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Effect of magnesium sulfate on the properties of magnesia castables

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Table of Contents

The experiment uses magnesia as the main raw material, magnesium sulfate as the binding agent, and silica as the auxiliary binding agent. The critical size of aggregate particles is 5 mm, and continuous particles are used for proportioning: 5 to 3 mm, 3 to 1 mm, 1-0 mm, and <0.074 mm. Keep the amount of silica powder added at 3%, and change the amount of magnesium sulfate: 1%, 2%, 3%, 4%, 5%. Calcined at 110℃×24 h, 1100℃×3 h and 1550℃×3 h. The physical properties of the specimens were studied and tested. The results show that the appropriate amount of magnesium sulfate is 3% to 4%.

Keywords: magnesia castable; magnesium sulfate; magnesia; silica powder

Preface

Magnesia castable is an alkaline refractory material. Its notable features are high refractoriness, strong resistance to erosion by alkaline slag and iron slag, and does not pollute molten steel. It is widely used in the steel industry. At present, the binding agent of magnesia castables is usually silica powder, aluminate cement or polyphosphate. Silica powder and aluminate cement produce a liquid phase at high temperatures, which is detrimental to the high-temperature performance of the material; polyphosphate There is a problem that it can easily lead to phosphorus increase in molten steel. In order to reduce the adverse effects of the above-mentioned binding agents, the author used magnesium sulfate as the main binding agent and silica as the auxiliary binding agent to study the effect of magnesium sulfate on the performance of magnesium castables.

Test

Test conditions

The main raw materials used in the test are fused magnesite and silica powder, and their chemical composition is shown in Table 1. Magnesium sulfate is 99.5% industrial grade.

Table 1 Chemical composition of raw materials %

nameAl₂O₃Mg0SiO₂CaOFe,0
Fused magnesia0.3695.020.581.470.51
Silica powder  94.15 1.42

Test methods

Magnesia is used as aggregate and fine powder. The aggregate particle size includes three-level particle gradation of 5~3mm, 3~1mm and 1~0mm, and the fine powder is ≤0.074 mm fine powder. The mass ratio of aggregate to fine powder is 7030, and the sample is cast in a 160 mm × 40 mm × 40 mm triple mold. After 24 hours of natural curing, it was demoulded and heated at 110℃×24h, 1100℃×3h and 1550℃×3h.

Test content

Keep the amount of silica powder added at 3%, and change the amount of magnesium sulfate: 1%, 2%, 3%, 4%, 5%. By testing the bulk density, compressive strength, linear change rate and slag resistance of samples treated at different temperatures, the impact of the addition of the binder magnesium sulfate on the performance of magnesia castables was analyzed and discussed.

Test results and analysis

Effect of magnesium sulfate on bulk density.

Magnesium sulfate contains 7 crystal waters and is stable in humid air below 48.1°C. When the temperature is higher than 48.1°C, -1 molecule of crystal water is lost and becomes magnesium sulfate hexahydrate. At 70°C~80°C, 4 molecules of crystal water are lost, and at 100°C, 5 molecules of crystal water are lost. It loses 6 molecules of crystal water at 150°C (reported to be 120°C), and loses all crystal water at 200°C, turning into powdery anhydrous magnesium sulfate. It is completely decomposed into MgO and SO₃ when heated to about 1200°C. The effect of changes in magnesium sulfate during the heating process on the volume density of the sample is shown in Figure 1.

Figure 1 Effect of magnesium sulfate addition amount on bulk density

It can be seen from Figure 1 that as the addition amount of magnesium sulfate increases, the volume density of the sample after treatment at 110°C × 24 hours gradually decreases. The binding agent magnesium sulfate loses 5 molecules of crystal water at 110°C, so as the addition amount increases, the water lost when the sample is treated at 110°C × 24 h gradually increases. When the sample loses water, the pores left by it gradually increase, and the volume density gradually decreases.

After treatment at 1100°C for 3 hours, the binding agent magnesium sulfate heptahydrate completely lost crystal water and decomposed into MgO and SO₃. As the addition amount of magnesium sulfate increases, the pores left by the loss of water and volatilization of SO₃ gas gradually increase, the weight of the sample gradually decreases, and the volume density gradually decreases. And generally lower than the bulk density after treatment at 110℃×24h.

After treatment at 1550℃×3h, as the addition amount of magnesium sulfate increases, the volume density of the sample gradually decreases, and the change range is not large. At 1550°C, MgSO₄ has completely decomposed, and the highly active MgO generated by the decomposition reacts with the silica powder in the sample to form forsterite. The reaction formula is: 2MgO+SiO₂→2MgO·SiO₂ (1)

Entering the full sintering stage, the density of the sample structure increases and the pores gradually decrease; at the same time, impurities in the material produce a liquid phase at high temperatures, and since the liquid phase fills the pores, the density of the structure increases. The above reasons should cause the bulk density to increase. However, when the material is treated at 1100°C for 3 hours, as the amount of magnesium sulfate increases, the pores left by the loss of water and the volatilization of SO₃ gas gradually increase. As a result of the mutual growth and decline, although the bulk density of the sample gradually decreases, the change amplitude is not large.

Effect of magnesium sulfate on compressive strength.

The binding agent magnesium sulfate mainly loses crystal water when treated at 110°C for 24 hours. The loss of crystal water will cause changes in the density and degree of bonding of the material structure, thus affecting the compressive strength of the sample. At 1100℃×3h, the decomposition of magnesium sulfate and the disappearance of the -Si-0-Si- bond reduce the internal bonding of the material. At the same time, due to the low heat treatment temperature, the material cannot be fully sintered, which also affects the improvement of the internal bonding degree of the material. 1550℃×3 h, since the material enters the sintering densification stage, the increase in the amount of magnesium sulfate is very beneficial to the sintering of the sample and helps to improve the compressive strength. The influence of magnesium sulfate addition amount on compressive strength is shown in Figure 2.

