Most of the simple shape, thick alumina brick products are manufactured by the molding process. The so-called molding method (also known as semi-dry pressing method), is to contain a small amount of binder, with a coarse particle and fine powder ratio of billet, placed in the metal model, in the press pressure and molding.
For alumina brick products, that is, molding, why the size of the mud should emphasize the coarse, medium and fine coordination, here, it is necessary to specifically explain.
First, the raw material is crushed to a certain fineness, the purpose is to make there are more contact surfaces between the particles, which is conducive to solid reaction and sintering, but for molding, it is not entirely the case. Because the finer the powder particles, the larger the specific surface area, the larger the loose volume of the powder, which makes it difficult to compact when molding, and easy to produce lamination.
On the contrary, if the mud is coarse particles, it is also difficult to compact, and because the contact surface between particles decreases, the distance between particle centers increases, which is not conducive to solid phase reaction. Therefore, it is required to match coarse and fine particles in ingredients; Second, it has been mentioned before that when the same particle size is used for packing, the voidage ratio is 26%$47.6%. To obtain the maximum packing density, particles with different particle sizes must be used to match, so that small particles can fill the gaps between large particles. Thirdly, proper particle collocation can reduce the shrinkage of the billet body, which is beneficial to control the size of the billet body. Of course, in the alumina brick shape of a very small body, the above three points are not necessarily overemphasized.
The coarse particles used in molding, most of which are added to the binder with very fine powder, are made into the coarse particles with good fluidity. The coarse particles are commonly called false particles. The methods of making false particles or granulation are as follows:
(1) Common law. The appropriate amount of the aqueous solution of the binder is added into the powder, mixed through the coarse sieve, depending on the cohesion of the binder, the particle size is more uniform;
(2) Pressure method. The powder mixed with the binder is first pressed into blocks and then crushed and screened into coarse grains. Its density and mechanical strength are high, which is commonly used in industrial production.
(3) Light burning method. The ball ground fine powder is made into mud with a small amount of water, calcined at a lower temperature, and then crushed through a coarse sieve;
(4) Spray drying method. The powder and binder are added to make slurry, which is then sprayed into the granulator for atomization. The droplets are dried by the hot air in the granulator.
The process of molding of alumina brick, summed up, should include the following steps:
(1) First, the raw materials are processed into fine powder and false particles;
(2) Then according to the billet requirements, particle gradation;
(3) Add additives, lubricants, binders and other evenly mixed into billet (or mud);
(4) Finally weigh into the mold, in the press compression molding.
