Phosphoric acid, phosphorous acid, and sodium hypophosphite are several important chemical raw materials for low-valent phosphorus compounds. They are characterized by their ability to undergo redox reactions to produce phosphate salts. Phosphoric acid is widely used both domestically and internationally in agriculture, food, and electronic chemistry. Currently, most wet-process phosphoric acid production methods use sulfuric acid as the extractant, decomposing phosphate rock to obtain phosphoric acid and calcium sulfate. Depending on the crystal form of calcium sulfate, production methods can be categorized in several ways. Below, we introduce four methods for wet-process phosphoric acid production.

1. Dihydrate Method: 

This method is carried out at relatively low temperatures (75~86℃) and low concentrations (20%~30% P2O5) to ensure the formation of calcium sulfate dihydrate. The reaction is carried out in a series or continuous reactor, with the slurry residence time in the system being 4~8 hours. To improve the phosphate rock decomposition rate, phosphate rock powder, washed dilute phosphoric acid, recycled slurry, and backflow acid are first added to the reactor to react and generate calcium dihydrogen phosphate. Then, sulfuric acid is added to react and generate calcium sulfate dihydrate precipitate. Most of the slurry after the reaction is returned, with a small portion sent for filtration. After filtering out the concentrated acid, it undergoes further washing with dilute acid and water to separate phosphoric acid and washed gypsum. Theoretically, the dihydrate wet process for phosphoric acid production can yield 28%–32% P₂O₅, but due to impurities in the ore and difficulties in controlling operating conditions, it generally only yields 20%–25% P₂O₅. The filtration intensity is 400–600 kg (dry basis)/(m²·h), and the total phosphorus recovery rate is 90%–96%. These figures can be even higher when the ore grade is high and the operation is well controlled.

2. Hemihydrate Process: 

When calcium sulfate exists in a hemihydrate form, it is called the hemihydrate process. The hemihydrate process can directly produce high-concentration phosphoric acid, reaching 38%–42% P₂O₅. It contains fewer impurities dissolved in the acid and can be directly used in compound fertilizers. The resulting hemihydrate gypsum can also be directly used as building materials. Therefore, compared to the dihydrate process, it has stronger economic advantages and better development prospects. However, its disadvantages include the high required temperature (93~120℃), leading to severe corrosion; and the hemihydrate gypsum hydrating and clogging the filter cloth in the filter, making filtration difficult.

3. Hemihydrate-Dihydrate Method: 

This method first precipitates calcium sulfate in a hemihydrate form, then forms large dihydrate crystals. This yields concentrated phosphoric acid, high filtration efficiency, and high phosphorus recovery rate. Additionally, the dihydrate-hemihydrate method is used to improve phosphorus recovery from phosphate rock while obtaining pure gypsum, which can be directly used as a raw material in industrial production.

4. Anhydrous Method: 

This method is carried out at high temperatures (120~130℃) and high acid concentrations, yielding phosphoric acid with 42%~45% P₂O₅. The difficulties of this method are: severe corrosion of equipment occurs at 120~130℃; external heating with steam is required to reach this reaction temperature; and separation of the acid and anhydrous product is difficult. Therefore, it remains in the experimental stage.

The production processes for phosphorus-containing compounds such as phosphoric acid, sodium hypophosphite, sodium tripolyphosphate, sodium pyrophosphate, and aluminum hypophosphite flame retardants have been continuously improving. Currently, domestically produced finished products have completely replaced imported products in terms of content, appearance, and impurities. However, downstream applications and processing technology are indeed a weakness for domestic manufacturers.