Cooling Air Velocity on Iron Ore Pellet Performance Based on Experiments and Simulations

Abstract

During the pellet cooling process, cooling air velocity is crucial for optimizing the cooling rate, evaluating the utilization rate of cooling heat energy, and improving pellet performance. As the simulated cooling air velocity increased, the gas temperature at the cooling endpoint decreased from 87 °C to 51 °C, and the solid temperature decreased from 149 °C to 103 °C. The total enthalpy of the recovered gas initially reduced and then increased while the heat recovery rate gradually increased. During the experiment, the inhomogeneity of pellet quality gradually increased with the rise in cooling air velocity. The effect of cooling air velocity on pellet properties is primarily reflected in the formation of cracks and low-melting liquid phases (FeO and fayalite). As the cooling air velocity increases, the softening onset temperature of the pellet decreases significantly. The melting zone decreases from 193 °C to 105 °C, and the permeability of the adhesive zone increases.

1. Introduction

In recent years, China’s iron and steel companies have gradually increased the proportion of pellets in the furnace, considering environmental protection, iron ore resource characteristics, and blast furnace smelting requirements [1,2,3]. However, to meet the demands of high pellet proportions in the blast furnace, producing high-quality pellets has become a primary focus for many steel companies [4]. The iron ore pellet production process mainly consists of green pellet preparation, drying and preheating, oxidation and roasting, and the cooling of the roasted pellets. Many steel companies and scholars have conducted systematic and in-depth research on green pellet preparation, drying and preheating, and oxidation roasting. However, the role of pellet cooling is often neglected [5,6,7,8,9]. As Chinese iron and steel enterprises increasingly use high gangue magnetite to prepare pellets, the impact of the cooling process on pellet performance has become more significant. Therefore, studying the cooling process of iron ore pellets is necessary.

Many scholars have conducted systematic and in-depth research on the cooling process of iron ore sintering [10,11,12,13]. The liquid phase formed within the sintered ore at high temperatures crystallizes during cooling. The cooling rate affects not only the size and structure of the crystals but also the stability between phases and the strength of the binder phase, which in turn influences the mechanical and metallurgical properties of the sinter [14,15,16,17,18]. As the SiO₂ gangue phase content in the pellets increases, the internal liquid phase of the roasted pellet also increases. This leads to a gradual increase in the formation of low melting point substances such as FeO-SiO₂, 2FeO-SiO₂-FeO, 2FeO-SiO₂-Fe₂O₃, and 2FeO-SiO₂-SiO₂, thereby intensifying the impact of cooling on pellet performance [19,20,21]. Experimental studies on the cooling process of iron ore pellets are relatively few. Previous research compared the effects of different cooling methods on pellet performance, finding that the strength of pellets after water cooling was significantly lower than after air cooling. Some studies have shown that the cooling rate affects the solidification properties of the pellets by comparing them before and after annular cooling. During the cooling process, pellets undergo secondary oxidation, with Fe₂O₃ grains continuing to grow into continuous crystals, thereby optimizing the mineral phase structure and increasing solidification strength. Air cooling is the primary method for cooling pellets in actual production. Changes in cooling air velocity not only affect the heat recovery efficiency of the annular cooling system but may also influence the solidification and metallurgical performance of the pellets. Therefore, it is necessary to investigate the impact of varying cooling air velocity on pellet performance.

Previous researchers have conducted detailed studies on the simulation and analysis of the iron ore pellet annular cooling system. Feng et al. [22] investigated the effect of process parameters such as pellet diameter, bed thickness, air flow rate, and temperature on pellet bed temperature distribution using the commercial software FLUENT 2010. Yang et al. [23] optimized iron ore pellet cooling using mathematical modeling and data mining techniques to visualize the steady-state thermal processes in the recirculating cooler. Current research [24,25,26] mainly focuses on heat recovery in the cooling process and the simulation of annular cooling equipment, often neglecting the influence of cooling air velocity changes on the cooling rate and pellet performance. The overall effect of cooling air velocity variation on pellet performance and heat recovery can be clarified by combining experimental studies with simulation analysis. This approach is crucial for ensuring pellet compressive strength and heat recovery efficiency.

