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Effect of firing temperature on properties of magnesia-iron-aluminum spinel bricks

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Abstract: In order to determine the firing temperature when preparing magnesia-iron-aluminum spinel bricks using sintered iron-aluminum spinel as the main raw material. High-purity magnesia with particle sizes of 5 to 3, ≤3, and ≤ 0.088mm (mass fractions of 36%, 28%, and 31% respectively) and ferro-aluminum spinel powder with particle sizes of ≤0.088 mm (mass fraction of 5%) prepared by reaction sintering are used ) as the main raw material. Using pulp waste liquid as a binding agent, after batching, mixing, shaping and drying, the magnesia-iron-aluminum spinel bricks were fired in a tunnel kiln at 1450, 1500, 1550, 1600 and 1650°C respectively. The bulk density, apparent porosity, compressive strength, room temperature flexural strength, thermal shock resistance and kiln-hanging properties were tested, and the physical phase and microstructure of the sample were analyzed. The results show that between 1450 and 1650°C, as the firing temperature increases, the normal temperature compressive strength and normal temperature flexural strength of magnesia-iron-aluminum spinel bricks gradually increase, while the thermal shock resistance gradually decreases. The magnesia-iron-aluminum spinel brick prepared when the firing temperature is 1550°C has a larger volume density and smaller apparent porosity, and has the best kiln-hanging properties. Its main crystal phases are MgO, FeAl₂O₄ and magnesia-iron-aluminum composite spinel.

Keywords: reaction sintered iron-aluminum spinel; firing temperature; magnesia-iron-aluminum spinel brick; kiln skin properties; cement rotary kiln

Alkaline refractory materials such as magnesia bricks, magnesia-alumina bricks, magnesia-chrome bricks, and dolomite bricks are widely used in industries such as building materials, steel, and non-ferrous metals. Among them, magnesia-chrome bricks are the most commonly used. In particular, cement rotary kilns use a large amount of magnesia-chromium bricks due to the need to form a self-protective kiln skin in the firing zone. However, the chromium in magnesia-chrome bricks can form highly toxic hexavalent chromium (Cr⁶*) under high temperature and alkaline conditions and pollute the environment. In order to reduce the harm to the environment, chromium-free refractory materials have made great progress in recent years. The products currently developed include magnesia-aluminum spinel bricks, dolomite bricks, magnesia-zirconium bricks and magnesium-iron-aluminum spinel bricks. However, due to its high CaO content, dolomite bricks are easily hydrated during production, storage, and transportation, which affects product performance. The spinel component in magnesia-alumina spinel bricks can easily cause burn flow of the kiln skin and erosion of spinel minerals under overheating conditions. As a result, the kiln-hanging properties are poor; although magnesia-zirconium bricks have good corrosion resistance and peeling resistance, they are not cost-effective. Through research on the performance of the above products and comparison of the results of use in cement rotary kilns, it was found that magnesia-iron-aluminum spinel bricks have the best performance. Compared with traditional directly bonded magnesia-chromium bricks, magnesia-iron-aluminum spinel bricks have good kiln-hanging properties, corrosion resistance, spalling resistance and thermal shock resistance. It is an ideal chromium-free green and environmentally friendly kiln lining material for the new dry process cement rotary kiln firing zone.

At present, domestic research on magnesia-iron-aluminum spinel bricks mainly uses fused iron-aluminum spinel as raw material, and there are few reports on using reaction-sintered iron-aluminum spinel as raw material to prepare magnesia-iron-aluminum spinel bricks. In this work, magnesia-iron-aluminum spinel bricks were prepared using reaction-sintered iron-aluminum spinel as raw material, and the effect of sintering temperature on the properties of magnesia-iron-aluminum spinel bricks was studied.

Test

Raw materials

High-purity magnesia with particle sizes of 5 to 3, ≤3, ≤0.088 mm and sintered iron-aluminum spinel powder of ≤0.088 mm are used as the main raw materials. The chemical composition is shown in Table 1.

