Effects of Adding Iron Ores-based Calcium Ferrites to the Sinter Mix on Sinter Quality and Reduction of CO₂, NO and SO₂

Abstract

Sinter plants account for more than 10, 40 and 70% of the total emission of CO₂, NO and SO₂ in steelworks, respectively. It is necessary to reduce the fuel ratio in the sinter mix to decrease the CO₂ emission from sinter plants, which will harmfully affect on the melt formation and sinter quality consequently. To overcome the loss in the amount of melt, the current study aims to clarify the effect of adding iron ores-based calcium ferrites to the sinter mix on the sinter quality and emission of harmful gases. The addition of calcium ferrites promoted a significant drop in the sintering temperature, while maintaining the porosity level. The presence of calcium ferrites led to the formation of finer pores, modifying the dominant pore size in the sinter from macro to medium/micropores. The amount of SFCA and SFCA-I phases was significantly increased from 9% of the standard sinter to at least 32%. The sinter properties such as reduction degree, RI and RDI were improved in the presence of calcium ferrites by at least 24, 74 and 26%, respectively. Due to the decrease in the sintering temperature, the required fuel ratio is expected to decrease by more than 30%, and consequently the identical reduction ratio was resulted in the emission of CO₂. The presence of calcium ferrites also contributed to the reduction in the emission of NO and SO₂ at least by 38 and 49%, respectively. A low-temperature sintering process could be designed by adding calcium ferrites to the sinter mix.

1. Introduction

The crude steel production is increasing worldwide,¹⁾ which indicates that integrated steel mills should consume more fossil fuels as a reducing agent and heat source to satisfy this demand. Meanwhile, the steelmaking companies are making great efforts to become more environmental-friendly. Based on the current situation, the reduction in the emission of greenhouse gases being mainly represented by carbon dioxide (CO₂) has been attempted by a new climate policy agreement worldwide.²⁾ Although the implementation during the years varies from country to country, the final objectives are to become carbon neutral by 2050.^3,4) In the steelworks, carbon is mainly consumed in the operation of the blast furnaces and sinter plants.⁵⁾ Furthermore, the decrease in the amount of carbon used in those areas would end up with a reduction in the emission of not only CO₂ but also other harmful gases such as nitrogen oxide (NO) and sulfur dioxide (SO₂).^6,7)

Iron ore sintering is an agglomeration process of a mixture of ore fines mainly consisting of iron ores, fluxes and fuel, which produces sinters with suitable chemical and physical properties for the operation of the blast furnace.⁸⁾ Several reactions occur during the sintering process, which led to the formation of several compounds such as calcium ferrite (CaO∙Fe₂O₃) and dicalcium silicate (2CaO∙SiO₂). Calcium ferrite might be considered to be a low melting temperature compound^8,9,10) and desirable bonding phase, which improves the strength of the sinter when formed at higher levels.^11,12)

The sintering process of iron ore accounts for about 10, 40 and 70% of CO₂, NO and SO₂, respectively, of the total emission in the steel industries.^13,14,15,16) CO₂ is mainly formed from the combustion of the fuel used as a heat source¹⁷⁾ to produce sinter. Therefore, the primary attempt to decrease the CO₂ emission from the sinter plant is to reduce the ratio of fuel present in the sinter mix.^5,18) However, the afore-mentioned reduction in the amount of fuel will decrease the energy supplied to the sinter bed, and then affect the sintering temperature profile, which will lead to a decrease in the amount of melt. This melt phase is highly required for bonding of the particles present in the sinter mix, which is directly related to the sinter strength.^11,12) Yabe et al.⁵⁾ reported that it was very difficult to decrease the CO₂ emission by a simple reduction in the fuel amount, while Umadevi et al.¹⁹⁾ showed that the sinter physical properties such as tumbler and reduction disintegration indices were the worst in case the coke rate was at the lowest level. Several researchers^{5,20,21,22,23,24,25)} tried to find a way to overcome the heat loss caused by the decrease in the fuel ratio. All the mentioned studies replaced the combustion heat from the carbon source with the reoxidation heat of metallic iron, FeO, or Fe₃O₄ to Fe₂O₃. For example, Yabe et al.⁵⁾ investigated the effect of replacing raw iron ore with pre-reduced iron ore on the sinter properties. Their results showed that it is possible to decrease coke consumption and maintain quality requirements such as productivity and reducibility. At present, several researchers^{26,27,28,29,30)} confirmed the ability of calcium ferrite to reduce the emission of NO generated during the sintering process. In the case of SO₂ emission, there is a lack of data regarding the reduction of SO₂ by the addition of calcium ferrite to the sinter mix.

The current study aims to overcome the heat loss caused by decreasing the fuel amount through the addition of low-temperature melting compounds, e.g. calcium ferrite. Therefore, it is necessary to clarify the effects of adding iron ores-based calcium ferrites to the standard sinter mix on the sinter quality and emissions of CO₂, NO and SO₂, which might suggest a low-carbon sintering process.

2. Experimental

2.1. Materials Preparation

The current study used two kinds of iron ores that are commonly applied in the sinter plant of steelmaking companies. X-ray diffraction (XRD, Bruker D8-Advance, Bruker, Massachusetts, USA) analysis was conducted to identify the major phases present in each iron ore. As is shown in Fig. 1, Ore A is a representative hematite-based iron ore (Fe₂O₃), while Ore B is a goethite-based ore (FeOOH). The chemical composition of both ores is shown in Table 1. In addition, the chemical composition of a mixed ore composed of 50 mass% Ore A and 50 mass% Ore B was calculated in Table 1. All the samples were analyzed by wet chemical titration³¹⁾ and Inductively Coupled Plasma - Atomic Emission Spectrometry (ICP-AES).³²⁾ As shown in Table 1, the two iron ores mostly differ in the content (mass%) of T.Fe and SiO₂. Thus, Ore A is considered to be a high-grade iron ore compared with Ore B. In addition, reagent-grade CaO (96% purity, SAMCHUN Chemical) was used. All the samples were crushed, and sieved in a particle size below 250 μm.

Fig. 1.

Phase composition of iron ores identified by X-ray diffraction.

Table 1. Chemical composition of the iron ores used (mass%).

Iron ores	T.Fe	FeO	M.Fe	Fe3+	SiO2	Al2O3	CaO	MgO	P2O5	S
Ore A	65.20	0.46	0.07	64.77	1.52	1.25	0.07	0.05	0.15	0.03
Ore B	57.00	0.17	0.07	56.80	5.39	1.59	0.06	0.08	0.08	0.06
Mixed ore	61.10	0.32	0.07	60.79	3.46	1.42	0.07	0.07	0.12	0.05

Table 2 represents the proximate and ultimate analyses of the coals used. As shown in Table 2, the rank of coal selected in the current research was high-grade anthracite, and mainly used as a heat source in the sinter plants of iron ores.