In this paper, the cooling experiments of pellets at different cooling air velocities were conducted using self-developed laboratory cooling equipment. The effects of cooling air velocity on the cooling rate, compressive strength, and microstructure of high-temperature pellets were examined. The pellets cooled at different cooling air velocities were analyzed for reduction index, reduction swelling index, and softening-melting properties. CFD numerical simulation was employed to simulate and analyze the effect of cooling air velocity variation on the cooling system’s temperature change, cooling rate, enthalpy change, and heat recovery, incorporating the annular cooling air velocity from the actual production process. Combining experimental and simulation analysis, this study investigates the change in pellet performance and heat recovery of the annular cooling system under different cooling air velocities. The findings provide theoretical support and a control basis for optimizing pellet quality and heat energy utilization in the annular cooling machine.

2. Experimental and Simulation

2.1. Raw Materials

The two magnetite concentrate fines used in the experiments were produced in Liaoning, China, with a high SiO₂ content. The SiO₂ content in concentrate-2 was 7.84%. For concentrate-1, 87.01% of the particles were less than 74 μm, and the Blaine surface area was 1837 cm²/g. For concentrate-2, 97.78% of the particles were less than 74 μm, and the Blaine surface area was 2188 cm²/g. Both concentrates met the pelletizing criteria, Table 1 shows the chemical composition and physical properties of iron ore pelletizing raw materials.

2.2. Experimental Procedures

The cooling experiments under laboratory conditions consisted of the following steps: first, pellet preparation, 10 kg of material was used for each pelletization experiment, of which the ratio of concentrate-1 to concentrate-2 was 3:7, and the bentonite dosage was 1.0%. Pellet preparation was carried out using a 1000 mm disc pelletizer with a disc side height of 80 mm, a speed of 450 rpm, an inclination angle of 47°, and a pelletizing time of 12 min. After the ball making, a round hole sieve is used to screen the pellets to achieve a particle size of 10–12.5 mm for the raw balls. Through the performance testing of the raw balls, the compressive strength exceeds 10 N, the wet falling strength is more than 4 times, and the moisture content is 9.5 ± 0.5%, qualifying them as green pellets, the pellet preparation shown in Figure 1a [27]. The green pellet drying process removes free moisture in a constant temperature oven at 105 °C for subsequent preheating and roasting; the drying of the green pellet process is shown in Figure 1b. The preheating and roasting experiments were carried out in a muffle furnace; 4 kg of pellets were used for each roasting experiment, and the pellets were packed in a hollow box with a high-temperature alloy. The roasting process was carried out at a heating rate of 10 °C/min, with a preheating temperature of 950 °C for 15 min and a roasting temperature of 1250 °C for 10 min; the preheating and roasting experiment is shown in Figure 1c.

After roasting, the high-temperature pellets must be immediately transferred to the cooling device, as shown in Figure 1d. The main component of the cooling device is a stainless-steel tube with a diameter of 50 mm wrapped in insulation cotton. The bottom of the tube features a high-temperature grate plate with a 5 mm aperture, which prevents the pellets from falling while allowing gas to pass through. The cooling gas is supplied by an air blower, entering from the lower part of the device through an anemometer in the pipe and passing through a porous baffle at the bottom to cool the pellets.

The thickness of the 4 kg pellets in the cooling unit is 800 mm, matching the thickness of the cooling unit layer. A stainless-steel funnel device is placed above the unit during the transfer process, and the sampling time should be limited to less than 1 min to minimize heat loss. A thermocouple is inserted after transferring the high-temperature pellets to the cooling unit. When the thermocouple reaches its maximum value, the fan is turned on to cool the high-temperature pellets. The thermocouple records the temperature change of the pellets at 1-min intervals.

The experimental cooling air velocity is determined according to the cooling rate, defined as the cooling temperature of the annular cooling particles per unit time. The cooling rate during pellet production is determined by Equation (1). T is the outlet gas temperature of each cooling section, and t is the cooling time. The cooling parameters of different cooling zones in the production process are shown in Table 2.

