Study on the Basic Characteristics of Iron Ore Powder with Different Particle Sizes

Abstract

In order to study in depth the differences in basic characteristics between iron ore fines commonly used by a steel company, and guide the sintering performance plant to choose the best ore allocation method, experimental studies on the basic characteristics of seven iron ore powders of three sizes were carried out using micro-sintering equipment, mainly including assimilation properties, liquid phase fluidity, and bonding phase strength. The results of the research showed that with the increase of the iron ore powder particle size, the assimilation of the seven iron ore powders showed an overall decreasing trend, deteriorating fluidity and decreasing bonding phase strength. Among them, the overall fluidity of iron ore powder A was poor, and the fluidity of iron ore powder B varied greatly between different particle grades, and the fluidity of iron ore powder C was more balanced and its bonding phase strength was high, while the overall bonding phase strength of iron ore powders B and E was low. The results of the study provide a theoretical basis for optimal ore allocation in sintering plants.

1. Introduction

In recent years, China’s steel industry has ushered in rapid development, and steel output has increased year by year. With the growth of iron and steel production, the consumption of iron ore is also increasing year by year. At present, iron ore relies heavily on imports in China. There are many kinds of minerals, and the grade and performance of iron ore are also different. The characteristics shown in sintering are not the same, so it is necessary to study the basic characteristics [1,2]. At present, the main charge for blast furnace ironmaking is sinter in China, and its quality seriously affects blast furnace production. According to actual production, the economic and technical indicators in sintering production not only are related to the content of each element of iron ore, but also depend on the basic characteristics of sintering under high-temperature conditions of iron ore [3,4]. In the production of iron ore entering the sintering process, it is not the ultra-fine-grained iron ore powder, but there are various grades. Therefore, it is more suitable for actual production to study the basic characteristics of several larger particle sizes of iron ore powder.

The purpose of this paper is to improve the overall quality of the sinter and to improve the production level. Because of the high-temperature basic characteristics of different iron ore powders, researchers from universities and enterprises have done some research by measuring the basic properties of different types—200-mesh iron ore powders, such as assimilation, liquid phase fluidity, and binder phase strength, and then comprehensively evaluating the sintering properties of the iron ore powders [5,6,7]. For example, Zhang et al. used fine-grained iron ore powder to study the basic characteristics of several iron ore powders commonly used in an iron and steel enterprise and discussed the granulation according to the micromorphology [5]. Liu et al. studied the influence of relevant parameters of basic characteristics of iron ore powder on the properties of sinter [6]. Wang et al. studied several typical basic characteristics of five kinds of iron concentrates and two kinds of iron powders, gave suggestions on complementary ore blending of iron ores and provided guidance for optimizing sintering and pellet ore blending [7]. According to the particle size distribution of iron ore powder in sintering production, there are not only fine particles, but more iron ore powder particles of larger size. To avoid differences in the sintering properties of the same iron ore powder due to different particle sizes, this study carried out the assimilation, liquid phase fluidity indices, and binding phase strengths of several iron ore powders commonly used in a steel mill with different particle sizes. The experiment accurately analyzed the basic characteristic data of iron ore powder of different grades.

Taking the obtained test results as a reference, it guides actual production, helps enterprises to optimize ore types and use them in a reasonable combination and comprehensively considers the choice to realize the optimized ore blending of iron ore powder sintering, thereby improving the output and quality of sintering ore and reducing the production cost of enterprises and guiding the adjustment of sinter raw material structure.

2. Materials and Methods

2.1. Materials Properties

The raw materials used in this test came from seven kinds of iron ore powders that are used daily in the sintering production of a domestic steel plant. The A, B, C, F, and G powders were from Australia, the D powder was from Brazil, and the E powder was from India. Among them, A, B, C, and F powders were limonite, D and E powders were hematite, and G was a new iron ore powder, which was martite. The chemical compositions are shown in

Table 1. Chemical compositions of iron ore powders used in the experiment (%).