Table 2. Proximate and ultimate analyses of fuels.

Fuels	Proximate analysis (mass%, db)				Ultimate analysis (mass%, db)
	Fixed Carbon	Volatile matter	Ash	Moisture	C	H	N	S
Anthracite	76.49	9.10	13.32	1.09	79.70	1.17	0.72	0.71
Coke	80.76	5.76	12.99	0.49	85.00	0.08	1.22	0.61

2.1.1. Calcium Ferrites (CFs)

The calcium ferrite was prepared based on the CaO–Fe₂O₃ phase diagram presented by Phillips.³³⁾ Philips³³⁾ reported that the lowest melting point of about 1206°C was achieved when the molar ratio of CaO to Fe₂O₃ is unity.(weight ratio = 26 to 74). Based on this, three calcium ferrites (CFs) were prepared using three kinds of iron ores; hematite-based Ore A, goethite-based Ore B and a mix between the two ores as previously presented in Table 1. To achieve the molar ratio of 1 to 1 for forming CaO∙Fe₂O₃ phase, the required amount of reagent-grade CaO (96% purity, SAMCHUN Chemical) was added with respect to 100 grams of each ore as shown in Table 3. The amount of Fe₂O₃ was evaluated by X-ray fluorescence (XRF) spectroscopy to guarantee the complete transformation of the goethite phase present in Ore B and Mix into hematite during the sample preparation.

Table 3. Added amount of reagent-grade CaO with respect to 100 grams of iron ore (grams).

Iron ores	Fe2O3 (g)	CaO (g)
Ore A	94.55	34.60
Ore B	89.80	32.87
Mixed ores	92.18	33.74

After the required amount of reagent-grade CaO was added to each iron ore, the three mixes were heated to prepare the calcium ferrites. As is represented in Fig. 2, the mixes were placed into an elevator furnace at 1400°C for 5 min to guarantee the complete melting of the mixes, which were then quenched in water, and dried in an oven of 100°C for 2 days. Finally, the samples were crushed, and sieved in a particle size below 250 μm for further analyses. As shown in Fig. 3, the XRD analysis confirmed the successful preparation of calcium ferrites whose major phase in the three CFs is CaO∙Fe₂O₃.

Fig. 2.

Preparation procedure of calcium ferrites. (Online version in color.)

Fig. 3.

Phase composition of calcium ferrites identified by X-ray diffraction.

2.1.2. Standard Sinter Mix (SSM)

The standard sinter mix was prepared in this study following the procedure of producing the sinter mix being used in steelmaking companies based on the fraction of iron ores present in the mix. Nowadays, the goethite-based iron ores are increasingly being used because of the depletion of high-grade hematite ores.^34,35) Therefore, in the current study, 20 mass% of Ore A and 80 mass% of Ore B were first mixed, which amounts to 92 mass% of the sinter mix. The remaining was occupied by reagent-grade CaO to keep the basicity [(mass% CaO)/[(mass% SiO₂)] of about 1.9 as shown in Table 4. The standard sinter mix was uniformly mixed for 3 hours employing the Turbula Mixer (T2F Nr. 120,942 Switzerland) at 34 revolutions per minute.

Table 4. Calculated chemical composition of the standard sinter mix (mass%).

T.Fe	FeO	M.Fe	Fe3+	SiO2	Al2O3	CaO	MgO	P2O5	S	Basicity (CaO/SiO2)
53.95	0.21	0.06	53.72	4.25	1.40	8.06	0.07	0.09	0.05	1.90

2.2. Experimental Procedure

The current research investigated several properties of the sinter. Firstly, it was attempted to map the change in the mineralogy of sinter consisting of phases and pores during the sintering process. The sinter properties such as reducibility and reduction disintegration were also evaluated. In addition, the emission of harmful gases such as CO₂, NO and SO₂ were quantified. All the experiments were performed for the sintered samples prepared by SSM and SSM containing iron ores-based CFs. The identical procedure was followed to clarify the effects of adding iron ore-based CFs to the SSM on the afore-mentioned sinter properties. The experimental method of each property was described in more detail.

2.2.1. Sintering Process of SSM and SSM Containing CFs

About 1 gram of standard sinter mix was shaped into a cylindrical tablet, with final dimensions of 10 mm Diameter x 4 mm Height, employing a steel mold under a pressure of 20 MPa. Then, the disc-shaped sample was placed inside an alumina crucible (15 mm Diameter x 10 mm Height). The disc-containing crucible was put into the vertical tube furnace when the experimental temperature of 1350°C was achieved. The afore-mentioned temperature was determined to simulate the maximum temperature achieved in an industrial sinter plant of iron ore.^36,37,38) The samples were exposed to a reducing atmosphere for simulating the combustion of coal during the sintering process.^39,40) The current study used a similar gas mixture that was employed by Hsieh et al.,³⁹⁾ a gas mixture composed of 75 vol% Ar, 23 vol% CO₂ and 2 vol% CO. The total gas flow rate was adjusted to be 1.5 L/min. The sample was kept inside the vertical furnace at the desired temperature and atmosphere for about 3 minutes. The defined time was set based on the previous studies that reported that the duration that the sample is exposed to the maximum temperature is very short.^36,41,42) Figure 4 represents the conditions implemented for the sintering experiment. After the samples were cooled, one sample was supplied for XRD analysis to identify the major phases present in the standard sinter, and the other sample was analyzed in terms of wet chemical analysis to evaluate the amount of total Fe, metallic Fe and Fe²⁺. Two more samples were used for the porosity analysis to evaluate the porosity level and size distribution of pores present in standard sinter by the average. The evaluation of porosity was performed in terms of mercury porosimeter (Automated Mercury Porosimeter, PoreMaster 33, Quantachrome Instruments, Florida, USA).

Fig. 4.

Heating pattern adopted for the sintering process experiment. (Online version in color.)

SSM was replaced with SSM containing different adding levels of calcium ferrites to understand the effect of adding calcium ferrite to the standard sinter mix on the sinter properties. Three different levels of replacement were selected; 10, 20 and 30 mass%. Figure 5 represents the scheme of adding CFs to SSM. Totally nine blends were prepared considering the three kinds of CFs and three replacement levels. To guarantee the uniform mix of SSM and CFs, each blend was mixed for 3 hours employing the Turbula Mixer (T2F Nr. 120,942 Switzerland) at 34 revolutions per minute. The basicity level of the sinter is expected to increase by increasing the amount of CF present in the sinter mix, as the relative amount of CaO will be increased in the sinter mix.

