Service life of fused zirconium corundum bricks

Casting temperature and casting method of fused zirconium corundum brick

The pouring process of fused zirconium corundum brick melts is much more complicated than most metal and other silicate melts because the process is carried out at relatively high temperatures (1770-1800°C). The viscosity of the fused zirconium corundum brick solution is higher than that of the metal melt, that is, the fluidity is poor. Practice proves. Overheating the melt can improve its fluidity. But avoid overheating, as overheating can cause shrinkage cavities of absorbed gas and product cracking, as well as structural delamination. As the temperature of the casting melt increases, the shrinkage cavities will increase accordingly, and shrinkage cavities will be formed in the castings, reducing the quality of the product.

The pouring temperature of the melt has an important influence on the formation process of the casting structure. If there is no overheating, the zonality of the structure disappears, and the shrinkage cavities are evenly distributed throughout the casting volume. But the casting density is very low, and the dense part is also very small. When the overheating temperature rises by 100°C, structural zonality will appear, shrinkage holes will be concentrated, the thickness of the dense part will also increase, and the density of each area and the entire fused zirconium corundum brick will also increase. If the temperature is increased further, the density of the material will increase, but it will lead to cracks and shrinkage cavities.

The molten refractory material changes from melting to solid, and the volume shrinkage rate is about 12% to 15%. If it is not handled properly, it will exist in the product in the form of shrinkage cavities, which will seriously affect the quality and performance of fused zirconium corundum bricks.

Four casting methods

Due to the temperature gradient during the coagulation process, chemical phase composition segregation will form on the cross section of the fused zirconium corundum brick ingot. The higher the zirconia content, the larger the ingot size and the greater the degree of segregation. The casting block is generally divided into three areas: the surface rapid cooling area (small and uniform crystals), the intermediate dense area (high concentration of zirconia, growing crystals) and the shrinkage cavity area (cavity). The first two areas are resistant to erosion. Condensation of molten liquid in sand mold will produce 12%? 15% volume shrinkage to form shrinkage cavities. There are four casting methods according to the size of the shrinkage cavity and the treatment method.

① Ordinary pouring method (PT for short) adopts ordinary riser pouring. The solidified portion has fine crystals, and the second solidified portion has coarse crystals. There are large shrinkage holes, and the position of the shrinkage holes is basically the same as that of the pouring holes on the brick surface. The bulk density is about 3.3g/cm3.

②The oblique pouring method (QX for short) tilts the mold to a certain angle, and the riser is placed on the upper end of the mold, so that the shrinkage cavity is condensed at one end of the product, and the position of the shrinkage cavity is close to the bottom of the brick. The molds are mostly dense, especially the lower half of the bevel. This method works for long bricks.

③Use the non-shrinkage casting method (WS for short) to cast sand with additional dimensions. Shrinkage holes are concentrated at the top. Cut off after annealing. The rest of the parts used are uniform in composition and dense in organization.

④Dense casting method (referred to as MS) The dense casting method is also called the strengthening casting method or the multiple supplementary casting method. In the secondary casting, shrinkage cavities are generated, and the risers are replaced at certain time intervals, and then recasting is performed. In this way, the shrinkage cavity volume can be greatly reduced and the full casting rate of the casting can be improved. The average bulk density of fused zirconia corundum bricks containing 33% zirconia can reach 3.6g/cm3.

The service life of fused zirconium corundum bricks should pay attention to the following aspects.

(1) Due to its dense structure and low porosity, molten refractories have no pores as buffers when subjected to thermal shock and uneven heating, and are prone to bursting. Therefore, when considering the selection of glass furnace materials, it is better not to use molten refractories in parts with large temperature changes.

(2) The thermal expansion curve of fused zirconium corundum brick has an abnormal sudden change near 1000°C, and the zirconia in the brick will undergo a reversible phase transition, accompanied by a large volume change. Therefore, when firing the glass kiln, the heating rate should not be too high or too fast in the temperature range of 900~1150 °C (usually the heating rate is required not to exceed 15 °C).

The microstructure changes of fused zirconium corundum bricks fired at high temperature are as follows:
(1) The coarse aggregate glass phase seeps out to the particle surface, reacts with Al2O3, and forms a mullite shell to seal the seepage channel, so that the peritectic reaction is carried out inside the particle;
②The glass phase of molten AZS powder reacts with active Al2O3, and the matrix becomes mullite. The microstructure of zirconia-bonded corundum bricks is characterized by the close combination of mullite and corundum-mullite eutectic.

