Chemical composition of fused zirconium corundum bricks

The chemical composition of fused zirconium corundum bricks is as follows:

Alumina (AI203): AI203 in AZS is corundum AI203, true density 3.99g/cm3, melting point 2050℃±10℃

Zirconium oxide (ZrO2): true density 5.60g/cm3, ZrO2 in AZS is baddeleyite, it has two forms: monoclinic phase below 1000℃, the transformation is reversible, and accompanied by 6.9% volume change, melting point 2715'℃, boiling point 4300℃, ZrO2 has the characteristics of maintaining high mechanical strength at high temperature, its load softening point is 4% deformation at 2000℃, but the thermal stability is slightly poor. ZrO2 has high chemical stability and does not react with metals, glass feldspar, silicates, etc. below 2000℃. In a reducing atmosphere, Zr02 partially transforms into ZrO (low melting point). Fe203 and Mn02 have a greater effect on Zr02 at high temperatures.

Silicon oxide (Si02): Melting point 1730℃. In an alkali-free environment, Si02 will inevitably generate mullite crystals with AI203. However, in AZS, the presence of mullite crystals is harmful, because the appearance of mullite will inevitably reduce the glass phase and corundum, destroying the high-performance petrographic structure of AZS, so the Si02 content must be limited, and it must be lower than or even much lower than 16.3% or 17% in AZS. It can be seen that although Si02 is the main component of AZS, it is a restricted component. The beneficial effect of Si02 is that it can generate a glass matrix-glass phase, which will play an important role in alleviating the casting cracks caused by the ZrO2 phase transformation. However, for high-zirconium AZS-41, the glass phase must also be limited.

Sodium oxide (Na20): The melting point of Na20 is 852℃. The effect of adding a small amount of Na20 to the AZS ingredients has been studied by Japanese researchers such as Yoshiki Bunpei. Figure 4-31 shows the effect of Na20 on the phase composition of ZAC ingots. As the amount of Na20 added increases, the thermal decomposition of mullite is promoted and the formation of mullite in the casting is inhibited. When Na20 is around 1.5%, all SiO2 forms a glass phase, and the content of matrix glass increases to about 20%. It can absorb the thermal stress in the casting and prevent cracks. The Na20 content in AZS-41 is low because the SiO2 content is low. Its suitable content is determined by SiO2/Na20=11~14. When SiO2 is 10%~13%, it is about Na200.98%1.18%. The amount of Na20 added has a significant effect on the amount of glass phase. When Na20 is reduced from 1.30% to 0.65%, the glass phase content drops from 20% to 12%, resulting in severe cracks.

Boron oxide (B203): relative density 2.46, melting point 450℃, softening temperature 220℃, it has two functions in AZS: one is fluxing, and the other is to improve the properties of the glass phase and prevent cracks in the casting. Because it can generate borosilicate glass with Si02, this glass has the advantages of small thermal expansion, high softening temperature, and strong chemical corrosion resistance. In fact, the addition of B203 can greatly reduce the viscosity of the glass phase. On the one hand, it can promote the melting of high melting point Zr02, and on the other hand (more importantly), it can absorb thermal stress in the casting in a larger range and effectively eliminate thermal cracks. However, the amount of B203 added is very small, generally not exceeding 0.25% or 0.5%, preferably 0.15%. Although adding B203 has the above benefits, it is no longer introduced in the products of many countries to minimize the amount of matrix glass.

Iron oxide (Fe203): can reduce the melting point of AZS batch materials; promote the formation of larger and uneven grains in castings; increase the content of glass phase, especially at high temperatures, it can increase the fluidity of glass phase and reduce the product's resistance to glass erosion. Some people pointed out that Fe203 in castings can release bubbles into the glass liquid during the use of the melting furnace, affecting the quality of the glass. Therefore, AZS has particularly strict restrictions on the content of iron oxide, requiring its content not to exceed 0.2%. Experience shows that the content of Fe203 should be less than 1% of the glass phase content, and the glass phase of AZS is generally 20%. So the content of Fe203 is 0.2% (the content of Fe203 in French ER1681 is 0.08%).

Titanium oxide (TiO2): can form eutectics with other components. This type of titanium compound has a volume change between 450 and 600°C, and it is easy to cause corundum to crack at 800°C. It has a fluxing effect on AZS and promotes the increase of the grain size of ZrO2 and other materials. Especially when melted under reducing conditions, TiO2 can reduce the viscosity and seepage temperature of the glass phase in AZS. Therefore, its content in AZS is strictly limited, strictly required to be below 0.2%, but my country can only require it to be below 0.5%, because the iron and titanium content of domestic zircon concentrate is 0.3% to 0.5%.