Table 1 Chemical composition of raw materials
Raw material w/%
SiO₂Fe₂O₃Al₂O₃CaOMgOFeO
High purity magnesia0.710.550.341.1697.000.24
Aluminum spinel1.190.1553.770.240.4244.23

Sample preparation

First add 36% (w) 5~3 mm high purity magnesia, 28% (w) ≤3mm high purity magnesia and 5% (w) iron-aluminum spinel powder (≤ 0.088 mm) into the mixer and mix 5 min, then add pulp waste liquid accounting for 6% of the total mass of raw materials and mix for 5 min, then add 36% (w) high-purity magnesia powder (≤0.088 mm) and mix for 8 min. The mixed mud was pressed into a 155 mm × 45 mm × 45 mm sample using a 2100 t automatic hydraulic press. After drying, it was fired in a tunnel kiln at 1450, 1500, 1550, 1600 and 1650°C.

Performance testing

Test the bulk density and apparent porosity of the sample according to GB/T 2997-2000; test the compressive strength of the sample according to GB/T5072-1985. Test the flexural strength of the sample according to GB/T3001-2000. According to YB/T376.1-1995, the normal temperature flexural strength of the test sample after being subjected to water cooling and thermal shock at 1100°C for three times is used to evaluate the thermal shock resistance based on the flexural strength retention rate after thermal shock. Spread 3 mm thick cement clinker slurry in the middle of the two prepared test blocks with dimensions of 30 mm × 30 mm × 60 mm and bond them together. Place them vertically in an electric furnace and keep them at 1500°C for 3 hours. After cooling, press GB/ T 3001-2000 measures the flexural strength at room temperature, and evaluates the kiln-hanging performance of the sample based on the bonding flexural strength. An X-ray diffractometer (D8ADVANCE) was used for phase analysis; a scanning electron microscope (Sirion 200) was used to observe the microscopic morphology of the sample.

Results and discussion

Effect of firing temperature on the normal temperature performance of the sample

Figure 1 shows the effect of firing temperature on the bulk density and apparent porosity of the sample. It can be seen from Figure 1 that as the firing temperature increases, the bulk density of the sample first increases and then levels off, and the apparent pores first decrease and then level off. At 1550°C, it has a larger bulk density and smaller apparent porosity; but after 1550°C, the bulk density and apparent porosity of the sample do not change much, indicating that the sample has been fully sintered.

Figure 1 Effect of firing temperature on sample volume density and apparent porosity

Figure 2 shows the effect of firing temperature on the normal temperature pressure resistance and normal temperature flexural strength of the sample. It can be seen from the figure that as the firing temperature increases, the normal temperature pressure resistance and normal temperature flexural strength of the sample gradually increase. Although the volume density of the sample fired at 1650°C is not the highest, its compressive strength and flexural strength are the highest.

Figure 2 Effect of firing temperature on the normal temperature strength of the sample

It can be seen from Figure 1 and Figure 2 that the magnesia-iron-aluminum spinel brick fired at 1550°C has better room temperature performance.

Effect of firing temperature on high temperature performance

Figure 3 shows the effect of firing temperature on the thermal shock resistance of the samples. It can be seen from Figure 3 that the thermal shock resistance of the sample decreases as the firing temperature increases. This is because the thermal expansion coefficients of alumina spinel and magnesia are very different. When subjected to thermal shock, the sample is prone to micro-cracks. When the sample is fired at 1450°C, relatively less ferro-aluminum spinel reacts. When subjected to thermal shock, the microcracks in the sample are more evenly distributed, which is beneficial to the release of thermal stress, so the thermal shock resistance is better. As the firing temperature increases, the amount of reacted ferro-aluminum spinel gradually increases. When subjected to thermal shock, the number of micro-cracks increases and main cracks are easily formed, thus reducing the thermal shock resistance of the sample.

