Effect of Alumina on Crystallization Behavior of Calcium Ferrite in Fe₂O₃-CaO-SiO₂-Al₂O₃ System

Abstract

Al₂O₃ is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al₂O₃ effect on the crystallization of calcium ferrite is rarely reported. In this work, a crystallization device was designed to investigate the crystallization behavior of calcium ferrite in Fe₂O₃-CaO-SiO₂-Al₂O₃ melt under non-isothermal conditions. XRD, SEM-EDS, and optical microscopy were used to identify the crystalline phase and the microstructure of samples. The result shows that the crystal morphology of SFCA changed in the order of strip, column, and needle as the Al₂O₃ content increased. The crystallization sequence of samples containing Al₂O₃ was observed as Ca₄Fe₁₄O₂₅ (C₄F₁₄) → Fe₂O₃ → Ca_3.18Fe_15.48Al_1.34O₃₆ (SFCA-I) → CaFe₂O₄ (CF) → Ca₅Si₂(Fe, Al)₁₈O₃₆ (SFCA) → γ-Ca₂SiO₄ (C₂S). The generation pathway of SFCA-I was found to be C₄F₁₄ + Si⁴⁺ + Al³⁺ → SFCA-I. Increasing the cooling rate can promote the formation of C₄F₁₄, SFCA-I, Fe₂O₃ and the amorphous phase. However, it prevented the crystallization of CF and SFCA while inhibiting the transformation of β-C₂S to γ-C₂S. When the Al₂O₃ content reached or exceeded 2.5 mass pct, the viscosity of Fe₂O₃-CaO-SiO₂-Al₂O₃ melt increased sharply, resulting in the decrease in the crystal size of calcium ferrite.

1. Introduction

High-basicity sinter is mainly utilized as a critical iron-containing material for blast furnace ironmaking, where calcium ferrite is the predominant binding phase [1,2]. The mineral composition and microstructure of the binding phase has an important influence on the quality of the sinter [3,4,5]. Most of the binding phase is mainly complex calcium ferrite.

Recently, with the increasing consumption of high-alumina iron ores, the investigations focused on the role of Al₂O₃ in the formation and crystallization of the binding phase have increased substantially [6,7,8]. Researchers [9,10,11] have found that adding a moderate quantity of Al₂O₃ can promote the formation of complex calcium ferrite.

In sinter, some studies [12,13,14,15] revealed two primary crystal forms of complex calcium ferrite as SFCA (Ca₅Si₂(Fe, Al)₁₈O₃₆) and SFCA-I (Ca_3.18Fe_15.48Al_1.34O₃₆). Compared to SFCA (column and lath), needle-shaped SFCA-I is more favorable for releasing internal stress to improve the strength of the sinter [16]. Furthermore, the microstructure and morphology of complex calcium ferrite also have an important influence on the strength of sinter [17]. Webster et al. [18] investigated the effect of Al₂O₃ on the formation process and thermodynamic stability of complex calcium ferrite. Liles et al. [19] investigated SFCA using the structural refinement approach, finding that Fe³⁺, Si⁴⁺, and Al³⁺ tended to occupy the tetrahedral positions of SFCA, while Fe³⁺, Ca²⁺ in the octahedral locations. The ion replacement is 2(Fe³⁺, Al³⁺) = Ca²⁺ + Si⁴⁺ on electric neutrality. In addition, lowering the temperature is aided in replacing Al³⁺ ↔ Fe³⁺.

Ding et al. [20] studied the crystallization kinetics of the CaO-Fe₂O₃ binary system by the DSC method using Avrami and Mo models. In addition, the crystalline surface activation energies of Ca₂Fe₂O₅ (C₂F) and CaFe₂O₄ (CF) were calculated using the Kissinger model. It was found that the crystallization rate of C₂F is faster than that of CF, while when the cooling rate increases, the crystallization of CF is accelerated, where inversely the crystallization of C₂F is inhibited. Yang et al. [21] investigated the role of Al₂O₃ in the crystallization behavior of the Fe₂O₃-CaO-Al₂O₃ melt during the cooling process. It was found that adding a tiny quantity of Al₂O₃ can improve the preferred orientation of solid solution in CaFe₂O₄, at the same time significantly affecting the fracture toughness of the sample. Park et al. [22] revealed the influence of Al₂O₃ on the reduction performance of the Fe₂O₃-CaO-Al₂O₃-SiO₂ pseudo-quaternary system through in situ observation by high temperature confocal laser scanning micro-scope, finding that the increase of Al₂O₃ led to the crystallization of Fe₂O₃. With the increase of cooling rate, the crystallization of SFCA was promoted, and the reducibility was improved.