Sample	TFe	SiO2	CaO	MgO	Al2O3	P	S	LOI
A	58.19	5.55	0.06	0.13	3.95	0.135	0.039	7.2
B	58.64	5.64	0.03	0.09	2.70	0.079	0.034	6.8
C	62.95	5.04	0.02	0.08	1.45	0.070	0.016	4.5
D	57.39	5.04	0.02	0.08	6.45	0.060	0.073	6.0
E	60.50	4.66	0	0.09	2.52	0.079	0.031	5.5
F	56.79	6.33	0.04	0.10	3.34	0.080	0.030	9.0
G	59.20	5.72	0.05	0.12	3.26	0.046	0.038	5.5

. From the data in the table, it can be seen that the iron grades of the ore powders were generally high and among them, C iron ore powder had the highest iron grade. The mass fraction of SiO₂ of F powder was the highest, accounting for 6.33%. The mass fractions of CaO and MgO were overall lower. The Al₂O₃ level of C ore powder was lower, and the rest were above 2.5%.

The pros and cons of the sintering effect are restricted by many factors, among which granulation is one of the most influential links. Granulation refers to the process in which fine particles adhere to coarse particles under the action of water or fine particles aggregate with each other and form small balls. Its purpose is to improve the particle size composition of the mix and reduce the content of fine particles in the mix [8]. If the granulation effect is too poor, the permeability of the material layer will be poor, thereby affecting the sintered ore yield. If the granulation effect is too good, the particle size composition of the mixture will be too large, the sintering gas permeability will be improved, the sintering speed will be accelerated, and the liquid phase generation during the sintering process will also be affected. Therefore, reasonable raw material particle sizes and mature granulation processes are an important part of iron ore sintering.

The particle size composition analysis of the mineral powder used in the experiment, as seen from Figure 1, showed the proportion of particle size less than −5 mm in all iron ore powders generally exceeded 70% and especially that in A iron ore powder it reached 86.77%. Therefore, the basic characteristics of iron ore powder of −1 mm, −3 mm, and −5 mm particle sizes were studied, and the basic characteristics of different particle sizes were studied to provide a reference for sintering production. The study of the structure of the sintered mix spheres showed that the mix granules were composed of core particles and adhering particles. It is called quasi-particles, and the formation of quasi-particles has a great relationship with the raw material particle size of the iron ore powder used. The ideal particle size of the nuclei was greater than 1−3 mm, while the intermediate particles had the size range of 0.25−1.0 mm, which were difficult to granulate [9]. As shown in Figure 2, the strength of the mixed pellets formed could be guaranteed.

2.2. Experiment Method

This experiment used a micro-sintering test device through infrared heating, as shown in Figure 3. The equipment was purchased from Chongqing Safety Production Science Research Co., Ltd. and was connected to a computer. The temperature was regulated by micro-sintering software in the computer, there was a special area outside the machine to display the temperature inside the furnace, and the temperature inside the furnace was also updated in real time on the computer. The interior of the furnace body generated infrared heat through a resistance wire, and the high temperature-resistant corundum tube fed the test sample into the interior of the furnace body to roast the test sample using high temperature. There was an industrial camera at a fixed position beside the furnace body for real-time shooting, and the flow state of the iron ore powder in the filmed video was transferred to the computer for further analysis. Finally, the assimilative temperature, liquid phase fluidity index, and binder phase strength of the seven ore powders were measured and studied with the aid of a micro-sintering integrated measurement system developed by the manufacturer. The test data obtained below were the average of 5 test results, among which the fluctuation range between the minimum assimilation temperature and the average value was ±15 °C, the difference between the liquid fluidity index and the average fluctuation range was ±0.5, and the fluctuation range of the bond phase strength and the average value was ±30 N. The error value increased slightly with the increase of particle size.

First, mineral powders and CaO reagents required for the test were placed into a drying oven set at 120 °C for 180 min and let stand to cool after drying. Then, −1 mm, −3 mm, and −5 mm sieves were used to sieve out the granular mineral powder in the test, which was placed in a dry bag for storage. After weighing the reagents needed for the test with an electronic balance at an accuracy of 0.001 g, using dry powder compaction, reagents were placed in molds, and a tablet press was used to place the mold containing the reagent under a pressure of 10 MPa for 120 s (the pressed sample size was described in subsequent experiments). Then, the sample was taken out and placed on a crucible. It was transferred to a furnace and calcined according to the set temperature rise curve shown in Figure 4 [10], with nitrogen atmosphere for 4−18 min and air atmosphere for the rest of the time. Then, the basic characteristic data of the ore powder were measured.

