The production methods of phosphoric acid in the industry

Phosphoric acid (H₃PO₄) is a critical industrial chemical used in a wide range of applications, from fertilizers to food additives and industrial cleaners. In the global market, phosphoric acid is produced mainly through two methods: the wet process and the dry (thermal) process. Each method serves different industrial needs and has its own advantages and challenges.

1. Wet Process

The wet process is the most commonly used method for producing phosphoric acid and is typically employed in large-scale production. This method starts with the extraction of phosphate rock, which is primarily composed of calcium phosphate minerals. The process involves the following steps:

Raw Material Preparation: The phosphate rock is first mined and then crushed into fine particles.

Acidulation: The crushed phosphate rock is reacted with sulfuric acid (H₂SO₄) in large reactors. This produces phosphoric acid, calcium sulfate (gypsum), and water.

Ca₃(PO₄)₂+3H₂SO₄→2H₃PO₄+3CaSO₄

Ca₃(PO₄)₂+3H₂SO₄→2H₃PO₄+3CaSO₄

Separation: The resulting mixture contains phosphoric acid and calcium sulfate. The gypsum is separated through filtration, leaving behind a diluted phosphoric acid solution.

Concentration: The phosphoric acid solution is then concentrated by evaporation to increase the acid concentration to the desired level (typically around 54% P₂O₅).

Purification: To obtain high-purity phosphoric acid, additional purification steps, such as solvent extraction or precipitation, may be used to remove impurities.

The wet process is highly efficient and cost-effective, making it the preferred method for large-scale production. However, it also generates significant amounts of gypsum as a byproduct, which can pose environmental challenges.

2. Dry (Thermal) Process

The dry or thermal process for producing phosphoric acid is less commonly used than the wet process but is crucial for producing high-purity phosphoric acid for certain applications, such as food-grade phosphoric acid and for specialized industrial uses. In this process, phosphate rock is directly reacted with carbon at high temperatures in an electric furnace.

Raw Material Preparation: The phosphate rock is finely ground and mixed with carbon (usually coke).

High-Temperature Reaction: The mixture is then heated in an electric furnace at temperatures of around 1,500–2,000°C. This results in the production of phosphoric acid vapors, which are collected by condensation, along with carbon monoxide as a byproduct.

Ca₃(PO₄)₂+5C+3O₂→2P₂O₅+5CO

Condensation and Purification: The phosphorus pentoxide (P₂O₅) produced is then dissolved in water to form phosphoric acid. The acid produced by this method is generally purer than that from the wet process, making it suitable for high-end applications.

While the dry process produces a higher purity acid, it is energy-intensive and less environmentally friendly due to the high temperatures involved and the potential for hazardous byproducts like phosphorus vapors.

3. Comparison of the Wet and Dry Processes

Factor Wet Process Dry (Thermal) Process

Raw Material Phosphate rock and sulfuric acid Phosphate rock and carbon

Main Products Phosphoric acid and gypsum (CaSO₄) Phosphoric acid and carbon monoxide (CO)

Purity of Phosphoric Acid Lower (for most grades) Higher (suitable for food and specialty uses)

Energy Requirement Moderate, requires acid production equipment High, requires electric furnace and high temperatures

Environmental Impact Gypsum waste disposal High emissions and energy consumption

Applications of Phosphoric Acid

Phosphoric acid is used across various industries, with the largest demand in the production of fertilizers. It is also used in food and beverage processing as an acidulant, in the pharmaceutical industry for drug synthesis, in metal treatment processes, and as a rust inhibitor in water treatment applications.

Fertilizers: The majority of phosphoric acid is used in the production of phosphate fertilizers, such as ammonium phosphate and superphosphate.

Food and Beverages: It is used as an acidulant to provide tangy flavors in carbonated beverages and processed foods.

Water Treatment: Phosphoric acid plays a crucial role in adjusting the pH of water and controlling scale formation in boilers.

Conclusion

Phosphoric acid production remains a vital part of the global industrial landscape. While the wet process dominates due to its efficiency and cost-effectiveness, the thermal process provides higher-purity phosphoric acid for specialized applications. Both methods have their strengths and are selected based on the intended use of the final product, highlighting the versatility of phosphoric acid in modern industries.