In this study, the crystallization mechanism of Al₂O₃ on the Fe₂O₃-CaO-SiO₂-Al₂O₃ system was examined in the non-equilibrium state. A crystallization device was introduced where the cooling rate can be precisely measured, and this approach may assure that the crystallization morphology and composition of the sample are more realistic. The finding of this study is benefit to understand the phase transition of the Fe₂O₃-CaO-SiO₂-Al₂O₃ system during the crystallization process. It lays a foundation for promoting the crystallization of SFCA to improve the quality of sinter.

2. Experimental Procedures

2.1. Preparation of Samples

Analytical grade reagents of Fe₂O₃ (≥99.9 pct, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), CaCO₃ (≥99.5 pct, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), SiO₂ (≥99.9 pct, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and Al₂O₃ (≥99.9 pct, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used to prepare the samples. The chemical composition of the initial samples is shown in

Table 1. Chemical Compositions of samples (mass pct).

Samples	Fe2O3	CaO	SiO2	Al2O3	LT (°C)
No. 1	75.64	20.36	4	0	1261
No. 2	75.45	20.31	3.99	0.25	1259
No. 3	75.26	20.26	3.98	0.50	1257
No.4	75.07	20.21	3.97	0.75	1256
No.5	74.88	20.16	3.96	1.00	1254
No.6	74.51	20.05	3.94	1.50	1250
No.7	74.13	19.95	3.92	2.00	1256
No.8	73.75	19.85	3.90	2.50	1276
No.9	73.37	19.75	3.88	3.00	1293

. A study revealed that [23], when the molar ratio of Fe₂O₃ to CaO in the binding phase is in the range of 1.25 to 1.59 in the actual sintering process, more liquid phase and the acicular-shape, column-shaped or columnar calcium ferrite would be generated. Therefore, the Fe₂O₃ to CaO molar ratio in this study was set at 1.3. In addition, 4.0 mass pct SiO₂ was chosen as the gangue in the iron ores, with Al₂O₃ content range from 0 to 3.0 mass pct. To ensure that the entire crystallization process was carried out in the pure liquid phase, the liquidus temperature (LT) of each sample was calculated using FactSage 8.2 software [24].

2.2. Sinter Process

At room temperature, Fe₂O₃, CaO, SiO₂, and Al₂O₃ were mixed evenly, as stated in Table 1. For improving precision, CaCO₃ was used to replace CaO with an equal-molar quantity for precise weighing. 20.0 g of sample and an appropriate amount of anhydrous ethanol (≥99.7 pct, Sinopharm Chemical Reagent Co., Ltd.) were mixed evenly, then roasted at 200 °C for 3 h in a drying oven under an air atmosphere. The sample was compressed into a cylindrical shape (Ø 20 × 20 mm) and sintered in a platinum crucible. From the previous research [25,26,27] it was found that if the sample was held above the TL for 2 h, a molten equilibrium liquid phase would be formed.

In this experiment, in order to obtain a complete equilibrium liquid phase, the sample was heated to 1350 °C at a heating rate of 5 °C/min and held for 4 h in air atmosphere. Subsequently, the samples were treated under the condition of various cooling rates (0.02 °C/s, 5 °C/s, 15 °C/s, and 65 °C/s.) [28] as presented in Figure 1.

For obtaining the order of different crystallization phases, once the samples were cooled to the target temperature at a cooling rate of 0.02 °C/s, water cooling was conducted to obtain an instantaneous mineral composition at the corresponding temperature.

