Wet Process Phosphoric Acid Production

In-line concentration measurement and quality monitoring of sulfuric and phosphoric acids 

Optimize Wet-Process Phosphoric Acid Production: Precision Acid Control for Maximum Efficiency

Phosphoric acid (H₃PO₄) is primarily produced by reacting phosphate rock with sulfuric acid, a method known as the wet process, which accounts for over 80% of global production. This multi-step operation—comprising reaction, filtration, and concentration—demands tight process control to ensure high yield and energy efficiency.

In the reactor, sulfuric acid (H₂SO₄) is used to acidify ground phosphate rock, converting tricalcium phosphate into phosphoric acid and forming calcium sulfate (gypsum) as a by-product. Maintaining the sulfuric acid concentration between 93–98% is critical: it directly influences both the reaction kinetics and the crystallization behavior of gypsum. Precise acid control at this stage also determines the strength of the resulting phosphoric acid, minimizing the energy load during evaporation.

Following solid-liquid separation and washing in the filtration step, the resulting phosphoric acid contains roughly 32% P₂O₅ (≈50% H₃PO₄). This intermediate-grade acid is then concentrated via evaporation to a commercial specification of 54% P₂O₅ (≈70% H₃PO₄). For food, pharma, and cosmetics applications, further purification steps are employed to reach high-purity grades.

Vaisala Polaris™ Process Refractometer helps to

• measure in real-time sulfuric and phosphoric acids concentration in-line, eliminating the need for sampling
• ensure that only phosphoric acid of high strength is produced and less energy is used by evaporators
• regulate the heat load in the reactor vessel
• control gypsum crystallization allowing crystal size and shape to optimize the efficiency of the filtration step
• reduce operation costs by monitoring the final phosphoric acid concentration 

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