Forming process of high alumina refractory bricks
Molding high-alumina refractory bricks is a precision process involving multiple steps, which requires strict control of raw materials, proportions and process parameters.
Before molding, check the model size. The model size ratio of ordinary high-alumina bricks: 1.5% for the upper part; 3.0% for the middle and lower pressure surfaces; 1.5% for the non-pressure surface.
Raw material selection and pretreatment:
High alumina: Select raw materials with Al₂O₃ content ≥48% (such as special grade and first grade alumina) to ensure refractory performance.
Calcination: The raw materials are calcined at high temperature (usually 1300~1500℃) to remove impurities and improve chemical stability.
Crushing and grinding: The calcined alumina is crushed and classified into coarse (3~1mm), medium (1~0.088mm), and fine (<0.088mm) to optimize the particle size distribution.
Batching and mixing:
Match design: According to the target Al₂O₃ content (such as 65%, 75% or higher), different grades of alumina are prepared and binders (such as soft clay or phosphate) are added.
Particle grading: coarse particles account for about 50%~60%, medium particles 20%~30%, and fine powder 15%~25% to increase the bulk density.
Mixing: After dry mixing, add water (or binder solution) and mix until the plastic state, and the moisture content is controlled at 4%~6%.
Molding process
High-pressure pressing: Commonly used friction brick press or hydraulic press (pressure 100~150MPa), through mold molding.
Mold design: Ensure accurate size and reasonable distribution of vent holes to prevent cracking.
Layered feeding: For complex-shaped bricks, layered feeding is used to ensure uniform density.
Vibration molding: Suitable for large-sized or special-shaped bricks, the particles are closely arranged through vibration.
Isostatic pressing (high-end products): Isotropic pressure, more uniform structure, but higher cost.
In order to reduce the porosity of high-alumina bricks and improve the quality of high-alumina bricks, the molding pressure must be increased. For ordinary high-alumina bricks, the molding pressure should reach 137MPa; the molding pressure of high-density high-alumina bricks should be in the range of 176~215MPa. In other words, only by using a friction brick press with a capacity of more than 250t can such an effect be achieved. The porosity of high-alumina bricks should meet the following requirements: I high-alumina bricks ≤21%; II high-alumina bricks ≤23%; III high-alumina bricks ≤22%.
Drying
Drying system: temperature 80~120℃, humidity gradient control, time 24~72 hours.
Purpose: slowly discharge free water to prevent cracking, residual moisture of the green body <1%.
Firing
High temperature sintering: firing in a tunnel kiln or shuttle kiln, temperature 1400~1600℃, insulation for 8~15 hours.
Firing curve: divided into four stages: preheating (dehydration), heating (crystal transformation), insulation (densification), and cooling.
Key reaction: the formation of mullite (3Al₂O₃·2SiO₂) and corundum (α-Al₂O₃) phases determines the strength and corrosion resistance of the brick.