2.3. Phase Determination

A part in each sample was ground to a particle size of less than 50 μm passing through the sieve completely for XRD determination. The mineral phase of the crystalline powder samples was identified using a Rigaku SmartLab X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). Cu Kα was used as the radiation source (40 kV, 150 mA) with a graphite curved monochromator in the diffracted beam path. The wavelength is 0.15406 nm, with a scanning speed of 10°/min, a scanning step length of 0.02°, and a scanning range (2θ) from 10° to 100°. XRD data were matched using Crystallographica Search-Match software (CSM3.0, Oxford Cryosystems Ltd., UK, Oxford).

The other part of the samples was embedded into the ethylenediamine-doping epoxy resin and polished for the microstructure observation. The mineral morphology and structure were observed by optical microscope (Optical Instrument Fifth Factory Co., Ltd., Shanghai, China) and scanning electron microscope (Zeiss GeminiSEM500, Berlin, Germany). The device is equipped with EDS (Ultim Max 170, Berlin, Germany) to detect elemental composition.

3. Results and Discussion

3.1. Effect of Al₂O₃

The composition change of the sample in equilibrium cooling process was obtained by heating to 1350 °C for 4 h and then cooling to room temperature at a rate of 0.02 °C/s. Figure 2, Figure 3 and Figure 4 and

Table 2. EDS results of the crystallized phases in the samples under 0.02 °C/s.

Sample No.	Marked Points	Crystal Phase	Elements (at Pct)
			Fe	Ca	Si	Al	O
0mass%	P1	C4F14	46.20	13.54	1.79	0.74	37.72
	P2	CF	39.38	21.93	1.38	0.94	36.37
	P3	γ-C2S	0.61	40.07	20.33	0.24	38.75
	P4	Fe2O3	62.63	0.06	1.45	0.95	34.91
1.0mass%	P2	CF	41.42	23.82	0.00	0.10	34.67
	P3	γ-C2S	0.48	41.01	19.91	0.31	38.29
	P4	Fe2O3	67.98	0.14	0.42	0.91	30.54
	P5	SFCA-I	47.64	14.79	2.42	2.15	33.01
1.5mass%	P2	CF	38.51	22.76	0.31	0.29	38.12
	P3	γ-C2S	0.86	37.95	17.98	0.26	42.96
	P4	Fe2O3	64.33	0.27	0.00	0.72	34.67
	P5	SFCA-I	43.01	14.34	3.83	3.65	35.16
2.0mass%	P2	CF	41.02	24.77	1.40	1.32	31.49
	P3	γ-C2S	0.64	42.53	21.15	0.35	35.33
	P4	Fe2O3	68.94	0.00	0.04	0.92	30.10
	P5	SFCA-I	47.06	14.56	3.20	3.20	31.98
2.5mass%	P2	CF	39.31	24.09	0.01	0.28	36.31
	P3	γ-C2S	0.35	39.08	19.78	0.16	40.63
	P4	Fe2O3	62.79	0.28	0.00	0.84	36.09
	P6	SFCA	41.35	13.26	3.83	5.00	36.56
3.0mass%	P2	CF	41.32	26.74	0.00	0.40	31.53
	P3	γ-C2S	0.94	45.13	17.11	0.00	36.82
	P6	SFCA	42.86	13.37	4.17	7.19	32.41

depicts the XRD patterns, optical micrograph, SEM, and EDS results of the crystalline samples containing varying amounts of Al₂O₃, respectively.

It is indicated that Ca₄Fe₁₄O₂₅(C₄F₁₄), CF, γ-Ca₂SiO₄(γ-C₂S), SFCA-I, SFCA and Fe₂O₃ were crystallized. The increase of Al₂O₃ content led to the gradual decreases of C₄F₁₄, Fe₂O₃, and γ-C₂S. SFCA-I first increased and then decreased. CF and SFCA increased gradually. The detailed result is as follows:

(1)

When Al₂O₃ was not added, Ca²⁺ reacted with Fe³⁺ and O²⁻ to form C₄F₁₄ and CF, while Ca²⁺ reacted with Si⁴⁺ and O²⁻ to form γ-C₂S;

(2)

When Al₂O₃ reached 0.5 mass pct, C₄F₁₄ disappeared, and CF had gradually increased, indicating that the preferentially crystallized C₄F₁₄ reacted with Al³⁺ and Si⁴⁺ to form SFCA-I;

(3)

When Al₂O₃ reached 2.0 mass pct, CF and Fe₂O₃ had gradually decreased, γ-C₂S had not changed significantly, and SFCA-I increased gradually. It shows that CF also participated in the generation of SFCA-I.

