Last atricle shows influencing factors of corundum refractory bricks in the forming process，this passage mainly shows the common problems of corundum refractory bricks in forming process.
The most common problems encountered during the molding process are:
(1) Body fracturing or spalling. This phenomenon is mainly caused by the fact that the air in the mud is not completely discharged except for the reason of the mold. Excessive moisture is also prone to spallation. Too much fine powder and too much pressure will cause spallation.
(2) The density of bricks is uneven. When the brick is pressed, there is frictional resistance between the particles, between the particles and the mold wall, which hinders the uniform transmission of pressure and causes uneven compression of each part of the brick, resulting in different compactness of each part.
In addition, the ratio (H/D) between the filling height of the brick and the compressed area is also an important factor causing uneven density. The greater the H/D, the more uneven the pressure transmission; in the pressing direction, the pressure decreases in turn from the pressing surface along the height of the brick, and the density distribution of the brick is correspondingly from tight to loose, as shown in Figure 1. If double-sided pressure is used, the density of the bricks is dense up and down and sparse in the middle. Therefore, in order to reduce the density difference of the formed bricks, it is necessary to pay attention to the reasonable design of the mold and the method of forming and pressing.
For products with complex shapes, corundum mud can be tamped with an air hammer to ensure the same density as semi-dry pressed bricks with a pressure of 80~100MPa. The main disadvantage of ramming is manual labor, which is gradually being replaced by vibration and isostatic pressing.
Using vibration molding, the product can basically be compacted in the early stage of vibration. The main advantage of this molding is that the brick does not expand elastically. The pressure exerted during the mud vibration is very uniform and reasonable, which can ensure the tightest accumulation of mud particles. The vibration parameter has a greater influence on the density than the pressure. The dispersion and particle distribution of the fine powder have an important influence on the compactness of the product. The use of large particles is easier to increase the density of the product than the small particles. Increasing the content of fine powder will reduce the density of the brick. . The best vibration parameters for forming corundum products are: the minimum moisture content of mud is 6.5%~7%. The amplitude is 1.0~1.25mm, and the vibration frequency is 2500~3000 times/min. The performance of vibration-formed corundum products is much better than that of compression-formed products, such as porosity corresponding to 18.6% and 21.9%, and compressive strength of 162MPa and 94MPa.
In recent years, the production of sintered corundum products has also adopted isostatic pressing. In order to ensure that the bricks have the required external dimensions, high strength and uniform high density, before isostatic pressing, the fine powder must be evacuated and vibrated in the model, which is different from vibration molding. The optimum time for alumina fine powder to reach densification through vibration is 60~80s; increasing the load can improve the efficiency of vibration densification, and the optimum value is 0.06~0.12MPa. The amplitude is increased from 0.1 to 1mm, which makes the bricks vibrate and compact, and the compression amount of the material increases, and vacuuming will also increase the compression value.
Vibration makes the brick compact, and its compression value is largely related to the properties of the fine powder. The bulk density of the powder decreases and the compression amount increases. The increase of the repose angle of the material can reduce the influence of the bulk density on the compression amount. For materials with a small natural repose angle, even if the material is freely fed into the mold, the material may be close to the densest arrangement in the air. In order to improve the properties of the material under vacuum vibration and hydrostatic pressure, spray drying or other methods should be used to granulate the material, that is, to make false particles, so that the material has a reasonable particle composition (0.1~0.3mm) and a relatively large heap. density.
In this way, by isostatic pressing of fine-grained corundum mud, large bricks of 2.41m×1.22m×1.22m can be obtained, and by quasi-isostatic pressing, material balls of φ20~90mm can be obtained. In addition, the semi-dry method of friction brick press must strictly implement the operation method of first light and then heavy, and increasing the number of blows is also an effective method to increase the density of corundum refractory bricks.
The formed corundum refractory bricks should be strictly inspected for appearance defects such as shape, dimensional tolerance, density, missing edges and corners, cracks, spalling, angles, and parallelism in accordance with standard regulations. Qualified bricks are sent to drying.
