The particle grading of high alumina brick raw materials directly affects its density through the following mechanisms, the core of which is to optimize the particle stacking efficiency and reduce the porosity:
I. The core role of particle grading
1. Filling the gaps Large particles (such as 8-5mm) form a skeleton, medium particles (5-3mm) fill the gaps between large particles, fine powder (3-0mm) further fills the secondary pores between medium particles, and micro powder
2. Reduce porosity Reasonable grading reduces the free space between particles and reduces the initial porosity of the green body after forming (the apparent porosity can be reduced from 25% to below 15%), laying the foundation for subsequent sintering densification.
II. High alumina brick Key parameters for grading optimization
1. Wide grading (covering coarse, medium, fine, and micro powder) is easier to achieve dense stacking than narrow grading. Rosin-Rammler distribution: Optimize the particle size ratio through a mathematical model, so that the particle size decreases exponentially and the filling efficiency is maximized.
2. Particle shape Angular particles (such as crushed bauxite) have a higher packing density than smooth particles due to interlocking, but they need to be combined with appropriate fine powder to fill the gaps between the edges.
III. Quantitative relationship between gradation and density
1. Ratio of coarse particles Too much coarse particles (> 40%) will lead to a loose skeleton and difficult penetration of fine powder; too little will not form an effective support structure.
2. Critical value of micro powder The proportion of micro powder (≤0.088mm) must be > 20% to effectively fill the microscopic pores generated by particle rearrangement during sintering.
IV. Actual cases and synergistic effects
1. High alumina bricks for cement kilns use coarse powder (8-5mm) 30% + medium powder (5-3mm) 30% + fine powder (3-0mm) 20% + micro powder (≤0.088mm) 20%, combined with vibration and pressure molding, the density reaches 2.75g/cm³, and the apparent porosity is ≤12%.
2. Collaborative grading with other processes + high pressure forming: Increase the proportion of fine powder to 50%, combined with 100MPa isostatic pressing, the density can reach 3.2g/cm³.
Grading + microwave sintering: shorten the sintering time through wide grading, while inhibiting excessive grain growth, and increase the density by 3%-5%.
V. Common problems and solutions
Low green body strength Insufficient fine powder, weak bonding between particles Increase micro powder to 20%-30%
Density after sintering does not meet the standard Grading imbalance, residual large pores Adjust the coarse: medium: fine ratio to 3:3:2 or 4:3:3
Sintering shrinkage is too large Too much fine powder leads to insufficient liquid phase Add 1%-2% sintering aid (such as B₂O₃).
