The Conditions Inside a Cement Kiln
A rotary cement kiln subjects its lining to a combination of loads that few other processes match. The brick sees a gas temperature well above the burning zone, a moving bed of clinker that abrades the hot face, an alkali and sulphate rich atmosphere, and repeated heating and cooling during start ups. Under these conditions, the lining survives because a stable clinker coating forms on the brick surface and shields it from both abrasion and chemical attack. High alumina bricks support that coating, keep their strength at temperature, and resist the alkali penetration that destroys lower grade materials.
Raw Materials and Firing
Calcined bauxite or fused alumina is combined with plastic clay and selected additives to create a pressable body with predictable firing shrinkage. The mix is batched, dry blended, wetted, then shaped by high pressure pressing, which produces a dense brick with low apparent porosity. Green bricks are dried in a tunnel dryer to remove moisture slowly and then fired between 1400 ℃ and 1550 ℃. This thermal cycle grows mullite crystals that bond the corundum grains, giving the brick its strength at temperature and its resistance to alkali vapour. Bricks are ground to tight dimensional tolerance after firing, because kiln shells are curved and thick joints are the first place where coating breaks away.
Typical Technical Specifications
Grade designations follow GB/T 2988, where the number states minimum alumina content. The values below are typical for rotary kiln and preheater duty.
| Property | LZ-55 | LZ-65 | LZ-75 |
|---|---|---|---|
| Al2O3 content, % min | 55 | 65 | 75 |
| Bulk density, g/cm3 | 2.30 to 2.45 | 2.45 to 2.60 | 2.60 to 2.75 |
| Apparent porosity, % max | 22 | 21 | 20 |
| Cold crushing strength, MPa min | 45 | 50 | 55 |
| Refractoriness under load at 0.2 MPa, ℃ min | 1450 | 1480 | 1500 |
| Thermal expansion at 1000 ℃, % | 0.6 | 0.7 | 0.8 |
Figures are typical acceptance ranges. Actual properties are confirmed on the mill test certificate for each production lot, along with alkali resistance test results where the duty requires them.
Where High Alumina Bricks Are Used in a Cement Plant
Rotary kiln burning zone and transition zone: LZ-75 for the highest thermal and chemical load, with LZ-65 in the transition where coating stability matters more than peak refractoriness.
Preheater tower and calciner: LZ-55 and LZ-65 shapes suit the cyclones and riser ducts, where alkali attack and gas abrasion dominate.
Clinker cooler: dense bricks with good abrasion resistance line the hot end walls and the area below the grate.
Silos and hoppers: lower density alumina shapes reduce structural load while limiting heat loss from stored clinker.
Installation, Coating and Wear Control
Kiln bricks are laid in rings with joints of 1 mm to 2 mm and a mortar matched to the brick alumina content. Expansion allowance is provided between rings and filled with ceramic fibre board so that brick growth during heat up does not distort the shell. First heat up is staged, with holding points that allow free and chemically bound water to escape gradually, and it is normally combined with the kiln start up schedule. In operation the coating is as important as the brick. A stable coating forms where the brick surface is dense, uniform and free of open joints, so a coating that peels in patches usually points to joint problems rather than to the brick itself. Inspection at each major shutdown records coating condition, joint width and remaining brick thickness per ring, so that single rings can be replaced before wear exposes the shell.
Frequently Asked Questions
Q: Why does the coating matter more than the brick in a kiln?
A stable coating shields the brick from abrasion and alkali attack. Without it, even a high grade brick wears quickly in the burning zone.
Q: Which grade is used in the burning zone?
LZ-75 is typical for the burning zone because it combines high refractoriness with low porosity, while LZ-65 is used in the transition zone.
Q: How does alkali attack damage a lining?
Alkali vapour enters open pores and reacts with the brick matrix, causing expansion and cracking behind the hot face. Lower porosity restricts that penetration.
Q: What joint thickness should be used?
Between 1 mm and 2 mm. Thick joints are the first place where coating breaks away and where gas and clinker fines penetrate.
Q: How is lining wear measured?
Remaining brick thickness per ring is measured at every major shutdown and compared with the previous record, which shows the wear rate and the ring that will need replacement first.