(4)

When Al₂O₃ reached 2.5 mass pct, the iron-rich SFCA-I was transformed into SFCA (high Si, high Al). Simultaneously, it promoted the precipitation of Fe₂O₃. Fe₂O₃ and CF increased, and Si⁴⁺ was mainly involved in generating SFCA, resulting in the decrease of γ-C₂S.

(5)

When Al₂O₃ reached 3.0 mass pct, CF and SFCA continued to increase, while γ-C₂S decreased and Fe₂O₃ disappeared.

3.2. The Sequence of Crystallization Phase

Due to the strong crystallization ability of calcium ferrite, the crystallization order in the liquid phase cooling process has yet to be understood. For obtaining the sequence of various phases crystallized in the Fe₂O₃-CaO-SiO₂-Al₂O₃ system, samples of No.1, No.6, and No.9 (0, 1.5, and 3.0 Al₂O₃ mass pct) were selected to further research as cooled to the different target temperature at a cooling rate of 0.02 °C/s, and followed by water cooling. Figure 5 depicts the XRD patterns of the collected samples. It shows that adding Al₂O₃ inhibited the formation of C₄F₁₄ and SFC, while it promoted the formation of Fe₂O₃, CF, and γ-C₂S simultaneous to the transformation of SFCA-I into SFCA. The detailed result is as follows:

(1)

When Al₂O₃ was not added, the crystalline phase of quenched samples was Fe₂O₃ and C₄F₁₄ at 1350 °C, while Fe₂O₃ and C₄F₁₄ increased at 1300 °C, simultaneously CF appeared; Fe₂O₃ and C₄F₁₄ increased at 1280 °C, while Fe₂O₃ decreased. CF increased at 1250 °C, simultaneously SFC and γ-C₂S appeared while Fe₂O₃ disappeared; CF increased at 1200 °C, and CF, SFC, and γ-C₂S increased at 1150 °C. So it can be considered the crystallization sequence was (Fe₂O₃, C₄F₁₄) → CF → (SFC, γ-C₂S).

(2)

When Al₂O₃ reached 1.5 mass pct, the crystalline phase of quenched samples was Fe₂O₃ and C₄F₁₄ at 1350 °C; SFCA-I appeared, simultaneously Fe₂O₃ increased at 1300 °C, but C₄F₁₄ disappeared; Fe₂O₃ and SFCA-I increased at 1280 °C; SFCA and CF appeared at 1250 °C, simultaneously Fe₂O₃ and SFCA-I decreased. At this time, SFCA was formed by the preferentially precipitated SFCA-I and Al³⁺ and Si⁴⁺ in the melt; CF and SFCA increased at 1200 °C, simultaneously γ-C₂S appeared; CF, SFCA, and γ-C₂S increased at 1150 °C. So it can be considered the crystallization sequence was (Fe₂O₃, C₄F₁₄) → SFCA-I → CF → SFCA → γ-C₂S.

(3)

When Al₂O₃ reached 3.0 mass pct, the crystalline phase of quenched samples was Fe₂O₃ and SFCA-I at 1350 °C; SFCA-I increased while Fe₂O₃ decreased, simultaneously CF appeared at 1300 °C; Fe₂O₃ and SFCA-I decreased at 1280 °C while CF increased, simultaneously SFCA appeared; CF and SFCA increased at 1250 °C while SFCA-I decreased, simultaneously Fe₂O₃ disappeared; SFCA-I decreased at 1200 °C while CF and SFCA increased, simultaneously γ-C₂S appeared; SFCA-I decreased at 1150 °C while CF, SFCA, and γ-C₂S increased. So it can be considered the crystallization sequence was (Fe₂O₃, SFCA-I) → CF → SFCA → γ-C₂S.

