In addition to particle grading, the density of high alumina bricks is also directly affected by the following raw material factors. Its core mechanism is to optimize the sintering behavior and structural density by regulating the physical and chemical properties of the raw materials:
I. Chemical composition and purity
1. Al₂O₃ content When the Al₂O₃ content is ≥85%, a continuous corundum skeleton (theoretical density 3.98g/cm³) is formed during sintering, and the density is significantly improved (can reach more than 3.0g/cm³); when the content is reduced to below 60%, the mullite phase increases (density 3.16g/cm³), and the upper limit of density decreases.
2. Impurity elements affect Fe₂O₃ (≤1.5%): a small amount promotes the formation of liquid phase (about 1300℃) and fills the pores; an excessive amount (＞3%) leads to an excess of liquid phase and an increase in porosity (the apparent porosity increases from 12% to 18%). TiO₂ (≤2.0%): forms a solid solution with Al₂O₃, inhibits abnormal grain growth, and improves density; excessive amounts form high-melting aluminum titanate (Al₂TiO₅), which hinders sintering. Alkali metal oxides (K₂O+Na₂O≤0.5%): reduce liquid viscosity and promote particle rearrangement, but excessive amounts lead to premature loss of liquid phase and residual pores.
2. High alumina brick Mineral composition and crystal structure
1. Bauxite phase diaspore type: fine-grained corundum is formed after calcination, with high sintering activity and a density of up to 2.85g/cm³;
Gibsburg type: coarse-grained corundum is generated after calcination, with low stacking efficiency, and high-pressure molding is required to reach 2.70g/cm³.
2. Kaolin conversion products: Kaolinite (Al₂Si₂O₅(OH)₄) decomposes into metakaolinite at 950℃ and generates mullite (3Al₂O₃・2SiO₂) at 1200℃. Its needle-like crystal interlaced structure can enhance the strength of the green body, but excessive mullite (＞40%) will increase the sintering shrinkage (from 1.5% to 3.0%), resulting in density fluctuations.
III. Particle morphology and surface characteristics
1. Particle shape Angular particles (crushed bauxite): bulk density 1.8-2.0g/cm³, strong mechanical bite, high green body strength (compressive strength 15-20MPa);
Spherical particles (fused corundum): bulk density 2.2-2.4g/cm³, but the bonding between particles is weak, and 10%-15% fine powder needs to be added to enhance the interface bonding.
2. When the surface hydroxyl (-OH) content of the surface active raw material is high, hydrogen bonding is easily formed during molding, and the green body density is increased by 5%-8%; however, the hydroxyl decomposes during sintering to produce water vapor. If it is not discharged in time, it will cause internal bubbling (porosity increases by 2%-3%).
IV. Additives and admixtures
1. Sintering aid B₂O₃ (0.5%-1.0%): reduce the sintering temperature (from 1600℃ to 1550℃), promote the formation of liquid phase, and increase the density by 0.1-0.2g/cm³;
TiO₂ (1.0%-2.0%): inhibit the coarsening of corundum grains (the average grain size is reduced from 50μm to 20μm), and refine the structure.
2. Strengthening agent silicon micropowder (≤5%): reacts with Al₂O₃ at 1450℃ to form mullite whiskers, bridge the pores, and reduce the apparent porosity by 3%-5%.
V. High alumina brick Raw material pretreatment process
1. Calcination system The calcination temperature of bauxite increased from 1500℃ to 1700℃, and the corundum phase content increased from 60% to 85%, but over-calcination caused the particle surface to become vitrified (density decreased by 0.1g/cm³).
2. Acid treatment and impurity removal The bauxite was soaked in 5% hydrochloric acid, and the Fe₂O₃ content decreased from 2.5% to 0.8%. After sintering, the density increased from 2.65g/cm³ to 2.80g/cm³.