(1)$v={\displaystyle \frac{T_1-T_2}{t}}$

In the actual production process of the pellet plant, the cooling rates are as follows: C-1 is 27.5 °C/min, C-2 is 35 °C/min, C-3 is 17.24 °C/min, and C-4 is 5.5 °C/min. When the air velocity is 6 m³/h, the corresponding cooling rate stability is approximately 20 °C/min. When the air velocity is 4 m³/h, the corresponding cooling rate stability is about 17 °C/min, and when the air velocity is 2 m³/h, the corresponding cooling rate stability is around 13 °C/min. Therefore, the pre-experimental parameters selected for the air velocities are 6 m³/h, 4 m³/h, and 2 m³/h.

The metallurgical properties of the iron ore pellets were evaluated using methods consistent with previous research. The pellets’ reduction index (RI), reduction swelling index (RSI), and softening-melting properties were primarily determined. The charge’s permeability index (S-value) was calculated according to Equation (2), where ΔP denotes the maximum pressure drop of the charge during the softening–melting experiments. The S-value represents the permeability of the charge layer during the softening-melting process, indirectly reflecting the viscosity of the slag [28].

(2)$\mathrm{S}={\int }_{T_s}^{T_d}∆P·dT(\mathrm{k}\mathrm{P}\mathrm{a}·°C)$

The microstructures of roasted pellets were characterized by optical microscopy (OPTON, Beijing, China) and scanning electron microscopy (ZEISS, Oberkochen, Germany) [29].

2.3. Simulation Analysis

We modeled the pellet cooling process using CFD numerical simulation to establish the benchmark mathematical model based on the on-site annular cooler. Figure 2a shows the schematic diagram of the actual annular cooler. The red frame in Figure 2a shows the cross-section of the cooling equipment making the base for our modeling. The corresponding three-dimensional model of the cross-section of the annular cooling equipment was established. In the simulation process, we approximated that the equipment is symmetrical in the y-z plane, so the 1/2 model was used as the object of study [30,31,32,33]. Figure 2(b-1) shows the physical model developed, and Figure 2(b-2) shows the mesh.

The gas phase is air, with density based on the ideal gas equation of state, gas viscosity determined using Sutherland’s law, thermal conductivity derived from kinetic theory, and heat capacity based on a piecewise polynomial formula. The solid phase comprises pellets with a constant density of 2100 kg/m³ and a particle diameter of 12 mm. The thermal conductivity of the solid phase is 9.8 W/(m·K), and the average specific heat is set to 1218 J/(kg·K). Based on the physical model, this study makes the following assumptions. The variation in pellet volume is neglected, and porosity is considered constant. The pellet region is regarded as a porous medium. Chemical reactions during cooling are ignored. Radiant heat transfer is negligible, and only thermal conductivity between gases, thermal conductivity between solids, and convective heat transfer between gases and solids are considered. The running speed of the trolley is ignored as it is much lower than the air velocity. The cooling gas temperature in the simulation process is set to 20 °C. Table 2 shows the actual cooling equipment parameters. The initial temperature of the solid phase is 1473 K (1200 °C). This study focuses on the heat transfer process between the cooling air and the pellets in the annular cooler, using enthalpy as an indicator of heat recovery efficiency [34].

Figure 2(c-1) shows the temperature distribution of the cross-section of the annular cooler, with low temperatures at the center and edges of the solid at the same height. A similar trend is observed in the gas temperature. Figure 2(c-2) shows the temperature variation with time at different locations in the cooling process. In the same annular cooler zones, the exhaust gas temperature gradually decreases with increasing pellet cooling time. Due to different air velocities in different annular cooler zones, there are significant differences in exhaust gas temperature. There is a sudden change when entering different annular cooler zones, and the bottom solid temperature decreases faster than the exit solid temperature with increasing cooling time. This is because the bottom solid material is in direct contact with the 20 °C cooling gas, resulting in fast heat transfer [35].

In contrast, the top solid material is heated by the gas, leading to slower heat transfer. The temperature change trend of the exported gas and solid phases is similar. Still, the solid temperature is less affected by the air velocity in the annular cooling zone, and the temperature difference between the two phases gradually decreases as the gas and solid phases exchange heat.

Figure 2(d-1) shows the actual temperature distribution of the annular cooler. Figure 2(d-2) shows the temperature distribution obtained by simulation. The simulation results closely match the exact temperature, indicating that the model can accurately predict and simulate field operations. The simulation process used the actual air velocity from Table 2, set to 1.0 as a standard, and analyzed the effects of cooling air velocity on cooling time, cooling rate, enthalpy change, and heat recovery of the cooling system at 0.70, 0.85, 1.00, 1.15, and 1.30 times the standard air velocity.

