Technical Requirements for Phosphate‑Bonded High‑Alumina Bricks (Phosphate Bricks)

Product characteristics: Unfired chemically bonded refractory bricks. Widely used in the transition zone, preheating zone, kiln hood and grate‑cooler of rotary kilns. They feature high medium‑temperature strength, good wear resistance and thermal shock resistance, but cannot withstand prolonged excessive high temperatures.

1. Service Conditions

1. Service temperature: Long‑term operating temperature ≤1300 °C; short‑term maximum temperature shall not exceed 1400 °C. Do not apply to the burning zone with sustained temperatures above 1450 °C.

2. Suitable locations: Areas subject to heavy wear and erosion, frequent start‑stop operations and large temperature fluctuations. Good resistance to alkaline dust and alkali corrosion.

3. Unsuitable conditions: Strong acidic media, long‑term temperatures above 1400 °C. For strong reducing atmospheres with high CO content, modified SiC phosphate bricks shall be adopted.

4. Ambient environment: Storage and construction sites must be dry and moisture‑proof. Moisture absorption will cause brick powdering and strength loss. Ambient humidity should preferably be below 70%.

2. Storage and Handling Requirements

1. Store in a dry warehouse, protected from rain and moisture. Do not store outdoors. Moisture‑absorbed bricks must not be installed in kilns.

2. Handle with care. Bricks with chipped or damaged edges and corners shall not be used for masonry.

3. Keep bricks dry. Do not wet brick surfaces with water.

3. Masonry and Construction Requirements

1. Matching mortar: Use special phosphate mortar only. Ordinary silicate refractory mortar is prohibited, as it will lead to bonding failure and brick falling.

2. Joint control: For ring masonry in rotary kilns, keep mortar joints at 2‑3 mm with full filling. Bricks shall fit closely to the kiln shell. Phosphate mortar shall not directly contact the steel shell; use alumina mortar as isolation backing between bricks and shell.

3. Construction ambient temperature: 5‑35 °C. Low temperature slows down curing.

4. Adopt ring‑masonry technique for rotary kilns. Carry out symmetrical sectional masonry with proper supports. Control expansion joints and reserve gaps for thermal expansion.

5. After masonry, cure naturally at room temperature for 24‑48 h to allow full curing of phosphate binder. Avoid water, rain and direct cold‑air blowing during curing.

4. Critical Heating‑up Requirements (Very Important)

Phosphate bricks contain crystal water. Excessively fast heating will easily cause bulging, cracking and spalling.

1. Start kiln heating only after curing is completed. Never fire directly after masonry without proper curing.

2. Recommended heating‑up rates:

- Room temperature ~ 300 °C: ≤15‑20 °C/h. Hold temperature plateaus to remove free water.

- 300 ~ 600 °C: ≤20‑25 °C/h. This stage is critical for removing crystal water from phosphate; rapid heating is forbidden.

- Above 600 °C: Increase rate to 25‑30 °C/h up to operating temperature.

3. Avoid prolonged shutdowns during heating‑up. Cool down slowly; do not apply rapid cooling, strong cold‑air blowing or large‑volume cold‑air intake through manholes.

5. Operation Requirements in Production

1. Avoid sustained kiln temperatures exceeding 1400 °C, as high temperature accelerates softening and damage of bricks.

2. Minimize drastic temperature swings. Strictly follow heating‑up and cooling curves for frequent start‑stop cycles.

3. Key inspection items: Check for brick loosening, bulging, corner spalling and flame leakage. Treat local damage in time to prevent further deterioration.

4. Kiln shutdown maintenance: Cool down slowly. Forced air cooling of the lining is prohibited.

6. Common Failure Causes

- Moisture absorption or rain exposure leading to brick powdering;

- Use of mismatched ordinary refractory mortar;

- Insufficient curing before rapid heating‑up;

- Too‑fast heating causing rapid vaporization of crystal water, resulting in bulging and cracking;

- Misapplication in high‑temperature burning zone.