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Technical comparison of processes to produce Dicalcium phosphate

The most commonly used process to produce dicalcium-phosphate is the neutralisation of calcium carbonate with phosphoric acid. Recently, the price of the high-grade rocks (MER<8) to produce phosphoric acid has significantly increased. Industry is therefore forced to use low grade rocks which complexifies the phosphoric acid production and decreases the quality of the phosphoric acid in terms of heavy metals and contaminants such as iron and aluminium oxides. Prayon has developed two further alternatives: hydrogen chloride acid process (Prayon-Ecophos process) and sulfuric acid process (Get more P process) to produce DCP without using phosphoric acid. The aim of this article is to present these three processes and to review their respective pros and cons in terms of plant operations and final product quality.

DCP based on H3PO4:
The neutralisation process consists of an addition of phosphoric acid on the calcium carbonate followed by a granulation. It can be performed in one or two steps. The resulting granules are dried in a rotary dryer and cooled down with a fluidized bed or a rotary cooler. The key operation parameters are the proportion between the phosphoric acid and the calcium carbonate to comply with the P2O5 content required on the final product as well as the moisture content on the granulation to comply with the required granule size. This process is flexible in terms of P2O5 content on the final product, it can be adapted, with a certain range, to the specification required. However, all the contaminants present on the phosphoric acid will be present on the final product. The DCP obtained via this process does not present a crystal shape but is amorphous with multiple stages of hydration. Some studies have shown that the biodigestibility of this DCP is around 60%.

DCP based on HCl:
Prayon-Ecophos process comprises several steps. The inlet materials are low grade rocks or tailings, hydrogen chloride acid, calcium carbonate, calcium oxide and water. The first step (module 1A) consists of a digestion of low-grade rock with diluted hydrogen chloride acid between 8 to 15%. At this stage, the P2O5 is present in the liquid phase together with aluminium oxide, iron oxide and fluor. The pH of the resulting slurry is slightly increased by addition of calcium carbonate to precipitate the aluminium oxide and partially the iron and fluor. This step needs to be controlled in order to reach 2000ppm of fluor content on the final DCP. The slurry is then filtrated through a filter press. The resulting liquid phase containing the P2O5 is sent to another module (module 1B), where the pH is increased with calcium carbonate to precipitate the P2O5 into DCP form. The method of addition of calcium carbonate is critical to reach the desired size of the crystals. The resulting slurry is filtrated with a vacuum filter. The solid part (DCP) is dried and the liquid part is sent to another module (module CCP -Calcium Chloride Purification). In this CCP module, the pH of the solution is increased to 7 or 10 (for magnesium removal) and then filtered. The resulting cake is cleared out and liquid phase containing CaCl2 is either sent to the sea or can be sent to module 4; where sulfuric acid is added to form HCl and gypsum. This HCl can be send back module 1A.

The operation is for sure more complex due to the numerous steps compared to the neutralisation process. The material of construction must be chosen carefully due to the presence of Cl- and low pH. However, the multiple precipitation/crystallisation ensure end-product with less than 2000ppm of F, 1ppm of Cd, 5ppm of Pb and 5ppm of As regardless of the rock quality. On this process the quantity of water is about 4t of water per ton of DCP which is higher than the average water consumption of a phosphoric acid plant (4 to 6 m³/ tP2O5). In some case this extra consumption is counterbalanced by the fact that the rock does not need to go through a water consuming beneficiation step. The final product obtained is a crystal with 2 stages of oxidation (dihydrate). Some studies have shown that the digestibility of such DCP is 82%.

DCP based on H2SO4:
Prayon – Get more P process, is similar to the Prayon-Ecophos process. The main differences are that sulfuric acid is used instead of hydrogen chloride acid and that only 2 modules are needed: module 2A for the attack and module 2B for the precipitation of the DCP (no need of the CCP module, nor the module 4). The mother liquid out of the module 2B is water and it can be recycled without treatment on the module 2A, decreasing water consumption to 1t/t DCP. The operation is simpler than the Prayon Ecophos process but more complex than the neutralisation process. The quality of the DCP is similar than the DCP obtained with the Ecophos process, except Our Ideas Make Profitable Plants for the Mg and sulphate content. If a DCP free of sulphate and magnesium is required some additional steps are required.

Conclusion
The neutralisation process is far from the simplest process but also the less flexible from the 3 presented process. Producing a DCP feed grade with this process requires a good quality phosphoric acid. To produce such acid, a high-grade rock of a purification step is required. Both DCP Prayon processes can produce feed grade DCP with low grade rock or even tailings. Those processes have the advantage to remove partially (up to 98%) the iron and aluminium such as heavy metals. Due to the increasing price of the high-grade rocks and the difficulty to supply it, those alternatives will be more and more use.

Kevin De Bois – Prayon Technologies