3. Results and Discussion

3.1. Effect of Varying Cooling Air Velocity on Pellet Solidification Properties

3.1.1. Effect of Varying Cooling Air Velocity on Cooling Time

Figure 3 shows the effect of cooling air velocity on pellet performance. As seen in Figure 3a, with the increase in cooling air velocity, the time required for the pellets to cool to room temperature gradually decreases, and the cooling rate becomes faster. When the air velocity is 6 m³/h, it takes 34 min to cool to below 100 °C; when the air velocity is 4 m³/h, it takes 41 min; and when the air velocity is 2 m³/h, it takes 51 min to reach below 100 °C.

The pellet cooling process can be divided into three stages. The first stage (1100–600 °C) is the rapid cooling stage, with a cooling rate of 20–50 °C/min. This temperature interval corresponds to C-1 and C-2, focusing on recovering residual heat in the annular cooling process. Second Stage (600–200 °C): In this stage, the cooling rate is reduced to 10–20 °C/min. The gas temperature is relatively low, the cooling time is shorter, and the hot air is used for the blast drying section. Third Stage (200–100 °C): In this stage, the temperature gradient between the pellets and room temperature is reduced, the cooling rate decreases to 0–10 °C/min, and the temperature drop process becomes slower, making it more challenging to recycle the subsequent hot air.

Cold compressive strength measurements were carried out on the cooled pellets under different air velocity conditions. Figure 3b shows the effect of air velocity changes on the compressive strength of the pellets after cooling. The lower the air velocity during the cooling process, the longer the cooling time, which can effectively improve the compressive strength of the pellets. When the air velocity is 2 m³/h, the average compressive strength of the pellets is 2189 N. When the air velocity is 6 m³/h, the average compressive strength of the pellets is 1656 N. By reducing the air velocity during the cooling process, the error value of pellet compressive strength is gradually reduced, and the uniformity of pellet compressive strength is improved.

3.1.2. Effect of Cooling Air Velocity on the Chemical Composition

Table 3 shows the effect of different cooling air velocities on the chemical composition of the pellets. The total iron (TFe) content of the pellets is approximately 64%. Changes in air velocity affect the quality of the pellets, particularly the FeO content. The FeO content of the pellets varied significantly, with the total FeO content of the roasted pellets gradually increasing as the cooling air velocity increased.

3.1.3. Effect of Cooling Air Velocity on the Microstructure of Pellets

The effect of air velocity variations on the structure of the cooled mineral phase of the pellets was analyzed using light microscopy. Figure 4 shows the impact of air velocity variation on the mineral phase structure during pellet cooling. Figure 4a–d depict the microstructure of the pellets under a cooling condition of 2 m³/h, Figure 4e–h show the microstructure under a cooling condition of 4 m³/h, and Figure 4i–l illustrate the microstructure under a cooling condition of 6 m³/h. The light microscope images reveal a more pronounced Fe₂O₃ recrystallization phenomenon. With an increase in cooling air velocity, the recrystallization process of the interconnection between grains is affected to a certain extent. The mineral phase becomes imperfectly crystallized, presenting a dispersed, point-like crystalline state. In severe cases, microcracks may appear inside the pellet due to the aggregation of internal stresses, resulting in a sharp reduction in the strength of the pellet.

The effect of cooling air velocity on the microstructure of the pellets was analyzed using a scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Figure 5a–c show the microstructure of the pellets after cooling at an air velocity of 2 m³/h, and Figure 5d–f show the microstructure after cooling at an air velocity of 4 m³/h. Figure 5g–i show the microstructure after cooling at an air velocity of 6 m³/h.

Figure 5a shows a denser structure compared to Figure 5d,g, with relatively fewer voids and smaller microcracks inside the pellet. This indicates that reducing the cooling air velocity helps release internal stresses during the cooling process and reduces the number of microcracks within the pellet. Conversely, a lower cooling rate contributes to the recrystallization and densification of the pellet. At an air velocity of 6 m³/h, more pore structures can be observed around the enlarged gangue phase. The rapid cooling and contraction of the veinstone phase leads to increased pore size and decreased pellet compressive strength.

