Introduction

Phosphoric acid (H₃PO₄) is one of the most important inorganic chemicals used worldwide. It plays a vital role in fertilizer manufacturing, food processing, pharmaceuticals, water treatment, metal finishing, detergents, and advanced materials such as lithium iron phosphate (LFP) batteries. Due to its wide range of applications, phosphoric acid is produced through different manufacturing technologies depending on the required purity, production capacity, and economic considerations.

The two major industrial routes for phosphoric acid production are:

1. Wet Process Phosphoric Acid (WPA)
2. Thermal Process Phosphoric Acid (TPA)

The wet process is the dominant technology, contributing more than 90% of global phosphoric acid production, mainly because of its suitability for fertilizer manufacturing and lower production cost. The thermal process, although more expensive, produces extremely high-purity phosphoric acid required for food-grade, pharmaceutical, and electronic applications.

Understanding the differences between these two processes is essential for chemical engineers, plant designers, and industry professionals involved in phosphoric acid production.

Wet Process Phosphoric Acid

Principle of Wet Process

The wet process involves the reaction of phosphate rock with sulfuric acid to produce phosphoric acid and calcium sulfate (gypsum) as a by-product.

The main chemical reaction is:

Ca₁₀(PO₄)₆F₂ + 10H₂SO₄ + 20H₂O → 6H₃PO₄ + 10CaSO₄·2H₂O + 2HF

In this process, phosphate rock is directly converted into phosphoric acid without producing elemental phosphorus.

The quality of wet-process acid depends strongly on the quality of phosphate rock used. Different impurities present in the rock dissolve into the acid, affecting the final product quality.

Wet Process Manufacturing Steps

1. Phosphate Rock Grinding

The first step is preparation of phosphate rock. The rock is crushed and finely ground to increase surface area and improve reaction efficiency.

Typical grinding requirements:

• Particle size: 90% passing 160–200 mesh
• Moisture control to improve grinding performance
• Uniform particle size distribution

Fine grinding improves phosphate recovery and reduces unreacted phosphate losses.

2. Acidulation Reaction

The ground phosphate rock is mixed with sulfuric acid inside a reactor.

Important operating parameters include:

During acidulation, phosphate minerals dissolve and calcium sulfate crystals are formed.

The process can operate in different modes:

• Dihydrate process (DH)
• Hemihydrate process (HH)
• Hemihydrate-Dihydrate process (HDH)

The selected technology influences P₂O₅ recovery, gypsum quality, and filtration performance.

3. Gypsum Crystallization

During reaction, calcium sulfate crystals form and grow. Proper crystal control is extremely important because it affects:

• Filtration rate
• P₂O₅ losses
• Moisture content of gypsum cake
• Plant productivity

Poor crystal formation can lead to:

• Difficult filtration
• High phosphate losses
• Increased operating costs

4. Filtration

The reaction slurry is sent to a vacuum belt filter where phosphoric acid is separated from gypsum.

Products obtained:

Liquid phase:

• Weak phosphoric acid

Solid phase:

• Phosphogypsum

Modern plants use advanced horizontal belt filters to achieve high recovery and low moisture gypsum.

5. Acid Concentration

The filtered acid usually contains approximately:

• 26–32% P₂O₅

It is concentrated through evaporation using steam.

Typical product concentration:

• 40–54% P₂O₅

Multiple-effect evaporators or forced circulation evaporators are commonly used for efficient concentration.

Advantages of Wet Process Phosphoric Acid

1. Low Production Cost

The biggest advantage of the wet process is economical production. It uses directly available phosphate rock and sulfuric acid, making it suitable for large-scale fertilizer plants.

2. High Production Capacity

Wet-process plants can produce thousands of tonnes per day.

Typical industrial capacities:

• 500 TPD
• 1,000 TPD
• 2,000 TPD
• 5,000 TPD+

3. Lower Energy Requirement

Unlike the thermal process, wet processing does not require an electric furnace. Therefore, energy consumption is significantly lower.

4. Suitable for Fertilizer Manufacturing

Wet-process acid is the preferred raw material for:

• DAP
• MAP
• NPK fertilizers
• SSP
• TSP

Limitations of Wet Process

The main disadvantage is impurity contamination.