Thermodynamically, the Gibbs free energy of formation of C₂S is lower than that of CF [27], and the reactions are as Equations (1) and (2), respectively. It shows that C₂S is more stable to form easier than CF. However, since the added SiO₂ content of 4mass pct is much smaller than Fe₂O₃, resulting in the probability of Si⁴⁺ reacting with Ca²⁺ is relatively small, simultaneously Si⁴⁺ also participates in the formation of SFC, SFCA-I, and SFCA. Therefore, the crystallization sequence of C₂S was late. Since the Gibbs free energy of C₄F₁₄, SFC, SFCA-I, and SFCA formations are not existing in the thermodynamic database, unfortunately it cannot be compared with other crystalline phases.

$$2\mathrm{CaO}+{\mathrm{SiO}}_2=2\mathrm{CaO}·{\mathrm{SiO}}_2 {\Delta }_{\mathrm{f}}{\mathrm{G}}_{\mathrm{m}}^{\mathsf{\theta }}=-118899-11.3\mathrm{T} (\mathrm{J}/\mathrm{mol})$$

(1)

$$\mathrm{CaO}+{\mathrm{Fe}}_2{\mathrm{O}}_3=\mathrm{CaO}·{\mathrm{Fe}}_2{\mathrm{O}}_3 {\Delta }_{\mathrm{f}}{\mathrm{G}}_{\mathrm{m}}^{\mathsf{\theta }}=-29700-4.81\mathrm{T} (\mathrm{J}/\mathrm{mol})$$

(2)

From the crystallization order of different samples containing Al₂O₃, a new generation path of SFCA-I was found in the crystallization process of the Fe₂O₃-CaO-SiO₂-Al₂O₃ quaternary system. The C₄F₁₄ reacts with Si⁴⁺ and Al³⁺ in the melt to form SFCA-I (C₄F₁₄ + Si⁴⁺ + Al³⁺ → SFCA-I), and SFCA-I reacts with Si⁴⁺ and Al³⁺ to form SFCA (SFCA-I + Si⁴⁺ + Al³⁺ → SFCA).

Figure 6 shows the corresponding cross-sectional optical micrograph, where the experimental results are consistent with the XRD results. Seven phases of Fe₂O₃, C₄F₁₄, CF, SFC, γ-C₂S, SFCA-I, and SFCA were co-precipitated in the melt. When the quenched temperature was lowered, equivalent to prolonging the crystallization time, the Fe₂O₃ grew up in a lump, and calcium ferrite (CF, C₄F₁₄, SFC) developed from strip to short column. Simultaneously needle-shaped SFCA-I transformed into column-shaped SFCA, and γ-C₂S developed from block to strip. γ-C₂S was generated at 1200 °C while degraded when lowered to 1150 °C.

Figure 7 and

Table 3. EDS results of the crystallized phases in the samples with different mass pct of Al₂O₃.

Sample No.	Marked Points	Phase	Elements
			Fe	Ca	Si	Al	O
0mass%	P1	Fe2O3	68.63	0.49	1.36	0	28.94
	P2	C4F14	51.13	14.23	1.16	0	33.48
	P3	Slag phase	35.81	22.88	5.90	0	35.41
1.5mass%	P1	C4F14	47.25	12.37	0.00	1.08	39.30
	P2	SFCA-I	44.04	12.68	1.08	2.20	39.99
	P3	Slag phase	44.52	15.88	2.71	2.55	34.34
3.0mass%	P1	Fe2O3	54.96	0.62	0.00	1.10	43.32
	P2	SFCA-I	39.52	11.80	2.31	4.84	41.53
	P3	Slag phase	36.85	15.91	3.18	4.12	39.95

show the SEM photos and EDS results of the samples, confirming the XRD results. With the increase of the mass percentage of Al₂O₃ in the melt, the crystalline phase would change, and Al₂O₃ promoted the transition from SFCA-I to SFCA while inhibiting the formation of C₄F₁₄.

3.3. Effect of Cooling Rate on Crystallization

Figure 8, Figure 9 and Figure 10 present the variations of XRD profiles of crystallized samples with different Al₂O₃ content under the cooling rates of 5 °C/s, 15 °C/s, and 65 °C/s, respectively. The corresponding optical micrographs of crystallized phases are presented in Figure 11.

Table 4. The crystalline phase of the corresponding samples in Figure 8, Figure 9 and Figure 10.