Table 4 shows the elemental composition at the spectrums marked in Figure 5. The white areas represent the Fe₂O₃ phase, the dark grey areas indicate the unreacted quartz (SiO₂) phase, and the light grey areas correspond to the garnet and pyroxene phases formed around the Mg- and Al-enriched silicate. Some pellets also show a greyish-white fayalite phase at the interface between SiO₂ and Fe₂O₃. The low-melting iron olivine phase is formed by the reaction of FeO with SiO₂ at about 1200 °C, while the solid-phase reaction of bentonite with quartz phases mainly forms garnet.

3.2. Simulation Analysis of Cooling Airflow on Cooling Performance

3.2.1. Effect of Cooling Air Velocity on Gas–Solid Phase Temperature

Figure 6 shows the effect of cooling air velocity on the gas–solid phase temperature. Figure 6a shows that the export gas temperature over time under different cooling air velocities follows the same trend: the higher the air velocity, the faster the cooling rate, and the shorter the time required to cool to the same temperature. Specifically, the gas temperatures after a cooling time of 48 min at 0.7, 0.85, 1.0, 1.15, and 1.3 times the air velocity were 87 °C, 71 °C, 62 °C, 55 °C, and 51 °C, respectively. As the air velocity increased, the gas temperature at a cooling time of 48 min decreased. When the air velocity increased from 0.7 to 1.3 times, the air velocity increased by 86%, and the gas temperature decreased by 41%. Increasing the air velocity from 0.7 to 1.0 times increased the air velocity by 43% and reduced the gas temperature by 29%, with the temperature decrease diminishing as the air velocity increased.

Figure 6b shows that the variation in solid temperature is similar to that of the gas temperature, although the solid temperature is higher. The solid temperatures after a cooling time of 48 min at air velocities of 0.7, 0.85, 1.0, 1.15, and 1.3 times were 149 °C, 129 °C, 117 °C, 109 °C, and 103 °C, respectively. An increase in the air velocity from 0.7 to 1.3 times resulted in a 31% decrease in solid temperature.

3.2.2. Effect of Cooling Air Velocity on Cooling Rate and Enthalpy

The cooling rate of pellet ore in different sections of the annular cooling equipment was calculated. The sections are defined as follows: C-1 is 0–14.5 min, C-2 is 14.5–24.5 min, C-3 is 24.5–39 min, and C-4 is 39–48 min. The effect of cooling air velocity on the cooling rate is shown in Figure 7a. The cooling rate of C-1 increases with increasing air velocity, while the cooling rates of C-2 to C-4 decrease with increasing air velocity. At air velocities of 0.7 and 0.85 times the standard air velocity, the cooling rate of C-2 is the highest. However, as the air velocity continues to increase, C-1 exhibits the highest cooling rate and the total cooling rate increases with the air velocity.

Figure 7b illustrates the effect of cooling air velocity on the cooling rate and heat recovery. The heat recovery of the flue gas from C-1 to C-3 in on-site production was analyzed, and the enthalpy produced by different cooling zones at various air velocities was calculated. The total enthalpy of the gas was also determined. The average air outlet temperature decreases as the inlet air velocity increases.

3.2.3. Effect of Cooling Air Velocity on Heat Recovery

The exhaust gas from the first zone of the annular cooler goes directly into the rotary kiln. The exhaust gas from the second zone goes directly into the preheating zone of the chain grate machine, and the exhaust gas from the third zone goes directly into the blasting and drying zone of the chain grate machine. The waste heat utilization rate of the annular cooler is calculated using Equation (3). Where ${\mathrm{Q}}_{1\mathrm{L}}^{\prime }$ is the heat recovery for one cooling circuit, ${\mathrm{Q}}_{2\mathrm{L}}^{\prime }$ is the heat recovery for two cooling circuits, and ${\mathrm{Q}}_{3\mathrm{L}}^{\prime }$ is the heat recovery for three cooling circuits. ${\mathrm{Q}}_{\mathrm{total}}$ is the total heat of the cooling circuit.