Fig. 5.

Sample preparation procedure: (a) Standard sinter mix and (b) Replacement of standard sinter mix with calcium ferrites at different levels. (Online version in color.)

The sample preparation for the experimental conditions of sintering and post-sintering analyses were the same as that previously used for the SSM. The only significant difference between the blends was the sintering temperature. SSM was sintered at 1350°C, while the SSM containing CFs were sintered at several temperatures to determine the proper one for each kind and adding level of calcium ferrite. The proper sintering temperature was decided based on the porosity level of the samples, those with similar porosity to the standard sinter, made from SSM at 1350°C, were selected. The temperature range was defined in terms of thermogravimetric-differential scanning calorimeter (TG-DSC, TGA/DSC 1 Star system, Mettler Toledo, Schwerzenback, Switzerland) analysis. A powder sample of each sinter blend weighing about 100 mg was placed in the TG-DSC furnace and then heated non-isothermally at a rate of 10°C/min from room temperature to 1450°C in an argon atmosphere.

2.2.2. Evaluation of Sinter Properties – Reducibility and Reduction Disintegration

The reducibility and reduction disintegration followed standard testing methods.^43,44) The reducibility tests were carried out to measure the ability of removing oxygen contained in the sinter by the indirect reduction with CO gas. One disc-shaped sample was non-isothermally heated at a rate of 10°C/min from room temperature to 900°C in a N₂ atmosphere in a vertical thermogravimetric analysis (TGA, Rubotherm, Bochum, Germany). After the target temperature was reached, the gas composition was changed into 70 vol% N₂ and 30 vol% CO, and then the sample was kept at a flow rate of 1 L/min for 3 hours. The experiments were carried out twice for repeatability, and the results were averaged. The percentage degree of reduction (R_t) of the sinter was evaluated according to Eq. (1):⁴³⁾   

$$R_{\mathrm{t}}(\%)=\left(\frac{0.111w_1}{0.43w_2}+\frac{m_1-m_t}{m_0\times 0.43w_2}\times 100\right)\times 100$$(1)

where w₁ and w₂ are the content of FeO and T.Fe in the sinter before reduction (%), respectively. m₀ is the weight of the sinter before heating (grams). m₁ is the weight of the sinter right before reduction (grams). m_t is the weight of the sinter after a certain reduction time (grams). The RI, also referred to as dR_t/dt (O/Fe = 0.9), was evaluated according to the following equation:⁴³⁾   

$$\mathrm{R}\mathrm{I}=\frac{d{R}_{\mathrm{t}}}{dt}\left(\frac{\mathrm{O}}{\mathrm{F}\mathrm{e}}=0.9\right)=\frac{33.6}{t_{60}-t_{30}}$$(2)

where t₆₀ and t₃₀ are the required time to achieve the degree of reduction of 60 and 30%, respectively.

The reduction disintegration test was performed to evaluate the strength of the sinter after going through a low-temperature reduction process for simulating the upper part of the blast furnace. About ten sintered samples were simultaneously tested. The samples were non-isothermally heated at a rate of 100°C/min from room temperature to 550°C in a N₂ atmosphere in a horizontal gold-image furnace. After the target temperature was reached, the gas composition was changed into 70 vol% N₂ and 30 vol% CO, and the sample was kept at a flowrate of 1 L/min for 30 minutes. After the samples were cooled to room temperature, the total weight was measured. Then, the samples were tumbled at 60 rpm for 15 minutes. Finally, the samples were sieved in different aperture sizes such as 6.3, 2.8 and 0.5 mm. The RDI_−2.8mm and RDI_+6.3mm were evaluated as the weight of samples smaller than 2.8 mm and larger than 6.3 mm, respectively, as represented by Eqs. (3) and (4).   

$$RD{I}_{-2.8\mathrm{m}\mathrm{m}}(\%)=\frac{m_0-m_1}{m_0}\times 100$$(3)

  

$$RD{I}_{+6.3\mathrm{m}\mathrm{m}}(\%)=\frac{m_2}{m_0}\times 100$$(4)

where m₀ is the weight of the sample after reduction and before tumbling (grams). m₁ and m₂ are the weight of samples retained in the sieves of 2.8 and 6.3 mm (grams), respectively.

2.2.3. Evaluation of the Fractional Conversion of Nitrogen to NO (X_NO) and of Sulfur to SO₂ (X_SO2) from Fuel Combustion in Sintering Process

Laboratory-scale equipment was performed to measure the fractional conversion of N and S to NO and SO₂, respectively, as shown in Fig. 6.

Fig. 6.

Schematic diagram of the experimental apparatus. (Online version in color.)

The experimental system mainly consists of a horizontal gold-image furnace, gas supply system and in-situ gas analyzer (QMS, Quadrupole Mass Spectroscopy, GAM 400, InProcess Instruments, Bremen, Germany). An alumina crucible was located in the middle of the reactor. A thermocouple was installed above the crucible to control the experimental temperature accurately.

Figure 7 summarizes the schematic representation of fuel addition to the SSM and the nine blends containing CFs. 5 mass% of fuel with respect to the weight of sinter mix was added. The fuel consisted of 50 mass% of anthracite coal and 50 mass% of coke. Totally ten combinations (sinter mix + fuel) were prepared. About 5 grams of each combination was placed inside an alumina crucible (65 mm Length x 20 mm Width x 10 mm Height), and then the crucible was charged into the gold-image furnace for which non-isothermal heating was performed. Table 5 summarizes the composition of each combination. The heating pattern used in this study was the same as previously applied by Cho et al.⁴⁵⁾ The temperature was raised at a rate of 100°C/min from room temperature to 1100°C and at 30°C/min from 1100°C to 1350°C. The composition of the gas injected into the furnace was always maintained at 79 vol% Ar and 21 vol% O₂ during the whole experiment. The total gas flow rate was adjusted to be 600 mL/min, and controlled by Mass Flow Controller (MFC). Outlet gases were analyzed in-situ by Quadrupole Mass Spectroscopy (QMS). All combustion experiments were carried out three times for repeatability, and the results were averaged.

Fig. 7.

Schematic representation of the fuel addition to the sinter mix. (Online version in color.)

Table 5. Amount of fuel added to the standard sinter mix and sinter mix containing calcium ferrites (grams).