Assimilation: 0.8 g of the iron ore powder sample was weighed to make a cylinder with a diameter of 8 mm, and 2.0 g of the CaO reagent were weighed and pressed into the cylinder with a diameter of 20 mm. The pressed iron ore powder sample was placed in the center above the pressed CaO sample, as shown in Figure 5. After entering the oven, a mouse was used to frame the sample in the program, and the software automatically recognized the line, as shown in Figure 6. Then, according to the preset heating curve of the system, the simulated sintering test was carried out. At this time, the camera conducted real-time monitoring. When the iron ore powder cake reacted with CaO and the height was reduced to 75% of the original height, the determination system determined that an assimilation reaction occurred at this time and the test was over. The sintering device was controlled to stop heating, and the measurement system recorded the temperature at the time when the test was stopped, which was the minimum assimilation temperature of the current particle-size mineral powder.

Liquid phase mobility: According to the content of SiO₂ and the mass fraction of CaO in the iron ore powder used in the experiment, the amount of CaO reagents to be added was calculated according to the binary alkalinity of 4.0. After weighing with a balance, the iron ore powder was mixed and stirred evenly [11]. Then, 0.8 g of the mixed sintered adhesive powder was weighed with a balance, poured into the mold, pressed into a cylinder with a diameter of 8 mm and placed in the middle of a crucible, as shown in Figure 7. Then, the crucible was placed on the corundum tube. A camera was fixed above to take pictures, and then, the picture taken by the lens in the measurement system were selected. The system automatically calculated the original area c of the sample before the test, that is, the surface area of the sample in Figure 7, and then sent it to the furnace body. After heating according to the set heating curve to reach the set temperature, the sample remained in the furnace for 4 min, and the test was over. Next, the device automatically moved the crucible out of the furnace, and the sample formed a liquid phase at this time. The camera fixed above was used to shoot the crucible vertically to obtain a picture of the crucible after the iron ore powder flowed. The captured picture can be seen in Figure 8. After the system determined the reaction crucible with the mouse frame of iron ore powder, the liquid phase area b was automatically measured, and the liquid phase mobility index was given, with the interface shown in Figure 8. The liquid-phase fluidity index a was obtained by Equation (1):

$$a=\frac{b-c}{c}.$$

(1)

Binding phase strength: According to the content of SiO₂ and the mass fraction of CaO in the iron ore powder used in the experiment, the amount of CaO reagents to be added was calculated according to the binary alkalinity of 2.0 [12]. The CaO reagents were mixed with the iron ore powder and stirred evenly after weighing with a balance. Then, 0.8 g of the mixed sintered adhesive powder was weighed with a balance, poured into a mold, pressed into a cylinder with a diameter of 8 mm and placed in the center of a crucible. As shown in Figure 9, the samples shown were −1 mm in size. The crucible was placed on a corundum tube, and the software was clicked to start execution button. The system controlled the corundum tube to feed into a heating furnace and controlled the furnace heating. After the sample stayed in the furnace for 4 min, the furnace body was automatically removed after the test. Then, the heated cake was taken out, and the sample was placed vertically on the press to measure the crushing strength. The button was pressed to start the test. The linear motor automatically added force and the display output real-time data, when it touched the sample. When the sample was broken, the maximum force number (strength value) was locked, and the linear motor ran in the reverse direction until the reset state stopped. The device shown in Figure 10 is a DL-2 type, and the manufacturer is Dalian Research & Design Instituteof Chemical Industry.

3. Results

3.1. Assimilation

The assimilation of iron ore powder refers to the ability of iron ore powder to react with CaO during the sintering process, which indicates the difficulty of generating a liquid phase. The lowest assimilation temperature characterizes the assimilation of iron ore powder. The lower the assimilation temperature, the better the assimilation of iron ore powder. The stronger the reactivity with CaO, the easier it is to generate a liquid phase and the more effectively the sinter will be consolidated [11]. The minimum assimilation temperature of various iron ore powders with different particle sizes was used in the experimental. The samples at the end of the experimental are shown in Figure 11. This can be obtained in Figure 12. It can be seen from Figure 12 that the assimilation temperature of B, E, and F iron ore powders was lower and the assimilation was better. The minimum assimilation temperature of D and G mineral powders was the highest among the seven mineral powders tested in this experimental, and the assimilation was poor. It is recommended to be used with other mineral powders with good assimilation properties. With the increase of particle size, the minimum assimilation temperature generally showed a decreasing trend. The minimum assimilation temperature largely decreased more significantly, when the particle size increased from −1 mm to −3 mm. Due to the increase in particle size, the contact area between individual mineral powder particles and calcium oxide increased. After the small cake was made, the surface tension of the mineral powder particles inside became smaller. After the heating reaction, the small cakes made of large particles were more prone to collapse than the small cakes made of small particles. However, the change in minimum assimilation temperature was not obvious, when the particle size increased from −3 mm to −5 mm. After analysis, since the change of the contact area between the two groups of selected iron ore powder particles and the CaO cake was not as obvious as the increase from −1 mm to −3 mm, the effect on assimilation was not so great.