Phosphate rock contains minerals that dissolve during acidulation, including:

• Iron
• Aluminum
• Magnesium
• Fluoride
• Sulfate
• Heavy metals
• Organic compounds

These impurities affect acid quality and limit direct use in food or pharmaceutical industries.

Additional purification steps are required for high-purity applications.

Thermal Process Phosphoric Acid

Principle of Thermal Process

The thermal process produces phosphoric acid through elemental phosphorus combustion.

Unlike the wet process, phosphate rock is first converted into phosphorus, which is then oxidized and hydrated to form phosphoric acid.

The process produces very high-purity acid because most impurities remain separated during phosphorus production.

Thermal Process Manufacturing Steps

1. Electric Furnace Production of Phosphorus

Phosphate rock is mixed with:

• Silica
• Coke

and heated in an electric furnace.

The reaction occurs at approximately:

1,400–1,500°C

Chemical reaction:

Ca₃(PO₄)₂ + 3SiO₂ + 5C → 3CaSiO₃ + 5CO + 2P

Phosphorus vapor is generated and separated from furnace gases.

2. Phosphorus Condensation

The phosphorus vapor is cooled and collected under water.

The collected phosphorus is stored safely before combustion.

3. Phosphorus Combustion

Liquid phosphorus is burned with dry air.

Reaction:

P₄ + 5O₂ → P₄O₁₀

Phosphorus pentoxide is produced.

4. Hydration

Phosphorus pentoxide reacts with water to produce phosphoric acid.

Reaction:

P₄O₁₀ + 6H₂O → 4H₃PO₄

The resulting acid has extremely high purity.

Advantages of Thermal Process

1. Highest Purity

Thermal phosphoric acid contains very low levels of:

• Iron
• Aluminum
• Magnesium
• Sulfates
• Fluoride
• Heavy metals

This makes it suitable for critical applications.

2. Food and Pharmaceutical Grade Production

Thermal acid is used for:

• Carbonated beverages
• Food additives
• Pharmaceutical products
• Laboratory chemicals

3. Electronic and Battery Applications

The growing demand for high-purity phosphoric acid for semiconductor manufacturing and lithium iron phosphate batteries has increased interest in thermal technology.

Limitations of Thermal Process

1. High Energy Consumption

The biggest challenge is electricity consumption due to furnace operation at extremely high temperatures.

2. High Capital Investment

The process requires expensive equipment:

• Electric arc furnace
• Gas cleaning system
• Phosphorus condenser
• Combustion tower
• Hydration system

3. Higher Production Cost

Due to high energy consumption and complex operation, thermal phosphoric acid is significantly more expensive than wet-process acid.

Comparison Between Wet and Thermal Process

Environmental Considerations

Wet Process Challenges

Major environmental concerns include:

• Large phosphogypsum generation
• Fluoride emissions
• Acidic wastewater
• Heavy metal management

Modern plants reduce environmental impact through:

• Fluoride recovery systems
• Gypsum utilization
• Wastewater recycling
• Improved filtration technology

Thermal Process Challenges

The main environmental concern is energy consumption.

Important improvements include:

• Renewable electricity usage
• Waste heat recovery
• Improved furnace efficiency

Future Trends in Phosphoric Acid Production

The phosphoric acid industry is moving toward:

Advanced Wet Process Technologies

Technologies such as HDH processes improve:

• P₂O₅ recovery
• Gypsum filtration
• Energy efficiency

Purification of Wet-Process Acid

New technologies such as:

• Solvent extraction
• Ion exchange
• Membrane purification

are helping convert wet-process acid into higher-purity products.

Growth of Battery-Grade Phosphoric Acid

The expansion of lithium iron phosphate batteries is increasing demand for ultra-high-purity phosphoric acid.

Conclusion

Wet process and thermal process phosphoric acid technologies serve different industrial requirements. The wet process remains the backbone of global phosphoric acid production because of its low cost, scalability, and suitability for fertilizer manufacturing. It is the preferred technology for producing DAP, MAP, NPK, and other phosphate fertilizers.

The thermal process, despite its high energy consumption and operating cost, continues to be essential where superior purity is required. Food-grade phosphoric acid, pharmaceutical products, electronic chemicals, and battery materials depend on thermal technology or advanced purification methods.

The selection between wet and thermal processes depends on product requirements, economics, raw material availability, and market demand. Future developments will focus on improving sustainability, reducing environmental impact, and producing high-purity phosphoric acid efficiently for emerging industries.