Most of the simple and thick alumina brick products are manufactured by molding process. The so-called molding method (also known as semi-dry pressing method) is to place a blank with a small amount of binder and a ratio of coarse particles to fine powder in a metal model, and press it on a press to form it.
For brick products, that is, compression molding, why the particle size of the mud should emphasize the combination of coarse, medium and fine, here, it is necessary to explain specifically.
First, crush the raw materials to a certain fineness, the purpose is to make more contact surfaces between the particles, which is conducive to solid-state reaction and sintering, but this is not entirely the case for compression molding. Because the finer the powder particles, the larger the specific surface area, and the larger the loose volume of the powder, which makes it difficult to press compactly during compression molding, and is prone to lamination. On the contrary, if the mud materials are all coarse particles, it is also difficult to compact, and because the contact surface between the particles is reduced, the distance between the particle centers is increased, which is not conducive to the full progress of the solid phase reaction. Therefore, it is required to have a mix of coarse and fine particles in the ingredients; second, as mentioned before, when the same particle size is used for packing, the void ratio is 26% to 47.6% (see Table 1). To obtain the maximum packing density, use Particles of different particle sizes must be used to match the small particles to fill the gaps between the large particles. Thirdly, proper particle matching can reduce the shrinkage of the green body, which is beneficial to control the size of the green body. Of course, when forming a green body with a small size, the above three points are not necessarily overemphasized.
Most of the coarse particles used for compression molding are made of finely ground powder and added with a binder to make coarse particles with good fluidity. Such coarse particles are customarily called pseudo particles. There are several ways to make fake granules or granulation:
(1) Common law. Add an appropriate amount of binder aqueous solution into the powder, mix and pass through a coarse-mesh sieve, and rely on the cohesion of the binder to obtain agglomerates with relatively uniform particle size;
(2) Pressurization method. The powder mixed with the binder is first pressed into blocks, then crushed and sieved into coarse grains. Its density and mechanical strength are high, which is a commonly used method in industrial production;
(3) Light burning method. The ball mill powder is made into a mud ball with a small amount of water, calcined at a lower temperature, and then crushed through a coarse-mesh sieve;
(4) spray drying method. The powder is made into a slurry by adding a binder, and then sprayed into the granulation tower for atomization, and the mist is dried by the hot air in the tower to form granules.
The compression molding process, in summary, should include the following steps:
(1) First process the raw material into fine powder and pseudo granules;
(2) Then according to the requirements of the billet, carry out particle grading;
(3) Then add additives, lubricants, binders, etc. and evenly mix it into a blank (or mud);
(4) Finally, it is weighed into the mold and pressed and formed on the press.