Cooling Rate (°C/s)	Al2O3 (Mass Pct)
	0	0.5	1	1.5	2	2.5	3
5	CF	CF	CF	CF	CF	CF	CF
	C4F14	SFCA-I	SFCA-I	SFCA-I	SFCA	SFCA	SFCA
	Fe2O3	Fe2O3
	β-C2S
15	CF	CF	CF	CF	CF	CF	CF
	C4F14	SFCA-I	SFCA-I	SFCA-I	SFCA-I	SFCA-I	SFCA-I
	Fe2O3	Fe2O3	Fe2O3	Fe2O3	Fe2O3	Fe2O3	Fe2O3
	β-C2S	β-C2S	β-C2S
65	CF	CF	CF	CF	CF
	C4F14	C4F14	C4F14	C4F14	SFCA-I	SFCA-I	SFCA-I
	Fe2O3			Fe2O3	Fe2O3	Fe2O3	Fe2O3
	β-C2S	β-C2S	β-C2S	β-C2S	β-C2S	β-C2S	β-C2S

shows six phases as CF, C₄F₁₄, SFCA-I, SFCA, Fe₂O₃, and β-C₂S.

It can be seen that CF, C₄F₁₄, and Fe₂O₃ were generated without Al₂O₃. With the increase of the Al₂O₃ content, the crystallographic phase transformed significantly as follows. C₄F₁₄ had gradually decreased as C₄F₁₄ reacted with Al³⁺ and Si⁴⁺ to form columnar SFCA-I. With further increasing the content of Al₂O₃, SFCA-I transformed to SFCA. Compared with SFCA, SFCA-I had a higher ratio of Fe₂O₃ to CaO in chemical composition. The crystallization of C₄F₁₄ and Fe₂O₃ was promoted during the transformation. Moreover, with the increase of Al₂O₃ content, the complex calcium ferrite first increased and then decreased. The two-dimensional crystal morphology of the minerals shows that CF was skeletal or corroded. Meanwhile, C₄F₁₄, Fe₂O₃, SFCA-I, and SFCA were existed in the morphology of strip, irregular block, column and needle, and short column, respectively.

It can be found that as increasing the cooling rate it shortens the crystal growth time, so the crystalline of some minerals would be inhibited, while the crystalline phase and morphology were also changed significantly. On the one hand, the crystal size would be narrowed. On the other hand, the formation of complex calcium ferrite and the conversion of SFCA-I to SFCA would be promoted, while the formation of SFC and γ-C₂S would be inhibited. It also promoted the formation of C₄F₁₄, Fe₂O₃, and the amorphous phase that filled around the complex calcium ferrite in an imperfect crystallization state. But when the cooling rate reaches 65 °C/s, it was found that C₄F₁₄ was easier to form than Fe₂O₃, so C₄F₁₄ should be crystallized with Fe₂O₃ first in the crystallization sequence. Therefore, the crystallization order of samples in the Fe₂O₃-CaO-SiO₂-Al₂O₃ melt containing Al₂O₃ should be C₄F₁₄ → Fe₂O₃ → SFCA-I → CF → SFCA → γ-C₂S.

To further confirm the phase composition, the SEM-EDS analysis of the sample with a cooling rate of 5 °C/s is shown in Figure 12 and

Table 5. The EDS results of the corresponding marked points in Figure 12.

Al2O3 (Mass Pct)	Marked Points	Phase	Elements (at Pct)
			Fe	Ca	Si	Al	O
0	P1	C4F14	52.06	12.37	0.79	0	34.78
	P2	CF	40.74	19.59	3.96	0	35.72
	P3	Slag phase	37.10	20.93	6.93	0	35.04
1.0	P2	CF	40.49	21.60	4.95	1.77	31.20
	P4	SFCA-I	48.28	14.24	2.09	0.92	34.48
1.5	P2	CF	42.16	19.75	4.89	2.06	31.14
	P4	SFCA-I	49.19	14.77	2.94	2.77	30.33
2.0	P2	CF	43.43	26.11	0	0.69	29.77
	P5	SFCA	47.35	14.22	3.40	4.42	30.62
2.5	P2	CF	39.39	21.58	4.51	3.94	30.58
	P5	SFCA	43.77	12.73	4.28	6.27	32.95
3.0	P2	CF	39.39	21.58	4.51	3.94	30.58
	P5	SFCA	40.10	13.17	4.26	6.25	36.22

. When the Al₂O₃ is not added, only C₄F₁₄ and CF phases were formed. With Al₂O₃ content increasing, C₄F₁₄, CF, SFCA-I, and SFCA appeared, which confirmed the experimental results in Figure 8.