Five problems that should be paid attention to when using fused zirconium corundum bricks

1. The fused zirconium corundum bricks produced in a reducing atmosphere have poor corrosion resistance.
The glass phase of fused zirconia corundum bricks will form a high viscosity layer in contact with high temperature glass liquid, which is the key to the good corrosion resistance of zirconia corundum bricks. This requires a higher softening temperature of the glass phase, so there are few other components in the brick except Al2O3, zirconia, silica and Na2O. However, due to the use of graphite-lined bricks and graphite electrodes during melting in an electric furnace, a reducing atmosphere will be created. Carbon and carbon monoxide will reduce a portion of Fe2O3, titania and silica in the melt to lower oxides, thereby increasing the number of components. According to the basic principle of comparison, this will lower the limit temperature of the liquid. When this brick is used at high temperature, the viscosity of the glass phase is greatly reduced, and a high-viscosity protective layer cannot be formed. Due to the low viscosity of the glass phase, it is easy to diffuse into the high temperature molten glass. Therefore, if the crystalline phase in the brick loses its binder, it will also fall into the molten glass, causing streaks and stones in the molten glass.

The fused zirconium corundum bricks produced by the oxidation method do not use graphite lining bricks. Although the electrodes still use graphite, they are melted by the long arc method, so that C combines with oxygen in the air in the arc to generate CO2 without entering the melt. The bricks produced by this method contain less than 0.005% carbon, so the corrosion resistance is greatly improved.
At the same time, the zirconia corundum brick produced by the reduction method will also generate bubbles in the glass. This is due to the fact that the suboxides and other substances such as carbon in the glass phase of the brick have strong reducing properties, and when they come into contact with the glass liquid, they will take away the dissolved oxygen in the glass liquid. This causes the soluble gas in the glass to change its composition and become a gas that is insoluble in the glass liquid, becoming bubbles. For example, SO3 dissolved in molten glass is reduced to SO2. SO2 has low solubility in molten glass, so it becomes bubbles. This reduces the quality of the molten glass.

2. Irregular changes in thermal expansion.

Fused zirconium corundum brick has the characteristics of stable and compact structure and strong resistance to molten glass erosion. There is abnormal expansion between 900 and 1200 ° C, which is caused by the transformation of zirconia monoclinic to tetragonal zirconia. It can provide reference for the baking and operation of glass tank kiln. Due to the influence of other components in AZS bricks, the crystal transition temperature fluctuates within 50°C. Therefore, in use, the temperature change between 950°C and 1150°C during heating and cooling should not be too large, generally not exceeding 15°C/h.

The abnormal expansion and contraction of zircon can be buffered by the glass phase without cracking. In the late 1970s, 0.15%~0.20% B2O3 was introduced into the composition, so that the glass phase contained in the AZS brick not only has better thermal shock resistance, but also has a buffering effect on abnormal expansion. But cracks can also appear if the temperature varies widely or fluctuates repeatedly within this temperature range. Therefore, when heating the kiln, the temperature should rise steadily in the range of 950~1150℃. When used in places where the temperature changes frequently (such as the feeding port, etc.), the outside should be protected by other bricks.

3. Electrical insulation
When using all-electric melting and electrofusion methods, electrode bricks are required to have good electrical insulation at high temperatures. The electrical insulation of zirconia corundum bricks is better and decreases with the increase of temperature. It can be seen that zirconium corundum bricks can be used as electrode bricks in the production of soda lime glass bath furnaces. However, AZS-41 bricks or zircon bricks should be used in the production of high borosilicate glass. Otherwise, not only will the power consumption be uncontrollable, but the bricks will melt, the electrodes will be damaged, and the molten glass will be contaminated.

4. Thermal conductivity
The thermal conductivity of zirconia corundum bricks is twice that of clay bricks. Therefore, when making pool wall bricks, the cooling air volume required near the liquid level should be twice that of making pool wall bricks when making clay bricks. So as to achieve the effect of full cooling, give full play to the advantages of zirconia corundum bricks.

The difference between AZS sintered bricks and Al2O3 sintered bricks

The corrosion resistance of glass at high temperature is: AZS-41#, AZS-36#, AZS-33#, α-β-al2o3, but the pollution resistance of AZS bricks is not as good as that of alumina electroforming bricks. The reason is that the alumina electroformed brick is very pure, and 90% is the nepheline metamorphic layer formed by Al2O3 on the surface of the brick in the molten glass. The viscosity of the nepheline metamorphic layer containing zirconia is not as high as that of easy melting, so the corrosion resistance at high temperature is not as good as that of AZS electroformed bricks. However, the greater the viscosity difference between the impurities flowing into the molten glass and the molten glass, the more difficult it is to dissolve and eliminate, while the nepheline metamorphic layer of the aluminum brick has a low viscosity and is easily dissolved in the molten glass, so the pollution to the molten glass is less.