metallic aluminum powder is a chemically active, flammable, explosive, and oxidizable granular powder. At room temperature, bare metal reacts with oxygen in the air to form Al2O3, which forms dense oxidation on the surface of each aluminum powder particle. membrane. The oxidation of aluminum by oxygen in the air is prevented, and the elemental aluminum in the oxide film is protected. Aluminum reacts with oxygen to form Al2O3, and reacts with water to form Al (OH) 3. Aluminum itself has the properties of amphoteric hydroxide. Aluminum powder is susceptible to oxidation when it is affected by changes in the external environment, such as: humid air, water, rain, or external forces, such as friction and collision, will destroy the oxide film on the surface of the aluminum powder, and continue the aluminum powder Oxidation oxidizes elemental aluminum in the original protective layer to generate Al2O3 and Al (OH) 3.
In magnesium-carbon brick, aluminum powder is added to the brick as an antioxidant to prevent carbon oxidation. The presence of carbon in magnesia-carbon bricks is mainly to prevent the slag from eroding into the bricks in order to improve the corrosion resistance of magnesia-carbon bricks. But carbon has the property of being easily oxidized. When the carbon in the magnesium carbon brick is oxidized, the erosion resistance of the brick changes accordingly. The oxidation of carbon will accelerate the damage of the bricks and reduce the service life.
In order to prevent carbon oxidation, easily oxidizable metal aluminum powder is added to protect the carbon in the brick. In order to improve the corrosion resistance, slag resistance, spalling resistance and high temperature resistance of magnesium carbon brick. The reaction mechanism is as follows:
1. When the Mg-C brick is heated, the activated aluminum in the brick reacts with carbon to form high-melting carbides, which re-aggregates the carbon, and finally generates Al4C3 and Al2O3. AL4C3 is contained on the surface of MgO, preventing the erosion of slag To improve the life of the brick.
2. At high temperature, magnesium carbon bricks oxidize activated aluminum particles to form Al2O3. This process is a phase transition process from metal to non-metal. The volume expansion of the oxidized Al2O3 compacts the pores in the brick, increases the density of the brick, and forms a ceramic combination, thereby improving the high temperature resistance of the brick.
Due to the great oxidation resistance of metallic aluminum powder in magnesia-carbon bricks, it is most important to choose a reasonable aluminum powder particle size distribution to ensure that the active aluminum loss to the aluminum powder is minimized during production and mixing. The main factor affecting the reduction of aluminum powder activity in production mixing is the particle diameter of aluminum powder. The reaction mechanism is as follows:
First, the higher the activity of aluminum powder in magnesium carbon brick, the better the oxidation resistance. Under normal conditions, the active aluminum content of aluminum powder has a direct relationship with the particle diameter. Large particle size has high activity, and small particle size has low activity. The reason is that the fine aluminum powder has a small particle size, a large specific surface area, a large oxide film area, and a low active aluminum content. Conversely, aluminum powder with a large particle size has a small specific surface area, a small surface oxide film area, and a high content of active aluminum.
2. When the fine aluminum powder and magnesia with large particles are mixed for 30 minutes, they are subject to friction and collision due to external forces. The oxide film on the surface of the aluminum powder particles is damaged or shear deformed. The aluminum powder particles without the protective film It reacts with oxygen to form Al2O3. In the continuous mixing by external impact, friction and collision, most of the fine aluminum powder will be oxidized to form Al2O3, which greatly reduces the activity of aluminum powder. Mixing is a process before the brick is formed. If the activity of aluminum powder is reduced after mixing, the content of active aluminum in the brick after molding will be greatly reduced, and the content of Al2O3 will increase, making the brick resistant. The sharp decrease of the erosion property has a great impact on the quality of the brick. The aluminum powder with large particles has a large unit volume, large mass, strong anti-friction and collision force, and the oxide film on the surface is less damaged, and the active aluminum basically remains unchanged, fully exerting the anti-oxidation effect of the aluminum powder.
3. The fine aluminum powder is in full contact with the phenolic resin during mixing. When the aluminum powder and the resin are evenly distributed, the fine aluminum powder with a large specific surface area reacts with water because the water content of the phenolic resin is between 2 and 4%. Al (OH) 3 is formed, which further reduces the activity of aluminum powder.
Production experiments prove that the fine aluminum powder with a particle size of -430 mesh (-33 microns) added to the magnesium-carbon brick has been basically oxidized to Al2O3 after 30 minutes of mixing, or reacted with water in the resin to form Al (HO) 3. Therefore, if the particle size of aluminum powder is less than -430 mesh (-33 microns), the content cannot be greater than 10%. Some manufacturers add 1.8% of aluminum powder to magnesia-carbon bricks, and some add 2.5%. The latter adds more than the former, but the antioxidant effect is not as good as the former. The main reason is that the particle size of the aluminum powder is too small. Contains -430 mesh (-33 microns) of too much fine aluminum powder, which is caused by oxidation during production and mixing, and increases raw materials.