To investigate the influence of cooling rate on the morphology of calcium ferrite (C₄F₁₄, SFCA-I, and SFCA) in the Fe₂O₃-CaO-SiO₂-Al₂O₃ melt under different Al₂O₃ content, the grain size of calcium ferrite in each sample in Figure 11 was measured using the Nano Measurer 1.2 software [29]. Thirty positions in each sample were selected, measured, and an averaged value was calculated. Figure 13 illustrates that with the increased cooling rate, the crystal size of calcium ferrite decreased significantly. Furthermore, when the Al₂O₃ content increased, the crystal size of calcium ferrite increased and subsequently decreased, which demonstrates that adding a small amount of Al₂O₃ promoted the formation of complex calcium ferrite. At different cooling speeds (5 °C/s, 15 °C/s, and 65 °C/s), the grain size achieved the maximum value (corresponding to 22.15 μm, 13.85 μm, and 9.25 μm, respectively) when the Al₂O₃ reached 2.0 mass pct. After Al₂O₃ reached 2.5 mass pct, it would increase a viscosity of the melt, which could be a primary reason for the decrease in the crystal size of calcium ferrite.

3.4. Discussion on Crystallization Mechanism

To further explain the effect of Al₂O₃ on the crystallization of calcium ferrite, FactSage 8.2 software was used to perform thermodynamic equilibrium calculations on the Fe₂O₃-CaO-SiO₂-Al₂O₃ system, even though C₄F₁₄, SFCA-I, and SFCA are lacking in the thermodynamic database. Future metallurgical workers are required to improve it.

Figure 14 and

Table 6. Changes of primary crystallization phase and temperature after adding Al₂O₃.

Al2O3 Content (Mass Pct)	Primary Phase	Primary Crystallization Temperature (°C)
0	M2O3	1261
0.5	M2O3	1258
1.0	M2O3	1253
1.5	M2O3	1250
2.0	α’-C2S	1251
2.5	α’-C2S	1252
3.0	α’-C2S	1253

show the primary crystallization temperature and crystallization amount of the thermodynamic equilibrium phase with different Al₂O₃ content. The result shows that without adding Al₂O₃, the phases are M₂O₃ (≥99.50 mass pct Fe₂O₃ and ≤0.50 mass pct Al₂O₃), α’-Ca₂SiO₄(α’-C₂S), and CaFe₄O₇(CF₂). With the Al₂O₃ content increasing, CF₂ disappeared while CF appeared. When Al₂O₃ content reached 2.0 mass pct, the primary crystallization phase transformed from M₂O₃ to α’-C₂S, and the transition temperature was 1250 °C. When Al₂O₃ content reached 3.0 mass pct, CF and Ca(Al, Fe)₆O₁₀ appeared.

The corresponding crystallization amounts of M₂O₃, α’-C₂S, and CF decreased from 39.01 to 24.19 mass pct, 11.41 to 11.12 mass pct, and 49.00 to 35.76 mass pct, respectively. Ca (Al, Fe)₆O₁₀ increased from 0.579 to 28.924 mass pct.

As shown in Figure 11, many spherical holes appeared in samples when the Al₂O₃ content reached 2.5 and 3.0 mass pct, which increased with the increase of the Al₂O₃ content and the cooling rate. Simultaneously, the crystalline size of minerals decreased. It can be considered that the increase of the melt viscosity resulted in a slow crystalline rate due to the hard mass transferring.

Figure 15 shows the viscosity diagrams of Fe₂O₃-CaO-SiO₂-Al₂O₃ melts with Al₂O₃ content at different temperatures, which is calculated using thermodynamical software, simultaneously combined by the Einstein-Roscoe formula (Equation (3)) [30], where the mass fraction of the solid phase was obtained as shown in Figure 14. The result shows that at all temperatures, the viscosity value increased with the increase of Al₂O₃ content. The viscosity value increased obviously. The viscosity increase would hinder the crystallization and mass transfer of complex calcium ferrite in the melt, resulting in poor crystalline morphology. Specially, after Al₂O₃ reached 2.5 mass pct, the viscosity of the melt increased sharply, which could be the main reason for the decrease in the crystal size of calcium ferrite.

$$\eta = {\eta }^0 {(1-\mathrm{c})}^{-2.5}$$

(3)

• η—solid-liquid mixing viscosity;
• η⁰—viscosity of pure liquid phase;
• c—a mass fraction of solid phase.