Samples	SSM (g)	CF (g)	Fuel (g)	Total weight (g)	Basicity level range (–)
100% SSM	4.76	–	0.24	5.0	1.9
90% SSM + 10% CF	4.28	0.48	0.24	5.0	2.2–2.4
80% SSM + 20% CF	3.81	0.95	0.24	5.0	2.6–3.0
70% SSM + 30% CF	3.33	1.43	0.24	5.0	3.0–3.8

To evaluate the amount of nitrogen and sulfur in coal that has been converted to NO and SO₂ during the experiment, fractional conversion of N into NO (X_NO) and S into SO₂ (X_SO2) were calculated in terms of the content of NO and SO₂ in outlet gas, which was determined by Quadrupole Mass Spectroscopy (QMS) as shown by Eqs. (5), (6), (7) and (8):   

$$\left[\mathrm{N}\right]\mathrm{g}\mathrm{a}\mathrm{s}=\left[\int \left(\frac{\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\hspace{0.17em} }\mathrm{p}\mathrm{c}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{N}\mathrm{O}\right)}{\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\hspace{0.17em} }\mathrm{p}\mathrm{c}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{A}\mathrm{r}\right)}\times Q\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\times p\mathrm{A}\mathrm{r}\right)\right]\times \frac{1\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{o}\mathrm{l}}{22\mathrm{\hspace{0.17em} }400\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{L}}$$(5)

  

$$X_{\mathrm{N}\mathrm{O}}=\frac{{\left[\mathrm{N}\right]}_{\mathrm{g}\mathrm{a}\mathrm{s}}}{W_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}\left(g\right)\times \frac{\left[\mathrm{w}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{p}\mathrm{c}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{N}\right]}{100}\times \frac{1\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{o}\mathrm{l}}{14\mathrm{g}\mathrm{\hspace{0.17em} }\mathrm{N}}}$$(6)

  

$$\left[\mathrm{S}\right]\mathrm{g}\mathrm{a}\mathrm{s}=\left[\int \left(\frac{\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\hspace{0.17em} }\mathrm{p}\mathrm{c}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{S}{\mathrm{O}}_2\right)}{\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\hspace{0.17em} }\mathrm{p}\mathrm{c}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{A}\mathrm{r}\right)}\times Q\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\times p\mathrm{A}\mathrm{r}\right)\right]\times \frac{1\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{o}\mathrm{l}}{22\mathrm{\hspace{0.17em} }400\mathrm{\hspace{0.17em} }\mathrm{m}\mathrm{L}}$$(7)

  

$$X_{{\text{SO}}_2}=\frac{{\left[\text{S}\right]}_{\text{gas}}}{W_{\text{sample}}\left(g\right)\times \frac{\left[\text{wt}\mathrm{\hspace{0.17em} }\text{pct}\mathrm{\hspace{0.17em} }\text{S}\right]}{100}\times \frac{1\mathrm{\hspace{0.17em} }\text{mol}}{32\text{g}\mathrm{\hspace{0.17em} }\text{S}}}$$(8)

where [N]_gas and [S]_gas are the moles of nitrogen and sulfur in the outlet gases (moles), respectively. Q_total is the total flowrate of inlet gas at standard state (mL/min), p_Ar is the partial pressure of Ar gas in the inlet gas, X_NO and X_SO2 are the fractional conversion of nitrogen and sulfur in the coal to NO and SO₂, respectively, and W_sample is the weight of sample used (grams).

3. Results and Discussion

3.1. Effects of Adding Iron Ores-Based CFs to the Standard Sinter Mix (SSM) on the Sintering Temperature and Porosity of Sinter

The standard sinter mix (SSM) was set as the reference material to evaluate the effects of adding iron ores-based calcium ferrites on several sinter properties. As previously described in the experimental section, ten different samples were prepared; one consisted of SSM and the other nine consisted of each calcium ferrite at three adding levels.

The melting behavior of SSM and CFs-containing SSM was estimated in terms of heat flow. Figure 8 represents the typical DSC curves of the samples. The temperature range was selected to amplify the temperature range that most reactions take place in the actual sintering process of iron ore. As is shown in Fig. 8, an endothermic reaction was started around 1200°C in all the curves, which is more evident in the samples containing 30 mass% of calcium ferrite. The referred phenomenon corresponds to the melting behavior of calcium ferrite, which is known to be around 1200°C.^8,46) The area inside this valley is related to the amount of melt generated, and this area also increased with increasing the amount of CF, which was previously expected because the calcium ferrites were intentionally added. In addition, it is possible to see the effect of adding CFs on the melting behavior of the sinter mix, especially in the highlighted zone. With respect to the SSM baseline, all the melt formation was shifted to a lower temperature range, which allows the samples containing calcium ferrites to be sintered at a lower temperature than the SSM which was sintered at 1350°C. Figure 9 summarizes how the calcium ferrites affect the melt formation temperature of the highlighted zone represented in Fig. 8. It could be seen that the addition of all kinds of CFs at different levels reduced the temperature below 1300°C, allowing a reduction of at least 50°C when compared with the standard sinter.

Fig. 8.

Melting behavior of sinter mixes containing different calcium ferrites in terms of TG-DSC: (a) Ore A-based, (b) Ore B-based and (c) Mix-based. (Online version in color.)

Fig. 9.

Change in the melt formation temperature with adding CFs to the SSM.

Based on the results shown in Fig. 9, the CFs-containing sinter mixes were sintered in the temperature range from 1210 to 1300°C to determine the proper sintering temperature of each blend. It is important to recall that the sintering temperature was decided based on the porosity level. From the porosity analysis, the lab-scale sinter prepared from SSM showed a porosity of about 24%. For the sake of comparison, an industrial sinter was also analyzed by the identical method and showed a porosity of 20%. The afore-mentioned results showed similar porosity between the sinter prepared in the lab and the industrial one, which confirmed the reliability of the sintering experiment performed in this study.

As shown in Table 5, the basicity level of the sinter increased by increasing the amount of CF present in the sinter mix, which was previously expected because the CFs were intentionally added to the sample. Therefore, further investigations regarding the effect of the amount of CF, at a fixed temperature, on any sinter property extend to the basicity level. Figure 10 represents the effect of adding CFs to the SSM on the sinter porosity at the selected temperature. It is clear that the calcium ferrite could behave in two different ways inside the sample; first of all, the increased amount of melt filled the pores among the particles during the sintering process, and secondly, the melt was more fluid and segregated in the bottom part of the sample, while leaving a porous zone in the upper part.³⁴⁾ Similar behavior was found by Wu et al.,³⁴⁾ which reported that low-melting-point materials filled the pores among the particles, and reacted with the nuclei ores during the sintering process. In addition, it was found that the liquid phase with high fluidity easily flowed down to the bottom of the sintering bed, resulting in the production of numerous large pores in the upper section. After conducting several sintering experiments, the proper temperature for sintering each sample containing CF was determined. Table 6 summarizes the conditions that the porosity was maintained at a similar level to that of the standard sinter.