Figure 3 Strength retention rate of samples burned at different temperatures after being subjected to water cooling and thermal shock at 1100°C for three times

Figure 4 shows the effect of firing temperature on the kiln hanging properties of the samples. It can be seen from Figure 4 that as the firing temperature increases, the bonding flexural strength between the sample and cement clinker first increases and then remains unchanged. The samples fired at 1550°C have the best kiln hanging properties. This is because the Fe₂O₃ and Al₂O₃ in the magnesia-iron-aluminum spinel bricks easily react with the CaO in the cement clinker to form C₂F, C₄AF and other low-melting point minerals with a certain viscosity. Can adhere to the hot surface of lining bricks to form a stable kiln skin. As the firing temperature increases, the amount of low-melting point minerals in the sample gradually increases, which improves the kiln-stickability of the sample.

Figure 4 The effect of firing temperature on the kiln hanging properties of samples

Phase and micromorphology analysis

Figure 5 shows the XRD pattern of magnesia-iron-aluminum spinel bricks prepared at a firing temperature of 1550°C. It can be seen that the main crystal phase of the sample is periclase, and the secondary crystal phases are Mg7.9Al15.43Fe0.58O32 and FeAl₂O₄.

Figure 5 XRD pattern of sample fired at 1550℃

Figure 6 is the SEM photo and EDS spectrum of the magnesia-iron-aluminum spinel brick fired at 1550°C. It can be seen from the figure: EDS analysis at point 1 inside the spinel ring shows that the main components are Al₂O₃, FeO and MgO, and their mass fractions are 55.00%, 43.00% and 1.50% respectively. This is consistent with the chemical composition of the alumina spinel raw material, indicating that the interior of the spinel ring is still composed of the alumina spinel raw material, but the MgO content has slightly increased. The mass fractions of Al₂O₃, FeO and MgO at point 2 on the spinel ring are 45.12%, 14.58% and 38.68% respectively. It shows that this ring is the diffusion zone of alumina spinel and magnesia, generating a spinel solid solution ring. Compared with the internal components of the spinel ring (see point 1), the Al₂O₃ and FeO contents decrease, while the MgO content increases significantly. Point 3 in the figure is magnesia adjacent to the spinel ring. The mass fractions of its main components MgO and FeO are 94.85% and 4.69% respectively, indicating that Fe²* diffuses into magnesia.

Figure 6 SEM photo and EDS spectrum of the sample fired at 1550 ℃

The above SEM-EDS analysis shows that during the preparation process of magnesia-alumina spinel bricks, Mg²* ions in magnesite will diffuse into the interior of the alumina-spinel. The Fe²* ions in the alumina spinel diffuse into the magnesite, thereby forming a spinel ring structure around the alumina spinel raw material. The interior of the spinel ring (see point 1) still maintains the original structure and composition of the aluminous spinel. However, due to the diffusion of some Mg²* ions into the iron-aluminum spinel particles, magnesia-iron-aluminum spinel is formed at the edge of the iron-aluminum spinel particles (see point 2), and the Fe²* ions diffuse into the surrounding MgO matrix (see Point 3), the accompanying volume expansion leads to the formation of micro-cracks, and the decomposition of iron-aluminum spinel and the mutual diffusion of Fe²* ions and Mg²* ions make the magnesia-iron-aluminum spinel brick have good structural flexibility.

Conclusion

(1) The firing temperature is 1450 ~ 1650 ℃. As the temperature increases, the normal temperature compressive strength and normal temperature flexural strength of magnesia-iron-aluminum spinel bricks gradually increase, and the thermal shock resistance gradually decreases. When the firing temperature is 1550°C, it has a larger bulk density and smaller apparent porosity, and the kiln-hanging properties are also the best.

(2) The main crystal phases of the magnesia-iron-aluminum spinel brick prepared when the firing temperature is 1550°C are MgO, FeAl₂O₄ and magnesia-iron-aluminum composite spinel (Mg7.9Al15.43Fe0.58O32). The mutual diffusion of Fe²* ions and Mg²* ions gives the magnesia-iron-aluminum spinel bricks good structural flexibility.

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