It can be seen from Figure 2 that as the addition amount of magnesium sulfate increases, the compressive strength of the sample gradually decreases after being treated at 110°C × 24 h. As the amount of the binding agent magnesium sulfate heptahydrate increases, the water lost by the sample when treated at 110°C for 24 hours gradually increases. The pores left by the sample losing water gradually increase, the structure becomes loose, and the density decreases. Under the action of pressure, the effective stress-bearing area of the unit section of the sample decreases, the pressure it bears to cause the sample to break decreases, and the compressive strength gradually decreases.

Figure 2 Effect of magnesium sulfate addition amount on compressive strength

After treatment at 1100℃×3h, as the amount of magnesium sulfate increases, the compressive strength gradually decreases and is generally lower than that after treatment at 110℃×24h. As the addition amount of magnesium sulfate increases, on the one hand, the pores left by the loss of water and volatilization of SO₃ gas gradually increase in the sample, and the total number of pores also increases compared with 110℃×24h. On the other hand, the chemical reaction that occurs when the auxiliary binder silica powder hydrates with water at room temperature is:

Hydroxyl groups, Si-OH bonds, are formed on its surface. During natural curing and 110℃×24h, the Si-OH bonds are dehydrated to form a siloxane network structure and polymerized into long chains of -Si-O-Si- bonds. The dehydration polymerization of silanol groups forms a strong -Si-O-Si- bonded three-dimensional spatial network structure, which strengthens the bonding of materials and improves the compressive strength of the sample after treatment at 110℃×24h. This three-dimensional spatial network structure can last until 700°C. That is, when the temperature is lower than 700°C, the strong -Si-O-Si- combined three-dimensional spatial network structure helps the material strength gradually increase. After treatment at 1100°C × 3 h, the disappearance of the -Si-0-Si- bond also intensified the reduction in material strength.

1550℃×3h, the sample is fully sintered, and as the magnesium sulfate addition amount increases, the compressive strength gradually increases. The highly active MgO generated by the decomposition of magnesium sulfate reacts with the silica powder in the sample to form forsterite, which enters the full sintering stage. The density of the sample structure increases, the pores gradually decrease, and the compressive strength gradually increases.

Effect of magnesium sulfate on linear change rate

The linear change rate mainly reflects the volume stability of the material at high temperatures. The sample only undergoes dehydration and decomposition reactions of magnesium sulfate at 1100°C for 3 hours. Due to the formation of pores inside the material, the material particles will be redistributed and arranged, which is mainly manifested by the close aggregation of the particles, causing the sample volume to shrink. 1550℃×3 h, the active magnesium oxide decomposed by magnesium sulfate promotes the sintering reaction of the material, and at the same time generates a large amount of liquid phase and fills the pores inside the sample, causing the volume shrinkage to increase. The effect of the addition amount of magnesium sulfate on the linear change rate is shown in Figure 3.

Figure 3 Effect of magnesium sulfate addition amount on linear change rate

As can be seen from Figure 3, the linear changes of the samples after firing at 1100℃×3 h and 1500℃×3h showed shrinkage, and the shrinkage after firing at 1550℃×3 h was greater than that at 1100℃×3 h. As the amount of magnesium sulfate increases, the linear change rate after firing at 1550℃×3h has a tendency to increase; the change rate after firing at 1100℃×3h is relatively small. This is because: at 1100℃×3 h, the sample is only affected by the volume shrinkage caused by the formation of pores due to the decomposition reaction. At 1550℃×3h, factors affecting the volume shrinkage of the sample are not only caused by the formation of pores due to decomposition reaction. More importantly, the active magnesium oxide formed by the decomposition of magnesium sulfate promotes the sintering of the material, and at the same time generates a large amount of liquid phase to fill the pores of the sample, aggravating the shrinkage and increase.

Effect of magnesium sulfate on slag resistance

The slag resistance performance is expressed by the slag height and slag penetration depth. The higher the slag height and the smaller the slag penetration depth are, the stronger the slag resistance ability of the material is. The test results at 1550℃×3 h are shown in Figure 4.

It can be seen from Figure 4 that as the addition amount of magnesium sulfate increases, the residue height of the sample first increases and then decreases, with 3% to 4% being better. The slag penetration depth of the sample first decreased and then increased, with a minimum of 4%.

The anti-slag performance is best when adding 4% magnesium sulfate. The slag resistance performance of the sample is mainly related to the number of pores. As the amount of magnesium sulfate increases, the highly active MgO generated by the decomposition of MgSO₄ increases. The more forsterite the MgO reacts with the silica powder in the sample, the more fully the material will be sintered, the denser the sample structure will be, and the fewer pores. The harder it is for slag to penetrate and corrode in the material, the better the slag resistance will be. When the amount of magnesium sulfate added is 5%, due to the excessive amount of magnesium sulfate, its decomposition to form pores has an adverse effect greater than the sintering effect of the material. The mutual trade-off results in the deterioration of the slag resistance performance of the sample.

Conclusion

1) As the magnesium sulfate content increases, the bulk density and compressive strength of the sample gradually become smaller after 110℃×24h and 1100℃×3h, and the range of the linear change rate at 1100℃×3h is relatively small.

2) As the magnesium sulfate content increases, the bulk density of the sample after burning

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