Fig. 10.

Effect of adding CFs to the SSM on the porosity level of sinter.

Table 6. Porosity level of standard sinter and sinter containing CFs.

Samples	Amount of CF (%)	Sintering temperature (°C)	Porosity level (%)
SSM	–	1350	23.6
SSM + Ore A-based CF	10	1285	28.5
	20	1275	18.9
	30	1270	24.7
SSM + Ore B-based CF	10	1280	22.3
	20	1280	15.9
	30	1235	29.6
SSM + Mix-based CF	10	1265	29.6
	20	1265	22.8
	30	1235	24.3

3.2. Change in the Pore Size Distribution and Phase Development in Sinter when Adding CFs to the SSM

The current study initially attempted to understand the effect of adding CFs to the SSM on the porosity of the sinter. However, the change in the pore size and phase formation induced by the addition of calcium ferrites could also be considered as an important parameter to explain a further change in the sinter quality. Several studies correlated the effects of pore size and phase development on the reducibility or reduction disintegration of sinter.^{36,47,48,49,50)} To guarantee the validity of evaluating the pore size distribution, the lab-scale and industrial sinters were compared. It is important to mention that the current study divided the size of the pores into three categories; macropores (> 100 μm), medium pores (< 100 and > 10 μm) and micropores (< 10 μm). Table 7 represents the pores size distribution of both samples. Based on the analysis of the pores, it was concluded that the lab-scale and industrial sinters are very similar in the dominant pore size, which was found to be the macro-pore. Figure 11 summarizes the effect of adding different calcium ferrites to the standard sinter mix on the pore size in the lab-scale sinter. The addition of CFs refined the pores present in the sinter, as is clearly seen from the change in the major, medium and micropores. For example, when adding the Mix-based CF, the macropores were almost disappeared in the sinter with levels below 10 mass%. Such a phenomenon was previously reported by several researchers,^51,52,53,54) which reported that the presence of a more fluid liquid phase (CF) would spread through a bigger area inside the sinter mix, increasing the contact between the sinter mix particles and melt phase, and consequently, the nucleation of finer pores would be promoted. The sintering temperature plays an important role in the formation of micropores as the fluidity of the liquid phase is a temperature-dependent property. As shown in Table 6, the sinter mix containing a higher level (30 mass%) of iron ore-based CFs was sintered at lower temperatures than that containing a lower level (10 mass%). Such a finding was important to avoid the intense flow of the melt downwards to the bottom of the sample, leaving a macroporous zone in the upper part of the sample behind.

Table 7. Comparison on the pore size distribution of lab-scale sinter and industrial sinter (%).

Samples	Macropores	Medium pores	Micropores
Lab-scale sinter (SSM)	60.7	29.0	10.3
Industrial sinter	71.7	21.6	6.7

Fig. 11.

Effect of adding CFs to the SSM on the pore size distribution of sinter: (a) Ore A-based CF, (b) Ore B-based CF and (c) Mix-based CF.

The effect of adding CFs to the SSM on the phase development of the sinter was evaluated in terms of XRD analysis, as shown in Fig. 12. It was expected that more liquid phase would be formed with adding calcium ferrite, and more oxides present in the sinter mix are likely to dissolve into this liquid phase, resulting in the formation of the most desirable phases such as SFCA and SFCA-I.^11,12,55) As represented in Table 8, the phases in each sample were quantified, which confirmed the expectation. The quantitative amount of liquid phase represented by SFCA and SFCA-I was significantly increased when adding the calcium ferrites to the sinter mix from about 9 to 32.6 mass% at least. In contrast to it, the amount of Fe₂O₃ was significantly decreased, which could explain the improved interaction between solid particles and the calcium ferrite phase. The method of quantifying used in this study was previously reported by Hariswijaya et al.,⁵⁶⁾ which consisted of calculating the integral value of the peaks identified by XRD of each phase using OringPro software. The evaluation was repeated at least 3 times to ensure accuracy. Finally, the average value was used as the representative quantity of each phase present in the sinter.

Fig. 12.

Effect of adding CFs to the SSM on the phase’s development of sinter: (a) Ore A-based CF, (b) Ore B-based CF and (c) Mix-based CF.

Table 8. Summary of the effects of adding calcium ferrites to the standard sinter mix on the sinter properties.

Samples	Amount of CF (%)	Fe2O3 (%)	Fe3O4 (%)	SFCA (%)	SFCA-I (%)
SSM	–	67.4	23.7	6.6	2.3
SSM + Ore A-based CF	10	50.7	15.2	21.6	12.5
	20	36.9	14.1	27.0	22.0
	30	27.1	12.2	32.8	27.9
SSM + Ore B-based CF	10	50.8	16.6	19.2	13.4
	20	41.0	18.4	25.6	14.9
	30	29.6	7.4	35.4	27.6
SSM + Mix-based CF	10	46.3	11.4	26.5	15.8
	20	35.9	12.4	30.4	21.3
	30	27.1	7.0	35.7	30.2

3.3. Effects of Adding Calcium Ferrites on the Reduction Behavior of the Sinter Measured by the R_t, RI and RDI

After understanding the change in the morphology of the sinter promoted by calcium ferrite, the effects of adding CFs to the SSM on the reduction behavior of the sinter were investigated in views of the degree of reduction (R_t), reducibility index (RI) and reduction disintegration index (RDI). To validate the lab-scale experiments, the identical procedure employed for the lab-scale sinters was conducted for an industrial sinter. R_t and RDI_−2.8mm of the industrial sinter were found to be 69 and 31%, respectively, which satisfies the minimum requirement requested by commercial sintering plants.⁸⁾ This confirms the reliability of the experimental procedure used in this study for the analysis of the lab-scale samples; SSM and CFs-containing SSM. Figure 13(a) compares the reduction degree of the standard sinter with that of the calcium ferrite-containing sinters. The addition of CFs to the SSM led to significant improvement in the reduction degree of the sinter. That is, the sinter prepared from the standard sinter mix showed a reduction degree of about 75%, while the CFs-containing sinters showed at least 90%. A combination of several parameters might be ascribed to this positive effect of calcium ferrite on the reduction behavior of the sinter. The current study investigated most of the parameters such as the change in the pore size from macro to medium/micro-pores,^36,49) the increase in calcium ferrite-related phases that improves the degree of reduction⁴¹⁾ and the decrease in the FeO content in the sinter,^19,38,57) as shown in Table 9. Although this study did not evaluate, another important parameter that might contribute to the reduction degree of the sinter is how the pores are distributed inside the sintered structure.⁵⁸⁾ It is very difficult to determine which parameter is dominant based on the results shown in Fig. 13(a). The results showed that all CFs at different levels promoted the reduction to similar values, even though the samples differ in their mineralogy. However, as a macro analysis of adding calcium ferrites, all the CFs promoted similar effect on the sinter mineralogy such as a decrease in the pore size, an increase in the amount of liquid phase and a decrease in the FeO content.

