Production process and application of phosphoric acid
Introduction
Phosphoric acid (H₃PO₄) is one of the most important industrial chemicals, with global production exceeding 50 million tons annually. As a key intermediate in phosphate chemistry, it serves as the foundation for numerous products ranging from fertilizers to food additives. This article examines the two primary production methods – wet process and thermal process – and explores their respective advantages, limitations, and applications across industries.
1. Production Processes
1.1 Wet Process Phosphoric Acid
The wet process accounts for approximately 90% of global phosphoric acid production due to its cost-effectiveness for fertilizer-grade acid.
Chemical Principle
The core reaction involves treating phosphate rock with sulfuric acid:
Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O → 3H₃PO₄ + 5CaSO₄·2H₂O + HF
Production Steps
- Raw Material Preparation: Phosphate rock (typically 30-40% P₂O₅ content) is ground to 100-150 μm particle size.
- Acidulation: Reacted with 93-98% sulfuric acid in a series of stirred reactors at 70-80°C.
- Filtration: Gypsum (CaSO₄·2H₂O) byproduct is separated using vacuum belt filters.
- Concentration: Evaporated from 28-32% P₂O₅ to 40-54% P₂O₅ in forced-circulation evaporators.
Technical Parameters
- Typical P₂O₅ recovery: 92-96%
- Energy consumption: 0.9-1.2 GJ/ton P₂O₅
- Impurities: 2-4% fluorosilicates, 1-2% iron/aluminum compounds
1.2 Thermal Process Phosphoric Acid
The thermal process produces high-purity acid (75-85% H₃PO₄) for food and technical applications.
Production Mechanism
- Phosphorus Production:
2Ca₃(PO₄)₂ + 6SiO₂ + 10C → P₄ + 6CaSiO₃ + 10CO (at 1400-1500°C) - Combustion & Hydration:
P₄ + 5O₂ → P₄O₁₀ P₄O₁₀ + 6H₂O → 4H₃PO₄
Key Process Stages
- Electric furnace: Reduces phosphate rock with coke and silica
- Condensation: Phosphorus vapor is condensed under water
- Combustion tower: P₄ burns in excess air at 1800-3000°C
- Hydration tower: P₄O₁₀ gas is absorbed in recycled phosphoric acid
Quality Specifications
- Purity: ≥85% H₃PO₄
- Heavy metals: <1 ppm As, <10 ppm Pb
- Color: APHA ≤20
2. Comparative Analysis
| Parameter | Wet Process | Thermal Process |
|---|---|---|
| P₂O₅ concentration | 28-54% | 75-85% |
| Energy consumption | Low | Very high |
| Capital cost | $200-400M/plant | $500-800M/plant |
| Product purity | Technical grade | Food/pharma grade |
| Byproducts | Gypsum (5-6t/t) | None |
3. Industrial Applications
3.1 Fertilizer Production (Wet Process)
- MAP/DAP: 60% of global output becomes (NH₄)₂HPO₄/NH₄H₂PO₄
- NPK fertilizers: Provides water-soluble phosphorus
- Superphosphates: Reacts with phosphate rock to create Ca(H₂PO₄)₂
3.2 Food Industry (Thermal Process)
- Acidulant: E338 in colas (0.05-0.1%) and processed cheeses
- pH control: Adjusts acidity in jams (pH 3.0-3.5)
- Chelating agent: Binds metal ions in meat products
3.3 Other Applications
- Metal treatment: Phosphating for steel (2-5% solutions)
- Detergents: Builder in TSP formulations (Na₃PO₄)
- Electronics: Etchant for silicon wafers (85% solutions)
- Pharmaceuticals: Excipient in dental cements
4. Environmental Considerations
- Wet process: Gypsum stacks may contain 0.2-0.5% residual P₂O₅
- Thermal process: Energy intensity (~12 MWh/ton P₄)
- Emerging technologies: Hemihydrate process reduces waste by 30%
Conclusion
While the wet process dominates bulk production for fertilizers, the thermal method remains essential for high-purity applications. Ongoing developments in purification techniques (solvent extraction, membrane filtration) are blurring the traditional quality divide between the two processes. Future advancements may focus on reducing the environmental footprint through improved byproduct utilization and energy recovery systems.
PolyblueChem is the supplier of Phosphoric acid, CAS:7664-38-2