Process Steps for Producing Alumina Bricks by Molding

Most simple, thick alumina bricks are manufactured using a molding process. Molding (also known as semi-dry pressing) involves placing a blank containing a small amount of binder, with a relatively coarse particle to fine powder ratio, into a metal mold and pressing it into shape on a press.

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For brick products, i.e., molding, why is it important to emphasize a combination of coarse, medium, and fine particle sizes in the clay? This needs to be explained in detail here. At the same time, the precautions in the molding process for alumina bricks become clear.

1. First, crushing the raw material to a certain fineness aims to increase the contact surface between particles, which is beneficial for solid-phase reaction and sintering. However, this is not entirely true for molding. Because the finer the powder particles, the larger their specific surface area, and the larger the loose volume of the powder. This makes it difficult to achieve a dense compaction during molding and easily leads to delamination. Conversely, if the clay material consists entirely of coarse particles, it is equally difficult to compact. Furthermore, the reduced contact area between particles and increased distance between particle centers hinder sufficient solid-phase reaction. Therefore, a mix of coarse and fine particles is required in the batching process.
2. Second, when using particles of the same size for packing, the porosity is 26%–47.6%. To achieve the maximum packing density, a mix of particles of different sizes must be used, allowing smaller particles to fill the gaps between larger particles.
3. Third, an appropriate particle size distribution can reduce green body shrinkage, which is beneficial for controlling the green body's dimensions.

Of course, when forming green bodies with very small dimensions, the above three points are not necessarily overemphasized.

Coarse particles used in compression molding are mostly made by adding a binder to finely ground powder to create coarse particles with good flowability. These coarse particles are conventionally called pseudo-granules. The methods for producing pseudo-granules, or granulation, are as follows:

• (1) Ordinary method: An appropriate amount of binder aqueous solution is added to the powder, mixed, and then passed through a coarse sieve. The binder's agglomerative effect yields relatively uniform granules.
• (2) Pressurization method: The powder mixed with the binder is first pressed into blocks, then crushed and sieved into coarse particles. This method has high density and mechanical strength and is commonly used in industrial production.
• (3) Light calcination method: Finely ground powder is mixed with a small amount of water to form mud balls, calcined at a low temperature, and then crushed and passed through a coarse sieve.
• (4) Spray drying method: The powder is mixed with a binder to form a slurry, which is then sprayed into a granulation tower for atomization. The droplets are dried by the hot air inside the tower to form granules. 

The compression molding process can be summarized as follows:

• (1) First, process the raw materials into fine powder and pseudo-granules.
• (2) Then, according to the requirements of the blank, perform particle size distribution.
• (3) Additives, lubricants, binders, etc., are added and mixed evenly to form a blank (or clay material).
• (4) Finally, weigh and pour into the mold, and shape under pressure on a press.

Rongsheng Refractory Materials Manufacturer specializes in producing various alumina refractory bricks and refractory castables. Contact us for free samples and quotations.