Fig. 13.

Effects of adding CFs to the SSM on the reduction behavior of sinter: (a) Degree of reduction and (b) Reducibility Index (RI).

Table 9. FeO content in the sinter (mass%).

Samples	Amount of CF (%)	FeO content (mass%)
SSM	–	7.9
SSM + Ore A-based CF	10	4.5
	20	4.5
	30	6.1
SSM + Ore B-based CF	10	4.8
	20	6.2
	30	1.6
SSM + Mix-based CF	10	3.1
	20	3.8
	30	2.3

Figure 13(b) summarizes the reducibility indices of the standard sinter and those containing CFs, which showed better degrees of reduction in Fig. 13(a) (sinter containing 20% of Ore A-based CF, sinter containing 10% of Ore B-based CF and sinter containing 20% of Mix-based CF). As shown, the CFs-containing sinters had a higher RI than the standard sinter. Two main factors might have influenced the rate of reduction, pore size and phase.^59,60) Maeda et al.⁵⁹⁾ reported that the reduction of calcium ferrite is much faster than the FeO one. From the results in Fig. 13(b), the Mix-based CF-containing sinter showed the highest rate because the combination of both parameters is the most optimal compared with other CFs. The Mix-based CF-containing sinter has 16.7 and 51.7% of micropores and liquid phase, respectively. Ore A-based CF-containing sinter had less amount of micro-pores (9.8%) than Mix-based CF-containing sinter, while Ore B-based CF-containing sinter lower amount of SFCA/SFCA-I (32.6%). In addition, when Ore A- and Ore B-based CFs-containing sinters were compared to investigate which factor was more dominant for the improvement of RI, it is believed that the amount of liquid phase present in the sinter is more effective than the amount of micropores. Ore A-based CF-containing sinter has 1.5 times more liquid phase than Ore B-based CF-containing sinter, while Ore B- based CF-containing sinter has 1.9 times more micropores than Ore A-based CF-containing sinter. Even though the liquid phase ratio is smaller than the micropores ratio, Ore A-based CF-containing sinter showed a higher RI value than Ore B-based CF-containing sinter.

Figure 14 summarizes the effect of adding calcium ferrites to the SSM on RDI_−2.8mm. It seemed that the addition of CFs promoted a significant decrease in the RDI_−2.8mm, which means that the sinter strength was improved, as shown in Fig. 14(a). The sinter prepared from the standard sinter mix showed a value of RDI_−2.8mm as about 5.3%. The CF-containing sinter that promoted the highest improvement was the sinter containing Ore A-based CF at 20% as adding level, with a value of RDI_−2.8mm below 1%. It is well-known from the previous studies about the effect of some phase constituents such as FeO, Fe₃O₄ and Fe₂O₃ on the reduction disintegration of the sinter.^19,36,47) In addition, Hosotani et al.³⁶⁾ reported about the positive effect of acicular calcium ferrite in the sinter on the RDI, while Sakamoto et al.⁴⁷⁾ mentioned the positive effect of the amount of micro-pores in the sintered structure. Such factors might be the reason for the improvement of RDI_−2.8mm by adding CFs to the SSM. As previously reported in this study, the addition of calcium ferrites decreased the size of pores in the sinter and increased the amount of calcium ferrite-related phases. As shown in Fig. 14(b), the plot of RDI_−2.8mm against the FeO content in the sinter confirmed the contribution of FeO level to the improvement of reduction disintegration. Even though the standard sinter contains a significant amount of FeO, it showed a high value of RDI_−2.8mm. The dominant pore size on its structure is macro-pores, which might be ascribed to the outlier data point of the standard sinter in Fig. 14(b).

Fig. 14.

(a) Effects of adding CFs to the SSM on the RDI_−2.8mm of sinter and (b) Relationship between RDI_−2.8mm and FeO content in sinter.

3.4. Influence of Adding CFs to the SSM on the Emission of CO₂

The first assumption made for evaluating the emission of CO₂ during the sintering process is that all the fuel present in the sinter mix will convert into CO₂. For it, it is necessary to correlate the ratio of fuel with sintering temperature for predicting the amount of CO₂ to be reduced. Umadevi et al.¹⁹⁾ showed an indirect relation between the two mentioned parameters. In their research, both the fuel ratio and sintering temperature were correlated to the FeO content in the sinter. The current study manipulated the data reported by Umadevi et al.¹⁹⁾ to derive a direct relationship between fuel ratio and sintering temperature. To make this relationship more reliable, the ignition temperature of the actual sintering process was considered to be data, in case the fuel ratio is zero. Through the combination of those two set points, one from Umadevi et al.¹⁹⁾ and the other from the ignition temperature, a linear relationship could be derived as represented in Eq. (9). The required amount of coke breeze could be estimated after the sintering temperature for each blend was decided.   

$$S.T.\left(°C\right)=46.4\times c+1\mathrm{\hspace{0.17em} }141.6$$(9)

where S.T. is the sintering temperature (°C) and c is the amount of required coke breeze (mass%).

The emission of CO₂ was predicted according to Eq. (9). Based on the equation derived by Umadevi et al.,¹⁹⁾ it was possible to predict the required amount of fuel to achieve a maximum sintering temperature. The current study determined the optimal temperature for several samples containing different kinds of CFs at adding levels. In common, all the samples maintained the porosity level in a similar range. The addition of calcium ferrites to the standard sinter mix changed the dominant pore size from macro to medium/micropores, which resulted in increasing the amount of liquid phase and decreasing the FeO content in the sinter. Furthermore, the best combination of calcium ferrite and the adding level was decided by the reduction behavior of the sinter. The selected samples are summarized in Table 10.

Table 10. Summary of the sinter properties of the best sample of each calcium ferrite added to the standard sinter mix.

