Prayon process

Central-Prayon A wet process for making phosphoric acid, similar to the Prayon process but using extra sulfuric acid in the crystallization of the gypsum in order to minimize losses of phosphoric acid. Developed jointly by the Central Glass Company (Japan) and Societe de Pray on (Belgium). 

F g- 23.14. Flow diagram of filtration section of a wet process phosphoric acid plant. (Prayon process, courtesy Davy Mckee Corp.)... 

The impurity content can be affected by the way in which the phosphoric acid manufacturing process is carried out single stage processes, or newer processes, which lead to the hemihydrate, have a higher impurity level, so that further purification steps are necessary. Two step processes, which either lead via the dihydrate to the hemihydrate (Central-Prayon process) or via the memihydrate to the dihydrate (Nissan process), provide products, which can be utilized without further purification steps. Should further purification of the phosphogypsum be necessary, two types of process are possible ... 

In the Prayon process, commonly used in Florida, the phosphate rock, ground to pass a 150-p.m (100-mesh) sieve, is treated with 30 to 46% phosphoric acid and 55 to 60% sulfuric acid. The rock and acid is circulated through reaction tanks to maintain the optimum time and temperature for the reaction and growth of phosphogypsum crystals. The phosphogypsum is filtered, washed with water, and pumped as a slurry to ponds from which the phosphogypsum settles to form the phosphogypsum stacks [5]. 

The hemihydrate process is similar to the Prayon process but uses higher temperatures and acid concentrations in the reaction tanks. This favors the initial formation of hemihydrate. In subsequent crystallization tanks the hemihydrate is mixed with gypsum suspensions where it recrystallizes as large crystals of the dihydrate, that is, as phosphogypsum. 

Three single-stage hemihydrate processes have been ip use in commercial-scale plants. They are the Norsk Hydro Licensing (formerly Fisons Fertilizer) Process, Prayon Process, and the Occidental Process. Average process inputs for hemihydrate processes are shown in Table 11.20.. 

This route has been investigated by Marchan (later Albright and Wilson), but the only commercial process is that developed at first by the Japanese Central Glass Company and Societe de Prayon, of Belgium and sub sequently jointly finished as the Central-Prayon Process. 

The various Prayon processes, as summarized in Table 11.23, are offered by Kvaerner Process Technology, London, England. Since 1984 Kvaerner (formerly Davy McKee) has engineered six plants of 100- to 1,310-tpd P2O5 capacity and provided engineering and supervision of construction for up to 59 wet-process acid plants in prior years. 

Prayon One of the Wet processes for making phosphoric acid by reacting phosphate rock with sulfuric acid. The byproduct is gypsum, calcium sulfate dihydrate. It uses a compartmentalized, multi-section, lined, concrete reactor, with finishing tanks in which the gypsum crystals mature. In 1990 one third of the wet process phosphoric acid made in the Western World was made in this way. The process was developed in 1977 by the Societe de Prayon, Belgium. Variations are known as PH2, PHI 1, and PH12. One variation uses solvent extraction with isopropyl ether and tri-n-butyl phosphate. 

Figure 11.13. Reaction Section in the Prayon Mark IV Process.

Such a solution has been adopted in the Singmaster and Breyer, Prayon, Fisons Fertilizers, Norsk Hydro Licensing, and Nissan C ( New Nissan") processes and a later development of the Nippon Kokan process. In commercial use are the Fisons HDH process, the Nissan C process, and a whole scale of Prayon s PH three-stage processes PH2, PH3, PHI 1, PH12 with very pure... 

Table 11.23. Features of the Prayon/Davy Processes Process/Features Dihydrate (DH) Hcmihvdrate [HH) Herrddihvdrate (HDH)...

In the early 1980s eight commercial plants in the United States and one in Canada were recovering uranium from wet-process phosphoric acid. At about the same time, a commercial uranium facility was constructed at the Chimie Rupel subsidiary-of-Societe de Prayon-inr Belgium, and others were planned in France by Rhone-Poulenc and APC, using their own technologies, and in Japan by a consortium headed by the Power Reactor and Nuclear Development Corporation, usirg its own process. 

Those using a DEPA/TOFK) mixture include the IMC/ Prayon, Freeport Uranium Recovery Co., Warning Mineral Corp., and COGEMA-APC processes. 

Figure 11.29. Prayon s Process for Phosphoric Acid Purification. 

Hoightaling, S. V. 1973. "DPG-Prayon Modern Dihydrate Process, Paper presented before the American Chemical Society, Division of Fertilizer and Soil Chemistry, Chicago, IL, U.S.A. 

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