May I ask a question? Regarding phosphoric acid! ～

In the production process of wet phosphoric acid, depending on the concentrations of phosphoric acid and sulfuric acid in the liquid phase as well as the temperature of the system, three hydrates of calcium sulfate exist in equilibrium with the solution: the dihydrate CaSO4·2H2O (gypsum), the hemihydrate α-CaSO4·0.5H2O (alpha-hemigypsum), and the anhydrous form CaSO4·II (anhydrous gypsum II). 　The production methods for wet phosphoric acid are often named after the form in which calcium sulfate appears. In industry, there are several methods for producing wet phosphoric acid: ① Wet phosphoric acid production using the dihydrate method (DH). This is the most widely used method in the world, and it includes multi-tank processes as well as single-tank processes. These processes can be further divided into those without slurry return and those with slurry return, as well as those with vacuum cooling and those with air cooling. For wet phosphoric acid produced by the dihydrate method, the typical value of W(P2O5) is 28–32%, with a total phosphorus recovery rate of 93–97%. The overall yield of phosphorus obtained by the dihydrate method is low, as the washing in this method is incomplete ； A small amount of phosphoric acid solution enters the voids in the calcium sulfate crystals; after calcium phosphate crystallizes, it grows alternately with the calcium sulfate crystal layer ； The formation of a calcium sulfate film on the surface of phosphate rock particles results in incomplete extraction of the phosphate rock. ② The half-hydrate-dihydrate method (HH/DH) for producing wet phosphoric acid involves first forming calcium sulfate into half-hydrate crystals, and then recrystallizing them into dihydrate form. This allows all of the P2O5 in the calcium sulfate lattice to be released, resulting in a total P2O5 recovery rate of 98–98.5%, while also improving the purity of phosphogypsum and expanding its applications. The semi-hydrate-dihydrate process can be further divided into two types based on the concentration of the acid in the product: the dilute acid process (HRC) and the concentrated acid process (HDH). In the former case, the hemihydrate crystals are directly hydrated to the dihydrate without filtration, followed by filtration and separation; the acid content of the product is 30–32% P2O5 ； The latter separates the product acid from the hemihydrate slurry; this acid has a W(P2O5) content of 45%, while the filter cake is sent to a hydration tank where it recrystallizes into the dihydrate form. ③ The total P2O5 yield of wet phosphoric acid produced by the dihydrate-hemihydrate (DH/HH) method reaches 99%. Phosphogypsum contains little crystalline water, which is advantageous for the production of sulfuric acid and cement; the acidity of the resulting product is around 35% W(P2O5). ④ The hemihydrate method (HH) for wet phosphoric acid yields high-concentration phosphoric acid with W(P2O5) = 40%–50%.

One more question: “③ With the dihydrate-hemihydrate (DH/HH) method, the overall yield of P2O5 in wet phosphoric acid production reaches 99%. Phosphogypsum contains little crystalline water, which is advantageous for the production of sulfuric acid and cement; the acidity of the resulting product is around 35% W(P2O5). ” The overall yield of P2O5 here is also high, and the concentration of the acid produced is high as well; then why are there so few manufacturers that use this method to produce phosphoric acid?

Hehe, once you take the cleaner’s position, remember to fill that role for me! :Lol: The dihydrate-hemihydrate process has low mineral consumption, high acid concentration, and low energy usage, but it requires more complex operational control, imposes high demands on the quality of phosphate rock, and the hemihydrate phosphoric acid system is quite corrosive. The production of wet-process phosphoric acid still relies mainly on the dihydrate process, which accounts for over 90% of global production capacity. This is mainly because the dihydrate process technology is relatively reliable, it allows for a wide range of operational conditions, corrosion of equipment materials is easy to address, and the investment required is low.

The current rapid extraction technology is actually another combination of the dihydrate method and the hemihydrate method; it represents another innovation by the Chinese and is well suited to the characteristics of phosphate mines in our country.

It refers to the different crystal forms of phosphogypsum. Simply put, dihydrate means that a phosphogypsum crystal contains two water molecules, while hemihydrate means it contains one and a half water molecules. Different forms of phosphogypsum crystals are formed mainly due to variations in the control range of the reaction temperature.