Refractory brick manufacturers will have various problems in the production of high-alumina bricks. Today, I will introduce the relevant content of black hearts and cracks in the production of high-alumina refractory bricks.
High alumina brick black heart
Black heart is a phenomenon that is easy to appear in the production of high alumina bricks. The characteristic of this brick is that the section color of the brick changes gradually, from yellow-green on the surface to gray-black on the inside. If the black-core brick is re-fired in an oxidizing atmosphere, the black-core part can be re-burned white.
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(1) Causes and conditions of black hearts
The main impurity oxides in high alumina bricks are Fe2O3 and TiO2. It can be speculated from the fact that the black heart bricks turn white after reburning in an oxidizing atmosphere, the cause of the black heart is iron and titanium impurity oxides, and the atmosphere is the condition for the black heart.
However, the coexistence of iron and titanium oxides or the presence of them separately has different effects on the coloring of the brick body. When titanium ions exist alone, the coloring is not significant, and the brick body is slightly blue, while when iron ions exist alone, the brick body is orange-brown. However, the coexistence of iron and titanium, even in an oxidizing atmosphere, can increase the coloring and sintering of alumina, resulting in black centers.
Under the condition of reducing atmosphere, the existence and coexistence of iron and titanium ions can make the brick core. However, when iron and titanium coexist, the coloration is deeper than when they exist alone. Therefore, the reducing atmosphere is easier to color the brick body than the oxidizing atmosphere, especially the coexistence of iron and titanium ions, especially in the reducing atmosphere.
(2) Ways to avoid black hearts from high alumina bricks
It can be seen from the above that there are two processes for the formation of black centers and whitening of high alumina bricks: one is that high temperature and reducing atmosphere make iron and titanium ions reduce and color, and the other is that low iron and titanium ions are re-oxidized and decolorized at lower temperature. Accordingly, the following methods can be used to avoid coloration:
①For high-titanium bauxite raw materials, use strong magnetic separation to remove iron, and try to avoid the coexistence of iron and titanium ions;
②Change the kiln loading method of high alumina bricks;
③Increase the amount of hot air to cause oxygen-enriched operation and avoid reducing atmosphere;
④ During the cooling process, the temperature is slowly lowered to re-oxidize and decolorize the low-valent iron and titanium ions.
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Causes of crack formation in high alumina bricks
High-alumina refractory bricks are all heterogeneous multiphase bodies, and there are relatively large cracks, fissures, etc., so that their actual thermal shock resistance indicators are not exactly the same as the calculated values, because the calculation is based on known parameters. Under the premise of large stress.
For high-alumina refractory bricks, the large stress gradient and short stress duration mean that fracture starts at the surface, but can also be stopped by pores or interfaces between particles before full failure. For these high alumina refractory bricks, increasing the porosity results in better thermal shock resistance.
We have noticed the following reasons for the formation of cracks in high alumina refractory bricks:
1. High-alumina refractory bricks usually belong to a multiphase system, and the linear expansion coefficients between different phases are often different, which will cause stress and lead to micro-cracks: at the same time, the stress generated when the refractory products (bricks) are cooled from the firing temperature is also source of microcracks. In addition, the stress formed during use can also cause high stress on the surface to produce micro-cracks, but it will not lead to the final fracture of high-alumina refractory bricks.
2. High-alumina refractory bricks usually use multi-stage ingredients and have a wide range of particle sizes such as coarse particles, medium particles and fine powders, so there are many pores, which often become the starting point of fracture.
3. The micro-cracks caused by the residual pores produced by the molding of high-alumina refractory bricks cannot be eliminated during firing and use.
4. The thermal stress and mechanical stress caused by thermal shock during the use of high-alumina refractory bricks will significantly exceed their mechanical strength.
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All the above-mentioned characteristics indicate that there are usually more pores in high-alumina refractory bricks, and high-alumina bricks can reach 0.78~2.5mm.
It can be seen that for high alumina refractory bricks, the conditions for crack propagation should be controlled rather than the nucleation conditions for cracks. That is to say, when the high-alumina refractory bricks produce initial cracks, the ability to eliminate thermal stress and make them have small structural damage should become the focus of our research:
It can be considered that it may be appropriate to apply fracture mechanics to analyze and study the fracture strength and crack propagation law of high-alumina refractory bricks, including the index of fracture resistance of high-alumina refractory bricks, the conditions for establishing fracture damage and the strength of high-alumina refractory bricks. calculation method, etc.
The Al2O3 on the surface of the high alumina brick is 1%-2% higher than that in the center, while the SiO2 is 1%-2% lower, and the NaO is more than 10 times higher than the center. From the phase analysis, it can be seen that the content of mullite in the epidermis is 12% lower than that of the central part, the content of corundum is 4%-5% higher, and the content of glass phase is 7%-8% higher. This shows that in the process of Na2O migration, Na+ volatilizes and adsorbs on the surface ofhigh-alumina bricks, which will promote the decomposition of mullite, reduce the mullite on the surface of high-alumina bricks, and increase the phase of corundum and glass. Due to the increase and early appearance of the liquid phase on the surface, the surface of the brick body shrinks prematurely, resulting in the generation of texture.
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Practice has shown that in order to avoid and reduce the occurrence of network cracks, the water absorption rate of grade 1 and 2 high-alumina rock clinker should be controlled below 4%-5% respectively, and the water absorption rate of cylindrical mill should also be controlled below 6%. The excess air coefficient is controlled between 1.1-1.2, and it should be adjusted by thermal engineering to make the firing of high-alumina bricks in a stable weakly oxidizing atmosphere.
What should I do if the refractory material cracks, falls off, and is easy to wear? If you want to solve the problem completely, you need to dig out the root cause and analyze the real reason. Zhengzhou Shennan Refractory Co., Ltd. has been engaged in refractory production and R&D services for many years, and can solve various difficult problems of high temperature furnace lining.Looking forward your consultation and technical exchange.
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