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Wet-Process Phosphoric Acid Production Processes

In the production of phosphoric acid by the hemihydrate-dihydrate wet process, phosphate rock is first crushed and ground to 80~100 mesh to prepare rock slurry, while 98% industrial sulfuric acid is diluted and proportioned according to the process ratio. The rock slurry reacts with sulfuric acid under stirring at 95~105 ℃ to undergo hemihydrate acidolysis, forming calcium sulfate hemihydrate crystals and crude phosphoric acid. The reaction slurry is sent to a vacuum filter to separate calcium sulfate hemihydrate from crude phosphoric acid with P2O5 content of 38%~42%, and the gypsum is washed to recover residual phosphorus. The calcium sulfate hemihydrate is mixed with water and converted into easily filterable calcium sulfate dihydrate crystals at 70~80 ℃, followed by re-filtration to separate phosphogypsum, which is then sent for stockpiling or resource utilization; the washing water is returned to the reaction system for phosphorus recovery. The crude phosphoric acid is purified by desulfurization, extraction and other procedures to remove impurities such as fluorine, sulfate radicals and heavy metals, and then concentrated by multi-effect evaporation to produce industrial-grade or purified phosphoric acid with P2O5 ≥ 54%. The tail gas containing HF and SiF4 is absorbed by water scrubbing to produce fluorosilicic acid and by-product fluorine salts, realizing the recovery of fluorine resources.

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Chemical Process Technology

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Product Description

 

I. Current Status of Wet-Process Phosphoric Acid Production

Wet-process phosphoric acid is the dominant production technology in the global phosphoric acid industry, accounting for 85%~90% of total output. Its core principle is to decompose phosphate rock with sulfuric acid. Alternative routes such as nitric acid process and hydrochloric acid process are rarely applied due to high cost and strict environmental constraints.The mainstream technical routes are mainly classified into three types:

  • Dihydrate Process

 It occupies a dominant position with mature technology, stable operation and good adaptability to phosphate rock. More than 80% of domestic plants adopt this process. Its disadvantages include relatively low phosphorus recovery rate of 92%~94%, high moisture content of phosphogypsum at 45%~50%, and difficulty in resource utilization.

  • Hemihydrate Process

It features energy saving and high efficiency. The reaction temperature is controlled at 95~105 ℃ to form calcium sulfate hemihydrate. The obtained phosphoric acid concentration reaches above 40% P2O5, the phosphorus recovery rate is 96%~98%, and the gypsum moisture content is only 25%~30%. However, it requires high-grade phosphate rock with P2O5 ≥ 30%, brings strong equipment corrosion and demands high process control precision, with domestic application proportion less than 10%.v

  • Hemihydrate–Dihydrate Process

As the mainstream upgrading direction, it combines the advantages of high acid concentration of hemihydrate process and easy solid-liquid separation of dihydrate process. The phosphorus recovery rate is over 98%, energy consumption is reduced by 15%~20% compared with the traditional dihydrate process, and by-product gypsum has better quality. It has become the preferred option for newly-built large-scale plants.

Industry trend: The traditional dihydrate process is being gradually upgraded, the hemihydrate–dihydrate process is becoming the mainstream, and supporting purification technologies are continuously optimized to improve product added value. Thermal-process phosphoric acid is being gradually replaced due to extremely high energy consumption and production cost, which is 2~3 times that of the wet process.

 

II.Process Description of Wet-Process Phosphoric Acid ( Hemihydrate–Dihydrate Process )

In the production of phosphoric acid by the hemihydrate-dihydrate wet process, phosphate rock is first crushed and ground to 80~100 mesh to prepare rock slurry, while 98% industrial sulfuric acid is diluted and proportioned according to the process ratio. The rock slurry reacts with sulfuric acid under stirring at 95~105 ℃ to undergo hemihydrate acidolysis, forming calcium sulfate hemihydrate crystals and crude phosphoric acid. The reaction slurry is sent to a vacuum filter to separate calcium sulfate hemihydrate from crude phosphoric acid with P2O5 content of 38%~42%, and the gypsum is washed to recover residual phosphorus. The calcium sulfate hemihydrate is mixed with water and converted into easily filterable calcium sulfate dihydrate crystals at 70~80 ℃, followed by re-filtration to separate phosphogypsum, which is then sent for stockpiling or resource utilization; the washing water is returned to the reaction system for phosphorus recovery. The crude phosphoric acid is purified by desulfurization, extraction and other procedures to remove impurities such as fluorine, sulfate radicals and heavy metals, and then concentrated by multi-effect evaporation to produce industrial-grade or purified phosphoric acid with P2O5 ≥ 54%. The tail gas containing HF and SiF4 is absorbed by water scrubbing to produce fluorosilicic acid and by-product fluorine salts, realizing the recovery of fluorine resources.

III. Core Advantages of the Process（hemihydrate–dihydrate process ）

1.Remarkable Cost and Energy Saving Performance

Compared with thermal-process phosphoric acid, it reduces production cost by 20%~30% and comprehensive energy consumption by about 70%. The further cuts energy consumption by 15%~20% and sulfuric acid consumption by 3%~4% against the traditional dihydrate route.

2.High Phosphorus Recovery and Resource Utilization

The phosphorus recovery rate exceeds 98%, much higher than 92%~94% of the conventional dihydrate process. It has strong adaptability and can utilize medium and low-grade phosphate rock to relieve the shortage of high-quality phosphate resources.

3.Controllable Product Quality and Wide Application

Purified crude phosphoric acid can be processed into industrial grade, feed grade, food grade and even electronic grade high-purity phosphoric acid, fully meeting the demands of phosphate fertilizer, fine phosphate chemicals, new energy (LiPF6) and other fields.

4.Strong Large-Scale Production Capacity

With mature and reliable technology, it can support large-scale single-line capacity above 150,000 tons per year. The plant features high automation level, stable continuous operation and excellent safety performance.

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