(3)$\mathsf{\eta }={\displaystyle \frac{{\mathrm{Q}}_{1\mathrm{L}}^{\prime }+{\mathrm{Q}}_{2\mathrm{L}}^{\prime }+{\mathrm{Q}}_{3\mathrm{L}}^{\prime }}{{\mathrm{Q}}_{\mathrm{total}}}}\times 100\%$

Figure 8 shows the effect of cooling air velocity on the heat recovery of the annular cooling system. The heat recovery efficiency is 93.17% at the actual cooling air velocity in the field, and it increases to 94.04% when the air velocity is increased by a factor of 1.15. The rate of increase slows when the air velocity ratio is more significant than 1.15. Simultaneously, the compressive strength of the pellets tends to decrease as the cooling air velocity increases. Therefore, within the range of qualified quality of actual pellet ore, the appropriate air velocity ratio should be selected between 1.0 and 1.15 of the existing parameters, which is more suitable.

3.3. Effect of Cooling Air Velocity on the Metallurgical Properties of Pellets

The effect of cooling air velocity on the metallurgical properties of the pellets is shown in Table 5. At a 2 m³/h cooling air velocity, the pellet reduction index (RI) is 66.8%. When the air velocity is 6 m³/h, the RI of the pellet is 71.5%. The higher RI is attributed to faster cooling, which results in more developed internal pores within the pellet, allowing the reduction gas to penetrate better and participate in the reaction. As the cooling air velocity increases, the pellet’s reduction swelling index (RSI) initially shows an increasing trend, followed by a decrease and then another increase. However, it remains below the 20% limit for malignant expansion. At a cooling air velocity of 2 m³/h, the RSI of the pellet is 16.50%. When the air velocity is 6 m³/h, the cooling rate is too fast, leading to a higher RSI of 18.90%. The softening-melting characteristics of the pellets cooled at different air velocities are shown in Figure 9. As the cooling air velocity increases, the pellet’s initial softening temperature (T10) decreases. At a cooling air velocity of 6 m³/h, the T10 is only 985 °C, whereas at 2 m³/h, the T10 is 1123 °C. Additionally, the softening interval gradually widens with increasing air velocity. With higher cooling air velocities, the S value, which indicates the permeability of the adhesion zone, gradually decreases. At a cooling air velocity of 6 m³/h, the S value reaches 191.39 kPa·°C, indicating the increased permeability of the adhesion zone.

4. Conclusions

The effect of cooling air velocity on the cooling rate, microstructure, energy recovery, and metallurgical properties of the pellets has been analyzed in detail through experiments and simulations. The following conclusions can be drawn:

• (1). With the increase in cooling air velocity in the experiment, the cooling rate of the pellets gradually increased, and the unevenness of pellet quality also increased. At an air velocity of 2 m³/h, the average compressive strength of the pellets is 2189 N. When the cooling air velocity is higher, the content of garnet, fayalite, and other low melting point phases increases.

• (2). As the simulated air velocity increased, the gas temperature decreased by 41%, and the solid temperature decreased by 31% for a cooling time of 48 min. The cooling rate increases with increasing air velocity. The total enthalpy of the flue gas decreases and then increases with the rise of the air velocity in different annular cooling zones from high to low, which are C-1, C-2, and C-3, respectively. The heat recovery rate increases with increased air velocity, and the heat recovery rate rises to 94.04% with increased air velocity by 1.15 times.

• (3). The effect of cooling air velocity on the metallurgical properties of the pellet is mainly reflected in the impact on the formation of low-melting liquid phases such as FeO and iron olivine. With the gradual increase in cooling air velocity, the softening start temperature of the pellet decreased significantly. With the increase in air velocity from 2 m³/h to 6 m³/h, the melting droplet interval decreased from 193 °C to 105 °C, and the melting droplet interval gradually became narrower. With the increase in cooling air velocity, the S value gradually becomes smaller, and the permeability of the adhesion zone increases.

Author Contributions

Conceptualization, J.Z. and Z.L.; methodology, software, L.M. and S.L.; validation, formal analysis, resources, Q.C. and L.H.; investigation, data collation: L.M. and H.J.; writing—original draft preparation, data collation, L.M.; writing—review and editing, J.Z., Z.L. and Y.W.; funding, J.Z., Z.L. and Y.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Schematic diagram of the cooling experiment for the iron ore pellets ((a) green pellet preparation; (b) drying of green pellets; (c) preheating and roasting; (d) cooling of iron ore pellets).