The mass fractions of MgO and Al₂O₃ also affected the assimilation of iron ore powder. The increase of the mass fraction of MgO in the sample led to the increase of the minimum assimilation temperature, because the MgO is a compound with a high melting point. In high-temperature solid-phase reactions, MgO forms a refractory material in a sintered material, which increases the liquidus temperature [13]. At the same time, MgO reduces the formation of calcium ferrite during sintering [14], so the increase of MgO content will make the assimilation worse. The content of Al₂O₃ also affects the assimilation. The more the content, the stronger the assimilation. Because of the mineralization reaction of Al₂O₃ with iron ore powder, the generated aluminate compound, semi-calcium aluminate (CA₂), has a low melting point and reacts with calcium ferrite (CF) and Fe₂O₃ to form calcium ferrite-containing aluminum (CFA). After the formation of CFA, an appropriate amount of SiO₂ solid solution promotes the development of acicular SFCA [15], and at the same time, Al₂O₃ can also accelerate ion diffusion, which promotes the transformation of magnetite to hematite, and promote the formation of composite calcium ferrite [16].

3.2. Liquid Phase Fluidity

The liquid phase fluidity of iron ore powder refers to the ability of iron ore powder to react with calcareous flux to form the liquid-phase bonding of surrounding materials during the sintering process. The quality of flow has a crucial influence on the quality of sinter, which is usually expressed by the fluidity index [17]. At 1250 °C, the iron ore powder with three particle sizes could not form a liquid phase. By increasing the test temperature, it was finally determined that the temperature was increased to 1350 °C, and particles with all three sizes had a liquid phase. The samples at the end of the test, shown in Figure 13, were −1 mm in size, and the fluidity indices of liquid phase under three grades of seven kinds of mineral powders as shown in Figure 14 were obtained. Different types of iron ore powders had different compositions, and the liquid phase fluidity indices measured in the microsintering system were also quite different. In the sintering production process, the liquid phase is too small to effectively bond other surrounding raw materials, which is not conducive to the sintering strength. On the other hand, if the liquid phase is excessively generated, it is easy to form a macroporous and thin-walled structure, which will lead to the poor gas permeability of sintered ore, and eventually lead to a decrease in strength. On the whole, the fluidity indices of B, F, and G iron ore powders were better [18]. It can be seen from Figure 14 that with the increase of iron ore powder size, the liquid phase fluidity index decreased accordingly, due to the increase in particle size and the number of mineral powder particles participating in the reaction decreases. This led to a reduced contact area with the CaO reagent and an ultimately poorer fluidity index.

The factors affecting the liquid phase fluidity index of iron ore powder are the mass fraction of MgO, the mass fraction of SiO₂, the mass fraction of Al₂O₃, and the sintering temperature. The assimilation of iron ore fines decreases with the increase of MgO mass fraction in iron ore fines, while reducing the liquid phase fluidity. This is due to the elevated magnetite content caused by the solid solution of Mg²⁺ in the magnetite lattice, which will reduce the mass fraction of Fe₂O₃, inhibit the formation of calcium ferrite and reduce the liquid phase fluidity of iron ore powder [14]. As the mass fraction of SiO₂ increased, the amount of CaO reagents added in the experimental also increased. The more binder phase calcium ferrite is formed during sintering, generating a larger area with a liquid phase, the better the corresponding liquid phase fluidity will be [10]. If the mass fraction of SiO₂ is too high, there will be calcium silicate compounds that increase the viscosity of the liquid phase and reduce the liquid phase fluidity of iron ore powder [11]. With the increase of Al₂O₃ content, the liquid phase fluidity index will also increase, but too high Al₂O₃ content will affect the liquid phase fluidity, because of the promotion of the formation of the silicate network, the viscosity of the liquid phase increases, and the fluidity of the liquid phase decreases [9,11,19]. In addition to the several factors described above, the sintering temperature also affects the liquid phase fluidity. As the sintering temperature increases, the formation of low-melting compounds will accelerate, thereby increasing the formation rate and improving the fluidity of the liquid phase.

