O 3 and Al 2 O 3 directions under the chemical potential until the CF and Al 2 O 3 disappeared, forming the layering structure shown in Figure 5.
observed that for 5h, the amount of CF in the samples increased with the increase in mole ratio of CF to Fe2O3 in the raw materials, as shown in Figure 6a.After sintering for 10 h, as shown in Figure 6b, the CF and Fe2O3 in the samples simultaneously disappeared when the mole ratio of CF to Fe2O3 in the raw materials was 1: 0.98, and the Ca2Fe15.50O25formed completely.This indicates the formula corresponding to Ca2Fe15.50O25 should be CaFe3.96O6.94(CF1.98),similar to the chemical composition of CaFe4O7.If the Ca2Fe15.50O25 as crystal structure is true, there will be a lot of Fe 3+ and O 2-vacancies in CF1.98, which is beneficial for its internal diffusion.For further reactions of CF1.98, 5% Al2O3 and 10% Al2O3 were added, respectively, to CF1.98, homogenized, pressed, and sintered at 1200 °C for 10 h.The XRD patterns of CF1.98-Al2O3 samples are shown in Figure 7.It can be seen that the products of the reaction after adding of 5% Al2O3 and 10% Al2O3 into CF1.98 were all CFA, indicating that the CFA was a solid solution with the same crystal structure although with differences in Al 3+ content.The reaction is as follows: CF1.98 was also confirmed as a precursor of CFA formation.

Reaction of CF with Al2O3
To further verify Reaction (2) with the CF-4%Al2O3 sample sintered at 1200 °C for different durations, the XRD patterns are shown in Figure 8.It can be seen that Ca2Fe15.50O25,CaAl2O4, CaAl4O7, and a little CFA were products of sintering process in which the CaAl2O4 transformed to CaAl4O7, and disappeared for 60min; simultaneously, the Ca2Fe15.50O25increased with time, while the CF decreased.The result is consistent with Reaction (2); the diffraction peak intensities of Ca2Fe15.50O25 and CFA in the CF-Al2O3 sample, as shown in Figure 8b, are clearly inferior to those in the CF-Fe2O3-Al2O3 sample.As shown in Figure 3, the Fe2O3 could act as a conductive layer of Al 3+ ; instead, the Al2O3 captured Ca 2+ from the CF to generate CA and CA2.It can be considered that the CA or CA2 was determined by the activity of Ca 2+ (or CaO) at the interface between the Al2O3 and CF, but CaO was not observed as it was in the CF-Fe2O3-Al2O3 sample.This further revealed that the appearance of CA or CA2 is a main reason to decrease the rate of CFA formation.

Stability of Products Generated in Reactions
Because the lack of thermodynamical data for the solid solution, the formation energy of the products was calculated by DFT on first-principles to estimate the stability.The result is listed in Table 5.It shows that the stabilities of CF and CF1.98 are lower than those of CFA, while those of CA, C2F, and CA2 are higher than those of CFA, indicating

Stability of Products Generated in Reactions
Because the lack of thermodynamical data for the solid solution, the formation energy of the products was calculated by DFT on first-principles to estimate the stability.The result is listed in Table 5.It shows that the stabilities of CF and CF 1.98 are lower than those of CFA, while those of CA, C 2 F, and CA 2 are higher than those of CFA, indicating that the appearance of CA, C 2 F, and CA 2 is not beneficial for CFA formation, simultaneously confirming that CF and CF 1.98 are thermodynamic precursors of CFA formation.The above reactions revealed that the formation of CFA is not the same as for the CF, C 2 F, CF 1.98 , CA, and CA 2 generated, respectively, by pairwise direct reactions among Fe 2 O 3 , CaO, and Al 2 O 3 ; it needs ion diffusion, especially in the case of Al 3+ , to the surface of precursors like CF and CF 1.98 , in which the best media for diffusion should be a solid solution like CF 1.98 or CFA with an ion vacancy; secondly CF, C 2 F, and Fe 2 O 3 have the ability to be replaced by Al 3+ rather than CA or CA 2 .

Conclusions
In this work, the formation of CFA in an Therefore, Al 3+ diffusion is promoted in the Fe 2 O 3 -CaO-Al 2 O 3 system, which contributes to an increase in the rate of CFA formation for promoting the low-temperature bonding phase in the sintering process of iron ore fines.

Figure 1 .
Figure 1.XRD patterns of synthesized CF and Hss.

Figure 1 .
Figure 1.XRD patterns of synthesized CF and H ss .

Figure 3 .
Figure 3. XRD patterns of samples for reactions of CF, respectively, with mixtures of Fe 2 O 3 -Al 2 O 3 (a) and H ss (b) for different durations at 1200 • C.

Figure 3 .
Figure 3. XRD patterns of samples for reactions of CF, respectively, with mixtures of Fe2O3-Al2O3 (a) and Hss (b) for different durations at 1200 °C.

Figure 4 .
Figure 4. SEM and EDS images of samples for the reactions of CF, respectively, with mixtures of Fe2O3-Al2O3 (a) and Hss (b) for different durations at 1200 °C.

Figure 4 .
Figure 4. SEM and EDS images of samples for the reactions of CF, respectively, with mixtures of Fe 2 O 3 -Al 2 O 3 (a) and H ss (b) for different durations at 1200 • C.