Samples	Amount of CF (%)	Degree of Reduction (%)	RI (–)	RDI−2.8mm (%)
SSM	–	74.7	0.4	5.3
SSM + Ore A-based CF	20	93.4	0.7	0.1
SSM + Ore B-based CF	10	94.5	0.7	2.6
SSM + Mix-based CF	20	93.5	0.9	3.9

By identifying the selected samples, it was possible to predict the required fuel ratio to achieve the sintering temperature proposed in this study, as shown in Table 6. To make it clear, the temperatures for the three selected samples were: 1275°C for Ore A-based CF at 20 mass%, 1280°C for Ore B-based CF at 10 mass% and 1265°C for Mix-based CF at 20 mass%. Then, according to the simple calculation by following Eq. (9), the amount of fuel ratio was evaluated, as summarized in Fig. 15. Finally, the primary assumption for predicting the emission of CO₂ was that all the fuel would convert into CO₂. This condition was made to simplify the calculation, even though the authors know that part of fuel converts into carbon monoxide (CO), and directly reduces the iron ore as well. Thus, the reduction in the emission of CO₂ would be exactly proportional to the decrease obtained in the fuel ratio. Table 11 represents the achieved reduction in the formation of CO₂. Based on the results of the current study, as a consequence of decreasing the required fuel amount, the CO₂ emissions were reduced by at least 33%.

Fig. 15.

Predicted fuel ratio in the samples containing calcium ferrites based on their sintering temperature.

Table 11. Reduction in CO₂ obtained by adding CFs to the SSM.

Samples	Amount of CF (%)	Fuel ratio (mass%)	CO2 reduction (%)
SSM	–	4.5	–
SSM + Ore A-based CF	20	2.9	35.6
SSM + Ore B-based CF	10	3.0	33.3
SSM + Mix-based CF	20	2.7	40.0

3.5. Role of Calcium Ferrite on the Formation of NO and SO₂

At present, there are three different techniques to monitor the emission of NO and SO₂ in the sinter plant; source control which focuses on the selection of fuels containing less amount of nitrogen and sulfur, end-of-pipe technology which treats the outlet gas from the sintering process and finally process control which modifies the parameters of sinter bed such as the addition of catalyst, the basicity of sinter mix, sintering time and so on. In the current study, the addition of CFs to the sinter mix could be classified as a source control method. Therefore, it is necessary to investigate how these calcium ferrites affect both the formation of NO and SO₂. Several previous studies already reported the positive effect of adding CFs on the decreased formation of NO,^{15,26,27,28,29,30)} while in the case of SO₂, there are no findings up to now.

Figure 16 summarizes the conversion of nitrogen into NO (X_NO) when adding CFs to the SSM. As confirmed in the previous studies results,^{15,26,27,28,29,30)} the addition of iron ores-based CFs promoted the reduction in the emission of NO during the sintering process with reference to the baseline represented by the standard sinter mix without any addition of calcium ferrites. The possible mechanism behind the reduction ability of calcium ferrites might be represented by Reaction (10),⁶¹⁾ where CFs act as a catalyst for the afore-mentioned reaction. Lv et al.¹⁵⁾ reported a similar mechanism performed by calcium ferrite, which had a strong catalytic effect on the reduction of NO in the presence of CO gas. Li et al.³⁰⁾ investigated the mechanism of NO reduction by CO gas on the surface of different Ca–Fe oxides such as CaO∙Fe₂O₃ and 2CaO∙Fe₂O₃. Both calcium ferrites had a good catalytic effect on the NO–CO reaction. Density functional theory (DFT) calculation and characterization were conducted to evaluate the catalytic activity of both calcium ferrites. Through the results, the presence of 2CaO∙Fe₂O₃ had a higher catalytic effect on the NO–CO reaction than CaO∙Fe₂O₃, mainly because the 2CaO∙Fe₂O₃ was in an electron-deficient state, which improved the oxygen tolerance not to reoxidize the reduced NO.   

$$\mathrm{N}\mathrm{O}\left(g\right)+\mathrm{C}\mathrm{O}\left(g\right)=\frac{1}{2}{\mathrm{N}}_2\left(g\right)+\mathrm{C}{\mathrm{O}}_2\left(g\right)$$(10)

  

$$\mathrm{\Delta }{G}^0=-372\mathrm{\hspace{0.17em} }300-98.36T\left(\text{J}/\text{mol}\right)\left(25-2\mathrm{\hspace{0.17em} }000°\text{C}\right)$$(11)

Fig. 16.

Effects of adding CFs to the SSM on the conversion of nitrogen to NO.

Figure 17 summarizes the effect of adding calcium ferrites to the SSM on the conversion of sulfur into SO₂ (X_SO2). The formation of SO₂ was promoted in the presence of calcium ferrites. The current study investigated the possible mechanism of calcium ferrite on X_SO2, as there are no previously reported researches. A new experiment was conducted by purging SO₂ gas of a fixed amount inside a gold-image furnace containing about 2 g of calcium ferrite. The outlet gas was in-situ analyzed during the experiment by QMS. The heating pattern used was the same as applied previously to evaluate X_NO and X_SO2. The concentration of SO₂ was changed during heating as shown in Fig. 18(a). It could be clearly seen that the amount of SO₂ was decreased in the temperature range from 600 to 1090°C, followed by an increase from 1090°C. The calculated area in both regions was found to be identical. To understand the phenomena identified during the reduction of SO₂, one experiment was stopped at 1000°C, and the sample was analyzed in terms of XRD as shown in Fig. 18(b). In addition to the CF phase, calcium sulfate (CaSO₄), iron sulfide (FeS) and magnetite (Fe₃O₄) were found to be formed. The identification of such phases and Reaction (12)⁶²⁾ confirmed the effect of calcium ferrites on the formation of SO₂ represented in Fig. 17. The CF caught the sulfur present in the sinter mix to form several compounds containing sulfur, and then this sulfur was released in the form of SO₂ at high temperatures. It is necessary to mention that the temperature where SO₂ concentration was changed in Fig. 18(a) is almost the same as that where the Gibbs free energy of Eq. (13) is equal to zero.   

$$\begin{array}{l}6{\text{SO}}_2\left(g\right)+5\text{CaO}\cdot {\text{Fe}}_2{\text{O}}_3\left(s\right)=\\ 5{\text{CaSO}}_4\left(s\right)+\text{FeS}\left(s\right)+3{\text{Fe}}_3{\text{O}}_4\left(s\right)\end{array}$$(12)

  

$$\mathrm{\Delta }{G}^0=-227\mathrm{\hspace{0.17em} }454+169.22T\left(\text{J}/\text{mol}\right)\left(25-1\mathrm{\hspace{0.17em} }400°\text{C}\right)$$(13)

Fig. 17.

Effects of adding CFs to the SSM on the conversion of sulfur to SO₂.

Fig. 18.

