Effect Of Magnesium Oxide On Spalling Resistance And Strength Of Alumina-Magnesia Refractories

Alumina-magnesia refractories have been widely used in ladles, seat blocks, permeable bricks, and tilting runners for hot metal desiliconization, achieving satisfactory results. Whether in the form of castables, precast blocks, or unfired bricks, these materials undergo not only sintering shrinkage and thermal expansion but also spinel formation during service. This leads to volume expansion and reduced material density, thereby affecting other properties.

Excessive expansion can cause cracking, lining bulging and spalling, decreased strength, and slag penetration. Insufficient shrinkage or expansion may also result in cracking, slag penetration, and spalling during service. Therefore, only with an appropriate magnesia content and well-controlled spinel formation rate and amount can the shrinkage caused by sintering be effectively compensated, ensuring high strength and preventing spalling, bulging, and slag penetration.

Accordingly, this study investigates the effect of magnesite content on the thermal expansion properties of unfired alumina-magnesia materials.

01 Raw Materials and Experimental Methods

1.1 Raw MaterialsHigh-purity alumina-magnesia materials were prepared using dense corundum, high-purity fused magnesia powder and activated α-alumina micropowder as raw materials.The chemical compositions of the raw materials are given in Table 1.

1.2 Specimen Preparation and Test Methods

The raw materials were batched according to the formulation shown in Table 2, using 2.5% dextrin as the binder and 3% water, followed by mixing. The mixture was then pressed into green specimens with dimensions of Φ15 mm × 50 mm under a pressure of 200 MPa. After natural drying for one day, the specimens were dried at 110 °C for 24 h to obtain the test specimens.

The bulk density and apparent porosity of the specimens were determined by the vacuum water displacement method. The thermal expansion rate from room temperature to 1640 °C was measured using a NETZSCH DIL 402E dilatometer, and the linear change rate after firing was calculated simultaneously (heating rate: 5 °C·min⁻¹). Phase analysis of the specimens was carried out using a Rigaku RINT 2200/PC X-ray diffractometer.

02 Results and Analysis

2.1 Effect of Fused Magnesia on the Properties of Alumina-Magnesia Materials

The effects of fused magnesia on the properties of alumina-magnesia materials are presented in Table 3. It can be seen from Table 3 that:

(1) With increasing content of fine fused magnesia powder, the densification of the material decreases after high-temperature treatment.

(2) The thermal expansion rate and the linear change rate after firing both increase with increasing content of fine fused magnesia powder.

(3) As the particle size of fused magnesia increases, the thermal expansion rate and the linear change rate after firing decrease, while the densification improves.

(4) When the addition of fine magnesia powder is 3%–7%, the linear change rate after firing can be controlled within the range of 0.5%–1.6%.

It can be seen from Figure 1 that, regardless of the addition amount of fused magnesia powder, the initial temperature at which the thermal expansion rate increases significantly is 1050 °C (see Curves 1, 2, 3, 4, 5, and 6). This is because the spinel formation reaction begins in the specimens at this temperature, accompanied by volume expansion.

The differences are as follows: the higher the content of fine magnesia powder, the faster the expansion rate increases (see Curves 2, 3, 4, and 5); the larger the magnesia particle size, the slower the expansion rate increases (see Curve 6).

For Sample No. 1 without magnesia powder, the increase in thermal expansion rate slows down above 1300 °C, which is mainly caused by the sintering shrinkage of the material at high temperatures.

2.2 Phase Composition of Sintered Alumina-Magnesia Materials and Its Effect on Material Properties

XRD analysis results show that, with increasing fused magnesia content in the batch, the spinel content in the sintered material increases. When the particle size of magnesia increases, the reaction rate decreases, and the amount of spinel formed decreases accordingly. This is consistent with the fact that a larger particle size reduces the specific surface energy and reactivity of the reactants, leading to a lower reaction rate. Therefore, the amount of spinel formed in Sample 6# is less than that in Sample 4#.

The theoretical expansion rate of the reaction between MgO and Al₂O₃ to form spinel is about 10%. Therefore, with increasing addition of fused magnesia powder, the extent of the spinel formation reaction in the material at high temperature increases, leading to increases in both the linear change rate after firing and the thermal expansion rate of the material.

Abnormal expansion of alumina-magnesia materials begins at 1050 °C, indicating that the spinel formation reaction also starts at this temperature. Since no spinel formation reaction occurs in Formula 1#, the slowdown in thermal expansion above 1300 °C is caused by sintering shrinkage.

The main reason why Sample 6# exhibits a lower thermal expansion rate, a lower linear change rate after firing, and higher densification than Sample 4# is that less spinel is formed in Sample 6#.

03 Conclusions

(1)Increasing the content of fine magnesia powder enhances the spinel formation reaction in the material, resulting in higher thermal expansion and linear change rates after firing, as well as a decrease in the bulk density of the material after sintering.

(2)Increasing the particle size of magnesia reduces the spinel formation reaction rate and the amount of spinel formed within a given time, leading to lower thermal expansion and linear change rates after firing, and an improvement in the densification of the material after sintering.

(3)When the linear change rate after firing is controlled at 0.5%–1.6%, a reasonable addition amount of fine magnesia powder is 3%–7%.

(4)For alumina-magnesia materials, the spinel formation reaction begins at 1050 °C, whereas the initial sintering temperature of corundum materials containing alumina micropowder is 1300 °C.

(5)By controlling the particle size and addition amount of magnesia, the linear change rate after firing and the thermal expansion rate of alumina-magnesia materials can be effectively managed, thereby improving thermal shock resistance and preventing spalling, bulging, and slag penetration.