5 )
, similarly, the phases CA 2 , CFA, Ca 2 Fe 15.50 O 25 , and Fe 2 O 3 can be confirmed through the EDS results and XRD patterns.It can be seen that there was a clear layering structure in order of CA 2 , CFA, Ca 2 Fe 15.50 O 25 , and Fe 2 O 3 , where Al 2 O 3 disappeared.To further confirm this result, it was characterized by EPMA, and the corresponding image and Al, Fe, and Ca mappings are shown in Figure 5.The Al, Fe, and Ca elements were distributed around the CA 2 , corresponding to CFA, Ca 2 Fe 15.50 O 25 , and Fe 2 O 3 , respectively, which is consistent with the result in Figure 4b.This can be seen in the result of Reactions (1), (2), and (4), in which Al 2 O 3 reacted with CaO after Reaction (2) to generate calcium aluminum oxide (CA or CA 2 ) as follows: CaO + Al 2 O 3 = CaAl 2 O 4 ∆G = −47172.5J•mol −1 (4) CaO + 2Al 2 O 3 = CaAl 4 O 7 ∆G = −63698.9J•mol −1 (Minerals 2024, 14, x FOR PEER REVIEW 8 of 14

Figure 5 .
Figure 5. EPMA image and Al, Fe, and Ca mappings of CF-Fe2O3-Al2O3 sample section sintered at 1200 °C for 120 min.

Figure 5 .
Figure 5. EPMA image and Al, Fe, and Ca mappings of CF-Fe 2 O 3 -Al 2 O 3 sample section sintered at 1200 • C for 120 min.

Figure 6 . 14 Figure 7 .
Figure 6.XRD patterns of CF-Fe 2 O 3 (F) samples sintered at 1200 • C for different durations by changing the raw material composition: (a) 5 h; (b) 10 h.For further reactions of CF 1.98 , 5% Al 2 O 3 and 10% Al 2 O 3 were added, respectively, to CF 1.98 , homogenized, pressed, and sintered at 1200 • C for 10 h.The XRD patterns of CF 1.98 -Al 2 O 3 samples are shown in Figure 7.It can be seen that the products of the reaction after adding of 5% Al 2 O 3 and 10% Al 2 O 3 into CF 1.98 were all CFA, indicating that the CFA was a solid solution with the same crystal structure although with differences in Al 3+ content.The reaction is as follows: CF 1.98 + Al 2 O 3 → CFA (6) CF 1.98 was also confirmed as a precursor of CFA formation.

( 1 )
Fe 2 O 3 -CaO-Al 2 O 3 system was investigated below 1200 • C by reactions among Fe 2 O 3 , CaO, and Al 2 O 3 ; CF with Fe 2 O 3 and Al 2 O 3 ; CF with Al-containing hematite; and CF with Al 2 O 3 for promoting the low-temperature bonding phase in the sintering process of iron ore fines.The conclusions are summarized as follows: It was observed in the Fe 2 O 3 -CaO-Al 2 O 3 sample sintered below 1200 • C that CF appeared at 1000 • C, while Ca 2 Fe 15.5 O 25 and CFA appeared at 1100 • C. (2) It was found that CF and Ca 2 Fe 15.5 O 25 are the precursors for CFA formation, and the chemical composition of Ca 2 Fe 15.5 O 25 phase was determined to be CaFe 3.96 O 6.94 (CF 1.98 ), similar to CaFe 4 O 7 .(3) It was revealed that the appearance of CA or CA 2 is a main reason to decrease the rate of CFA formation in Fe 2 O 3 -CaO-Al 2 O 3 samples.

Table 1 .
Chemical compositions (wt%) and sintering conditions for the synthesis of CF and H ss .

Table 2 .
To simulate the environment of the actual sintering process, the mole ratio of Fe 2 O 3 to CaO was set at around 3:1.The reactants including Fe 2 O 3 , CaCO 3 , and Al 2 O 3 were first weighed, homogenized in a mortar grinder (Fritsch Pulverisette 2, Idar-Oberstein, Germany) for 30 min, and pressed into a tablet of 5 mm in diameter and approximately 5 mm in height at 5 MPa, and sintered at 700 • C, 800 • C, 900 • C, 1000 • C, 1100 • C, and 1200

Table 2 .
Mass fraction (%) of reactants in various reactions for the formation of CFA.

Table 2 .
Mass fraction (%) of reactants in various reactions for the formation of CFA.

Table 3 .
The calculating parameters of formation energy using DFT.

Table 4 .
EDS results of samples for the reactions of CF, respectively, with the mixtures of Fe 2 O 3 -Al 2 O 3 and H ss for different durations at 1200 • C.

Table 4 .
EDS results of samples for the reactions of CF, respectively, with the mixtures of Fe2O3-Al2O3 and Hss for different durations at 1200 °C.

Table 5 .
Formation energy of products in the reactions for formation of CFA (eV).According to the above results, the mechanism of CFA formation in the Fe 2 O 3 -CaO-Al 2 O 3 sample can be considered as follows: First, Fe 2 O 3 reacts with CaO to generate CF or C 2 F; simultaneously, Al 2 O 3 reacts with CaO to generate CA or CA 2 , wherein the product is determined by the activity of the reactants.In addition, the contact between Fe 2 O 3 and Al 2 O 3 may form a solid solution of H ss .Further, CF and C 2 F react, respectively, with Fe 2 O 3 to generate CF 1.98 and CF by Reaction (1) and Reaction (7), as follows: Ca 2 Fe 2 O 5 + Fe 2 O 3 = 2CaFe 2 O 4 (7) CA reacts with Al 2 O 3 to generate CA 2 by Reaction (8), as follows: CaAl 2 O 4 + Al 2 O 3 = CaAl 4 O 7 (8) while CA 2 reacts with CaO to generate CA by Reaction (9), as follows: And then, H ss reacts with CF by Reaction (3) to generate CFA; simultaneously, Al 2 O 3 reacts with CF 1.98 by Reaction (6) to also generate CFA.