The thermal and wet process phosphoric acid

The differences between thermal process phosphoric acid and wet process phosphoric acid,CAS,7664-38-2.

Fundamental distinction

The core difference lies not in the chemical identity of the final product—which is phosphoric acid (H₃PO₄) in both cases—but in the production pathway. This pathway dictates the impurity profile, cost structure, and ultimately, the field of application. The two processes are fundamentally different in their chemical engineering and economics.

Wet Process Phosphoric Acid

1. Production Process & Chemistry

The wet process is a acid digestion method. It involves reacting sulfuric acid (H₂SO₄) with pulverized phosphate rock (primarily fluorapatite, Ca₅(PO₄)₃F) in a series of reactors.

The overall chemical reaction is:

Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O → 3H₃PO₄ + 5CaSO₄·2H₂O + HF

The hydrofluoric acid (HF) generated simultaneously reacts with silica impurities in the rock to form hydrogen hexafluorosilicate (H₂SiF₆). The calcium sulfate (gypsum) is filtered out as a solid by-product. This filtration step is critical and challenging, as the gypsum crystals must be formed in a way that makes them easy to filter and wash to recover the maximum amount of phosphoric acid.

2. Characteristics

Purity: Low. This is the most defining characteristic. The acid is contaminated with a multitude of ions originating from the phosphate rock. Key impurities include:

Sulfates (SO₄²⁻): From excess sulfuric acid.

Fluorine compounds: Primarily as H₂SiF₆ (fluosilicic acid) and other fluoride complexes.

Cations: Iron (Fe³⁺), Aluminum (Al³⁺), Magnesium (Mg²⁺), Calcium (Ca²⁺).

Heavy Metals: Traces of cadmium, uranium, and other radioactive elements present in the ore can also be co-extracted.

Concentration: The acid produced directly from the "filter" stage is relatively dilute, typically containing 68-75% H₃PO₄ (equivalent to 42-54% P₂O₅). It can be evaporated to higher concentrations, but the impurities remain.

Appearance: Often colored, ranging from a light green to a dark brown or black, due to the presence of dissolved iron and organic matter.

Cost: The process is less energy-intensive and therefore significantly cheaper than the thermal process.

3. Primary Applications

Due to its low purity, wet process acid is unsuitable for sensitive applications. Its dominant use (over 90%) is in the production of agricultural fertilizers, such as:

Diammonium Phosphate (DAP)

Monoammonium Phosphate (MAP)

Triple Superphosphate (TSP)

It is also used in the production of industrial-grade sodium phosphates (for detergents and water treatment) and for metal treatment processes where high purity is not a concern.

Thermal Process Phosphoric Acid

1. Production Process & Chemistry

The thermal process is a volatilization and oxidation method. It is a two-step process that first produces elemental phosphorus, which is then converted to pure phosphoric acid.

Step 1: Reduction in an Electric Arc Furnace. Phosphate rock, along with coke (a carbon reductant) and silica (a flux), is fed into a powerful electric arc furnace at temperatures exceeding 1400-1500°C. The reaction produces gaseous phosphorus and carbon monoxide:

2Ca₅(PO₄)₃F + 15C + 9SiO₂ → 15CO + 3P₄ (g) + 9CaSiO₃ (slag) + CaF₂

The molten calcium silicate slag is tapped off. The phosphorus vapor is condensed under water into a liquid form known as "yellow phosphorus."

Step 2: Combustion and Hydration. The liquid phosphorus is pumped to a combustion chamber, where it is burned in air to form phosphorus pentoxide:

P₄ + 5O₂ → P₄O₁₀The resulting P₄O₁₀ fumes are then hydrated (absorbed) in a carefully controlled manner in demineralized water to produce phosphoric acid:P₄O₁₀ + 6H₂O → 4H₃PO₄

2. Characteristics

Purity: Extremely high. Since the process involves vaporizing phosphorus, all the mineral impurities from the rock (metals, fluorine, etc.) are left behind in the furnace slag. The resulting acid is exceptionally pure.

Concentration: The acid produced can be made at very high concentrations, typically 85% H₃PO₄ or higher, as standard.

Appearance: A colorless, water-white, and viscous liquid.

Cost: The process is highly energy-intensive, primarily due to the enormous electricity demand of the electric arc furnace. This makes thermal phosphoric acid significantly more expensive than the wet process variant.

3. Primary Applications

The high purity and cost justify its use in applications where impurities would be detrimental:

Food Industry: As an acidulant, pH regulator, and flavor enhancer in soft drinks (e.g., colas), jams, and processed cheeses. It must meet strict food-grade specifications (e.g., FCC, FAO/WHO).

Pharmaceutical Industry: As an ingredient in dental cements, as a pH adjuster in syrups, and in the synthesis of active pharmaceutical ingredients (APIs).

Electronics Industry: For precise etching of silicon wafers and for cleaning glass substrates in LCD and OLED displays. For this, an even higher "electronic grade" purity is required.

High-Purity Chemicals: As a precursor for analytical reagents and high-grade sodium and potassium phosphates.

Summary

In essence, the choice between thermal and wet process phosphoric acid is a classic trade-off between purity and cost.

Wet Process Acid is the workhorse for agriculture, offering a cost-effective solution for fertilizer production, where impurities are tolerated.

Thermal Process Acid is a high-purity specialty chemical, essential for food, pharmaceutical, and electronic applications where the presence of even trace impurities is unacceptable, justifying its high production cost.

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