The isothermal cross-sections of Fe₂O₃-CaO-SiO₂-Al₂O₃ systems with varying Al₂O₃ content at different temperatures are shown in Figure 16. With the Al₂O₃ content increasing, the liquid phase is divided into three regions, named L_α, L_α + L_β, and L_β, where the content of Al₂O₃ increased from 1.0 to 3.0 mass pct. When the red component point is at 1250 °C, the primary crystal region is transformed from L_α + M₂O₃ to L_α + L_β + α’-C₂S. There may be two reasons for the deterioration of crystallization. On the one hand, the viscosity of L_α + L_β + α’-C₂S is higher than that of L_α+M₂O₃. In addition, the crystallization of α’-C₂S leads to the reducing of initial Ca²⁺ and Si⁴⁺ in the melt, which is not conducive to the crystallization of complex calcium ferrite. It can be seen from Figure 16 that when Al₂O₃ is 2.0 mass pct, not only a certain amount of liquid phase is retained, but also Ca²⁺ and Si⁴⁺ are not reduced too much. This also explains that when Al₂O₃ was 2.0 mass pct, the crystal size of calcium ferrite was the largest, as shown in Figure 13.

4. Conclusions

In this work the influences of Al₂O₃ content, cooling rate and the crystallization sequence of the Fe₂O₃-CaO-SiO₂-Al₂O₃ system during the cooling process were investigated. On this basis, the influence mechanism of Al₂O₃ content and cooling rate on the crystallization of complex calcium ferrite (C₄F₁₄, SFCA-I, SFCA) was also proposed. The main conclusions are as follows:

(1)

Al₂O₃ has an important effect on the composition of the crystal phase of the Fe₂O₃-CaO-SiO₂-Al₂O₃ system. Adding alumina promoted the crystallization of Fe₂O₃, γ-C₂S, SFCA-I, and SFCA, while it inhibited the crystallization of C₄F₁₄ and SFC. However, the content of CF first decreased and then increased. This is mainly because of the formation of complex calcium ferrite and the transformation of SFCA-I to SFCA.

(2)

The crystallization sequence in Fe₂O₃-CaO-SiO₂-Al₂O₃ melt under different Al₂O₃ content was investigated, where the corresponding crystalline order is (Fe₂O₃, C₄F₁₄) → CF → (SFC, γ-C₂S), (Fe₂O₃, C₄F₁₄) → SFCA-I → CF → SFCA → γ-C₂S, and (Fe₂O₃, SFCA-I) → CF → SFCA → γ-C₂S under the Al₂O₃ content of 0 mass pct, 1.5 mass pct, and 3.0 mass pct respectively. It can be concluded that the C₄F₁₄ reacts with Si⁴⁺ and Al³⁺ in the melt to form SFCA-I (C₄F₁₄ + Si⁴⁺ + Al³⁺ → SFCA-I), and then SFCA-I reacts with Si⁴⁺ and Al³⁺ to form SFCA (SFCA-I + Si⁴⁺ + Al³⁺ → SFCA).

(3)

As the cooling rate increase, C₄F₁₄, SFCA-I, Fe₂O₃, β-C₂S, and the amorphous phases are increased while CF and SFCA are reduced, and the crystal transformation from β-C₂S to γ-C₂S can be effectively inhibited. However, when the cooling rate was increased from 15 °C/s to 65 °C/s, C₄F₁₄ was found to crystallize before Fe₂O₃.

(4)

The crystal size of complex calcium ferrite first increases and then decreases by an increase of Al₂O₃ content, and the order of crystal morphology evolved is from the strip, columnar to needle-shaped. When Al₂O₃ content reached or exceeded 2.5 mass pct, the viscosity of Fe₂O₃-CaO-SiO₂-Al₂O₃ melt increased sharply, resulting in the decrease in the crystalline size of calcium ferrite.