3.3. Binding Phase Strength

Binding phase strength refers to the liquid phase formed by iron ore during sintering. The power consolidates the surrounding core minerals that have not yet melted. It plays a key role in judging the quality of sintered ore. Therefore, in the sintering process, the strength of sinter depends largely on the strengthening of the binding phase, and the consolidation of sinter is achieved by the binding phase [12]. The samples at the end of the test are shown in Figure 15. The samples shown were −1 mm in size, and the binding phase strengths of the seven iron ore powders at three particle sizes are shown in Figure 16.

The test results showed that the binding phase strengths of the seven iron ore powders were quite different. With the increase of particle size, the binding phase strength of iron ore powder decreased. The binding phase strength of each particle size of C iron ore powder was promising With stable and little change in strength, the iron ore powder can be added appropriately in the production. In addition, F and G iron ore powders also have outstanding binding phase hardness. Compound calcium ferrite had a significant increase in strength. Fe₂O₃ in iron ore powder and CaO contacted more fully from the kinetic conditions more conducive to the amount of composite calcium ferrite generation. Iron ore powder with a small particle size mixed with CaO. Iron ore powder particles and CaO contacted more fully, so the bonding phase strength was higher. The liquid phase fluidity and the mass fraction of SiO₂ had also a certain influence on the strength of the binder phase. For example, powder B had a high fluidity index and a low bonding phase strength, due to the fact that too good liquid phase fluidity will lead to poor bonding phase strength. Because there is too much liquid phase, it produced a thin-walled macroporous structure, making it brittle, so reasonable liquid phase fluidity can increase the strength of the bonding phase strength and the increase in the mass fraction of SiO₂ can enhance the binding phase strength. The SiO₂ mass fraction also affects the binding phase strength, and within a certain range, an increase in the SiO₂ mass fraction can increase the liquid phase production, which can improve the binding phase strength [20].

4. Conclusions

(1)

Through the experimental research on the basic characteristics of seven kinds of iron ore powder, the content of MgO, SiO₂, Al₂O₃, and other substances in the iron ore powder and its own microscopic morphology affected the basic characteristics of the iron ore powder.

(2)

The basic characteristics experiment based on the original particle size of ore powder was mainly divided into three particle sizes for comparative analysis. As for the assimilation temperature, with the gradual increase of particle size, the assimilation temperature showed a decreasing trend as a whole. However, due to the large particle size in the process of sample preparation and the positional relationship between the particles, the morphologies of the samples could not be generally unified, so there was a force majeure factor in the assimilation temperature measurement process. Therefore, the assimilation temperature alone cannot determine the performance of the mineral powder.

(3)

Next, the system temperature was set to 1350 °C in the fluidity index determination. In order to show the flow characteristics of each mineral powder more intuitively, with the increase of particle size, the flow index from large to small, it was also observed that the fluidity indices of iron ore powders B, F, and G showed a polarized arrangement, while iron ore powder A showed the smallest fluidity index under each particle size of iron ore powder, similar to iron ore powder E. Therefore, the proportions of these ore powders should be appropriately reduced in the sintering to ensure that the final index of the sintered ore conforms to the actual production on site.

(4)

The final bonding phase strength was the best feature of the data visualization of each mineral powder. The strength of each ore powder varied significantly, and the binder phase strength of each grade of iron ore powders C and F was the best among all the ore powders. The SiO₂ content of C was lower than that of F. If the low-silicon scheme is used for ore blending, the quality of sinter can be guaranteed by increasing the addition ratio of C iron ore powder. Iron ore powder B and iron ore powder E had the worst strength among all the ore powders. Such a binding phase strength cannot guarantee the quality of sintered ore, and it is recommended to reduce the amount used in the subsequent production. It needs to be used with iron ore powder with better basic properties.

According to the basic characteristics of iron ore powder for sintering and blending, the assimilation and fluidity of iron ore powder should be moderate, ensuring that there is sufficient liquid phase and suitable liquid viscosity during the sintering process. The ability to adhere to surrounding substances ensures that the sinter strength meets the requirements of blast furnace ironmaking.