Figure 2. Schematic diagram of cooling simulation of the iron ore pellets ((a) schematic diagram of pellet cooling equipment; (b) modelling of iron ore pellet cooling; (c) iron ore pellet cooling baseline model; (d) accuracy calibration of cooling models of iron ore pellet).

Figure 3. Influence of cooling air velocity on pellet performance ((a) cooling time; (b) pellet strength).

Figure 4. Light microscopic analysis of pellets at different cooling air velocities ((a–d) 2 m3/h; (e–h) 4 m3/h; (i–l) 6 m3/h).

Figure 5. SEM-EDS analysis positions of pellets at different cooling air velocities ((a–c) 2 m3/h; (d–f) 4 m3/h; (g–i) 6 m3/h).

Figure 6. Effect of cooling gas on gas–solid phase temperature ((a) gas temperature; (b) temperature of pellet).

Figure 7. Effect of cooling air velocity on cooling rate and enthalpy ((a) cooling rate; (b) enthalpy).

Figure 8. Effect of cooling air velocity on heat recovery.

Figure 9. Softening–melting characteristics of pellets at different cooling air velocities.

Table 1

Chemical composition and physical properties of iron ore pellet raw materials.

Raw Material	Chemical Composition/wt.%						LOI/%	Physical Property
	TFe	FeO	SiO2	Al2O3	CaO	MgO		Below 74 μm Particle Size/%	Blaine Surface Area/(cm2·g−1)
Concentrate-1	67.07	19.65	5.06	0.22	0.54	0.51	−1.35	87.01	1837
Concentrate-2	65.43	29.05	7.84	0.14	0.25	0.29	−3.40	97.78	2188
Bentonite	3.58	0.52	55.00	14.18	3.92	3.02	12.66	100	-

Note: Loss on ignition at 1025 °C for one hour in air.

Table 2

Actual cooling parameters of cooling equipment.

Cooling Parameters	Cooling 1 Zone (C-1)	Cooling 2 Zone (C-2)	Cooling 3 Zone (C-3)	Cooling 4 Zone (C-4)
Temperature of outlet gas/°C	1100	700	350	100
Cooling area/m2	46	30	46	28
Cooling times/min	14.5	10	14.5	9
Cooling air velocity/(Nm3·h−1)	198,000	180,000	180,000	180,000

Table 3

Chemical composition of iron ore pellets at different cooling air velocities.

Pellets	Air Velocity/(m3·h−1)	Chemical Composition/wt%									LOI/%
		TFe	FeO	CaO	SiO2	MgO	Al2O3	TiO2	K2O	Na2O
Pellet-1	2	64.31	0.29	0.34	6.45	0.38	0.33	0.02	0.05	0.10	<0.05
Pellet-2	4	63.75	0.46	0.33	7.31	0.38	0.36	0.02	0.03	0.07	<0.05
Pellet-3	6	63.86	0.55	0.33	7.29	0.38	0.33	0.02	0.04	0.08	<0.05

Table 4

Elemental composition at the spectrums marked in Figure 5.

Spectrum	Chemical Composition/wt%								Mineral Phase
	Fe	Ca	Si	Mg	Al	K	Na	O
1	81.30	-	-	-	-	-	-	18.70	Hematite
2	-	-	62.42	-	-	-	-	37.58	Quartz
3	3.68	4.85	33.33	3.50	10.42	0.53	1.66	42.04	Silicate
4	17.69	2.22	15.31	8.43	14.34	0.75	2.50	38.76	Garnet

Table 5

Metallurgical properties of pellet with different cooling air velocities in the experiment.

Material	Air Velocity/(m3·h−1)	Metallurgical Properties
		RI/%	RSI/%	T10/°C	T40/°C	TS/°C	Td/°C	ΔP/kPa	S/kPa·°C
Pellet-1	2	66.82	16.50	1123	1211	1330	1523	0.348	563.64
Pellet-2	4	63.00	10.68	1048	1187	1327	1522	0.370	611.52
Pellet-3	6	71.53	18.90	985	1194	1412	1517	0.236	191.39

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