The synthesis of phosphoric acid from phosphorus and oxygen

Phosphoric acid (H₃PO₄) is an essential inorganic acid widely used in agriculture, industry, and food processing. One of the most common industrial methods for synthesizing phosphoric acid is the thermal or furnace process, which involves the reaction of elemental phosphorus with oxygen, followed by hydration. This route produces high-purity phosphoric acid suitable for technical and food-grade applications.

1. Overview of the Thermal Process

The thermal process for phosphoric acid production begins with elemental white phosphorus (P₄), which is obtained from phosphate rock through a high-temperature reduction process. The phosphorus is then combusted in the presence of oxygen to form phosphorus pentoxide (P₂O₅), which is subsequently hydrated to yield phosphoric acid.

2. Combustion of Phosphorus

The first stage of synthesis is the oxidation of elemental phosphorus:

P₄ + 5 O₂ → 2 P₂O₅

This highly exothermic reaction occurs in a combustion chamber, typically at temperatures exceeding 1000 °C. The white phosphorus is sprayed into a combustion furnace where it reacts with a controlled stream of dry air or pure oxygen. The result is the formation of phosphorus pentoxide as a fine, white, powdery solid.

3. Hydration of Phosphorus Pentoxide

In the second stage, the phosphorus pentoxide reacts with water to produce phosphoric acid:

P₂O₅ + 3 H₂O → 2 H₃PO₄

This reaction is also exothermic and requires careful temperature control. The hydration is usually conducted in a hydration tower, where P₂O₅ gas or mist is absorbed into water or dilute phosphoric acid. To ensure complete conversion and prevent the formation of polyphosphoric acids, the water-to-oxide ratio must be carefully managed.

4. Purification and Concentration

The crude phosphoric acid produced is then filtered and concentrated. Because the thermal process does not involve insoluble minerals or impurities, the resulting acid is much purer than that produced via the "wet process" (which uses sulfuric acid and phosphate rock). The acid can be concentrated to around 85% by weight for industrial or food-grade use.

5. Advantages of the Thermal Process

High Purity: The product is virtually free of heavy metals and other contaminants, making it ideal for food and pharmaceutical use.

Controlled Composition: The thermal process allows precise control of acid concentration and quality.

Environmental Considerations: Though energy-intensive, the process generates fewer waste streams compared to wet processing methods.

Conclusion

The synthesis of phosphoric acid from phosphorus and oxygen is a well-established industrial method that yields high-purity acid through the controlled oxidation of elemental phosphorus and subsequent hydration of phosphorus pentoxide. This thermal process, despite its high energy demands, plays a key role in producing phosphoric acid for specialized applications where quality and purity are critical.