Methods for the Preparation of Phosphoric Acid

outline: Phosphoric acid is a basic chemical raw material widely used in the production of phosphates, fertilizers, food additives, animal feed, detergents, electronics, flame retardants, and other industries. Pure phosphoric acid is a colorless crystalline substance with a melting point of 42.3°C; it is a high-boiling acid that is readily soluble in water. Phosphoric acid is a triprotic weak to moderate acid that undergoes stepwise ionization; it is non-volatile, resistant to decomposition, and exhibits virtually no oxidizing power. It displays the typical properties of an acid. Industrially, concentrated sulfuric acid is commonly reacted with calcium phosphate to produce phosphoric acid; the slightly water-soluble calcium sulfate precipitate is then filtered off, leaving a phosphoric acid solution as the filtrate.

　　Background and Overview[1]

　　Phosphoric acid is a basic chemical raw material widely used in the production of phosphates, fertilizers, food additives, animal feed, detergents, electronics, flame retardants, and other industries. Pure phosphoric acid is a colorless crystalline substance with a melting point of 42.3°C; it is a high-boiling acid that is readily soluble in water. Phosphoric acid is a triprotic weak to moderate acid that undergoes stepwise ionization, is non-volatile and resistant to decomposition, and exhibits virtually no oxidizing power. It displays the typical properties of an acid. Industrially, concentrated sulfuric acid is commonly reacted with calcium phosphate to produce phosphoric acid; the slightly water-soluble calcium sulfate precipitate is then filtered off, leaving a phosphoric acid solution in the filtrate.

　　Report One,

　　A method for producing crystalline phosphoric acid by concentrating phosphoric acid with phosphorus pentoxide, comprising the following steps:

　　Pure yellow phosphorus with a purity of 99.95% and an arsenic content of 152 × 10⁻⁶ is thoroughly melted at 70°C. Based on a ratio of 1 kg of yellow phosphorus to 6.3–6.6 m³ of air, a pump delivers the molten phosphorus to the yellow-phosphorus burner, where it is mixed with compressed air at 0.7 MPa and injected into the phosphorus-fired boiler. Inside the boiler, the yellow phosphorus is rapidly atomized and burns vigorously.

　　The P2O5 generated during combustion is introduced at the top of the hydration tower, where deionized water is initially sprayed for cooling. Once a dilute acid is formed, the spray is switched to dilute phosphoric acid. Dilute phosphoric acid is continuously sprayed from the top and evenly overflows down the inner walls, circulating to absorb the P2O5. After 100% H3PO4 is obtained, a suitable amount of deionized water is added at the tower top to maintain the spray, and the 100% H3PO4 is then sent to the arsenic-removal unit. There, HCl gas is introduced at 0.5% of the total weight of the phosphoric acid, with the temperature maintained at 130°C for 120 minutes. The product is then cooled to 32°C and forwarded to the crystallizer, where phosphate seed crystals equivalent to 0.4% of the total weight of the phosphoric acid are added and the mixture is stirred. The resulting crystalline phosphoric acid product is analyzed as follows: P2O5 72.42%, SO42− 25×10−6, Fe 8×10−6, As 0.5×10−6, Cl− 2×10−6, Pb 2×10−6.

　　Report Two,

　　A process for producing phosphoric acid by direct reduction of phosphate ore using double-layer composite pellets, the method being as follows:

　　Chemical composition of the raw materials: phosphate rock contains 25% P₂O₅; silica sand contains 98% SiO₂; and coke powder contains 75% fixed carbon. The three raw materials are blended according to a CaO/SiO₂ molar ratio of 0.4, with a carbon addition equal to 1.5 times the theoretical amount required for the reduction of P₂O₅ in the phosphate rock. The mixture is then co-ground until more than 80% passes a 200-mesh sieve, after which 2% sodium humate binder is added and water is incorporated to form pellets. The moisture content of the pellets is 12%, and their diameter ranges from 10 to 16 mm. These pellets are subsequently subjected to a second pelleting process, during which they are coated with a layer of coke powder that passes more than 80% through a 200-mesh sieve. The weight of the coke powder accounts for 10% of the pellet weight. The pellets, meeting the specified chemical composition and physical specifications, are dried and consolidated at 100°C, then fed into a φ0.8 m × 10 m radial-draft rotary kiln. The reduction temperature inside the kiln is maintained at 1300°C, and the material remains in the kiln for 1.5 hours. Under these conditions, the phosphorus volatilization rate within the pellets exceeds 80%. The resulting kiln gas containing P₂O₅ is subsequently hydrated and absorbed to produce phosphoric acid with a concentration of 85% H₃PO₄.