Aluminum phosphate is one of the series of phosphate binders and the most commonly used phosphate binder. In addition to aluminum phosphate, the orthophosphate binders include zirconium phosphate, magnesium phosphate, chromium phosphate, and composite phosphate binders.
Properties of Aluminum Phosphate
Aluminum phosphate is a salt, non-toxic, and has no erosive effect on human skin.
Aluminum phosphate is usually prepared by reacting phosphoric acid with more active aluminum hydroxide. With the different P2O5/Al2O3 molar ratios (expressed by M) in the reactants, the reaction products are also other (that is, the degree of oxygen substitution in phosphoric acid is different). There is aluminum dihydrogen phosphate [Al(H2PO4)3 or Al2O3·3P2O5·6H2O, also known as the first aluminum phosphate]. Aluminum mono hydrogen phosphate [Al2(HPO4)3 or 2Al2O3·3P2O5·3H2O, also known as the second aluminum phosphate or aluminum hydrogen phosphate] and aluminum orthophosphate (AlPO4 or Al2O3·P2O5, also known as the third aluminum phosphate or aluminum phosphate).
With the difference in the degree of substitution of hydrogen in phosphoric acid by Al2O3, the properties of various aluminum phosphates are also very different.
Generally speaking, acid aluminum phosphate (ie M>1) has good cementing properties: when the M value is small, the solubility is low or even insoluble, and cannot be used as a binder. Usually the M=3~5 of the aluminum phosphate binder, that is, the main component is aluminum dihydrogen phosphate.
The viscosity of aluminum phosphate solution is related to temperature, concentration, and composition. The viscosity increases with the increase of the amount of aluminum dihydrogen phosphate contained in it. At the same concentration, the viscosity decreases with increasing temperature.
Preparation of Aluminum Phosphate
Aluminum phosphate has two kinds of liquid and solid
Liquid aluminum phosphate is a colorless transparent viscous aqueous solution, M is about 3, density 1.47~1.48g/cm3, pH=1~2.
When the M value is different, the concentration of concentrated phosphoric acid with a concentration of 100% H3PO4 reacts with the aluminum hydroxide to form aluminum mono-hydrogen phosphate, and the required amount is different. If it is prepared with 85% industrial phosphoric acid or 60% phosphoric acid, you only need to divide the required amount of H3PO4 reaction by the concentration of phosphoric acid to get the required amount of phosphoric acid at a certain concentration.
When preparing the aluminum phosphate solution, put the pre-weighed dilution water (hot water when the temperature is low) into an acid-resistant container. Then slowly pour in pre-weighed phosphoric acid, and stir with a stirring rod that does not react with acid to mix phosphoric acid and water evenly. Immediately add aluminum hydroxide in batches, stirring continuously while adding, until all the aluminum hydroxide is dissolved. An aluminum phosphate solution can be obtained after natural cooling.
Solid aluminum dihydrogen phosphate is a white powder, the crystal form is rhombohedral, the content of P2O5 is 57%~67%, and the content of Al2O3 is 17%~22%.
Neutralize phosphoric acid and aluminum hydroxide to prepare a high-concentration aluminum dihydrogen phosphate solution, and then remove part of the water by vacuum evaporation or spray drying at about 95°C at room temperature to obtain solid aluminum dihydrogen phosphate. Ordinary aluminum dihydrogen phosphate powder is extremely hygroscopic and easy to agglomerate, so it should be stored properly.
Cementing Mechanism and Application of Aluminum Phosphate
Aluminum phosphate binder is a kind of thermosetting material, and its cementation mechanism belongs to condensation polymerization. When the material combined with aluminum phosphate is heated, the acid aluminum phosphate decomposes and dehydrates, and polymerizes, resulting in a bonding effect.
Aluminum phosphate is generally used as a binder for acidic and neutral materials such as silicon, clay, high alumina, corundum, zirconium, and silicon carbide. But it is generally not used for alkaline materials, especially magnesia materials (magnesium dihydrogen phosphate can be used as a binder), because aluminum phosphate reacts violently with magnesia, and the mixture coagulates rapidly.
Aluminum phosphate used as a binder has the following characteristics: non-toxic and tasteless, does not corrode the skin, is soluble in water or an aqueous solution at room temperature, and has extremely high viscosity. Materials containing aluminum phosphate binder can be stored for a certain period of time as long as the water does not lose, and the plasticity does not decrease. Drying to above 100°C can produce considerable strength, and no low-melting material will be formed at high temperatures. Adding an appropriate amount of coagulant can make it harden at room temperature, so it can also be used for pouring materials.
In the end
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