Role of calcium ferrite on the formation of SO₂: (a) Change in the SO₂ concentration during heating in the presence of calcium ferrite and (b) Phase composition of calcium ferrite at 1000°C identified by X-ray diffraction. (Online version in color.)

A further investigation on the formation of SO₂ was conducted. Figure 17 summarized the effect of adding CFs to the SSM on X_SO2 when the sample was heated up to 1350°C to understand the general behavior of calcium ferrites. However, as shown in Table 6, the sintering temperature of the CFs-containing sinter mix was at most 1285°C which could really affect the formation of SO₂ as shown in Fig. 19(a). That is, a slight change in the temperature would significantly affect the total emission of SO₂. Therefore, the conversion of sulfur to SO₂ for each sample was evaluated at their sintered temperature as represented in Fig. 19(b). Due to the decrease in the temperature by adding the CFs, the formation of SO₂ was decreased compared with the standard sinter. An important point to mention is that the actual effect of calcium ferrite on the promoted formation of SO₂ was maintained if the samples sintered at the same temperature are compared between them. As an example, when 10 and 20% of Ore B-based CF is present in the sinter mix, both were sintered at 1280°C, which was applied to the Mix-based CF at the adding levels of 10 and 20%.

Fig. 19.

Effects of sintering temperature on the conversion of sulfur to SO₂: (a) Formation pattern of SO₂ with temperature and (b) Conversion of sulfur to SO₂ at different sintering temperatures.

3.6. Design of a Low-Temperature Sintering Process by Adding Preformed Calcium Ferrites to the Standard Sinter Mix

The effects of adding calcium ferrites to the standard sinter mix are summarized in Table 12. Several properties evaluated in this study were compared. The first improvement to be mentioned was the decrease in the sintering temperature by 70°C at least. In addition, in view of the formation of phase in the sinter, the liquid phase was significantly increased from 8.9 to at least 32.6% when adding CFs to the sinter mix. The reduction behavior of the sinter was also improved in the CFs-containing sinter compared with the standard sinter. For example, when the sinter contained 20% of Ore A-based CF, the degree of reduction (R_t) and RDI_−2.8mm were enhanced from 74.7 to 93.4 and from 5.3 to 0.1%, respectively. The emission of NO and SO₂ was reduced by the action of calcium ferrites through different mechanisms. In the case of NO, CFs acted as a catalyst for the NO–CO reaction while in the case of SO₂, CFs decreased the sintering temperature, which reduces the emission range of SO₂.

Table 12. Summary of the effects of adding calcium ferrites to the standard sinter mix on the sinter properties.

Properties of sinter	SSM	SSM + Ore A-based CF (20%)	SSM + Ore B-based CF (10%)	SSM + Mix-based CF (20%)
Sintering temperature (°C)	1350	1275	1280	1265
Liquid phase (%)	8.9	49.0	32.6	51.7
FeO content (mass%)	7.9	4.5	4.8	3.8
Rt (%)	74.7	93.4	94.5	93.5
RDI−2.8mm (%)	5.3	0.1	2.6	3.9
Required fuel amount* (mass%)	4.5	2.9	3.0	2.7
Reduction in CO2** (%)	–	35.6	33.3	40.0
XNO (%)	46.6	41.4	42.8	42.5
Reduction in NO*** (%)	–	42.7	38.8	45.3
XSO2**** (%)	21.4	13.0	14.1	18.5
Reduction in SO2*** (%)	–	60.9	56.1	48.1

*  Based on the Eq. (9)

**  Based on the assumption that all fuel converts into CO₂

***  Considering the required fuel ratio and the X_NO/X_SO2

****  Considering the effect of temperature, as shown in Fig. 19(b)

Finally, the important factor to be highlighted is that the decrease in the sintering temperature allows a considerable saving in the ratio of the fuel in the sinter mix. This decrease in fuel consumption will consequently promote a significant reduction in the emission of CO₂, NO and SO₂ from the sinter plants. The addition of CFs improved all the sinter properties, which provided a method of preparing a sinter with better quality than the actual standard sinter. Therefore, the suggested low-temperature sintering process in the current research could provide a high-quality sinter that emits a lower amount of harmful gases to the atmosphere.

4. Conclusions

The current research investigated the effects of adding calcium ferrites to the standard sinter mix on several properties of the sinter as a possible method of reducing the amount of fuel used in the sintering process, which would contribute to decreasing the emission of CO₂, NO and SO₂. From the findings, the following conclusions were obtained.

(1) The addition of calcium ferrites significantly decreased the sintering temperature below 1300°C, while maintaining the porosity level in the same range as the standard sinter. The presence of calcium ferrites also promoted the change in the dominant pore size from macro to medium and micro-pores. In addition, with increasing the amount of CF in the sinter mix, the amount of liquid phases such as SFCA and SFCA-I were significantly increased. The standard sinter contained about 9% of liquid phase while the CFs-containing sinter mix showed at least 32%.

(2) Such combined change in the mineralogy of sinter made by the addition of calcium ferrites provided better sinter quality such as reduction degree (R_t), reducibility index (RI) and reduction disintegration index (RDI), that is, in view of the reduction behavior of the sinter.

(3) The reduction degree of the sinter was improved to 90% from 75% in the standard sinter. The amount of finer pores, high level of the liquid phase and low FeO content were considered to be important factors affecting the mentioned improvement. In the case of RDI, the presence of finer pores and FeO content in the sinter were determinant parameters for a better RDI. Except for one sample, all the sinters containing CFs showed a better RDI than the standard sinter. The most impressive was shown by the addition of 20% of Ore A-based CF, which showed a 0.1% of RDI_−2.8mm while that of the standard sinter was 5.3%.

(4) The addition of calcium ferrites promoted a decrease in the emissions of CO₂, NO and SO₂ for different reasons. Due to the reduction in the amount of required fuel, less amount of CO₂ was expected to form. At least 33% of the CO₂ emission could be reduced when compared with the standard sinter. At least 39% of NO emission was reduced by the catalytic effect of calcium ferrite on the reduction of NO by CO. The emission of SO₂ was reduced by 48% at least due to the decrease in the sintering temperature of sinter mixes containing calcium ferrites since the formation profile of SO₂ was highly affected by temperature.

(5) Based on the obtained results in this study, a low-temperature sintering process could be suggested. The addition of calcium ferrites to the standard sinter mix allowed the decrease in the ratio of required fuel in the sinter mix. The presence of CFs in the sinter mix produced a sinter of better quality while emitting lower amount of harmful gases such as CO₂, NO and SO₂. Therefore, it is possible to decrease the use of carbon, and consequently reduce the emission of CO₂, which might be a significant advance in view of the environment protection.

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