The causes of network cracks on high alumina bricks

The network cracks on the surface of high-alumina bricks are the defects that often occur in the production of it. The causes of network cracks are complex. The impurity content (especially R₂O content), the sintering degree, the critical particle size, the amount of fine powder added, the mixing quality of the mud, the humidity and temperature of the drying medium of the reduced body, the shrinkage of the reduced body during the firing process, the secondary mullitization reaction and the use of corundum recrystallization will all cause the surface of the product to produce reticulated lines.

The sintering of high alumina brick is liquid phase sintering. The formation temperature and quantity of the liquid phase important factors leading to inconsistent shrinkage and surface network cracks. And also the heating rate during sintering and the atmosphere conditions.

The degree of sintering, the sintering atmosphere and the volatile matter in the gas phase have a great influence on the network cracks on the surface of high aluminum products. The clinker with poor sintering will continue to shrink during the sintering process of the products. This can cause the products to crack and produce cracks.

The effects of water absorption

The degree of surface reticulation cracking of the products is closely related to the water absorption of the clinker used. The greater the water absorption of the clinker, the greater the intensity of the mesh. When using clinker with high water absorption to make bricks, the clinker itself will continue to complete the sintering process during the firing process of the products. The shrinkage of the products is large and uneven, so it is easy to crack and mesh. Among the clinker with poor sintering, the secondary mullitization is not sufficient. And the secondary mullitization of the clinker itself is still continuing during the sintering process of the products. This is an internal factor of inconsistent shrinkage of products. Resulting in an increase in the number of cracks in the product network and the degree of cracking.

The effects of firing atmosphere

The firing atmosphere in the kiln is also one of the reasons for the network cracks of high-alumina products. When firing high-alumina products, the atmosphere in the kiln needs weak oxidation flame. The excess air coefficient is controlled between 1.1 and 1.2. The experimental results show that the network cracks on the surface of the products tend to decrease with the increase of the excess air coefficient. However, the fluctuation of network crack scrap is large. Therefore, the fluctuation of excess air coefficient should not be too large.

In addition, the network cracks on the surface of the product mostly occur on the brick surface within the gap of the code brick. It is estimated that when the coefficient of excess air in the kiln is small or reducing atmosphere is generated, CO is easy to stay in these places due to the small brick gap, thus reducing Fe₂O₃ in the products to FeO. The brick surface facing the channel has relatively smooth air flow, so it will not be affected by the change of atmosphere and there is no network crack. In particular, frequent changes in the nature of the combustion atmosphere should be avoided during the firing process. Because this alternating change will damage the surface.

The chemical analysis

According to the chemical analysis of the surface and the center of the products with more surface network cracks, the A1₂0₃ on the surface of the brick body is 1% 2% higher than the center; SiO₂ is 1%~2% lower, and Na₂O is more than 10 times higher than the center. According to the material analysis, the content of mullite in the epidermis is 12% lower than that in the center, the content of corundum is 4%~5% higher, and the content of glass phase is 7%~8% higher.

This shows that in the migration process of Na₂O, with the volatilization of Na+and adsorption on the surface of high-alumina brick, Na+will promote the decomposition of mullite, resulting in the reduction of mullite, the increase of corundum and the increase of glass phase on the surface of products. As the amount of liquid phase on the surface increases and appears ahead of time, the brick surface shrinks prematurely, resulting in the generation of reticulation.

The solutions of network cracks on high alumina bricks

The practice shows that in order to avoid and reduce the occurrence of network cracks, the water absorption of engineering and engineering grade high-alumina rock clinker should be controlled below 4% and 5% respectively. The water absorption of the drum mill should also be controlled below 6%. The excess air coefficient should be controlled between 1.1 and 1.2. Moreover, the firing of high-alumina refractory bricks should always be in a stable weak oxidation atmosphere through thermal adjustment.