Introduction
Aluminum dihydrogen phosphate (H₂Al(PO₄)₂) is a key high-temperature binder in the refractory industry. It primarily functions by forming an initial bond during room-temperature hardening and then developing a ceramic-phase structure through high-temperature sintering. This process bonds the refractory aggregates into a solid whole, giving the material excellent high-temperature resistance (stable above 1600 °C), slag erosion resistance, and thermal shock resistance.
Typical applications include:
Refractory castables/plastisols: Used for lining blast furnaces, ladles, and similar equipment to resist erosion from molten iron and slag.
Refractory coatings: Applied to furnace surfaces to provide insulation and resist penetration by high-temperature gases.
Repair compounds: Used for rapid repair of damaged refractory parts, forming a chemical bond with the original material.
Compared with binders such as water glass and cement, aluminum dihydrogen phosphate offers significant advantages in high-temperature strength and resistance to acid slag erosion. It is a core refractory raw material for high-temperature industries such as iron and steel, glass, and ceramics.
function
1. Ambient hardening and initial bonding
H₂Al(PO₄)₂ in aqueous solution dissociates into Al³⁺ and H₂PO₄⁻, which adsorb onto or weakly chemically bond with hydroxyl groups (–OH) on the surface of the refractory aggregate, forming the initial adhesion.
As water evaporates and the solution becomes more concentrated, H₂PO₄⁻ gradually polymerizes into polyphosphates, while Al³⁺ and phosphate form a network-like colloidal structure, which imparts room-temperature strength to the material.
2. High-temperature sintering and ceramization combination
When heated to 200–600 °C, H₂Al(PO₄)₂ gradually decomposes,The aluminum phosphate (AlPO₄) produced reacts with oxides in the refractory aggregate (such as Al₂O₃ and SiO₂) to form a low-melting-point eutectic. This eutectic creates a glassy-phase bond between the particles, significantly enhancing the high-temperature strength of the material.
As the temperature further increases above 1000 °C, the glassy phase gradually crystallizes into high-temperature-resistant crystals such as mullite (3Al₂O₃·2SiO₂), which provides the refractory material with excellent thermal shock resistance and structural stability.
Application
1. Refractory Castables and Plastisols
Application:
Used in high-temperature areas such as kiln linings, blast furnace iron troughs, ladle permanent linings, and similar components.
Advantages:
Good compatibility with aggregates such as corundum and spinel; excellent workability after mixing (can be cast or rammed into shape).
Hardening speed can be adjusted by adding accelerators or retarders, making it suitable for on-site construction.
The ceramic-phase structure formed at high temperatures provides resistance to slag erosion (e.g., slag in iron and steel smelting, and molten glass in glass kilns).
2. Refractory Paints and Coatings
Application:
Applied to the outer walls of kilns or the surfaces of metal components, where it provides thermal insulation and resistance to washout.
Advantages:
Forms a dense coating after drying, which prevents the penetration of high-temperature gases and reduces heat loss.
Bonds strongly to metal substrates and can withstand frequent heating and cooling cycles (offers better thermal shock resistance than silicate-based binders).
3. Refractory Pounding and Repair Material
Application:
Used for repairing damaged parts of blast furnace cylinders, rotary kiln linings, and similar equipment.
Advantages:
Can be manually or mechanically pounded and molded at room temperature to quickly fill gaps.
After firing, it forms a chemical bond with the original refractory material, preventing peeling at the repair site.
Key technical points in the application
1. Proportioning Optimization
The concentration of aluminum dihydrogen phosphate solution is usually controlled between 50% and 60%. A concentration that is too low leads to insufficient adhesion, while a concentration that is too high causes the material to harden too quickly.
The particle size distribution of the refractory aggregate should be matched with the binder dosage (generally 5% to 15%) to avoid excessive shrinkage at high temperatures caused by an excessive amount of binder.
2. Curing System Control
During the room temperature drying stage (25–60 °C), rapid water loss must be avoided to prevent cracking of the material.
During heating and sintering, the heating rate should be controlled (for example, the rate should not exceed 5 °C/min in the 100–300 °C range) to prevent rapid evaporation of water, which would lead to high porosity.
3. Hydrolysis Resistance and Modification
In long-term humid environments, aluminum dihydrogen phosphate may hydrolyze to produce Al(OH)₃ precipitates, reducing adhesion. Therefore, it is necessary to add water repellents (such as sodium methylsilanolate) or perform water-repellent treatments.
Doping with oxides (e.g., Cr₂O₃, ZrO₂) can improve the coefficient of thermal expansion at high temperatures, further enhancing resistance to thermal shock.
Typical Applications
Iron and Steel Industry: Refractory castables for blast furnace linings that can withstand high temperatures of 1400–1600 °C and resist erosion from molten iron and slag.
Glass Industry: Refractory coatings applied to the roofs of glass kilns, capable of resisting erosion by volatile substances such as Na₂O in molten glass.
Ceramic Sintering Furnaces: Refractory plastisols used on the inner walls of furnaces, maintaining structural stability during sintering at 1300–1500 °C to extend the furnace's service life.
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