Phosphoric Acid Production

What It Is

Phosphoric acid (H₃PO₄) is the intermediate chemical used to manufacture most phosphate fertilizers. It is produced by reacting phosphate rock with acids, most commonly sulfuric acid, in large industrial plants. From phosphoric acid, fertilizers such as diammonium phosphate (DAP), monoammonium phosphate (MAP), and triple superphosphate (TSP) are made.

How It Works

There are two main industrial routes:

1. Wet Process (dominant, ~90% of global production)
   • Finely ground phosphate rock is digested with concentrated sulfuric acid.
   • Reaction produces phosphoric acid and a by-product called phosphogypsum.
   Ca₁₀(PO₄)₆F₂ + 10H₂SO₄ + 20H₂O → 6H₃PO₄ + 10CaSO₄·2H₂O + 2HF
   • The phosphoric acid is filtered, concentrated, and used directly in fertilizer manufacture.
2. Thermal Process (minor, specialty use)
   • Phosphate rock is burned in an electric furnace with coke and silica.
   • Produces elemental phosphorus, which is oxidized and hydrated into very pure phosphoric acid.
   • Used mainly for food and industrial-grade phosphates, not fertilizers (too costly).

Where It Happens

Major phosphoric acid complexes are located in phosphate-rich countries and export hubs:

• Morocco (OCP Group): World leader in integrated mining and acid production.
• China: Dozens of medium and large plants supplying domestic fertilizer demand.
• United States (Florida): Historically significant, though declining.
• Saudi Arabia (Ma’aden): State-backed production integrated with phosphate mining.
• India: Imports rock and produces phosphoric acid for DAP/MAP.

Environmental Considerations

• Phosphogypsum waste: Produced in massive volumes (5 tonnes per tonne of P₂O₅). Contains heavy metals and naturally occurring radioactive material. Usually stored in stacks, creating long-term environmental risks.
• Fluoride emissions: Must be scrubbed to prevent air pollution.
• Water and energy use: Plants are resource-intensive and can strain local environments.

Why It Matters

Phosphoric acid production is the bottleneck of the phosphate fertilizer industry. Without it, phosphate rock is unusable in large-scale agriculture. Global trade in phosphoric acid (e.g., Morocco to India) is just as important as trade in finished fertilizers.

Outlook

• Short term: Morocco and China will continue to dominate production.
• Medium term: Environmental constraints in the US and EU could reduce capacity, increasing reliance on imports.
• Long term: Managing phosphogypsum waste and reducing emissions are critical challenges. Research into alternative beneficiation and recycling methods is ongoing but far from scale.

Phosphoric acid will remain essential for global agriculture, but its environmental footprint ensures it will be under increasing scrutiny.

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