Some Factors Affecting the Strength of Unfired High-Alumina Bricks Bonded with Phosphoric Acid

The heat treatment temperature of phosphoric acid combined unfired high-alumina bricks is 350-450℃, so that non-hygroscopic AlPO4 is formed in the unfired bricks. Adding 10% of binding clay to phosphoric acid combined unfired bricks can improve the high-temperature volume stability of unfired bricks. At the same time, it improves the molding performance and baking strength of unfired bricks. Some industrial alumina and aluminum hydroxide can be added to phosphoric acid combined unfired high-alumina bricks to adjust the reaction rate with phosphoric acid. For unfired bricks, the phosphoric acid concentration is 40-60%, and the addition amount is 4-6%. If the concentration is too high and the amount is too small, the mixture will be dry and the molding performance will be poor; if the concentration is too low and the amount is too much, the mixture will be thin and harden slowly. With the increase of phosphoric acid concentration and addition amount, the load softening point tends to decrease, the linear change after high-temperature firing also increases, and the compressive strength after high-temperature firing also decreases.

Unfired High-Alumina Bricks Combined with Phosphoric Acid

Some Factors Affecting the Strength of Unfired High-Alumina Bricks Combined with Phosphoric Acid

1.) Raw material calcination temperature, binder dosage, powder fineness, molding pressure and heat treatment temperature.

2.) The amount of phosphoric acid solution added should be 4.5-5% of pure phosphoric acid according to the prescribed ratio. Those below the lower limit have poor binding ability, and those above the upper limit are easy to vitrify at high temperature (1300℃) to form brittle materials.

3.) Powder fineness directly affects the amount of phosphoric acid and the bonding strength of the product. The higher the powder fineness, the more sufficient the chemical reaction with the phosphoric acid solution, and the higher the chemical bonding strength.

4.) The influence of heat treatment temperature. At a temperature of 20-100℃, hydrogen phosphate is first formed. After heating, the excess phosphoric acid and the crystal water in the phosphate are continuously evaporated and concentrated, thereby accelerating the precipitation and precipitation of the phosphate and exerting its cementing effect. At 200℃, more polyphosphate Al(PO3)3•H2O is formed, which is an unstable reversible product. At 300-400℃, in addition to Al(PO3)3, there is also AlPO4 salt. At 500℃, Al(PO3)3•H2O disappears, and all forms β-Al(PO3)3 and AlPO4.

5.) Above 300℃, the strength can reach stability, with good moisture resistance.

6) Below 300℃, the strength changes due to moisture and rescue measures.

Characteristics of Phosphate-bonded High-Alumina Bricks

The physical and chemical indicators of phosphate-bonded high-alumina bricks are: refractoriness ≥1970℃, 2 kgf/cm2 load softening point temperature greater than 1350℃. Volume capacity ≥2.75g/cm2, room temperature compressive strength ≥61.8MPa. It has high high temperature compressive strength, good thermal shock stability, and low wear resistance. The main characteristics of phosphate-bonded high-alumina bricks are as follows:

1. Structural characteristics

Phosphate-bonded high-alumina bauxite clinker contains a large amount of α~Al2O3. When it is mixed with phosphoric acid, it begins to combine at 0℃~120℃. In the range of 124℃~427℃, aluminum phosphate and pyrophosphate are generated. At 510℃, most of the aluminum phosphate is generated, which becomes the connecting aggregate of high-alumina bricks. AlPO4 has the spatial skeleton structure of SiO2, and [PO4][AlO4] are both tetrahedral structures. According to the "Thomson Law" in 1947, the aluminum in the phosphate formed at high temperature tends to be combined to the fourth power. Aluminum in silicates formed at low temperature or high pressure tends to combine in the sixth power. Therefore, aluminum phosphate has a stable structure before it decomposes at temperatures below 1500°C. This stable structure has the characteristics of chemical stability, wear resistance, thermal stability, and high temperature strength below 1500°C.

2. Thermodynamic properties

The Al2O3 content in phosphate-bonded high-alumina bricks is generally above 80%. Therefore, Al2O3 plays a leading role in chemical thermodynamic properties. CaO has a greater affinity for oxygen than Al, while Si has a smaller affinity than Al. However, from the phase diagram of CaO~Al2O3~P2O5, a eutectic can only be formed at 1440°C. This provides a theoretical basis for the use of phosphate-bonded high-alumina bricks in preheating zones, decomposition zones, and cooling zones.

3. Chemical stability

The structure of the main mineral components of phosphate-bonded high-alumina bricks is cristobalite-type AlPO4, which is stable before decomposing into AI2O3. However, at higher temperatures, its mechanical strength is slightly reduced. This is because AlPO4 gradually decomposes above 1500%. Since AlPO4 decomposes above 1500°C, the high-temperature performance of phosphate-bonded high-alumina bricks is affected, and the hot strength drops sharply, thereby reducing their hot wear resistance.