Sulfuric Acid in Phosphoric Acid Production

Phosphoric acid (H₃PO₄) is a key intermediate in the production of phosphate fertilizers such as monoammonium phosphate (MAP) and diammonium phosphate (DAP). The most common method for producing phosphoric acid is the wet process, which relies heavily on sulfuric acid (H₂SO₄) to convert naturally occurring phosphate rock into a water-soluble form.

The Chemistry Behind Sulfuric Acid

The core reaction involves treating phosphate rock (mainly fluorapatite, Ca₅(PO₄)₃F) with concentrated sulfuric acid. The chemical reaction is:

This yields phosphoric acid and calcium sulfate (gypsum) as a byproduct. The gypsum is separated via filtration, and the resulting phosphoric acid is concentrated for use in fertilizer production.

Process Overview: Wet-Process Phosphoric Acid

1. Grinding: Phosphate rock is finely ground to increase surface area.
2. Reaction: Sulfuric acid is added in a reactor, typically under controlled temperature (70–80°C) and agitation.
3. Filtration: The slurry is filtered to separate gypsum from the phosphoric acid solution.
4. Concentration: The acid is evaporated to reach desired P₂O₅ levels (typically 26–32%).
5. Storage and Use: The acid is stored or further processed into MAP, DAP, or other phosphate-based fertilizers.

Why Sulfuric Acid Is Indispensable in Phosphoric Acid Production

Sulfuric acid (H₂SO₄) is not just a reagent—it is the driving force behind the wet-process route for phosphoric acid production.

1. High Reactivity with Phosphate Rock

Sulfuric acid reacts vigorously with phosphate rock, even when the ore contains impurities such as silica, carbonates, or fluorides. This high reactivity ensures:

• Complete conversion of calcium phosphate into soluble phosphoric acid
• Formation of gypsum (CaSO₄) as a manageable byproduct
• Reduced need for pre-treatment of lower-grade ores

This makes sulfuric acid ideal for processing a wide range of phosphate rock qualities, including those with variable mineralogy.

2. Energy Integration and Process Synergy

Modern sulfuric acid plants are designed to recover large amounts of heat during the conversion of sulfur to sulfuric acid. This heat is typically used to:

• Generate high-pressure steam
• Support evaporation and concentration stages in phosphoric acid production
• Reduce external energy demand for the entire fertilizer complex

This energy synergy not only improves overall efficiency but also lowers the carbon footprint of the operation. Some facilities even use this recovered energy to generate electricity for internal use or grid supply.

3. Process Simplicity and Scalability

The wet-process method, which uses sulfuric acid, is significantly less energy-intensive than the thermal process (which requires burning elemental phosphorus at high temperatures). Key advantages include:

• Lower capital and operating costs
• Simpler equipment and maintenance
• Scalability for small, medium, and mega fertilizer plants
• Compatibility with integrated fertilizer production (e.g., MAP, DAP)

This simplicity has made the wet process the dominant global method for phosphoric acid production, accounting for over 90% of global output.

Environmental and Operational Considerations

• Byproduct Management: Gypsum must be handled properly to avoid environmental contamination.
• Emission Control: Modern plants use mist eliminators and scrubbers to reduce SO₂ and fluoride emissions.

• Water Use: Efficient water recycling systems are essential to minimize consumption and wastewater discharge.

Conclusion on Sulfuric Acid in Phosphoric Acid Production

Sulfuric acid stands as the indispensable backbone of phosphoric acid production, particularly via the wet process method. Its exceptional reactivity with a wide range of phosphate rock qualities ensures complete conversion to phosphoric acid, while its synergy with on-site energy systems boosts process efficiency and reduces operational costs. The wet process, powered by sulfuric acid, offers a simplified, scalable, and less energy-intensive route—making it the dominant global method for producing phosphate-based fertilizers.

For fertilizer manufacturers and industrial investors, securing a consistent supply of high-purity sulfuric acid is not just a technical requirement—it’s a strategic priority for ensuring the sustainability, productivity, and competitiveness of their operations.